Microstructure and property evaluation of high-pressure die-cast Mg-La-rare earth (Nd, Y or Gd) alloys

Microstructure, castability and tensile properties were investigated in high-pressure die-cast Mg-La-Nd, Mg-La-Y and Mg-La-Gd alloy series, with a constant La concentration at approximately 2.5 wt.% and the concentrations of Nd, Y or Gd were varied. All three alloy series had a dendritic microstructure with a Mg-La-rich eutectic with increasing Nd, Y or Gd content and containing a Mg12RE intermetallic phase. The morphology of the eutectic at ternary alloying additions of equal to or less than 1.0 wt.% was lamellar but became increasingly divorced at higher ternary concentrations. This was however more obvious in Mg-La-Y and Mg-La-Gd than Mg-La-Nd alloys. The hot tearing susceptibility in all three alloy series increased markedly with even micro-alloying additions of Nd, Y or Gd, and began to decrease again in alloys with more than 0.5 wt.% Y or 1.0 wt.% Gd, but did not decrease significantly for Mg-La-Nd. A model using the temperature-fraction solid curves as input parameters was used to estimate hot tearing susceptibility for Mg-La-Nd alloys. Tensile testing at room temperature showed that Mg-La-Nd alloy series had higher 0.2% proof stress and lower elongation to failure than either the Mg-La-Y or the Mg-La-Gd alloy series for Nd concentrations greater than 1 wt.% due to a greater effectiveness of grain boundary reinforcement. (C) 2014 Elsevier B.V. All rights reserved.