Fine microstructure

Pharr, G. M., Harding, D. S., and Oiiver, W. C., Mechanical Properties and Deformation Behavior of Materials Having Ultra-Fine Microstructures, M. Nastasi, D. M. Parkin, and H. Gieiter, Eds., Kluwer Academic Pubiishers, Netherlands, 1993. 

Milnes and Mostaghaci [5.5] compared the consequences of different drying methods on the density, the sinter rate and micro structures of sublimated TiO-, suspensions. Evaporation of water in a micro-oven and by radiation heating leds to strongly bound agglomerates, while freeze drying resulted in softly bound secondary clusters. The freeze dried powder reached in 2 h of sintering 98 % of the theoretical density, while differently dried powders needed twice as much time and had a less fine microstructure. 

The PCA process uses supercritical fluid drying to help preserve fine microstructures in the material. Supercritical fluid drying is a technique that has been used for many years to dry biological materials and, more recently, aerogels (qv). The original solvent is replaced by exchange with a supercritical fluid, such as C02, and the system is depressurized above the critical temperature of the SCF. SCFs have no vapor—liquid interface. Thus fine microstructures are... 

The first carbonitride alloys based on Ti(C,N)—Ni—Mo were introduced in 1970 followed by (Ti, Mo)(C,N)-based compositions having fine microstructures that provided a balance of wear resistance and toughness (4). Continued research on the titanium carbonitride alloys, often called TiC—TiN cermets, in the 1980s led to the development of complex cermets having a variety of additives such as molybdenum carbide(2 1) [12069-89-5], Mo2C, TaC, NbC, zirconium carbide [12020-14-3], ZrC, hafnium carbide [12069-85-1], HfC, WC, vanadium carbide [12070-10-9], VC, chromium carbide (3 2)... 

The successful application of hard coatings on cutting tool substrates is due to the combination of physical and mechanical properties of the coating. From a functional standpoint, chemical stability, hot hardness, and good adhesion to the substrate are essential optimum coating thickness, fine microstructures and compressive residual stresses can further enhance their performance. CVD A1203 and PVD TiAIN provide the necessary chemical inertness required to machine irons and steels. 

High a// ratio, fine microstructure, normally low conversion rates... 

Accelerated nitridation, increased /1-content More homogenous reaction. High a-content, fine microstructure, high strength and KIC Coarse microstructure, low strength and KIC... 

The key to most of the functional properties reported is a fine microstructure of the metal particles (i.e. in the nanometer scale) which is uniformly dispersed within a ceramic matrix. In some cases the particle size needed is in the range of a few nanometers in order to enhance the surface properties, while in other cases optimization is needed between the demand for single domain particles while minimizing unwanted surface states. 

Several techniques have been developed to achieve ceramics and composites with fine microstructure ... 

In Fig. 19.8(a) and Fig. 19.8(c) the fine microstructures of hot-pressed reference samples resulting from ball and attritor milling and sintering are... 

It is instructive to consider the free-energy hierarchy and the metastable phase equilibria when crystallization of an amorphous material is discussed. Koster and Herold [56] discussed these aspects of crystallization and showed that crystallization reactions of amorphous alloys can be classified into the following three types polymorphic, primary and eutectic crystallization reactions. Among these three types, the slowest crystal growth process is expected for primary crystallization and thus, primary crystallization is ideal for tailoring fine microstructures upon decomposition of amorphous alloys. 

Unless a wet cell or cryo stage is used, the fine microstructure is much altered by dehydration in the instrument (J10,S41). However, localized drying occurs in any paste even before it is placed in a high vacuum, as soon as the RH falls below saturation. The water is lost initially from the wider pores, which are probably represented disproportionately on fracture surfaces. The state of the cement paste in a practical concrete may thus vary on both a macro and a micro scale between dry and saturated. 

Samples sintered from B powder at 1400°C have the maximum value of modulus of elasticity (493 GPa). These samples have relatively high density (4.42 g/cm ) and the same fine microstructure as in [7]. These results correlate with the data of the work [4], where samples with the same density and modulus of elasticity were obtained under higher pressure and temperature (7.2 GPa, 2000°C). 

For the present purpose, our first observation is that under certain circumstances, it is possible that the minimizer will be represented as the limit of a sequence of increasingly fine microstructures. Alternatively, this can be stated by noting that in some cases there are no smooth deformations that minimize the energy. Instead, we extend the space of possible minimizers by including those deformations which are piecewise continuous and are allowed to have a vanishingly small thickness. 

We report on the powder metallurgical fabrication of bismuth-antimony solid solution and the thermoelectric properties of the fabricated composites. The solid solution powders were prepared by mechanical alloying (MA) aiming at large reduction of the thermal conductivity with the very fine microstructures obtained through MA process. The prepared bismuth-antimony powders (Bi-7.5at%Sb) have been sintered by hot pressing. 

Comparing the requirements for high strength (uniform, fine microstructure) to those needed to improve toughness (nonhomo-geneous, duplex microstructure) reveals the problem in achieving both simultaneously. 

Shelkovnikov et al. (1976) reported low values like 10-100 Oe. Hardt (1966) found that evaporated films with a fine microstructure had He values from 30 to 340 Oe depending on the Co/Sm ratio, the annealing temperature (760-940 K) and the presence of a magnetic field. The only conclusion he can draw is that He decreases for higher Co concentrations (above the SmCos stoichiometry). 

Another additive that has been investigated to improve the performance of HA is Hthium (Li). The addition of lithium can increase the microhardness and produces a fine microstructure in HA. Fanovich et al. [1998] found that the addition of 0.2 wt% of Li to HA produced the maximum microhardness (5.9 GPa). However, the addition of high amounts of Li to HA results in abnormal grain growth and large pores. Furthermore, Li addition to HA results in the formation of /3-TCP upon sintering. 

Dense parts made by hot-pressing are generally considered to have a very fine microstructure (see Fig. 12g) and the best mechanical properties. Completely pure silicon carbide does not exhibit any noticeable plastic behavior up to its decomposition temperature and can therefore be densified only under diamond synthesis conditions... 

A detailed analysis of different microstructures [29-31,33] shows that they can be explained by the number of p nuclei in the starting powder, even at very low P Si3N4 contents. In Fig. 8 the calculated volume fractions of growing nuclei which are necessary in the starting composition to produce two different fine microstructures in the sintered materials are given. This calculated content is the lowest P-Si3N4 content in the starting powder required to explain the microstructure. For crystallite sizes between 50 and 100 nm [20,35], the necessary amount of the p fraction in... 

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