Tweed microstructures II. In several phases of the Ni-Al system

Abstract

In this second in a series of papers on tweed microstructures in phases of the Ni-Al system, we describe the electron microstructure of the 3R and 7R martensites, the γ phase and the newly discovered Ni₅Al₃ phase in an alloy of average composition 63 at.% Ni-Al. The alloy was subjected to ageing treatments at temperatures ranging from 300 to 850°C for periods of up to 15 days, which resulted in the precipitation of the γ and Ni₅Al₃ phases from the supersaturated β phase. The metastable Ni₂Al phase has also been found to occur, and its effect on the tweed microstructure in the β phase is examined. We conclude that Ni₂Al precipitation is not the cause of tweed in β-Ni-Al. The cubic γ phase exhibits a tweed microstructure almost identical to that of the β phase. The tweed microstructures of the tetragonal 3R martensite and orthorbombic Ni₅AI₃, differ from those of the cubic phases in ways which reflect their different symmetries. These differences provide further clues to the origin of tweed microstructures.