LAUSR

Strain-rate cycling tests associated with ultrasonic oscillation were carried out between the strain rates (e) of 1.1×10−5 s−1 and 5.6×10−5 s−1 for NaBr:Li+ or K+ single crystals from 77 K up to the room temperature. Superposition of oscillatory stress during plastic deformation causes a stress drop (Δτ). By using the stress change (Δτ') due to the strain-rate cycling keeping the stress amplitude constant, Δτ'/Δlne was given as a measurement of the strain-rate sensitivity (λ) of flow stress. The relative curve of Δτand λ has a stair-like shape for the two kinds of crystals at low temperature (T). That is to say, the curve has two bending points and two plateau regions and λ between the two bending points decreases gradually with increasing Δτ. The activation energy (G0) etc. concerned with the break-away of a dislocation from the isotropic strain around additive cation could be obtained on the basis of three relative curves (τp vs. T, λp vs. T, and τp vs. V) fitting the Cottrell-Bilby relation to the experimental results. τp is Δτ value of first bending point at low stress decrement and λp is the difference between λ at first plateau place and at second one on the curve of Δτ vs. λ. V is the activation volume obtained from λp. As a result, it was found that Li+ ions are slightly weaker than K+ ones as weak obstacles to dislocation motion in the crystals. Furthermore, the additive Li+ or K+ ions act as still weaker obstacles to dislocation motion in NaBr host crystal as against NaCl because of a slight reduction in G0.