The effects of alloying additions and heat treatment on creep properties and microstructure of high-pressure die-cast magnesium-rare-earth alloys

Poor elevated temperature creep properties have limited the use of traditional Mg alloys, such as Mg-Al alloys, in the automotive industry primarily to ambient temperature applications in automobiles. Grain boundary reinforcement was commonly used to improve elevated creep properties of Mg alloys by preventing grain boundary sliding. However, the main creep mechanism is still under debate. More recently, precipitation hardening, solid solution strengthening and/or the diffusion rate of solute in the matrix have been proposed as the key factors that influence creep. High-pressure die-cast (HPDC) Mg-rare earth (Mg-RE) alloys are an ideal choice for elevated temperature automotive parts, such as powertrains, due to their excellent elevated temperature creep resistance and high production rate. In this investigation the differences in elevated temperature creep properties of three high-pressure die-cast Mg-rare earth (Mg-RE) alloy series, Mg-La-Nd, Mg-La-Y and Mg-La-Gd have been determined. A consistent concentration of La (0.45at.%) was used to maintain similar grain boundary strengthening for all the alloys and also to assist with castability. The ternary RE concentration was varied and the RE elements were chosen to have differing solubility in Mg. This was done to investigate the influence of solid solution strengthening and precipitation hardening on creep. The creep stress exponent in combination with electron microscopy was used to determine that the main creep mechanism was diffusion-controlled dislocation climb. Dislocations were shown to be decorated by precipitates in all three alloy series. It was shown that the Mg-La-Nd alloy series had significantly worse creep resistance at 177°C and 90 MPa when compared with the Mg-La-Y or Mg-La-Gd alloys series. For alloys with approximately 0.2 at.% ternary RE or greater, the Mg-La-Y and Mg-La-Gd alloys had minimum creep rates of approximately 5 x 10^-10 s^-1 in comparison with Mg-La-Nd alloys which had 1 x 10^-8 s^-1. Alloys with high concentrations of ternary RE such as Mg-0.45La-1.18Y (at.%) and Mg-0.45La-0.87Gd (at.%) reached 0.1% creep strain following 600 h of creep testing at 90 MPa and 177°C. The relatively high Nd-concentrated alloy Mg-0.45La-0.63Nd had significantly worse creep properties reaching 1 % creep strain in less than 350 h. The morphology of the microstructure of three HPDC Mg-La-RE alloy series with varying ternary alloying concentrations was compared. It was found that all alloys had relatively similar morphologies with respect to average grain size, volume fraction of eutectic present at grain boundaries and the intermetallic phase present in the eutectic. The age hardening response of the alloys revealed that the Mg-La-Nd alloys reached peak-aged conditions sooner than Mg-La-Y or Mg-La-Gd but also overaged more rapidly. This was used to indicate that Mg-La-Nd alloys had the poorest thermal stability/fastest diffusion rate of solute out of the three alloys series investigated. The precipitates formed dynamically during creep testing and were finer in Mg-La-Y and Mg-La-Gd alloys than in Mg-La-Nd alloys. It was shown that it is possible to solution treat for a relatively short duration, or solution treat and then age HPDC Mg-La-RE alloys without causing any surface blistering to the casting. However, creep properties as well as yield strength were negatively affected unless relatively high concentrations of solute in solid solution or a relatively high number density of precipitates were present. This was the result of a reduction in grain boundary reinforcement that was caused by the intermetallic becoming less continuous and also by the removal of the supersaturated region of solute in the matrix near the grain boundaries. It was concluded that improvements to elevated temperature creep resistance of HPDC Mg-La-RE alloys could be achieved by building on a base Mg-RE alloy (Mg-La) that had sufficiently good castability and grain boundary reinforcement with low diffusive/thermally stable soluble ternary RE additions in solid solution