X.Z. Qin

Interfacial precipitation of the M5B3-type boride in Ni-based superalloys

Abstract Microstructural characteristics of an experimental Ni-based superalloy with boron addition subjected to a long-term ageing treatment were systematically investigated by various kinds of transmission electron microscopy technique. Based on detailed electron diffraction analyses, we found that there are many nanosized M5B3 precipitates in our long-term ageing alloys, which keeps a good orientation relationship with the γ/γ′ matrix. Furthermore, the precipitation characteristics of M5B3 phase were clarified. It is found that the M5B3-type boride prefers to precipitate at the γ/γ′ interfaces and low-angle grain boundaries. These interfacial nanosized precipitates can play the role of pinning effect and are expected to be advantageous for postponing the γ′ rafting and low-angle grain boundary migration to some extent at high temperature.

Degeneration of Primary MC Carbide in a Cast Ni-Base Superalloy

During long-term thermal exposure, the degeneration of primary MC carbide is a diffusion-controlled process. With the element exchange between the primary MC carbide and the γ matrix, three MC degeneration reactions, that is, γ + MC → γ + M23C6 (Reaction I), MC + γ →η+ M23C6 (Reaction II) and MC + γ →η+ M6C (Reaction III), sequentially operates. Of them Reaction III has never been reported in the previous references and is kinetically most difficulty. It is also shown that the grain boundary MC decomposes much more rapidly than the grain interior MC and that Reaction III is not as often seen at the grain boundary as in the grain interior.

Microstructural characterization of the M23C6 carbide in a long-term aged Ni-based superalloy

Abstract The microstructural features of M23C6 carbide in a long-term aged heat- and corrosion-resistant Ni-based superalloy have been investigated in detail using various kinds of transmission electron microscope (TEM) techniques. It is found that TEM contrast, which is related to structural and chemical inhomogeneities inside the grains, always exists in the interior of grains in the alloy. The structure of these inhomogeneous regions has been determined to be the same as that of the γ′ and t-M23C6 phases, where t-M23C6 indicates a transitional and metastable phase. Although possessing the same structure as the M23C6 phase, the chemical composition of the t-M23C6 is different from that of the M23C6 phase. Compared with M23C6, t-M23C6 is richer in Ni, Co, Al and Ti but poorer in W, Mo and Cr. This phenomenon of structural and chemical inhomogeneity demonstrates that pristine M23C6 carbide (p-M23C6) precipitated in standard heat-treated samples is unstable. Therefore, upon long-term ageing treatment, Ni, Co, Al and Ti may locally enrich inside the p-M23C6 phase, finally forming the γ′ phase, which can be described by the decomposition reaction p-M23C6 → M23C6 + γ′.

Effects of Nb/Ti Ratio on the Microstructures of Two Experimental Ni-Based Cast Superalloys

Effects of Nb/Ti ratio on the microstructures of two experimental Ni-based cast superalloys of the Nb/Ti ratio 0.46 and 0.03 were investigated. The Nb/Ti ratio had a significant effect on the dendritic microstructures, especially the γ precipitate, of the two alloys under the as-cast condition. The amount and composition of primary MC carbides were closely related to the Nb/Ti ratio for that the Nb atoms have a stronger tendency to form the MC carbides than the Ti atoms. During thermal exposure, the primary MC carbides gradually degraded through two reactions: firstly MC + γ M6C + γ and then MC + γ M23C6 + γ. The Nb/Ti ratio had little influence on the reaction formation but obvious influence on the decomposition degree of the primary MC carbides decomposition. In addition, the second M6C and M23C6 carbides on the boundaries (GBs) gradually formed a continuous chain during thermal exposure. The γ-free zones appeared along the GBs as the growth of the second carbides and γ precipitates on the GBs when thermal exposure time last up to 3000 h.

Microstructural characterization of the η-Ni3(Ti, Al) phase in a long-term-aged Ni-based superalloy

Abstract Microstructural features of a geometrically closed packed hexagonal η-Ni3(Ti, Al) phase precipitated in a long-term-aged Ni-based superalloy are characterised using various kinds of transmission electron microscope (TEM) techniques. The defect features in the η grain interior are clearly revealed at the atomic scale. A nano-sized γ′ phase is found at the defect regions of the micro-sized η phase. The interior γ′ phase keeps a good orientation relationship (OR) of (1 1 1)γ′//(0 0 0 1)η and [1 0]γ′//[2 0]η with the η-Ni3(Ti, Al) phase. However, there is no definitive OR between η-Ni3(Ti, Al) grains and the surrounding matrix. By means of first-principle calculations, we reveal that a symmetrical interfacial feature between γ′ and η corresponds to a lower energy state.