Atom Probe Tomography Investigations of Microstructural Evolution in an Aged Nickel Superalloy for Exhaust Applications
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TRODUCTION
THE development of nickel superalloys has been driven by the need for corrosion-resistant materials with excellent high-temperature performance for use in gas-turbine engines for aircraft propulsion and power generation. The first jet engines were made from steel but as engine efficiency increased, so too did engine operating temperatures. New materials were required to make turbine blades able to withstand the higher temperatures and corrosive jet engine environment. Nickel superalloys were developed to fulfill these requirements, and have been extensively used by aerospace industries since 1929.[1] The unique combination of high-temperature strength and excellent corrosion resistance means that use of nickel superalloys is not limited to the aerospace sector but is in fact applicable in a wide range of extreme environments. For example, Inconel 625 is a nickel superalloy originally designed for high-strength steam piping for supercritical steam power plants.[2] However, in recent years, the alloy has been used to make exhausts for Formula 1 class racing engines.[3]
HAZEL GARDNER, JAMES O. DOUGLAS, MICHAEL P. MOODY, and PAUL A.J. BAGOT are with the Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK. Contact e-mail: [email protected] STELLA PEDRAZZINI is with the Royal School of Mines, Imperial College London, Exhibition Road, London SW7 2AZ, UK. DIDIER DE LILLE is with the Good Fabrications Ltd., Long Crendon, Aylesbury HP18 9BA, UK. Manuscript submitted August 28, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The excellent corrosion resistance of Inconel 625 enables exhaust tubings to withstand the corrosive environment created by the exhaust gases. Tight fuel allowances in the sport mean that in order to reach competitive speeds, an energy recovery system (ERS) must now be used in conjunction with the internal combustion engine (ICE).[4] This increases the thermal efficiency of the power unit (ERS + ICE) to ~ 50 pct. The high-temperature behavior of Inconel 625 must as a result be well understood to enable the exhaust system to cope with the rising heat loads. Nickel superalloys owe their corrosion resistance and high-temperature strength to the careful choice of alloying additions. For example, aluminum, chromium, and molybdenum are added to improve corrosion resistance. Niobium also enhances corrosion resistance,[5] while titanium degrades it.[6,7] In the case of Inconel 625, formation of the ordered c¢¢ strengthening phase is promoted by the addition of niobium and aluminum.[8] This coherent c¢¢ phase has a DO22 crystal structure and can more than double the yield stress.[9] c¢¢ (Ni3(Nb, Ti, Al)) forms in preference to c¢ (Ni3Al) in alloys with lower aluminum and titanium content such as Inconel 625.[10] Aging controls the size, volume fraction, and morphology of the c¢¢ precipitates, which in turn determines the mechanical properties of the alloy. Although the exhaust is initially in a solution-treated state, it operates between 600 C and 1000 C which mea
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