High Resolution Electron Microscope Study on Formation and Growth Processes of Atomic Medium Range Order in Amorphous Alloys

点击翻译按钮获取中文摘要

This study has been made as a two years project study from 1990 to 1991 in order to investigate structure and morphologies of atomic medium range order (MRO) in amorphous alloys, especially to investigate its formation and growth processes. As an amorphous alloy system we chose Pd-Si system. since the system has been well known as having a lot of X-ray and neutron diffraction data. In this study, eutectic Pd_<82>Si_<18> alloy ribbons made by rapid-quenching and Pd_<82>Si_<18> thin films prepared by Ar-beam sputtering were used as specimens. Structure observations of as-formed and annealed specimens were made by high resolution electron microscopy (HREM) and analytical electron microscopy. For thinning ribbon specimens, an ion-milling equipment was used which was funded by this research project. Obtained results are as follows:(1)MRO structures were observed in both of the ribbon and sputtered specimens. Calculated HREM images based on a fcc-MRO structure model were consistent with thos … More e observed.(2)Size and distribution of MRO domains were dependent of the cooling rate in rapid-quenching or in the vapor deposition.(3)On Annealing up to 573K, a MRO domain growth was observed. At 573K the MRO domain grows to a size of about 5nm, giving a splitting of the second halo diffraction ring into two diffuse rings in the selected area diffraction. Clear fcc diffraction patterns are obtained from the domains by the nano-beam diffraction. Si atoms in the MRO domains diffuse out to the matrix region on annealing at 573K.(4)On annealing at 623K clear fcc diffraction patterns start to appear in the selected area diffraction. The precipitation of alpha-Pd with a size of about 8nm starts at this temperature.The MRO structure and its growth on annealing are observed in both ribbon and thin film specimens in the same morphology in the same temperature ranges. The "structural relaxation" in an amorphous alloy on annealing can be understood as a phenomenon in which a free-volume reduction occurs mainly by the growth of MRO and a chemical ordering of atoms in the matrix proceeds by the local compositional variation on annealing. Less