Second phase particles carbides

Adding inert second phase particles results in dispersion hardening. Submicron oxide or carbide particles act as pinning sites that block the motion of dislocations. Intermetallic phases can be made to form, which not only block the motion of dislocations, but also help to stabilize grain boimdaries. The hardening effect of a dispersed phase is not as effective as a coherent precipitated phase, but dispersion hardening can be used in systems that do not... 

In this type of composite materials the main load-bearing constituent is the matrix. Small hard particles are distributed evenly in the matrix to block dislocations. The mechanism of the strengthening is described by several authors, among others several years ago by Kelly (1964) and later by Ashby and Jones (2005a). The matrices are made of metals and polymers. The particles are of different origin silica powder or fine sand are used for polymer matrices and, for metallic matrices, second phase precipitates and particles made of oxides, nitrides, carbides and borides. Precipitates are small particles which crystallise from impurities dissolved in metal alloys. The crystallization during the cooling process results in very small and hard particles, closely distributed in the metal matrix. When ceramic particles... 

Technical alloys frequently consist of different phases i.e., regions with differing chemical composition or crystal structure. As we will see later (in section 6.4.4), particles of a second phase that are enclosed by a matrix of a first phase are especially important to influence mechanical properties. One example for this is iron carbide (cementite, FesC) that increases the strength of steels when precipitated as fine particles. 

Two modifications of the duidized-bed reactor technology have been developed. In the first, two gas-phase duidized-bed reactors coimected to one another have been used by Mobil Chemical Co. and Union Carbide to manufacture HDPE resins with broad MWD (74,75). In the second development, a combination of two different reactor types, a small slurry loop reactor followed by one or two gas-phase duidized-bed reactors (Sphetilene process), was used by Montedision to accommodate a Ziegler catalyst with a special particle morphology (76,77). This catalyst is able to produce PE resins in the form of dense spheres with a diameter of up to 4—5 mm such resins are ready for shipping without pelletization. 

It was shown that the efficiency for radial mixing depends on the gas phase as well (O. Eichstadt, Continuous Mixing of Fine Particles within Fluid Dynamic Vertical Tube Mixers, Dissertation, in German, ETH-Zurich, 1997). At best they operate with low volume concentration and for particles between 20 and 200 pm. Static mixers have been used for very abrasive free-flow materials such as silicon carbide. Since any rotating equipment is avoided inside static mixers, abrasion is limited. As will be shown below, mixture quality is dependent on feed consistency and residence time within the static mixer. Since the latter is very short in static mixers (seconds or fractions of a second), short-time feeding precision has to be very high to achieve high-quality mix. 

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