The Influence of Grain Size on Low-Stability Pre-Transitional Structural-Phase States of NiAl Intermetallide

Using the Monte Carlo method, the influence of grain size (model cell dimensions) on the peculiarities of pretransitional, low-stability structural-phase states of NiAl intermetallide in the region of structural-phase transformations is investigated during thermal cycling (heating and cooling). An analysis of the temperature dependences of the long-range order parameters shows that during heating the maximal long-range order is observed in an alloy with the maximal grain size, while the minimal - in the alloy with the smallest grain size. In order to achieve disordering of the alloy by increasing its grain size, it has to be increasingly overheated. Under cooling, long-range order primarily appears in a fine-grained alloy. The larger the grain size, the wider the temperature interval of the structural-phase transformation. The peculiarities of formation of the structural-phase states in the course of cooling as a function of the grain size (model cell dimensions) indicate that the first ordered regions appear in the fine-grained alloy. As the grain size increases, the temperature at which long-range order appears becomes lower, in other words, a still higher overcooling is required for the system's atomically-ordered states to be formed.