Dynamic recrystallization and recovery during high-pressure torsion: Experimental evidence by torque measurement using ring specimens

Abstract

a b s t r a c tSeveral pure metals exhibit softening when imparting large strains at room temperature. This study investigates the nature of this strain softening. Torque measurements during high-pressure torsion using ring specimens, which appear to be more suitable than disk specimens for the evaluation of the strain response on the in situ flow stress, suggest that the softening in aluminum occurs mainly by dynamic recrystallization and recovery, whereas no appreciable dynamic softening occurs in copper. The softening in aluminum is associated with decreasing the dislocation density and increasing the grain size and missorientation angles, whereas copper exhibits ultrafine grains of higher dislocation density and few nanotwins. A significant static recrystallization is detected in HPT-processed pure copper by argon irradiation during ion milling.

Microstructural evolution of pure copper subjected to friction sliding deformation at room temperature

“…A HAB spacing of 300 nm is however in the typical range of 250-400 nm previously reported for deformation to large strains by both high pressure torsion [24,25], equal channel angular extrusion/pressing [26][27][28][29] and accumulative roll-bonding [30] of Cu, where in each case due to the large shear strains involved some sample heating can be expected. All these observations suggest that these fine grains represent a dynamically recovered high strain deformed structure resulting in a near-saturation in microstructural refinement.…”

Microstructure and microhardness of OFHC copper processed by high-pressure torsion

“…A comparison with earlier studies on HPT of pure Cu of 99.99% purity or lower [46,[56][57][58] reveals three important differences in the current study using very high purity OFHC Cu: (1) the present results show a local maximum microhardness at a very low equivalent strain of ~2, (2) thereafter the microhardness decreases to a new local minimum at equivalent strains between ~3 and ~8 and (3) the steady-state microhardness begins at equivalent strain above ~22 whereas it occurred at equivalent strains above ~15 in the earlier studies on lower purity material [56,57].…”

“…Numerous reports are now available showing conventional hardening without recovery in pure Cu [15,[40][41][42][43][44][45][46][47] although there are two investigations showing the occurrence of softening when post-HPT annealing is conducted at elevated temperatures [35,48]. These results suggest, therefore, that the lower stacking fault energy of Cu prevents the occurrence of softening with rapid recovery as observed in high-purity Al.…”

High-pressure torsion of thick Cu and Al–Mg–Sc ring samples

“…It was shown that this softening occurs dynamically during the HPT processing [39]. Because Cu has a higher melting temperature, softening was not observed during processing at room temperature but it does occur when it was processed at elevated temperature equivalent to room temperature of pure Al [40].…”