Impact of short range ordering on the anomalous four-stage strain hardening behavior of low solid-solution hardening Ni–Cr alloys

Abstract

The effect of short range ordering (SRO) on the strain hardening behavior of low solid-solution hardening Ni–Cr alloys with high stacking fault energies (SFEs) was systematically investigated under uniaxial compression to a high true strain ε t of 0.9 at room temperature. An unexpected four-stage (including Stages A, B, C and D) changing law in strain hardening rate was revealed, which is completely different from the case of conventional high-SFE alloys whose strain hardening rate just monotonously declines as the strain increases. Observations on the deformation microstructures strongly demonstrate that the recovery of strain hardening rate in Ni–Cr alloys (Cr ≥ 20 at.%) at Stages B (0.11 < ε t < 0.21) and D ( ε t ≥ 0.65) should be ascribed to the transformation in dislocation slip mode from wavy slip to planar slip, and the occurrence of the extensive deformation twins (DTs) together with the secondary DTs, respectively. Besides, in the later period of Stage C (0.21 < ε t < 0.65), microbands (MBs) and DTs have been locally formed in the Ni–Cr alloys with high SRO degree, which delays the decrease of strain hardening rate to a certain extent. Based on these experimental observations, the impacts of SRO on the formation of planar slip bands, MBs and DTs are analyzed, and the effect of SRO on the four-stage strain hardening behavior is also discussed systematically based on the different deformation mechanisms. In brief, an enhancement of SRO in low solid-solution hardening Ni–Cr alloys with high SFEs has been convincingly confirmed to be capable of improving the strain hardening capacity.