Christoph Leyens

Titanium and titanium alloys

A titanium powder or alloy powder produced by introducing a TiCl4 vapor into a continuum or flowing stream of sodium metal at a velocity not less than sonic velocity of the vapor wherein the sodium is present in an amount greater than stoichiometric sufficient to maintain substantially all the reaction products below the sintering temperature thereof and wherein said Ti powder has a packing fraction in the range of from about 4% to about 11%. The powders without fines have a particle diameter in the range of from about 3.3 to about 1.3 microns based on a calculated size of a sphere from a BET surface area in the range of from about 0.4 to about 1.0 m2/g.

Titanium and titanium alloys : fundamentals and applications

Foreword.List of Contributors.1. Structure and Properties of Titanium and Titanium Alloys (M. Peters, et al.).2. Beta Titanium Alloys (G. Terlinde and G. Fischer).3. Orthorhombic Titanium Aluminides: Intermetallic with Improved Damage Tolerance (J. Kumpfert and C. Leyens).4. gamma-Titanium Aluminide Alloys: Alloy Design and Properties (F. Appel and M. Oehring).5. Fatigue of Titanium Alloys (L. Wagner and J.K. Bigoney).6. Oxidation and Protection of Titanium Alloys and Titanium Aluminides (C. Leyens).7. Titanium and Titanium Alloys - From Raw material to Semi-finished Products (H. Sibum).8. Fabrication of Titanium Alloys (M. Peters and C. Leyens).9. Investment Casting of Titanium (H.-P. Nicolai and Chr. Liesner).10. Superplastic Forming and Diffusion Bonding of Titanium and Titanium Alloys (W. Beck).11. Forging of Titanium (G. Terlinde, et al.).12. Continuous Fiber Reinforced Titanium matrix Composites: Fabrication, Properties and Applications (C. Leyens, et al.).13. Titanium Alloys for Aerospace Applications (M. Peters, et al.).14. Production, Processing and Application of gamma(TiAl)-Based Alloys (H. Kestler and H. Clemens).15. Non-Aerospace Applications of Titanium and Titanium Alloys (M. Peters and C. Leyens).16. Titanium and its Alloys for Medical Applications (J. Breme, et al.).17. Titanium in Dentistry (J. Lindigkeit).18. Titanium in Automotive Production (O. Schauerte).19. Offshore Applications for Titanium Alloys (L. Lunde and M. Seiersten).Subject Index.

Influence of substrate material on oxidation behavior and cyclic lifetime of EB-PVD TBC systems

EB-PVD NiCoCrAlY/P-YSZ TBCs on several polycrystalline, directionally solidified, and single crystalline (SX) substrate alloys were thermally cycled at 1100°C. TBC spallation does not correlate solely to TGO thickness, but depends also very much on the substrate alloy. The longest lifetimes are achieved on Hf-containing alloys while SX alloys suffer from early TBC spallation. The formation of the thermally grown oxide was investigated in detail by TEM. A mixed layer of alumina and zirconia exists in the as-coated condition. After initial slight thickening, the thickness of this mixed layer remains constant over a long period of time. During thermal exposure, a continuous layer of pure α-alumina forms and grows underneath the mixed zone by oxygen inward diffusion.

EB-PVD Thermal Barrier Coatings for Aeroengines and Gas Turbines

Ceramic thermal barrier coatings (TBCs) offer the potential to significantly improve efficiencies of aero engines as well as stationary gas turbines for power generation. On internally cooled turbine parts temperature gradients of the order of 100 to 150 °C can be achieved. Today, state-of-the-art TBCs, typically consisting of an yttria-stabilised zirconia top coat and a metallic bond coat deposited onto a superalloy substrate, are ntainly used to extend lifetime. Further efficiency improvements require TBCs being an integral part of the component which, in turn, requires reliable and predictable TBC performance. Presently, TBCs fabricated by electron beam physical vapor deposition are favoured for high performance applications. The paper highlights critical research and development needs for advanced TBC systems, such as reduced thermal conductivity, increased temperature capability, lifetime prediction modelling, process modelling, bond coat oxidation, and hot corrosion resistance as well as improved erosion behaviour.

Effect of composition on the oxidation and hot corrosion resistance of NiAl doped with precious metals

Cast NiAl alloyed with Cr, Pt, Pd, Ir and Ru was tested in 1-h cycles at 950°C under hot corrosion conditions and at 1150°C in oxygen. For comparison, Hf-doped NiAl variants and a cast NiPtAl alloy resembling the composition of commercial aluminide coatings were included. Cr was the only element that reduced hot corrosion attack of NiAl significantly. However, at higher temperatures, addition of Cr to Hf-doped NiAl accelerated the alumina scale growth rate and promoted spallation of the oxide scale. The results from initial detailed characterization indicate that rejection of chromium at the metal-oxide interface gives rise to the formation of chromium-rich precipitates in the alloy, which apparently modify its oxidation behavior. This suggests that for NiAl-based substrates, hot corrosion resistance and exceptional scale spallation resistance may be mutually incompatible goals.

