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  • 1.
    Fargas, G.
    et al.
    CIEFMA/EEBE, Departament de Ciència dels Materials i Enginyeria Metal·lúrgica, Universitat Politècnica de Catalunya, Barcelona 08019, Spain , .
    Roa, J.J.
    CIEFMA/EEBE, Departament de Ciència dels Materials i Enginyeria Metal·lúrgica, Universitat Politècnica de Catalunya, Barcelona 08019, Spain.
    Sefer, Birhan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. CIEFMA/EEBE, Departament de Ciència dels Materials i Enginyeria Metal·lúrgica, Universitat Politècnica de Catalunya, Barcelona 08019, Spain.
    Pedersen, R.
    Division of Subtractive and Additive Manufacturing, University West, S-46186 Trollhaettan.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Matteo, A.
    CIEFMA/EEBE, Departament de Ciència dels Materials i Enginyeria Metal·lúrgica, Universitat Politècnica de Catalunya, Barcelona 08019, Spain.
    Influence of cyclic thermal treatments on the oxidation behavior of Ti-6Al-2Sn-4Zr-2Mo alloy2018In: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189, Vol. 145, p. 218-224Article in journal (Refereed)
    Abstract [en]

    Ti-6Al-2Sn-4Zr-2Mo is one of the most common titanium alloys for aerospace industry. This alloy experiences oxidation phenomenon at elevated temperatures. In the present study, cyclic thermal treatments were performed in air at 500, 593 and 700 °C, up to 500 cycles, in order to determine the oxidation kinetics and to analyze the oxide scale and alpha-case formation. Moreover, results were compared to those achieved under isothermal conditions to elucidate differences between both thermal conditions. In this sense, metallographic techniques and X-ray diffraction, together with a detailed advanced characterization of the microstructure by Field Emission Scanning Electron Microscopy and Focus Ions Beam, were used to analyze surface oxidation evolution. Results pointed out that cyclic treatments induced a strong increase of the weight gain compared to isothermal treatments. The analysis of the oxide scale revealed the formation of not only rutile, as isothermal treatments, but also anatase. Thickness of the oxide scale was higher for cyclic conditions, while alpha case did not exceed values reached by isothermal treatments and even became lower at 500 °C.

  • 2. Gaddam, Raghuveer
    et al.
    Sefer, Birhan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Oxidation and alpha–case formation in Ti–6Al–2Sn–4Zr–2Mo alloy2015In: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189, Vol. 99, p. 166-174Article in journal (Refereed)
    Abstract [en]

    Isothermal heat treatments in ambient air were performed on wrought Ti–6Al–2Sn–4Zr–2Mo (Ti–6242) material at 500, 593 and 700 °C for times up to 500 hours. In presence of oxygen at elevated temperatures simultaneous reactions occurred in Ti–6242 alloy, which resulted in formation of an oxide scale and a layer with higher oxygen concentration (termed as alpha–case). Total weight gain analysis showed that there was a transition in the oxidation kinetics. At 500 °C, the oxidation kinetics obeyed cubic relationship up to 200 hours and thereafter changed to parabolic at prolonged exposure times. At 593 °C, it followed parabolic relationship. After heat treatment at 700 °C, the oxidation obeyed parabolic relationship up to 200 hours and thereafter changed to linear at prolonged exposure times. The observed transition is believed to be due to the differences observed in the oxide scale. The activation energy for parabolic oxidation was estimated to be 157 kJ/mol. In addition, alpha–case layer was evaluated using optical microscope, electron probe micro analyser and microhardness tester. The thickness of the alpha–case layer was found to be a function of temperature and time, increasing proportionally, and following parabolic relationship. The activation energy for formation of alpha–case layer was estimated to be 153 kJ/mol.

  • 3.
    Gaddam, Raghuveer
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sefer, Birhan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Study of alpha case depth in Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V2013In: 7th EEIGM International Conference on Advanced Materials Research: 21–22 March 2013, LTU, Luleå, Sweden, IOP Publishing Ltd , 2013, article id 12002Conference paper (Refereed)
    Abstract [en]

    Titanium alloys, mostly Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) and Ti-6Al-4V (Ti-64) are used in aero engine applications, because they possess high specific strength. The future concept in designing aircraft engines results in higher pressure, which increases the efficiency of aircraft engines by achieving high thrust and lowering the fuel consumption. Nevertheless, higher pressure in the engine means increase of service temperature. These conditions enforce new requirements on the materials used for manufacturing the engine components (compressors). Ti-6242 is mostly used in compressors where the service temperature is in the range of 400-450°C. It is well known that titanium alloys above 480°C for longer service time have tendency to form a hard and brittle oxygen stabilized surface layer (α-case). This layer has impact on the mechanical properties of the surface, by lowering the tensile ductility and the fatigue resistance. Factors that contribute for growth of α-case are: presence of oxygen, exposure time, temperature and pressure. In order to extend the service temperature of titanium alloys, it is required to understand the formation of α-case at high temperatures for long exposure times. In the present study, isothermal oxidation experiments in air were performed on forged Ti-6242 alloy at 500°C and 593°C up to 500 hours. Similar studies were also performed on Ti-64 sheet at 593°C and 700°C. Alpha case depths for both alloys were quantified using metallography techniques and compared.

