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Austenitic stainless steels are used extensively in harsh environments, including for high-pressure gaseous hydrogen service. However, the tensile ductility of this class of materials is very sensitive to materials and environmental variables. While tensile ductility is generally insufficient to qualify a material for hydrogen service, ductility is an effective tool to explore microstructural and environmental variables and their effects on hydrogen susceptibility, to inform understanding of the mechanisms of hydrogen effects in metals, and to provide insight to microstructural variables that may improve relative performance. In this study, hydrogen precharging was used to simulate high-pressure hydrogen environments to evaluate hydrogen effects on tensile properties. Several austenitic stainless steels were considered, including both metastable and stable alloys. Room temperature and subambient temperature tensile properties were evaluated with three different internal hydrogen contents for type 304L and 316L austenitic stainless steels and one hydrogen content for XM-11. Significant ductility loss was observed for both metastable and stable alloys, suggesting the stability of the austenitic phase is not sufficient to characterize the effects of hydrogen. Internal hydrogen does influence the character of deformation, which drives local damage accumulation and ultimately fracture for both metastable and stable alloys. While a quantitative description of hydrogen-assisted fracture in austenitic stainless steels remains elusive, these observations underscore the importance of the hydrogen-defect interactions and the accumulation of damage at deformation length scales.

The effects of internal hydrogen on the deformation microstructures of 304L austenitic stainless steel have been characterized using electron backscattered diffraction (EBSD), transmission Kikuchi diffraction (TKD), high-resolution scanning transmission electron microscopy (HRSTEM), and nanoprobe diffraction. Samples, both thermally precharged with hydrogen and without thermal precharging, were subjected to tensile deformation of 5 and 20 pct true strain followed by multiple microscopic interrogations. Internal hydrogen produced widespread stacking faults within the as-forged initially unstrained material. While planar deformation bands developed with tensile strain in both the hydrogen-precharged and non-precharged material, the character of these bands changed with the presence of internal hydrogen. As shown by nanobeam diffraction and HRSTEM observations, in the absence of internal hydrogen, the bands were predominantly composed of twins, whereas for samples deformed in the presence of internal hydrogen, ε-martensite became more pronounced and the density of deformation bands increased. For the 20 pct strain condition, α′-martensite was observed at the intersection of ε-martensite bands in hydrogen-precharged samples, whereas in non-precharged samples α′-martensite was only observed along grain boundaries. We hypothesize that the increased prevalence of α′-martensite is a secondary effect of increased ε-martensite and deformation band density due to internal hydrogen and is not a signature of internal hydrogen itself.

Twinning plays an important role in governing the balance between strength and ductility in hexagonal-close-packed (HCP) metals. Here, we report a combined experimental and theoretical study of twin nucleation from a single dislocation in HCP crystals. Specifically, high-resolution transmission electron microscopy has been used to identify { 11 2 ¯ 1 } twin nuclei in HCP rhenium, providing evidence of their nucleation from a dislocation. The favorability of this dislocation-based nucleation mechanism is rationalized by an anisotropic elasticity model of dislocation dissociation, parametrized by density functional theory calculations, which suggests the conditions for disconnection nucleation and propagation, under which this { 11 2 ¯ 1 } twinning mechanism is expected to be effective. The analysis serves to advance our understanding of the origin of the unique predominance of { 11 2 ¯ 1 } twinning in rhenium, which correlates with the high strength and ductility featured by this metal. It also provides new insights into design strategies that may be effective in activating this twinning mode and enhancing the balance between strength and ductility in HCP alloys more broadly.

Ductile rupture or tearing usually involves structural degradation from the nucleation and growth of voids and their coalescence into cracks. Although some materials contain preexisting pores, the first step in failure is often the formation of voids. Because this step can govern both the failure strain and the fracture mechanism, it is critical to understand the mechanisms of void nucleation and the enabling microstructural configurations which give rise to nucleation. To understand the role of dislocations during void nucleation, the present study presents ex-situ cross-sectional observations of interrupted deformation experiments revealing incipient, subsurface voids in a copper material containing copper oxide inclusions. The local microstructural state was evaluated using electron backscatter diffraction (EBSD), electron channeling contrast (ECC), transmission electron microscopy (TEM), and transmission kikuchi diffraction (TKD). Surprisingly, before substantial growth and coalescence had occurred, the deformation process had resulted in the nucleation of a high density of nanoscale (≈50 nm) voids in the deeply deformed neck region where strains were on the order of 1.5. Such a proliferation of nucleation sites immediately suggests that the rupture process is limited by void growth, not nucleation. With regard to void growth, analysis of more than 20 microscale voids suggests that dislocation boundaries facilitate the growth process. The present observations call into question prior assumptions on the role of dislocation pile-ups and provide new context for the formulation of revised ductile rupture models. While the focus of this study is on damage accumulation in a highly ductile metal containing small, well-dispersed particles, these results are also applicable to understanding void nucleation in engineering alloys.

