In-situ observation of surface relief formation and disappearance during order-disorder transition of equi-atomic CuAu alloy using laser scanning confocal microscopy

In-situ observation of the formation and disappearance of the surface relief associated with the twinning during the order-disorder transitions among CuAu-I (L1 ₀), CuAu-II (PAP) and disordered fcc phases was conducted using Confocal Scanning Laser Microscopy equipped with a gold image furnace. The Retro effect was confirmed in poly-crystal samples, however no evidence was found in single-crystal samples. Also observed in poly-crystal samples are that the disordering temperature detected by the disappearing of relieves is different from grain to grain, and that grain boundary cracking alleviates the Retro effect. The observed phenomena were explained based on the crystallographic orientation relationship among grains investigated by FESEM/EBSD in terms of the elastic strain effect around grain boundaries induced by transition. It was confirmed that in each grain the surface relieves correspond to a set of two {011} planes having a 〈100〉 axis perpendicular to both planes in common. It was also found that the larger the average strain of two neighboring grains is, the lower the transition temperature. This observation was explained by the stress effect on the stability of a phase.