Microtwin Structure and Its Influence on the Mechanical Properties of REBCO Coated Conductors

The elastic properties of REBCO tape (i.e., REBa₂Cu₃ O_7-d, RE = Y, Sm and Gd) have been investigated by means of diffraction techniques using synchrotron radiation. Total local strain A_hkl^l, which consists of thermal strain A_hkl^T and lattice strain A_hkl, was analyzed using a microscopic structure model, with two types of microtwin configuration along the tensile axis, i.e., with the [100] or [110] crystal axis oriented parallel to the longitudinal direction of the tape. Experimental results were: (a) slope dA_h00^l/dA is larger than dA_0k0^l/dA (where A is the external tensile strain), which is attributed to the elastic modulus E_a along the a-axis being smaller than E_b along the b-axis; (b) both dA_h00^l/dA and dA_0k0^l/dA are smaller than unity, but dA₁₁₀^l/dA is almost unity depending on the configuration of microtwin; (c) the thermal strain for different planes is different as A_h00^T ≠ A_0k0^T due to the elastic behavior of the superconducting layer being modified because it is constrained by the substrate and the outer metallic layer; and (d) a large Poisson ratio is attributed to this constraint state. It is suggested that the microtwin structure is critical to understand and model this unusual stress/strain behavior.