The peculiarities of the formation and the development of reorientated bands in crystals are considered. Reorientated bands are investigated experimentally in MgO single crystals under the compression testing. It is shown that the misorientation between the material in the band interior and the surrounding volume can reach some degrees. The data of electron microscopy investigations of dislocation structure of such bands are performed. The disclination model is proposed for the propogation of the reorientated bands. This model permits to explain the laws of the band development, for example, the formation of discrete areas with reoriented crystal lattice ahead the terminated band.

The average length of dislocation segments between pinning points is estimated in plastically prestrained MgO crystals from measurements of the internal friction and the Young's modulus defect at 100 kHz. The results coincide satisfactorily with those obtained from in situ straining experiments in the high voltage electron microscope.

The stress -strain ~Eiaracteristics of MgO single crystals in compression are investigated in the temperatxre range from -196 "C to 2400 "C o n samples of the same shape and size. It is shown that the stress -strain curves may bc subdivided into three basic types, cach of them corresponding to the definite temperature range. The work hardening rate a t stage I is tempcrature independent and a t stage T I lineary decrcases with temperature increase in overall temperature range where work hardening is observed. The high temperat,urc maximum in the temperature dependence on the proportional limit is found which is not due to the effects of impurities. The cffect of geometrical form of spccimens on stress-strain parameters and the local true strain distribution i n compressed specimens have been studied.

A deformation of MgO crystals at high temperature Significantly increases the plasticity in a subsequent deformation at room temperature. It was shown by deformation experiments inside the high voltage electron microscope that this effect is most probably due to the homogeneous dislocation structure formed by straining at high temperature, but not to changes in the processes controlling the mobility of individual dislocations. Straining of MgO single crystals at 800 to 1400 "C by 0.5 to 1% significantly increases the plasticity of the material in a subsequent deformation at room temperature (BEREZHKOVA, PERSTNEV). The effect may result from particularities of the dislocation structure formed during the high temperature deformation, or, as a aecond explanation, from changes of the processes governing the dislocation mobility. In order to elucidate this problem, micro-samples of the predeformed material were strained in-situ in the high voltage electron microscope at room temperature.The specimens for in-situ experiments were prepared in the following manner. From the samples predeformed in compression up to about 1% a t 1000 "C thin plates of the dimensions 10 x 0.2 x 1.6 mms were cleaved. Afterwards, they were thinned down by chemical polishing (ELKINGTON et al.;APPEL et al. 1977). Taking into account that the investigation in the strailling device for the HVEM is carried out in tension, the plates were cleaved in two different orientations: the tensile axis of the micro-samples was 1) perpendicular to the axis of the previous compression (type A), or 2) parallel to it (type B) (Fig. 1). The method of the in-situ deformation in the HVEM was described by APPEL et al. 1977 and MESSERSCHNIDT, APPEL 1976a, b, 1979a. The specimens were deformed by small load increments. The changes in the dislocation structure were recorded photographically after stopping the drive, i. e. in the process of relaxation. Four in-situ straining experiments were carried out.The results of the in-situ experiments were analysed with respect to two aspects: 1) development of the total dislocation ensemble and 2) investigation of the processes governing the mobility and multiplication of individual dislocations. The dislocation structure formed during the high temperature deformation contained a large number of edge dislocation dipoles. In contraat to the structure arising from a room tempera-

Interaction processes are studied of impulse laser irradiation (λ = 1.06 μm) with the MgO monocrystal. The structural crystal transformations after the exposure are studied by the X‐ray structural analysis (XRA), time‐of‐flight cathodoluminescence (10−8 s), and other optical techniques. The process of the crystal defect formation on the whole volume of the sample (effect of the side opposite to the irradiated one) is ascertained to the exposure of the impulse laser irradiation onto the MgO surface in the melting regime. The laser impulse heating of the MgO surface leads to the process of recrystallization when a material is divided into blocks whose dimensions are much less than the ones of the outcoming (non‐irradiated) samples; at the same time a growth of microstrains is observed in the whole volume of the samples. Formation and transformation of point structural defects occur side by side with density dislocation growth ≈︁ 1011 cm−2. Cathodoluminescence (CL) measurements show changes in the spectrum (2.4 to 2.6), (3.0 to 3.1), (3.3 to 3.4) eV caused by laser irradiation which may be explained by point‐defect formation in MgO. The results obtained allow to understand the process of the defect formation by impulse laser onto the MgO crystal surface.