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Inhaltsverzeichnis

Frontmatter

Plenary

Frontmatter

A Brief History of the Development of Grain Refinement Technology for Cast Magnesium Alloys

Grain size control during casting is important for ensuring the best and most consistent mechanical and corrosion performance is achieved in magnesium alloy components. Also, it is important for the fabrication of wrought Mg products with desired surface quality. Industrially, this means employing grain refinement methods. The important role of grain refinement was realized in the 1930s. However, developing a good understanding of the solidification mechanisms for improved refinement technologies involved in successful refinement has been the subject of ongoing research ever since. This paper provides a brief history of developments both scientifically and industrially, and summarizes current issues related to the development of improved grain refiners.

D. H. StJohn, P. Cao, M. Qian, M. A. Easton

Creep

Frontmatter

Compressive Creep Properties of Wrought High Temperature Magnesium Alloys in Axial and Transverse Orientation — A Neutron Diffraction Study

Magnesium alloys developed for high temperature applications may become a material of choice in structural components exposed to elevated temperatures, for example, around automotive front end structures. Performance of wrought materials, however, often depends on the direction of the load with respect to the extrusion direction.This study focused on investigating compressive creep response of three extruded magnesium alloys: AE42, EZ33 and ZE10 in the longitudinal and transverse directions. In-situ neutron diffraction experiments on evolution of primary and secondary creep strain at 175°C, 50MPa load for the duration of 24 hours was studied for several crystallographic planes.The results suggest that the AE42 alloy exhibited significant strain fluctuations on the basal (0002) and <math display='block'> <mrow> <mo stretchy='false'>(</mo><mn>10</mn><mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mn>2</mn><mo stretchy='false'>)</mo> </mrow> </math>$$(10\bar 12)$$ planes, possibly related to plastic deformation processes of grain boundary sliding or twinning. In contrast, EZ33 and ZE10 alloys had a significantly lower and more uniform strain deformation in both elastic and plastic regime. Micro structure analysis revealed that the AE42 alloy consisted of large Al11RE3 particles with Mg17Al12 phase forming around Al-RE particles and in intergranular regions, while the aluminum-free alloys contained complex intermetallics on the grain boundaries, as well as Zn-RE nanoscale dispersoids throughout the matrix.

D. Sediako, L. Bichler, M. van Hanegem, S. Shook

Creep Behaviour of Mg Binary Solid Solutions

Specimens of cast Mg-0.8 at.%Gd, Mg-2.2 at.%Zn and Mg-2.5at.%Al alloys were tested in compression at an initial strain rate of ~1.5×10−5s−1 at room temperature and 180°C. The Mg-Zn alloy, and more so the Mg-Gd alloy, exhibited a largely athermal behaviour, in contrast with the Mg-Al alloy which softened considerably at the higher temperature. The athermal behaviour of the Zn- and Gd-containing alloys can be accounted for by their strong tendency to developing short range order.

Saeideh Abaspour, C. H. Cáceres

Influence of Yttrium on Creep Behavior in Nano-Crystalline Magnesium Using Molecular Dynamics Simulation

Due to their light weight, magnesium (Mg) and its alloys have great potential for reducing vehicular mass and energy consumption. However, the use of Mg alloys is currently restricted to low-temperature automotive components application. This work strives to gain a better understanding of the effect of yttrium (Y) (up to 3at%) on creep behavior of columnar nanocrystalline Mg with a grain size of 5nm and 10nm. Using molecular dynamics (MI)) simulations, nanocrystalline Mg with various local concentrations of Y was subjected to a constant-stress loading, ranging from 0–500MPa, at different initial temperatures, ranging from 473–723K. Our simulations reveal that the secondary stage creep rate <math display='block'> <mrow> <mo stretchy='false'>(</mo><mover accent='true'> <mi>&#x03B5;</mi> <mo>&#x02D9;</mo> </mover> <mo stretchy='false'>)</mo> </mrow> </math>$$(\dot \varepsilon )$$ decreases by 71% with the addition of only 1at% Y at 500MPa and 623K. With the addition of alloying elements such as Y, the creep rate in the secondary region decreases and the creep deformation mechanism is changed from the void nucleation, growth, and coalescence to GB rotation/sliding. The analyses of the diffusion coefficient and energy barrier reveal a stronger contribution to the overall deformation by the grain boundary diffusion at the low-temperature (423K) and by the lattice diffusion at the higher-temperature (723K).

M. A. Bhatia, K. N. Solanki

Aging Behavior and Microstructural Evolution in Mg-3Nd-0.2Zn-0.5Zr Alloy

The aging behavior and micro structure evolution of Mg-0.2Zn-3Nd-0.5Zr alloy were studied in this paper. Transmission Electron Microscopy investigations showed four sets of precipitates formed during aging. These precipitates are β” which forms on prismatic planes during early stages of precipitation, γ” which forms on basal plane, and β” which forms on prismatic and pyramidal planes. In addition, some unknown precipitates in the form of dark spots were detected in 8 hours aged sample. The results from nanoindentation showed an activation volume in the range 80–100 b3 that is consistent with prismatic glide as the rate controlling mechanism in the system. Furthermore, a small amount of dislocation climb along with dynamic recovery occur at room temperature upon deformation in this system.

Amirreza Sanaty Zadeh, Xiangyu Xia, Alan A. Luo, Joseph E. Jakes, Donald S. Stone

Cast Materials

Frontmatter

FE Modelling of Tensile and Impact Behaviours of Squeeze Cast Magnesium Alloy AM60

In response to the need for reduced global emissions, the transportation industry has been steadily increasing the magnesium content in vehicles. This trend has resulted in experimental documentation of numerous alloy and casting combinations, while comparatively little work has been done regarding the development of numerical material models for vehicle crashworthiness simulations. In this study, material mechanical behaviour was implemented into an existing material model within the nonlinear FEA code LS-DYNA to emulate the mechanical behaviour of squeeze cast magnesium alloy AM60 with a relatively thick section of 10 mm thickness. Model validation was achieved by comparing the numerical and experimental results of a tensile test and Charpy impact event. Validation found an average absolute error of 5.44% between numerical and experimental tensile test data, whereas a relatively large discrepancy was found during Charpy evaluation. This discrepancy has been attributed to the presence of microstructure inhomogeneity in the squeeze cast magnesium alloy AM60.

Sante DiCecco, William Altenhof, Henry Hu

High Temperature Deformation of Magnesium Alloy TX32-0.4Al-0.8Si

The effect of aluminum and silicon as micro-alloying elements in TX32 magnesium alloy on its hot deformation behavior has been studied by conducting uniaxial compression tests at various combinations of temperatures and strain rates in the ranges 300–500 °C and 0.0003–10 s−1. A processing map has been developed and the effect of processing conditions on deformation behavior has been analyzed. Two processing windows that enable good hot working of the alloy are identified at (1) 390–500 °C/0.0003–0.005 s−1(Domain 1), (2) 430–500 °C/0.3–10 s−1 (Domain 2). The kinetic analysis is obeyed in these two domains and the relevant apparent activation energy values are found to be 215 and 170 kJ/mole respectively. These are higher than that for self-diffusion in magnesium suggesting that intermetallic particles present in the matrix generate back stress.

C. Dharmendra, K. P. Rao, N. Hort, K. U. Kainer

Primary Production and Shaping

Frontmatter

Doing Projects in a Foreign Language — Communications Management, Issues and Strategies

The Qinghai Salt Lake Industry Co. Ltd. (QSLIC) smelter project involves a Chinese state-owned client, a Canadian engineering company, their American technology partner, several Chinese design institutes and international vendors. Proper technical communication plays an important role during the development of a project in a foreign language and across cultures. Using the QSLIC project as an example, this paper presents the role of Communications Manager and personal qualifications required, as well as technical communications management, issues and strategies, lessons learnt while doing smelter projects in China and Chinese business culture and etiquette.

