Internal stresses in tension and compression, in extruded and artificially aged 7.7at%Al alloy, were determined by means of in situ neutron diffraction. Measurements were performed on grains which had their c-axis normal to, parallel to and at 62º from the stress axis. The results were consistent with basal slip and {10▪2} twinning. The twinning manifested itself clearly by variations in the integrated intensity of diffraction peaks during loading. Most of the observed variations in scattered peak intensity could be explained by referring to the lattice reorientation produced by {10▪2} twinning. The calculated stress tensors which corresponded to these intensity variations were used to show that a critical resolved shear stress criterion was applicable to {10▪2} twinning, and that the onset of twinning did not correlate with the stress normal to the twin plane. Intensity variations, which could not be explained by {10▪2} twinning, occurred in cyclic tension tests. It was suggested that this was due to {10▪1} twinning, which yielded a contribution to the grain strain tensor that was consistent with the straining direction. It was suggested that such twinning had not previously been observed in polycrystalline samples because it appeared to be essentially elastic; so that it disappeared almost entirely upon removal of the applied stress.

Study of the Mechanical Properties of Mg-7.7at%Al by in situ Neutron Diffraction. M.A.Gharghouri, G.C.Weatherly, J.D.Embury, J.Root: Philosophical Magazine A, 1999, 79[7], 1671-95