Cooperative freezing of the L12 ordered domains at the critical cooling temperature of Ni3Fe alloy

Abstract

Abstract It is well known that Ni3Fe transforms from a disordered solid solution to an ordered intermetallic with L12 superstructure when the alloy is cooled slowly. Here we elucidate the underlying cooperative phenomenon and the atomistic mechanism of this ordering process based on simulations using embedded atom potentials. As the simulated alloy is cooled from the disordered state to the critical cooling temperature (T c), Ni atoms with L12 order [denoted as Ni(L12 ⩾ 1) atoms] increase significantly along with Ni atoms having the least deviation from L12 local order (denoted as Ni([IP]3) atoms). The ordering (up to T c) occurs predominantly through random increase in Ni(L12 ⩾ 1) atoms throughout the system, as indicated by absence of long-range order. At T c, L12 ordered domains formed by Ni(L12 ⩾ 1) atoms ‘freeze’, i.e. these domains, collectively, achieve a threshold strength against thermal fluctuations. This is indicated by (i) dissipation of large-scale fluctuations of Ni(L12 ⩾ 1) atoms at T c and (ii) the growth of the L12 domains through propagation (at the expense of atoms with non-L12 local environment) as the alloy is cooled below T c. The stability threshold of the L12 ordered domains at T c is qualitatively consistent with (i) the critical slowing down, i.e. a significant increase in annealing time (to about 41 days) at 497 °C close to T c (∼500 °C) and (ii) sharp changes in bulk properties (due to loss of stability of the domains) when the alloy is heated across T c to about 550 °C. Further, the experimental long-range order parameter values as a function of reduced temperature are in reasonable agreement with the corresponding values of the simulated alloys. The contribution of Ni([IP]3) atoms to ordering in the actual alloy is potentially significant since such atoms together with nearest neighbours constitute about 75% of the total atoms in the simulated alloys at T c.