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Oxidation behaviors of three Ni-based model alloys and pure Ni in the temperature range of 700–1200 °C are investigated to reveal effects of Cr, Al, and Si on the oxidation resistance of Ni-based superalloys. The formation and integrity of consecutive chromia or alumina layers are important for excellent oxidation resistance. The addition of 20 at.% Cr can effectively improve the oxidation resistance of Ni-based alloys by forming a thin chromia film below 1000 °C, while adding 15 at.% Al has a beneficial effect on the oxidation resistance of Ni-based alloys at temperatures above 900 °C. The addition of 2 at.% Si to Ni-Al alloy is insufficient to form a protective SiO2 layer but can accelerate the formation of alumina, which enables Ni-Al alloy to form a consecutive inner alumina layer at a relatively low temperature of 800 °C and further improve the oxidation resistance above 800 °C. [ABSTRACT FROM AUTHOR]

In this study, we investigate the movement of dislocations in Ni-based superalloys under the influence of lattice misfit stresses between the ordered γ'-cuboids and the disordered γ-matrix during thermal exposure in the absence of an applied load. This study focuses on a different condition than the conventional creep testing, and thus offers a unique opportunity to study the intrinsic behavior of the alloy. The dislocation density increases substantially with time during thermal exposure, leading to the formation of various configurations of dislocation networks on the {100} γ/γ′ interfaces, including <110> diamond-shaped, <110>/<100> mixed polygon-shaped and <100> square-shaped networks. During thermal exposure, the b = 1 2 < 110 > native dislocations first move and evolve into 60° mixed dislocations along the <110> directions on the {100} γ/γ′ interfaces, forming the <110> diamond-shaped dislocation networks. In the case of longer thermal exposures, the dislocations further evolve into pure edge dislocations along the <100> on the {100} γ/γ′ interface, leading to the evolution of the <110> diamond-shaped dislocation networks into <100> square-shaped networks, with the mixed <110>/<100> dislocation networks as an intermediate stage during the transition. These movements occur by sweep glide in the {111} planes and diagonal climb on the {100} planes for the edge and mixed dislocations and by cross-slip for the screw dislocations. The driving force for all of these movements is the interaction between the normal misfit stresses and the edge components of the Burgers vectors of the dislocations to relax the misfit stresses. An analysis based on the elastic strain energy considerations is presented to explain the driving forces for the dislocation movements. [ABSTRACT FROM AUTHOR]

To determine the influence of Re and Ru on the phase morphologies and microstructure in Ni-based superalloys, a series of model alloys derived from the 4th generation TMS-138 superalloy have been investigated with systemically changing Re and Ru additions. All alloys are subjected to solid solution and aging treatments to achieve the typical γ/γ′ two phases microstructure. Phase transition temperatures are measured by using differential scanning calorimetry. The Re is found to increase the γ′-solvus temperature with a slope of roughly 5.4 ℃ per wt% Re in Ru-free alloys and 2.3 ℃ per wt% Re in 2Ru alloys, while the Ru exhibits much weaker effect. Microstructure and composition of the two phases are examined to understand the Re and Ru effects on γ′ size, shape, volume fraction, the elemental partition behavior, and the γ/γ′ lattice misfit. The results show that the addition of Re affects size and shape of γ′ precipitates, and also reduces γ/γ′ lattice misfit significantly. Meanwhile, Re increases partition coefficients of Cr and Ru. In comparison, the addition of Ru seems to have little effect on morphology of γ′ precipitates and the γ/γ′ lattice misfit, but Ru appears to alter partitioning behaviors of other elements. Because Re and Ru are two key elements for high generation Ni-based single crystal (SX) superalloys, this work might provide guidelines for designing next generation of SX superalloys. [Display omitted] • Re effectively increases the γ′ solvus temperature of superalloys, while Ru has slight effect on the γ′ solvus temperature. • Re addition decreases the size of γ′ phase and γ/γ′ lattice misfit, facilitating cubic precipitation of γ′ phase. • Ru addition only has little effect on the γ′ morphology and γ/γ′ lattice misfit of superalloys. • Partitioning behaviors of other alloying elements depend on the Re and Ru additions, especially Ru. [ABSTRACT FROM AUTHOR]

Two distinct evolutionary processes of secondary γ′ precipitates in a Ni-based single crystal superalloy were revealed by means of Cs-corrected high-resolution transmission electron microscopy (HRTEM) and corresponding X-ray energy dispersive spectroscopy analyses. Upon creep testing at elevated temperatures, it was determined that large quantities of secondary γ′ precipitates were formed. The precipitates were found to select two distinctive forms and routes: one randomly dispersed in the γ matrix (2nd γ′ γ ), and the other formed at nodes of dislocation networks (2nd γ′ d ) in regular arrays. These two types of secondary γ′ precipitates exhibited different evolutionary morphologies and mechanisms. The 2nd γ′ γ precipitates progressively evolved in shape from spherical to cuboidal and then to butterfly-like shapes, and the mean size also increased gradually from 11 nm to 34 nm. The 2nd γ′ d precipitates exhibited stable cuboidal morphology and a mean size of ∼30 nm. These differences are attributable to the different element transport conditions for the two types of precipitates, i.e., an uphill diffusion for the 2nd γ′ γ precipitates and a dislocation network pipe transportation for the 2nd γ′ d precipitates. The 2nd γ′ γ precipitates show severe segregation of γ′-rich elements of Ni and Al and deficiency of γ′-poor elements of Cr compared with the 2nd γ′ d precipitates. [ABSTRACT FROM AUTHOR]

In their comment (Scripta Mater. (2016)), Cormier et al. raised questions on the findings reported in our earlier publication (Acta Mater. (2016) of the formation of 2nd γ′ d and 2nd γ′ γ precipitates in a Ni-based single crystal superalloy during creep testing at elevated temperatures, and suggested that the 2nd γ′ d particles were mere observations of surface protuberances of the primary γ′ rafts and that the 2nd γ′ γ precipitates were possibly formed during cooling after the creep test. To clarify these, we conducted new experiments, and present the new evidence and our responses to the points raised in this short communication. [ABSTRACT FROM AUTHOR]

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. Unlabelled Image • The elastic stretching alters the partitioning of elastic strains in two phases, whereas the misfit remains unchanged. • The elastic strain energy and hydrostatic stress anisotropies are ruled out to be responsible for rafting. • The anisotropic dislocation on γ'/γ interfaces wtith γ' phase cuboid morphology unchanged were created by pre-straining. • The dislocation anisotropy is effective in causing rafting in Ni-based superalloys by causing misfit anisotropy. [ABSTRACT FROM AUTHOR]

Abstract 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. Graphical abstract Unlabelled Image Highlights • Different anisotropic dislocation network structures were created by pre-straining at an elevated temperature. • The pre-existing anisotropic dislocation structures have no influence on the rafting behavior of the γ′ phase cuboids. • The pre-existing dislocations promote the formation of TCP phases during creep testing. • All pre-strained samples showed significantly reduced creep life. [ABSTRACT FROM AUTHOR]