Understanding the high temperature reversed yield asymmetry in a Mg-rare earth alloy by slip trace analysis

In this work electron backscattered diffraction-assisted slip trace analysis was utilized to investigate the origin of the high temperature reversed yield asymmetry in a Mg-rare earth alloy. This material was tested at room and high temperature, in tension and compression, both in the as-extruded condition, in which the alloying elements are in solid solution, as well as following two aging treatments which gave rise to different precipitation levels. A statistically relevant number of slip traces were analyzed after each test in order to estimate the influence of the testing conditions and the precipitation state on the relative activity of basal, prismatic and pyramidal systems. In tension, basal slip was found to be the dominant deformation mechanism under all the testing conditions investigated. In compression, twinning predominated always at room temperature and was replaced by basal slip in the aged samples at high temperature. A pronounced reversed yield asymmetry, i.e., the presence of a higher yield stress in compression than in tension, was observed at high temperature in the aged alloy samples. Careful microstructural examination of the spatial distribution of slip traces by high resolution scanning electron microscopy (SEM) revealed increased intra- and intergranular basal slip localization in those specimens under tensile deformation at high temperature. The high temperature reversed yield asymmetry observed in the aged samples was thus attributed to the suppression of twinning in compression and to enhanced softening in tension due to basal slip localization.