Further experiments on the behaviour of single crystals of zinc subjected to alternating torsional stresses

Object of Experiment .—From the results of a previous experiment on a single crystal of zinc, it was concluded that the formation of twins in zinc occurred on planes of the 101ˉ 2 type and that the particular operative twinning plane (of the six available) was determined chiefly by the direction of slip on the original basal plane and possibly, to some extent, by the relative magnitudes of the normal stresses on the possible twinning planes. In this previous experiment the orientation of the crystal was such that slip on the original basal plane occurred in one direction only and one pair of complementary twins only was observed. From the results it was predicted that if a test were made on a crystal of suitable relative orientation of the crystallographic and straining axes such thatall three slip directions became operative, then theoperative twinning planes should change with the slip direction. The present experiment was planned in order to test this prediction. Again, in the previous experiment, of the two possible pairs of complementary twinning planes associated with any one slip direction, it appeared probable that the choice of the operative pair was influenced by considerations of normal stress on the twinning plane. The present experiment would, it was hoped, throw further light on this aspect of twinning.