Microstructure and Texture in Steels

The effects on texture formation of rolling above and below the T

nr

(no-recrystallization temperature) are considered. Rolling above the T

nr

leads to the appearance of the ‘cube’ component, while the absence of recrystallization accompanied by strain accumulation is responsible for the appearance of the fcc ‘rolling fibre’. The latter consists of the Cu (copper), S, Br (brass) and Goss (as well as intermediate) components. On transformation, the cube is converted into the rotated cube, Goss, and rotated Goss. After lower temperature finishing, the Cu and Br are converted into the ‘transformed Cu’ and ‘transformed Br’, respectively. Some attention is paid to the Kurdjumov-Sachs and Nishiyama-Wassermann correspondence relations and the importance of variant selection (departure from these relations) is discussed. Finally, the effect of operating parameters such as cooling rate, grain size and solute level are also considered.

This paper is an introduction to the mathematical estimation of the crystallographic texture and microstructure resulting from the displacive transformation of austenite in steels, under the influence of an externally applied system of stresses. It begins with an introduction to the problem, a description of the phenomenological theory of martensite crystallography, and the application of this theory along with a variant selection criterion to determine the texture due to solid-state, displacive transformation. It is demonstrated that there remain difficulties which make a complete closure between theory and experiment unlikely. Progress is needed in relating the chemical and mechanical driving forces for phase transformation to the evolution of overall volume fractions of different crystallographic variants.

Interstitial free (IF) steel with an ultrafine microstructure has been produced by three different routes: (i) cold rolling, (ii) accumulative roll-bonding (ARB) and (iii) martensitic transformation followed by cold rolling. The microstructure refines with increasing strain without saturation to a value of about 100 nm at an equivalent strain (ε

VM

) of 8, which is the maximum strain investigated. At all strains a microscopic analysis by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) shows that the microstructure is subdivided by dislocation boundaries and high-angle boundaries. For both cold rolled samples and ARB samples the flow stress increases as the boundary spacing decreases. For the finest structures this leads to a flow stress at room temperature in the range 900–1000 MPa. Finally structure-property relationships are discussed especially the effect of post-processing treatments by annealing and by low strain deformation.

This paper will discuss the background and the basic concept of the grain boundary engineering (GBE) proposed by one of the present authors in the early 1980s. It is shown that the grain boundary microstructure which can be quantitatively described by newly introduced microstructural parameters, i.e. the grain boundary character distribution (GBCD) and the grain boundary connectivity, can bridge the gap between structure-dependent properties of individual grain boundaries and bulk properties of polycrystalline materials. In order to prove the importance of GBCD and its applicability to GBE, we overview basic studies of GBE performed in the last two decades, showing the GBCD-controlled bulk properties and possible factors affecting GBCD, such as the grain size, texture, material purity/composition, for various kinds of materials including ferrous and non-ferrous materials. Recent achievements of GBE for structural materials are discussed paying a particular attention to the control of intrinsic and extrinsic brittleness caused by structure-dependent intergranular fracture. As the most recent successful achievements, GBE by high magnetic field applications is introduced. Finally an example of GBE for functional materials is given taking the most recent study on polysilicon for high performance solar cells.

The development of recrystallization texture in three low carbon sheet steels, conventional IF steel, fine grained IF steel, and DP steel, for automotive application was investigated using EBSD technique. The recrystallization texture at the early stage of recrystallization was very similar to that at the final stage in all three steels. A strong α-fiber texture with {112}<110> peak and weaker γ-fiber texture with {111}<110> peak were developed during the deformation of all three low carbon steels. After recrystallization, they changed to a strong γ-fiber texture with strong {111}<112> component. However, the intensity of the texture components varied in the different steels. The grains of γ-fiber orientations developed from deformed grains of similar orientations and grew rapidly, consuming the neighboring grains to form a strong γ-fiber texture. In conventional IF steels, some recrystallized grains formed at grain boundaries showed weak correlation with {111} orientation, however, the growth rates of these random grains were too slow to affect the final recrystallization texture. In the fine grained IF steel, the intensity of the {001}<110> component after recrystallization was higher than that in the conventional IF steel. Strain induced boundary migration (SIBM) was observed in the fine grained IF steel. The intensity of the γ-fiber texture was much weaker in the DP steel than in the IF steels. Nucleation of the {110}<001> and {001}<100> recrystallized grains from the shear bands in {111}<112> or {110}<110> grains was expected. The austenite transformation during the intercritical annealing of the DP steel was found to have little effect on the formation of its final recrystallization texture.

