Low temperature deformation and fracture behaviour of [100] and [110] chromium single crystals

Abstract

The fracture mechanisms of [100] and [110] chromium single crystals at uni-axial tension in a temperature interval between −75 °C and 50 °C have been studied with scanning electron microscopy. The significant findings are that [100] single crystals exhibit the sub-critical crack growth by cleavage at temperatures above the lower limit of the brittle-to-ductile transition while [110] crystals exhibit cleavage crack branching after a few per cent of plastic deformation. The preferable direction of cleavage crack propagation along the {100} planes is [110]. Planes of microcracking and delamination at the brittle fracture are {112} planes. Fracture energies of [100] and [110] single crystals were estimated by measuring sub-critical cracks and cleavage cracks before branching. The temperature dependence of [100] crystal surface energy was described as the Arrenius's function with activation energy U o = 0.2 eV. The nucleation of cleavage crack is discussed as a thermoactivated process with the same nature as yield of the BCC metals.