Grain size materials

The creep rate is inversely proportional to the square of the grain size d. Thus large-grain-size materials are more resistant to creep than finegrained ceramics. This is well documented experimentally. 

Catalysts with perovskitic structure guarantee a good compromise between stability and activity and have a relatively low cost, so they can constitute a valid alternative to supported noble metals, with particular reference to the reactions of partial or total oxidation of hydrocarbons (catalytic combustion). The traditional route used to synthesize perovskites was first introduced by Delmon and co-workers in the late nineteen sixties [1]. It enables to obtain i) mixed oxides over a wide range of composition ii) good control of the stoichiometry iii) an excellent interspersion of the elements in the final product iv) very small grain size materials. 

Studies performed at the University of Missouri-Rolla in conjunction with Rockwell Scientific have shown FSP to produce a hne-grain-size material and create low-temperature, high-strain-rate superplasticity in aluminum and titanium alloys. The PNNL is currently investigating the application of this FSP-induced superplasticity in the fabrication of large, integrally stiffened structures. 

It has been shown that coarse grain size materials are more susceptible to SCC than fine grain size material. A patch-type of relationship connecting grain diameters and the stress required to initiate a SCC crack is well-known 9pr = 9o + Kd, where 9q and K are constants. K is related to the surface energy associated with formation of new surface ... 

Unlike melting and the solid-solid phase transitions discussed in the next section, these phase changes are not reversible processes they occur because the crystal stmcture of the nanocrystal is metastable. For example, titania made in the nanophase always adopts the anatase stmcture. At higher temperatures the material spontaneously transfonns to the mtile bulk stable phase [211, 212 and 213]. The role of grain size in these metastable-stable transitions is not well established the issue is complicated by the fact that the transition is accompanied by grain growth which clouds the inteiyDretation of size-dependent data [214, 215 and 216]. In situ TEM studies, however, indicate that the surface chemistry of the nanocrystals play a cmcial role in the transition temperatures [217, 218]. 

The commercial sintered spinel and M-type ferrites have a porosity of 2—15 vol % and a grain size in the range of 1—10 ]lni. In addition, these materials usually contain up to about 1 wt % of a second phase, eg, CaO + Si02 on grain boundaries, originating from impurities or sinter aids. 

Hot pressing produces compacts that have superior properties, mainly because of higher density and finer grain size. Closer dimensional tolerances than can be obtained with pressing at room temperature are also possible. Hot pressing is used only where the higher cost can be justified. It has been usehil in producing reactive materials. One use is the combination of P/M and composites to produce hot-pressed parts that are fiber reinforced. 

The microstmcture and imperfection content of coatings produced by atomistic deposition processes can be varied over a very wide range to produce stmctures and properties similar to or totally different from bulk processed materials. In the latter case, the deposited materials may have high intrinsic stress, high point-defect concentration, extremely fine grain size, oriented microstmcture, metastable phases, incorporated impurities, and macro-and microporosity. AH of these may affect the physical, chemical, and mechanical properties of the coating. 

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