INFLUENCE OF THE MARTENSITIC PHASE TRANSFORMATION ON THE ULTRASONIC ATTENUATION AND VELOCITY IN SINGLE CRYSTAL LITHIUM

Abstract
A gated coherent phase sensitive ultrasonic technique was employed to measure the ultrasonic attenuation and velocity in single crystals of ⁶Li and ⁷Li in the vicinity of the martensitic phase transformation temperature. To avoid straining the samples, electromagnetic transducers were used to generate and detect the acoustical signals. The velocity and the attenuation were measured at 10MHz for temperatures between 20K and 180K for waves associated with the three independent elastic constants C_L=(C₁₁+C₁₂+2C₄₄) / 2, C₄₄ and C' = (C₁₁ - C₁₂) / 2. Transition temperatures between 60K and 80K were observed. In every case for all modes studied, the transition was very abrupt and no premartensitic effects were observed. The transformation was accompanied by a large increase in the ultrasonic attenuation. The attenuation typically changed by 5dB/cm, 15dB/cm and 55dB/cm for the C_L, C₄₄ and C' modes respectively. The velocities associated with the elastic constants C_L and C₄₄ decreased by 8% and 20% respectively at T_c while the velocity associated with C' increased by about 10% to 20% at T_c. For C_L and C₄₄ the velocity continued to decrease as the temperature was lowered to 20K. Generally on reversion the crystals return to the original body-centered orientation. The results will be compared with other studies of the martensitic transformation in lithium.