Precipitation creep

It is shown that solute atoms differing in size from those of the solvent (carbon, in fact) can relieve hydrostatic stresses in a crystal and will thus migrate to the regions where they can relieve the most stress. As a result they will cluster round dislocations forming atmospheres similar to the ionic atmospheres of the Debye- Huckel theory ofeleeti oly tes. The conditions of formation and properties of these atmospheres are examined and the theory is applied to problems of precipitation, creep and the yield point."... 

If you see the amps on your precipitator creeping up, or spiking up, something is beginning to short-circuit the electric grids, or insulators. Most commonly, corrosion products are falling off the walls of the precipitator vessel. In my experience, this is the most common cause of precipitator failure. 

The solution-precipitation creep model was first proposed by Raj and Chyung 32 they assumed two cases ... 

Wakai, F., Step model of solution-precipitation creep , Acta Mater., 1994,42, 1163-72. 

Melendez-Martinez, J J., Gomez-Garcla, D., and Dominguez-Rodriguez, A., Acritical analysis and a recent improvement of the two-dimensional model for solution-precipitation creep application to silicon nitride ceramics , Phil. Mag., 2004, 84, 2305-16. 

Raj R. and Chyung C.K., "Solution-Precipitation Creep in Glass-Ceramics," ActaMetaU., 29,159-66 (1981). 

AISI 321 and 347 are stainless steels that contain titanium and niobium iu order to stabilize the carbides (qv). These metals prevent iatergranular precipitation of carbides during service above 480°C, which can otherwise render the stainless steels susceptible to iatergranular corrosion. Grades such as AISI 316 and 317 contain 2—4% of molybdenum, which iacreases their creep—mpture strength appreciably. In the AISI 200 series, chromium—manganese austenitic stainless steels the nickel content is reduced iu comparison to the AISI 300 series. 

As we saw in Chapter 10, the stress required to make a crystalline material deform plastically is that needed to make the dislocations in it move. Their movement is resisted by (a) the intrinsic lattice resistance and (b) the obstructing effect of obstacles (e.g. dissolved solute atoms, precipitates formed with undissolved solute atoms, or other dislocations). Diffusion of atoms can unlock dislocations from obstacles in their path, and the movement of these unlocked dislocations under the applied stress is what leads to dislocation creep. 

Climb unlocks dislocations from the precipitates which pin them and further slip (or glide ) can then take place (Fig. 19.3). Similar behaviour takes place for pinning by solute, and by other dislocations. After a little glide, of course, the unlocked dislocations bump into the next obstacles, and the whole cycle repeats itself. This explains the progressive, continuous, nature of creep, and the role of diffusion, with diffusion coefficient... 

If the particle Re is well above the creeping flow range, mean drag may be increased or decreased by freestream turbulence. The most significant effect is on the critical Reynolds number. As noted in Chapter 5, the sharp drop in Cd at high Re results from transition to turbulence in the boundary layer and consequent rearward shift in the final separation point. Turbulence reduces Re, presumably by precipitating this transition." ... 

Surface cracks in boiler tubes, creep damage in power plant components and precipitate analysis in components subjected to high temperature and stress have been successfully assessed by this technique and necessary further inspection scheduled, depending upon the severity of the observed defect. The following is an example of the type and severity of defects and the necessary action taken in the context of power plant operations. 

Fig. 4.10 Models of two-phase creep (a) solution-precipitation and (b) matrix flow.

Solution-precipitation theory cannot be used to justify creep asymmetry or high tensile stress exponents for ceramic matrix composites. The theory suggests that creep is symmetric in stress and that the stress exponent is equal to 1. Justification of creep asymmetry by solution-precipitation would require other parameters in Eqn. (4) to depend on the sign of the applied stress. A nonlinear dependence on stress would be required. Diffusion and devitrification may play a role in this regard however, the data needed to support this possibility have yet to be obtained. 

Above the threshold, deformation occurs as a consequence of direct particle interaction. Several mechanisms of interaction have been suggested solution-precipitation flow of fluid between particles and cavity formation at the particle matrix interface. These theories of creep suggest several rules to improve creep behavior (1) increase the viscosity of the matrix phase in multiphase materials (2) decrease the volume fraction of the intergranular phase (3) increase the grain size (4) use fiber or whisker reinforcement when possible. As the creep rupture life is inversely proportional to creep rate, lifetime can be improved by improving creep resistance. 

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