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Scandium metal processing for aerospace application / Sephira Riva

DOI (Published version): 10.23889/SUthesis.39867

Abstract

The use of scandium has been conventionally restricted to minor additions in alloys for structural applications. The term ‘scandium effect’ came thus to indicate the properties improvements caused by the precipitation of scandium intermetallics in the matrix.The development of High-Entropy Alloys (HE...

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Published: 2018
URI: https://cronfa.swan.ac.uk/Record/cronfa39867
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Abstract: The use of scandium has been conventionally restricted to minor additions in alloys for structural applications. The term ‘scandium effect’ came thus to indicate the properties improvements caused by the precipitation of scandium intermetallics in the matrix.The development of High-Entropy Alloys (HEA, multi-principal component alloys combining compositional complexity with simple crystal structures) opened the door for the development of new systems, and therefore for new applications for lightweight metals such as scandium.This work is a thorough investigation of the potential of scandium-based in- termetallics in the growing field of HEAs.The synthesis and characterisation of multi-principal component alloys contain- ing scandium as active alloying element illustrate the compound-forming ability of this element. The high mixing entropy of the studied systems (comprising alloys of scandium with first raw metals or with hcp-structured elements) cannot efficiently inhibit the precipitation of stable intermetallic compounds. Among them, of in- terest is the so-called W -phase, so-far only reported for Al-basedCu-containing commercial alloys.Small scandium additions to Al2CoCrFeNi, Al0.5CoCrCuFeNi and AlCoCr- Cu0.5FeNi cause the segregation of a secondary phase along grain boundaries. This hexagonal Laves phase, formed by scandium in combination with Al, Co, Cr, Cu, Fe and Ni, does not disrupt the HEA matrix and is extremely stable. Moreover, it deeply affects microstructure and mechanical properties – for ex-ample, by enhancing the HEA stability with the postponement of a T-dependent phase exsolution by roughly 150 °C. Furthermore, a synergistic effect in the main phase stabilisation takes place when the Sc-doped sample is pressed at 9.5 GPa: no transition occurs and the intermetallic dissolves in the matrix.Preliminary investigations performed by spark-plasma sintering of different additives (among which Sc2O3, used as a source of scandium metal) in combination with the Al CoCrFeNi alloy led to the discovery of a promising nanodiamond HEA composite.
Keywords: Scandium, Metallurgy, High-Entropy Alloys, HEA Composites
College: College of Engineering