ON THE INTERPRETATION OF HIGH-RESOLUTION ELECTRON-MICROSCOPY IMAGES OF PREMARTENSITIC MICROSTRUCTURES IN THE NI-AL BETA-2 PHASE

The premartensitic microstructures of quenched Ni-Al β2 (B2) phase have been examined at room temperature by high resolution electron microscopy. The alloy of Ni63Al37 (which transforms to a close-packed 7R martensite at Ms ≅ 200 K) exhibits a fine-scale mosaic assembly of non-uniformly distorted and micromodulated domains. The atomic structure simulated for these domains involves {110} 〈110〉 shear-plus-shuffle displacements. This inhomogeneous microstructure is shown to be directly related to the ensuing transformation deriving from the unusually low energy of the ∑4-TA2 phonon mode of the β2 parent phase, and its anomalous temperature-dependent incomplete softening as T → Ms. Analysis of the microstructural development provides important insights into the nature of heterogeneous nucleation of the martensitic transformation. © 1990.