Microstructural development and tensile behavior of a hot isostatically pressed nickel-based superalloy

A powder nickel-based superalloy, FGH4096, was hot isostatically pressed at a super-solvus temperature. The microstructure of the as-fabricated sample was investigated by using a range of characterisation techniques and the properties evaluated by tensile testing. Prior particle boundaries (PPBs) decorated by near-continuous precipitates were observed throughout the sample. The PPB precipitates were identified to be face centred cubic (FCC) TiC through transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) analysis. In addition to γ, γ′ and TiC, a new phase formed and homogeneously distributed within prior powder particles after HIPing. TEM analysis reveals that it is tetragonal CrMo₂B₂ or Cr₃Mo₃B₄ [a = b = 5.825 Å, c = 3.115 Å, α = β = γ = 90°, space group = P4/mbm], both of which have the same crystallographic structure and lattice parameter. A great number of fine γ grains were found to form around PPBs and a high density of annealing twins were present throughout the as-HIPed sample, indicating that full recrystallization has happened during HIPing. With the presence of PPBs, the as-HIPed sample failed in an inter-particle de-bonding fracture mode but in a highly dimpled way, giving rise to both high strengths and good ductility at both room temperature and high temperature. Deformation happened by dislocation slipping through grains. Dislocations, however, did not cut through PPB precipitates but instead accumulated around these precipitates, which may have caused stress concentration along PPBs and eventually led to inter-particle de-bonding.