Time-resolved synchrotron diffractometry of phase transformations in high strength nickel-based superalloys

Phase transformations in prototype high strength polycrystalline nickel-based superalloys of varying Ti/Nb ratio are studied using time-resolved, high resolution X-ray synchrotron diffractometry. The dissolution kinetics of the ordered phase Ni₃(Al, Ti, Nb, Ta) upon heating to the solutioning temperature of ∼1200 °C and its reprecipitation on cooling are deduced; effects of varying Nb and Ti alloy composition on the reaction kinetics are identified. Heating to 800 °C does not alter substantially the fraction of the strengthening phase Ni₃(Al, Ti, Nb, Ta) but further heating causes its rapid dissolution. At higher temperatures, evidence is provided for the formation of further ordered phases; Ni₃(Ti, Ta) is proposed and possibly Ni_0.45Ta_0.55; cooling causes their dissolution and reprecipitation of Ni₃(Al, Ti, Nb, Ta), so that it seems probable that the reactions are coupled. The unforeseen high temperature precipitation of further ordering by phases other than Ni₃(Al, Ti, Nb, Ta) implies the possibility of a contribution by them to the high temperature mechanical behaviour of these materials, which until now has been thought to be solely due to Ni₃(Al, Ti, Nb, Ta). The MC carbide, probably TiC, is stable even at the solution heat treatment temperature; no evidence of reactions involving other carbides such as M₂₃C₆ is found.