This paper investigates the evolution of texture and microstructure at the metal-vapour interface during transformation annealing in vacuum. Interrupted annealing experiments were carried out on cold rolled Mn, Al and Si alloyed ultra low carbon steels. The textures were characterized by X-ray diffraction and SEMEBSD techniques. The results show the variation in the surface texture components obtained after BCC recrystallisation and double α−γ−α transformation annealing. The recrystallisation texture consists mainly of a <111>//ND fibre, while the transformation texture at the surface exhibits a <100>//ND fibre in combination with components of the <110>//ND fibre. It was revealed that the latter specific surface texture was present in a monolayer of outer surface grains which were in direct contact with the vapour atmosphere. This observed phenomenon could be explained by considering the role of surface energy anisotropy occurring during phase transformation annealing.

Cold rolled and annealed interstitial free high strength (IFHS) steels show various kinds of precipitates. Out of these, FeTiP is the most deleterious, since its formation leads to a loss of strength as well as degradation of the formability of the steel. This effect is much more pronounced in batch annealed IFHS steels than in their continuous annealed counterparts. The harmful effects of FeTiP precipitation in batch annealed steels can be largely avoided by the use of appropriate coiling and annealing temperature, depending on the composition of the steel.

The substrate steel texture appears to control the texture of galvanized and galvannealed coatings on interstitial free steels. Again, the powdering resistance of coatings depends on the type of textures produced in them. An attempt has been made to correlate the textures of the substrate and the coating with the forming behavior of the latter. In the galvannealing operation, using lower galvannealing power (lower strip temperature) improves the coating formability. The formation of an almost continuous {01.3}<uv.w> fiber and lower amount of iron present in the coating layer could be the possible reasons for this behavior.

Microalloying elements like Al, B, Nb, Ti, V can be used to optimise the microstructure evolution and the mechanical properties of advanced high strength steels (AHSS). Microalloying elements are characterised by small additions < 0.1 mass% and their ability to form carbides or nitrides. They can increase strength by grain refinement and precipitation hardening, retard or accelerate transformations and affect the diffusion kinetics. Thus, by their addition the AHSS with their high requirements to process control can be adopted to existing processing lines. Different combinations of microstructural phases and different chemical compositions have been investigated for AHSS in order to combine high strength with excellent formability.

Recent trends in automotive industry towards improved passenger safety and reduced weight have led to a great interest in AHSS (Advanced High Strength Steel), and DP, TRIP, CP, MA and high-Mn TWIP (TWinning Induced Plasticity) steels are particularly promising due to their superior toughness and ductility. The properties of low SFE (Stacking Fault Energy) austenitic high Mn FeMnC steel exhibiting twinning-induced plasticity have recently been analyzed in detail. It is argued that although the mechanical properties of TRIP and TWIP steels are often assumed to be solely due to effects related to straininduced transformation and deformation twinning, respectively, other mechanisms may also play an essential role such as point-defect cluster formation, planar glide, pseudo-twinning, short range ordering, and dynamic strain ageing, e.g. in the case of TWIP steel. At low strain rates, the plastic deformation of TWIP steels is often controlled by the movement of very few well-defined localized deformation bands. The formation and propagation of these Portevin-LeChatelier (PLC) bands lead to serrated stress-strain curves, exhibiting a small negative strain rate sensitivity.

The present contribution offers a critical analysis of the mechanical properties of high-Mn TWIP steels and focuses on their potential as automotive materials. In addition, the challenges related to the production and applications of high-Mn TWIP steels are discussed. The new insights in the properties of TWIP steels result from the use of new experimental techniques combining high sensitivity infrared thermo-graphic imaging and optical in situ strain analysis. Finally the importance of the use of TEM (Transmission Electron Microscopy) in understanding the development of deformation microstructures in TWIP steel is also illustrated.