  • 4.
    Sefer, Birhan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    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 Treatment2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    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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  • 5.
    Sefer, Birhan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Oxidation and Alpha–Case Phenomena in Titanium Alloys used in Aerospace Industry: Ti–6Al–2Sn–4Zr–2Mo and Ti–6Al–4V2014Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Titanium and its alloys are attractive engineering materials in aerospace industry because of their outstanding mechanical properties such as high specific strength and excellent corrosion resistance. Ti–6Al–2Sn–4Zr–2Mo (Ti–6242) and Ti–6Al–4V (Ti–64) are two alloys commonly used for manufacturing components in jet engines, such as fan blades, disks, wheels and sections of the compressor where the maximum temperature is in the range of 300–450 °C. At temperatures above 500 °C and in oxygen containing environments, these alloys are oxidizing rapidly. Oxidation normally involves formation of an oxide scale on top of the metal and a hard and brittle oxygen enriched layer beneath the scale which is known as “alpha–case”. The alpha–case layer has a detrimental effect on important mechanical properties such as ductility, fracture toughness and especially the fatigue life when an engine component is subjected to dynamic loading. In order to increase the understanding of the oxidation and alpha–case phenomena, the behaviour of these two alloys after long time exposure in air at elevated temperature has been investigated. Heat treatments were performed on Ti–6242 and Ti–64 alloys in ambient air at 500, 593 and 700 °C for times up to 500 hours. The oxide scale and the alpha–case layer were analysed, characterized and compared for both alloys. It was found that the oxide scale and alpha–case thickness are functions of temperature and time. Faster and more complex oxidation kinetics was noted in Ti–64, whereas in Ti–6242 the oxidation kinetics was found moderate at all tested temperatures and times. Discrepancies in the oxidation kinetics are believed to be because of the differences in the chemical composition and the microstructure of the two alloys. In addition, different morphology of the oxide scales was observed after 500 hours exposure time at 700 °C. The thickness of the alpha–case layer was measured using conventional metallographic and microscopic techniques. It was found that in both alloys the alpha–case growth obeys parabolic law with respect to time at all three tested temperatures. In addition, the diffusion coefficients and the activation energy of oxygen diffusion were estimated in the temperature range of 500–700 °C for the two alloys. Electron probe micro analyser (EPMA) was used to measure the oxygen concentration along the thickness of the alpha–case layer. It was found that the oxygen concentration decreases along the alpha–case layer. The oxygen concentration profiles were used to estimate the alpha–case thicknesses and it was found good agreement with the optically measured values for almost all investigated temperatures and times. Only at 700 °C in the time interval 300–500 hours in Ti–6242 a difference between the results from the two methods was found. Moreover, the EPMA concentration profiles of the main α and β alloying elements, before and after heat treatment at 700 °C for 500 hours, revealed microstructural changes and an increase of the α–phase volume fraction in the two investigated alloys.

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  • 6.
    Sefer, Birhan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Department of Materials Science and Metallurgical Engineering, Universitat Politècnica de Catalunya.
    Dobryden, Illia
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Division of Surface and Corrosion Science, KTH Royal Institute of Technology.
    Almqvist, Nils
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Research and Technology Centre, GKN Aerospace Engine Systems.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Chemical Milling of Cast Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo Alloys in Hydrofluoric-Nitric Acid Solutions2017In: Corrosion, ISSN 0010-9312, E-ISSN 1938-159X, Vol. 73, no 4, p. 394-407Article in journal (Refereed)
    Abstract [en]

    The behavior of cast Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo during chemical milling in hydrofluoric-nitric (HF-HNO3) acid solutions with 1:3 and 1:11 molar ratios was investigated using electrochemical and atomic force microscopy (AFM) techniques. Faster corrosion rate in 1:3 solutions was measured for Ti-6Al-4V than for Ti-6Al-2Sn-4Zr-2Mo, whereas in 1:11 solution Ti-6Al-2Sn-4Zr-2Mo exhibited higher corrosion rate. Scanning Kelvin probe force microscopy measurements revealed difference in the Volta potential between the α-laths and the β-layers in the Widmansttäten microstructure indicating operation of microgalvanic cells between the microconstituents when in contact with HF-HNO3 solution. The AFM topography measurements demonstrated faster corrosion of the α-laths compared to the β-layers, in both alloys. In 1:3 solutions, higher α/β height difference was measured in Ti-6Al-4V, whereas in 1:11 solution, the difference was higher in Ti-6Al-2Sn-4Zr-2Mo. The results revealed that the chemical milling behavior of the two investigated alloys is controlled by the microscopic corrosion behavior of the individual microconstituents.

