Anomalous temperature-dependent strength and the operative slip modes in an extruded Mg-Y sheet

Abstract

This work is intended to further understand the controversial temperature dependencies of various slip modes in Mg alloys, and their effects on the mechanical properties at high temperature (HT). A systematical and statistical investigation on the temperature-dependent macroscopic deformation behavior and the corresponding grain-scale slip activity was performed for both an extruded Mg-10Y (wt.%) sheet and a pure Mg sheet during tension at 25–300 °C. The alloy's strength increased by up to 44 MPa (14.0%) at HT compared to that at 25 °C and this was accompanied by decreased pyramidal II <c+a> slip activity; both phenomena were opposite to that for pure Mg. The critical resolved shear stress (CRSS) ratios were estimated based on the ∼1700 sets of observed slip traces, and a positive temperature-dependent CRSS_{pyr II}/CRSS_bas was found in Mg-10Y. Compared to pure Mg, Mg-10Y exhibited pronounced strain hardening at HT due to enhanced slip-slip interactions, including multiple slip and cross slip, increased GND accumulation, and Y solute-dislocation interactions. The significant pyramidal II <c+a> slip activity (up to 30% frequency), its thermal hardening and pronounced strain hardening nature are proposed to be the key mechanisms for the observed anomalous strength increase in Mg-10Y. The grain-scale experimental evidence for <c+a> dislocation activity and its correlation to mechanical properties were revealed in this study and compared to recent atomic-scale simulations.