An anomalous (partial) stacking fault was observed within deformation twins in these hexagonal close-packed metals. It was pointed out that the energy which was necessary for the formation of a partial stacking fault was large as compared with that of an I2 stacking fault which was produced by the dissociation of a perfect dislocation. It was proposed that there were 2 ways in which to generate partial stacking faults. One was to move single partial dislocations through each closely-packed plane layer-by-layer. Another was to form it as a by-product of a large-scale deformation process such as twinning. The former process was thought to be unlikely because perfect dislocations were rarely observed to dissociate in the matrix of deformed hexagonal close-packed materials with low c/a ratios. In the present case, no isolated stacking fault of any type was seen in samples under a wide range of deformation conditions. Stacking faults were found only within various types of twin. It was also noted that partial stacking fault were not necessarily contained within the basal planes, and could sometimes be irrationally oriented. The formation of such anomalous stacking faults was closely related to the twinning process in hexagonal close-packed structures. It was suggested that, although these new stacking faults were not expected to play as important a role as normal stacking faults in deformation, they provided a clue to the improved understanding of twinning in hexagonal close-packed metals. As the phase shift for the (10•1) reflection of a partial stacking fault was only half of that for an I2 stacking fault, the bright field contrast of the partial stacking fault, using this reflection, was fairly weak in the kinematic region. The partial dislocation which surrounded a partial stacking fault was unique in that the strain which was associated with it was too small to produce a significant distortion contrast. The dislocation was virtually invisible. It was also suggested that it moved only by means of diffusion.

S.G.Song, G.T.Gray: Philosophical Magazine A, 1995, 71[2], 263-74