When a solid is plastically deformed in a hard testing machine and the cross‐head is arrested, a continuous decrease of load is observed. An interpretation and analysis of this stress relaxation phenomenon is made. This effect can be used to study mechanisms of plastic deformation in solids, and details are given in the paper.

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By applying the theory of thermally activated nucleation to the switching of ferroelectric domains, a method is developed to experimentally obtain the value of both the activation enthalpy, ⌬H, and activation volume, V * , for the thermally activated process involved in ferroelectric switching. The method was applied to the switching of a soft lead zirconate titanate and values of ⌬H = ͑0.16± 0.02͒ eV and V * = ͑1.62± 0.16͒ ϫ 10 −25 m 3 were obtained at the coercive field. These values imply that the energy, ⌬U, required for the formation of switching nuclei is mainly supplied by the work done by the electric field. A comparison of these values with those obtained from theoretical considerations suggests that the switching is achieved by the sideways expansion of nuclei formed at the domain boundaries in the form of low amplitude and long wavelength fluctuations of the domain walls.

The influence of the orientation with respect to the direction of stresses has been investigated over the temperature range 77 to 413 °K. At temperatures below 300 °K the limit of the proportionality of strain to stress and the rate of work‐hardening depend markedly on the orientation. At temperatures above 300 °K the yield stress is practically independent of the orientation and the crystals can experience a large elongation without necking. Crystals with the tensile axis orientated in a direction near the centre of the unit stereographic triangle exhibit three work‐hardening stages. Slip lines and etch pit observations have shown that profuse cross‐slip of screw dislocations is a characteristic feature of the deformation of molybdenum single crystals. An explanation, based on the mobility of the jogs in screw dislocations, is proposed for the orientation dependence of the yield stress.

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