Nanocrystalline intermetallic compounds in the Ni–Al–Cr system synthesized by mechanical alloying and their thermodynamic analysis

Abstract

Mechanical alloying (MA) of the ternary Ni–Al–Cr mixtures having the Ni50Al50, Ni25Cr25Al50 and Ni50Cr25Al25 compositions was investigated. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometry (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show that for all three compositions the gradual interdiffusion of elements during MA leads to the development of different phases. The final structures for the Ni50Al50 and Ni25Cr25Al50 compositions are nanocrystalline NiAl and (Ni, Cr)Al intermetallic compounds, respectively. However MA of the Ni50Cr25Al25 composition led to the formation of a Ni3Al intermetallic compound as well as Cr which transformed to the (Ni, Cr)3Al intermetallic compound after subsequent heat treatment. The structural transition upon annealing was investigated. Furthermore, thermodynamic analysis using the Miedema model for all three systems is carried out and discussed in detail.