Deling Xian

Impact of Site Elevation on Mg Smelter Design

Site elevation has many surprising and significant impacts on the engineering design of metallurgical plant of all types. Electrolytic magnesium smelters maybe built at high elevation for a variety of reasons including availability of raw material, energy or electric power. Because of the unit processes they typically involve, Mg smelters can be extensively impacted by site elevation. In this paper, generic examples of the design changes required to adapt a smelter originally designed for sea level to operate at 2700 m are presented. While the examples are drawn from a magnesium plant design case, these changes are generically applicable to all industrial plants utilizing similar unit processes irrespective of product.

Phillip W. Baker

Purification of Highly Contaminated Magnesium Melt

The steering wheel core is chosen as the first target for the development of a purification technology for highly contaminated magnesium melt, because it contains abundant foreign matter such as polyurethane, copper electrodes, and steel inserts, which have high potential to form non-metallic inclusions and to deteriorate the corrosion resistance of recycled alloys. Various melt treatment technologies have been investigated for refining AM50 magnesium alloy contaminated with polyurethane. The NMI content in magnesium alloy scrap contaminated with polyurethane was effectively reduced by a sequential refining process consisting of filtration, fluxing, and gas bubbling treatments. The filtration step reduced most large inclusions such as carbon residues from the decomposed polyurethane. The subsequent fluxing and gas bubbling treatments effectively removed the small inclusions such as carbonates and oxides.

Byoung-Gi Moon, Bong-Sun You, Ki-Ho Koh

Selective Laser Melting of Magnesium and Magnesium Alloys

Selective Laser Melting (SLM) offers the possibility to create three dimensional parts by having full freedom of design. Therefore prototypes can be produced faster and conventionally manufactured parts can be shaped individually, including an optimized design regarding potential loads and parts weight. The manufacturing of biocompatible metals like 316L and TiA16V4 is already industrially established. Because of the corrosive and mechanical properties of magnesium and the advantages of the SLM process, using magnesium is of great interest for manufacturing individual biodegradable implants. Recent investigations on SLM of magnesium have not led to successful operation so far. Due to the low vaporizing temperature, manufacturing non-porous and three dimensional parts from magnesium was not possible yet. Following a new strategy, using an industrial SLM system with an overpressure building chamber, investigations on SLM of magnesium are now carried out in order to overcome these difficulties and produce fully dense three dimensional parts.

Matthias Gieseke, Christian Noelke, Stefan Kaierle, Volker Wesling, Heinz Haferkamp

Evolution of the Magnetherm Magnesium Reduction Process

The magnetherm process was developed and implemented as an improvement of the solid state silicothermic magnesium reduction process. This process was used commercially for over 35 years. The reduction plant operation and equipment are described as they existed at the time of the industry wide curtailment.

James C. Sever, Marlyn Ballain

Research on new type materials preparation for magnesium production by Silicothermic process

During the magnesium preparation process of the silicothermic process, dolomite was made into pellets after calcination process, about 5% fine materials would be produced which can’t be used and lead to high energy consumption and high-cost. Based on in-situ reduction of dolomite-based desulfurization theory, this paper proposed a new technology that dolomite was made into the pellets first and then was calcined to prepared the pellets with higher reactivity and stability. The results indicate that: In contrast with dolomite pellets prepared by single binder, dolomite pellets prepared by composite binder are dominant in functions without fine materials produced, the size of pellets is uniform, the falling strength can reach 2.5 times per 0.5 m, and the compressive strength gets 80N as well.

Wen Ming, Zhang Ting-an, Dou Zhi-he, Ren Xiao-dong, Zhang Rui, Zhou Lian

Texture and Twinning

Frontmatter

Effect of grain size and basal texture on tensile properties and fracture characteristics of extruded AZ31 alloy

With different tilting angles corresponding to the basal planes of the extruded Mg-3%Al-1%Zn alloy (0°, 30°, 45°, 60°, 90°), uniaxial tensile tests of the extruded alloy were conducted at room temperature to discuss the effects of basal texture and grain size on its tensile properties and fracture characteristics. Compared to the coarse-grained sample, the fine-grained sample showed obvious increase in the yield stress since the tilting angles were 0° and 90°. However, the total elongation of the fine-grained sample was higher than that of the coarse-grained specimen since the tilting angles ranged from 30° to 60°. According to the observation of fracture surface, a dimple feature was observed in the fine-grained samples. In the contrast, a plate-like feature was recognized on the fracture surface of the sample with coarse grains.

Hsiang-Ching Chen, Truan-Sheng Lui, Li-Hui Chen

Interaction between a Mg17Al12 precipitate and twin boundary in magnesium alloys

Interactions between Mg17Al12 precipitates and <math display='block'> <mrow> <mo>&#x007B;</mo><mn>10</mn><mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mn>2</mn><mo>&#x007D;</mo><mo>&#x003C;</mo><mn>10</mn><mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mo>&#x003E;</mo> </mrow> </math>$$\{ 10\bar 12\} < 10\bar 1\bar 1 >$$ twin boundaries (TBs) in magnesium were studied by molecular dynamics simulations. The results obtained agree well with experimental observations in which precipitates can be entirely engulfed by <math display='block'> <mrow> <mo>&#x007B;</mo><mn>10</mn><mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mn>2</mn><mo>&#x007D;</mo><mo>&#x003C;</mo><mn>10</mn><mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mo>&#x003E;</mo> </mrow> </math>$$\{ 10\bar 12\} < 10\bar 1\bar 1 >$$ twins without being sheared. Structural analysis of the TBs in the atomic scale shows that the TBs are extremely incoherent during twin growth and highly deviate from the <math display='block'> <mrow> <mo>&#x007B;</mo><mn>10</mn><mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mn>2</mn><mo>&#x007D;</mo> </mrow> </math>$$\{ 10\bar 12\}$$ twinning plane as previously observed in a number of experiments. The simulation studies indicate that <math display='block'> <mrow> <mo>&#x007B;</mo><mn>10</mn><mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mn>2</mn><mo>&#x007D;</mo><mo>&#x003C;</mo><mn>10</mn><mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mo>&#x003E;</mo> </mrow> </math>$$\{ 10\bar 12\} < 10\bar 1\bar 1 >$$ twinning was accomplished solely by atomic shuffling that converts the parent lattice to the twin lattice without involving twinning dislocations, resulting in zero shear strain at the TBs.

B. Li, S. N. Mathaudhu

Microstructure and Texture Evolution in a Magnesium Alloy During Extrusion at Various Extrusion Speeds

An AM30 magnesium alloy was extruded by using a lab-scale flat die at ~450 °C and various ram speeds: 5 mm/min, 10 mm/min, 20 mm/min, 30 mm/min, and 50 mm/min, respectively. Microstructure and texture in the representative locations inside the die and the extrudate of the AM30 at different ram speeds were examined by electron backscatter diffraction (EBSD). Significant dynamic recrystallization (DRX) occurred inside the die, whereas static recrystallization (SRX) took over in the extrudate outside the die. Profuse <math display='block'> <mrow> <mo>&#x007B;</mo><mn>10</mn><mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mn>2</mn><mo>&#x007D;</mo><mrow><mo>&#x2329;</mo> <mrow> <mn>10</mn><mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> </mrow> <mo>&#x0232A;</mo></mrow> </mrow> </math>$$\{ 10\bar 12\} \left\langle {10\bar 1\bar 1} \right\rangle$$ extension twinning activated during extrusion at low ram speed of 10 mm/min, but twinning was hardly observed at high ram speed of 50 mm/min. DRX and SRX led to different microstructure evolution at different extrusion speeds. Possible mechanisms that govern the DRX and the SRX were analyzed.

