Microstructure characteristics and compressive properties of NiAl-based multiphase alloy during heat treatments

The alloy of Ni–Al26–Cr15–Co6–Ti3–Mo2–Nb0.5–Ho0.1 (at.%) was heat-treated and studied by microstructure examination and compressive test. The results reveal that in the as-cast alloy, α-Cr, σ and μ phases are segregated along the NiAl phase boundary and form into network. Small Ni2AlTi, Cr2Nb and Ni2Al3Ho particles precipitate at NiAl grain boundary. The subsequent solid solution treatment with rapid and intermediate cooling rate transforms the net-like precipitating phases into discontinuous ones, but the rapid cooling leads to micro-cracks along phase boundary. However the lower cooling rate results in spherical α-Cr particle and needle-like Ni3Al phase in NiAl matrix. The short-term aging after solid solution treatment promotes the coarsening of Ni3Al lathes and α-Cr particles, and an orientation relationship of [0 0 1¯]β||[1¯ 1 0]λ′ and (1 1 0)β||(1 1 1)γ′ is found between lath-shaped Ni3Al phase and NiAl matrix. In addition, the μ phases are substituted by σ and some Cr3Ni2 phases. The long-term aging treatment reduces Ni3Al, σ and μ phases and coarsens the α-Cr phase further. The solid solution treatment and short-term aging improve the mechanical properties of the NiAl-based multiphase alloy significantly.

► The α-Cr, σ and μ phases precipitate and form into network in as cast alloy.
► Rapid and intermediate cooling rate after solution treatment break the precipitates network; the lower cooling rate results in some α-Cr and Ni3Al precipitates.
► Short-term aging results in massive Ni3Al lathes and α-Cr particles, but long-term aging treatment coarsens the globule-shaped α-Cr particle and decreases the lath-shaped Ni3Al, μ and σ phase.
► The solid solution with short-term aging treatment improves mechanical properties of the alloy significantly.