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The important factors limiting the fourth-generation Ni-based single-crystal superalloys (Ni-SXs) are the increases in cost and density. A cheaper and lighter potential alternative element of Ru, Co's comprehensive effects were studied. The effects of Co and inhomogeneity between dendrite core (DC) and interdendritic region (ID) on creep performance at 1100 °C and 137 MPa in a 4th generation Ni-SXs were systematically investigated. The results show that the Ni-SXs with 9 wt% Co has the longest creep lifetime. Co additions alleviate the overall element segregations between DC and ID, especially Re, Cr, and W. And reverse partitioning behavior of Co also changes the partitioning ratio of elements between γ and γ′ phases, resulting in the inhibition of TCP phases precipitation for most creep time to avoid excessive consumption of solid solution strengthening elements and local stress concentration. The decrease of the lattice constant of γ reduces lattice misfit between γ and γ′ with the addition of Co, leading to the enlargement of dislocation network spacing and less resistance for movement or shearing of dislocation. The decrease of γ′ solvus temperature with Co addition promotes the dissolution of γ′ at high working temperature, resulting in severe topological inversion of γ/γ′ microstructures at ID. In addition, the main creep cracks of all Ni-SXs appear at ID, so the inhomogeneity between DC and ID is concerned. • The precipitation of TCP phases are inhibited with Co addition, which is a basis of low-cost and low-density alloy design. • The comprehensive effects of Co on high temperature creep properties are investigated and an optimal Co content exists. • The increase of Co affected the γ′ phase content, lattice mismatch, element distribution and TCP phase. • The effect of Co on microstructures evolution varies at different creep stages. • The inhomogeneity between dendrite core and interdendritic region causes creep cracks originate from interdendritic region. [ABSTRACT FROM AUTHOR]

The plastic deformation behavior of three various orientation specimens of a nickel-based single-crystal (SX) superalloy were investigated by in-situ electron back-scattered diffraction (EBSD) at room temperature. Tensile properties, slip morphology, the evolution of Schmid factor, lattice rotation, and local strain were thoroughly discussed. In [1 1 13] specimen, the interaction of multi slip behavior induced local strain concentration, which restricted the plasticity but enhanced the strain hardening. Due to the single slip behavior, lower tensile strength and higher elongation were discovered in [9 1 7] and [2 1 3] specimens. Furthermore, during the plastic deformation of [1 1 13] specimen, lattice rotation is confined around [0 0 1] pole and shows a dispersive distribution, which is triggered by the multi slip behavior. A modified Sachs model considering multi slip behavior has been developed to predict the lattice rotation, this model is also applicable to severely deformed grains in polycrystalline materials. [ABSTRACT FROM AUTHOR]

The structural and mechanical response of pure body-centered cubic (BCC) Nb under shock loading were systematically studied. Shock-induced orthorhombic (α") martensitic transformations were observed in single crystals Nb with different orientations, in which dense lenticular α" martensite variants distribute uniformly along different directions by forming a hierarchical structure. Neighbouring martensite variants share a {111}<110> α" twinning relationship between each other, and high-density dislocations are found distributing along the α"/BCC(β) interface. The formation of α" martensite was dominated by a mechanism of collinear shuffling of alternating {110} β planes along with <1 1 ¯ 0> β direction by a displacement δ. The collision between martensite variants along different direction induce abundant microcracks, with BCC-to-ω and BCC-to-FCC phase transformations occurring at the crack tips. These findings not only provide the experimental evidence of shock-induced α" martensitic transformation in pure BCC metals, but also link the microstructure interaction of materials to their macroscopic shock failure directly, which may have a broad impact on the study of shock deformation in different materials. [ABSTRACT FROM AUTHOR]