Modeling of particle size evolution during mechanical milling
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INTRODUCTION
M E C H A N I C A L alloying (MA) is a versatile process for the production of powders having unique microstructures. It has been used for some time for the production of Ni-base superalloys and dispersion-hardened AI powders, i~ 5] More recently, the process has been used to synthesize intermetallics and other inorganic nonmetallics.~6 ~0~And when high-energy mills are used for such synthesis, the powders produced manifest even more unusual characteristics. Examples include nanocrystalline structures i~H61 (resulting from the extensive plastic deformation occurring during MA), amorphous materials i~7 221 (whose formation is facilitated by the considerable stored energy of cold work and the large degree of interphase surface area per unit volume concurrent with the fine microstructures generated by MA), and alloys with extended solubilities [23'24'25](likely for the same reasons amorphous phases form). When ductile materials are mechanically alloyed, the process can conveniently be divided into several stages, as emphasized by Benjamin and Volin. t261 Plastic deformation first causes particle flattening. This is followed by a period where particle welding is dominant and the average particle size increases while the number of particles decreases. The structure of the particles has a lamellar nature. As the particles work harden, they fracture more readily. During this stage where fracture and welding take place at approximately the same rate, the particles now contain a number of differently oriented lamellar regions. Subsequent milling leads to further microstructural refinement. This period is often referred to as "steady state processing." However, the term is somewhat of a misnomer as particle size and size distribution are not invariant during this stage, although it is true that these characteristics change at a lesser rate than they do in the earlier stages of the process. B.J.M. AIKIN, formerly Graduate Student, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA, is Research Associate, Case Western Reserve University, NASA Lewis Research Center, Cleveland, OH 44135. T.H. COURTNEY, formerly Professor, Department of Materials Science and Engineering, University of Virginia, is Professor and Chair, Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931. Manuscript submitted January 27, 1993. METALLURGICAL TRANSACTIONS A
We have been interested in modeling the MA of ductile materials. This has involved studies from both a fundamentaP 27-3~ and an empirical perspective. With respect to the latter, the authors have been interested primarily in the respective fracture and welding tendencies during the early MA stages and how these depend upon material mechanical properties and process variables. Knowing these factors, the kinetics of the alloying process can be characterized with potential benefit for predictive purposes and/or process refinement and optimization. We have used experimental observation
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