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Dispersion Strengthening

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Quick
When the solubility of a material is exceeded by adding too much of an alloying element, a second phase forms and a two-phase alloy is produced. The boundary between the two phases is a surface at which the atomic arrangement is not perfect. In metals, this boundary interferes with the slip of dislocations and strengthens the material. The general term for strengthening by the introduction of a second phase is dispersion strengthening.


Details

More than one phase must be present in any dispersion-strengthened alloy. The continuous phase, which is usually present in larger amounts, is called the matrix. The second phase, usually present in smaller amounts, is the precipitate. In some cases, two phases form simultaneously. These structures are defined differently, calling the intimate mixture of phases a microconstituent.

There are some general considerations for determining how the characteristics of the matrix and precipitate affect the overall properties of a metal alloy:

Good	Poor	The matrix should be soft and ductile, although the precipitate should be strong. The precipitate interferes with slip, while the matrix provides at least some ductility to the overall alloy.
		The hard precipitate should be discontinuous, while the soft, ductile matrix should be continuous. If the precipitate were continuous, cracks could propagate through the entire structure. However, cracks in the discontinuous, brittle precipitate are arrested by the precipitate-matrix interface.
		Precipitate particles should be small and numerous, increasing the likelihood that they interfere with the slip process.
		Precipitate particles should be round, rather than needlelike or sharp-edged, because the rounded shape is less likely to initiate a crack or to act as a notch.
		Large amounts of precipitate increase the strength of the alloy.

Two-phase materials are also produced for reasons other than strengthening; in these cases, the above criteria may not apply. For example, the fracture toughness of materials may be improved by introducing a dispersed phase. Incorporating a ductile phase in a ceramic matrix or a rubber phase in a thermosetting polymer improves toughness; forming a dense network of needle-shaped precipitates in some titanium alloys helps impede the growth of cracks. Producing globules of very soft lead in copper improves machinability.