Tensile behavior of polycrystalline zirconium at 4.2°K

Abstract

Tensile tests were conducted at 4.2°K on both longitudinal and transverse specimens machined from zirconium plate. In this plate the basal planes of the grains were strongly aligned parallel to and distributed nearly uniformly about the rolling direction. The metal had higher ductility at 4.2°K than at room temperature. Prestraining transverse specimens at 4.2°K produced higher room temperature damping capacity than prestraining at 77°K. All stress-strain curves for the 4.2°K tests exhibited characteristic sudden drops in the load. Transverse specimens showed a much higher incidence of twinning, greater strain hardening, more plastic deformation between serrations, and fewer serrations than longitudinal specimens. The experimental evidence is in agreement with Kula and DeSisto’s conclusions that twinning per se is probably not responsible for the discontinuous yielding. The present observations are also consistent with Basinski’s hypothesis that the load drops are related to thermal softening. A simple calculation shows that at temperatures near absolute zero, the heat generated in a specimen during deformation is much more than that necessary to cause an instability and consequent load drop. Hence it is concluded that all the heat generated is not retained in the specimen and a large fraction must be lost to the surroundings. An estimate of the magnitude of the temperature rise associated with a typical load drop was made.