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

摘要

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 1 223-1 273 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, 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 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 σ_c. In addition, the dislocation density, p, 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 p with stress must be reflected in a large change in n and Q_c through a change in σ_i.