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.