DISPERSION-STRENGTHENED METALSParticular composites in which a stable material, usually an oxide, is dispersed throughout a metal matrix. The particles are less than 39 (iin (1 (xm) in size, and the particle volume fraction ranges from only 2 to 15%. The matrix is the primary load bearer while the particles serve to block dislocation movement and cracking in the matrix. Therefore, for a given matrix material, the principal factors that affect mechanical properties are the particle size, the interparticle spacing, and the volume fraction of the particle phase. In general, strength, especially at high temperatures, improves as interparticle spacing decreases. Depending on the materials involved, dispersion-hardened alloys are produced by powder-metallurgy liquid-metal, or colloidal techniques. They differ from precipitation-hardened alloys in that the particle is usually added to the matrix by nonchemical means. Precipitation-hardened alloys derive their properties from compounds that are precipitated from the matrix through heat treatment.
There are a rather wide range of dispersion-hardened-alloy systems. Those of aluminum, nickel, and tungsten, in particular, are commercially significant. Tungsten thoria, a lamp-filament material, has been in use for more than 30 years. Dispersion-hardened aluminum alloys, known as SAP alloys, are composed of aluminum and aluminum oxide and have good oxidation and corrosion resistance plus high-temperature stability and strength considerably greater than that of conventional high-strength aluminum alloys. Another dispersion-hardened metal, TD nickel, has dispersion of thoria in a nickel matrix. It is 3 to 4 times stronger than pure nickel at 1600 to 2400°F (871 to 1316°C). TD-nickel-chromium also has been produced for increased resistance to high-temperature oxidation.
Other metals that have been dispersion-strengthened include copper, lead, zinc, titanium, iron, and tungsten alloys. The copper is used for resistance-welding (spot-welding) tips.
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