Mechanical and Creep Behavior of Advanced Materials

A brief outline of the research activities during the past three decades in the Nuclear Materials Research Group at NC State University is summarized that comprises of creep mechanisms in materials, reliability of solders in electronic packaging, dynamic strain aging, radiation embrittlement of ferritic steels, anisotropic biaxial creep of hexagonal closed packed metals with emphasis on Zircaloy cladding, in-service monitoring of structural materials using ABI, non-interactive Nuclear Magnetic Resonance studies of dynamical behavior of point and line defects during deformation, and radiation effects in nanocrystalline materials. All the described aspects are the outcomes of research experiences of the author starting from his MS thesis at Cornell University on creep of alpha iron and doctoral thesis research on NMR studies followed by post-doctoral research at the University of California in Berkeley on creep and superplasticity, and University of Newcastle, Australia on radiation effects on yield point phenomena in steels.

Biaxial creep of Nb-modified Zircaloy-4 (HANA-4) tubing is investigated at varied ratios of hoop and axial stresses at a constant temperature of 500 °C using internal pressurization superimposed with axial load while monitoring the hoop and axial strains using non-contact laser telemetric extensometer and linear variable differential transducer, respectively. Steady-state creep rates along the hoop and axial directions were evaluated in the power-law creep regime from which the creep locus was derived at a constant energy of dissipation. The resulting creep locus was compared with that predicted by the anisotropy parameters, R and P in the Hill’s formulation for generalized stress for anisotropic materials. Crystallographic texture of the tubing was characterized using electron backscatter diffraction technique. Research is supported by NSF grant #DMR0968825.

Impact toughness of Zr-2.5Nb pressure tube material was evaluated as a function of specimen geometry, orientation and test temperature. Two types of samples were used. Curved impact specimens were machined directly from the tube section thereby retaining the pressure tube curvature and flat impact specimens were machined from a flattened tube section subjected to stress relieving treatment. In both the cases the specimen thickness was the actual pressure tube thickness that is 3.5 mm and the other dimension of the impact specimen was 55 mm × 10 mm. The tests were carried out for both as-received and hydrided pressure tubes between room temperature and 300 °C. As the Zr-2.5Nb pressure tube material is anisotropic, the studies were also carried out using specimens with crack growth either along axial or along transverse direction of the tube. The paper describes the results generated and discusses the materials impact toughness behaviour under different conditions.

The 14YWT FCRD NFA-1 is a nanostructured variant of ODS ferritic steels. It is processed by ball milling FeO and argon atomized Fe-14Cr-3W-0.4Ti-0.2Y (wt%) powders, followed by hot extrusion, annealing and cross-rolling to produce ≈10 mm thick plates. The plate contains a bimodal distribution of highly textured, pancake-shaped, generally submicron, grains. NFA-1 also contains a large population of microcracks lying in planes normal to the plate thickness direction. The microcracks form on {001} planes and propagate in \( \left\langle {110} \right\rangle \) directions along low angle subgrain boundaries formed during high-temperature deformation. Tensile tests in directions parallel to the extrusion (L) or cross-rolling (T) manifest high strength and good ductility over a wide range of temperatures. In contrast, loading in the short plate thickness (S) direction, perpendicular to the microcrack faces, manifests a much lower strength, and almost zero ductility, with flat, faceted cleavage fracture surfaces up to ≈100 °C. However, tensile ductility in the S orientation increases at higher temperatures above with a brittle-to-ductile transition (BDT). The L, T and S properties are reasonably similar (isotropic) above ≈200 °C. At lower temperatures, deformation in both tensile and fracture toughness tests is accompanied by extensive delamination due to propagation of the microcracks. Delamination has relatively modest effects on tensile properties, but actually improves fracture toughness, either by relaxing triaxial stress in thin delaminated ligaments near the tip, or crack deflection, depending on the specimen orientation. Elastic-plastic toughness (K_Jc) of NFA-1 undergoes a cleavage BDT at ≈−175 °C, with stable crack tearing initiation just beyond general yielding at higher temperatures.

Some of the earliest characterization of steady-state creep in aluminum, was developed at UC Berkeley by John Dorn, his students and postdoctoral scholars, one being K.L. Murty. Research of the group included classic five power-law creep, Harper-Dorn creep, power-law breakdown and viscous glide. Many of the models and theories persisted for a relatively long period of time due to the thoughtfulness of the work. This paper discusses the more recent developments in these phenomena that may lead to new interpretations in aluminum creep, as well as other crystalline materials.

