Microstructural characteristics and self-accommodation of the martensite in equiatomic ZrPd and near-equiatomic NiZr alloys

Thermoelastic martensitic transformation plays an important role in shape memory effects and superelasticity. The thermoelastic martensite reduces the elastic strain energy during transformation via a combination of multiple variants (i.e., self-accommodation). We investigated the microstructural characteristics and self-accommodation of the martensite in equiatomic ZrPd and near-equiatomic NiZr alloys by X-ray diffraction analysis, scanning electron microscopy–electron backscatter diffraction analysis, and transmission electron microscopy (TEM) observation. The space group of the equiatomic ZrPd and near-equiatomic NiZr alloys at room temperature was determined to be Cm . In the equiatomic ZrPd alloy, two habit plane variants (HPVs) were bounded by a {021} Cm compound twin. The minimum self-accommodation unit necessary to relax the strain energy due to the B2-to- Cmcm martensitic transformation is a pair of HPVs with plate- and polygonal-like morphologies. In addition, the strain energy resulting from the martensitic transformation from the Cmcm to the Cm phase is released by the formation of a {001} Cm compound twin as a lattice invariant shear (LIS). The near-equiatomic NiZr alloy had coarse variants with a grain size of several hundred micrometers and thin variants with a width of a few tens of nanometers; the interface of these variants was a {021} Cm deformation twin. TEM in situ heating observations revealed that the near-equiatomic NiZr alloy transformed from the Cm to the Cmcm structure around 573 K and that the {001} Cm compound twins induced as an LIS disappeared. Clarifying the thermoelastic martensitic transformation in such Zr-based alloys is useful for the development of high-temperature shape memory alloys. Graphical abstract a Inverse pole figure map of the Cm martensite. b Lattice correspondence variants map. c and d Pole figures taken by {021} Cm , and {111} Cm planes, respectively.