Dorn dislocation creep

The creep strength of AljNb is comparatively low - a stress of 10 MN/m produces 1 % strain in only 500 h and fracture in 2300 h - whereas the yield stress compares favorably with the superalloys. This illustrates the fact that the difference between the yield stress and the creep strength is much more pronounced for intermetallics than for conventional alloys. Creep of Al3Nb is controlled by dislocation climb which is accompanied by subgrain formation. The observed creep behavior corresponds to that of conventional disordered alloys and the creep rates are described by the known constitutive equations. This will be discussed in more detail with respect to NiAl (Sec. 4.3). The secondary creep rate follows the power law, i.e. Dorn equation for dislocation creep... 

The creep behavior of the ternary B2 phase (Ni,Fe)Al was studied in detail as a function of stress, temperature, composition, and grain size (Rudy and Sauthoff, 1985 Rudy, 1986 Jung etal., 1987). At high temperatures, e.g. 60% of the melting temperature or higher, the secondary creep at rates between about 10" s" and 10" s exhibits power law behavior, i.e. the observed secondary creep rates are described by the familiar Dorn equation for dislocation creep [Eq. (2)] (Mukherjee etal., 1969). 

At stresses below the flow stress where the dislocation density is independent of stress, p is constant and n equal to one is predicted. This is referred to as the Harper-Dorn creep. 

In Eq. (10), V is the atomic vibration frequency, p is the dislocation density, a is the area swept out by a dislocation during an event, b is the Burgers vector, and Q is the activation energy. [A mistake in the reference has been corrected in Eq. (10).] Following the development of the Dorn creep equation, Eq. (10) can be generalized to... 

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