Creep behaviours of Cr25Ni35Nb and Cr35Ni45Nb alloys predicted by modified theta method

Creep behaviours of Cr25Ni35Nb and Cr35Ni45Nb alloys were investigated under 22-50 MPa at 1173 K, 1198 K and 1223 K. The stress dependence of minimum creep rate and rupture life obeyed power law, and the stress exponent values were similar for both alloys indicating similar deformation and fracture mechanism. These alloys also obeyed Monkman-Grant and Modified Monkman-Grant Relations, and Cr25Ni35Nb exhibited higher MG and MMG constants. It can be concluded that Cr35Ni45Nb has higher creep strength than Cr25Ni35Nb, whereas the creep ductility is not as good as Cr25Ni35Nb alloy. A modified theta equation was proposed to estimate the creep behaviours, where the random errors of experimental data from the fitted creep curves were smaller than traditional theta equation. The obtained theta parameters revealed good linear relations as a function of stress, and the predicted creep behaviours were in good agreement with experimental ones.