Enthalpy-driven retardative diffusion stabilizes nanoprecipitates in ultralight magnesium alloys

Tang S.; Kou Z.; Zhao Y.; Luo T.; Xin T.; Feng T.; Zhao Y.; Wilde G.; Lan S.; Li X.; Jiang B.; Ferry M.

Abstract

Ultralightweight body-centered cubic (BCC) magnesium-lithium-aluminium (Mg-Li-Al) alloys, strengthened by metastable nanoprecipitates, can achieve yield strengths exceeding 350 MPa with densities lower than 1.4 g/cm3. However, their strength decreases as the ultrafine microstructure suffers from rapid coarsening at ambient temperature and transformation into incoherent equilibrium phase at elevated temperatures. To employ these alloys in structural applications, it is imperative to enhance their poor thermal stability. Here we show that minor silver (0.33at.%Ag) addition stabilizes metastable nanoprecipitates in a BCC Mg-38at.%Li-4at.%Al alloy via retarding the diffusion of Al and Li to inhibit the coarsening and transformation of these precipitates, thereby remarkably enhancing the thermal stability from ambient temperature to as high as 170 °C (∼0.52Tm). Specifically, minor Ag addition maintains a high yield strength of ∼500 MPa after high temperature exposure close to 0.52Tm, resulting in a stabilized specific strength of ∼370 kN·m/kg, which surpasses almost every other lightweight engineering alloy. The selection of Ag is guided by an enthalpy-driven alloying strategy, viz. a more negative average enthalpy of mixing implies stronger binding and hence an enhanced diffusion activation barrier. First-principles calculations validate the interrelation between average enthalpy of mixing and local binding strength (and diffusivity). This enthalpy-driven alloying strategy can be extended to other precipitation-hardened alloy systems, leveraging stabilized nanoprecipitates for elevated-temperature applications.