The structure of rapidly solidified Al- Fe- Cr alloys
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I.
INTRODUCTION
IMPROVED strength at elevated temperatures has been a continuing goal in aluminum alloy development. ~A microstructure containing a dispersion of an insoluble constituent in the alloy is most useful for attaining high strength and good structural stability at elevated temperatures .2 The thermal stability of high strength aluminum alloys in use today is rather poor, and the strength decreases rapidly with increasing temperature and with increasing soaking time at temperatures above 170 to 180 ~ Thus, the supersonic aircraft industry (2.7 to 3.3 roach) has had to replace aluminum alloys with more expensive titanium alloys to achieve good performance of parts and structures designed to be exposed to elevated temperatures. The use of thermally stable aluminum alloys with increased strength at elevated temperatures has the advantage of lower cost and lower component weight. In various research projects, transition metal alloying additives were used. These elements have in common: 2 1. High solubility in molten aluminum. 2. Low solid-solubility in aluminum. 3. A low diffusion rate in aluminum. 4. A tendency to form stable precipitates at elevated temperature. These qualities act to retain the microstructural stability of the alloy at service temperatures by the introduction of a fine particulate dispersion of hard and stable intermetallic. The inherent low solid solubility of such dispersion forming elements prevents their introduction in the desired amount by traditional metallurgical techniques. Rapid solidification, which allows a high degree of supersaturation, seems to be a solution to the problem. The development of powder atomization technology as well as other rapid solidification techniques is thus expected to result in improved performance of aluminum alloys at elevated temperatures. Iron and chromium, among other elements, meet the requirements listed above and furthermore, the intermetallic compounds they form with aluminum are of fairly low density. Thus, appreciable amounts of these alloying elements may be added without a prohibitive increase in the density R. YEARIM is MSc Student, Department of Materials Engineering, Technion, Haifa, Israel. D. SHECHTMAN, Faculty Member, Department of Materials Engineering, Technion, Haifa, Israel, is presently on sabbatical leave at National Bureau of Standards, Washington, DC 20234. Manuscript submitted December 29, 1981. METALLURGICAL TRANSACTIONS A
of the final product, z Precipitation processes and some of the mechanical properties of certain aluminum alloys containing mainly Ti, Zr, Mn, Cr, Ni, V, or Fe have been investigated in the past. 2-7 The main encouraging results were high yield and tensile strengths and the retention of strength after exposure to relatively high temperature. The goal of this work was to investigate systematically four aluminum alloys, supersaturated with iron and chromium. The compositions of the four alloys are (in weight percentage): Table I.
Alloy 1 2 3 4
Fe 7 7 7 7
The Composition of the Alloys
Cr 1 1 0.5 1.5
T
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