Effect of age hardening on the deformation behavior of an Mg-Y-Nd alloy: In-situ X-ray diffraction and crystal plasticity modeling

The present study investigates the deformation behavior of an extruded Mg-Y-Nd (WE54) alloy in as-extruded and aged conditions. Via age-hardening at 250 °C for 16 and 500 h or annealing treatments at 400 °C for 24 h, precipitates are formed within the grains or at the grain boundaries. To characterize microstructural changes with the age-hardening conditions, we employ electron-backscattered diffraction, transmission electron microscopy, and optical microscopy. In the as-extruded material, we observed an uncommonly low activity of {101¯2}〈101¯1¯〉 tension twinning in comparison with other Mg alloys. The tension twinning activity substantially increased after precipitation hardening and the accompanying reduction of alloying element concentration in solid solution. Consistent with the microstructural observations, the increase in twin activity clearly manifests in the compression flow curves. While the as-extruded and 16 h/250 °C samples exhibited a classical decreasing hardening rate throughout straining associated with crystallographic slip, the 500 h/250 °C and the 400 °C annealed sample featured a characteristic increase in the hardening rate associated with twinning. In order to determine the impact of the different heat treatments on the individual slip and twinning modes, in-situ energy-dispersive X-ray synchrotron diffraction experiments during loading and elasto-plastic self-consistent modeling were conducted. We find that plate-shape precipitates on the {101¯0}α planes harden 〈a〉 basal slip more than the other slip systems, while the reduced solute concentration in the 500 h 250 °C and 24 h 400 °C samples results in a significant decrease in the critical resolved shear stress for {101¯2}〈101¯1¯〉 tension twinning.