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

The precipitation of eta phase and its effects on stress rupture properties of K4750 alloy were investigated. During aging at 850 degrees C for 50-500 h, thin plate-like eta phase were firstly precipitated at grain boundaries (GBs) and around MC carbides, then grew into grain interiors to form a Widmanstadter morphology. The ri phase around MC carbides was considered to be the product of MC degradation which was in reaction of MC + gamma -> M23C6 t eta The eta phase had an orientation relationship with gamma matrix: {0001}(eta)//{111}gamma, < 11<(2)over bar>0 >(eta) // < 1<(1)over bar>0 >gamma. As the growth of phase, gamma' phase was absorbed by eta phase and led to the formation of gamma' precipitation free zones (PFZs). This was mainly because both gamma' and t phases were enriched in Ti and Ni, and the dissolution of gamma' released Ti and Ni for the formation of n phase. As the aging time prolonged, stacking faults (SFs) were induced in some coarsening gamma' phase. The SFs within gamma' offered sites for the nucleation of eta phase, which facilitated the formation of eta phase in grain interiors. To clarify the effects of eta phase on stress rupture properties, samples were carried out the standard heat treatment (SHT), SHT +850 degrees C/300 h and SHT + 850 C/300 h + SHT, respectively. And eta phase could only be observed in the sample having been subjected to SHT +850 degrees C/300 h. After above three heat treatments, the stress rupture life of K4750 alloy tested at 750 degrees 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 eta phase and the degradation of MC carbides, which would be analyzed in this paper.