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

The elastic properties of REBCO tape (i.e., REBa 2 Cu 3 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 dA 110 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.