The influence of Co on creep deformation anisotropy in Ni-base single crystal superalloys at intermediate temperatures

In order to investigate the influence of Co on creep deformation anisotropy, some Ni-base single crystal superalloys have been creep tested at 800°C and 735 MPa. It is found that, although Co-containing alloys have higher creep resistance when the tensile direction is within 5° of 〈100〉, they are much more sensitive to tensile orientation than Co-free alloys, which have similar creep rupture lives for tensile orientations within 25° of 〈100〉. It is revealed that for Co-containing alloys the 111〈112〉 slip system is operative during first-stage creep, when heterogeneous deformation occurs. Also, the degree of heterogeneous deformation correlates very well with creep rupture lives. It may thus be concluded that Co is not essential in improving creep rupture properties of Ni-base superalloys.