Microstructure, excess solid solubility, and elevated-temperature mechanical behavior of spray-atomized and codeposited
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INTRODUCTION
DISPERSION-strengthened, elevated-temperature aluminum alloys derive their strength and thermal stability from the presence of a dispersion of nanometer-sized particles which effectively impede dislocation motion during deformation. The strengthening characteristics of these particulates at elevated temperatures are dependent on their ability to resist coarsening, and therefore, low diffusivities and limited or no equilibrium solid solubility are desirable, tl] Two approaches can be utilized to synthesize aluminum alloys containing a dispersion of thermally stable, nanometer-sized phases. The first approach involves energetically blending a mixture of fine aluminum alloy powders with a ceramic phase, typically oxides, carbides, or nitrides, to produce a matrix containing a distribution of well-dispersed, incoherent fine particles. Since these ceramic particles typically have no solubility in the aluminum matrix, they M. GUPTA, Graduate Student, F.A. MOHAMED, Professor, and E.J. LAVERNIA, Associate Professor, are with the Departments of Materials Science and Engineering and Mechanical and Aerospace Engineering, University of California, Irvine, CA 92717. J. JUAREZISLAS, Research Scientist, is with the lnstituto De Fisica, Universidad Nacional Autonoma de Mexico, Laboratorio De Cuernavaca, Cuernavaca, Morelos, Mexico. W.E. FRAZIER, Head of Metals and Ceramics Processing Team, is with the Naval Air Development Center, Warminster, PA 18974-5000. This article is based on a presentation made in the symposium "Spray Processing Fundamentals: Coating and Deposition" presented as part of the 1990 TMS Fall Meeting, October 9, 1990, in Detroit, MI, under the auspices of the TMS Synthesis and Analysis in Materials Processing Committee. METALLURGICAL TRANSACTIONS B
provide effective high-temperature strengthening. The origin of this approach may be traced to work on sintered aluminum powders during the late 1940s and early 1950s that eventually became the predecessor of various mechanically alloyed aluminum products investigated ever since. I2m The second approach involves precipitating a fine dispersion of transition metal aluminide phases from the matrix through solid-state reactions. Since the addition of transition elements to a A1 under the nearequilibrium conditions present during ingot solidification commonly results in the formation of coarse, embrittling second phases as a result of their limited liquid and solid solubilities, these are usually added under rapid solidification (RS) conditions. The highly nonequilibrium conditions present during RS lead to the formation of extended solid solutions and metastable phases; the former can subsequently be decomposed into a fine dispersion of thermally stable precipitates, t4j The addition of transition elements to aluminum alloys, in combination with RS processing, has been successfully utilized to produce manifold aluminum alloys containing complex second-phase dispersoids based on additions of Fe, Ce, V, Si, Cr, Mo, Ti, and Zr. t5-~21 Among the family of
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