Mechanical properties and microstructure evolution during deformation of ultrafine grained zirconium at low temperatures

Mechanical properties of ultrafine grained (UFG) zirconium (grain size 200 nm), produced by a combination of extrusion, wire drawing and specific annealing, were studied at temperatures 4.2 - 300K in uniaxial compression and compared with coarse grained (CG) Zr. In parallel, investigations by X-ray diffraction (texture) and transmission electron microscopy were undertaken in order to reveal the evolution of the microstructure with increasing strain. Volume fractions of twins have been determined for UFG and CG Zr. It has been found that the activity of twinning is smaller in UFG Zr in comparison with CG Zr at ambient and lower temperatures, but the contrary is true for very low temperatures (4.2K), where twinning increases with decreasing grain size. The influence of twinning on mechanical properties of UFG Zr has been discussed, too

Anomalous Evolution of Strength and Microstructure of High‐Entropy Alloy CoCrFeNiMn after High‐Pressure Torsion at 300 and 77 K

Ultrafine and nanocrystalline states of equiatomic face‐centered cubic (fcc) high‐entropy alloy (HEA) CoCrFeNiMn (“Cantor” alloy) are achieved by high‐pressure torsion (HPT) at 300 K (room temperature, RT) and 77 K (cryo). Although the hardness after RT‐HPT reaches exceptionally high values, those from cryo‐HPT are distinctly lower, at least when the torsional strain lies beyond γ = 25. The values are stable even during long‐time storage at ambient temperature. A similar paradoxal result is reflected by torque data measured in situ during HPT processing. The reasons for this paradox are attributed to the enhanced hydrostatic pressure, cryogenic temperature, and especially large shear strains achieved by the cryo‐HPT. At these conditions, selected area electron diffraction (SAD) patterns indicate that a partial local phase change from fcc to hexagonal close‐packed (hcp) structure occurs, which results in a highly heterogeneous structure. This heterogeneity is accompanied by both an increase in average grain size and especially a strong decrease in average dislocation density, which is estimated to mainly cause the paradox low strength.© 2019 The Author