Structural-Phase Сhanges During the D1a→A1 Phase Transition in the Ni4Mo Alloy

Abstract

The results of studying the structural and thermal effects in the region of the D1a→A1 order-disorder phase transition in the Ni4Mo alloy are presented. It is found that the D1a→A1 transition occurs in a narrow temperature range of 16 °C while the long-range order parameter п in this range decreases from 1 to 0.85 and, upon reaching ТК, decreases abruptly to 0. Studying the thermal effects allows obtaining the temperature dependence of the specific heat Ср(Т) is obtained and determination of the enthalpy and entropy of the D1a→A1 order-disorder phase transition in the Ni4Mo alloy. It is found that the main contribution to the entropy of transformation is configurational. The nonconfiguration contribution is not large and contributes to the ordering processes. It is established that the beginning of the deviation on the temperature dependence of Cp(T) from the linear one correlates with the beginning of the formation of the two-phase state D1a +A1 and the beginning of a decrease in the atomic long-range order in the D1a. Based on calculations based on the first a principles of thermodynamic parameters of compounds, the enthalpy of formation in the Ni-Mo system is obtained. It is found that the most stable compound in this system is Ni3Mo. Theoretical calculations of the compound formation enthalpy give a value of -8.3 kJ/mol.