Flow stress and work hardening of Mg-Y alloys

Abstract

Flow stress and work hardening behaviour of Mg-Y binary alloys have been studied under uniaxial tension and compression at 4 K, 78 K and 298 K. Electron microscopy observations show that Yttrium forms multi-atomic clusters distributed randomly in Mg matrix. During thermomechanical processing, Y accumulates at the grain boundaries and controls recrystallization texture, which acquires lower intensity of the basal component and higher degree of randomness in comparison to polycrystalline Mg. Tensile and compressive yield stress of the alloys is temperature dependent and decreases as temperature increases from 4 K to 298 K. The grain size and solution strengthening contributions to the yield stress have been analyzed for all alloys and deformation temperatures. Critical resolved shear stress (CRSS) of individual deformation modes has been obtained from the modelling of the flow stress. These results show that Y strengthens most the <c+a> slip system, followed by pyramidal I, prismatic, twinning and basal systems. All slip systems exhibit monotonic increase of CRSS with Y concentration, but at different rates. Analysis of the solution strengthening component of the yield stress suggests that it is determined by activity of the basal slip participating in yielding. A correlation between the athermal hardening rate, Θh, and the amount of slip in individual deformation modes leads to the conclusion that higher athermal hardening rates are produced by a balanced contribution of basal and non-basal slip, whereas lower Θh is associated typically with less activity of <c+a> slip. Improved ductility of Mg-Y alloys is reflected in the type of dislocation microstructure developed during plastic flow and the character of the fracture surfaces. These alloys' characteristics are linked to the mechanical property data to obtain better understanding of the mechanisms responsible for the enhanced plasticity of Mg-Y alloys.