Plastic Instability of Tantalum Single Crystals Compressed at 4.2 K

Serrations due to slip were observed on the stress-strain curves of zone-refined tantalum single crystals compressed at 4.2 K. It was found on an oscillograph that each rapid stress drop was accompanied by a rapid temperature rise of about 40 K. This suggests that a discontinuous slip is caused by the plastic instability due to the heat generated during the deformation. Using a deformation model involving a temperature rise due to the work done, calculations of the stress-strain curve and the corresponding temperature change were made for the present crystal, which gave a rapid stress drop and a rapid temperature rise in agreement with the experiments within a factor of 1.5. The present calculation shows that shows that no triggering action is necessary for the initiation of a discontinuous stress drop. A criterion for the thermal instability to occur was also discussed.