HVEM In‐situ Straining Experiments on MgO Specimens Predeformed at High Temperature

Abstract

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-