Grain Size Hardening Effects in Mg-Gd Solid Solutions

Abstract

Pure Mg and alloys with 0.4, 1.3, and 3.8 at. pct Gd were cast with grain sizes between 700 and 35 µm and tested in tension and compression after solid solution heat treatment and quenching. The grain structure of the castings was random, that is, there was no preferred orientation, unlike the situation in extrusions and forgings usually reported in the literature. The results are compared to earlier work on Mg-Zn alloys. A tension–compression asymmetry in which the yield strength in compression is less than in tension was observed in pure Mg but was reversed for the concentrated alloys. The Hall–Petch stress intensity factor, k, first increased then decreased with the amount of Gd in solution. It is noted that defining the friction stress by extrapolating the data to infinite grain size should be treated with caution in Mg and its alloys: nevertheless, a rationale involving solid solution softening/hardening and twinning is offered for the observed values of the friction stress. The reversion of the tension–compression asymmetry is explained by the operation of $\left\{ {10\bar{1}1} \right\}$ (contraction) twinning in the concentrated alloys in place of $\left\{ {10\bar{1}2} \right\}$ (extension) twinning in pure Mg and the dilute alloys. It is argued that the activation of $\left\{ {10\bar{1}1} \right\}$ twinning in the more concentrated alloys accounts for their lower k-value.