Scripta METALLURGICA Vol. 26, pp. 435-436, 1992 Pergamon Press plc et MATERIALIA Printed in the U.S.A. All rights reserved

STRENGTH RETENTION FOLLOWING ELEVATED TEMPERATURE EXPOSURE OF A RAPIDLY SOLIDIFIED ALUMINUM ALLOY

Blandino C Go and Brian Wilshire*

Advanced Metallics Research, Rohr Industries, Chula Vista, CA 91912, USA

*Department of Materials Engineering, University College, Swansea SA2 8PP, UK

(Received November 4, 1991) (Revised November 26, 1991)

The demand for high-efficiency high-performance commercial and military aircraft has been a motivating factor in the continued development of advanced aluminum alloys. In particular, the majority of the programs supported by several United States government agencies, especially the Air Force and Navy, have focused on the procurement of particle-strengthened aluminum alloys characterized by superior properties at elevated temperatures.

No aluminum alloy manufactured by traditional ingot-forming routes has been found to possess the combination of tensile and creep-rupture properties, formability, corrosion resistance and thermal stability currently required for many nacelle structural components, thrust reversers and engine pylons. Indeed, even the aluminum alloys produced by conventional powder processing methods are not suitable for applications involving long-term exposure at temperatures of around 589K. For this reason, attention has been directed to the use of state-of-the-art powder metallurgy processing techniques, since rapid solidification methods can avoid problems associated with segregation, solid solubility limitations, etc.

A rapidly-solidified AI-8.1Fe-I.7V-1.3Si alloy, initially referred to as FVS0812 but re-designated as alloy 8009 by the Aluminum Industries Association, has been developed and produced by Allied Signal Inc. to meet the property specifications now defined for high-performance aircraft applications. In order to assess the microstructural stability of this particle-strengthened alloy, the present study was undertaken to determine the effects of elevated temperature exposure on tensile properties.

The material was produced in the form of rapidly-solidified thin ribbons by a melt spinning technique using a water-cooled wheel. Powder formed by pulverization of the thin ribbons was then hot compacted in a vacuum press, after which the compacts were extruded to break up the thin oxide coating on the powder particles in order to promote metal-to-metal bonding. The extruded billets were subsequently forged, prior to hot roiling to plate and sheet. Figure 1 shows the fine AI2(Fe,V)3Si dispersion present in the as-received sheet, which had a grain size of the order of 0.5~tm.

Fig. I Transmission electron micrograph showing the particle dispersion in the as-received alloy 8009 sheet.

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436 SOLIDIFIED A1 ALLOY Vol. 26, No. 3

Tensile coupons were prepared from the 1.016mm sheet, with the specimen gauge lengths machined to be parallel to the rolling direction. Using a muffle furnace, these specimens were heated in air to pre-selccted exposure temperatures and held for known times before air cooling. The exposure conditions studied were 2, 30, 100 and 1000 hours at 589K and also 1000 hours at 700K. After exposure, the tensile coupons were tested at room temperature and at 589K according to ASTM E8-81. Tensile testing was performed on a Model 810 MTS machine at a constant crosshead speed of 1.06 x 10Sms ' .

Figure 2 shows the effects of increasing exposure time at 589K on the yield strength and tensile strength of alloy 8009 at room temperature and at the exposure temperature. Clearly, the material exhibits excellent stability, retaining its strength properties even after exposure times of 1000 hours and more. For comparison purposes, Figure 2 also includes room temperature tensile data for a conventionally-produced AI-Cu alloy, designated 2219-T6, which displays a 36% loss of strength after 100 hours and a 53% loss of strength after 1000 hours exposure (1).

The strength deterioration observed for particle-strengthened alloys during prolonged exposure at elevated temperatures is generally attributable to particle coarsening. In the case of alloy 8009, TEM studies established that the initial mean particle diameter of approximately 30rim was not changed discernibly, even after a 1000 hour exposure at 700K. This observation is fully consistent with the low particle coarsening rate of 2.9 x 10-~m3 per hour at 700K determined independently for this material (2), indicating the impressive resistance to coarsening of the uniform silicide dispersion developed in alloy 8009.

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LEGEND o - RT TENSILE = - 589"K TENSILE + - RT YIELD x - 589°K YIELD * - 2219-76 RT TENSILE {Ref v - 2219-76 RT YIELD (Ref

Test coupons, exposed at 589°K in air for different times, tested at room tcmpecatua¢ and at 589°K

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0.0 200.0 400.0 600.0 800.0 lO00.O 1200.0

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Fig.2 The effects of exposure time at 589K on the yield strength and tensile strength of alloy 8009 at room temperature and at the exposure temperature. Room-temperature tensile data are also included for alloy 2219-T6 subjected to identical exposure conditions.

Conclusions

The U.S. Air Force has established property goals for the thermal stability of high-peffotTnance aluminum alloys, specifying retention of mechanical properties after exposure times of 100 and I000 hours at temperatures up to 589K. The present resuhs show that alloy 8009 exceeds the property goals defined for all exposure conditions, as a result of the microstructural stability of the uniform dispersion of fine silicidc panicles developed in this rapidly-solidified AI-Fe-V-Si alloy.

Acknowled2ement

The present work was carried out as pan of the corporate research strategy of Rohr Industries Inc.

References

1. Aerospace Structural Metals Handbook, (1988) 2. P.S. Gilman, Private Communication.