X-ray analysis of twin dominated deformation in an aged Mg-7Sn-3Zn-0.04Na alloy

In-situ transmission X-ray diffraction based compression deformation experiments are performed to study the twinning behaviour in as-extruded (non-aged) and peak-aged Mg-7Sn-3Zn-0.04Na alloy. The axial lattice strains were measured in the parent grains, twins and the precipitates as a function of applied stress. The critical shear stress for achieving 5% twinning was found to be increased by ∼26 MPa by the presence of 7% of Sn and by ∼50 MPa by the presence of the particles formed after 10 h of aging. The evolution of twin volume fraction with plastic strain is similar in both non-aged and aged conditions, indicating no change in the relative activities of slip and twin in the ideally oriented (101¯0) parent grains. Predictions made in previous studies of twin thickening stresses and twin bypass stresses agree reasonably well with the measured values, given the experimental error. Considerable relaxation was seen in the precipitate lattice reflections. This is attributed to relaxation effects continuing during X-ray data collection. Macroscopic flow curves confirm that precipitate hardening in the present system is particularly sensitive to relaxation effects. This is likely to be an important consideration for fatigue loading of precipitate hardened samples.