The detection of dislocations by low temperature heat conductivity measurements

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Abstract
The lattice heat conductivity of a specimen after straining is reduced by the dislocations produced by the deformation. A theory by Klemens enables the dislocation density to be calculated from the decrease in the lattice conductivity at liquid helium temperatures. In the present experiments copper–zinc single and polycrptds (7, 16 and 30% Zn) have been deformed by various amounts and the increase in the dislocation density has been determined by heat conductivity measurements. The results are shown to be in qualitative agreement with current theories of work hardening. In stage I of the stress/strain curve there is only a slight increase in dislocation density, but there is s rapid increme in stage II. Whereas in both these stages the dislocation density is independent of the zinc content, the onset of stage III is strongly dependent on it. During stage III there is very little increase in dislocation density, In stage I the dislocation density is proportional to the stress, whereas in stage II it is proportional to (stress)2. Transmission electron microscope experiments on some of the specimens show that the Klemens theory overestimates the dislocation density by a factor of about 6. Cornpasison of electrical resistivity data with the lattice thermal conductivity results gives further evidence for suggesting that the electrioal resistivity is affected by stacking faults rather than by dislocations. Experiments on specimens taken at various stages during a fatigue test show that the dislocation density increases during the first two hundred fatigue cycles, but thereafter it remains constant.

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