High-resolution transmission electron microscopy investigation of the face-centered cubic/hexagonal close-packed martensite transformation in Co-31.8 wt pct Ni alloy .2. Plate intersections, extended defects, and nucleation mechanisms

The face-centered cubic/hexagonal close-packed (fcc/hcp) martensite phase transformation in a Co-31.8 wt pet Ni alloy was studied by high-resolution transmission electron microscopy (HRTEM). The HRTEM was used to study the structure and properties of intersections between martensite plates and other defects observed in the alloy such as stacking fault tetrahedra (SFT) and Z-type defects. The HRTEM was also used to attempt to determine if various proposed mechanisms for the fcc/hcp martensite transformation were operating. There is evidence to suggest that the reflection mechanism proposed by Bollmann and the dipole mechanism proposed by Hirth are active in the fcc/hcp martensitic transformation, although the evidence is not completely certain in either case. Growth of the hcp phase by a four- or six-plane mechanism as proposed by Mahajan et al. is possible in theory but was not observed in this study. Transformation by previously proposed pole mechanisms was also not observed in this study, although evidence for a new type of pole mechanism was found. The formation of SFT along the fcc/hcp martensite interface was observed to occur by the cross-slip of Shockley partial dislocations out of the fcc/hcp interface onto conjugate fee matrix planes, followed by further cross-slip to form the SFT, as previously observed for grain boundaries in fee alloys.