Improvement of γ' coarsening model in high γ' volume fraction Ni-base superalloys containing different Ta/W ratio

Abstract

The current research was carried out to improve the growth model of γ' precipitates in Ni-base superalloys with the high volume fraction of γ' precipitates and large lattice misfit (δ) of γ/γ'. The alloys were designed based on the Ni-base superalloys of CM247 LC with various Ta/W weight ratios from 0.15 to 2.3. Many studies performed so far about the kinetics of the γ' coarsening have been related to the NiCrAl or NiTiAl systems which incorporate insignificant lattice misfit of γ/γ'. However, in the current work, the coarsening kinetics of the γ' precipitates with the composition of Ni3(Al,Ta) was investigated in the superalloys containing high contents of Ta and high δ. For this purpose, the designed superalloys were exposed to long-term aging at 1000 °C for 200 h after being cast by Vacuum Induction Melting (VIM) followed by direct aging heat treatment. Microstructural observations were performed by an SEM, while lattice parameters of the γ and γ' phases were determined using XRD. The results indicate that cubic coarsening law is not valid for the γ' precipitates in high volume fraction condition, while the coarsening exponent (n) in the superalloys having a high volume fraction of γ' and large δ is a non-integer exponent satisfying 1.3 < n < 1.6 criterion. Meanwhile, under long-term aging at 1000 °C, the minimum coarsening rate (k) occurs for the γ' precipitates with the Ta/W ratio of about 0.5, because a combined effect of the maximum values of δ and high ordering degree in the γ' phase is recorded at this ratio. In fact, the elastic coherency strains in the γ matrix around the γ' precipitates increase with increasing the absolute value of δ and consequently the γ' coarsening rate is hindered. Moreover, the variations in the coarsening rate coefficient (k) in terms of δ revealed that the γ/γ' interface is coherent in the range of 0.1%< δ < 0.3%, while this interface becomes incoherent over 0.3%.