Solid-state crystalline-glassy cyclic phase transformations of mechanically alloyed Cu 33 Zr 67 powders
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I. INTRODUCTION
FOR more than 5000 years, most metallic alloys have been fabricated by melting and casting techniques.[1] The mechanical alloying (MA)[2,3] process is rather a newcomer to the world of metallurgy and materials science. Whether it can be called a revolution or simple evolution, the MA method has been successfully employed for producing wide varieties of several advanced materials, including metal nitrides,[4,5] carbides,[6,7] hydrides,[8,9] silicates,[10] equilibrium[11] and nonequilibrium[12] phases, composite and nanocomposite materials,[13–16] and nanocrystalline materials.[17,18] Amorphous alloys are a class of materials which have been also fabricated by the MA method.[19–26] It has been reported that further milling of some amorphous alloy powders, e.g., Ti75Al25,[27] Al80Fe20,[28] Fe78Al13Si9,[29] and Ti50Al25Nb25,[30] leads to amorphouscrystalline phase transformations (crystallizations) and the formation of crystalline phases. The structural change(s) upon milling after crystallization was not studied until 1997, when El-Eskandarany et al.[31] reported the first novel approach for the possibility of cyclic amorphous-crystallineamorphous phase transformations in mechanically alloyed Co75Ti25 amorphous alloy powders. It has been believed that this phenomenon does not have any obvious analogues with the periodic redox reactions or with diffusive-reactive phenomena known in chemistry. The present study has been undertaken to investigate the structural changes that take place upon room-temperature high-energy ball milling a mixture of elemental Cu33Zr67 powders. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and differential M. SHERIF EL-ESKANDARANY, formerly Visiting Professor, Institute of Materials Researcher, Tohoku University, is Professor, Inoue Superliquid Glass Project, Exploratory Research for Advanced Technology, Japan Science and Technology Corporation, Sendai 982-0807, Japan; he is also a Permanent Staff Member, Mining and Petroleum Engineering Department, Faculty of Engineering, Al-Azhar University, Nasr City 11371 Cairo, Egypt. A. INOUE, Professor and Dean, is with the Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Manuscript submitted July 12, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
thermal analysis have been used to monitor the structural changes and the thermal stability of the mechanically alloyed powders. II. EXPERIMENTAL PROCEDURE Pure elemental powders (99.9 pct or better) of Cu (20 m) and Zr (50 m) were balanced to give the nominal composition of Cu33Zr67 (at. pct) and were mixed in a glove box under a purified argon atmosphere. The mixed powders of the starting reactant materials were then sealed into a tempered chrome steel vial (250 mL in volume) together with 50 tempered chrome steel balls (10 mm in diameter). The ball-to-powder weight ratio was maintained as 14:1. The MA process was performed in a high-energy planetary ball mill (Frit
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