Towards an improved understanding of plasticity, friction and wear mechanisms in precipitate containing AZ91 Mg alloy

This work reports a combined experimental and atomistic simulation study on continuous precipitates (CPs) and discontinuous precipitates (DPs) affecting the scratch induced wear in AZ91 magnesium alloy. Nanoscratching experiments complemented by atomic simulations were performed to understand the directional dependence and origins of plasticity, friction and wear mechanisms in AZ91 alloys with reference to nanocrystalline HCP magnesium. Post scratch deformation analysis was performed using electron back scattering diffraction, scanning electron microscope and molecular dynamics (MD) simulation. The direction of orientation of the precipitates was observed to make a significant influence on the deformation behaviour. For example, regardless of the precipitates type (CP or DP), a ductile-brittle transition becomes pronounced while scratching along the longitudinal direction of precipitates, whilst a fully ductile response was obtained while scratching along the transverse direction of the precipitates. However, regardless of the direction of orientation, DPs showed a higher wear resistance and coefficient of friction compared to the CPs. These observations were supported by the quantitative analysis of the planar defects such as coherent twins, extrinsic and intrinsic stacking faults in the deformation zone as well as types 1/3〈11¯00〉 and 1/3〈12¯10〉 dislocations extracted from the MD data.These observations will facilitate an improved design of AZ91 alloys in particular and intermetallic precipitate containing alloys in general.