Processing and microstructural characterization of Al-Cu alloys produced from rapidly solidified powders
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I. INTRODUCTION
RAPIDLY solidified alloy powders offer a number of benefits for producing materials with novel microstructures. For example, the scale of the microstructure is generally much smaller than that produced by conventional molten-metallurgy routes. Moreover, by mixing powders of different composition prior to consolidation, microstructures with controlled heterogeneity can be produced. In this article we describe the development of such materials in the Al-Cu system with a wide range of composition. The motivation for this work is to better understand the mechanical behavior of alloys and particulate composites that contain hard, brittle particles on the scale of microns and larger. Such materials are known to exhibit high rates of strain hardening during the elastic-plastic transition.[1] It has been postulated that the hardening rate is enhanced due to the inhomogeneous distribution of the second phase. Continuum constitutive models which assume a bimodal distribution of the particles into particle-rich regions (or clusters) and particle-poor regions, do indeed predict an increased rate of hardening when compared with a material containing a uniform distribution of particles with the same overall volume fraction.[2] However, real materials processed by conventional techniques are complex, and the assumption of a bimodal distribution is clearly a poor approximation to these microstructures. In order to test these models through experiment, materials with intentionally bimodal particle distributions are needed. The production of such materials is the subject of this article. K.T. CONLON, formerly Graduate Student, Department of Materials Science and Engineering, McMaster University, is Postdoctoral Fellow, Neutron Program for Materials Research, National Research Council of Canada, Chalk River, ON, Canada K0J 1J0. E. MAIRE, formerly Postdoctoral Fellow, Department of Materials Science and Engineering, McMaster University, is Research Associate, GEMPPM, INSA de Lyon, 69621 Villeurbanne, France. D.S. WILKINSON, Professor, is with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada L8S 4L7. H. HENEIN, Professor and Director, is with the Advanced Materials and Processing Laboratory, University of Alberta, Edmonton, AB, Canada T6G 2G6. Manuscript submitted February 9, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
The powders used in this study are produced by a novel process known as impulse atomization (IA).[3,4,5] This new atomization technique allows us to achieve a narrow dropletsize distribution using a uniform orifice-size nozzle. By varying the orifice size from 12.5 to 2000 mm, a tailored droplet-size distribution can be designed. The basic approach for IA is to periodically accelerate the fluid in a crucible through orifices located at the crucible bottom. A plunger immersed in the fluid and placed near the nozzle supplies the acceleration. As a result of the oscillation of the plunger, segments of fluid emerge from the nozzle. These segments further br
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