Investigations on the Plastic Instability in an HCP Mg-Li Alloy

Plastic instability is reported in hexagonal close-packed (HCP) LA41 magnesium alloy during tensile tests. Serration amplitude associated with plastic instability is measured to increase with increasing strain and decrease with increasing strain rate. The model of dynamic strain aging (DSA) is proposed to be the controlling mechanism. Moreover, it is reported that annealing could reduce the flow instability, which has potential practical applications. 1. Introduction Plastic instability associated with the production of Portevin-Le Chatelier (PLC) bands, which is also know as serrated flow or jerky flow, is a topic of current theoretical and engineering interest. Lots of studies in this area are on aluminum alloys; however, due to soaring research on magnesium alloys, similar results have been reported in Mg-Ag [1], Mg-Gd [2], Mg-Y-Nd [3], Mg-Nd [4], and Mg-Al-Zn [5, 6] alloys, respectively. Microscopically, plastic instability was believed to result from intensive interactions between mobile dislocations and solutes atoms. Cottrell [7] assumed that solute atoms had sufficient mobility to move with mobile dislocations and serrated flow was caused by the drag effect of solute atoms. However, other finding revealed that solute atoms were not expected to have such mobility [8]. Later investigations proposed that the dynamic strain aging (DSA) process during deformation might be the major controlling mechanism [9–14]. Recently, Picu [15] has suggested a new perspective by considering the effect of solute clusters, which are formed on forest dislocations, on the strength of dislocation junctions to shed more light on this long-lasting yet intriguing issue. Macroscopic features, such as spatiotemporal localization of plastic deformation and propagation of deformation bands, were also investigated systematically. Experimentally, Chmelík et al. [16] monitored in situ collective dislocation motion by acoustic emission (AE) and laser extensometry techniques. They concluded that nucleation and propagation of PLC bands were dominated by different dislocation processes. Shabadi et al. [17] and Zhang et al. [18] examined dynamics deformation bands by more sophisticated laser speckle technique. Dablij and Zeghloul [19] characterized the PLC effect by measuring deformation band strains and band propagation velocities. These macroscopic features are also explained theoretically. Particularly, Lebyodkin et al. [20] simulated localized deformation bands patterns by a discrete model based on statistical behavior of stress drops, while Ananthakrishna’s extended model [21]