The growth and morphology of directionally solidified nickel base γ/γ′- σ superalloys

Abstract

A series of Ni-Nb-Al-Cr(γ/γ′- σ) alloys in the composition ranges Nb 19.3 to 23.2 wt pct, Al 2.5 to 5.2 wt pct and Cr 0 to 7.05 wt pct have been directionally solidified under high thermal gradient (G) at both steady state and under conditions of abruptly or gradually changing growth rate(ft). The critical ratio of G andR, (g/r)*, to achieve two-phase plane frontin- situ composite growth increases as chromium and niobium (Cb) concentration deviates from the trough or surface of two-fold saturation. Interlamellar spacing of composites tend to decrease with increasing chromium content. Structures produced at steady state growth in whichG/R < (G/R)* are consistent with previous work and can be related to the location of the alloy composition with respect to the line of two-fold saturation. For alloys, which at lowG/R exhibited σ dendrites, any perturbation in growth velocity (atG/R > (G/R)*) precipitated a single phase σ (Ni₃Nb) band. For alloys which at lowG/R exhibited γ dendrites a similar effect was achieved only when growth rate was reduced abruptly by more than an order of magnitude. Interlamellar spacing of two alloys (approximately Ni-20 wt pct Nb-2.5 wt pct Al-6 wt pct Cr) was studied and for abrupt reductions in growth rate in which bands were not produced, it was observed to decay slowly to the new steady state value over distances which are inconsistent with the assumption of simple niobium diffusion control. A gradual increase in growth velocity for one of these alloys resulted in extremely slow adjustment of interlamellar spacing occurring over a period greater than one hour. An abrupt increase in growth velocity for all alloys caused immediate adjustment of interlamellar spacing to the new steady state value.