Long range order, antiphase domain structures, and the formation mechanism of α1 (“Bainite”) plates in A CuZnAl alloy

Abstract

The inheritance of long range order during the β 2 (B2) or β 3 ( L2 1) → α 1 (“Bainite”) transformation in a Cu-26.67Zn-4Al (wt%) alloy has been demonstrated. The associated antiphase domain (APD) structures in the α 1, plates were studied by TEM dark field imaging using both nearest-neighbor (NN) and next-nearest-neighbor (NNN) superlattice reflections. At temperatures below the β 2 → β 3 ordering temperature, both NN and NNN APDs are inherited by the α 1, plates during their initial formation and subsequent early stage plate thickening. NNN order is lost, although NN order and its APD structure are preserved in subsequently thickened layers of the plates. As a result, the α 1 ordered structure is composite, consisting of an 18R central region and an ordered 9R surrounding outer layer. At temperatures above the β 2 → β 3 transition temperature, the parent β 2 possesses only NN order. Similar inheritance of NN order and associated APDs is also found at lower transformation temperatures. The observed evolution of long range order and the APD structure of the α 1 plates strongly suggests that both the initial transformation and early growth stages of the plates involve a shear mechanism. Long range diffusion, which is effectively confined to the NNN sublattice due to the presence of NN order at the interface, eventually takes over and controls subsequent plate thickening. The thickness of the 18R central zone in α 1 plates decreases significantly with increasing transformation temperature, implying that the shear-diffusional transition is shifted to an earlier transformation stage as the transformation temperature becomes increasingly higher than the martensitic transformation temperature, M s . The lifetime of long range order in the α 1, plates also decreases with increasing transformation temperature, as internal diffusion in the plates is enhanced. The implications of these observations are discussed and compared with earlier results on similar transformations in binary CuZn and AgCd alloys. Accordingly, it is believed that, for the first time, an aggregate of substantial documentation has been obtained for the case where a solid state phase transformation begins by a shear mechanism and ends up being dominated by classical diffusion. This behavior can be rationalized in terms of ordering temperatures in relation to the M s temperature of a given alloy. The essential point in this case, is that a “mixed” transformation is clearly involved, the description of which in classical terms, e.g. shear or diffusion controlled, depends upon the time frame of observation during the course of total transformation.