Currently, special attention is drawn to materials with a complex of special properties and the ability to preserve them in extreme conditions. Creating materials with a set of target properties and a given structural-phase state of the system is far from being a trivial task. Classical studies of low-stability pre-transitional states have been performed on β-alloys of the Ni–Al system. The structural-phase characteristics of the BCC alloy during thermal cycling were investigated by a Monte Carlo simulation. Using the Monte Carlo method, an irreversibility of the processes has been shown which take place during thermal cycling in the course of the structural-phase transformations in BCC alloys, such as NiAl intermetallic compounds of the Ni–Al system. Monte Carlo simulations show that the structural-phase characteristics of the BCC NiAl alloy are formed during cooling from a solid solution. It is noted that during heating and cooling the alloy undergoes various structural-phase states, which indicates the irreversibility of the processes during heating–cooling.