Characteristics of Particles and Particle Dispersoids

Characteristics of particles and particle dispersoids, (Courtesy of the Stanford Research Institute prepared hy C. E. Lapple.)... 

Fig. 2-2. Characteristics ui particles and particle dispersoids Reprnduci d by penni >-ien of SRI International, Menlo Park, California, 1959-...

The primary distinguishing characteristic of gas dispersoids is particle size. The generally accepted unit of particle size is the micrometer, pm. (Prior to the adoption of the SI system, the same unit was known as the micron and was designated by p.) The particle size of a gas dispersoid is usually taken as the diameter of a sphere having the same mass and density as the particle in question. Another common method is to designate the screen mesh that has an aperture corresponding to the particle diameter the screen scale used must also be specified to avoid confusion. 

The mechanical alloying of mixtures of ductile and brittle components results in the brittle particles being trapped at the layered welded interfaces of the ductile component (Benjamin 1970, Maurice and Courtney 1994). Continued milling results in fi-acture of the brittle particles and the development of a uniform distribution of particles of the brittle phase within the matrix of the ductile phase. This is in contrast to the distribution in conventional powder systems where the dispersoid particles are confined to the prior particle boundaries. Mechanically alloyed CDS alloys are an important practical example of a ductile/brittle system. If the volume fraction of the brittle phase is of the order of 0.5, the characteristic layered structure does not form. Rather, the microstructure consists of a uniformly distributed nanocrystalline mixture of the two phases (Schaffer and McCormick 1990a). 

Aluminum alloy microstructures are developed as a result of alloy composition and thermomechanical treatment. From a corrosion perspective, the dominant features of alloy microstructure are grain structure and the distribution of second phase (intermetallic) particles as constituent particles, dispersoids, or precipitates. Such particles have electrochemical characteristics that differ from the behavior of the surrounding alloy matrix, making alloys susceptible to localized forms of corrosion attack that has been termed microgalvanic corrosion. 

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