Reconstruction of martensite variant microstructures in grains of NiTi and NiTiHf alloys transforming with and without external stress

When martensitic transformations in a SMA polycrystal proceed under external stress, polycrystalline grains are strained in reversible manner so that external recoverable strain is observed in thermomechanical loading experiments. The recoverable strain is accommodated by martensitic transformation into martensite variants arranged in such a way that strain is compatible at every internal interface in martensite as well as at grain boundaries. The arrangement of martensite variants in grains, called martensite variant microstructures, are significantly affected by external stress.

We have reconstructed martensite variant microstructures formed in grains of nanocrystalline NiTi wires and laser powder bed fusion fabricated NiTiHf alloys cooled without stress and under various external tensile stresses by nanoscale orientation mapping in TEM as well as by electron backscattered diffraction in SEM. The reconstruction methods will be introduced and the observed martensite variant arrangements will be discussed.

It will be shown that, although the NiTi and NiTiHf alloys undergo B2-B19’ martensitic transformations into (001) compound twinned martensite, the martensite variant microstructures they create in grains are completely different. The external recoverable strains which depend on the stress applied on cooling are accommodated by martensite variant arrangement in a completely different manner in NiTi and NiTiHf alloys. Potential reasons for that will be discussed.