Structure/Property Relationships and Applications of Rapidly Solidified Aluminum Alloys
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Kear,
B.C.
Giessen,
and
M. Cohen,
editors
411
STRUCTURE/PROPERTY RELATIONSHIPS AND APPLICATIONS OF RAPIDLY SOLIDIFIED ALUMINUM ALLOYS
C. M. ADAM Pratt & Whitney Aircraft, Government Products Division, P.O. Box 2691, West Palm Beach, Florida, USA
ABSTRACT During the last five years Pratt & Whitney Aircraft has developed rapid solidification powder metallurgy and consolidation techniques to produce advanced aluminum alloys. A centrifugal rotary atomization device with forced high velocity helium convective cooling has been developed to pilot-plant stage, to produce aluminum alloys of novel compositions for advanced gas turbine engine applications. Rapidly solidified aluminum alloys solidify as6 5 spherical droplets up to 100 gm diameter with cooling rates of 10 - 10 K/sec, and demonstrate new microstructural features which have been exploited to develop elevated temperature mechanical properties. Alloys have been developed for 400 - 500°F fan and compressor applications that have traditionally used titanium alloys, and this paper reviews the microstructural evolution of rapidly solidified structures during thermomechanical processing.
INTRODUCTION
In a parametric study of the specific properties of a wide range of commercially available engineering alloys as a function of increasing temperature, it is clear that aluminum alloys dominate at temperatures up to 200'F (93°C), titanium alloys at temperatures of 200 to 800'F (93 to 427°C), precipitation hardening stainless steels at temperatures of 800 to 1200'F (427 to 649°C), and nickel-base superalloys at temperatures above 1200°F (649°C). In recent programs [1, 2], it was anticipated that aluminum-base alloys could be extended in operating temperature to 650'F by taking advantage of some recent developments in rapid solidification technology and powder metallurgy. The underlying premise to this anticipation was that new aluminum alloy precipitation hardening systems might be developed, based on metastable transition intermetallics, produced by the decomposition of supersaturated rapidly solidified alloys. The decomposition reaction kinetics should be slow enough at the temperatures used for powder consolidation for the subsequent strengthening phases to be developed after further thermomechanical processing. Virtually all the binary aluminum systems have been investigated, so that the effects of rapid solidification for systems involving the transition elements are now well known. The design of alloys involving the transition elements then required sufficiently rapid solidification to produce the necessary precursor single phase supersaturated solutions, following the principles outlined by Jones [3], or fine microeutectic metastable structures such as those described in this paper. MECHANICAL PROPERTY GOALS Replacement of titanium alloys in gas turbine fan blade and low temperature compressor applications require development of aluminum alloys with far superior strength/density and stiffness properties than those available from conventional alloys. Fig. 1 a
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