Influence of bondcoat pre-treatment and surface topology on the lifetime of EB-PVD TBCs

Abstract In the present study two types of bondcoats (BC) were used, a NiCoCrAlY overlay coating produced by electron beam physical vapor deposition (EB-PVD) and a platinum aluminide diffusion coating (Ni,Pt)–Al. The coatings were deposited onto nickel-base IN100 and Rene142 substrates and were pre-treated in vacuum or Ar–H gas mixture prior to the deposition of an EB-PVD yttria partially stabilized zirconia top coat. The specimens were thermal cyclically tested in air at 1100 °C (50 min at 1100 °C/10 min forced air cooling). The microstructures of the thermally grown oxide (TGO) layers prior and after testing were examined by scanning electron microscopy and electron energy dispersive spectroscopy. Heat-treatment in Ar–H significantly improved the lifetime of the thermal barrier coatings (TBC) system with IN100+PtAl BC, whereas reduced lifetimes were found on IN100+NiCoCrAlY BC and Rene142+PtAl. High Y-contents of the NiCoCrAlY BC significantly reduced TBC lifetimes. Furthermore, the results for IN100+PtAl and Rene142+PtAl demonstrated that early TBC failure was associated with high TGO surface roughness of the metal/oxide interface, indicating that surface roughness has a significant effect on TBC system performance.

Intermetallic Ti-Al coatings for protection of titanium alloys : oxidation and mechanical behavior

Abstract Intermetallic Ti-Al coatings were deposited onto near-α titanium alloy TIMETAL 1100 using magnetron co-sputtering technique. Two coating systems were investigated: gradient layers with increasing Al content towards the surface of the coatings and a multilayer system consisting of three single layers of Ti 3 Al, TiAl and TiAl 3 . The overall coating thickness was 4 μm and 16 μm for both systems. Isothermal oxidation tests at 750 °C revealed good oxidation resistance and effective oxygen prevention from the substrate by the coatings. Room temperature tensile tests after long-term exposure to air at 600 °C proved the beneficial influence of the coatings on ductility of the base material. The coatings are highly ductile under creep conditions, thus keeping oxygen away from the substrate alloy even at high straining. In some cases creep lifetime was considerably prolonged. No detrimental influence of the Ti-Al coatings on the fatigue properties of TIMETAL 1100 was found for the 4-μm multilayer coatings, whereas fatigue limit under repeated strain was slightly decreased for the 16-μm coatings.

Sputtered intermetallic Ti-Al-X coatings : phase formation and oxidation behavior

Intermetallic Ti–Al–X (X=Cr, Nb) coatings were deposited on conventional near-α titanium alloy TIMETAL 1100 and on γ-TiAl based Ti–48Al–2Cr–2Nb by magnetron sputtering for oxidation protection. Low temperature deposition process leads to a metastable coating structure which transforms into a two-phase microstructure during high temperature exposure. Oxidation behavior was tested in interrupted weight gain tests in laboratory air at 750°C for TIMETAL 1100 and at 900°C for Ti–48Al–2Cr–2Nb. Oxidation behavior of the coatings shows the same tendency for both substrates and oxidation temperatures. Nb-containing coatings exhibited only poor protection due to partial spallation of the coatings during cooling. Maximum oxidation resistance was achieved by Ti–63Al–7Cr coatings on both substrates. For the γ-TiAl based Ti–Al–Cr coatings oxidation resistance is improved with increasing Cr content. Despite high oxidation resistance of the Cr-containing coatings rutile was always found in addition to protective alumina in the oxide scale.

Magnetron-sputtered Ti–Cr–Al coatings for oxidation protection of titanium alloys

Abstract Ti–Cr–Al coatings with a nominal composition of Ti–51Al–12Cr (in at.%) were deposited on the near-α titanium alloy TIMETAL 1100 (Ti–6Al–2.75Sn–4Zr–0.4Mo–0.45Si, in wt.%) by dual-source magnetron sputtering. Substrate bias voltage between 0 and 80 V was applied during coating deposition. The microstructure of the coating changes from fine columnar to a nearly structureless morphology by increasing the substrate bias voltage. Due to the specific shape of the electrical field around the samples the morphology of the as-coated samples is divided into three characteristic zones, the width of which are essentially dependent on the height of the bias voltage. All-around coated disk-shaped samples were isothermally tested in a thermobalance at 750°C in dry air to investigate their oxidation kinetics. Oxidation resistance of the coatings was maximal at zero bias voltage and slightly decreases up to 40 V. For those coatings deposited at 60 and 80 V strong spallation was observed, which is caused by bubble formation on heating due to argon incorporated during deposition.

Two-source jumping beam evaporation for advanced EB-PVD TBC systems

Abstract The continuous increase of the turbine inlet temperature in gas turbines necessitates new TBCs with a temperature capability beyond the current partially yttria stabilized material coatings. The present paper focuses on two-source jumping EB-PVD processed novel candidate layers for future TBC applications. It is shown that mixtures of oxides with widely different vapor pressures can be manufactured by this technique. The microstructure of the layers depends strongly on deposition conditions and on materials properties as well. Partial yttria stabilized zirconia coatings show no differences in microstructure and phase formation if deposited either by two-source jumping-beam or by one-source one-beam evaporation, while for ceria stabilized zirconia coatings large differences mainly in chemistry are found; depending on the jumping frequency multilayers are formed. In mixed silica zirconia coatings, crystalline zircon (ZrSiO 4 ) was formed neither in the as coated condition nor after annealing. Finally, jumping beam experiments allowed a detailed understanding of the growth kinetics of EB-PVD TBCs and the formation of a feather-like structure within the columns.