Q. Ma, S. J. Horstemeyer, B. Li, Z. McClelland, P. T. Wang, M. F. Horstemeyer

The Texture and Microstructure Evolution of Mg-Zn-Ce Alloys

The texture evolution in four Mg-Zn-Ce alloys was compared to that of Mg-3Al-1Zn (AZ31) alloy following rolling and subsequent isothermal annealing. All the as-cast and homogenized alloys were rolled through two stages that can be characterised as rough rolling and finish rolling, respectively. To investigate the effect of finish rolling temperature, one finish rolling pass with 65% reduction in thickness was performed at 300 °C and 450 °C. Of the studied compositions, the Mg-1Zn-lCe, which had the highest Ce/Zn ratio, showed the weakest as-rolled texture and homogenous shear banding/twinning. Changing the Zn content changed particle size and, in alloys subject to texture weakening, the static recrystallization mechanism altered. On annealing, the maximum intensity of basal pole figures decreases as recrystallization progresses. The Mg-1Zn-1Ce (with the highest Ce/Zn), texture weakening is maintained even after full recrystallization, when grain coarsening occurs. However, in the Mg-4Zn-1Ce and AZ31 alloys, texture strengthening occurs when grain coarsening occurs, and the double split basal peak is replaced by a single peak. In these two alloys, grain coarsening is also accompanied by a bimodal grain size whereas in the Mg-1Zn-1Ce alloy, the grain coarsening leads to a uniform grain size. It is concluded that the differences between the Ce bearing alloys is related to Zn; increasing Zn decreases the solubility of Ce, which can influence the texture changes.

M. Sanjari, A. Farzadfar, T. Sakai, H. Utsunomiya, E. Essadiqi, In-Ho Jung, S. Yue

Twin boundary migration creating zero shear strain: In-situ TEM observations and atomistic simulations

Atomistic simulations were conducted to study the migration of <math display='block'> <mrow> <mo>&#x007B;</mo><mn>10</mn><mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mn>2</mn><mo>&#x007D;</mo><mo>&#x003C;</mo><mn>10</mn><mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mover accent='true'> <mn>1</mn> <mo>&#x00AF;</mo> </mover> <mo>&#x003E;</mo> </mrow> </math>$$\{ 10\bar 12\} < 10\bar 1\bar 1 >$$ twin boundary. A bi-crystal that satisfies the twin orientation relationship was constructed and a tensile strain was applied in parallel to the <c> axis of one crystal. Under the tensile strain, the twin boundary starts to migrate but the migration (twin growth) does not produce any observable shear strain on the bicrystal. In-situ transmission electron microscopy (TEM) observations of a single crystal Mg under tension and compression confirm that during twinning and detwinning, no shear strain is produced. The specimen uniformly elongates and narrows during twinning, and widens across the width during detwinning.

B. Y. Liu, B. Li, Z. W. Shan

Microstructure Characterization of Weakly Textured and Fine Grained AZ61 Sheet

Formability in magnesium alloy sheet is strongly limited by a strong basal texture in the as-rolled material, which is difficulty to remove by thermal processing. We introduce a new process to the control of texture by combining Thixomolding and Thermomechanical Processing (TTMP). Plates of AZ61L with a divorced β-Mg17Al12 eutectic are produced by Thixomolding, resulting in a non-textured, fine grained (2.8 µm) precursor. Sheet produced from the plate by single pass warm-rolling exhibits a weaker texture, and more isotropic tensile deformation than generally observed in AZ-series alloy sheet. Recrystallization annealing produces a further reduction in texture and average grain size (2.3 µm) and results in nearly isotropic room temperature deformation, a yield strength of ~ 220 MPa, and an elongation of ~ 23%. Particle stimulated nucleation of new grains by the β-phase during both dynamic and static recrystallization, is critical for achieving the low levels of texture. The influence of β-phase distribution in microstructure development is discussed.

T. D. Berman, W. Donlon, C. K. Hung, P. Milligan, R. Decker, T. M. Pollock, J. W. Jones

Wrought Materials I

Frontmatter

Flow Behavior and Hot Workability of Pre-Extruded AZ80 Magnesium Alloy

The hot deformation behavior of pre-extruded AZ80 magnesium alloy has been studied using the processing map technique. Compression tests using Gleeble-3800 thermal simulator were performed in the temperature range of 250–450°C and the strain rate range of 0.001–10 s−1. The flow stress data were used to develop processing maps at true strains of −0.1 to −0.8 according to the well-known dynamic material model and instability criterion. A single dynamic recrystallization (DRX) domain occurs in the range of 420–450°C and 0.1–1.0 s−1, which are the optimum forming conditions for the hot working of this alloy. There are two flow instability regimes occurring at 250–450°C and 0.004–10 s−1 and 433–450°C and 0.002–0.014 s−1. The former occurs at low temperatures and/or high strain rates and is associated with adiabatic shear bands or cracks, flow localization, and deformation twinning, while the latter at higher temperatures and lower strain rates is due to abnormal grain growth and wedge cracking.

Lei Gao, Alan A. Luo, Shiyi Wang, Xiaoqin Zeng

Inverse strain rate sensitivity of bendability of an AZ31 sheet in three-point bending

Three-point bending tests were performed on as-rolled and annealed (at 150 °C) AZ31 sheet specimens at various displacement rates (1.0, 5.0 and 50.0 mm/min) at room temperature. The as-rolled specimens present a negative sensitivity, i.e., the bending angle decreases as the strain rate increases; however, the annealed specimens show a positive sensitivity, i.e., the bending angle increases as the strain rate increases. Such an inverse strain rate sensitivity of sheet bending may significantly impact the sheet forming of Mg alloys.

B. Li, S. J. Horstemeyer, A. L. Oppedal, P. T. Wang, M. F. Horstemeyer

Annealing of Cold and Warm Rolled AZ31B Magnesium Alloy Sheets

Annealing experiments have been performed on warm and cold rolled AZ31B magnesium alloy sheets at 200°C in several time intervals. The effect of the amount of deformation, the annealing time, and the presence of intermetallic phases were evaluated to obtain a microstructure constituted of fine recrystallized grains. Micro structural evolution was followed using optical microscopy, scanning electron microscopy combined with energy dispersive spectroscopy (SEM-EDS), X-ray diffraction and Vickers hardness measurements. After annealing for 60 min, it was found that static recrystallization mostly takes place on cold-rolled sheets with high thickness reduction, refining the grain size below 10 urn. Twins that were observed in warm rolled specimens nearly disappear and micro structure became suitable for further rolling process. Coarse phases of Mg17Al12 were found inside grains and fine precipitation of Al4Mn and AlMn were detected at grain boundaries, which presented strong pinning effects to prevent the grain growth.

Litzy L. Choquechambi Catorceno, Denise Adorno Lopes

Gas-Pressure Bulge Forming of Mg AZ31 Sheet at 450°C

Magnesium (Mg) sheet materials, such as wrought AZ31, possess low densities and high strength- and stiffness-to-weight ratios. These properties suggest that the use of Mg sheet is viable for reducing vehicle weight, an important goal of the automotive industry. Magnesium exhibits poor ductility at room temperature, but high-temperature forming processes may be used to manufacture complex vehicle closure panels. Tensile tests are the most common method of characterizing the plastic deformation of sheet materials. However, gas-pressure bulge tests may be more representative of the stress states that occur during the manufacture of sheet metal components. This study investigates the plastic deformation of AZ31 sheet during both biaxial and plane-strain gas-pressure bulge forming at 450°C. The heights and thicknesses of formed specimens are measured and compared. The deformation behaviors of the AZ31 sheet are related to observations of grain growth and cavitation that occur during forming.

