Jump-like twin boundary movement in indium crystals

This is an investigation of the specific features of twin boundary movement by creep in indium crystals at room temperature. It has been shown that in the most structurally perfect crystals, broadening of twin layers is jump-like. Jumps in the kinetic relations for boundary displacement are the most pronounced in the stress range where the initial boundary velocity becomes as high as (5 × 10⁻⁵–10⁻⁴) cm/s. The effect of the applied stress on the average jump density and the characteristic boundary motion velocities, that is the initial velocity, the velocities at the jump and just before it were investigated. Dislocation structure deterioration as well as impurity increase lead to changes in the boundary motion character, namely to monotonic reduction of boundary velocities with time. It has been ascertained that the jump-like nature of the boundary motion is due to boundary interaction with obstacles present in the crystal and unambiguously associated with periodically changing boundary structure: defect formation and eventual defect inclusion into the twinned material.     

Effect of the magnetic flux trapping upon the temperature dependence of the flow stress change at the superconducting transition

The effect of the superconducting transition on the flow stress in single and polycrystals of Pb-In and Pb-Sn alloys has been studied in the temperature range 1.8 K to T_e. Unlike earlier efforts, these experiments involved investigation of the magnetic flux trapping. Two types of temperature dependences of the magnetic flux trapping, B₀(T), were revealed. In Pb-1.9 at.% In and Pb-6 at.% Sn alloys (subjected to various thermal treatment), B₀ monotonically rises with lowering temperature. In Pb-1.8, 3 at.% Sn single crystals the flux trapping arises at high strains (> 50%) and low temperatures (<2.5 K). In this case B₀ magnitude depends on the strain rate. The difference in the B₀(T) behaviour is associated with dissimilar flux pinnig by pinning centres. The date on B₀(T) obtained give an insight into the two types of the Δσ(T) dependences; taking into account of the flux trapping allowed the Δσ values to be corrected, so that as a result, a single Δσ_SN(T) curve was obtained for all the alloys and structures studied.     

Dynamics of twin layer broadening in indium

The mobility of plane-parallel twin layer boundaries in indium crystals has been studied in the 235–390 K temperature range under stresses, τ, of (0.25–20) g/mm² (static load) and (35–335) g/mm² (crest values under impact load). For τ/G = (0.2–0.6) × 10⁻⁵ (G is the shear modulus), the process of twin layer broadening has been shown to be thermally activated; the process parameters and their stress dependence have been found. – Possible mechanisms of twin layer broadening have been analysed, and it has been concluded that a pole mechanism is inadequate to treat the results obtained. The twin broadening is considered to be due to nucleation and motion of twinning dislocations along boundaries in each subsequent twinning plane; it has been shown that the thermal activation parameters measured while broadening a twin layer, can be inconsistent with the elementary acts of broadening process (nucleation or motion of twinning dislocations). A deep gap between data on stress dependence of the activation energy of twin layer broadening for indium and calcite crystals and the Sumino theory is explained by the determining influence of the real imperfect structure of specimens on the process studied.