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This work investigates the effect of Ru on the precipitation of the topologically close-packed (TCP) phase from the γ-γʹ microstructural degradation analysis. The addition of Ru is found to delay the precipitation time of the TCP phase from 100 h to 1000 h after thermal exposure at 950 °C. The effect of Ru on the γ-γʹ microstructural degradation is also investigated. Adding Ru slightly decreases the coarsening rate of the γʹ size and promotes the reduction of the volume fraction of γʹ phase. The reduction of the volume fraction of γʹ phase causes the ''inverse partitioning'' effect and increases the solution limit of the TCP forming element in the matrix. These help inhibit the precipitation of the TCP phases in Ni-based single crystal superalloys during thermal exposure.

This work investigates the effect of microstructural degradation on the overtemperature rupture performance in Ru-containing Ni-based single crystal superalloys. Unlike the initial samples, the rupture lives in the degraded sample are largely lost, and adding Ru in the degraded sample increases the rupture life at 1120 °C/100 MPa and 1180 °C/100 MPa. Based on the difference in the microstructure and microstructural evolution in the initial and degraded samples. Their influence on the rupture performance can be derived. The enlarged width of the γ matrix is the major reason for the reduction of the rupture life and increase in the strain for the degraded sample compared to the initial samples. The existence of the dislocation network and the inhibition of the precipitation of the TCP phase are the major reasons for the lower rupture life reduction in the 5Ru degraded sample than in the 3Ru degraded sample. The mobility of the dislocation increases with the temperature, weakening the benefit effect caused by Ru.

Hydrogen-fueled and hydrogen-hybridized aircraft engines are a new trend in the aviation industry for environmental reasons. Single crystalline Ni-based superalloys are the most commonly used engine materials and their hydrogen embrittlement properties need urgent investigation. In this study, the hydrogen embrittlement behavior and underlying fracture mechanism of a second-generation Ni-based single crystal superalloy with electrochemical hydrogen pre-charge were investigated. The superalloy showed tremendous susceptibility to hydrogen embrittlement with reduced strength and ductility. A large number of micropores and cracks on the fracture surface are found in hydrogen-charged specimens, leading to embrittlement and ultimate cracking. More dislocations, stacking faults and DSBs are observed in specimens with hydrogen uptake. Hydrogen-induced micropores first form at the γ/γ′ interface and then propagate into the γ′ phase, leading to cracking, which was analyzed using in situ environmental studies with a transmission electron microscope. Hydrogen reduces the cohesive strength between the γ- and γ′-phase and accelerates crack propagation along the voids. Hydrogen embrittlement fracture in Ni-based single crystal superalloys is due to synergistic hydrogen-enhanced local plasticity, strain-induced vacancies and decohesion in the hydrogen-induced cracking process.

In this work, the effect of directional solidification methods on solid solution windows in two Ni-based single-crystal superalloys is investigated. The sample prepared using the liquid metal cooling (LMC) method was found to contain smaller dendrite spacing, lower dendrite segregation and eutectic, and a smaller size of γ′ phase than that of the high-speed rapid solidification (HRS) method. The alloy-2 contained higher Cr, Co and lowered Re exhibited similar dendrite spacing and volume fraction of eutectic, and the lower dendrite segregation and smaller size of γ′ phase than that of the other alloy-1. The solution treatment window is broadened by adopting the LMC method or changing the composition to alloy-2. This is mainly realized by decreasing the lower limit temperature, and the upper limit temperature remains unchanged. According to the eutectic only contains the melt temperature, a similar eutectic is responsible for the unchanged upper limit temperature. At this state, the lower bound of the temperature is the dissolution of the γʹ phase in the dendrite core. The smaller size of γ′ phase and the elements' partition ratio are responsible for the change in the solid solution window.

It is known that Ni-based superalloys experience microstructural rafting during high temperature straining. This paper presents a comparative study of the different rafting models by means of finite element modeling and experimentation investigation. It was found that uniaxial elastic loading does not alter the misfit or misfit isotropy of the structure, but causes repartitioning of the misfit into elastic strains in the γ and γ' phases. This causes an elastic strain energy anisotropy and hydrostatic stress anisotropy among the vertical and horizontal γ phase channels. Both anisotropies predict a raft structure that is contrary to the experimental observation. Through a carefully designed pre-treatment at 750 °C and under a uniaxial stress of 750 MPa, an anisotropic dislocation structure was created on the {001} γ'/γ interfaces without altering the cuboidal morphology of the γ' phase. This allows the evaluation of the influence of dislocations on the rafting behavior of the alloy. It was found that rafting occurred in the pre-treated samples containing anisotropic dislocation structures during thermal exposure without applied stress, instead of isotropic corsening, and that the raft structures conform to the expectations base don the anistropic dislocation strucutres. This demonstrates that an anisotropic dislocation structure on the {001} γ'/γ interfaces is a direct cause of rafting. This is attributted to the fact that dislocations on γ'/γ interfaces help to relax the local misfit strains, causing some to expand at the expense of others. At the same time, the formation of the anisotropic dislocation structure is a direct result of the anisotropy of elastic strains induced by the applied bia stess in during the pre-treatment. These explain the complex interplay of the elastic and plastic models reported the literature.

This study investigated a novel hypothesis on the influences of residual or pre-dislocation structures on creep life of Ni-based single crystal superalloys. Different types of anisotropic dislocation structures were created by pre-straining the samples at a relatively low temperature under different constant bias load conditions. After such pre-straining treatment, the samples were subjected to creep testing at higher temperatures under uniaxial tension. It was found that the pre-straining treatment was effective in creating different anisotropic dislocation structures without affecting the γ′ phase cuboid morphology and that the samples containing different anisotropic dislocation structures have all significantly reduced creep lives. The difference in the anisotropy of the dislocation structures has little influence on the reduction of the creep life or the rafting morphology of the γ′ phase cuboids. By using a Re-free alloy, the influence of the TCP phase precipitation on creep life is also clarified. These observations suggest that interruption to high temperature straining is the main contributor to the reduced creep lives. This finding raises a serious concern on the validity, or reliability, of creep life assessment based on continuous laboratory creep tests for real life applications, where the services are almost always interrupted into intermittent sessions. Keywords: Ni-based superalloys, Creep, Dislocation structures, Anisotropy, Pre-straining treatment