The effect of grain size on the tensile and creep properties of Mg-2.6Nd-0.35Zn-xZr alloys at 250 degrees 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 degrees C, the maximum long-service temperature for these alloys. Grain sizes ranging from 102 to 920 mu m 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 mu m). An addition of 1.8% Zr reduced the grain size to 84 mu m 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 degrees C (720 min). Both the yield strength and ultimate tensile strength at 250 degrees C followed the Hall-Petch relationship while the ductility increased linearly with decreasing grain size. The steady-state creep rate at 250 degrees 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 degrees C, the T6 grain size of these alloys should be limited to about 174 mu m. However, when the T6 grain size is <100 mu m, it may result in a significant increase in the steady-state creep rate.