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

The elastic properties of REBCO tape (i.e., REBa2Cu3 O7-d, RE = Y, Sm and Gd) have been investigated by means of diffraction techniques using synchrotron radiation. Total local strain Ahkll, which consists of thermal strain AhklT and lattice strain Ahkl, 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 dAh00l/dA is larger than dA0k0l/dA (where A is the external tensile strain), which is attributed to the elastic modulus Ea along the a-axis being smaller than Eb along the b-axis; (b) both dAh00l/dA and dA0k0l/dA are smaller than unity, but dA110l/dA is almost unity depending on the configuration of microtwin; (c) the thermal strain for different planes is different as Ah00T ≠ A0k0T 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.