Microstructure evolution of wrought nickel based superalloy GH4037 in the semi-solid state

Abstract

Study on semi-solid processing of superalloy is little reported due to the difficulties in fabricating semi-solid billets. In this manuscript, a novel method called semi-solid isothermal treatment of wrought superalloy (SSITWS) was firstly proposed in order to fabricate semi-solid billets of superalloy. The effects of isothermal temperature and soaking time on microstructure of Ni-based superalloy GH4037 in the semi-solid state were investigated. The results show that the near-globular microstructure of wrought GH4037 superalloy can be obtained by the semi-solid isothermal treatment. The average grain size increased and the roundness decreased with the prolonged soaking time at 1350 °C and 1360 °C. When the isothermal temperature was elevated to 1370 °C or 1380 °C, the microstructure evolution was complex due to the combination of breaking up mechanism and coarsening mechanism. At 1390 °C, the dendritic grains were obviously observed in the microstructure. With the increasing of isothermal temperature and soaking time, the liquid boundary film coarsening behavior occurred in the microstructure. With the increasing of soaking time, the size of intragranular liquid droplets increased, but the amount of them decreased. When the isothermal temperature was high, the intragranular liquid droplets disappeared. The coarsening rate constant at 1360 °C was lower compared to that at 1350 °C. However, when the isothermal temperature was higher than 1370 °C, the coarsening kinetics was not suitable for describing the microstructure evolution. The discrete carbide precipitates appeared in the grain boundaries. The distribution of Ti and Mo were higher in the grain boundaries than that in solid grains, and the distribution of other elements was homogenous in the grain boundaries and inside the grains.