Effect of Heat Treatment on Microstructure and Mechanical Properties of Dual Two-Phase Ni₃Al and Ni₃V Intermetallic Alloys

So-called Ni base dual two-phase intermetallic alloys are composed of primary Ni₃Al (L1₂) phase precipitates among eutectoid microstructures consisting of the Ni₃Al and Ni₃V (D0₂₂) phases. In this article, microstructural refinement of an alloy with a nominal composition of Ni₇₅Al₁₀V₁₅ (in at.%) was attempted by various heat treatment processes. When the alloy was continuously cooled down after solution treatment, fine and cuboidal Ni₃Al precipitates were developed by rapid cooling while coarse, rounded and coalesced Ni₃Al precipitates were developed by slow cooling. When the alloy was isothermally annealed at temperatures above the eutectoid temperature, the morphology of the Ni₃Al precipitates changed from fine and cuboidal one to large and rounded one with increase in annealing time. When the alloy was annealed at temperatures below the eutectoid temperature, the Ni₃Al precipitates were grown keeping cuboidal morphology. The morphological change from the cuboidal to rounded Ni₃Al precipitates was induced by the transition from the growth driven by elastic interaction energy between the precipitate and matrix to that by the surface energy of the precipitate. Fine and cuboidal Ni₃Al precipitates generally resulted in high hardness.