Cobalt based alloys exhibit excellent corrosion resistant and mechanical properties. The mechanical properties were insensitively investigated while the hardening mechanism due to CW is not fully understood because of microstructure complexity and mixing of several hardening mechanisms. The present study was focused on the HAVAR Co based alloy, and the microstructure evolution due to cold rolled process at 20%, 44% and 85% reduction of thickness. It was found that annealed state (reference state) is a solid solution with FCC crystallographic structure, equiaxed grains, very low dislocation density and few twins. At low degree of cold rolled process (up to 44%), the microstructure contains higher dislocation density, large twins that contain sub-twins and stacking faults. The density of these faults increased as a function of cold rolling. At higher degree (85%), the matrix contains very high dislocation density, twins, sub-twins and fine grains or broken grains which are described as mosaic structure of sub grains. The overall changes in the microstructure are the hardening mechanisms that are responsible from the increased of the yield stress from 400 MPa for the annealed state, up to 1900 MPa for the 85% cold rolled degree.
