Re-recognition of the Effects of Phosphorus and Boron on the γ″ and γ′ Phases in IN718 Alloy

The effects of P and B on the matrix strength and precipitations of γ′ and γ″ phases in the grain interior were re-recognized in this study. The combination addition of P and B markedly accelerated the precipitations of γ′ and γ″ phases and strengthened the matrix of IN718 alloy when air-cooled from high temperature, while made no difference when water-quenched from high temperature. The effect of single addition of P on the precipitations of γ″ and γ′ phases was the same with that of the combination addition of P and B, while the single addition of B had no effect on the precipitations of the two phases. Therefore, it was P rather than B which accelerated the precipitations of γ′ and γ″ phases. P could take part in the precipitations of γ′ and γ″ phases, which was revealed by electrochemical extraction and quantitative analysis of chemical composition. It also revealed that P atoms were dissolved in the γ matrix to a relatively high degree at the temperature that γ′ and γ″ phases began to precipitate, and consequently the precipitations of γ′ and γ″ phases were accelerated. The first-principle calculation indicated that P decreased the formation enthalpies of γ′ and γ″ phases when it occupied the Ni lattice sites in the two phases, which explained the effect of P on the γ′ and γ″ phases.