Strain-rate sensitivity of the flow stress of ultrafine-grain aluminum at temperatures 4.2–295K

Abstract

The influence of the grain size on the dislocation interaction mechanisms that control the deformation of aluminum in the temperature range 4.2–295K is studied. For this, samples of coarse-grain (CG) and ultrafine-grain (UFG) aluminum obtained by equi-channel angular pressing were deformed by stretching at constant rate ε̇ as well as in the rate-cycling regime along the deformation curve. The effects of temperature on strain hardening and the stain-rate sensitivity of the flow stress of CG and UFG material are compared. It is shown by means of thermal-activation analysis of the experimental data that the dependence of the rate sensitivity parameter m=[∂lnσ∕∂lnε̇]T on the grain size and temperature of the aluminum is explained by a change of the dislocation mechanisms controlling the plastic deformation of the aluminum. In the temperature interval 4.2–40K the plastic deformation of the UFG and CG aluminum is due to a single mechanism of intersection of forest dislocation. In the interval 120–295K for CG and 77–140K for UFG aluminum the increase in the parameter m is explained by the activation of the mechanism of transverse slip of dislocations. At temperatures above 140K the high rate sensitivity of the stress and low rate of strain hardening of UFG aluminum can be due to the activation of the grain-boundary diffusion and grain-boundary slip-through.

Influence of Decreased Temperature of Tensile Testing on the Annealing-Induced Hardening and Deformation-Induced Softening Effects in Ultrafine-Grained Al–0.4Zr Alloy

“…The m -value determined from the stress-jump magnitude m = Δlnσ/Δln is equal to about 0.045 in both HPT and HPT + AN + 0.25 HPT states. The m -value obtained is comparable (slightly higher) with the m -values reported for HPT-processed Al ( m ~0.03) [ 24 ], ECAP-processed Al ( m ~0.04) [ 25 , 26 ], and the HPT-processed Al–Cu–Zr alloy ( m ~0.033–0.038) [ 27 ].…”

Influence of Strain Rate on the Effect of Plasticization of High-Strength Ultrafine-Grained Al-Cu-Zr Alloy

“…The obtained value well agrees with the literature data [34][35][36], which show that the value of the strain-rate sensitivity coefficient for the UFG-metals with the FCC lattice is within the range ∼ 0.01−0.03. The values of m ∼ 0.03 and ∼ 0.04 were obtained at the room temperature for the high-purity (4N) Al structured by the HPT [19] and for the commercially pure UFG Al (99.5%) structured by the equal channel angular pressing (ECAP), respectively [27]. For the 6082 aluminum alloy in the UFG-state formed by ECAP, the study [26] has obtained the value m = 0.03.…”

“…The increased value of the coefficient m is usually corelated to activated grain boundary sliding, which results in more uniform microplastic flow in the sample preventing macrolocalization processes with neck formation. It provides for increased plasticity [26,27] or even leads to superplasticity at the room temperature, as it was observed, for example, for the Al−30Zn (wt%) UFG-alloy [19].…”