The crystallography of the bainite transformation in beta brass

Abstract

The crystallography of low-temperature (plate) bainite formation in beta brass was studied in alloys containing 40–45 wt.% zinc. Habit plane of the bainite plates were determined using single surface trace analysis for plates formed between 200 and 400°C in these alloys. It was found that the mean position of the habit plane pole scatter region varied with temperature of formation and composition of the alloy. Except for the habit plane near (123)β, most of the variation is in a region near the sub-zero martensite habit plane, (2 11 12)β. The former is observed for the first time in plate bainite formation in beta brass. Application of the phenomenological theory of martensite crystallography to the bainite formation using stacking fault shear as the lattice invariant shear shows that the variation of the habit plane could be due to the variation in tetragonality, c α a α and relative volume, ν of bainite produced at different temperatures in these alloys. The habit plane scatter near (2 11 12)β falls within the ranges 0.90 < c α a α < 1.00 and 1.00 < ν < 1.02. These ranges are consistent with the structure proposed for lowtemperature bainite by previous workers. The habit plane near (123)β is consistent with an axial ratio ~0.96 and relative volume ~0.935. The latter corresponds to a dilatation parameter, δ ~0.975 relative to the undistorted martensite habit interface.