In recent years there has been an increased emphasis on the development of new advanced high strength sheet steels (AHSS), particularly for automotive applications. Descriptive terminology has evolved to describe the “First Generation” of AHSS, i.e. steels that possess primarily ferrite-based microstructures, and the “Second Generation” of AHSS, i.e. austenitic steels with high manganese contents which include steels that are closely related to austenitic stainless steels. First generation AHSS have been referred to by a variety of names including dual phase (DP), transformation induced plasticity (TRIP), complex-phase (CP), and martensitic (MART). Second generation austenitic AHSS include twinninginduced plasticity (TWIP) steels, Al-added lightweight steels with induced plasticity (L-IP

®

), and shear band strengthened steels (SIP steels). Recently there has been increased interest in the development of the “Third Generation” of AHSS, i.e. steels with strength-ductility combinations significantly better than exhibited by the first generation AHSS but at a cost significantly less than required for second generation AHSS. Approaches to the development of third generation AHSS will require unique alloy/microstructure combinations to achieve the desired properties. Results from a recent composite modeling analysis have shown that the third generation of AHSS will include materials with complex microstructures consisting of a high strength phase (e.g. ultra-fine grained ferrite, martensite, or bainite) and significant amounts of a constituent with substantial ductility and work hardening (e.g. austenite). In this paper, design methodologies based on considerations of fundamental strengthening mechanisms are presented and evaluated to assess the potential for developing new materials. Several processing routes will be assessed, including the recently identified Quenching & Partitioning (Q&P) process developed in the authors’ own laboratory.

Focus is on the multi-level character of existing or currently developed models for polycrystal deformation. A short overview is presented of two-level models ranging from the full-constraints Taylor model to the crystal-plasticity finite element models, including the description of a few recent and efficient models (GIA and ALAMEL). Validation efforts based on experimental cold rolling textures obtained for an aluminium and a steel alloys are discussed.

The ideal orientations of textures that develop at large strains can be identified with the help of crystal plasticity simulations. In this short review, an overview is presented on these types of simulations that helped in the identification of the deformation texture components of fcc, bcc and hcp materials in pure shear (rolling) as well as in simple shear (torsion) during the last 20 years. The technique is based on the so-called persistence parameter that was introduced by Tóth, Gilormini and Jonas in 1988 [

Acta Metall.

, 36, 3077–3091]. The formation of textures and several texture effects can be understood with the help of the persistence parameter together with the rotation field of orientations in Euler space and the divergence quantity. The stability of ideal orientations is especially investigated and it is shown that simple shear distinguishes from pure shear in a very particular way; all ideal orientations of simple shear are positioned at orientations where the divergence is zero while in rolling they are situated within a negative divergence field.

In the present paper the specifications and potentials of the 3 dimensional x-ray diffraction (3DXRD) method is shortly described and examples of applications are reviewed. The main focus is however on 3DXRD results leading to advancements in recrystallization modelling. 3DXRD measurements have shown that all investigated individual grains have different recrystallization kinetics – not two grains are alike. This is found for samples deformed both to high and very high strains. Based on the experimental results, a JMAK model has been advanced to incorporate distributions of growth rates or anisotropic growth directionality. Effects of the new modelling are analysed and compared to standard JMAK modelling. Also effects of experimentally observed distributions of nucleation sites are analysed using JMAK simulations. Finally it is discussed in more general terms how modelling may be advanced through experimental verifications.

An efficient digital FFT-based viscoplastic method was applied to calculating the viscoplastic stress-strain response on a 3D image of a serial sectioned nickel alloy. A single strain step under uniaxial tensile loading was calculated using crystal plasticity. Analysis of the results indicated higher stresses near grain boundaries than in the bulk of grains. All types of grain boundary gave similar results and no special cases, such as the twin boundaries, were identified. A new analysis for clusters of high stress points was introduced; the distribution of sizes of high stress clusters was found to be close to log-normal.

