Taira Suzuki

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.

Ultrasonic attenuation and magnetic properties of superconducting niobium in the mixed state

Abstract The ultrasonic attenuation coefficient was measured in superconducting niobium as a function of temperature and magnetic field. The observed temperature dependence of attenuation in zero magnetic field agreed rather well with the BCS theory, and the magnitude of the energy gap at 0°K was found to be (3.5 ± 0.1) kT c . In a magnetic field, the attenuation coefficient began to increase at the initial penetration field, H c1 , and reached a value corresponding to the normal state at the upper transition field, H c2 . The variation of the attenuation was compared with the behavior of the magnetization of the same specimen, assuming that the effective energy gap in the mixed state is proportional to the square root of the magnetization. Good agreement was found when temperature was close to T c , whereas a remarkable discrepancy occurred at lower temperatures. The discrepancy was particularly large near H c1 . Finally, following the recent theory given by Maki, two parameters, κ 1 , κ 2 , and their temperature dependence were determined.

Ultrasonic attenuation in superconducting niobium

A study of ultrasonic attenuation using longitudinal waves of 30 Mc/s was carried out for superconducting niobium, whose resistance ratio varied from 100 to 830. The measured temperature dependence of αsαn was found to deviate from the BCS relation with increasing purity, when ql < 1. A similar deviation was also found for the magnetic field dependence of αsαn. These results cannot be explained by an anisotropic or a two gap model.

Softening of Al Single Crystals due to Superconducting Transition

Using a 3 He cryostat, the flow stress increment Δτ sn associated with superconducting-normal transition has been investigated in the temperature range between 0.5 K and T c (1.183 K) for Al single...

Anomalous attenuation of ultrasonic waves in superconducting niobium

Abstract Anomalous absorption at H c1 and a deviation of the temperature variation of α s /α n from the BCS relation have been found in the measurements of the ultrasonic attenuation in superconducting niobium.

The Effect of Superconducting Transition on Plastic Properties of Lead and Lead Alloys

The flow stress change Δτ associated with superconducting transition has been measured on lead and lead alloy crystals against the kind and the concentration of solute atoms as well as the amount of deformation and temperature. Δτ increases with applied stress rapidly in the beginning and slowly in later stages of deformation. The temperature dependence of Δτ cannot simply be expressed as proportional to (1- B s / B n ), where B n and B s are the electronic drag coefficients of a dislocation in normal and superconducting states, respectively. Δτ of lead alloys is dominantly affected by the size misfit between solute and solvent atoms. As to the solute concentration c , Δτ∝ c n , where n is first about 1/2 but increases with deformation. These results are discussed theoretically by taking account of the influence of the electron viscosity on both terms of the activation energy and the attempt frequency in the rate equation describing the thermally activated motion of a dislocation.

On the Superdiffusion of Hydrogen in Va-Metals

Experimental procedures for the determination of superdiffusion coefficients of hydrogen in V a -metals (V, Nb, Ta) under external tensile stress parallel to are described in detail. Results are analyzed in the light of the lattice activated tunneling theory. The activation energy E a and the tunneling matrix element | J | are obtained experimentally as follows: E a [meV]: 21 for V, 15 for Nb, 12 for Ta, and | J |[meV]: 0.44 for V, 0.085 for Nb, 0.065 for Ta. The superdiffusion mechanism of hydrogen in these metals is theoretically discussed.

Dislocation Motion in the Field of a Random Distribution of Point Obstacles: Solution Hardening

Deformation rates of crystals, according to many experimental and theoretical studies, are controlled by thermal activation processes for surmounting various barriers. So far, the problems have been discussed using the following implicit solutions: (1) the processes are quasi-static and do not involve any dynamical aspects; (2) the frictional forces produced in materials are neither extremely large nor small. If the frictional forces due to the material are very small, the hypothesis (1) does not hold any more. This is the case of deformation of superconductors as described in the next chapter. Weertman [3.1] dissussed creep of ice on the basis of visco-elastic deformation theory developed by Eshelby [3.2]. According to ultrasonic attenuation experiments [3.3], B is 107 times as large as that for metals. In this case, although the frictional force is still proportional to the velocity of dislocations, the time of motion in the area between barriers becomes longer than that needed to surmount each of them. In other words, the barriers are of no importance for the discussion of the rate of deformation. Accordingly, such a linear visco-elastic deformation may be put aside in the present discussion. As for studies of plastic deformation of crystalline materials, the past treatments of the case, where the frictional forces are very small, should be carefully reconsidered. In the present chapter, we discuss problems within the above framework, (1) and (2), and those outside of this will be discussed in the next chapter.

Dislocations in bcc Metals and Their Motion

Copper (and its alloys) and iron or steel form two groups of metals that have been utilized since the ancient times. More recently, aluminium and its alloys have found many applications, thus forming three important metallic groups. When we compare the low-temperature flow stresses of these three kinds of metals, we find that that of Fe is much higher than the other two: at liquid nitrogen temperature usual steels can no longer be plastically deformed without fracture. From the fact that Fe has body-centered cubic (bcc) structure while Cu and Al have face-centered cubic (fcc) structure, such a difference in low-temperature plasticity is considered to be due to the difference in crystal structure. In fact, while all pure fcc metals exhibit weak temperature dependence of the yield stress, metals with bcc structure show, without exception, a strong temperature dependence. In this chapter, we describe characteristic plastic behavior of bcc metals and its interpretation.

Anomalous Plasticity of Alloys at Low Temperatures

A statistical dislocation model is proposed of the anomalous temperature dependences of flow stress below about 50 K found in many alloys. A thermal unit process is assumed to consist of a triggering thermal unpinning of an obstacle and mechanical unzippings following it. The dimensions of the unit vary with temperature and stress. A selfconsistency condition on fluctuation and relaxation of the distribution of dislocation configurations is taken into account. The model can explain the experiments well without detailed assumptions. The anomalies are found to occur, due to the onset of cooperative two dimensional propagations of unzipping motion of dislocations assisted by inertial effects.