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.