Structure of Ni20Cr13Al alloy with and without carbon addition annealed at 800°C

The phase analysis of Ni20Cr13Al showed the presence of the (hypostoichiometric) γ′ (Ni3Al) phase in chill-cast alloy. The spheroid particles of γ′ change their shape to cuboidal at the very beginning of the annealing at 800°C. This change is accompanied by a decrease of volume fraction of γ′, and at the same time by an increase of hardness. The annealing also leads to the precipitation of Cr-rich α-phase. The precipitation starts at grain boundaries. After longer times at the annealing temperature, the particles appear in the grain interior as well. In the vicinity of grain interior particles, electron microdiffraction patterns contain the spots corresponding to Ni2Cr long-range order. At grain boundaries, the precipitation of Cr-rich phase induces the growth of Ni3Al particles and grain boundary migration. Carbides (Cr23C6) are very rare in an alloy with only traces (impurities) of carbon while in carbon enriched (0.02%) alloys they appear after 500 h of annealing mainly in association with the Cr-rich particles.