STRUCTURAL INTERPRETATION OF THE NUCLEATION AND GROWTH OF DEFORMATION TWINS IN ZR AND TI .2. TEM STUDY OF TWIN MORPHOLOGY AND DEFECT REACTIONS DURING TWINNING

A TEM study of mechanical twinning in Ti and Zr was performed. A large number of [0001]-dislocations which are rarely seen in the matrix, and a new type of stacking fault were found within deformation twins in both materials. Matrix dislocations were observed to actively interact with the tips of twin embryos and step ledges of growing twin lamellae. Twin embryos bounded by straight coherent and inclined semi-coherent boundaries were observed. These results as well as the morphological variation of twin lamellae during deformation do not agree with twinning dislocation theories but support a step-wise nucleation and growth mechanism proposed for deformation twinning in h.c.p. structures. Extended discussions are provided to address unanswered questions in the literature concerning deformation twinning.