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• The orientation rotation path of nearly [001] oriented specimens during the intermediate temperature creep was revealed. • The dominant slip system at different creep stages at intermediate temperature was determined. • The influence of {111} 〈110〉 and {111} 〈112〉 slip systems on different creep stages at intermediate temperature was evaluated. • The orientation sensitivity of creep life in near [001] oriented Ni-based single crystal superalloys at intermediate temperatures was unveiled. Although blades with a deviation angle of less than 15° between the blade stacking axis and the [001] orientation are qualified in the industry, the creep life of samples near [001] orientation exhibits significant anisotropy at intermediate temperatures. Those crystals having orientations within 15° from precise [001] exhibited significantly longer lives when their orientations were closer to the [001]-[101] boundary of the stereographic triangle than to the [001]-[111] boundary. Here, we first investigated the orientation rotation path of specimens near [001] orientation during creep at 750 °C/750 MPa, then revealed the dominant slip systems at different creep stages. Subsequently, we evaluated the effect of orientation deviation from precise [001] on creep properties. Finally, our research revealed the orientation sensitive mechanism of creep life in near [001] oriented Ni-based single crystal superalloys at intermediates. [ABSTRACT FROM AUTHOR]

Three-dimensional atom-probe tomography and scanning electron microscopy were utilized for an in-depth examination of the formation of secondary γ′ precipitates and the composition difference between secondary γ′ precipitates and primary γ′ precipitates in a Re-containing Ni-based single crystal superalloy. It was found that the secondary γ′ precipitates was formed during the air cooling after primary aging of 1180 °C/4h and would grow up during the secondary aging at 870 °C. Al and Ta were mainly distributed in γ′ precipitates (including the primary and the secondary γ′ precipitates) and Mo, Cr, Co and Re were more distributed in γ matrix. Compared with the primary γ′ precipitates, the secondary γ′ precipitates contained more γ-forming elements, such as Re, Cr, W and Mo, while the content of Ta was less. The distribution of W in the secondary γ′ precipitates was much higher than that in the primary γ′ precipitates and was equal to that in γ matrix. Based on the diffusion constant of alloying elements in Ni, the diffusion distance of various elements was as followed during the air cooling after primary aging: Re < W < Co < Mo < Ni < Ta < Cr < Al. The insufficient diffusion distance of γ-forming elements was an important factor leading to their enrichment in the secondary γ′ precipitates. And the Re element might hinder the diffusion of other γ-forming elements, such as Cr and Mo. • 3D-APT was utilized for an in-depth examination for secondary γ′ precipitates. • Secondary γ′ precipitates were formed and grown during air cooling. • The chemical composition of secondary γ′ precipitates was clarified. • Secondary γ′ precipitates contained more γ-forming elements and W element. • The diffusion distance of elements in γ matrix was calculated during air cooling. [ABSTRACT FROM AUTHOR]

The topologically close-packed (TCP) phases with R/P intergrowth structure formed at the final stage of directional solidification in a Ru-containing nickel-based single crystal (Ni-SX) superalloy were investigated. The nodular TCP phases were distributed in the interdendritic regions of the as-cast microstructure. Precipitation of the β-NiAl phases and the subsequent incomplete peritectic reaction (Liquid + β-NiAl → γ′ + β-NiAl residual) resulted in the accumulation of the TCP-forming elements (Cr, Co, Mo, W, Ta, and Re) in the residual liquid around the β-NiAl phase, which provided a favorable nucleation condition, leading to the precipitation of TCP phase in the vicinity of the β-NiAl phases. In addition, the TCP phases were found to be the R/P intergrowth structure with an orientation relationship of [2,7,10] R //[001] P and (32 2 ¯) R //(200) P , (4 4 ¯ 2) R ∼3.2° from (1 1 ¯ 0) P. Further, the formation mechanism of these TCP phases with R/P intergrowth structure was systematically discussed in terms of crystal structure, interface characteristics, and chemical composition. The result can provide theoretical guidance and experimental data for the composition design and heat treatment process optimization of the novel Ru-containing high-generation Ni-SX superalloys. [Display omitted] • TCP phase with R/P intergrowth structure was found in an as-cast Ru-containing Ni-based single crystal superalloy. • The microstructural characteristics and the formation mechanism of TCP phase with R/P intergrowth structure were studied. • The accumulation of TCP phase formers in the residual liquid near the β-NiAl phases led to the precipitation of TCP phase. • Homologous crystal structure, coherent R/P interface, and similar composition promoted the nucleation of P phase on R phase. [ABSTRACT FROM AUTHOR]

The Ni-based single crystal superalloy in [011] orientation exhibited poor creep properties at intermediate temperatures. Detailed knowledge of the damage mechanism was still not deeply understood. Here, the damage mechanism resulting from the orientation rotation was proposed. The deformation microstructure depended on the distance from the fracture; at the farther position, there were micro-twins, while at the nearer position were stacking faults. Both micro-twins and stacking faults all originated from the activity of a single {111} <112> slip system. The activity of a single {111} <112> slip system would lead to orientation rotation, which was asynchrony due to the sample being laterally constrained. The asynchrony of orientation rotation would cause nonuniform deformation and localized force inhomogeneity, leading to premature failure of the specimen. • The microstructure evolution of a [011]-oriented Ni-based single crystal superalloy at 750 °C/750 MPa creep was studied by multi-scale characterization. • The deformation microstructure depended on the distance from the fracture. • The activity of a single {111} <112> slip system dominated the deformation process and led to orientation rotation. • The damage mechanism resulted from the orientation rotation, in which the asynchrony of orientation rotation would cause nonuniform deformation and localized force inhomogeneity, leading to premature failure of the specimen. [ABSTRACT FROM AUTHOR]

Two Nickel-based single crystal superalloys (one Ru-free and one containing 3 wt.% Ru) were comparatively investigated by the three-dimensional atom probe tomography and the nanoindentation atomic force microscope to reveal the effect of Ru addition on the microstructure and the mechanical properties of γ' phases after standard heat treatment. The results showed that the Ru addition neither altered the morphology and the volume fraction of γ' phases, nor changed the interface structure and the partition coefficients of alloying elements between γ and γ' phases, however, it enhanced the hardness and elastic modulus of γ' phases. First-principles calculation indicated that no matter whether the Ni site or Al site was occupied by Ru, the elastic modulus of γ' phases increased, which might be ascribed to the solution strengthening effect of Ru on γ' phases. [Display omitted] • The effect of 3 wt.% Ru addition on elemental partitioning coefficients was determined by 3D-APT. • The effect of 3 wt.% Ru addition on on the elastic modulus of γ′ precipitates was studied by NI-AFM. • The strengthening mechanism resulted from Ru element partition on γ' phases was clarified. [ABSTRACT FROM AUTHOR]