Understanding the sigmoidal primary creep behaviour in a multi-component solid-solution Co-base alloy between 700 – 850°C

This investigation was motivated by the lack of understanding of the role of solid solution strengthening and substructure evolution during the creep of multi-component alloys. Thus, we studied the tensile creep behaviour of a multi-component solid-solution Co-base alloy (Co41-Ni23-Cr26-W4.7-Fe2-Mn1-C0.5) at 650 – 900°C and stress (σApplied) range: 80 – 400 MPa. Uncommon sigmoidal primary creep (SPC) was witnessed at 700–850°C and in the intermediate stress range. The stress exponent (∼5 – 7) and activation energy (∼270 kJ/mol ≈ Qlattice) suggest that dislocation creep controls the creep rate. Transmission electron microscopy (TEM) studies revealed structural evolution with creep strain. At 750°C and 160 MPa, at creep strain (ε) = 0.25%, multiple pinning points along the dislocation length were seen, and a thorough analysis confirmed them as jogs. Friction stress estimation from dislocation curvature from multiple TEM micrographs revealed that effective stress on dislocations is high (>0.6σApplied) while solid solution strengthening is nominal during creep. Overall analysis suggests that the creep rate in the multi-component alloy is recovery controlled while jogs' motion determines the SPC creep rate.