Plastic properties and defect structure of LiF-LiF:Mg layered single crystals at T = 4.2 K

The compressive stress-strain diagrams are obtained for layered single crystals of the LiF-LiF:Mg type with different orientations of the reinforcing layers at T = 4.2 and 300 K. The strength characteristics and specific features of the defect and dislocation structures of the crystals strained in liquid helium are studied and compared with those for the crystals strained at 300 K. It is established that the layered single crystals remain plastic at T = 4.2 K. Under a strain ε > 1%, there arise microcracks and macrocracks responsible for complete fracture of the crystals. It is revealed that long-term (for ten years) storage of the layered single crystals at 300 K substantially affects the magnesium impurity structure in the reinforcing layers and leads to a considerable enhancement of their strength characteristics. This effect is taken into account when analyzing the strength properties of the layered single crystals in the temperature range T = 300–4.2 K. The inference is made that the results obtained in studying the defect microstructure of the model layered single crystals at the initial stage of plastic deformation can be used to predict the strength properties of the composites at T = 4.2 K.