Crystallization characteristics of the Ti 53 Cu 47 amorphous alloy
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DUCTION
RECENTLY, research on nanocrystalline materials (NCMs) has been conducted extensively.[1–4] In general, NCMs are materials that have grain sizes of 1 to 100 nm. Because of their ultrafine grains, the NCMs have a large interfacial or surface area. Since the interfacial surface or energy plays an important role in determining the free energy of NCMs, and because there are a large number of atoms located at the interfaces, NCMs can be considered to have a structure in between those of crystalline and amorphous materials. It is expected that NCMs would exhibit novel physical,[5,6,7] mechanical,[8,9] and chemical properties.[10] There are many methods to synthesize NCMs, e.g., vapor condensation,[1,11,12] mechanical milling,[13,14] rapid quenching,[15,16,17] etc. In addition, a method to synthesize NCMs by means of crystallization from the amorphous ribbons has also been developed recently.[18,19] With this method, it is easy to obtain grains with various nanoscale dimensions by controlling the heat-treatment condition. Since dense nanocrystalline alloys are difficult to obtain from the process of vapor condensation followed by in-situ compaction, direct crystallization from the amorphous alloys makes it possible to produce nanocrystalline alloys without porosity. Furthermore, by proper control of the heating condition, the crystallization method is also able to create different kinds of interfacial structures in NCMs. It provides a better chance to investigate the nature of the interfacial structure (coherent, semicoherent, or noncoherent) in NCMs. Because of the aforementioned advantages, the crystallization method has C.-L. LEE, formerly Graduate Student, Department of Materials Science and Engineering, National Tsing Hua University, is Team Leader, Nanya Technology Corp. Luchu, Taoyuan, Taiwan, R.O.C. F.-R. CHEN, Professor, Department of Engineering and System Science, and T.-P. PERNG, Professor, Department of Materials Science and Engineering, are with the National Tsing Hua University, Hsinchu, Taiwan, R.O.C. Manuscript submitted August 28, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS A
become one of the most important methods to synthesize NCMs. The crystallization process from the amorphous phase involves nucleation and growth. A high nucleation rate and a low growth rate are necessary to manufacture NCMs from amorphous alloys. Therefore, to obtain NCMs, knowledge of the crystallization behavior of amorphous alloys is necessary. The studies of crystallization behavior have been frequently discussed in the literature.[20,21,22] In order to monitor the change of the heat content of materials during the crystallization process, differential scanning calorimetry (DSC) is often applied. When it is used in isothermal heating, the results of DSC are very useful for obtaining the kinetics parameters of crystallization. If the DSC result is combined with the observation by transmission electron microscopy (TEM), the structural evolution of the ribbon at various stages of annealing can be obtained. The descript
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