The tensile properties of single crystals of high-purity iron at temperatures from 100 to ─253°C

Abstract

An account is given of the deformation and fracture characteristics of single crystals of high-purity iron of various orientations when tested in tension at temperatures from 100 to ─253°C. At temperatures down to ─124°C, the crystals were fully ductile, giving chisel-edge fractures and 100% reduction in area. At ─196°C, depending on the orientation of the stress axis, the behaviour covered the whole range from fully ductile with 100% reduction in area to completely brittle with cleavage fractures and no apparent deformation. Between these limits, mixtures of slip, twinning and cleavage were obtained. At ─253°C, the crystals gave cleavage fractures, and, over most of the orientation range studied, this occurred without prior deformation. It is shown that the resolved shear stresses required to produce slip or twinning at ─196°C vary with the orientation; in particular, the values are higher for orientations within 20 to 25° of the [001] than elsewhere in the unit triangle, and these values are not reached in this region before failure occurs by cleavage. A mechanism is put forward to explain this in terms of ‘locking’ of dislocations. The cleavage strength resolved normal to the cleavage plane is not constant with change in temperature, as is commonly supposed, but is substantially higher at ─253°C than at ─196°C.