Phase stability, microstructure and mechanical properties of Li containing Mg-based bulk metallic glass composites

Abstract

Abstract 5–15 at.% Li was substituted into a glass forming Mg–Cu–Gd alloy and the effect on glass forming ability, phase development and mechanical properties were studied. Li was found to have a negative impact on glass forming ability, evident from a reduction in the critical diameter. This resulted from a strong tendency to precipitate the body centered cubic (bcc) Mg 7 Li 3 phase. The substitution of Li in amounts less than or equal to 9 at.% promoted a two phase glass matrix-bcc crystal composite. Yielding and work hardening were enhanced by raising the amount of Li, causing a maximum plastic strain of approximately 1.6% for 15 at.% Li, attributed to the presence of the bcc crystals. Deformation response is analyzed within the framework of: (1) a need to match microstructure length scales, and (2) a reduced shear modulus in the crystalline phase, as compared to the matrix. A particular focus of this study was thus to consider both the length scale and modulus mismatch ideas applied to composite bulk metallic glasses by Hofmann et al. (Nat. Lett., 2008, 451, 1085–1089) and whether they can explain the large plasticity that Li containing Mg-based alloys have previously displayed. While a comparison of the alloys reported here with those in the literature suggests that the first of these constraints may be being met, deformation is concentrated within the crystalline phase rather than being shear band mediated. The activation energy for dislocation propagation within the ductile precipitates may be lower than for shear transformation zone nucleation in the matrix therefore, providing a constraint for the future design of tough glass matrix composites.