Lianxu Yu

Effect of B on microstructure and properties of low thermal expansion superalloy

Abstract The effect of B on microstructure and various properties including coefficient of thermal expansion (CTE), HV hardness, and both smooth and notch stress rupture properties of modified Thermo-Span alloy was studied. The results show that B hardly dissolves in matrix. Increasing B content constrains the formation of Laves phase and grain boundary (GB) precipitation of Laves and G phases, but promotes the formation of M(Co, Fe)NbB boride. In low B doped alloy, its intrinsic high susceptibility to intergranular cracks leads to reduced rupture life and notch sensitivity. Increasing B improves grain boundary cohesion, tying up vacancies and reducing GB diffusion, which constrains the nucleation and propagation of intergranular microcracks, prolongs the rupture life and eliminates the notch sensitivity in the new alloy. Compared with conventional Thermo-Span alloy, the B doped modified alloy shows lower CTE and improved notch sensitivity.

Precipitation behaviour of β and γ′ in high Al low thermal expansion superalloy

Abstract The precipitation behaviour of β and γ′ in a low thermal expansion superalloy IN783 was investigated. It was demonstrated that coarse γ′ precipitates do form during β aging at 845°C. During aging at lower temperatures, not only fine γ′ particles are formed, the coarse γ′ particles also change from spherical to cuboidal. Therefore, a bimodal distribution of γ′ is formed in the matrix after standard aging treatment. The formation of β consumes Al atoms, which constrains the formation of coarse γ′ in the zone around β during aging at 845°C. During γ′ aging, a fine γ′ precipitating zone is formed around β. Prolonging the β aging duration markedly promotes the formation of β at grain boundaries and in the matrix, significantly enlarges the fine γ′ precipitating zone and influences the precipitation of coarse γ′.

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.

The role of primary Laves phase on the crack initiation and propagation in Thermo-Span alloy

Abstract Thermo-Span alloy is an oxidation resistant low thermal expansion superalloy and shows good mechanical properties, which has been used to fabricate components in gas turbine engine to control the clearance between static and rotating part, and thus improve the efficiency. Owing to the insufficient Cr, it was not applicable for this alloy to employ long homogenisation treatment at high temperature, and consequently, the primary Laves phase formed during the solidification process might be remained in this alloy. However, few studies were carried out on studying the role of primary Laves phase in this alloy. This paper studied the effects of primary Laves phase particles on the crack initiation and propagation during tensile test at room temperature, 650 and 750°C of the rolled bar made from casting Thermo-Span alloy. It was found that at room temperature, cracks were mainly related with the Laves phase larger than 5 μm and the Laves clusters. At 650°C, cracks were formed preferentially at large Laves particles, but they would propagate faster at grain boundaries. At 750°C, as the hardening effect of γ’ phase was not effective, and the stress concentrated at large Laves phase would be released by grain deformation, so that large cracks were no longer widely spread and more intergranular cracks especially at surface of the specimen were found.