Refinement of precipitate distributions in an age-hardenable Mg–Sn alloy through microalloying

It has been known for some time that trace additions (<∼0.1 at.%) of selected elements to some age-hardenable alloys can have a disproportionate effect on the hardening response. This is now known to be related to the formation of heterogeneities (clusters or precipitates) on the matrix lattice at which the nucleation of hardening precipitates may be enhanced leading to a refinement in the particle distribution. In this work, qualitative thermo-kinetic criteria for choosing microalloying additions for precipitation-hardenable alloys are outlined and applied to a model Mg–Sn alloy. Additions of Na and In + Li are chosen, and an acceleration and enhancement of the ageing process is observed for both sets of additions at 200°C. The microstructures are examined using transmission electron microscopy and the microalloying additions are shown not to affect the identity or crystallography of the Mg2Sn phase formed, although a substantial refinement of the distributions is observed. In the case of Na additions, the number density of particles is increased by two orders of magnitude, resulting in hardening increment increases of 270%. The In + Li additions lead to increases of approximately one order of magnitude in number density and 150% increases in the hardening increment. The generality of the observed effects and the resulting hardening responses are discussed.