THE HARDENING MECHANISMS OF HAVAR Co-BASED ALLOY

The mechanical properties of high strength Co-based alloys were intensively investigated. However, the hardening mechanisms due to cold work and heat treatments are not fully understood because of the microstructural complexity and the combination of several hardening mechanisms. The present study was focused on the Co based alloy HAVAR and the correlation between the mechanical properties and the hardening mechanisms. The microstructure investigation included XRD, SEM, TEM and the mechanical properties were determined by tensile test.

It is found that the annealed condition is a solid solution single phase with FCC crystallographic structure. It contained equiaxed grains, very low dislocations density and small quantity of large FCC M₂₃C₆ carbides. The cold rolled microstructure contained very high dislocation density, fine deformation bands, twins, stacking faults and the phase constitution includes FCC matrix and strain induced HCP phase. Heat treatment between 500°C and 700°C invokes homogeneous precipitation of fine cubic M₂₃C₆ carbides with sub-nanometer size. This secondary hardening mechanism enhances the yield strength and can eliminate the ductility. The treatment added carbide precipitation but did not disrupt the complex cold rolled microstructure.

Keywords: Co based alloy, HAVAR, heat treatments, TEM, microstructure, carbide