On the measurement of dislocation damping forces at high dislocation velocity

In direct mobility experiments in single crystals, dislocation velocity is studied as a function of stress by the application of short-duration stress pulses. The stress pulse consists of a loading wave, followed microseconds later, by an unloading wave. At high velocities, dislocation inertia effects become important if the dislocation damping force is a decreasing function of dislocation velocity. In general, the magnitude of this force can be determined only if the relative velocity between the applied stress wave and the dislocation is considered

Temperature Dependent Viscous Drag in Close-packed Metals

The influence of viscous drag on dislocation motion in close-packed metals is examined. Three experimental measurements of the viscous drag are discussed, i.e. internal friction, strain rate versus stress, and stress-time-displacement measurements. Experimental results of each of these methods are compared. Theories of dislocation-phonon and dislocation-electron interactions leading to viscous drag are briefly described. The experimentally-determined dislocation drag coefficient is qualitatively in agreement with the predictions of damping through dislocation-phonon interactions. It is concluded that additional theoretical work is needed for a quantitative comparison of theory and experiment. Additional experimental work to determine the temperature dependence of the drag coefficient below 66°K is needed to resolve discrepancies in different theories of the dislocation-electron interaction

An experimental study of the mobility of edge dislocations in pure copper single crystals

The velocity of edge dislocations in 99.999% pure copper crystals has been measured as a function of stress at temperatures from 66°K to 373°K by means of a torsion technique. The range of resolved shear stress was 0 to 15 megadynes/cm^2 for seven temperatures (66°K, 74°K, 83°K, 123°K, 173°K, 296°K, 373°K). Dislocation mobility is characterized by two distinct features: (a) relatively high velocity at low stress (maximum velocities of about 9000 cm/see were realized at low temperatures), and (b) increasing velocity with decreasing temperature at constant stress. The relation between dislocation velocity and resolved shear stress is: v=v_0(τ_r)/τ_0)^n, where v is the dislocation velocity at resolved shear stress τ_r, v_o is a constant velocity chosen equal to 2000 cm/sec, τ_0 is the resolved shear stress required to maintain velocity v_0, and n is the mobility coefficient. The experimental results indicate that τ_0 decreases from 16.3 × 10^6 to 3.3 × 10^6 dynes/cm^2 and n increases from about 0.9 to 1.1 as the temperature is lowered from 296°K to 66°K. The experimental dislocation behaviour is qualitatively consistent with an interpretation on the basis of phonon drag

Dislocation Mobility in Copper and Zinc at 44°K

The torsion technique for dislocation mobility studies in close-packed metallic crystals developed by Pope, et al. (1) was first extended to low temperatures by Gorman, et al. (2). With this method, dislocation displacements are observed by x-ray diffraction on a crystal surface which was previously bonded directly to the torsion machine. Therefore a bonding agent must be utilized which is sufficiently strong to transmit the torsional stress pulse but pliable enough to prevent damage to the very soft test crystal. Various mixtures of organic solvents were found to have suitable properties when cooled to their glass-transition temperatures, and with these bonding agents mobility experiments were extended down to 66°K (2, 3, 4, 5). The torsion tests were carried out in an apparatus in which the test crystal could be cooled to any temperature down to the freezing point of nitrogen

Comments on the Measurement of Dislocation Mobility and the Drag Due to Phonons and Electrons

Experimental methods for the measurement of intrinsic interactions between moving dislocations and the crystal lattice are considered. It is emphasized that the stress pulse method is applicable at stress levels greater than about twice the static flow stress, while internal friction experiments may be used to explore the interaction at very low stress levels and small dislocation velocities. Recent results of low temperature stress pulse measurements in Cu are presented. The interactions deduced from measurements between 4.2°K and 400°K in some FCC metals are compared to theoretical predictions. Suggestions are made for future theoretical and experimental work on unresolved aspects of the intrinsic interactions

Dislocation Interactions with Thermal Phonons and Conduction Electrons

Experimental methods for the measurement of intrinsic interactions between moving dislocations and the crystal lattice are considered. It is emphasized that the stress pulse method is applicable at stress levels greater than about twice the static flow stress, while internal friction experiments may be used to explore the interaction at very low stress levels and small dislocation velocities. Recent results of low temperature stress pulse measurements in Cu are presented. The interactions deduced from measurements between 4.2°K and 400°K in some FCC metals are compared to theoretical predictions. Suggestions are made for future theoretical and experimental work on unresolved aspects of the intrinsic interactions

Experimental Measurement of the Drag Coefficient

The drag coefficient is related to the dissipative viscous force which acts on a dislocation in motion, The magnitude of the drag coefficient for a dislocation of known Burgers vector is determined by measurement of the viscous force at a known dislocation velocity, or by measurement of the energy dissipation brought about by the viscous force. We discuss here these measurements and explore the special conditions which make possible the determination of the drag coefficient

On the Damping of Screw Dislocation Motion in FCC Crystals by Phonon Viscosity

The phonon viscosity mechanism, as it applies to screw dislocations moving on the {111} planes of face-centered-cubic crystals, is examined. Formulas are derived for changes in the elastic stiffnesses of cubic crystals with the impression of an arbitrary elastic strain. The result is specialized to the case of the strain field of a screw dislocation in a face-centered-cubic crystal. Lattice energy absorption from a moving screw dislocation is then considered through the relaxation of the elastic stiffnesses. Using the formula for energy absorption, a dislocation damping coefficient, B, is found. B is independent of temperature above the Debye temperature, and at lower temperatures has different forms for anisotropic and isotropic crystals. The damping coefficient for an edge dislocation is qualitatively similar to that of the screw dislocation. The results are consistent with recent measurements of the damping coefficient for dislocations in copper

Dislocation mobility in pure copper at 4.2 °K

Torsional stress pulses of several microseconds duration were applied at 4.2 °K to cylindrical single crystals of copper containing freshly introduced dislocations. Dislocation displacements were measured by means of a double-etch technique, and subsequently the dislocation damping coefficient B was determined to be equal to 0.8 × 10-5 dyn sec/cm2. While B decreases monotonically with decreasing temperature, the value of B at 4.2 °K is greater than that predicted from theoretical calculations of the interaction between a moving dislocation and the conduction-electron gas in copper