Investigation of the stress rupture behavior of GTD-111 superalloy melted by VIM/VAR

The effects of vacuum induction melting (VIM) and vacuum arc remelting (VAR) processes on the microstructure and stress rupture properties of Ni-based GTD-111 superalloy were investigated. Samples of GTD-111 master alloy were melted in VIM and VAR furnaces and then poured into a preheated ceramic mold for VIM melt or into a water-cooled copper mold for VAR melt. The as-cast samples were examined radiographically to ensure that no casting defects were present in the final castings; the samples were then heat-treated using a standard heat-treatment cycle. The microstructure of the samples was investigated using optical microscopy and scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy for microanalysis. On the basis of standard ASTM-E139, stress rupture tests were carried out at 1000°C under a stress of 300 MPa. The results showed that a γ matrix, fine γ′ precipitates, a γ–γ′ eutectic structure, carbide particles, and some harmful phases such as σ and η phases were present in the as-cast samples. The γ′ precipitates with cubic morphology appeared in the matrix after the standard heat-treatment process. The extent of segregation and the amount of γ–γ′ eutectic structure formed in the VAR-prepared sample were less than in the VIM-prepared sample. The results of stress rupture tests showed that the rupture time for the VAR sample was 43% longer than that for the VIM sample.