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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]

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]

Although their mechanical behavior has been extensively studied, the atomic-scale deformation mechanisms of metallic nanowires (NWs) with growth twins are not completely understood. Using our own atomic-scale and dynamic mechanical testing techniques, bending experiments were conducted on single-crystalline and twin-structural Ni NWs ( D = ∼40 nm) using a high-resolution transmission electron microscope (HRTEM). Atomic-scale and time-resolved dislocation nucleation and propagation activities were captured in situ . A large number of in situ HRTEM observations indicated strong effects from the twin thickness ( TT ) on dislocation type and glide system. In thick twin lamella ( TT > ∼12 nm) and single-crystalline NWs, the plasticity was controlled by full dislocation nucleation. For NWs with twin thicknesses of ∼9 nm < TT < ∼12 nm, full and partial dislocation nucleation occurred from the free surface, and the dislocations glided on multiple systems and interacted with each other during plastic deformation. For NWs with twin thicknesses of ∼6 nm < TT < ∼9 nm, partial dislocation nucleation from the free surface and the gliding of those dislocations on the plane that intersected the twin boundaries (TBs) were the dominant plasticity events. For the NWs with twin thicknesses of ∼1 nm < TT < ∼6 nm, the plasticity was accommodated by a partial dislocation nucleation process and glide parallel to the TBs. When TT < ∼1 nm, TB migration and detwinning processes resulting from partial dislocation nucleation and glide adjacent to the TBs were frequently observed. [ABSTRACT FROM AUTHOR]

Understanding the high-temperature initial oxidation mechanism and its dynamic processes is generally important because a small amount of initial oxidation can induce rapid corrosion and cause catastrophic failure. We reveal the mechanism of initial oxidation in a third-generation Ni-based superalloy by in situ visualizations and investigations of the nano- and atomic-scale dynamic processes from room temperature to 900 °C with a Cs-corrected environmental transmission electron microscope. The initial oxidation starts from the γ/γ′ interface at low temperatures. The high-temperature oxidation process with deficient oxygen prefers oxidation sites at the cross-junctions of the γ/γ′ interfaces. The growth rate of the oxide nanoparticles depends on the oxidation temperatures, with a low rate below 600 °C and a rapid-growth speed at temperatures higher than 700 °C. Mass transfer from the γ′ and γ phases to the γ/γ′ interface, particularly at the cross-junctions of these interfaces, is observed to cause oxide accumulation. This study provides a direct observation of the interphase, interface-junction-initiated outward oxidization including Al and Cr elements. These results shed light on the initial oxidation mechanisms of materials containing a second phase with interphase interfaces and the materials at deficient oxygen conditions as well as those with protection coating layers. [Display omitted] [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]