Effect of Cold Working and Aging Treatment on the Mechanical Properties of Co-Ni Alloy

Co-Ni alloys in which Co, Ni, Cr and Mo are principal elements, exhibit an impressive combination of high strength, high toughness, and excellent corrosion resistance. In this study, the effects of cold-rolled combined with the heat-treatment ranged from 673 to 973 K from 1hour to 10 hours on the mechanical properties and microstructures of Co-Ni alloys were investigated systematically. The relations between the aging temperature and mechanical properties were concluded. The initial ultimate tensile strength of 790 MPa increased to 1808 MPa by cold rolled 80 pct. After aging the cold-rolled alloy (80 pct ) at temperature 773K for 4hours, the ultimate tensile strength and the hardness reached to 2220MPa and 759(HV), respectively. It is found that the material was hardened by the cold working and aging which provided the second hardening. However, TEM observations and X-ray diffractions suggested that no structural change could be found. The cold deformation introduced platelets of a few atomic layers in thickness less than 100 nm, which were identified as stacking faults. A high density of nanoplatelets and dislocations, piled up in the vicinity of twin plate strengthened materials. The aging treatment provided the second major source of strengthening after cold-working (and only after cold-working) by the formation of secondary twins. The ultimate strength resulted from that the intersection of deformation twins and secondary twins blocking the dislocation movement.