The effect of grain size on the tensile and creep properties of Mg-2.6Nd-0.35Zn-xZr alloys at 250 °C

A systematic study has been made of the effect of grain size on the tensile and creep properties of Mg-2.6Nd-0.35Zn-xZr (in wt%) alloys (ML10 or ZM6) at 250 °C, the maximum long-service temperature for these alloys. Grain sizes ranging from 102 to 920 μm were produced in the T6 state after grain refinement with Zr. The sand cast Mg-2.6Nd-0.35Zn alloy solidified as largely columnar grains (900±430 μm). An addition of 1.8% Zr reduced the grain size to 84 μm but most grains were still irregular rather than equiaxed. The grain size increased linearly with 1/Q (Q: growth restriction factor) calculated from the soluble Zr content only. Significant grain hexagonalization occurred during solid solution treatment at 530 °C (720 min). Both the yield strength and ultimate tensile strength at 250 °C followed the Hall-Petch relationship while the ductility increased linearly with decreasing grain size. The steady-state creep rate at 250 °C increased significantly with decreasing grain size but followed the general power law at each stress level evaluated (50-90 MPa). The effect of grain size on creep is related to the applied stress where the grain size effect exponent p showed a linear dependency on applied stress. To satisfy the requirements for both the tensile and creep properties at 250 °C, the T6 grain size of these alloys should be limited to about 174 μm. However, when the T6 grain size is <100 μm, it may result in a significant increase in the steady-state creep rate.