Spark plasma sintering of a nanocrystalline Al-Cu-Mg-Fe-Ni-Sc alloy
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increasing demand to manufacture weightefficient structures that are damage tolerant and can operate at elevated temperatures has fueled both the development of novel alloy compositions and the introduction of radically different processing approaches. A case in point is the family of commercial Al alloys, in which the operating temperature is generally limited to temperatures close to ;250 °C. Inspection of the literature on elevated temperature Al alloys reveals two important findings. First, adequate elevated thermal stability will only be attained via nonequilibrium microstructures that incorporate, for example, insoluble Fe dispersoids.[1] Second, successful implementation of these materials will strictly depend on the ability to conceive of processing strategies that successfully retain these nonequilibrium microstructures. One such strategy, spark plasma sintering (SPS), is the topic of the current study. Al-Cu alloys have been extensively used in aerospace, aeronautical, and automotive applications due to their high strength, up to intermediate temperatures (;200 °C).[2] In these alloys, the normal aging sequence after a hightemperature solution treatment is as follows:[3] supersaturated solid solution / Guinier–Preston (GP) zones / u0 / u9 / u, where the metastable phases u0 and u9 are responsible for the high strength observed in this type of alloy.[4] The addition of alloying elements in Al-Cu alloys (such as Mg or Ag) has also been investigated, resulting in the formation of novel metastable intermetallic phases (such as V Al2Cu).[5] In the case of low Cu, high Mg alloys, the main ´N ˜ IGA, Postdoctoral Candidate, LEONARDO ALEJANDRO ZU AJDELSZTAJN, Research Scientist, and ENRIQUE J. LAVERNIA, Dean of Engineering, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Contact e-mail: [email protected] Manuscript submitted October 19, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
strengthening phase changes from platelike u9 (metastable form of Al2Cu) to rod-shaped S9 (metastable form of Al2CuMg),[6] giving rise to a more stable system. Further development of alloys with a low Cu/Mg ratio gave rise to the 2618 aluminum alloy (Al-Cu-Mg-Fe-Ni) for intermediate temperatures (;230 °C), where the addition of Fe and Ni (1 wt pct each) promotes the formation of stable Al9FeNi intermetallics.[7] The most famous application of the 2618 alloy is in the fuselage of the recently retired supersonic civil transportation vehicle Concorde.[8] Al-Sc alloys are of interest because of their superior strength.[9] The primary source of strengthening in this type of alloys is derived from the precipitation of a uniform distribution of nanosized Al3Sc particles.[10] These particles are resistant to coarsening, and remain coherent up to 30 nm.[11] The advantages of the Al-Sc system are: low diffusivity of Sc in Al, a very low interfacial energy between Al3Sc and the Al matrix, and a very high specific strengthening effect of Sc in Al (on an atom-to-atom basis).[12] Nan
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