Stress Exponent of Minimum Creep Rate and Activation Energy of Creep for Oxide Dispersion-strengthened Nickel-based Superalloy MA754

Abstract

The stress exponent of the minimum creep rate, n, and the activation energy of creep, Qc, were obtained for the oxide dispersion-strengthened nickel-based superalloy MA754 by conducting creep tests at 1223–1273 K in the stress range of 130–190 MPa. The values of n and Qc in MA754 were determined to be 26 and 962 kJ/mol, respectively. The causes of these high values were determined by measuring the internal stress, σi, using the strain dip test. The ratio of σi to applied stress, σa, was very high at lower stresses, while at higher stresses, σi reached its saturated value with increasing stress. In this stress range, the ratio of σi to σa decreased drastically with increasing stress, which reflected the large increase in creep rate. Such a large increase in creep rate with increasing applied stress led to an unpredictably higher value of n. With increasing temperature, the saturated σi decreased, which resulted in a relatively large creep rate. The larger creep rate at high temperatures led to a large value of Qc. In addition, the dislocation density, ρ, of the interrupted creep specimens at the time of the minimum creep rate increased with increasing stress and reached the saturated value. This change in ρ with stress must be reflected in a large change in n and Qc through a change in σi.