Alexander J. Carpenter, Jon T. Carter, Louis G. Hector, Eric M. Taleff

Corrosion I

Frontmatter

Efficiency of a New Hexavalent Chromium-Free Chemical Pickling Process Based on Organic and Inorganic Acids on Magnesium Alloys Mg-Y-RE-Zr and Mg-Zn-RE-Zr

The removal of surface contaminants as result of corrosion attack, inadequate or incorrectly applied surface protection coating (HAE, DOW 19) on Mg-Y-RE-Zr and Mg-Zn-RE-Zr alloys was investigated by a new chemical pickling process based on organic and inorganic acids with addition of anticorrosion compounds and wetting agents. Various combinations of acetic, citric, oxalic, tartric acids with nitric, sulphuric, phosphoric and hydrofluoric acids were investigated. The influence of the composition, pH and time of pickling, on the surface morphology, roughness, composition (of contaminant film and of alloy surface) were evaluated using Bruker Dektak 150 profilometer, Scanning electron microscopy and X-Ray Photoelectron Spectroscopy.The experimental results show that the best pickling efficiency was obtained with a combination of tartaric acid, nitric acid and a zirconium fluride for removal of ~ 20 urn of the contaminated surface. After completely removal of the incorrectly coatings, the alloy surfaces obtained were remarkably cleaned with a low roughness (between 3 and 4 urn) and without any attack at the grain boundaries and surface, and without any surface contaminant. An anti-corrosive effect was obtained by the formation of a protective layer on the surface of the alloy during the pickling. This new tartaric + nitric acids based pickling is an interesting alternative to the environmentally undesirable Cr6+- surface pickling.

Hélène Ardelean, Antoine Seyeux, Sandrine Zanna, Philippe Marcus, Sophie Pettier, Nathalie Le Pottier, Daniel Lecuru

Galvanic Corrosion of Mg-Zr Alloy and Steel or Graphite in Mineral Binders

The dismantling of UNGG nuclear reactor generates numerous nuclear wastes such as fuel decanning commonly composed of Mg-Zr alloy. A conditioning strategy consists in encapsulating these wastes into a hydraulic binder in a suitable state for storage. The eventual presence of steel and graphite accompanying the magnesium wastes could imply corrosion by galvanic coupling. This work is an experimental investigation of the galvanic coupling between Mg-Zr alloy and steel or graphite using ZRA electrochemical method in Portland cement or geopolymer pastes. The lowest corrosion activity of magnesium alloy while coupled to graphite or steel cathode has been observed in geopolymer pastes. Indeed, in this binder, an efficient corrosion protection of the magnesium alloy maintains the galvanic current very low during all the hardening process. In geopolymer paste, current densities of anodised Mg-Zr alloy is not dependent of the cathode/anode surface ratio in the range of 0.1 to 5 due to the dominance of the anode resistance.

David Lambertin, Adrien Rooses, Fabien Frizon

The influence of Mg-Zr master alloy microstructure on the corrosion of Mg

In this study, sixteen Mg-Zr alloys were produced to investigate the role of Zr on corrosion of Mg. Alloys were produced using two different commercial Mg-Zr master alloys commonly used for grain refining Mg, but which contain different Zr particle size distributions. It is seen that the master alloy with a smaller Zr particle size leads to an alloy containing more Zr in solid solution. The ratio of Zr in solid solution and in particle form was observed to have a marked effect on the corrosion of Mg.

D. S. Gandel, M. A. Easton, M. A. Gibson, T. Abbott, N. Birbilis

The Use of AC-DC-AC Methods in Assessing Corrosion Resistance Performance of Coating Systems for Magnesium Alloys

The potential utility of AC-DC-AC electrochemical methods in comparative measures of corrosion-resisting coating system performance for magnesium alloys under consideration for the USAMP “Magnesium Front End Research and Development” project was previously shown in this forum [1]. Additional studies of this approach using statistically-designed experiments have been conducted with focus on alloy types, pretreatment, topcoat material and topcoat thickness as the variables. Additionally, sample coupons made for these designed experiments were also subjected to a typical automotive cyclic corrosion test cycle (SAE J2334) as well as ASTM B117 for comparison of relative performance. Results of these studies are presented along with advantages and limitations of the proposed methodology.

Robert C. McCune, Vinod Upadhyay, Yar-Ming Wang, Dante Battocchi

Corrosion Behavior of Cerium-Based Conversion Coatings on Magnesium Alloys Exposed to Ambient Conditions

Exposure of CeCCs on AZ31B and AZ91D Mg alloys to ambient sunlight, temperature, and humidity was done to determine the effect on corrosion resistance. It was found that the CeCCs changed from pale yellow to almost translucent after 24 hours of sunlight exposure. The effect of the solar electromagnetic radiation on the morphological, chemical and optical properties of these coatings was investigated using SEM and UV-Vis characterization techniques. In addition, the corrosion performance of CeCCs before and after ambient exposure was studied by ASTM B117 neutral spray testing and electrochemical polarization measurements. In general, the changes in appearance did not adversely affect the corrosion performance of the coatings.

Carlos E. Castano, Surender Maddela, Matthew J. O’Keefe

Nanostructured Materials

Frontmatter

Thermal Stability of Ultra-Fine Grained Magnesium Alloy Processed by Extrusion and ECAP

The mechanical properties and thermal stability of ultra-fine grained (UFG) structure of magnesium alloy AZ31 during annealing was investigated. UFG specimens were prepared by a combined two-step severe plastic deformation process: the extrusion (EX) and the equal-channel angular pressing (ECAP). This combined process leads to micro structure refinement and enhanced microhardness. Specimens were annealed isochronally at temperatures 150 – 400 °C for 1 hour and isothermally at temperatures 170 – 210 °C. The evolution of microstructure and mechanical properties were studied by light and scanning electron microscopy and microhardness measurements. The coarsening of a fine-grained structure at higher temperatures was accompanied by a gradual decrease of the microhardness.

Jitka Vrátná, Miloš Janeček

Corrosion II

Frontmatter

Formation of Vanadate Conversion Coating on AZ31 Magnesium Alloy

Magnesium alloys have high strength-to-weight ratios, excellent castability, machinability, weldability, and thermal stability, and good damping capacity. Therefore, Magnesium alloys are recognized as alternatives to Al alloys and steel to reduce the weight of structural materials. However, a major obstacle to the widespread use of magnesium alloys is its poor corrosion resistance, particularly in wet environments. Therefore, further surface treatment of magnesium and its alloy is important in meeting several industrial specifications. In the present investigation, a chromate-free, corrosion-resistant conversion coating using vanadium based solution was applied to AZ31 magnesium alloy. Conversion coating was carried out in vanadium based solution. The effect of vanadium concentration and treatment time on the coating film was characterized by SEM, EDX and XRD. The corrosion characteristics of anodic films were evaluated using potentiodynamic polarization test in deaerated NaCl solution. Corrosion resistance property was improved with conversion coating treatment.

