Laves phases in Mg-Al-Ca alloys and their effect on mechanical properties

Abstract

Mg-Al-Ca alloys with Laves phase reinforcement are suitable for structural applications. The composition, crystal structure, and distribution of Laves phases can be tuned by the alloy composition and heat treatment, which subsequently influence their mechanical properties. In this study, three model alloys Mg-6Al-2Ca, Mg-5Al-3Ca and Mg-4Al-4Ca were investigated, which include C15, C36, and C14 Laves phases. The as-cast alloys have interconnected Laves phases that form a skeleton structure. After annealing, they became more rounded particles, while the metastable C36 phase was transformed to C15. The Laves phases in different crystal structures exhibit distinct ranges of chemical compositions and lattice parameters. Well defined orientation relationships were observed between small C15 platelets and the Mg matrix (Mg(0002) // C15(111), Mg[112̅0] // C15[112̅]). Another pair of parallel orientations was found between Mg(11̅01) and the c-plane of hexagonal C36/C14. Nevertheless, most coarsened Laves phases have incoherent interfaces with the matrix and hinder dislocation slip transfer. The annealed alloys have lower yield strength than their as-cast counterparts, but higher ductility or ultimate tensile strength. The yield strengths of as-cast alloys are correlated to the interconnectivity of the skeleton, whereas those of annealed alloys are related to the spacing between Laves phases.