Effect of Li addition on the plastic deformation behaviour of AZ31 magnesium alloy

The enhancement in the workability, if any, of a dilute Mg alloy, AZ31, with the Li addition is investigated by examining the hot deformation behaviour of AZ31 alloyed with 1, 3 and 5wt% Li. Compression tests were conducted in the temperature, T, range of 150–400°C and strain rates, ε̇, ranging from 10−3 to 10+2s−1. Experimental results show that the deformation behaviour can be divided into three T regimes. In the low T regime (150–200°C), an increase in the Li content in AZ31 decreases the twin density with a concomitant enhancement in the non-basal or cross-slip activity. While high Li alloys exhibit low flow stress at low ε̇ in this regime, an increase in ε̇ increases strain hardening rate and twinning activity. Flow localization and shear band formation were observed in most of the alloys in this regime. Deformation in the intermediate T regime (250–300°C) leads to dynamic recovery (DRY) at low ε̇ and dynamic recrystallization (DRX) at high ε̇, which are the softening mechanisms in Li containing alloys. In high T regime (350–400°C), these alloys exhibit softening mechanisms similar to intermediate T regime but high Li alloys surprisingly exhibit higher flow stresses, which is attributed to the higher amount of Li in the solid solution, a result of the dissolution of Li containing precipitates. Non-basal slip and twinning at high ε̇ occur uniformly with homogeneous DRX. A low activation energy for deformation of high Li alloys indicates that the Friedel-Escaig (F-E) mechanism as the rate controlling deformation mechanism. The onset of twinning was examined by the appearance of first local maxima before peak strain in (d2σ/dε2) vs. ε curves. Mechanisms responsible for the dependence of critical stress for the onset of DRX on T and ε̇ are also discussed.