S. A. Salman, K. Kuroda, M. Okido

Joining and Friction Processing

Frontmatter

Microstructure modification and performance improvement of Mg-RE alloys by friction stir processing

Friction stir processing (FSP) is a severe plastic deformation (SPD) processing, which is very useful to refine grain size and secondary phase as well as change the texture of metal materials. Many FSP research were focused on aluminum alloys, while there are few reports on FSP of magnesium alloys, esp. on precipitation-hardening Mg-RE alloys. This paper overviewed the micro structures and mechanical properties of several FSPed Mg-RE alloys, such as Mg-Gd-Zn-Zr, Mg-Gd-Ag-Zr, and Mg-Nd-Zn-Zr with or without long period stacking ordering (LPSO) structure. The effects of processing parameters, such as rotation rate and traversing speed, on microstructure and mechanical properties were evaluated. It shows that FSP can effectively lend to performance improvement by micro structure modification, including obtaining remarkable finer and more homogenized grains, changing distribution and volume percentage of secondary phase etc.

Yujuan Wu, Liming Peng, Feiyan Zheng, Xuewen Li, Dejiang Li, Wenjiang Ding

A Multi-stage Approach for Predicting Fatigue Damage in Friction Stir Spot Welded Joints of Mg AZ31 Alloy

In this work, we propose a model for predicting fatigue damage in friction stir spot welded (FSSW) joints made of Mg AZ31 alloy. In this modeling approach, an attempt is made to capture failure mechanisms due to the influence of variation in welding parameters including tool plunge depth, tool rotation speed, and tool pin diameter. As such, the fatigue model presented here is a deterministic approach, where fatigue lifetimes are estimated based on specific geometrical and micro structural information. In particular, the model addresses the observed variation in failure mechanisms commonly observed in Mg FSSW coupons under a range of applied loading. Further, a distinction is made between fatigue crack incubation, micro structural small and physically small fatigue crack growth, and finally long crack growth of the coupon. The fatigue model presented here showed good correlation for fatigue lifetimes for variation in welding conditions.

H. M. Rao, J. B. Jordon

Wrought Materials II

Frontmatter

Recrystallization Behavior of a MgAlCa Alloy During Thermomechanical Processing and Subsequent Heat Treatment

Microstructure refinement via static and dynamic recrystallization during thermomechanical processing is crucial to the development of wrought magnesium product with desirable combinations of strength and ductility. Micro structural evolution in thixomolded AXJ810 is investigated during warm rolling and subsequent annealing at 300°C and 450°C using scanning electron microscopy and electron backscatter diffraction. The rolled sheets exhibited a partially recrystallized micro structure with a strong basal texture. Significant grain refinement was observed during both static and dynamic recrystallization. Retarded grain growth and strong texture randomization were observed only in regions containing intermetallic particles.

V. M. Miller, T. M. Pollock

The Influence of Deformation Mechanisms on Rupture of AZ31B Magnesium Alloy Sheet at Elevated Temperatures

Gas-pressure bulge tests were conducted on Mg alloy AZ31B wrought sheet until rupture at temperatures from 250 to 450°C. The rupture orientation was observed to change with forming pressure, which controls the forming strain rate, at 350 to 450°C. This phenomenon is a result of associated changes in the mechanisms of plastic deformation. At slow strain rates (≤ 3 × 10−2 s−1), cavity interlinkage associated with grain boundary sliding (GBS) creep induced rupture along the sheet rolling direction (RD). At fast strain rates (≥ 3 × 10−2 s−1), flow localization (necking) associated with dislocation-climb-controlled (DC) creep induced rupture along the long-transverse direction (LTD), a result of mild planar anisotropy. Biaxial bulge specimens tested at 250 to 300°C ruptured explosively, hence preventing any further analysis.

Aravindha R. Antoniswamy, Alexander J. Carpenter, Jon T. Carter, Louis G. Hector, Eric M. Taleff

Effect of the Extrusion Conditions on the Microstructure and Mechanical Properties of Indirect extruded Mg-Zn-Y Alloy with LPSO Phase

Mg alloys are the lightest commercial structural alloys and have excellent specific strength and stiffness characteristics. Due to its good castability, the die casting process has been established for the fabrication of various automobile components such as instrumental panels and seat frames. In particular, the application of Mg alloys to a conventional direct extrusion process is not considered to be cost-effective mainly due to their low extrudability.The indirect extrusion process is free of friction between the billet and the container; hence, the extrusion pressure as well as surface cracking can be reduced considerably compared to the direct extrusion process, leading to its implementation at lower temperatures and higher speeds. In this study, Mg-Zn-Y alloy was subjected to the direct and indirect extrusion processes and the effects of the processing conditions on the microstructure and mechanical properties were investigated.

Jonghyun Kim, Yoshihito Kawamura

Investigation of Mechanical Properties and Deformation Behavior of CaO Added Mg-6Zn-1.2Y Sheets

Recently, CaO added Mg-Al alloy sheets exhibited improved strength and elongation due to the effects of dispersion hardening and grain-refining by intermetallic compounds such as Al2Ca and (Mg,Al)2Ca and improved melt cleanliness. However, there is little study on these effects in Mg-Zn alloy system. In this study, as-cast ingots were prepared by melting Mg, Zn, Mg-25Y master alloy and Mg-3CaO master alloy in an electric resistance furnace under SF6+CO2 atmosphere. Sheet specimens were prepared by hot rolling process after indirect extrusion. The mechanical properties of CaO added Mg-6Zn-1.2Y specimens varied significantly with annealing process while the variation of mechanical properties of Mg-6Zn-1.2Y specimen was small. In addition, the texture of specimens with CaO (Ca in ICP analysis) was different from that of specimen without CaO. Therefore, we discussed the effect of microstructure and texture on mechanical properties and deformation behavior of CaO added Mg-6Zn-1.2Y sheet.

Hyun Kyu Lim, Young-Ok Yoon, Shae K. Kim

Phase Formation

Frontmatter

Bounds to Hardening by Solid Solution, Precipitation and Short Range Order in Mg Binary Alloys

Miedema’s coordinates are used to rank 4 model binary alloys considering the respective values of enthalpy of formation and the tendency to developing random solid solution, precipitation, short range order (SRO) and intermetallic compounds. The terminal solid solubility generally increases whereas the tendency to order decreases with decreasing heat of formation, and hardening by near-random solid solution and/or precipitation is expected to be dominant for solutes with low tendency to order, such as Al. For solutes with an intermediate tendency to order, hence solubility, such as Zn, or to form compounds, such as Gd, SRO is predicted to dominate the hardening. For solutes whose very large heat of formation leads to very high melting point intermetallics forming congruently, such as Sb, the terminal solid solubility is too low for any solute based hardening to be feasible. Implicancies for alloy design and selection regarding solute or precipitation hardening, SRO and creep resistance are discussed.

C. H. Cáceres, Saeideh Abaspour

A new magnesium alloy system: TEXAS

A new TEXAS alloy system (Mg-Sn-Nd-Ca-Al-Si) is presented in order to extend the range of applications for magnesium alloys. The alloy has been produced by permanent mould direct chill casting, a process that provides a homogenous distribution of alloying elements throughout the entire casting. This work presents microstructural features and a new Mg-Sn-Ca phase with the morphology of hexagonal platelets. Additionally mechanical properties and the corrosion behaviour of TEXAS alloys are presented in as cast and heat treated conditions.

Björn Wiese, Chamini Mendis, Carsten Blawert, Eric Nyberg, Karl Ulrich Kainer, Norbert Hort

Effect of Sn additions on the age hardening response, microstructures and corrosion resistance of Mg-0.8Ca (wt%) alloys

The as-cast Mg-3Sn-2Ca alloy shows a good creep resistance that is better than that of AE42 alloy. The high content of alloying additions in these alloys results in low ductility due to the presence of primary intermetallic particles. The effect of reduced amount of Ca and Sn, below the maximum solid-solubility limit of each alloy additive, was studied to determine the potential of the Mg-0.8Ca-xSn alloys in structural applications. The microstructure of the alloys resulting from different processing routes was characterized and the corrosion resistance of the alloys measured to determine the best combination of alloy composition for further alloy development.

