Displacive-replacive phase transformation in a Ni_62.5Al_37.5 β₂-phase studied by high-resolution electron microscopy and microdiffraction

Abstract

The structure of the metastable Ni₂Al phase, which has long been a matter of controversy, has been carefully re-examined by means of high-resolution trans-mission electron microscopy (HREM) and electron microdiffraction. First, it is concluded that the as-quenched Ni_x Al_100−x (60 ⩽ x ⩽ 65) material already exhibits a partial ω-type collapse in a one-dimensional fashion which and is consistent with the anomalous dip in the phonon dispersion curve. Ni₂ Al precipitates are formed on annealing by thermal decomposition of the high-temperature Ni_xAl_100−x B2 phase and still retain the smali ω-type shuffle. The amount of displacement in the well developed Ni₂Al phase was estimated to be between 20 and 50% of the ideal ω collapse; this was determined by means of a combined technique of HREM and microdiffraction together with dynamical calculations of HREM images and diffraction intensities.