Synthesis of nanodispersed phases during rapid solidification and crystallization of glasses in Ti 75 Ni 25 alloys
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I.
INTRODUCTION
DEVITRIFICATION of metallic glasses is one of the popular techniques for producing nanocrystals.[1,2] Several titanium intermetallic compounds, both in nanoscale and large grain sizes, have been produced by this route.[3,4] However, by judiciously controlling the parameters involved during rapid solidification processing, nanocrystals can also be produced directly from the liquid state, eliminating the devitrification step. In order to achieve this goal, it is necessary to understand microstructural evolution under nonequilibrium processing conditions and the underlying mechanisms so that a suitable strategy can be formulated. We have recently reported such an approach for the synthesis of nanodispersions of Ti2Ni in the TiNi matrix close to equiatomic composition.[5] In this article, we first investigate the microstructural evolution under nonequilibrium conditions in a set of Ti75Ni25 alloys with different amounts of Si impurity and endeavor to demonstrate that nanocrystals dispersed in a glassy matrix can be produced directly from the melt by controlling the process parameters. Further, we present a detailed study of the structural and microstructural developments during subsequent heat treatments. The understanding gained allows us to evolve an effective heat treatment, whereby crystalline nanocomposites can be produced. II.
EXPERIMENTAL PROCEDURE
The Ti75Ni25 alloys were rapidly solidified by the melt spinning technique. The primary process parameter varied in the present study was wheel speed, which ranged from
R. NAGARAJAN, NRC Postdoctoral Fellow, is with the Department of Mechanical Engineering, Naval Post Graduate School, Monterey, CA 93943. K. AOKI, Professor, is with the Department of Materials Science, Kitami Institute of Technology, Kitami, Hokkaido 090, Japan. K. CHATTOPADHYAY, Professor, is with the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted April 22, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
30 to 60 m/s. The exact compositions of the alloys as analyzed by an electron probe microanalyzer under different processing conditions are listed in Table I. Two of the alloys contained Si as the impurity picked up from the quartz nozzle during processing and provided an opportunity to study the effect of a small amount of Si. The samples were characterized by the X-ray diffraction technique using a Huber Guinier diffractometer, scanning electron microscopy (JEOL* 840), and transmission electron microscopy *JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.
(TEM) (JEOL 2000 FX II). A detailed transmission electron microscopic examination of all the samples was carried out. The samples for transmission electron microscopic examination were thinned from both sides, making them electron transparent using a GATAN ion mill. The thermal stability of the alloys was investigated using a PERKINELMER** DSC IIC differential scanning calorimeter. **PERKIN-ELMER is a trademark of Perkin-Elmer Physical Electronics, Eden Prairie,
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