The Mechanism of η Phase Precipitation in A286 Superalloy During Heat Treatment

In this research, the mechanism of eta (η-Ni₃Ti) phase precipitation in iron-nickel-based A286 superalloy was assessed during aging heat treatment in the temperature range between 650 and 900 °C for the times of 1-30 h. Optical microscopy, scanning electron microscopy, differential thermal analysis, and x-ray diffractometry were used to describe the η phase transformation. The results showed that the major precipitates at temperatures below 840 °C were γ′ and η. The η phase started to precipitate at the expense of the γ′ phase after prolonged aging. The η phase existed in the samples aged at temperature higher than 760 °C with cellular morphology. The η volume fraction increased with increasing heat treatment time. In addition, when the aging temperature was increased from 760 to 820 °C, the η volume fraction increased and then decreased after 840 °C. The η phase morphology also changed from cellular to Widmanstätten-type during aging. The time-temperature-precipitation diagrams of these morphologies are presented. The results indicated the differences in precipitation mechanisms of η phase at 840 and 860 °C.