The deformation of lithium, sodium and potassium at low temperatures: Tensile and resistivity experiments

Abstract

Stress-strain tests have been made on Li, Na and K at temperatures between 293 and 4.2°K. At higher temperatures the behaviour of all three metals is very similar. They are soft and after the yield point very little hardening is observed. At low temperatures Li and Na transform to f.c.c. and h.c.p. structures respectively on deformation, the character of their stress-strain curves changes accordingly and they harden continuously throughout the test. At 20 and 4.2°K Li deforms with many regions of discontinuous slip and at 4.2°K it fractures in a brittle fashion. Whilst K remains b.c.c. down to 4.2°K it also exhibits an unusual type of hardening at 20 and 4.2°K which is very temperature dependent. The change in the electrical resistivity during low temperature deformation was also measured. There was no detectable change when K was deformed at 4.2°K. This offers further evidence that a large proportion of the resistivity change which is observed when metals are deformed is due to stacking faults. Both Li and Na give considerable resistivity changes when they are deformed at 4.2°K and much of this change is ascribed to the scattering of electrons at the faulted regions within the transformed phases.