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Title: High temperature oxidation behaviour of nickel-base superalloys
Author: Arnold, K.
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2017
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The Ni-base superalloys are a popular range of materials for study following consolidation by additive manufacturing (AM) techniques, such as selective laser melting (SLM). However, very little work has been done to assess the high temperature oxidation behaviour of Ni-base superalloys fabricated by SLM, despite the fact that this class of alloy is designed primarily for operation at temperatures >650°C. In the present work, the isothermal oxidation behaviour of the Ni-base superalloys Alloy 718 and Alloy 625 was studied following consolidation by SLM. A third Ni-base superalloy, Haynes 230, which is doped with a small amount of the reactive element La, was also studied following SLM-consolidation. The same three alloys were studied in wrought form for comparison purposes. Also studied following consolidation by SLM were oxide dispersion strengthened (ODS) derivatives of Alloy 625 and Haynes 230, which contained a 0.5 Wt. % addition of Y2O3, added by mechanical alloying (MA), and developed during the project for which the present work was conducted. Comparators for the ODS variants of Alloy 625 and Haynes 230 were fabricated by spark plasma sintering (SPS). All of the alloys were oxidised in laboratory air at 900°C and the oxidation kinetics determined using thermogravimetric analysis (TGA), or from scale thickness measurements. The work has shown that SLM-consolidated Alloy 718 oxidised slightly faster than wrought Alloy 718. SLM-consolidated Haynes 230 oxidised ~3x faster than wrought Haynes 230 alloy, but SLM-consolidated Alloy 625 oxidised ~2x slower than wrought Alloy 625. The ODS variant of Alloy 625, in SLM-consolidated and SPS-consolidated forms, oxidised ~10x more slowly than wrought Alloy 625. The SLM-consolidated ODS variant of Haynes 230 oxidised at approximately the same rate as wrought Haynes 230, but in SPS-consolidated form the ODS variant of Haynes 230 oxidised ~10x faster than wrought Haynes 230. The improvement in the oxidation resistance of the ODS variant of Alloy 625 is attributed to the well-known reactive element effect, which occurs when alloys are appropriately doped with reactive elements. The reduction in the oxidation resistance of the SPS-consolidated ODS variant of Haynes 230 is attributed to overdoping of the alloy with reactive elements, which is known to decrease the oxidation resistance of nickel-base alloys. It is proposed that SLM-consolidation improves the oxidation resistance of the ODS variant of Haynes 230 by ‘slagging off’ reactive elements from the alloy during consolidation, but for the same reason, the oxidation resistance of Haynes 230 is reduced by SLM-consolidation.
Supervisor: Tatlock, G. J. ; Jones, A. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral