Single crystals of cobalt have been deformed in tension over the temperature range 20°C to 600°C. On heating a transformation from the hexagonal close packed (hep) structure to the face centred cubic (fee) structure occurs at 430°C. The deformation behaviour in the hep phase has been compared to that for other hep metals, and it has been shown that only basal slip occurs even in unfavourable orientations. Twinning may occur, but has not been found to be associated with an increase in the work hardening rate. Similarly the properties in the fee phase have been compared to those of fee metals. The effect of deformation on the transformation has been studied on specimens which have been thermally cycled through the transformation during tensile tests. It has been found that the (111)/ (0001) transformation habit plane may be controlled by deformation. Recrystallisation may occur if two slip systems operate, but this is a function of crystal orientation. Slip may be induced on an unfavourably orientated glide plane in the fee phase, and this indicates that deformation dislocations on the (0001)[subscript: h], /(111)[subscript: c] plane are not affected by the transformation. However, glide dislocations on any {111} plane, which does not form the basal plane on cooling do not affect the work hardening behaviour in the hep phase. In previously deformed specimens, the flow stress is a function only of the deformation history, i.e. the yield point in the hep phase may be raised by a factor of 10 or the yield point in the fee phase may be lowered by a factor of 3. However, the work hardening rate depends only on the crystal structure and is always higher (by 20 to 100 times) in the fee phase than in the hep phase. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate