The effect of variations in the atomic composition on the low-stability pre-transitional structural phase states of the NiAl intermetallic compound

The analysis of the influence of the deviation of the atomic composition from the stoichiometric on the intermetallic state during the cooling process using the Monte-Carlo method using the example of the NiAl intermetallic compound showed that the deviation is a significant factor in the pre-transition weakly stable structural-phase states before the transformation. The values of the short-range order parameter of alloys of nonstoichiometric composition are significantly lower in absolute value of the corresponding values of the stoichiometric intermetallic composition, therefore the tendencies to the appearance of atomic order in the intermetallic compounds of non-stoichiometric composition are significantly less compared to the stoichiometric alloy. The behavior of the temperature dependences of the long-range order parameter during cooling of alloys of non-stoichiometric compositions differs substantially from the behavior of the corresponding dependence of the intermetallic compound of stoichiometric composition. When cooling alloys of non-stoichiometric compositions, significant overcooling is required to establish a long-range order, and the appearance of ordered phases occurs at much lower temperatures. Moreover, the temperature dependence curve of the long-range order parameter of the Ni45Al55 alloy lies substantially below the corresponding curve of the Ni55Al45 alloy, and this implies that the establishment of the long-range order in these nonstoichiometric alloys occurs in different ways. It is noted that the deviation of the composition of the system from stoichiometric causes a significant refinement of ordered and disordered regions.

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