EFFECT OF DISLOCATION SHAPE-MEMORY IN BCC AND FCC METALS AFTER SMALL DEFORMATIONS AT 4,2 K

The effect of dislocation shape memory Is studied experimentally al 4. 2-300 K on niobium, iron, nickel and copper samples after their torsio nal deformation from 0.1 to 0.4 % at 4.2 K. after the above mode of de formation, the effect of dislocation shape memory Is observed both in bcc and in fee metals, but its intensity and temperature are dependent on the defect structure formed during the low-temperature deformation and on the friction forces of the crystalline lattice. The degree of the shape restoration is much higher hi bcc metals than in fee ones. I t is found that the shape restoration in bcc metals has three stages. In fcc metals the presence of the dislocation shape memory sad its ext ent are dependent on the energy of stacking defects (SDE): In low SDE metals (e.g., Cu) the shape restoration proceeds in two stages; in hig h SDE metals (e.g., NI) no shape restoration is observed. Physical mec hanisms accounting for the stages of dislocation shape memory are prop osed. The temperature dependences of the shape restoration rates and t he mechanical properties of Nb, Fe, Cu and Ni are compared. It is assu med that the character of changes in the thermally activated dislocati on shape memory may serve a criterion of bcc metal liability to brittl e fracture at low temperature-deformation.