Acquiring knowledge about microstructures and textures is crucial for the improvement and development steel products, because these two characteristics are controlling factors for the properties of steel. Diffraction techniques using X-rays, electrons or neutrons are suitable to study microstructures (e.g. phase relationships) and textures (crystallographic orientations). X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) are generally available techniques within an industrial research environment.

Different examples from daily research within Corus RD&T are shown, where these diffraction techniques have been used. For the development of new hightech multi-phase steels we study phase composition and microstructure to optimise product properties. Retained austenite may easily be detected and quantified by XRD, using the right approach. Moreover, using an area-sensitive 2D-detector, such as e.g. the GADDS system, the textures of ferrite and austenite components may be identified simultaneously during a single measurement. Micro XRD (beam diameter down to 100 μm) allows us to study small-scale welding joints within TRIP steels and how these welds evolve with time. Even more interesting is the possibility to study recrystallisation and phase transformations in situ at high temperature in real time, e.g. to make phase transformations visible during intercritical annealing.

Another fascinating and beneficial attribute of XRD and EBSD is to study coatings/platings such as Ni in relation to the underlying steel substrate. Not only it is possible to identify the thickness of a plating or the phases in a coating, but textural and microstructural relations between the substrate and the surface layer can also be visualised. The understanding of microstructure opens the way to grain boundary engineering in coated products to help improve the corrosion properties.

The formability of sheet metals is strongly influenced by, and can be predicted from, crystallographic texture, and is generally assessed in terms of an r-value and/or n-value off-line from tensile test samples. There is interest in the development of a non-destructive, cheap and simple to operate system for texture assessment. Ultrasonic velocity is directly related to a material’s elastic modulus and metal single crystals can have significantly different elastic properties along their principal crystal axes. Hence, if a polycrystalline sample has preferred texture then variations in ultrasonic velocity with angle to the rolling direction are expected. In this work the ultrasonic velocity anisotropy, measured using a noncontact electro-magnetic acoustic transducer (EMAT) system, with respect to sheet rolling direction was determined and compared to calculated elastic modulus anisotropy, using quantified texture components (from X-ray diffraction or EBSD and their known individual anisotropies), and mechanically measured modulus values, at 0°, 45° and 90° to the rolling direction, for aluminium and steel sheets. Predictions of elastic anisotropy based on surface texture determination, as characterised by X-ray diffraction or surface EBSD, gave poor correlations with EMAT velocity anisotropy for aluminium sheets that contained significant through thickness texture variations, however, accounting for this using multiple EBSD scans through thickness gave good correlations. For steel it was found that the EMAT velocity anisotropy matched the measured modulus variation with angle, with differences between samples with different textures (as-rolled and heat treated conditions) being observed. However the predicted modulus variation did not show much difference between samples, resulting in some discrepancies with the EMAT velocity and measured modulus values. Results from this work, and data from the literature, suggest that monitoring the recrystallisation process in aluminium using an EMAT sensor is much more straight forward than for steel due to aluminium showing greater differences in elastic modulus, and hence ultrasonic velocity, anisotropy between the as-rolled and recrystallised textures.

High energy X-rays are well known due to there high penetration power particular in materials testing devices. For diffraction experiments high energy X-rays with more than 50 keV can be obtained at storage rings or using a tungsten X-ray tube. According to the high penetration power, these beamlines offer a very high photon flux and an excellent brilliance. That means measurements can be carried out fast. As an example, the complete texture measurement at one position of a steel shaft with 34 mm in diameter has taken 45 minutes nondestructively. On the other hand the high photon flux allows to measure foils or thin wires down to 50–100 μm. These new and fast options make it possible to measure in situ textures under tension, compression and at high temperatures. We have used 100 keV X-ray to measure the texture transition as well as the phase transition in a steel sample. The experiments were done at the high energy beamline BW5 (Hasylab at Desy/Hamburg). 100 keV X-rays have a wavelength of 0.1240 Å which means due to the Bragg’s law very low scattering angles. Using a MAR345 image plate detector one obtains a set of complete Debye-Scherrer cones in a 2θ-range of 7° in about 1 sec. At room temperature we found 100% ferrite. During heating up till the austenite region we were able to investigate the thermal expansion and the texture relation between ferrite and austenite, which follows in our case the Kurdjumov-Sachs model. Furthermore, the program package MAUD offers the possibility to follow the phase transition, so that the composition at all temperatures can be documented during heating. It has to be pointed out that the texture influence on the quantitative phase analysis can be included by MAUD, so that even for strong crystallographic textures the relation ferrite/austenite can be given very well.

