THE INFLUENCE OF LOCAL DEFECTS IN PB-BI SINGLE CRYSTALS ON PLASTICITY MECHANISMS AT 0.5-295 K

Nicolai V.Isaev, Vasiliy D.Natsik, Vitaliy V.Pustovalov, Vera S.Fomenko, Sergei E.Shumilin

B.I.Verkin Institute for Low Temperature Physics and Engineering NAS of Ukraine, 47, Lenin Prosp., Kharkov, 310164, Ukraine, e-mail:pustovalov@ilt.kharkov.ua

According to the theory of yield stress of solid solutions based on the concept of thermally activated motion of dislocations through local impurity obstacles, the effective stress monotonically increases at lowering temperature. At liquid hydrogen-helium temperatures region plasticity displays anomalies which cannot be explained within thermoactivation concepts. To interpret the low temperature anomalies in fcc and hcp metals, it is assumed that inertial and quantum-mechanical mechanisms of dislocation motion are effective. Experimental evidence has been obtained to support this assumption, though the picture is not yet clear completely.

In this report the kinetics of low temperature plastic deformation was investigated comprehensively on Pb-Bi single crystals (0.1; 0.5; 1.0; 3.0; 6.0 at.% Bi). The tension axis was oriented near [110]. The strain hardening curves were taken at the constant tension rate 10^-4s^-1 in the temperature range T=0.5-295 K. The experimental low temperature deformation setup was equipped with a He⁴-cryostat to provide T4.2 K and a He³-cryostat to provide T=0.5-4.2 K. The parameters measured were the yield stress and the strain rate sensitivity of the flow stress at 10 and 100 times increase of the strain rate. Below T_c the measurement was made on superconducting and normal states of samples, the transition to the normal state was effected by switching on the magnetic field.

The studies of the temperature dependencies of the yield stress and strain rate sensitivity have revealed anomalous plasticity typical of solid fcc and hcp metal solutions. The thermoactivation analysis of the dependencies obtained shows that at 25-150 K the plastic deformation of Pb-Bi alloys is controlled by the thermally activated motion of dislocations through impurity obstacles. A quantitative comparison has permitted empiric estimation of the parameters of dislocation - impurity interaction and of the internal stresses. It is shown that the considerable deviation of the temperature dependencies of the plasticity parameters from the regularities of the simple thermally activated process below 25 K is caused by the mechanisms of thermally inertial (25-10 K) and quantum inertial (~1 K) motion of dislocations through impurity obstacles. The analysis of the concentration dependence of the threshold temperature at which the low temperature anomaly occurs has permitted empirical estimation of the electron and phonon components of the dynamic drag coefficient of dislocations.

Below the superconducting transition temperature, the effect of change of the flow stress (Dt_NS) was studied, which occurred as the superconducting sample went over into normal state. It is shown that the dependence of the Dt_NS upon the Bi concentration is the evidence in favor of effective operation of the thermoinertial mechanisms of dislocation motion.