Effect of dislocation shape memory in bcc and fcc metals after small deformations at 4.2 K
Abstract
Experimental investigations have been carried out of the dislocation shape memory in the temperature interval 4.2–300 K for niobium, iron, nickel, and copper samples deformed at 4.2 K by torsion from 0.1 to 0.4%. After such a treatment, the dislocation shape memory is found to exist both in bcc and fcc metals, but the intensity and temperature of its emergence depend on the defect structure formed as a result of low-temperature deformation, as well as on the friction forces in the crystal lattice. It is shown that the extent of shape restoration in bcc metals is much larger than in fcc metals. The process occurs in three stages in bcc metals. In fcc metals, the emergence and extent of the dislocation shape memory depend on the packing defect energy (PDE). In metals with a low PDE (e.g., copper), the shape is retrieved in two stages, while in metals with a high PDE (e.g., nickel), a restoration of the shape is not observed. Physical mechanisms for the observed stages of dislocation shape memory are proposed. The temperature dependence of the shape restoration and mechanical properties of Nb, Fe, Cu, and Ni are compared. It is suggested that the nature of variation of the thermally activated part of the dislocation shape memory may serve as a criterion for the tendency of bcc metals to brittle fracture upon low-temperature deformation.