Metastable Phases in Metals Produced by Ion Implantation

Abstract

Metallic alloys are usually produced by equilibrium or quasiequilibrium methods. The constituents of the alloy including any desired impurities can be mixed together, melted, and the resultant liquid allowed to solidify and cool. Because the cooling rates are normally fairly slow, the final alloy is usually stable against thermal annealing at operating tempera­ tures and times. In some cases, an alloy may be rapidly cooled or quenched in order to produce a material with a particular physical property. This alloy is metastable with respect to thermal annealing, and the property will be lost if the material is heated to a sufficiently high temperature. The alloy was produced, however, from an equilibrium phase, perhaps a liquid, at a higher temperature. This limits conventional studies of metastable alloys to those that can be formed by quenching from an equilibrium phase. Ion implantation is a method for introducing impurities into the surface layer of a solid in a nonequilibrium fashion. Ions of the desired impurity are accelerated to an energy (generally in the range of 20-200 keY) related to the desired depth of the implanted layer and directed onto the surface of the material to be implanted. The ions penetrate the surface of the material and come to rest some distance below the surface. The impurity concentration distribution in depth is usually almost Gaussian with the peak of the distribution at a depth of a few hundred to a few thousand angstroms, depending on the ion-target combination and the