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 5 wt% Li. Compression tests were conducted in the temperature, T, range of 150-400 degrees C and strain rates, (epsilon) over dot, ranging from 10(-3) to 10(+2) s(-1). Experimental results show that the deformation behaviour can be divided into three T regimes. In the low T regime (150-200 degrees 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 (epsilon) over dot in this regime, an increase in (epsilon) over dot 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 degrees C) leads to dynamic recovery (DRY) at low (epsilon) over dot and dynamic re-crystallization (DRX) at high (epsilon) over dot, which are the softening mechanisms in Li containing alloys. In high T regime (350-400 degrees 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 (epsilon) over dot 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 (d(2)sigma/d epsilon(2)) vs. epsilon curves. Mechanisms responsible for the dependence of critical stress for the onset of DRX on T and (epsilon) over dot are also discussed. (C) 2016 Elsevier B.V. All rights reserved.