As-cast microstructure and redistribution of elements in high-temperature diffusion annealing in cobalt-base superalloy GH5605

Abstract

The dendritic morphology, elements segregation index, precipitates morphology, and precipitates types in GH5605 ingot produced by vacuum induction melting and electroslag remelting were investigated by using optical microscopy (OM), field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS) spectrum analysis and the results of thermodynamic and kinetic calculations by Thermal-Calc and JMatPro sofeware. To study the effects of high-temperature diffusion annealing on GH5605 ingot, the annealing system was investigated and the microstructure and macrostructure characteristics of GH5605 ingot were analyzed before and after the diffusion annealing by differential scanning calorimetry (DSC) and thermal compression simulation tests in Gleeble 3800 test machine. In the OM results, dendrites are not obvious, and secondary dendritic arms cannot be distinguished in the GH5605 surface but they are gradually clearer toward the center area. The EDS results show that element segregation index is comparably small in GH5605 ingot; every element segregation index is in the range of 0. 9-1. 4 which is not as large as those of nickelbased superalloy. The main segregating elements during solidification are Cr and W which mainly segregate in the dendritic regions.According to the FESEM results, the precipitate phases include austenite and grain boundary carbide M23C6 and because of the Cr and W segregation at dendritic arms, an unexpected eutectic phase comprising austenite and M23C6 appears, and the alternating lamellae of austentite and M23C6 develop a lathlike morphology. Different macrostructure and microstructure characteristics including the morphology of dendritic, elements segregation index, grain size, morphology and the amount of eutectic phase were analyzed and compared in different annealing times. The high-temperature diffusion annealing system is optimal at 1210 ℃/8 h, at which the dendrites and elemental segregation are substantially eliminated, and the eutectic phase is almost dissolved.