Composition Dependence of Bainite Morphology in Cu–Zn–Al Alloys

Abstract

The morphology of bainite structure is examined by optical and electron microscopy for Cu–Zn–Al alloys with various compositions. It depends not only on the heat treatment for the bainite formation but also strongly on the electron atom ratio, e⁄a, and the Ms temperature of the alloy. In alloys with e⁄a smaller than 1.46, the 9R bainite plates are formed with a pair of the variants having (551) and (55\bar1) habit planes in a form of long hinge. As e⁄a increases, the numbers of bainite plates decrease with increasing the size. The trace of habit plane keeps linear until the second structural change, 9R to fcc, takes place in the plates. As the Ms temperature decreases or the aging temperature increases, this change becomes easier to take place. If e⁄a is larger than 1.46, the bainite plate is transformed with γ2 particles or a complex eutectic structure consisting of α and γ2 phases is constructed. In a specimen with e⁄a=1.47, incommensurate γ phase is observed in electron microscopy and the bainite plates are considerably smaller than those in the specimen with e⁄a=1.44. The observed morphological change due to the alloy composition is discussed by using a pseudo-binary phase diagram constructed with the form of Cu–(Znkx–Alx), where the ratio k of the atomic concentrations of Zn and Al is assumed to be unchanged.