Effects of grain size and heat treatment on the tensile properties of Mg–3Nd–0.2Zn (wt%) magnesium alloys

This paper reports the tensile properties and deformation behavior of a newly cast magnesium alloy Mg–3Nd–0.2Zn–xZr (NZ30Kx, x=0, 0.1, 0.3, 0.5, 1.0, 2.0 wt%) with different grain sizes and heat treatment conditions. Zirconium (Zr), as an effective grain-refiner in NZ30K alloys, is found to significantly improve the yield strength (YS), ultimate tensile strength (UTS) and elongation of these alloys. The NZ30K alloys show a significant response to heat treatment, achieving 60 and 47% increases in YS and UTS, respectively, in the peak-aged condition (14 h at 200 °C), compared with those of the as-cast alloy. Both grain refinement and age hardening significantly improve work-hardening at strains below 1%, leading to higher yield strength. At large strains, however, only grain refinement continues to show a slight strain hardening. The strain-hardening exponents of both NZ30Kx–T4 and NZ30Kx–T6 alloys decrease with reducing grain size at large strains. The Hall–Petch relationships of true flow stresses of NZ30Kx–T6 alloys at low strains (0.001–0.04) show a tendency of two-slope feature as plastic strains. All tensile failures occur prior to global instability, indicating the existence of localized damage.