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
Abstract: 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 during chemical treatment in hydrofluoric-nitric acid (HF/HNO3) solutions. The explored phenomena are related to manufacturing and service conditions of aero-engine components. During manufacturing and operation, the alloys are running at elevated temperatures in oxygen containing environment. Under these conditions there are formations of an oxide on the surface and an oxygen enriched layer below the oxide, commonly referred as alpha-case. The alpha-case is a hard and brittle layer that is detrimental to the mechanical properties and must therefore be minimized or completely eliminated. A conventional method for elimination of alpha-case is chemical 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 carried out in this study. Both alloys developed rutile type of oxide structure and Ti-6Al-2Sn-4Zr-2Mo exhibited stronger oxidation resistance than Ti-6Al-4V. Transition from parabolic to linear oxidation rate was observed at 700 ºC and ≥ 200 hours for both alloys. The difference in the oxidation kinetics of the two alloys is suggested to be related with the chemical composition of the alloys. The oxygen enriched layer, i.e. alpha-case layer, was characterised and its thickness was measured using conventional metallographic and microscopic techniques. Parabolic relationship of the alpha-case layer growth rate with time was observed for both alloys. The oxygen diffusion parameters and activation energies were estimated in the temperature range of 500-700 ºC. Additionally, the oxidation at 700 ºC for 500 hours resulted in microstructural changes and element re-distribution. The bulk and alpha-case layer hardness at micro- and nano-scale were measured using microhardness and nanoindentation techniques. The alpha-case layer had 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 the influence on low cycle fatigue (LCF) strength of cast Ti-6Al-2Sn-4Zr-2Mo alloy was investigated. Short and long chemical processing times (5 and 60 minutes) and three imposed total strain ranges in fatigue tests were evaluated. Significant drop in fatigue life was observed for the samples etched before LCF testing, as compared to the non-etched samples. The influence from etching was found to be most detrimental for fatigue samples tested at the lowest strain ranges. 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 crack initiation site was discerned in the non-etched samples. Inspection of the surface of the etched samples revealed selective and severely etched prior β grain boundaries and pit formation at the triple joints of the priorβ grain boundaries. These surface defects were considered as stress raisers promoting an earlier fatigue crack initiation. The influence of two different molar concentration ratios (1:3 and 1:11) of HF and HNO3 acids on the corrosion behaviour of cast Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo were investigated using electrochemical and atomic force microscopy (AFM) techniques. The corrosion of the two alloys was a function of the HF/HNO3 concentration ratio and also of the alloys’ chemical compositions. 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 for selective dissolution was the formation of local micro-galvanic cells between the α-phase and the β-phase. Moreover, the Volta-potential was measured using scanning Kelvin probe force microscopy (SKPFM) and the obtained maps revealed difference in the Volta-potential between the α-laths and the β-laths in both alloys. This observation strengthened the likelihood for formation and operation of micro-galvanic cells between the α-phase and the β-phase when the alloys were in contact with HF/HNO3 solution. Keywords: Titanium alloys, oxidation, oxide, alpha-case, chemical milling, corrosion.
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