Martensitic transformations and microstructures in splat-cooled NiAl

The martensitic transformation in splat-cooled Ni–37.5at.%Al and Ni–35at.%Al material is investigated by different transmission electron microscopy (TEM) techniques. It was found that the extreme conditions of the rapid quench produce much smaller grains with a higher dislocation density than in bulk material. The martensite start temperature Ms is also increased slightly, probably due to internal stresses, leading to small martensite plates often grouped into self-accommodating configurations inside the Ni–37.5at.%Al B2 grains. The 3R martensite microstructure of completely transformed grains in Ni–35at.%Al reveals several peculiar features again pointing towards internal stress configurations. Among these are stepped martensite–martensite interfaces within a former B2 grain, alternating sequences of martensite microtwin stackings on both sides of a grain boundary between former B2 grains and small arrowhead configurations with a 14M-like stacking observed at such grain boundaries.