Pathways to Titanium Martensite

Abstract

The structural relationship between the parent and product phases in the martensitic transformation from the parent β phase is described. The atomic movements leading to the martensite are accomplished by a long-range shear {112} < 111 > that transforms the parent to the product lattice and a short-wavelength displacement or shuffle {110} < 110 > that induces the correct stacking. The microstructures arising out of these paths depend on which of these modes initiates the transformation. When the shear precedes the shuffle, conventional martensite forms as dislocated laths or internally twinned plates. A signature of the {110} < 110 > shuffle that follows or accompanies the shear is present in both cases as stacking fault-related domains. The shuffle displacement precedes the shear with increasing β stabilizer addition, resulting in a nanodispersion of a structure with orthorhombic symmetry, which we have designated as O′. Al, Sn, Zr and O additions promote the shuffle. The O′ dispersion acts as embryos for the formation of nanomartensite on cooling or with the application of stress. The resulting continuous and controlled strain incorporation into the lattice within the constrained nanoembryos results in nonlinear superelasticity or the invar and elinvar effects. The stability of the bcc parent is discussed in terms of phonon mode or related elastic constant softening.