C. L. Mendis, D. Tolnai, C. Blawert, N. Hort

Thermodynamics of phase formation in Mg-La-Ce-Nd alloys

Experimentally validated thermodynamic descriptions have been developed for the ternary Mg-La-Ce, Mg-La-Nd, and Mg-Ce-Nd systems by selecting key alloys in both systems and analyzing the phase formation in both the as-cast and heat treated state by SEM/EDS and DSC. These results were combined to form the validated thermodynamic Calphad-type description for quaternary Mg-La-Ce-Nd alloys. It is shown that for these light rare earth elements (La, Ce, Nd) the intermetallic phases with Mg exhibit significant mutual solid solubility in the ternary systems, extending into the quaternary alloy system. This is reflected by considering the shared crystal structures in the thermodynamic modeling. Simulated solidification paths of three Mg-La-Ce-Nd alloys with different La:Ce:Nd ratios and a common total content of 5 wt.% rare earth (RE) metals are evaluated using computational thermodynamics. Unexpected and distinctly different solidification behavior of these three alloys is revealed. The sequence La→Ce→Nd in the periodic table is not at all reflected in a monotonous solidification behavior. The demonstrated individual impact of each of these elements forbids treating the RE additions as a mere wt.% sum of RE elements.

Rainer Schmid-Fetzer, Joachim Gröbner, Artem Kozlov, Milan Hampl, Mark A. Easton, Suming Zhu, Mark A. Gibson, Jian-Feng Nie

Phase Stability Investigation of the Mg-Zn-Sm System

Phase equilibrium of the Mg-Zn-Sm ternary system at the Mg-rich region at 400°C was investigated. One ternary phase Z was confirmed, with a composition formula of Mg3Zn10Sm and crystal structure of P63/mmc. Zn was found to have large solubility in both Mg3Sm (~35.6 wt.%) and Mg41Sm5 (~4 wt.%) phases. An isothermal section of 400°C in the Mg-rich corner was constructed, which provides important understanding of phase relationships in the Mg-Zn-Sm system.

Xiangyu Xia, Amirreza Sanaty Zadeh, Chuan Zhang, Xiaoqin Zeng, Donald Stone, Alan A. Luo

In situ synchrotron diffraction of the solidification of Mg-RE alloys

Mg-RE alloys have a good potential to be used in bio-applications as degradable implants. Their macroscopic characteristics are strongly dependant on the microstructure, which can be tailored through the alloy composition and the solidification parameters. In situ synchrotron diffraction provides a unique tool to follow the phase formation, possible meta-stable and stable phase transformations and grain growth during cooling. In the present study Mg alloys containing Gd, Y and Nd were investigated to characterize the solidification phenomenon during cooling from 680°C to room temperature. Samples were melted and solidified in a Bähr 805 dilatometer modified for in situ synchrotron measurements. The molten samples were contained in steel crucibles, and the temperature was controlled by type S thermocouples during cooling. The results provide an experimental validation of thermodynamic calculations; and provide experimental input for refining the thermodynamic models, which contribute to the better understanding of the microstructure evolution to control desirable macroscopic characteristics.

D. Tolnai, C. L. Mendis, A. Stark, G. Szakács, B. Wiese, K. U. Kainer, N. Hort

Nucleation Kinetics of the γ-Phase in a Binary Mg-Al Alloy

The binary magnesium-aluminum system has been studied extensively the last decades. Surprisingly, the kinetics of nucleation of the γ-precipitates (Mg17Al12) in the α-matrix have not been determined experimentally in detail. The main goal of this research is to investigate the precipitation kinetics of the γ-phase in a Mg-Al alloy during annealing, and to understand the relation between the precipitate volume fraction and the mechanical properties of the material. Comprehensive micro-structural analysis was undertaken to reveal the number density, volume fraction, and morphological changes of the γ-precipitates after solution heat treatment (homogenization) and during subsequent annealing below the solvus temperature. As a result, the incubation time and steady state nucleation rate of γ-precipitates in the α-matrix were determined by fitting the experimental data to the classical nucleation theory.

Mehdi Lalpoor, J. S. Dzwonczyk, N. Hort, S. E. Offerman

Solidification

Frontmatter

Computational Multi-Scale Modeling of the Microstructure and Segregation of Cast Mg Alloys at Low Superheat

It is well known that casting at low superheat has a strong influence on the solidification structures of the cast alloy. Recent studies on casting magnesium AZ alloys at low superheat using the Magnetic Suspension Melting (MSM) process have shown that the cast alloy exhibit a fine globular grain structure, and the grain size depend on the cooling rate. This paper describes a stochastic mesoscopic model for predicting the grain structure and segregation in cast alloys at low superheat. This model was applied to predict the globular solidification morphology and solute redistribution of Al in cast Mg AZ31B alloy at different cooling rates. The predictions were found to be in good agreement with the observed grain structure and Al segregation. This makes the model a very useful tool for optimizing the solidification structure of cast magnesium alloys.

Laurentiu Nastac, Nagy El-Kaddah

Effect of Casting Defects Distribution on the Beginning of Tensile Fracture in Semi-solid Injected Magnesium AZ91D Alloy

Semi-solid process is useful for magnesium alloys because processing temperatures lower than conventional casting processes result in decreased combustibility. Additionally it can decrease casting defects by the increased viscosity and decreased solidification shrinkage. In this study, casting defects of semi-solid injected AZ91D specimens were observed by X-ray CT tomography and tensile test was carried out. Thus, relations between casting defects and fracture starting point were investigated. As a result, the specimens were not always fractured at the site of the largest defect; meanwhile the defects situated near the surface or perpendicularly elongated to the tension axis exerted a potent influence on fracture.

Yuichiro Murakami, Kenji Miwa, Naoyuki Kanetake, Shuji Tada

Effect of inoculation method of refiner on the grain refinement of AZ91 alloy

Grain refinement involving inoculation of a grain refiner was investigated for application to a commercial Mg alloy. Cylindrical pellets composed of a mixture of MnCO3 and carbon powder with several different ratios were plunged into a commercial AZ91 alloy. The results show that the addition of the mixed pellets can more efficiently refine AZ91 compared to the individual use of carbon powder or MnCO3. In particular, the addition of mixed pellets with a 3:7 ratio between MnCO3 and carbon provides an excellent refining efficiency, decreasing grain size from 460µm to 52µm of the AZ91 alloy. This is attributed to inoculation of MnCO3 and carbon, which brings about the formation of heterogeneous nucleants such as MgO, Al4C3, and Al8Mn5 before solidification of a-Mg and also to melt agitation by the release of CO2 gas

Jun Ho Bae, Young Min Kim, Chang Dong Yim, Ha-Sik Kim, Bong Sun You

Nanocomposites and Metal Matrix Composites

Frontmatter

Nanoparticle Addition to Enhance the Mechanical Response of Magnesium Alloys Including Nanoscale Deformation Mechanisms

In this study, various magnesium alloy nanocomposites derived from AZ (Aluminium-Zinc) or ZK (Zinc-Zirconium) series matrices and containing Al2O3, Si3N4, TiC or carbon nanotube (CNT) nanoparticle reinforcement (representative oxide, nitride, carbide or carbon nanoparticle reinforcement, respectively) were fabricated using solidification processing followed by hot extrusion. The main aim here was to simultaneously enhance tensile strength and ductility of each alloy using nanoparticles. The magnesium-oxygen strong affinity and magnesium-carbon weak affinity (comparison of extremes in affinity) are both well known in the context of magnesium composite processing. However, an approach to possibly quantify this affinity in magnesium nanocomposite processing is not clear. In this study accordingly, Nanoscale Electro Negative Interface Density or NENID quantifies the nanoparticle-alloy matrix interfacial area per unit volume in the magnesium alloy nanocomposite taking into consideration the electronegativity of the nanoparticle reinforcement. The beneficial (as well as comparative) effect of the nanoparticles on each alloy is discussed in this article. Regarding the mechanical performance of the nanocomposites, it is important to understand the experimentally observed nanoparticle-matrix interactions during plastic deformation (nanoscale deformation mechanisms). Little is known in this area based on direct observations for metal matrix nanocomposites. Here, relevant multiple nanoscale phenomena includes the emanation of high strain zones (HSZs) from nanoparticle surfaces.

