The thermoelastic martensitic transformation in β′ Ni-Al alloys: II. Electron microscopy

Abstract

The substructure of martensite in a 63.4 at. pct Ni-Al alloy was studied by transmission electron microscopy with particular attention to the “mixed” martensite morphologies observed in Ni-Al alloys, {111} <11•2>_fct transformation twinning was the only mode of lattice-invariant shear found and the measured twin thicknesses lead to an inhomogeneous shear value,m ₂, in excellent agreement with that predicted from the crystallographic theory. The internal twins in the martensite plates forming characteristic mixed morphologies were found to be at definite angles to one another and these angles were carefully measured to be 96 deg and 125 deg for the “fork” type or “spear” type martensite morphologies. The corresponding angles as predicted from the Bowles-Mackenzie theory are 94.2 deg and 124.4 deg. The interface plane between the martensite variants that form the spear type morphology was found to be a {011}_β' type plane with the internal twins in each variant twin related to each other across this mirror plane.