Effect of Nd Additions on Extrusion Texture Development and on Slip Activity in a Mg-Mn Alloy

Two Mg-1 wt pct Mn alloys containing 0.5 wt pct and 1 wt pct Nd have been processed by indirect extrusion at temperatures ranging from 548 K (275 °C) to 633 K (360 °C) and speeds between 2.8 and 11 mm/s. The microstructure and the texture of the extruded bars were analyzed in order to understand the effect of the processing parameters and of the rare-earth (RE) alloying additions on the texture development. Increasing the Nd content results in weak textures in which the predominant orientations are a function of the extrusion conditions. This may be explained by the occurrence of particle pinning of grain boundaries and by the nucleation of grains with a wider range of orientations. Mechanical tests were carried out in tension and in compression in all the processed samples at 10⁻³ s⁻¹ and room temperature. It was found that larger RE amounts give rise to the disappearance of the yield asymmetry and to an anomalously high activity of tensile twinning, especially at the lowest extrusion temperatures. This has been attributed to an increase of the critical resolved shear stress of basal slip due to the presence of Mg₃Nd coherent and semi-coherent intermetallic prismatic plates.