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HIGH-TEMPERATURE X-RAY-DIFFRACTION STUDIES OF LATTICE-DYNAMICS IN NI3(FE,NB) ORDERED ALLOYS

A study is performed on characteristics of lattice dynamics in the initially ordered ternary Ni3(Fe1-xNbx) (x = 0 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 U(f)2BAR, the Debye characteristic temperature THETA(D), and linear thermal expansion coefficient alpha. It is found that the U(f)2BAR, THETA(D), and alpha 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 U(f)2BAR and alpha and decrease in THETA(D). Moreover, the addition of Nb stabilizes Ni3Fe ordered structure and shifts the order-disorder transition to higher temperature.