A novel way of amorphous phase formation during mechanical alloying of copper and cadmium powders
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INTRODUCTION
IT is well known that amorphous phase formation can be induced by mechanical alloying.[1–9] In general, there are two different ways of producing metastable phases, including amorphous phases, by ball milling. In one approach, an amorphous phase is formed directly from the reaction between two elemental metals when a powder mixture of an appropriate composition is subjected to milling.[1–5] It is believed that kinetic factors allow the amorphous phase formation in this case, in preference to the formation of possible equilibrium intermediate phases, even though the driving free energy for the formation of such phases may be larger than that for the formation of an amorphous phase. A different situation occurs when a powder of an equilibrium intermediate phase (or phases) is destabilized by ball milling and transformed into a corresponding amorphous phase.[3,6–9] In this case, it is believed that the free energy of the equilibrium intermediate phases is raised to a level above that of the possible amorphous phases in the same composition range, due to accumulation of structural defects including grain boundaries and free surfaces, thereby raising the free energy above that of an amorphous state (i.e., a frozen liquid at that temperature).[3] In a previous study, Lee and Koch[10] demonstrated that, in the nickelzirconium system, an amorphous phase could also be formed by ball milling a mixture of two equilibrium intermetallic compounds or of a mixture of pure nickel (or zirconium) and an intermetallic compound. This may be considered as a third way of amorphization by ball milling. However, as pointed out by Lee and Koch, the amorphization of these mixtures appears to proceed by initial amorphization of the individual intermetallic compounds, followed by alloying of the two amorphous phases, or initial amorphization of an intermetallic phase, which is then enriched by the pure metal without occurrence of crystallization. If this is the case, this third way involves the deD.L. ZHANG, Lecturer, is with the Department of Technology, University of Waikato, Private Bag 3105, Hamilton, New Zealand. T.B. MASSALSKI, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890. Manuscript submitted December 19, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
stabilization of intermetallics and, thus, resembles the second. In the present article, we also report a case of amorphization by ball milling a mixture of a pure metal and an equilibrium intermediate phase. However, in this case, an admixture of the pure metal appears to be a necessary condition for the amorphous phase formation. This unusual behavior was discovered during mechanical alloying of copper and cadmium powders. The copper-cadmium system itself is unusual in that there is a rather large size difference (16.5 pct) between the copper and cadmium atoms.[11] For this reason, there is very little primary solid solubility on both sides of the phase diagram, and the intermediate pha
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