Time-temperature-precipitation behavior of Ni-Fe base wrought superalloy 706

The time-temperature-precipitation (TTP) diagram is essential in the design of heat treatments for any precipitation strengthened superalloy. Some TTP diagrams have been already presented for Ni-Fe-base superalloy 706, which has been used for high temperature services. However, the gamma'-gamma '' to-precipitate that is the most important strengthening agent in this alloy is not seen in the literature. Moreover, effects of aluminum, titanium and niobium, important substitutional elements in gamma' and gamma '' precipitates, on the TTP behavior are not clear. In this study, the TTP and the time-temperature-hardness (TTH) diagrams are presented of the commercial Alloy 706 and experimental alloys containing only one or two of the key elements in a temperature range from 600 to 900 degrees C. The alloys containing Ti were all age-hardenable, especially at temperatures between 700 and 800 degrees C. The observation by an optical microscopy, scanning electron microscopy and transmission electron microscopy revealed the gamma', gamma '', gamma'-gamma '' co-precipitates and eta precipitates in those alloys. Among the three elements, titanium plays a most important role in the precipitation strengthening behavior of Alloy 706, Furthermore, both aluminum and niobium are virtually ineffective without incorporating titanium with them. Niobium promotes the gamma '' formation and suppresses the a formation. Aluminum enhances the formation of stable gamma'-gamma '' co-precipitates, more effectively in the co-existence of titanium and niobium.