Uniaxial creep tests were performed on Zircaloy-4 sheet in the temperature range of 500–600 °C at high stresses (>10⁻³E, E is the elastic modulus) to uncover the rate-controlling mechanism. Stress exponents and stress-dependent activation energies, respectively, in the range of 9.3–11 and 220–242 kJ/mol were obtained from the steady state creep rate data. TEM analyses on the deformed specimens revealed extensive hexagonal screw dislocation networks on the basal planes indicating recovery of screw dislocations by cross-slip to be the dominant mechanism. Furthermore, analysis of the creep data in the light of Friedel’s cross slip model for HCP metals and the activation volume of the operating deformation mechanisms measured using stress relaxation tests favor cross-slip of screw dislocations as the rate controlling mechanism in the creep testing conditions employed in this study. In addition, transitions in creep mechanisms of Zircaloy-4 are presented along with its application to the evaluation of the total strain accumulated in Zircaloy-4 fuel cladding during dry storage.

Tensile and creep properties of materials are usually evaluated using standard test methods. These methods are material intensive. Small specimen testing methods such as, Impression Creep, Small Punch Creep and Ball Indentation allows us to determine the mechanical properties of materials. Creep deformation behavior can be studied using Impression Creep. Small Punch Creep test method is used to evaluate creep deformation and fracture properties of materials. Tensile properties and fracture toughness can be evaluated using Ball Indentation method. Compared to the standard test methods, these methods have several advantages in materials development, structural integrity assessment and characterization of mechanical properties of different but narrow microstructural zones in weld joints. This paper presents a review of the recent advances in these three small specimen testing methods and discusses their relative advantages and limitations using ferrous and non-ferrous alloys.

TerraPower has revitalized the manufacturing of HT9, and optimized its heat treatment to be used as fuel claddings in the Traveling Wave Reactor. TerraPower initiated a comprehensive test program to compare the mechanical and thermal creep properties of the optimized TerraPower HT9 to the historical HT9. The uniaxial tensile tests show TerraPower HT9 has improved yield strength compared to historical HT9 across all temperatures. Charpy impact toughness tests on TerraPower HT9 show lower upper shelf energy compared to historical HT9, but the ductile to brittle transition temperature (DBTT) between the steels remain similar. Three point bend tests between room temperature and 400 °C show comparable fracture toughness to those of historical HT9. Thermal creep test data suggest TerraPower HT9 have improved creep strength compared to historical HT9.

Controlling mechanisms for creep of zirconium and zirconium alloys continue to be debated. In previous studies, the authors analyzed cumulative zirconium and zirconium alloy creep data over a broad range of stresses (0.1–115 MPa) and temperatures (300–850 °C) based on a literature review and experiments. Zirconium obeys traditional power-law creep with a stress exponent of approximately 6.4 over stain-rates and temperatures usually associated with the conventional “five-power-law” regime. The measured activation energies for creep correlated with the activation energies for zirconium self-diffusion. Thus, dislocation climb, rather than the often assumed glide mechanism, appears to be rate controlling. The stress exponents of the creep data in the five-power-law regime for Zircaloy-2 and Zircaloy-4 were determined to be 4.8 and 5.0, respectively. Further advances in the understanding of the controlling mechanisms for zirconium and zirconium alloys will be presented based on a review of the literature over the past decade.

The creep behaviour of polycrystalline materials is now well described through a series of equations that delineate various mechanisms describing intragranular and intergranular flow processes. The recent ability to fabricate metals with exceptionally small grain sizes within the submicrometer and nanometer ranges has raised questions concerning the applicability of these flow processes in these new and advanced materials. This report examines this problem by initially reviewing the fundamental mechanisms occurring in high temperature creep and then considering the extension of this approach to materials having ultrafine grain sizes.

For structural applications, ductility is essential along with high strength in nanocrystalline (nc) materials. In general, ductility is controlled by strain hardening and strain rate sensitivity. In conventional materials which are coarse grained, the deformation is mainly dislocation based and accumulation of these dislocations results in work hardening. The deformation mechanisms that are operative in nc materials are distinct and the strain hardening ability is limited in nc materials. Strain rate sensitivity (SRS) and activation volume are the two key parameters which govern the underlying deformation mechanisms in nc materials. Higher SRS value could be an indication of better ductility levels. In general, nanocrystalline single phase fcc metals showed increased SRS, where as bcc metals showed decreased SRS. The addition of second phase effects the overall SRS of the nano composite/alloy. Since producing nc materials in bulk quantities is a challenge, nanoindentation, which can be performed on smaller sized samples, is an useful technique to study SRS and activation volume. Strain rate sensitive characteristics of Al and its alloys are reviewed in this paper. Our earlier work as well as the available literature data on these alloys showed that the nature and structure of the second phase dispersions greatly influence the SRS.

