Microstructural study of equiatomic PtTi martensite and the discovery of a new long-period structure

The martensitic phase in an equiatomic PtTi alloy was studied by electron microscopy, neutron and X-ray diffraction, and differential scanning calorimetry. The expected orthorhombic B19 structure with Type I twinning on {1 1 1} planes was found plus a new 33 long-period microtwin stacking based on this B19 structure, also including plate twinning. The measured microtwin volume fraction for the B19 is found to be larger than the value calculated from crystallographic theory, which can be explained by the fact that no martensite lattice parameters at the transformation temperature are known. An atomic model is suggested for the new long period structure in agreement with the electron diffraction and high resolution electron microscopy data.