Physical metallurgy and mechanical properties of ductile ordered alloys (Fe,Co,Ni)3 V

Abstract

The physical metallurgy and mechanical properties of long-range ordered (LRO) alloys based on the quasiternary system Fe₃ V-C0₃ V-Ni₃ V are reviewed comprehensively. Ordered crystal structures in (Fe, Co, Ni)₃V alloys are characterized by specific sequences of stacked close-packed ordered layers, and their stacking character can be altered systematically by controlling the electron concentration (e/a) of the alloys. As e/a decreases, the stacking character changes from purely hexagonal, through different, ordered mixtures of hexagonal and cubic layers, to purely cubic. Partial substitution of Fe for Co and Ni lowers e/a, thus stabilizing the L1₂ -type cubic ordered structure. Alloys with hexagonal ordered structures exhibit brittle fracture and very little ductility, but alloys with the cubic ordered structure are ductile. The mechanical behaviour of these ductile LRO alloys is reviewed, with emphasis on deformation at high temperatures. Unlike conventional solid-solution strengthened alloys, the yield strengths of the LRO alloys increase with temperature, peaking around the critical ordering temperature T_c (65G–950°C). Further investigation suggests that this anomalous temperature dependence is caused by a thermally activated process, rather than a disordering process. The creep behaviour of the LRO alloys can be described by a power-law rate equation. All the alloys exhibit a dis continuous change in the steadystate creep rate in the vicinity of T_cc, and the formation of LRO lowers it by two orders of magnitude. Other metallurgical and mechanical properties of the LRO alloys are also discussed, including workability, fatigue behaviour, hydrogen embrittlement, oxidation and corrosion, irradiation effects, and microstructural features. The review emphasizes the correlation of these properties with metallurgical variables in these alloys. The potential development of (Fe, Co, Ni)₃ V alloys as new structural materials for high-temperature applications is also discussed.