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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 Ni 3 (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 Ni 3 (Al, Ti, Nb, Ta) but further heating causes its rapid dissolution. At higher temperatures, evidence is provided for the formation of further ordered phases; Ni 3 (Ti, Ta) is proposed and possibly Ni 0.45 Ta 0.55 ; cooling causes their dissolution and reprecipitation of Ni 3 (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 Ni 3 (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 Ni 3 (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 M 23 C 6 is found.

A family of novel polycrystalline Ni-based superalloys with varying Ti:Nb ratios has been created using computational alloy design techniques, and subsequently characterized using atom probe tomography and electron microscopy. Phase chemistry, elemental partitioning, and c¢ character have been analyzed and compared with thermodynamic predictions created using Thermo-Calc. Phase compositions and c¢ volume fraction were found to compare favorably with the thermodynamically predicted values, while predicted partitioning behavior for Ti, Nb, Cr, and Co tended to overestimate c¢ preference over the c matrix, often with opposing trends vs Nb concentration.

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