Mikrostruktur und mechanische Eigenschaften der reibgeschweißten γ‐TiAl‐Feingusslegierung Ti‐47Al‐3.5(Mn+Cr+Nb)‐0.8(B+Si)

Microstructure and mechanical properties of friction welded γ‐TiAl based alloy Ti‐47Al‐3.5(Mn+Cr+Nb)‐0.8(B+Si) in investment cast condition. This paper describes properties of joints produced by friction welding of the intermetallic γ‐TiAl based alloy Ti‐47Al‐3.5(Mn+Cr+Nb)‐0.8(B+Si) in investment cast and hot‐isostatically pressed condition. The effect of friction welding parameters on microstructure and local properties are examined and discussed. It is found that the properties of the joint are essentially affected by properties of as‐cast Ti‐47Al‐3.5(Mn+Cr+Nb)‐0.8(B+Si) base material, both at room temperature and 700 °C.     

Oxidation‐Resistant Coatings for Application on High‐Temperature Titanium Alloys in Aeroengines

High‐temperature application of titanium alloys in aeroengines is often limited by their insufficient resistance to the aggressive environment. Magnetron‐sputtered Ti–Al based coatings were developed in order to increase the maximum service temperature of conventional titanium alloys from the present 520–600 °C, the temperature limit set by the mechanical capabilities of most advanced alloys. The coatings not only demonstrated excellent oxidation resistance but also demonstrated beneficial effects on mechanical properties. Most importantly, the fatigue behavior of the substrate alloys was not degraded, a major hurdle for coating application on titanium alloys so far. Initial results on Cr‐containing Ti–Al based coatings indicated significant potential for application on titanium aluminides.     

Protective coatings on orthorhombic Ti2AlNb alloys

Abstract

The oxidation behaviour of an orthorhombic Ti–22Al–25Nb alloy, bare and with protective coatings, was investigated at 750°C in air under quasi-isothermal and thermal cycling conditions. As found by post-oxidation analysis, the uncoated substrate material was severely degraded by formation of spalling oxide scales and ingress of oxygen and nitrogen causing nitride precipitation, internal oxidation and interstitial embrittlement. Metallic Ti–51Al–12Cr coatings as well as nitride coatings based on Ti–Al–Cr–Y–N, either monolithically grown or with superlattice structure, provided an effective diffusion barrier against oxygen. The excellent oxidation resistance of the TiAlCr coatings was associated with the ternary Ti(Al,Cr)₂ Laves phase promoting the formation of a protective alumina scale. The different intermetallic phases formed in the interdiffusion zone caused neither cracking nor spallation of the protective coating. Both, monolithically grown TiAlCrYN and superlattice TiAlYN/CrN coatings, exhibited slow, but nearly linear oxidation kinetics at 750°C in air. In the subsurface region of the substrate a niobium rich phase and the α₂-phase formed. At the coating/substrate interface pores and a thin, fine-grained TiN layer were found.     

Role of rare earth oxide coatings on oxidation resistance of chromia-forming alloys

Rare earths (RE) have been used to increase high temperature oxidation resistance of chromia and alumina forming alloys. The RE can be added as elements (or oxides) to the alloys or applied as oxide coatings to the alloy surface. This paper presents the effect of different RE oxide coatings and lanthanum chromite coatings on the high temperature oxidation behavior of Fe20Cr and Fe20Cr4Al alloys. The oxidation resistance of the Fe20Cr alloy increased with increase in ionic radius of the RE element in the coating. The RE oxides decreased chromia growth rate more than alumina growth rate. In extended cyclic oxidation tests that were carried out from peak temperatures of 900 °C, 1,000 °C and 1,100 °C to room temperature at cooling rates of 300 °C/s and 1,000 °C/s, the La₂O₃ coating increased cyclic oxidation resistance of the Fe20Cr alloy significantly more than the Pr₂O₃ coating. The role of RE in increasing overall oxidation resistance of chromia forming alloys is discussed.     

