Influence of severe plastic deformation at cryogenic temperature on grain refinement and softening of pure metals: Investigation using high-pressure torsion

Several metals were severely deformed at cryogenic temperature in liquid nitrogen and at room temperatures in air using high-pressure torsion (HPT). Extra grain refinement to the nanometer level and extra hardening were achieved after cryogenic-HPT in niobium (Fig. 1), which has a high melting temperature. In copper, which has a moderate melting temperature, nanograins formed during cryogenic-HPT but self-annealing, i.e., abnormal softening and grain coarsening to the micrometer level, occurred within a few hours after the cryogenic-HPT (Fig. 2(a) and (b)). In low-melting-temperature metals such as zinc, magnesium and aluminum, cryogenic-HPT led to extra softening and/or formation of coarser grains because of enhanced static recrystallization (Fig. 3). The effect of impurities on grain size, hardness-strain behavior and self-annealing was also studied after cryogenic-HPT (Fig. 2(a) and (b)).