Severe plastic deformation techniques are known to produce grain sizes up to submicron level. This leads to conventional Hall-Petch strengthening of the as-processed materials. In addition, the microstructures of severe plastic deformation processed materials are characterized by relatively lower dislocation density compared to the conventionally processed materials subjected to the same amount of strain. These two aspects taken together lead to many important attributes. Some examples are ultra-high yield and fracture strengths, superplastic formability at lower temperatures and higher strain rates, superior wear resistance, improved high cycle fatigue life. Since these processes are associated with large amount of strain, depending on the strain path, characteristic crystallographic textures develop. In the present paper, a detailed account of underlying mechanisms during SPD has been discussed and processing-microstructure-texture-property relationship has been presented with reference to a few varieties of steels that have been investigated till date.

The textures and grain sizes within the heat affected zone (HAZ) and weldmetal regions of single phase bcc (Fe-3 wt% Si; 430 stainless steel) and fcc (AA5182 and 5251) alloys in spot and metal inert gas (MIG) welds have been determined using electron back-scattered diffraction (EBSD); optical and scanning electron microscopy (SEM) techniques. In all the situations studied, the grains that developed into the columnar weldmetal grains from the HAZ had a misorientation of less than 10° between <100> and the maximum thermal gradient (∇T

max

). The initial weldmetal columnar grain width can be related to the HAZ grain diameter at the fusion boundary by a simple relationship involving the proportion of grains with the favourable low misorientation texture in weldmetal and HAZ. This relationship does not hold throughout the whole weldmetal region due to competitive growth between columnar grains (steel) and a columnar-to-equiaxed transition (CET; Al-based alloys).

The present paper deals with the evolution of texture in austenitic stainless steels during annealing after 95% cold rolling. After 95% cold rolling, the texture is mainly of the brass type {110} <112> along with a scatter towards the S orientation {123} <634> and Goss orientation {011} <100>. Weak evidence of Cu component is observed at this high deformation level. During annealing, recovery is observed before any detectable recrystallization. After recrystallization, the overall texture intensity was weak; however, there are some discernible texture components. There was no existence of the brass component at this stage. Major components are centered on Goss orientation and Cu component {112} <111> as well as the BR component {236} <385>. Also, there are some few orientations which come up after recrystallization i.e. {142} <2 –1 1> and {012} <221>. With increase in annealing temperature the textural evolution shows emergence of weak texture with another new component i.e. {197} <211>. The evolution of texture was correlated with the deformation texture through twin chain reaction.

Iron aluminides of different composition were deformed at high temperatures by extrusion through a round and rectangular die, approximating tension and plane strain deformation, respectively. Depending on temperature and composition dynamic recrystallization takes place. To investigate post-deformational recrystallization annealing treatments have been applied. Global and local textures were measured by neutron and electron back scatter diffraction (EBSD), respectively. The EBSD method allows the separation of deformation and recrystallization texture components. Moreover, the texture development is correlated with the microstructure evolution derived from EBSD mappings. The development of the deformation and recrystallization textures is discussed by means of different models on polycrystal deformation and recrystallization, respectively. Conclusions concerning the anisotropy of the elastic and plastic properties are drawn.

The model proposed by Miedema is widely used to calculate the enthalpy and Gibbs energy of formation under non-equilibrium conditions, but ignores the possible effect of interfacial component of Gibbs energy change on phase evolution. The latter may be significant in aggregates with ultrafine or nanometric grain size. In the present study, Miedema model is extended to calculate enthalpy and Gibbs energy of ternary Al-Cu-Ti and Al-Cu-Nb systems after incorporating interfacial energy component as a function of grain size. This exercise allows determination of probable composition range of solid state amorphization operative below a critical grain/crystallite size for a given solid solution.