Using electron backscatter diffraction, over 1000 grain boundary/twin interactions were investigated in pure Re after uniaxial compression. Crystallographic factors such as grain boundary disorientation angle, displacement gradient tensor accommodation, and twin plane and shear vector alignment were taken into account as well as the macroscopic Schmid factor. It was found that the crystallographic relationship between coincident twin pairs at grain boundaries fall into two categories. In the first category, the twin pairs satisfied two criteria: the twin plane alignment was maximized across the boundary and the twin variant was kept constant across the boundary. In the second category, comprising approximately 5% of the characterized interactions, twin variant selection appeared to be driven by the macroscopically applied stress state, as resolved by the Schmid factor. In comparison to published results on twin/grain boundary interactions in Mg and Zr, it was found that twins transmit across grain boundaries in Re far more readily, regardless of the grain boundary disorientation angle. This is likely to be a function of the anomalously low twin boundary energy in Re as well as the difference in magnitude of the twinning-induced deformation that must be accommodated at the boundary, with the {11 2 ¯ 1} 26 ¯ > system being dominant in Re and the {10 1 ¯ 2} 11 ¯ > being dominant in Mg and Zr.

While rhenium has proven to be an ideal material in fast-cycling high-temperature applications such as rocket nozzles, its prohibitive cost limits its continued use and motivates a search for viable cost-effective substitutes. We show that a simple design principle that trades off average valence electron count and cost considerations proves helpful in identifying a promising pool of candidate substitute alloys: The Mo–Ru–Ta–W quaternary system. We demonstrate how this picture can be combined with a computational thermodynamic model of phase stability, based on high-throughput ab initio calculations, to further narrow down the search and deliver alloys that maintain rhenium’s desirable hcp crystal structure. This thermodynamic model is validated with comparisons to known binary phase diagram sections and corroborated by experimental synthesis and structural characterization demonstrating multiprinciple-element hcp solid-solution samples selected from a promising composition range.

The microstructural evolution of Re under tension was investigated using EBSD, in-situ and ex-situ TEM. The samples had a texture that suppressed possible { 11 2 1 } 〈 11 2 6 〉 twinning, while favoring type 〈 11 2 0 〉 basal dislocations during tensile straining. In-situ tensile straining was performed on rhenium both at room temperature and at 920 °C to investigate the low strain region of deformation, where it was found that type b = [ 11 2 0 ] screw dislocations operated on the basal planes in loosely aligned slip bands. Through Schmidt factor analysis, EBSD had shown that pyramidal slip activated appreciably only at strain values above half of the failure strain. At failure stresses, type b = [ 11 2 0 ] basal screw dislocations observed in dislocation slip bands and type b = [ 11 2 3 ] pyramidal screw dislocations formed dislocation nets interfering with type glide. HAADF STEM imaging was used to view the morphology of tension-induced { 11 2 1 } 〈 11 2 6 〉 twins at higher strain values. Twin transmission with { 11 2 1 } 〈 11 2 6 〉 twins changing twin planes between parent and matrix orientations was observed. The twin structure observed in tension were representative of classical twin structures, contrary to the { 11 2 1 } 〈 11 2 6 〉 type twins seen in compression which consisted of twin aggregates. Our results suggest that the compression-tension asymmetry in Re is likely due to the twin favorability of the microstructure forcing the creation of many more twins during compression as compared to tension. The exceptionally-high work hardening rate in Re was found to be mainly due to large scale twin formation coupled with basal slip activity during initial straining.

This work demonstrates an experimental methodology for reusing anode material from end of life commercial lithium ion batteries (LiB) in order to create new LiB anodes. End-of-life LiB cells were safely opened and assessed as a source of anode material. Anode material extracted from LiB cells through a basic mechanical separation was cycled stably with minimal processing. The LiB cells produced with recovered anode material showed equivalent cycling capacity and lower first cycle capacity loss than similarly produced virgin graphite anodes, regardless of recycled material source or morphology, as shown by SEM imaging. The effects of some graphite pre-lithiation were seen, mainly in a lowered initial voltage of the cells before the first cycle. A methodology for scaling-up this laboratory process for industrial recycling is discussed.

During in situ tensile loading of a Mg–2.5 wt%Y alloy, a large kink band was observed. The orientation of the kink band varies continuously with a maximum misorientation angle of 18° with respect to the matrix grain. The formation of the kink band is accompanied by the nucleation of numerous { 1 0 1 ¯ 2 } twins at the kink boundaries. A mechanistic model is proposed to account for this unique kink band and concomitant twin nucleation via dissociation of basal dislocations. This mechanism is aided by the presence of Y, which suppresses { 1 0 1 ¯ 2 } twinning and simultaneously decreases the stacking fault energy.