A Study Of Metastable Alloys Formation In Solid Thin Films

Through the years there has been a growing interest in metastable alloys formation in solid thin films since the first synthesis of amorphous alloy in the early 60's of this century. In this thesis, I have studied systematically a variety of metasatble alloys including amorphous and metastable crystalline phases formed in the binary metal thin films, and attempted to clarify such questions from thermodynamic and kinetic points of view, and as possible relationship to electronic structures. Several new metastable crystalline phases were observed by means of ion mixing of the thin films with their structures and compositions determined, and were accordingly classified into five categories: (1) super saturated solid solution; (2) hcp- I around A3B (A=fcc) through a transition of fcc→hcp; (3) fcc-I around AR3 (B=bcp) via hcp→fcc; and (4) hcp-Ⅱ and fcc-Ⅱ phases formed around AB3-4(B=bcc) via a two step transition of bcc→hcp→fcc. A thermodynamic model and a crystallographic model were proposed to calculate their free energies. Besides, relationship between the electron/atom ratio and the composition was given for some of the phases. Amorphous alloys phases were obtained in the Y-Mo, Y-Ta and V-Zr multilayered films with large positive heats of mixing. It was found, for the first time, that the formation of the amorphous alloys is interface-controlled: the fraction of the interfacial atoms determines the initial energetic state of the multilayered films, and thus determining the maximum possible composition range of the amorphous alloys, i.e. the more the interfacial atoms, the broder the composition range. The initial energies of the multilayered films were evaluated by using a simple model for an interface to interpret the formation of the amorphous alloys in these systems. A modified thermodynamic model, which can be used for the binary metal systems with either a negative or a positive heat of mixing, was proposed by the authors. The influence of the sign of the heat of formation and the atomic size mismatch between the constituent metals on the spontaneous vitrification was discussed in detail on the basis of the experimental results and the theoretical calculations. Zhengjun Zhang (Materials Physics) Directed by Professor Baixin Liu...