ATOM-PROBE STUDIES OF NANOSTRUCTURED ALLOYS

This paper reviews our recent atom probe studies of nanostructured alloys which are fabricated by three methods: (1) crystallization from metallic glasses, (2) sputter film deposition, and (3) phase decomposition. Alloys with nanograin structures display various interesting magnetic or mechanical properties which are not attainable in conventional alloys. In the first half of this paper, we focus our discussion on the evolution of nanocrystalline structure from metallic glasses by the primary crystallization process. By atom probe analyses, we found evidence that heterogeneities of one of the alloying elements are present in the initial stage of crystallization. In the later stage of crystallization, one of the slow diffusers controls crystal grain growth. In the second half of this paper, we discuss the nanostructure of Co-Cr sputtered thin films and Cr-Fe alloys, in which phase separation plays a critical role for controlling magnetic properties. Co-Cr has an equilibrium two-phase region below 400 degrees C, at which temperature the bulk diffusion is negligible. However, by producing alloy thin films by the sputter deposition method at elevated temperatures (200 degrees C-400 degrees C), we have observed the presence of the equilibrium two-phase state and improved magnetic properties for these films. Cr-Fe films and bulk alloys both show giant magnetoresistance (GMR). The MR value changes depending on the microstructural evolution, and the origins of MR are discussed on the basis of the atom probe results.