Irradiation creep of Zr-alloy nuclear reactor core components affects the reactor performance and also limits the reactor life in cases where those components cannot easily be replaced. For Zr-2.5Nb pressure tubing irradiation creep has been extensively studied for a range of temperatures, between 250 and 350 °C, and dose rates, between 1 × 10¹⁶ and 2 × 10¹⁸ n m⁻² s⁻¹ (E > 1 meV), using data from various materials test reactors and power reactors. These studies have shown that irradiation creep is controlled by a complex combination of slip and diffusional mass transport (often referred to as irradiation growth in the absence of stress). Irradiation creep is dependent on the crystallographic texture, the dislocation structure, and the grain structure; the importance of each being a function of irradiation temperature and displacement damage rate. Data will be presented, together with mechanistic modelling, to show what factors affect creep under different irradiation conditions.

This work investigated the solid-states phase transformation behavior of Zr–Cr–Fe alloys during rapid cooling from β-phase region. Scanning electron microscopy (SEM) was used to characterize the microstructure evolution of Zr–Cr–Fe alloys containing different Mo and Bi contents. The results show that two different phase transformation modes were involved during β to α transformation for different domains within prior β grains: (i) Martensitic transformation resulting in lath-shaped grains occured within prior β grain interiors. (ii) Massive transformation generating massive-shaped grains initiated along two adjacent prior β parent grain boundaries. Alpha (α) lath width reduced with increasing Mo concentration while Mo strongly retarded massive phase transformation. Interestingly microstructures exhibited no significant variation in the case of specimens containing different Bi contents irrespective of the phase transformation modes.

Modified 9Cr–1Mo steel exhibited dynamic strain ageing (DSA) in the temperature range from 523 to 673 K and it was established on the basis of plateau/peak in yield and tensile strength, minima in ductility and serrations in stress–strain curve. High density of dislocations and typical features like dislocation debris, kinks and bowing of dislocations was observed in the regime of DSA. This steel exhibited cyclic softening irrespective of the strain amplitude, strain rate, and temperature. The observed cyclic softening is associated with many factors like cell formation at room temperature and additionally annihilation of array of dislocations at 573 K, in addition to coarsening of carbides at 873 K.

The curved thin-walled structures of multi film-cooling holes with different curvatures were adopted to simulate film-cooling turbine blades. The low cycle fatigue (LCF) characteristic was studied based on the theory of crystallographic slip damage. Results show that there is obvious stress interference among cooling holes. Two slip bands around the holes were found linear at approximately 45° and 135° to the loading axis. The maximum resolved shear stress reduces with the increase of curvature radius. Meanwhile, the LCF life positively increases with the changing of curvature radius. When curvature radius is less than 13 mm, it makes a remarkable effect on the resolved shear stress and LCF life; however, when the curvature radius exceeds 13 mm, it can be replaced by the plate structure. Furthermore, an exponential curve is found fitting for the relation between the curvature radius and the logarithmic fatigue life.

This article reviews the discovery of new phases of carbon (Q-carbon) and BN (Q-BN) and addresses critical issues related to direct conversion of carbon into diamond and h-BN into c-BN at ambient temperatures and pressures in air without any need for catalyst and presence of hydrogen. The Q-carbon and Q-BN are formed as a result of quenching from super undercooled state by using high-power nanosecond laser pulses. We discuss the equilibrium phase diagram (P vs. T) of carbon, and show that by rapid quenching kinetics can shift thermodynamic graphite/diamond/liquid carbon triple point from 5000 K/12 GPa to super undercooled (4000 K) carbon at atmospheric pressure in air. Similarly, the hBN-cBN-Liquid triple point is shifted from 3500 K/9.5 GPa to as low as 2800 K and atmospheric pressure. It is shown that nanosecond laser heating of amorphous carbon and nanocrystalline BN on sapphire, glass and polymer substrates can be confined to melt in a super undercooled state. By quenching this super undercooled state, we have created a new state of carbon (Q-carbon) and BN (Q-BN) from which nanocrystals, microcrystals, nanoneedles, microneedles and thin films are formed. The large-area epitaxial diamond and c-BN films are formed, when appropriate planar matching or lattice matching template is provided for growth from super undercooled liquid state. Scale-up processing of diamond, c-BN and diamond/c-BN heterostructures and related nanostructures such as nanodots, microdots, nanoneedles, microneedles and large-area single-crystal thin films will have tremendous impact on applications ranging from abrasive and tool coatings to high-power devices and myriad of biomedical applications.

