Abstract
Polycrystalline specimens with a modulated structure of a Cu-4 mass%Ti alloy were stretched in the temperature range from 4 to 70 K and at 293 K, and resulting microstructure were observed. Relationships between deformation twinning and serrated stress-strain curves were studied. Results obtained were summarized as follows:
(1) Serrated deformation or sharp load drops on the load-elongation curves were observed during stretching at temperatures below about 40 K. They occurred at stress levels lower than that corresponding to the proportional limit. The number of load drops increased with increasing stress level or strain and also increased with decreasing deformation temperature. No indications of serrated deformation were observed in stretching at 68 K and 293 K. (2) Deformation twins were observed in specimens stretched up to certain stress levels at all the test temperatures. Volume fraction of deformation twins increased with increasing stress level or strain. It increased with decreasing deformation temperature for the same amount of strain. (3) Sharp load drops detected at stress levels below the proportional limit are due not to “discontinuous slip” not to “thermal instability” but to the deformation twinning. The twinning stress can be given by detecting the first small load drop. The observed twinning stress has a tendency to increase with decreasing deformation temperature at the cryogenic temperature range. This temperature dependence of the twinning stress is explainable in terms of temperature dependence of the stacking-fault energy, which can be estimated from the electron concentration of this alloy.
