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

The elastic properties of REBCO tape (i.e., REBa2Cu3O7-d, RE =Y, Sm and Gd) have been investigated by means of diffraction techniques using synchrotron radiation. Total local strainAhkll, which consists of thermal strainAhklTand lattice strainAhkl, 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) slopedAh00l/dAis larger thandA0k0l/dA(whereAis the external tensile strain), which is attributed to the elastic modulusEaalong thea-axis being smaller thanEbalong theb-axis; (b) bothdAh00l/dAanddA0k0l/dAare smaller than unity, butdA110l/dAis almost unity depending on the configuration of microtwin; (c) the thermal strain for different planes is different asAh00T≠A0k0Tdue 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.