A newly developed laser powered heating stage for commercial SEMs in combination with automated EBSD-data acquisition was used to investigate the α-γ-α phase transformation in steel. This novel experimental setup can be used to achieve more information about microstructure and orientation changes. First, the results on the α-γ-α phase transformation in a microalloyed steel are presented.

Using a new analytic flow function, an analysis of the deformation field in symmetrical and asymmetrical rolling has been carried out. The asymmetry concerns the differences in the angular speeds of the rolling cylinders. The flow function describes the trajectory of the material flow from which the velocity field and the velocity gradient is obtained by partial derivations. The new flow function takes also into account the “discontinuity” at the entry of the material into the die. By introducing a non-homogeneous velocity distribution at the end of the flow line, the shear component in the rolling plane and in the rolling direction that is characteristic to the asymmetric rolling is naturally introduced into the deformation process. The varying velocity gradient along selected flow lines is incorporated into the viscoplastic self-consistent polycrystal plasticity model to simulate the development of the deformation texture. The effect of multiple passes as well as the asymmetries on the evolution of the deformation textures is studied for bcc iron.

Evolution of microstructure and texture was studied in severely plastically deformed (up to an equivalent strain of 6.4) high purity (99.99%) Ni sheets processed through Accumulative Roll Bonding (ARB). As received Ni plates (~ 10 mm in thickness) were cold rolled to ~ 80% reduction in thickness (~ 2 mm) and vacuum annealed at 600°C for one hour and these were used as the starting materials (average grain size ~ 25 μm) for the subsequent ARB processing. ND and TD plane normal sections of the ARB processed sheets were subjected to Electron Back Scatter Diffraction (EBSD) and Transmission Electron Microscope (TEM) studies. The ARB processed Ni sheets were found to be filled with ultrafine grains (average grain size ~ 400 nm) after 8 cycles of ARB. Extensive shear band formation was observed particularly in the high cycle ARBed materials. The deformation textures were found to be quite inhomogeneous at the low cycle regime of the ARB. However, the deformation texture achieved remarkable homogeneity after 6 and 8 cycles of ARB and S ({123} <634>) component of the deformation texture was found to be quite strong.

In the Ultra-steel project in NIMS, high-Si and Al type ultrafinegrained (UFG) weathering steel was created by the multi-pass warm rolling method, and the corrosion resistance and microstructure of the rust were estimated. The Si and Al-bearing UFG steels exhibited excellent corrosion resistance than carbon steel (SM). The EPMA and TEM analyses showed that Si and Al were mainly existing as nano-oxides in the inner rust layer formed on the UFG steels. The Al K

α

X-ray spectrum of the test sample exhibited the peak at the same position as that of Al

2

O

3

, which suggests that Al is present in the inner rust in Al

3+

state. In the same way, Si was identified as Si

2+

in the complex Iron oxides of inner rust using EPMA.

The EIS (Electrochemical Impedance Spectroscopy) measurement was conducted for the corrosion test samples to find that the corrosion resistance (Rt) of Si and Al-bearing UFG steel was much larger than that of SM. In the developed steel, the nano-complex oxides were made in the lower layer of iron rust, that increased Rt and suppressed the corrosion. Finally, it was found that High-Si and Al type UFG weathering steel showed excellent properties in strength, toughness and corrosion resistance.

AA 1050 Aluminium, AISI 304L and AISI 316L austenitic stainless steel were deformed at different strain and strain path. Deformed microstructure in AISI 304L and AISI 316L austenitic stainless steel shows significant amount of deformation twin and Strain Induced Martensite (SIM). AA 1050 Aluminium shows grain interaction between neighbouring grains. In this study effort has been made to understand these microstructural developments. It has been found that biaxial strain path and high strain shows higher amount of deformation twins in AISI 316L stain less steel, strain induced martensite in AISI 304L stainless steel and grain interaction in AA 1050 Aluminium.

