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, Q c, were obtained for the oxide dispersion-strengthened nickel-based superalloy MA754 by conducting creep tests at 1 223- 1273 K in the stress range of 130-190 MPa. The values of n and Q c 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, σ i, using the strain dip test. The ratio of σ i to applied stress, σ a; a, was very high at lower stresses, while at higher stresses, σ i reached its saturated value with increasing stress. In this stress range, the ratio of σ i to σ a;a 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 σ i decreased, which resulted in a relatively large creep rate. The larger creep rate at high temperatures led to a large value of Q c. 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 Q c through a change in σ i;. © 2012 ISIJ.