Phase precipitation and phase stability in Nimonic 263

Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermodynamic calculations were employed to study the phase stability and phase precipitation in the Ni-based superalloy Nimonic 263. The major p recipitates at temperatures below 900 degreesC were gamma-prime and eta (Ni 3Ti), with the eta phase starting to precipitate at the expense of the gamm a-prime phase after prolonged annealing. The eta precipitates had a thin-pl ate morphology and formed a regular Widmanstatten pattern. They had a fixed orientation relationship with the fee matrix. Very fine particles of M23C6 also precipitated intragranularly, and they had a cube-on-cube orientation relationship (OR) with the fee matrix. Intragranular precipitation of a ve ry fine cylindrical MC carbide was also observed. Grain-boundary (GB) preci pitates were mostly M23C6, With a small fraction of MC. Based on the experi mental results, a time-temperature-transformation (TTT) diagram was constru cted. The experimental results were compared to thermodynamic predictions, and good a,agreement was observed. The calculations correctly predicted alm ost all the major phases and their stability temperature ranges. The experi mental and calculation results, together, depict a clearer picture of phase stability in the alloy.