  • 7.
    Sefer, Birhan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Gaddam, Raghuveer
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. GKN Aerospace Engine Systems Sweden.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Study of the Alpha-Case Layer in Ti–6Al–2Sn–4Zr–2Mo and Ti–6Al–4V by Electron Probe Micro Analysis2014Conference paper (Refereed)
    Abstract [en]

    Titanium and its alloys are susceptible to oxidation when exposed to elevated temperatures and oxygen containing environments for long exposure times, e.g. in jet engines [1–3]. In such conditions oxygen rapidly reacts with titanium, stabilizing α–titanium and forming solid solution due to the high solubility of oxygen in titanium (14.5 wt.%) [4]. The oxidation results in simultaneous formation of oxide scale on top of the metal and a brittle oxygen enriched layer beneath the scale, commonly referred as alpha–case. Alpha–case layer reduces important mechanical properties such as ductility, fracture toughness, and most severe reduces the fatigue life of jet engine components when subjected to dynamical loadings [5]. Therefore, the alpha-case layer in aerospace applications is usually removed by chemical milling [1–3] or prevented by using vacuum environments and high temperature coatings [1–3,6–9]. In the present study alpha–case in Ti–6Al–2Sn–4Zr–2Mo and Ti–6Al–4V alloys was developed by performing isothermal heat treatments at 700 °C in ambient air for 500 hours. The developed alpha–case layer was evaluated metallographically and by using instrumental techniques. It was found that the alpha–case development is a function of alloy composition and microstructure. The oxygen and the main alloying elements concentration profiles were measured using Electron Probe Micro Analyzer (EPMA) in both alloys. Based on the analysis of the concentration profiles an increase of the amount of alpha phase in the two alloys was found as a result of beta to alpha phase transformation.

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  • 8.
    Sefer, Birhan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Gaddam, Raghuveer
    Rovira, Joan Josep Roa
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Mateo, Antonio
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Chemical milling effect on the low cycle fatigue properties of cast Ti-6Al-2Sn-4Zr-2Mo alloy2016In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 92, no 1, p. 193-202Article in journal (Refereed)
    Abstract [en]

    The current research work presents the chemical milling effect on the low cycle fatigue properties of cast Ti-6Al-2Sn-4Zr-2Mo alloy. Chemical milling treatment is one of the final steps in manufacturing titanium alloy components that removes the brittle alpha-case layer formed during various thermal processes. The treatment includes immersion of the components in solutions containing hydrofluoric (HF) and nitric (HNO3) acids in relevant molar ratios. Although this treatment demonstrates advantages in handling components with complex net geometries, it may have detrimental effects on the surface, by introducing pitting and/or intergranular corrosion and thereby adversely affecting in particular the fatigue strength. The first series of specimens were tested in as-machined condition. Two more series were, prior to fatigue testing, subjected to 5 and 60 minutes chemical milling treatment. It was found that the fatigue lives were substantially decreased for the chemically treated specimens. The fractographic investigation of all mechanically tested samples revealed multiple fatigue crack initiation sites in the chemically milled samples. These cracks were located either at the prior beta grain boundary or the prior beta grain boundary triple joints. The prior beta grain boundaries were found to have deep ditch-like appearance which depth increased with increasing milling time. These ditch-like grain boundaries acts as stress raisers and thereby promote early fatigue crack initiation and thus lower fatigue life.

  • 9.
    Sefer, Birhan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Rovira, Joan Josep Roa
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Mateo, Antonio
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Evaluation of the bulk and alpha-case properties in Ti-6Al-4V at micro- and nano-metric length scale2016In: 13th World Conference on Titanium: August 16-20, 2015 * Manchester Grand Hyatt * San Diego, California / [ed] V. Venkatesh; A.L. Pilchak; J.E. Allison; S. Ankem; R. Boyer; J. Christodoulou; H.L. Fraser; M.A. Imam; J. Kosaka; H.J. Rack; A. Chaterjee; A. Woodfield, John Wiley and Sons , 2016, p. 1619-1624, article id 271Conference paper (Refereed)
    Abstract [en]

    In the present study the hardness of individual alpha (α)-Ti grains in Ti-6Al-4V was measured by nanoindentation using Berkovich tip indenter. Additionally, alpha-case layer was induced by performing isothermal heat treatment at 700°C in air for 500 hours. The average hardness of the α-Ti grains found in the bulk material and in the alpha-case layer were 6.7 ± 0.7 GPa and 9.4 ± 1.4 GPa, respectively. The high hardness of the α-Ti grains in the alpha-case layer is due to solid solution strengthening caused by interstitial oxygen diffusion. The thickness of the developed alpha-case layer was estimated metallographically and compared with that measured from a hardness profile performed along the layer. Moreover, electron back-scattered diffraction was used to determine the local crystallographic orientation, the texture of the alloy microstructure, as well as phase fraction changes, where the nanoindentation measurements were performed.

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