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 D1_a → 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 D1_a superstructure to single-phase А1 through the diphase (D1_a → A1) region occurs in the Ni₄Mo alloy. A more complex D1_a → (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 (η_max = 0.9) is impossible to obtain by annealing.