Examination of deformation mechanism maps in 2.25Cr-1Mo steel by creep tests at strain rates of 10^-11 to 10^-6 s^-1

The deformation mechanism map of 2.25Cr-1Mo steel was examined by creep data obtained over a wide range of creep rates down to 10^-11 s^-1. The stress dependence of minimum creep rates of the steel is similar to that of particle strengthened materials: low, high, and low stress exponent, respectively, in high (H), intermediate (I), and low (L) stress regions. The stress exponent and activation energy for creep rate suggest dislocation creep controlled by lattice diffusion as the deformation mechanism in regions I and L, including service conditions of the steel. Transition to diffusion creep occurs at a lower creep rate than what is expected in the deformation mechanism maps. Region H appears above athermal yield stress. During loading in this region, athermal plastic deformation takes place by dislocation glide mechanism, and then dislocation creep starts. The dislocation creep in region H is different from the one in regions I and L due to the plastic deformation during loading. A modified creep mechanism map of 2.25Cr-1Mo steel is proposed on the basis of the experimental results.