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The distribution of Rhenium (Re) in a Ni-based single-crystal superalloy is studied by sub-angstrom resolution transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). It is found that Re atoms segregate at the tensile stress regions near the interfacial dislocation cores, forming the “Cottrell atmosphere”, and the segregation process is facilitated by dislocation pipe diffusion. In situ TEM and scanning electron microscopy (SEM) straining studies reveal that the Re-decorated dislocation networks along the phase boundaries act as mechanical walls that effectively block dislocation motion and crack propagation. Furthermore, the degree of Re segregation can be regulated by thermal treatment. Theoretical analysis demonstrates that this remarkable alloying effect originated mainly from the interactions between the local composition strain of Re and the dislocation strains, leading to significantly stabilized interfacial dislocation networks. These results provide a new perspective on understanding the origin of the Re effect on mechanical properties in Ni-based superalloys and will be beneficial to both improving creep properties of Ni-based superalloys and designing high-performance Re-free superalloys. [ABSTRACT FROM AUTHOR]

Topologically close-packed (TCP) phases, although inherently hard, are commonly perceived as detrimental in various alloys. In Ni-based single crystal superalloys (Ni-SXs), the primary adverse impact of TCP phases on material mechanical properties stems from their direct propagation, disrupting the original γ/γ′ dual-phase structure and leading to cleavage along the TCP phases under applied stress. Understanding the specific reasons for the straight propagation of TCP phases through the complex phase structure is essential for mitigating or potentially reversing their detrimental effects on material performance, a challenge that remains elusive. This study addressed this challenge by employing ultra-high spatial and energy resolution transmission electron microscopy (TEM) characterization, coupled with in situ scanning transmission electron microscopy (STEM) heating experiments in a fourth-generation Ni-based single crystal superalloy. Our research unveiled a significant phase-separation process during the incubation stage preceding σ phase formation at elevated temperatures, driven by the diffusion of atoms. Notably, domains of the γ′ phase, characterized by a scale of tens of nanometers, emerged within preexisting γ phases, and vice versa. This phenomenon reconstructed the original γ/γ′ dual-phase structure, leading to a finely refined dual-phase structure. The refined structure effectively reduced the diffusion distance of alloy elements necessary for forming σ phases. Consequently, the σ phases nucleated randomly and propagated directly through the newly refined γ/γ′ dual-phase structure. Throughout the thickening process, the notable difference in the diffusivity of alloying elements eventually resulted in the growth of a highly defective σ phase structure. [Display omitted] [ABSTRACT FROM AUTHOR]

This work investigates the evolution of the crystal structure of the topologically closed packed orthorhombic P phase precipitate in a Re-containing Ni-based single crystal superalloy during thermal exposure. The P phase is formed with a thin needle morphology. The precipitate is formed from the matrix with an initial complex atomic arrangement which continues to evolve during the process of thermal exposure. Based on the experimental evidence of this study and the theoretically predicted structure reported in the literature, a mechanism of the structural transformation is proposed. The initial structure is composed of a parallelogram (P) atomic arrangement configuration, which gradually evolve into a rectangle (R) atomic arrangement configuration in the [100] P projection. In the [010] projection, the initial structure is composed of alternating rows of a larger parallelogram (P') and a larger rectangle (R') configurations, which gradually evolve into an intricate structure of P'-R' along the length [001] P direction and P'P'-R'R' along the transverse [100] P direction. The initial structure is formed for its structural similarities to the γ phase to minimize lattice mismatch. The final structure is evolved over time to conform to its thermodynamically more stable state. The structural evolution is achieved by collective atomic shuffling. These intricate atomic arrangements gives rise to the very faint and highly dense parallel striation lines along the transverse [010] P direction in [100] P view and along the length [010] P direction in [010] P view under transmission electron microscopy observation. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Advanced single-crystal superalloys, developed to improve the high temperature capability of gas turbines, often underperform in low-temperature conditions. This limitation compromises the overall reliability of turbine blades under complex service environments. This study introduces a novel strategy to enhance the overall performance of a fourth-generation single-crystal superalloy. Rapid liquid-nitrogen quenching following the aging treatment promotes extensive nucleation of ultrafine γ′ particles (γ′ γ) with an average size of ∼10 nm, precipitated under extremely low supersaturation, as evidenced by high-resolution transmission electron microscopy (HRTEM) and atom probe tomography investigations. The combination of differential scanning calorimetry analysis, density functional theory calculations, and kinetic models verifies the high stability of γ′ γ particles at low-temperature conditions around 900 °C. HRTEM observations reveal Orowan bypassing and dislocation cutting of γ′ γ particles, leading to additional precipitation hardening by limiting dislocation motion in the γ phase and shear of primary precipitates. Consequently, a bimodal precipitation microstructure comprising γ′ γ particles and primary γ′ phase significantly reduces the strain rate of low-temperature creep and nearly doubles the creep rupture life at 800 °C/735 MPa. Simultaneously, high-temperature properties at 900 °C/392 MPa and 1100 °C/137 MPa remain comparable to those of the original microstructure as the primary γ/γ′ microstructure is minimally affected by rapid cooling. This advancement improves overall performance and redefines the importance of small precipitates, broadening microstructure design concepts for future superalloy development. [Display omitted] [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]

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]

The chemical and structural evolution within dislocation networks are investigated in a nickel-based single crystal superalloy crept under 1373K and 137MPa. The local region within dislocation networks can be divided to two separated parts: gridlines which compose the arrangements of dislocation networks and mesh regions as intervals between gridlines. With evolution of dislocation networks, precipitation of nano-sized γ′ phase (γ′ n) are observed in mesh region, while gridlines always display the FCC feature of γ phase. The formative mechanism of γ′ n is revealed as increased segregations of Cr, Co and Re along gridlines which enhance the contents of γ′-rich elements such as Al in mesh region to promote the local phase transition from FCC to highly ordered L1 2. However, growth of γ′ n requires massive diffusion of γ′-rich elements that is confined by the chemical barrier along gridlines with strong segregations of Cr, Co and Re. The mesh region of γ′ n and gridlines of γ reinforce each other forming potential γ/γ′ substructures to stabilize the dislocation networks during creep. Denser dislocation networks formed in dendrite core under larger local lattice misfit are accompanied with facilitated formation of γ/γ′ substructures which derive higher creep resistance than interdendritic region. Image, graphical abstract [ABSTRACT FROM AUTHOR]