Precipitation hardening in Inconel* 625

The hardness of the nickel based superalloy Inconel 625, aged at 625, 700, and 760°C for different intervals of time ranging from 1 to 335 h, has been measured. Peak hardening is found to occur much earlier at 760°C than at 700°C. Also the peak hardness is higher at 700°C than at 760°C. The results have been discussed in terms of precipitation. Scanning electron microscopy revealed the presence of precipitates in specimens aged at 760°C for a longer time. Electron probe microanalysis results show these precipitates to be rich in Ni, Nb, and Mo indicating that these are γ″ precipitates of Ni₃ (Nb, Mo) type. Transmission electron microscopy confirmed that these are γ″ precipitates. It also suggests that nucleation takes place heterogeneously on dislocations and stacking faults. Longer aging causes somewhat uniform nucleation but still preferentially on the secondary defects. At 700°C γ′ precipitates have been observed in addition to γ″ precipitates. The orientation relationship between the precipitates and the matrix has also been determined.