Ion—Irradiation Induced Phase Changes in Metallic Systems

Abstract

It is well known that irradiation of thin metallic film structures by high—energy ion beams produces a variety of metallurgically interesting modifications of the atomic structure and morphology of the irradiated material. The modifications include changes in the composition profile of the film structure due to ion mixing, creation of nonequilibrium defect populations within the phases present, and even changes in the phases themselves. The latter type of modification is the primary concern of this paper. In order to address the question as to why phase changes occur, one must in fact also develop an understanding of the evolution of composition profiles and defect distributions within the phases present since these are the factors which control the thermodynamic stabilities of the competing phases. This subject can be naturally divided into two cases according to whether the initial configuration of the film structure is chemically homogeneous or inhomogeneous. In the former case, ion irradiation does not alter the overall composition profile. Neglecting effects such as preferential sputtering of one component from a free surface, the composition of the film remains uniform throughout the sample during irradiatio, and ion mixing produces only local concentration fluctuations. In the latter case of an initially inhomogeneous material (e.g. a bilayer or multilayer film with two or more components), the overall composition profile evolves under irradiation and the phases present are subjected changes in average composition.