Design and development of an experimental wrought aluminum alloy for use at elevated temperatures
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I.
INTRODUCTION
AGEDaluminum alloys have the highest specific strengths of all commercially available engineering alloys at temperatures up to 100 °C. Between 100 and 200 °C, however, mechanical properties decline rapidly due mainly to coarsening of the fine precipitates on which the alloys depend for their strength. One approach to improve the elevated temperature performance of aluminum alloys has been the use of rapid solidification technology to produce powders or foils containing high supersaturations of elements such as iron or chromium that diffuse slowly in solid aluminum. In this regard, several experimental materials are now available which have promising creep properties up to 350 °C. Clearly, techniques of this kind will be needed if aluminum alloys are required to operate in such temperature regimes. Alloys produced by rapid solidification processing tend to be relatively costly and more difficult to prepare than wrought materials fabricated from ingots. Accordingly, an attempt has been made to design a new wrought alloy that might combine the high strength at ambient temperatures with good creep performance in the range 150 to 200 °C. This paper outlines the scientific basis for this design and presents results of a preliminary evaluation of the microstructure and properties of the alloy in the form of extruded bar. II.
HIGH-STRENGTH
EXISTING ALUMINUM ALLOYS
The 7000 series of age hardenable alloys that are based on the A1-Zn-Mg-Cu system develop the highest room temperature tensile properties of any aluminum alloys produced from conventionally cast ingots. Values of 0.2 pct proof stress and tensile strength normally exceed 500 MPa and 550 MPa if the alloys are aged to the T6 condition. However, the strength of these alloys declines rapidly if they are exposed to temperatures exceeding 100 to 120 °C (Figure 1)." Alloys of the 2000 series such as 2014 (AI-
I.J. POLMEAR is Deputy Vice-Chancellor and formerly Chairman, Department of Materials Engineering, Monash University, Melbourne, Australia. M.J. COUPER is with Central Laboratory, BBC Brown Boveri & Co. Ltd., Baden, Switzerland. Manuscript submitted January 20, 1987. METALLURGICALTRANSACTIONS A
4.3 pct Cu-0.5 pct Mg-0.8 pct Si-0.6 pet Mn)* and 2024 * Unless stated otherwise, alloy compositions and additions are expressed in percentages by weight.
(A1-4.3 pct Cu-1.5 pct Mg-0.6 pct Mn) perform better above these temperatures but are not normally used for elevated temperature applications. One wrought aluminum alloy commonly favored for elevated temperature applications is 2618 (A1-2.4 pct Cu-l.5 pct Mg-1 pct Fe-1 pct Ni-0.2 pct Si) and it has particular application in forgings. Typical room temperature tensile properties of 2618 in the T6 condition are 0.2 pct proof stress: 380 MPa, tensile strength: 440 MPa. The mechanical properties of 2618 become superior to the 7000 alloys above 120 °C (Figure 1). For temperatures exceeding approximately 220 °C, however, another 2000 series alloy 2219 (A1-6.3 pet Cu-0.3 pct Mn-0.18 pct Zr-0.1 pct V) displays
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