ELECTRON-MICROSCOPY AND NEUTRON-SCATTERING STUDIES OF PREMARTENSITIC BEHAVIOR IN ORDERED NI-AL BETA-2 PHASE

High resolution electron microscopy and elastic and inelastic neutron scattering examinations of the β2(B2) phase of Ni62.5Al37.5 show a direct relationship between the evolution of premartensitic structural configurations and anomalous changes in lattice dynamical behavior as the β2 cools toward its martensitic transformation temperature Ms. The resulting microstructure is a fine-scale mosaic assembly of non-uniformly distorted and modulated domains, in which {110}〈110〉 shear-plus-shuffle displacements give rise to (110)B2 micromodulations with wavelengths of about 1.3 nm. These displacements are derived from the unusually low energy of the Σ4〈ζζ0〉-TA2 phonon mode and its anomalous temperature-dependent incomplete softening at ζ=0 (namely 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 7R(5, 2) martensite but are generally too weak to act as potent nucleation centers. Similar ISD configurations develop at defects with higher strain amplitudes (dislocations, grain boundaries etc.) and these are the most likely sites for heterogeneous nucleation. © 1990.