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 system-atical 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 ℃. The alloy's strength increased by up to 44 MPa (14.0 %) at HT compared to that at 25 ℃ and this was accompanied by decreased pyramidal II ❬c + a❭ slip activ-ity; 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 hard-ening 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 reasons 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.

Key words: Mg-RE alloy, Anomalous temperature dependency CRSS, Slip trace analysis, Pyramidal ❬c + a❭slip