Muralidharan Paramsothy, Manoj Gupta

Properties of Extruded Disintegrable Metal Composites

Recently, a lightweight, disintegrable material with high strength and high ductility has been successfully used in oilfield tool applications to save well operation time and cost. The material is a powder metallurgy composite consisting of a metal matrix and mechanical/chemical reinforcements. Warm extrusion was used to produce rods and tubes of the material from cold pressed green preform. Disintegration rates of the extruded materials with different compositions range from 0 – 300 mg/cm2/hr in 3% KCl at 93.3 °C with a strength of > 60 ksi (414 MPa) and ductility of 7 – 15%. This paper presents the micro structure, disintegration, and mechanical properties of the extruded composite. The results prove that disintegration properties can be controlled by altering the chemical composition of the reinforcements while maintaining good strength and ductility.

Bobby Salinas, Zhiyue Xu, John Welch

Effect of Fiber Volume Fractions on Corrosion Resistance of Mg AM60 Alloy-Based Composites in NaCl Solutions

Magnesium-based MMCs with their high stiffness-to-weight ratio bring a great interest to automotive and aerospace industries. Besides the enhanced mechanical properties of the MMCs, corrosion resistance is often a concern for applications being used in harsh environment. In this study, Magnesium alloy AM60 matrix composite reinforced with 7 vol%, 11 vol% and 22 vol% Al2O3 fibers were squeeze casted. The corrosion behavior in aqueous solutions containing 3.5% NaCl was investigated in comparison with magnesium alloy AM60. The scanning electron microscopy (SEM) was employed for the microstructure analysis of the composites and matrix alloy AM60 before and after corrosion test. The results show that the addition of Al2O3 fiber deteriorated the corrosion resistance of the matrix and the corrosion rate increases as the fiber volume fraction increased. The formation of the interfaces between the reinforcement and the matrix should be the primary mechanism for the corrosion to easily take place.

Xuezhi Zhang, Xiaoping Niu, Henry Hu

Synthesis of Disintegrable Metal Composite for Oilfield Applications

Lightweight metal composites were traditionally developed for weight-critical applications. Recently, significant efforts have been made to improve the corrosion performance. In this study we report the synthesis of a novel type of high-strength metal composites with enhanced corrosion rate for use in self-disintegratable tools in oil and gas wells. The composites were fabricated through a powder metallurgy procedure by consolidating reactive metal powders that were coated with nanoscale metallic and/or ceramic coatings. The interaction between the metal matrix and coating was studied using X-ray diffraction, differential scanning calorimetry, and electron microscopy. The composites exhibit simultaneous high strength (up to 460 MPa) and two orders of magnitude increase in the corrosion rate (i.e., 250 mg/cm2/hr) under saline water. The corrosion behavior and associated field applications are described.

Zhihui Zhang, Bobby Salinas, Caleb Newman, Zhiyue Xu

Wrought Materials III

Frontmatter

Effect of Precipitation on Dynamic Recrystallized Grain Size in a Magnesium Alloy

Magnesium, the lightest structural metal has specific tensile strength and rigidity superior to iron and aluminum showing a rising demand for use in the automotive industries to lighten the structure. But, magnesium and its alloys have limited ductility and poor formability at room temperature due to an insufficient number of operative slip and twinning systems associated with its hexagonal close packed structure. A fine grain structure and random texture can improve the ductility or formability. The aim of this work is to investigate the effect of precipitates on micro structure evolution during hot deformation of Mg-Al-Sn alloy. Mg-Al-Sn alloy has been designed using thermodynamic modeling software, FactSage, based on forming precipitates at deformation temperature of 300 °C. The formation of precipitates, mostly Mg2Sn, during the dynamic recrystallization process may slow down the grain growth resulting in finer grain size and grain size homogenization.

Abu Syed Humaun Kabir, Jing Su, In-Ho Jung, Stephen Yue

Texture Development in an Extruded Magnesium Alloy During Compression Along the Transverse Direction

Wrought magnesium alloys generally exhibit a certain extent of mechanical anisotropy at room temperature because of the presence of texture. In the present study the work hardening behavior and texture evolution of an extruded AM30 magnesium alloy were studied after compression along the transverse direction (TD) at varying strain levels. The initial samples were chosen to have a strong basal texture with the basal planes parallel to the extrusion direction. During compression a stage of accelerated work hardening characterized by an increasing work hardening rate appeared, and the c-axes of the grains were observed to re-orient parallel to the compressive loading direction. With increasing compressive strain extension twins first formed and then disappeared at higher strains.

D. Sarker, D. L. Chen

Evolution of microstructure during caliber rolling of AZ31 alloy

An AZ31 alloy has been processed by caliber rolling, in which a repetitive oblique shear strain is applied with a fixed reduction ratio, resulting in increase in strength and ductility by simultaneous grain refinement and texture modification. A total of 18 passes were made at 473K through successive grooves with reduction. A dramatic increase in the tensile YS was achieved with no substantial change in the elongation. Evolution of microstructure by caliber rolling has been studied by TEM. Samples after 4, 6, 8 and 18 passes were examined. After 4 passes the sample showed clear grain boundary structure and grain refinement. After 6 passes, a severely deformed structure is observed, indicating accumulation of strain. Spots with concentration of strain were observed in basal planes. Activation of prismatic slip was observed. After 8 passes the strain appears more severe. After 18 passes, along with concentration of strain, sharp grain and subgrain boundaries appeared, with subgrains of about 100–200nm size. The small grain and subgrain size, along with accumulated strain, is responsible for the high strength.

Alok Singh, Hidetoshi Somekawa, Tadanobu Inoue, Toshiji Mukai

Increasing volume fraction of precipitates and strength of a Mg-Zn-Y alloy by pre-ageing deformation

Pre-ageing deformation was applied to a Mg-Zn-Y alloy to refine the size of ß′1 precipitates. Reductions in the precipitate length and diameter were accompanied by a substantial increase in the volume fraction of the ß′1 precipitates from 0.5% (no prestrain) to 2.3% (5% pre-strain). This contrasted with Mg-Zn alloys, in which the precipitate volume fraction in the peak aged condition was not affected by pre-ageing strain. The increase in precipitate volume fraction contributed significantly to an increase in the yield strength from 217 MPa (no pre-strain) to 286 MPa (5% pre-strain). Precipitate strengthening via Orowan looping was the most significant contributor to the overall strength of the alloy.

Julian M. Rosalie, Hidetoshi Somekawa, Alok Singh, Toshiji Mukai

Mapping the Mechanical Properties of Alloyed Magnesium (AZ 61)

In this work, an advanced form of nanoindentation is used to map the mechanical properties of AZ 61. The probed area includes both alpha (Mg-rich) and beta (Al12Mg17) phases. The measured moduli of the two phases compare well with expected values. The hardness of the beta phase is three times greater than that of the alpha phase. The hardness map reveals an area of intermediate hardness surrounding the beta phase. This zone is a mesophase which is chemically distinct and may influence the mechanical behavior of the alloy in unexpected ways.

