Precipitation of η phase and its effects on stress rupture properties of K4750 alloy

Abstract

The precipitation of η phase and its effects on stress rupture properties of K4750 alloy were investigated. During aging at 850 °C for 50–500 h, thin plate-like η phase were firstly precipitated at grain boundaries (GBs) and around MC carbides, then grew into grain interiors to form a Widmanstädter morphology. The η phase around MC carbides was considered to be the product of MC degradation which was in reaction of MC+ γ→ M23C6+ η. The η phase had an orientation relationship with γ matrix: 0001η//111γ,<112‾0>η//<11‾0>γ. As the growth of η phase, γ′ phase was absorbed by η phase and led to the formation of γ′ precipitation free zones (PFZs). This was mainly because both γ′ and η phases were enriched in Ti and Ni, and the dissolution of γ′ released Ti and Ni for the formation of η phase. As the aging time prolonged, stacking faults (SFs) were induced in some coarsening γ′ phase. The SFs within γ′ offered sites for the nucleation of η phase, which facilitated the formation of η phase in grain interiors. To clarify the effects of η phase on stress rupture properties, samples were carried out the standard heat treatment (SHT), SHT+850 °C/300 h and SHT+850 °C/300 h + SHT, respectively. And η phase could only be observed in the sample having been subjected to SHT+850 °C/300 h. After above three heat treatments, the stress rupture life of K4750 alloy tested at 750 °C/360 MPa was 506.00 h, 99.55 h and 447.08 h, and the elongation was 13%, 22% and 8.7%, respectively. The change of stress rupture properties was closely related to the formation of η phase and the degradation of MC carbides, which would be analyzed in this paper.