Precipitation hardening in Inconel 625

The hardness of the nickel based superalloy Inconel 625, aged at 625, 700, and 760 degrees 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 deg rees C than at 700 degrees C, Also the peak hardness is higher at 700 degre es C than at 760 degrees C. The results have been discussed in terms of pre cipitation. Scanning electron microscopy revealed the presence of precipita tes in specimens aged at 760 degrees C for a longer time. Electron probe mi croanalysis results show these precipitates to be rich in Ni, Nb, and Mo in dicating that these are gamma " precipitates of Ni,(Nb, Mo) type. Transmiss ion electron microscopy confirmed that these are gamma " precipitates. It a lso suggests that nucleation takes place heterogeneously on dislocations an d stacking faults. Longer aging causes somewhat uniform nucleation but stil l preferentially on the secondary defects. At 700 degrees C gamma' precipit ates have been observed in addition to gamma " precipitates. The orientatio n relationship between the precipitates and the matrix has also been determ ined. MST/4356.