Climb-Controlled Creep

The secondary creep behavior of (Ni,Fe)Al shows analogous characteristics. In the Ni rich phases and in the binary NiAl a well-defined substructure is found after creep. The subgrain size is of the order of 10 pm, and the dislocation density within the subgrains is about 10 m . In agreement with this, stress exponents between 4 and 4.5 have been found for the Ni-rich phases, i.e., these phases behave like class II alloys with dislocation climb controlling the creep. 

In this mechanism creep occurs by dislocation motion, i.e., glide and climb. For the climb-controlled process the creep rate can be expressed as... 

K using an activation energy of 590 kJ mol . The data are represented by a straight hne with a stress exponent of n 4. In this case, the deformation is attributed to dislocation-climb-controlled intragranular creep (Fig. 6.47). 

At relatively high stresses, beyond the elastic region or the shear moduli, creep is controlled by dislocation-glide movement and by glide in adjacent planes following climb. Real materials contain various internal obstacles (such as dislocations) or... 

Here, Qc is the activation energy for creep and n is the stress exponent. A similar expression may be given for climb-controlled creep ... 

The double power-law creep equation (Eq 8) separates the creep component into climb-controlled and combined climb-glide-controlled. During the field use conditions where the thermal excursions are milder and stresses are lower compared to typical accelerated thermal cycling... 

The water-weakening effect observed in the stress-strain curves of specimens deformed in either regime is not reflected in the observed microstructures. This may be because the small difference in strength of wet and dry specimens (only a factor of about 2) is a consequence of only a small change in the relative activities of different deformation mechanisms, such as glide versus climb or dislocation mobility versus diffusion-controlled creep. However, these observations are not consistent with the assertion of Mackwell et al. (1985) that recovery due to climb is significant... 

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... 

Further below, time-dependent deformation (creep) iiutiated by climb will be extensively discussed. In this section, an example of dislocation climb is illustrated. Figure 3.70 shows dislocation climb in an AI2O3-YAG specimen. Here, climb was assisted by thermal activation. Such a dislocation network, resulting from the reaction of dislocations from the basal and pyramidal slip systems, involves dislocation climb. It is a diffusion-controlled deformation mode characterizing creep deformation and, in this particular case, the activation energy determined is Q = 670 kJ/mol. 

The subscripts and superscripts 1 and 2 refer to cross-slip and climb, respectively. Dislocation motion must overcome significant structural barriers or must cross-slip or climb past obstructions. At the lower temperatures, dislocation crossslip and climb both occur. At the higher temperatures, dislocation climb becomes a rate-controlling mechanism and classic values of the stress exponent (n = 4.5) are obtained. The creep-activation energy is that of cation diffusion. 

Low temperature (in relation to the melting point of the material) creep of metals is usually controlled by dislocation movements, because their structures contain sufficient active slip systems and have small Peierls stresses (the force needed to bring about dislocation movement) [4-8]. Deformation can also be controlled by dislocation climb, a process requiring vacancy diffusion. At high temperatures, deformation in metals is usually controlled by diffusion creep mechanisms that do not involve dislocation movement. In ceramics, however, diffusion creep may be the dominant mechanism under most processing conditions due to the small number of slip planes, the high Peierls stresses, and to the need to move stoichiometric amounts of the different atomic species present in the material (both anions and cations for an ionic compound). 

When there is no primary stress, voids grow by absorption of vacancies, dislocations climb in aU directions and the material simply increases in volume with an isotropic volume expansion this is the swelling phenomenon. But in the presence of a stress and independently of thermal creep, a plastic creep strain is also observed under irradiation in the direction of the applied stress [29]. Several, but mainly two, irradiation creep mechanisms caused by dislocation motion are invoked depending on whether they involve pure dislocation climb, or slip controlled by climb... 

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