Ferritic-martensitic steels with good high temperature mechanical properties have many promising applications in fossil and nuclear power plants. In this work, a F92 steel was tensile tested from room to elevated temperatures (up to 700 °C). This material exhibited higher strength than traditional P92 steels. The reasons for the observed changes in mechanical properties were investigated by studying the microstructural characteristics in undeformed and deformed specimens using transmission electron microscopy. The microstructural evolution accelerated significantly under loading as temperature increased. For instance, the deformed microstructure at 600 °C showed early stages of M₂₃C₆ precipitate formation under loading. The M₂₃C₆ precipitates exhibited more coarsening tendency whereas the MX-type precipitates retained their size. As coarsening of M₂₃C₆ precipitates progressed at elevated temperatures, the strength gradually decreased as the solid solution strengthening deteriorated by removing W and Mo from the solid solution matrix.

The effect of various surface finishes on the interfacial fracture toughness of SAC405 solder joints under shock loading conditions was investigated using sandwich compact tension specimens. The surface finishes investigated in this study were organic solderability preservative (OSP), immersion silver (ImAg), nano-tin dispersed in an organic carrier (OrM), and gold over nickel (NiAu). It was found that the ImAg surface finish appears to enhance the fracture toughness, while the NiAu surface finish appears to have no effect compared to bare copper. For the OSP and OrM surface finishes, more data will need to be collected to reach a conclusion regarding their effect. The effects on fracture toughness could not be attributed to the thickness or morphology of the intermetallic compound layer found at the solder joint interface. Future work will focus on understanding the mechanisms by which immersion silver enhances the fracture toughness. Overall, the results of this study may help designers make more informed decisions about which surface finishes can be used to reduce shock failures in consumer electronic devices.

Cast duplex stainless steel piping in light water nuclear reactors experience thermal aging embrittlement during operational service. Interest in extending the operational life to 80 years requires an increased understanding of the microstructural evolution and corresponding changes in mechanical behavior. We analyze the evolution of the microstructure during thermal aging of cast CF-3 and CF-8 stainless steels using electron microscopy and atom probe tomography. The evolution of the mechanical properties is measured concurrently by mechanical methods such as tensile tests, Charpy V-notch tests, and instrumented nanoindentation. A microstructure-based finite element method model is developed and utilized in conjunction with the characterization results in order to correlate the local stress-strain effects in the microstructure with the bulk measurements. This work is supported by the DOE Nuclear Energy University Programs (NEUP), contract number DE-NE0000724.

Kanthal APMT^® steel (Fe–22Cr–5Al–3Mo) was developed mainly for using as high temperature furnace elements. This kind of high-Cr ferritic steels is not considered to have good weldability because of a variety of metallurgical issues. Friction stir welding (FSW), a solid state welding process, was applied to a Kanthal APMT^® plate in a bead-on-plate configuration using a PcBN tool with a tool rotation rate of 600 RPM and a traverse speed of 25.4 mm/min. Microstructure and mechanical properties were evaluated to determine the weld quality and examine the feasibility of applying FSW as a joining technique for this steel. Microstructural characteristics were mainly studied by optical microscopy and transmission electron microscopy. The stir zone contained equiaxed grain structure with an average grain size of 13.7 μm. Interestingly, Vickers microhardness profile across the processed zone has revealed no significant change in microhardness.

This research focuses on work with emphasis on direct measurements of stresses during mesoscale microstructural deformation of nickel based and zirconium based alloys during 3-point bending tests for fracture toughness at elevated temperatures. A novel nano-mechanical Raman spectroscopy measurement platform was designed for temperature, stress, and chemistry mapping at micro to nanoscale for different temperature and loading conditions. During the 3-point bending test to measure fracture toughness of micron sized samples, notch tip plastic stresses as a function of microstructure, load, and temperature, with micron scale resolution were measured. The temperature field distribution was correlated to stress distribution and residual microstructure stresses around the area of the notch tip. The mechanical properties which include the elastic modulus, hardness and stress-strain relation at the plastic zone around the notch tip were also measured and compared before and after the bending tests. A new finite element method formulation that incorporated different elastic and plastic material properties from indentation experiments at different locations was validated using the experiments. We find that residual stress is an important indicator of scatter in material failure data.

Low-stress high-temperature tensile-creep behavior of AZ31 Mg alloy was investigated to characterize microstructure evolution, uncover dominant creep mechanism and find a correlation with common creep models. The stress exponent, inverse grain size exponent and activation energy value were evaluated. Cavity nucleation from stress concentration sites, types of fracture surfaces and microstructural evidence of grain migrations were observed in crept samples that are indicative of Rachinger mechanism of grain boundary sliding (GBS). Experimental data reveal a reasonable correlation with Langdon’s model. Further analysis on fracture behavior of this alloy in a wider range of stresses show that they follow Monkman-Grant model in predicting the fracture time.