Mechanically induced solid-state devitrifications of Zr 70 Pd 20 Ni 10 glassy alloy powders
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I. INTRODUCTION
WHETHER it can be called a revolution or simple evolution, the worldwide interest in metallic glassy alloys has been sustained to a great degree by the clear benefits seen in the use of such advanced materials in a number of application areas.[1– 4] The unique achievement of Duwez et al.[5] in 1960, when they obtained Au-25 at. pct Si glass by applying a very high cooling rate (⬎105 K/s) to the metallic melt, has led to subsequent studies for fabrication of a wide variety of glassy and amorphous materials at slower rates of cooling[6] or even at room temperature (mechanical alloying (MA)).[4,7] So far, glassy alloys have attracted many in the materials-science community due to their desirable physical, chemical, and mechanical properties, which make them pioneer materials for many industrial applications. Zr-based glassy alloys, with their unique short-range atomic order, exhibit many interesting amorphization and crystallization behaviors that make them ideal noncrystalline alloys for fundamental studies. Most of the Zr-based glassy alloys have been obtained by the liquid casting technique, where the solidification occurs so rapidly that the atoms are frozen at their liquid configuration. Most striking is the evolution of new phases and structures not given in the standard texts and not apparently produced by the conventional technique of cooling from the melt. It has been reported that some Zr-based glassy alloys devitrificate into a metastable icosahedral quasi-crystalline phase upon annealing above their glass transition temperature (Tg).[8–14] It is believed that the formation of such a metastable phase is attributed to the original precipitation of a small mole fraction of potential nanoclusters during the casting procedure.[13] M. SHERIF EL-ESKANDARANY, Professor of Materials Science and Metallurgy, formerly with the Inoue Superliquid Glass Project, Exploratory Research for Advanced Technology, Japan Science and Technology Corporation, Sendai 982-0807, Japan, is with the Department of Mining and Metallurgy, Faculty of Engineering, Al-Azhar University, Nasr City 11371, Cairo, Egypt. Contact e-mail: [email protected] J. SAIDA, formerly Group Leader, Inoue Superliquid Glass Project, is with the Institute of Materials Research, Tohoku University, Sendai 980-8577, Japan. A. INOUE, Professor and Dean of Institute for Materials Research, is with Tohoku University. Manuscript submitted July 8, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
In a previous work,[15] we attributed the precipitation of such nanoicosahedral clusters to the inability of the solidification procedure, which occurs so rapidly, to overcome any homogenization opportunities of the obtained glassy phase. In contrast to the melt-spun technique, ball milling the elemental powders of the same composition does not allow such an opportunity for the cluster formation. This can be understood from the milling mechanism and the high capability of the MA to homogenize the glassy powders at the atomic scale throughout a long period
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