Deformation behaviour and mechanical properties of severely deformed copper at cryogenic temperaturesThursday (29.09.2016) 15:15 - 15:30 Part of:
Advances in power supply, transport or magnet applications request progress in different fields of materials science – especially the development of superconducting materials. Nevertheless, support structures like shells or central formers for the typically ceramic superconductors are important. The support framework for high temperature superconducting cables are usually based on conventional metallic alloys and have to fulfil both structural requirements at the operating temperature (typically 77 K and below) as well as functional demands like high electrical conductivity in the case of failure of the superconductor. Hence, these materials determine the overall performance of the cable.
Accordingly, the objective of the present study is the deformation behaviour and mechanical properties of pure copper (OFHC) at cryogenic temperatures which mechanical properties were initially improved by severe plastic deformation (SPD) using equal channel angular pressing (ECAP) at room temperature. The evolution of microstructure and mechanical properties is systematically investigated as a function of ECAP passes (up to four times) in order to obtain different degrees of grain refinement. The deformation behaviour of the differently prepared Cu materials was investigated by means of tensile tests at temperatures ranging from room temperature down to 4.2 K. The microstructure altered by deformation was analysed by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Depending on the degree of grain refinement by the ECAP process and the temperature of the deformation test, different amounts of deformation twinning are observed. Thus, varying work hardening behaviour is found as well. At 77 K and below, coarse grained copper exhibits deformation by twinning in addition to dislocation slip. Hence, the onset of plasticity is only slightly altered by decreasing temperature while ultimate tensile strength is significantly increased due to dynamic refinement of the microstructure during deformation. In contrast, ECAP processed copper exhibits higher yield strength in the whole temperature range in combination with unincisive work hardening.