Evidence for incomplete martensitic transformation in Ni0.62Al0.38 by neutron diffraction

High-resolution neutron powder diffraction was used to investigate the evolution of the crystal structure of Ni0.62Al0.38 with a martensitic transformation temperature, MS, of 65 K as a function of temperature. Although the β2 parent phase has the propensity to nucleation instability, no precursor effects are observed until ∼40° above MS. Below MS, the transition does not occur homogeneously and it is shown that the martensite coexists with the austenite over a wide temperature range. The low temperature phase seems to be largely described by the 7R structure that accomodates the transition from the parent to the martensite and minimizes the lattice mismatch between the two.