Effect of Strain Path on Microstructural Evolution and Mechanical Behaviour of Zinc during Equal Channel Angular Pressing

Kurt Mills

doi:10.25907/01038

Zinc (Zn) is generating considerable interest for use in high-performance applications, including as bioresorbable implants and as a battery anode material for Zn-ion batteries. However, it typically lacks the requisite strength and ductility for these applications. In this study, equal channel angular pressing (ECAP) was investigated to improve the mechanical properties. The ECAP strain path, referred to as the route, is altered by changing the billet orientation between passes, with four standard pressing routes being A, BA, BC, and C (referred to as RA, RBA, RBC, and RC, respectively). Studies on other alloy systems show that the pressing route can have significant impacts on mechanical properties, uniformity of grain refinement and crystallographic textures. However, effects from different pressing routes on the microstructure and mechanical properties of Zn have not been reported to date. In this research, trials with commercially pure Zn revealed that ECAP resulted in an approximate 50% reduction in grain size compared to the initial material. However, RA had a notably higher average grain size and more regional variability across billet sections than the other routes. The pressing route impacted processability, with nonredundant strain paths (RA and RBA) tending to exhibit more cracks that led to failures compared to redundant strain paths (RBC and RC). Overall, RC yielded intact samples with minimal roughening or cracking, whereas RBC and RA exhibited some roughening and occasional cracking. Contrastingly, RBA induced intense cracking, which led to failure, so that mechanical properties could not be ascertained for this condition. For pressing routes that gave intact samples for tensile tests (RC, RBC, and RA), there was an ~80% improvement in the yield strength (YS) and approximately 50% increase in ultimate tensile strength (UTS), while ductility remained >80% with four ECAP passes. Different textures developed under each pressing route, with two prominent texture components present (Y and B fibre textures) related to the deformation via simple shear. For RA, a unique texture formed, dominated by the B fibre with less intense contribution from the Y fibre. This contrasts with the other routes, which were dominated by the Y fibre. In the case of RBA and RBC, the B fibre was absent. Further investigations into the textural evolution during pressing revealed that both Y and B fibre textures formed after the first ECAP pass, with the Y fibre more dominant. It was determined that the Y fibre formed due to the alignment of the crystal’s c-axis with the applied shear plane, though some minor misorientation was noted (~30⁰), while the B fibre formed due to the parallel alignment of the basal plane with the shear plane. For RBA and RBC, only the Y fibre remained after the second pass as the 90⁰ rotations employed for these routes resulted in the perpendicular realignment to the shear plane of prior B fibre textured grains, effectively suppressing this fibre. The Y fibre was much less susceptible to changes in the applied shear plane, maintaining similar intensity throughout the process. Meanwhile, under RC, the texture was stable over the four passes, due to iso-planar shear across all passes. RA was unique in that the B fibre intensity was incrementally increased with each pass, as the changes in the applied shear plane direction under this strain path strengthen B-fibre alignment, while the Y fibre remained stable for this route. To further understand the effects of ECAP on the microstructures and mechanical properties of Zn alloys, a follow-up study was conducted into the influence of pressing routes on a more complex multiphase Zn-14Al alloy with potential to exhibit superplastic behaviour. Zn-Al alloys show potential in battery anode applications to effectively suppress dendrite growth, while also improving corrosion resistance. A single ECAP pass gave the greatest strength increment of ~20% with comparable ductility (~150% elongation) to samples subject to four passes, while there were notable differences in mechanical properties for different routes. RA yielded the highest strength increment with an approximate ~15% increase in the UTS compared to the initial extruded condition, while RC led to an equivalent reduction of ~15%. However, all pressing routes resulted in intense grain refinement, which significantly enhanced ductility by ~50%. The textures that formed in Zn-14Al billets after four ECAP passes were like those for the pure Zn. However, the B fibre was more intense, with an increase in intensity from ~5% to ~13% in RA and increasing from <3% to ~7% in RC, where the B fibre now dominates. However, there was little change in the intensity of the Y fibre across all routes, maintaining an intensity of around 4.5%, though RBA also exhibited some minor B fibre texture (~3%), which was not present in pure Zn. The addition of Al to Zn increases the c/a ratio so that basal slip is enhanced, enabling easier alignment of the B fibre with the applied shear plane and strengthening this texture component. A preliminary assessment of the corrosion performance for commercially pure Zn and Zn- 14Al revealed significant differences related to the pressing route, with an almost order of magnitude difference in the measured corrosion rates. Several key parameters were identified which contributed to the differences in the measured corrosion properties, with the average grain size, dislocation density, crystallographic textures and residual stresses highlighted. The role of average grain size was evident, whereby significant grain refinement of the as-received material by ECAP led to substantial improvements in the corrosion resistance. Those strain paths predicted to generate lower residual stress, RBC and RC, exhibited higher corrosion resistance than those with higher residual stresses, RA and RBA. Meanwhile, differences in the crystallographic textures and dislocation densities for different routes were also an important influence on corrosion responses, which warrants further in- depth study.

Effect of Strain Path on Microstructural Evolution and Mechanical Behaviour of Zinc during Equal Channel Angular Pressing

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