AbstractElastic and inelastic neutron-scattering and high-resolution electron-microscopy examinations of the β2 (B2) phase of NixAl(100-x) alloys show a direct relationship between the anomalous changes in lattice dynamical behavior and the evolution of static premartensitic structural configurations as the β2 cools toward its martensitic transformation temperature, Ms. The resulting microstructure is a fine-scale mosaic assembly of nonuniformly distorted and modulated domains, in which {110}<110> shear-plus-shuffle displacements give rise to the {110}B2 micromodulations of ∼1.3 nm wavelength. These displacements are derived from the unusually low energy of the Σ4<ζζ0>–TA2 phonon mode and its anomalous temperaturedependent incomplete softening at ζ = 0 (viz., the elastic constant C) and at ζ ≈ 0.16. These inhomogeneously strained domains (ISDs) are believed to be centered on low strain-amplitude defects. They are viewed as strain embryos of the product 7M(5, 2) martensite but are generally too weak to act as potent nucleation centers. Similar ISD configurations develop at defects with higher strain amplitudes (e.g., dislocations, grain boundaries) and these are the most likely sites for heterogeneous nucleation to occur.
Microstructural development from 9-14 years: evidence from the ABCD Study
