Structural evolution in mechanically alloyed Al-Fe powders
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INTRODUCTION
A number of "far from equilibrium" synthesis techniques have been developed during the past few years to produce materials with improved properties; these include rapid solidification (RS) processing, vapor deposition, and mechanical alloying (MA). tl,21 Even though the principles utilized in these various processing techniques are quite different, the main objective of all of them is the same--improved microstructural and constitutional flexibility/control, thereby allowing enhanced physical and mechanical properties. Mechanical alloying is a solid-state powder processing technique that involves repeated welding, fracturing, and rewelding of powder particles in a dry, high-energy ball mill. p,4,5] Mechanical alloying has been shown to be a powerful technique in allowing even further excursions from equilibrium than RS,[2] in addition to its more traditional application of producing oxide-dispersion-strengthened (ODS) nickel- and iron-base alloys33,41 The presence of these nonequilibrium phases allows greater flexibility and control of the final equilibrium microstructure, thereby producing materials with properties superior to those obtained by more conventional ingot metallurgy and casting techniques. The nonequilibrium phases synthesized by MA include supersaturated solid solutions, metastable crystalline or quasicrystalline intermediate (intermetallic) phases, and amorphous phases, t4,61Mechanical alloying also results in a highly refined microstructure with a grain size down to nanometer levels. [7,81
D.K. MUKHOPADHYAY, Graduate Student, Department of Metallurgical and Mining Engineering, is with the University of Idaho. C. SURYANARAYANA, Professor, Department of Metallurgical and Mining Engineering, and Associate Director, Institute for Materials and Advanced Processes, and F.H. FROES, Professor, Department of Metallurgical and Mining Engineering, and Director, Institute for Materials and Advanced Processes, are with the University of Idaho, Moscow, ID 83844-3026. Manuscript submitted August 22, 1994. METALLURGICALAND MATERIALSTRANSACTIONS A
Alloying of A1 with Fe increases the high-temperature strength due to a dispersion of second-phase particles,t9] an effect that can be enhanced by increasing the volume fraction of the second phase after increasing the solid solubility extension of Fe in A1 by techniques such as RS E91or MA.pOJ The A1-Fe phase diagram is shown in Figure 1 and the equilibrium solid solubility of Fe in A1 at room temperature has been reported to be 0.025 at. pct.t"]* *All compositions in this article are expressed in at. pct unless otherwise mentioned.
Intermetallic compounds generally have very high melting points and therefore can potentially be used for hightemperature structural applications, but they are brittle at room temperature. It has been shown that nanostructured materials have improved ductility over their coarse-grained counterparts, t12~Generally, synthesis of intermetallics is not achieved directly by MA;[131 rather, a subsequent annealing is required. H
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