High-Temperature X-ray Diffraction Studies of Lattice Dynamics in Ni$\text{}_{3}$(Fe,Nb) Ordered Alloys

A study is performed on characteristics of lattice dynamics in the initially ordered ternary Ni$\text{}_{3}$(Fe$\text{}_{1-x}$Nb$\text{}_{x}$) (x = O to 10 at.%) alloys using high temperature X-ray diffractometry for the temperature range from 300 K to 1300 K. Aspects investigated included temperature and concentration dependencies of the full mean square atomic displacements Ū²$\text{}_{f}$, the Debye characteristic temperature Θ$\text{}_{D}$, and linear thermal expansion coefficient α. It is found that the Ū²$\text{}_{f}$, Θ$\text{}_{D}$, and α parameters show anomalous change at temperatures where the alloys are subjected to order-disorder phase transitions. It is further found that Nb addition results in the considerable increase in the parameters Ū²$\text{}_{f}$ and α and decrease in Θ$\text{}_{D}$. Moreover, the addition of Nb stabilizes Ni$\text{}_{3}$Fe ordered structure and shifts the order-disorder transition to higher temperature.