Features of the Order–Disorder Phase Transition during the Ordering of Ni4Mo and Ni4W Alloys with D1a Superstructure

Abstract

Results are presented from studying atomic long-range order in Ni4Mo and Ni4W alloys via X-ray diffraction analysis. The phase transitions in these alloys are confirmed to be of the D1a → A1 type. The temperature dependences of the long-range order parameters are found to be similar in Ni4Mo and Ni4W alloys. The difference between their properties is due to different characters of the order–disorder phase transition. A transition from the single-phase state with D1a superstructure to single-phase А1 through the diphase (D1a → A1) region occurs in the Ni4Mo alloy. A more complex D1a → (A1 + δ) peritectoid transformation takes place in the Ni4W alloy. It is shown that an order parameter equal to unity can be obtained in Ni4Mo alloy through long-term ordering annealing, while such long-range order in Ni4W alloy (ηmax = 0.9) is impossible to obtain by annealing.