Interaction Between Precipitate Basal Plates and Tensile Twins in Magnesium Alloys

A textured Mg-Al-Zn alloy rolled plate was solution-treated and then aged at 320 degrees C for 2 and 116 hours, respectively. Afterwards, the three conditions were tested at room temperature in compression along the transverse direction to activate {10 (1) over bar2} twinning. Both aged specimens exhibited a yield stress about 10 MPa higher than that of the solution-treated condition, with the increase of the yield stress attributed to the extra stress required for the twins to grow in the presence of particles. In order to understand the mechanism responsible for such strengthening, the effect of the precipitate basal plates on the critical resolved shear stress (CRSS) for twin growth was estimated using four different calculation approaches: Orowan stress to bow twinning dislocations around particles, elastic back-stress resulting from unsheared precipitates inside the twin, strengthening of basal slip within the twin (related to plastic relaxation), and stress to bow a twinning super-dislocation loop capable of further expansion. These methods give an order of magnitude difference in the calculated strengthening effect that spans the measured CRSS increase. The last two methods give the best estimates of the CRSS increase for twin growth depending on the aging time.