Effect of Sc Addition on the Mechanical Properties of Mg-Sc Binary Alloys

Abstract

The addition of rare earth (RE) alloying elements is a promising method for improving the strength, ductility and overall formability of magnesium (Mg) alloys. However, the underlying mechanism for this phenomenon remains unclear. An investigation on the effect of the rare earth element, scandium (Sc), on binary Mg-Sc alloys has been pursued.

Tension and compression tests were performed on a series of dilute binary Mg-Sc alloys at temperatures of 298 K, 78 K and 4.2 K. As a reference, pure Mg was also investigated for comparative purposes. Differences in tension and compression stress-strain curves highlighted distinct activated mechanisms, where slip dominated in tension and twinning governed compression. The observed increase in ductility and prolonged necking was attributed to a weaker basal texture, enhanced twinning and non-basal slip. A decreased work hardening rate suggests an improvement in dislocation recovery with Sc addition. In compression, Mg-Sc alloys followed Fleischer’s theory of solution hardening, where stress scales with concentration, c, as c^1/2; however, there was a very weak fit with both Fleischer and Labusch models under tension. The strengthening rate displayed by Mg-Sc was relatively weak compared to previously studied Mg-RE systems. However, considering the estimated misfit parameters, the size and modulus misfit was not enough to account for the strengthening rate. The results suggest that hardening of the twinning mode may influence strength.

Constitutive modelling, based on a self-consistent plasticity model, was used to characterize the deformation behaviour. The simulations predicted an increased relative activity of non-basal <c+a> slip with Sc addition, supporting experimental results and proposed mechanisms in literature. The results of Mg-Sc alloys have been connected to theories that identify a decrease in stacking fault energy (SFE) as the determining factor for increased strength and ductility of Mg-RE alloys. A comparison of the SFE of previously studied REEs with Sc, demonstrated strong evidence towards the theory’s validity. Sc has been shown to only moderately reduce the SFE of Mg and hence, the present experimental results have shown a moderate increase in strength and ductility. Additional modelling and detailed dislocation analysis are suggested as future steps to further support this theory.