A new approach, the

Facet

method, to describe the yield loci of textured materials is proposed. It is based on an analytical expression of plastic potentials in strain rate space of which the parameters are identified by fitting to the predictions of multilevel micromechanical models. The chief advantage of this new formulation is that it automatically ensures convexity of the anisotropic yield loci. Furthermore, the approach can be easily extended to formulate stress-space based plastic potentials. This is quite an efficient property especially for the implementation in finite element codes that are used for simulation of industrial metal forming processes because it offers a significant reduction in computation time needed for yield checks compared to that of strain rate space based plastic potential expression. In our work, the

Facet

method is applied in combination with the Taylor-Bishop-Hill micromechanical model, with both strain rate as well as stress space formulations, to various model textures and industrial materials. In this paper, the equipotential surfaces and the corresponding yield loci will be presented for a sharp rotated-cube model texture and a moderately sharp industrial grade IF steel sheet texture. A brief quantitative assessment of the new method with respect to the original model data will be presented.

The Hall-Petch relationship was studied in interstitial-free steel subjected to Φ = 90° Equal Channel Angular Extrusion for up to N = 8 passes via route B

C

processing. The composite equation indicates that although low-angle grain boundaries provide the maximum strengthening up to N = 8 passes, the contribution from high-angle boundaries also increases with greater pass number. Alternatively, the evolution of boundary misorientation in as-ECAE microstructures and its effect on mechanical properties up to N = 3 passes is also understood by using a misorientation scaling factor within the original Hall-Petch equation.

Equal channel angular extrusion is now a well know process to generate ultra-fine grain microstructure in bulk materials. Since the material undergoes a large deformation, the process is also associated with evolution of characteristic texture. Most of the studies carried out on this subject aim at studying single phase materials. However, such a study is very relevant for two-phase materials owing to the possible enhancement of super-plastic properties. In the present work, a thorough investigation of evolution of microstructure and texture has been carried out to elucidate the deformation mechanisms and subsequent texture evolution in a model two-phase material, namely (α + β) brass. A detailed analysis of texture evolution in both α and β (B2) phases will be presented.

Grain growth kinetics was studied for commercially pure magnesium subjected to equal channel angular extrusion (ECAE). The specimens were ECAE processed upto 4 passes at 523 K following all the three important routes, namely A, B

c

and C. Texture and microstructures of the samples were studied using Electron Back Scattered Diffraction (EBSD) technique in a Field Emission Gun – Scanning Electron Microscope (FEG-SEM). It was observed that the grain size significantly reduces after ECAE. ECAE process produces a slightly rotated B and C

2

fiber. Static annealing leads to normal grain growth with unimodal distribution of grains through out the temperature range. Average activation energy for grain growth in the temperature range studied is found to be less than the activation energy for lattice diffusion and grain boundary diffusion of magnesium. No significant change in texture during isochronal annealing for 1 hour i.e., the predominant deformation texture remains same.

In the present study, solidification microstructure and texture evolution in grain-refined Ti-6Al-4V and γ-TiAl alloys via trace boron addition are compared with their baseline counterparts. Boron addition resulted in dramatic grain refinement by almost an order of magnitude. The texture developed in these alloys is also markedly different from the baseline alloys.

Dual phase (DP) steels have been prepared from low carbon steel (0.14% C) at intercritical temperature 740°C and time is varied from 1 minute to 30 minutes followed by water quenching. These steels have been characterized by optical microscopy, FE-SEM, hardness measurements, tensile properties and electron backscattered diffraction (EBSD) studies. Tensile properties of a typical dual phase steel are found to be 805 MPa ultimate tensile strength with 18% total elongation. Martensite volume fraction of D P steel (determined by EBSD technique) prepared at 740°C for 6 minutes is found to be 10.2% and the grain size of ferrite and martensite found to be 14.39 micron and 1.05 microns respectively. Abrasive wear resistance of dual phase steels has been determined by pin on drum wear testing machine. DP steels have been found to be 25% more wear resistant than that of normalized steel. Short intercritical heating time followed by water quenching gives higher wear resistance by virtue of smaller and well dispersed martensite island in the matrix of ferrite.