Elevated-temperature stability of mechanically alloyed Cu-Nb powders
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I.
INTRODUCTION
W H E N two ductile phases are mechanically alloyed, they develop a convoluted lamellar (Damascus steel) microstructure for intermediate milling times. Interlamellar spacings found in this stage depend on the mechanical properties of the processed materials and are also process dependent (i.e., spacings depend on mill power and type). But spacings are generally in the range of several to several tenths of micrometers. Consolidation of milled powders inevitably necessitates their exposure to elevated temperatures. During this exposure, microstructural changes occur. In addition to recovery/ recrystallization of the heavily cold-worked materials, the microstructural morphology is altered. Lamellae often evolve into spheres, and the structure coarsens in general. As properties are affected by structural changes, it seems worthwhile to ascertain and classify the kinds of structural evolution that might take place in mechanically alloyed powders during elevated-temperature exposure. This article attempts to do this for a simple binary alloy, Cu-Nb. These elements are nearly insoluble in each other at room temperature. Near the melting point of Cu, a modest amount of Nb (ca. 1.5 wt pct) can dissolve in Cu. Many of the kinds of structural changes that occur in mechanically alloyed structures can be predicted a priori-recrystallization, for example. And the general means by which lamellae evolve into spheres are likewise known. The subsequent coarsening behavior of these spherical dispersions ought to be that of Ostwald ripening, extensively studied in the past. However, there are likely to be peculiarities and nuances associated with the "coarsening" of mechanically alloyed "lamellar" structures. The R.J. COMSTOCK, Jr., Graduate Student, is with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903. T.H. COURTNEY, formerly Professor, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA, is Professor and Chair, Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, M1 49931. Manuscript submitted November 15, 1993. METALLURGICALAND MATERIALS TRANSACTIONSA
descriptions of shape instabilities in plates--which eventuate in their transforming to spheres--have been couched in terms of ideal structures (Section II); that is, ones for which the broad faces of the plates are presumed smooth and devoid of curvature. This is not the case for mechanically alloyed powders. Figure 1 is a micrograph of a Cu-20 vol pct Nb mechanically alloyed powder (the material of this study). The many twists and bends in the lamellae lead to local variations in curvature not considered in the "idealized" descriptions of lamellar instabilities. The variations in lamellar thickness, noticeable also in Figure 1, can further complicate these descriptions. Considerations such as this are addressed in this article. However, before presenting the specifics relative to this work, it is worthwhile to summarize
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