Thermodynamics-Based Selection and Design of Creep-Resistant Cast Mg Alloys

Atomic level thermodynamics arguments that account for the generally weak age hardening response while suggesting that extending the athermal regime through short-range order (SRO) is a most feasible path to increasing the creep strength of many current alloys are presented. The tendency, or otherwise, of many solutes to develop SRO in dilute solid solutions rationalizes a number of observations in current multicomponent Mg alloys, and in particular the retention of linear strain hardening at high temperatures, while it disputes the viability of several micromechanisms often considered active, such as pinning of edge dislocations by mobile solute clouds, dynamic precipitation of thermally stable precipitates, or atomic size effects on the diffusivity. Potential solutes are sorted out and ranked based on the sign and value of the enthalpy of mixing of binary solid solutions using the Miedema phenomenological scheme. Due to their large negative energy of mixing and reasonable solubility (>1 at. pct) at similar to 473 K (similar to 200 degrees C), Y and Gd appear as the best candidates to increase the creep strength through SRO, followed by Nd and Ca, in close agreement with data reported in the literature. The feasibility of enhancing the age hardening response through homogeneously nucleated, coherent precipitates, in some cases despite the negative energy of mixing of the alloy, or via internally ordered precipitates mimicking those present in Mg-Th alloys is considered by making parallels with the Al-Zn and the Al-Cu alloy systems. The possible optimization of the strengthening of high pressure die cast alloys combining SRO and intergranular eutectics or of heat-treatable cast alloys through internally ordered precipitates and SRO is discussed. (C) The Minerals, Metals & Materials Society and ASM International 2015