Atomic Structure of Alloys Rapidly Quenched from the Melt

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High resolution electron microscopy, field ion microscopy (FIM), analytical electron microscopy (STEM X-ray microanalysis by energy dispersive X-ray spectrometer; STEM-XMA) and in situ high voltage electron microscopy (HVEM) were successfully used to investigate the structure and stability of some glass-forming alloys. Thin ribbons of Fe78P15C7, Fe80P13C7, Ni80P15B5 and Ni75B15Si10 alloys were obtained by rapid quenching from the melt by the twin-roll method. In the untilted interference images (high resolution transmission electron microscope images; HR-TEM) and FIM images of rapidly quenched state and of annealed state, regular fringes (HR-TEM) and circular arrays of image spots (FIM), which suggested the existence of local orders, i.e., microcrystallites, were sometimes observed, depending on the quenching methods and the quenching or annealing conditions. These results suggest the existence of different atomic structures in the so-called amorphous alloys which yield diffraction halos. In all the amorphous alloys examined in this study, the transformation from the amorphous state to the stable crystalline state (ST-state) passed through intermediate stages where metastable crystalline phases (MS-phases) appeared. The STEM-XMA and the selected area diffraction (SAD) analysis showed that the MS-phase in the Ni80P15B5 amorphous alloy had basically the same structure and composition as Ni3P, which was one of the ST-phases in the recrystallized state of this alloy. Furthermore, a slight expansion in the lattice suggested that the MS-phase contained a little amount of B. The STEM-XMA and the SAD analysis also showed that the MS-II phase which was one of the MS-phases in Ni75B15Si10 amorphous alloy, i.e., the MS-phase which appeared at higher temperature by in situ heating in a HVEM, did not contain Si, and the MS-II phase was considered to be a compound in the system Ni–B. The HR-TEM showed that the MS-phases, in general, contained many internal defects.