Jennifer L. Hay, Phillip Agee

Microstructure

Frontmatter

Effects of Alloying Elements and Cooling Rate on Morphology of Phases in CaO Added Mg-Al-Si Alloys

Mg2Si formed by the addition of Si in Mg-Al alloys is the very useful intermetallic compound. However, Mg2Si phases in the Mg-Al-Si alloys are prone to forming undesirable, coarse Chinese script shape, which will deteriorate the mechanical properties of Mg alloys. Therefore, in order to modify the Chinese script shaped Mg2Si phases in the Mg-Al-Si alloys, many methods have been studied such as alloying addition of Ca or P. This study was aimed at improving the mechanical properties of CaO added Mg-Al-Si alloys by the addition of alloying elements and varying cooling rate to change of morphology of Mg2Si and CaMgSi phases. The micro structure s of specimens were observed by OM and SEM, and the phase analysis was performed by XRD, TEM and EDS. To evaluate the mechanical properties of tested alloys, tensile tests were carried out at room and high temperatures.

Young-Gil Jung, Hyun Kyu Lim, Young-Ok Yoon, Shae K. Kim, Do Hyang Kim

Microstructure and Mechanical Properties of Die Cast Magnesium-Aluminum-Tin Alloys

Mg-Al-Sn alloy system offers good combination of strength, ductility, castability and corrosion resistance for potential automotive structural applications. This report summarizes the micro structure and mechanical properties of two Mg-Al-Sn alloys, AT72 (Mg-7Al-2Sn1) and AT96 (Mg-9Al-6Sn), prepared using high pressure die casting process. The microstructure of as-cast Mg-Al-Sn alloys was investigated using computational thermodynamics modeling and experimental techniques. Both alloys show improved mechanical properties and excellent die castability compared with the conventional AZ91 (Mg-9Al-1Zn) and AM50 (Mg-5Al-0.3Mn) alloys. The AT72 alloy is currently used in developing large thin-wall die cast door inners and cast magnesium wheels.

A. A. Luo, P. Fu, X. Zeng, L. Peng, B. Hu, A. K. Sachdev

Study on Microstructure and Mechanical Property of Squeeze Casting Az91d Magnesium Alloy

Squeeze casting characterized by gentle die filling and solidification under high pressure is recognized one of the potential processes for the production of thick-walled parts that are amendable for heat treatment to enhance mechanical properties of magnesium alloys. In this study, squeeze casting experiments under different applied pressures were carried out for AZ91D magnesium alloy and the temperature variations at different locations in the casting were measured during the solidification process. The micro structure morphology was observed by using OM. The grain size and mechanical property of the casting were determined. The effects of pressure on the cooling curves, micro structure and mechanical property were discussed.

Yanda Li, Zhiqiang Han, Alan A. Luo, Anil K. Sachdev, Baicheng Liu

Tailoring Precipitates in Mg-6Zn-2Gd Based Alloy Subjected to High Pressure Torsion

The precipitates formed at lower ageing temperatures are very unstable at higher application temperatures, thus are not ideal for the high temperature applications. A traditional approach in solving the problem is from alloy design and development route by increasing solute concentrations or addition of micro-alloying elements. Recently, high pressure torsion was also employed to tailor the precipitate microstructures of the age-hardening alloy that are significantly different from those obtained by conventional isothermal ageing treatments. In this contribution, an Mg-6Zn-2Gd-0.4Zr (wt. %) alloy has been subjected to high pressure torsion (HPT). The precipitate micro structure evolution during HPT processing has been characterised using TEM, with an aim to elucidate the effect of HPT processing on nucleation and growth of precipitates, and the mechanisms dominating the evolution of precipitate microstructure. This investigation demonstrates a new perspective for application of HPT to tailor precipitate microstructure for unique properties in service.

J. H. Li, P. Schumacher

Microstructure and Phase Evolution in Mg-Gd and Mg-Gd-Nd Alloys With Additions of Zn, Y and Zr

Microstructure and phase evolution in Mg-Gd and Mg-Gd-Nd based alloys with additions of Zn, Zr and Y were analyzed in the as-cast, solution treated and aged conditions. Alloys has been investigated after solution treatment at 540°C for 24hr followed by isothermal aging at 175°C up to 32 days by using of Vickers hardness, optical microscopy, scanning electron microscopy equipped with EDS, X-ray diffraction and transmission electron microscopy. It was found that the as-cast alloys contained primary α-Mg matrix, eutecticlike structures, cuboid-like phases and Zr-rich clusters. The homogenized and quenched alloys contained primary α-Mg solid solution, smaller amount of divorced eutectic compounds, enlarged cuboid-like particles and Zr-rich clusters. The eutectic phase was Mg5Gd prototype with the composition Mg5(GdxNd1-x, x≈0.2). The compositions of the cuboid shaped particles are characterized by enlarged amount of Gd and can be written as Mg2(Gd x Y1-x) with x≈0.85 in the Mg-5Gd based alloy, and Gd4(YxNd1-x) with x≈0.5 in the Mg-6Gd-3.7Nd based alloy. The cuboid shaped particles grew during aging and reached ~3µm average size. Precipitation of ß″ and ß′ phases during aging was observed. Mg-6Gd-3.7Nd based alloy reached a maximum value of microhardness after 16 days of aging; in Mg-Gd based alloy, microhardness increased more slowly and reached a maximum value after 32 days of aging.

S. Khawaled, M. Bamberger, A. Katsman

Twin Roll Casting

Frontmatter

Influence of Temperature and Rolling Speed on Twin Roll Cast Strip

Reducing vehicle weight and emissions by lightweight design is a major goal of the automotive industry. Magnesium as the lightest structural metal offers a significant weight saving potential compared to steel and aluminum. Cast magnesium components are widely used, e.g. as engine blocks or gear box housings. The application of magnesium sheets is currently hampered by the low formability of magnesium which means that a large number of rolling passes is required to roll a DC cast slab to final gauge sheet. This large number of rolling steps is the main reason for the high cost of magnesium sheets. An alternative, cost-efficient production route for magnesium sheets with improved properties is feedstock production by twin roll casting (TRC). In this paper we report the results of twin roll casting experiments on the magnesium alloy AZ31 (Mg-3Al-lZn-Mn) and discuss the influence of the process on the micro structure and texture of the strips.

G. Kurz, J. Bohlen, L. Stutz, D. Letzig, K. U. Kainer

Mathematical Modeling of the Effect of Roll Diameter on the Thermo-Mechanical Behavior of Twin Roll Cast AZ31 Magnesium Alloy Strips

Although the Twin Roll Casting (TRC) process has been used in the aluminum sheet production industry for more than 60 years, the usage of this process to fabricate magnesium sheets is still at its early stages. Similar to other manufacturing processes, the development of the TRC process for magnesium alloys has followed a typical route of preliminary studies using a laboratory-scale facility, followed by pilot-scale testing and most recently attempting to use an industrial-scale twin roll caster. A powerful tool to understand and quantify the trends of the processing conditions and effects of scaling up from a laboratory size TRC machine to an industrial scale one is develop a mathematical model of the process. This can elucidate the coupled fluid-thermo-mechanical behavior of the cast strip during the solidification and then deformation stages of the process. In the present study a Thermal-Fluid-Stress model has been developed for TRC of AZ31 magnesium alloy for three roll diameters by employing the FEM commercial package ALSIM. The roll diameters were chosen as 355mm, 600mm and 1150mm. The effect of casting speed for each diameter was studied in terms of fluid flow, thermal history and stress-strain evolution in the cast strip in the roll bite region.

Amir Hadadzadeh, Mary Wells

Backmatter

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