The Influence of Variations in Atomic Composition on Low-Stability Pretransitional Structural-Phase States of a NiAl Intermetallic Compound

An analysis of the influence of a deviation of atomic composition from stoichiometry performed by the Monte Carlo method using a NiAl intermetallide as an example in the course of its cooling demonstrates that such deviation is an important factor in the region of pretransitional low-stability structural-phase states preceding transformation. The short-range order parameter values in non-stoichiometric alloys in their absolute values are essentially lower than those of a stoichiometric intermetallide; hence the tendency towards atomic ordering in non-stoichiometric intermetallics is noticeably lower than that in the stoichiometric alloys. The behavior of the temperature plots of the long-range order parameter in non-stoichiometric alloys during their cooling is considerably different from that of a stoichiometric intermetallide. In the case of cooling of non-stoichiometric alloys, a considerable degree of overcooling is required for the long-range order to set, and the formation of ordered phases occurs at much lower temperatures. Moreover, the temperature plot of long-range parameter of the Ni45Al55 alloy lies somewhat lower than that of the Ni55Al45 alloy, which implies that setting of the longrange order in these non-stoichiometric alloys occurs in different ways. It is underlined that a deviation of the system's composition from stoichiometry gives rise to an essential refinement of the ordered and disordered regions.