Self-accomodation and morphology of 14M (7R) martensites in an Ni370at.%Al alloy

The self-accomodation mechanism and morphology of the unique 14M (7R) martensite was studied using single crystals of an Ni370at.%Al alloy. The self-accomodating group was identified by the X-ray diffraction technique, assisted by computer simulations, and it was found that the elastic strains around martensites are effectively cancelled out by making a variant group around the {110&}_B2 pole, being similar to other β phase alloys. As a result of extensive observations using a scanning electron microscope and an optical microscope, the basic morphology of the self-accomodating group was found to consist of parallelogram bounded by type I and type II twin bounderies, which is different from the conventional diamond morphology. This basic morphology was supported by the theoretical considerations based on the phenomenological crystallographic theory, and it was shown to be favored over the diamond morphology. Furthermore, the developing mechanism of self-accomodation is proposed, on the basis of the above results.