Chemical order relaxation in a substitutional solid alloy around the critical temperature

The relaxation rate for phase transformations depends on how the system is far from its thermodynamic equilibrium. If, for example, temperature in the system approaches to the phase transition point, the relaxation rate becomes close to zero. We theoretically investigate the laser-induced kinetics of chemical order-disorder transformations (CODTs) in substitutional solid alloys at temperatures $T$ satisfying the condition that $|T--{T}_{c}|\phantom{\rule{0.16em}{0ex}}\ensuremath{\ll}\phantom{\rule{0.16em}{0ex}}{T}_{c}$, where ${T}_{c}$ is the CODT critical temperature. In these studies, we use the equation obtained by Metiu, Kitahara, and Ross, that is, $d\ensuremath{\eta}/dt=--(\mathrm{\ensuremath{\Gamma}}/2{k}_{B}T)\ensuremath{\partial}F/\ensuremath{\partial}\ensuremath{\eta}$, where $0l\ensuremath{\eta}l1$ is the chemical order parameter, \ensuremath{\Gamma} is the frequency of atomic jumps, ${k}_{B}$ is the Boltzmann constant, and $F$ is the free energy of the system, which we combine with the Landau theory for second-order phase transitions. If the two lowest terms are retained only in a Taylor expansion of $F$, the $\ensuremath{\eta}(t)$ dependence can be found analytically. Such an approach is assumed to be valid for sufficiently small $\ensuremath{\eta}l0.5$. As an example, we simulate CODTs in the thin-film (40-nm thickness) binary alloy of ${\mathrm{Fe}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}$ ($x=0.6$). Our simulations show that both extensive chemical ordering and disordering in the solid alloy are feasible under short-pulse laser irradiation, at least, at a nanosecond time scale. This finding can be useful for improving the properties of functional alloyed materials and for extension of their potential applications.