In-situ Observation of Surface Relief Formation and Disappearance during Order-Disorder Transition of Equi-atomic CuAu a
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In-situ Observation of Surface Relief Formation and Disappearance during Order-Disorder Transition of Equi-atomic CuAu alloy using Laser Scanning Confocal Microscopy Seiji Miura†, Hiroyuki Okuno*, Kenji Ohkubo and Tetsuo Mohri Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, JAPAN *) Graduate Student, Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University. † ) corresponding author. Tel. & Fax: +81-11-706-6347 E-mail address: [email protected] (S. Miura)
ABSTRACT In-situ observation of the formation and disappearance of the surface relief associated with the twinning during the order-disorder transitions among CuAu-I (L10), 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 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.
INTRODUCTION Many studies have been dedicated to understand the nature of order-disorder transition in various alloy systems. In the first-order phase transition it has been accepted that the phase transition temperature goes down with increasing a cooling rate, which is explained in terms of the delay of nucleation. This is also expected for the case of heating, while for the order-disorder transition of equi-atomic CuAu it was found, by both DSC analysis and electrical conductivity measurements, that the transition temperature decreases with increasing the heating rate [1-5]. This phenomenon has been called “Retro effect”. It has been known that there are three phases for equi-atomic CuAu alloy; high temperature disorder fcc, low temperature CuAu-I with L10 structure and CuAu-II with a periodic antiphase (PAP) structure in the middle [6]. By extending the Johnson-Mehl-Avrami model to competitive nucleation and growth of three phases, it was shown that the Retro effect can be explained qualitatively [7]. However, this study includes several unknown factors, which obscures the applicability to the general case.
S4.4.2
Traditional macroscopic experimental technique such as the DSC (or DTA), electrical conductivity or thermal expansion
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