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

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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.