Structural-Phase Transformations of an fcc-Alloy During Thermal Cycling

Using an intermetallic compound of the Ni–Al system as an example, it is shown by the Monte Carlo technique that the processes developed during thermal cycling in the course of structural phase transformations in FCCalloys are irreversible. As a result of a heating–cooling cycle, a certain hysteresis is observed, whose presence suggests an irreversibility of these processes, which is indicative of the difference in the structural-phase states in the stages of heating and cooling. An analysis of the atomic and phase structure of the intermetallic system during its heating–cooling, i.e., in the course of order–disorder and disorder–order phase transformations has supported the difference in its structural-phase states in the stages of heating and cooling. Upon completion of the disorder–order phase transition, two antiphase domains with B2 superstructure are formed in the system. It is demonstrated that to ensure an order–disorder transition the system has to be somewhat overheated in contrast to a commonly acknowledged phase transformation temperature, while to achieve a disorder–order transition it has to be somewhat overcooled with respect to this temperature.