Boron effect on phase transformation of σ and M23C6 in nimonic 105 superalloy

Abstract

In this study, the formation and transformation of σ and M23C6 phases in Nimonic 105 superalloy at different level of boron were investigated. So alloys with two levels of boron were prepared and the microstructure were analyzed using optical and scanning electron microscopy and X-ray diffraction. It was found that the time of M23C6 formation, M23C6 transformation into σ phase and grain growth mechanism were influenced by boron addition. Boron segregated at the grain boundaries and changed the morphology of M23C6 from continuous to discontinuous one. M23C6 transformed to needle-like σ precipitates at 750 °C after 360 and 240 h heating and the σ volume fraction reached to 2.2 and 3.4 vol% after 500 h in the alloys containing 0.013 and 0.003 wt% B, respectively. Furthermore, the time-temperature- precipitation (TTP) diagrams of M23C6 phase were presented. It was observed that time and temperature formation of M23C6 precipitation shifted to higher times and temperatures as a result of boron addition.