Metals having a dispersion of a finely divided non-metallic phase have been known for many years the idea being to provide the strengthening that is produced by precipitation hardening without the drawback that the second phase goes into solution as the temperature rises thus limiting the operating temperature.
The Evolution of Metal Matrix Composites
An early example was SAP (sintered aluminium powder) which was made by pressing and sintering heavily oxidised aluminium flake powder, the sintered material being subsequently hot worked to break up the Al2O3 films and disperse them in finely divided form. There was some improvement in the properties but it was insufficient to make large scale use economical. The picture was, however, changed dramatically as newer procedures for getting very much finer dispersions of the non-metallic phase have been developed, and metal matrix composites (MMC) as such materials are now called, represent a major step forward in the search for improved materials i.e. with better mechanical properties especially at elevated temperatures.
Powder metallurgy is the most important route by which such composites are produced. In the majority of cases so far developed the strengthening phase is a stable oxide usually of another metal and the term ODS - oxide dispersion strengthening is in everyday use.
The Internal Oxidation Process
A number of different processes are available for producing the very fine dispersions required. In one process an alloy of the matrix metal with the metal that forms the stable oxide is heated in an atmosphere that is reducing to the matrix metal but oxidising to the second metal. The latter is converted to oxide uniformly distributed throughout the matrix. In the case of precious metals - Ag, Pt etc heating in air can be used and a range of electrical contact materials consisting of silver with a dispersion of e.g. Cd oxide, Sn oxide, and/or In oxide are now widely used. The internal oxidation as the process is called occurs as a result of the diffusion of oxygen through the silver lattice and with large sections, this is a lengthy process. However, if powder is used a relatively short oxidising cycle is required so that the pressing and sintering of internally oxidised powder is the best procedure. In this case the object is not to improve the strength but the electrical properties, i.e. the resistance to contact welding.
Using Salt-Based Precursors
In other cases the matrix metal sometimes in the form of salt is mixed with a solution of a salt of the metal with the more stable oxide and mixture is heated in an atmosphere that is reducing to the matrix metal but oxidising to the second metal. ODS platinum and tungsten are made in this way.
Other composites use fibres or whiskers as the strengthening agent. The most recent process that represents a major step forward in materials for very high temperature applications, gas turbines for jet engines in particular, is mechanical alloying. This process involves milling, usually in an attritor, a mixture of a metal powder and a refractory material for long periods during which the refractory particles are broken up and incorporated in the metal. The 'alloyed' powder is subsequently compacted, sintered, and normally wrought by extrusion or hot rolling. Superalloys made in this way are now in service, and mechanically alloyed aluminium alloys are under trial. In the case of aluminium another mechanical alloy is made by a similar milling process starting with a mixture of aluminium powder and graphite which during the milling process is incorporated in the metal as aluminium carbide, Al4C3.
Another class of wrought sintered material that is beginning to make an impact is based on particulate material - powder or chopped ribbon - that has been solidified and cooled at a very high rate such that metastable non-equilibrium microstructures result. They may be microcrystalline or amorphous. The process is applicable only to certain alloys, and one important feature is that the matrix metal can retain in solid solution a much higher than the equilibrium percentage of the alloying element. Providing that the densification and mechanical working is carried out at a temperature low enough to avoid destroying the non-equilibrium structure, remarkably enhanced mechanical properties can be achieved. A major development programme is underway with alloys of aluminium, titanium, and magnesium, the hope being that their use in aircraft structures will significantly reduce the weight and increase the payload.