Magnesium single crystals of three different orientations were deformed by tension at room temperature to investigate the geometrical criterion resulting from the Schmid law for activation of basal and non-basal slip systems. Changes of crystallographic orientations of investigated single crystals were systematically measured during deformation and the geometrical criterion was examined in respect to the changes of crystallographic orientations during deformation. The geometrical criterion itself failed to explain lack of activity of non-basal slip systems in the deformed magnesium single crystals. Therefore the ratio of activation volumes of soft to hard slip systems is considered to be introduced for a better understanding of the deformation in hcp metals.
Magnesium single crystals of purity (99.8 wt%) deformed to stage B on the work-hardening curve at the temperature of 293 K and at a strain rate of 10^{-3} s^{-1} were investigated. The modified Bridgman method was used to obtain the crystals of the preferred orientation of (0001)〈11\overline{2}0〉 as primary slip system. By using the method based on the experimental gradient matrix, the activity of slip systems was obtained in magnesium single crystals deformed to shear strain 1.2 γ, where well developed stage B of work hardening was observed. It was shown that primary (0001)〈11\overline{2}0〉 slip system was dominant in the whole investigated range of the examined deformation. The observation and analysis of etch pits on the {\overline{1}2\overline{1}0} plane showed the heterogeneous distribution of dislocations formed during deformation into walls of dislocations perpendicular to the (0001) slip plane. The suggested model of work hardening of magnesium single crystals, which is worth taking into consideration, shows the influence of the long-range stress field derivating from the groups of dislocations arranged in dislocation walls.
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