SUPERLOCALIZATION OF PLASTIC-DEFORMATION IN CRYSTALS AT HIGH-TEMPERATURES

Plastic strain distribution and dislocation structure in alkali halide (LiF, LiF:Mg, NaCl, KCl) and ferroelectric (NaNO2) crystals under con stant tensile or compressive stress at high temperatures (T> 0.6T(m)) have been studied. It was found that plastic deformation instabilities at instantaneous deformation occur? namely deformed samples exhibit m icrobands with very high local shear strain of about 10(3)%. The local strain rate in microbands is much more than 5 s(-1). These narrow mic robands (between 2 and 10 mu m in different crystals) are oriented in accordance with the slip systems in crystals. The average distance bet ween microbands is usually from 20 to 100 mu m. The surface steps corr esponding to local high shear strains are observed in the form of wavy slip. Sometimes, a change in the relief (a depression or a bump) of t he surface parallel to the Burgers vector is also observed inside the microbands. The increase of local strain during deformation occurs mai nly due to the creation of new microbands rather than by shear in alre ady existing ones. Possible mechanisms of plastic instability in highl y strained microbands are discussed. (C) 1997 Elsevier Science S.A.