Environment Related Surface Phenomena and their Influence on Properties of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo Oxidation at Elevated Temperature and Corrosion During Chemical Treatment
Sammanfattning: This doctoral thesis covers investigation of the surface phenomena of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo alloys related with oxidation at elevated temperature and corrosion duringchemical treatment in hydrofluoric-nitric acid (HF/HNO3) solutions. The explored phenomenaare related to manufacturing and service conditions of aero-engine components. Duringmanufacturing and operation, the alloys are running at elevated temperatures in oxygencontaining environment. Under these conditions there are formations of an oxide on the surfaceand an oxygen enriched layer below the oxide, commonly referred as alpha-case. The alpha-caseis a hard and brittle layer that is detrimental to the mechanical properties and must therefore beminimized or completely eliminated. A conventional method for elimination of alpha-case ischemical processing in HF/HNO3 solutions, known as chemical milling.Isothermal oxidation treatments in air at 500, 593 and 700 ºC for up to 500 hours were carriedout in this study. Both alloys developed rutile type of oxide structure and Ti-6Al-2Sn-4Zr-2Moexhibited stronger oxidation resistance than Ti-6Al-4V. Transition from parabolic to linearoxidation rate was observed at 700 ºC and ≥ 200 hours for both alloys. The difference in theoxidation kinetics of the two alloys is suggested to be related with the chemical composition ofthe alloys. The oxygen enriched layer, i.e. alpha-case layer, was characterised and its thicknesswas measured using conventional metallographic and microscopic techniques. Parabolicrelationship of the alpha-case layer growth rate with time was observed for both alloys. Theoxygen diffusion parameters and activation energies were estimated in the temperature range of500-700 ºC. Additionally, the oxidation at 700 ºC for 500 hours resulted in microstructuralchanges and element re-distribution. The bulk and alpha-case layer hardness at micro- and nanoscalewere measured using microhardness and nanoindentation techniques. The alpha-case layerhad higher hardness due to the solid solution strengthening effect of the diffused oxygen.The effect of chemical milling in 1:11 HF/HNO3 solution on the surface integrity, and theinfluence on low cycle fatigue (LCF) strength of cast Ti-6Al-2Sn-4Zr-2Mo alloy wasinvestigated. Short and long chemical processing times (5 and 60 minutes) and three imposedtotal strain ranges in fatigue tests were evaluated. Significant drop in fatigue life was observedfor the samples etched before LCF testing, as compared to the non-etched samples. The influencefrom etching was found to be most detrimental for fatigue samples tested at the lowest strainranges. The fatigue life reduction was correlated with the number of crack initiation sites.Multiple crack initiation sites were observed for the etched samples, whereas only one crackinitiation site was discerned in the non-etched samples. Inspection of the surface of the etchedsamples revealed selective and severely etched prior β grain boundaries and pit formation at thetriple joints of the prior β grain boundaries. These surface defects were considered as stressraisers promoting an earlier fatigue crack initiation.The influence of two different molar concentration ratios (1:3 and 1:11) of HF and HNO3acids on the corrosion behaviour of cast Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo were investigatedusing electrochemical and atomic force microscopy (AFM) techniques. The corrosion of the twoalloys was a function of the HF/HNO3 concentration ratio and also of the alloys’ chemicalcompositions. The AFM measurements revealed selective and faster dissolution rate of the α-phase than the β-phase in the Widmanstätten microstructure. It was considered that the reason forselective dissolution was the formation of local micro-galvanic cells between the α-phase and theβ-phase. Moreover, the Volta-potential was measured using scanning Kelvin probe forcemicroscopy (SKPFM) and the obtained maps revealed difference in the Volta-potential betweenvithe α-laths and the β-laths in both alloys. This observation strengthened the likelihood forformation and operation of micro-galvanic cells between the α-phase and the β-phase when thealloys were in contact with HF/HNO3 solution.Keywords: Titanium alloys, oxidation, oxide, alpha-case, chemical milling, corrosion.
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