Dynamic dislocation phenomena in single crystals of Cu-10.5-at.%-Al alloys at 4.2 °K

Abstract

Single crystals of Cu-10.5-at.%-Al alloys with the [¯12¯5] {1¯2¯1} {¯2¯10} orientation experience discontinuous elongations under tensile stress at 4.2 °K. The processes are initiated at a resolved shear stress of 3.01 Kg ${\mathrm{mm}}^{-2\mathrm{}}$ on the {¯1¯11} glide system. They introduce slip bands with a mean width of 47 $\overline{\mathrm{m}}$ and produce a mean integrated shear displacement of 5.7 μm corresponding to the motion of 3.5 × ${10}^4$ dislocations in near-edge orientations. Simultaneous elongation-time and stress-time records have been obtained with capacitive and ceramic piezoelectric transducers located near the site of relaxation in the cryostat. After an initial interval of 30 $\overline{\mathrm{s}}$ec, elongation proceeds at a constant rate of 1.1 cm ${\sec }^{-1\mathrm{}}$ for 400 μsec. During this process, the shear stress is 0.024 kg ${\mathrm{mm}}^{-2\mathrm{}}$ lower than at the onset of relaxation. The rate of elongation, which is determined by the mechanical properties of the system, gives a value for the product of the number and velocity of the moving dislocations of 3.76 × ${10}^7$ cm ${\sec }^{-1\mathrm{}}$. Observations of the surface topography of the slip bands and of the dislocation-etch-pit distributions within them are used to establish a model for the sequence of events during the relaxation process. With this model, the mean velocity of the dislocations is found to be 2.1 × ${10}^4$ cm ${\sec }^{-1\mathrm{}}$ at a resolved shear stress of 2.98 kg ${\mathrm{mm}}^{-2\mathrm{}}$ and a local steady-state temperature of 17 °K.