Investigations of ferritic/martensitic super heat resistant 11‐12 %Cr steels for 650 °C power plants

The investigations of advanced ferritic/martensitic 11–12 %Cr steels for 650 °C power plant components focus on the improvement of high‐temperature creep properties with respect to chemical composition. The claim of the DFG research work was the development of new heat‐resistant 12 %Cr ferritic‐martensitic steels with sufficient creep and oxidation resistance for a 650 °C application by using basic principles and concepts of physical metallurgy on the basis of the state of art and to overcome the usual trial and error industrial alloy development. Efforts are focussed on a 100,000h creep strength of 100MPa at 650 °C in combination with a sufficient corrosion resistance by a Cr content of 12 % with contents 4‐5 %W, 3.4‐5,5 %Co, V, B and 1 %Cu as well as the choice of Ta or Ti instead of Nb. The results demonstrate that the aim is not to realize with the used alloying concept. In the long term range all 12 %Cr melts have a lower creep rupture strength than the advanced 9 %Cr piping steel P92. A high creep strength could be reached with a 0.06 % Ta alloyed 11 %Cr melt, which is in addition alloyed with a higher C and B content and as well as with lower W and Co portions. The results indicate in accordance with the finding of other steel researcher that a lower Cr content allows more effectiveness for the alloying partners respectively for the generation of more stable precipitates.     

Oxidation resistance of TiN,CrN, TiAlN and CrAlN coatings deposited by lateral rotating cathode arc

In this paper, four kinds of hard coatings, TiN, CrN, TiAlN and CrAlN (with Al/Ti or Al/Cr atomic ratio around 1:1), were deposited on stainless steel substrates by a lateral rotating cathode arc technique. The as-deposited coatings were annealed in ambient atmosphere at different temperatures (500–1000 °C) for 1 h. The evolution of chemical composition, microstructure, and microhardness of these coatings after annealing at different temperatures was systematically analyzed by energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and nanoindentation experiments. The oxidation behaviour and its influence on overall hardness of these four coatings were compared. It was found that the ternary TiAlN and CrAlN coatings have better oxidation resistance than their binary counterparts, TiN and CrN coatings. The Cr-based coatings (CrN and CrAlN) exhibited evidently better oxidation resistance than the Ti-based coatings (TiN and TiAlN). TiN coating started to oxidize at 500 °C. After annealing at 700 °C no N could be detected by EDX, indicating that the coating was almost fully oxidized. After annealed at 800 °C, the coating completely delaminated from the substrate. TiAlN started to oxidize at 600 °C. It was nearly fully oxidized (with little residual nitrogen detected in the coating by EDX) and partially delaminated at 1000 °C. Both CrN and CrAlN started to oxidize at 700 °C. CrN was almost fully oxidized (with little residual nitrogen detected in the coating by EDX) and partially delaminated at 900 °C. The oxidation rate of the CrAlN coating is quite slow. After annealing at 1000 °C, only about 19 at.% oxygen was detected and the coating showed no delamination. The Ti-based (TiN and TiAlN) coatings were not able to retain their hardness at higher temperatures (≥ 700 °C). On the other hand, the hardness of CrAlN was stable at a high level between 33 and 35 GPa up to an annealing temperature of 800 °C and still kept at a comparative high value of 18.7 GPa even after annealed at 1000 °C, indicating a very promising applicability of this coating for high speed dry machining and other applications under high temperature environments.     

Oxidation behaviour of an intermetallic Ti–Al–Cr–Zr bond coat on a γ–TiAl based TNB alloy with 7YSZ thermal barrier coating

Abstract

Intermetallic titanium aluminide alloys are attractive light-weight materials for high temperature applications in automotive and aero engines. The development of γ-TiAl alloys over the past decades has led to their successful commercial application as low pressure turbine blades. The operating temperatures of γ-TiAl based alloys are limited by deterioration in strength and creep resistance at elevated temperatures as well as poor oxidation behaviour above 800 °C. Since improvement in oxidation behaviour of γ-TiAl based alloys without impairing their mechanical properties represents a major challenge, intermetallic protective coatings have aroused increasing interest in the last years.

In this work, a 10 μm thick intermetallic Ti–46Al–36Cr–4Zr (in at.-%) coating was applied on a TNB alloy using magnetron sputtering. This layer provided excellent oxidation protection up to 1000 °C. Microstructural changes in this coating during the high temperature exposure were extensively investigated using scanning and transmission electron microscopy. The coating developed a three-phase microstructure consisting of the hexagonal Laves-phase Ti(Cr,Al)₂, the tetragonal Cr₂Al phase and the cubic τ-TiAl₃ phase. After long-term exposure the three-phase microstructure changed to a two-phase microstructure of the hexagonal α₂-Ti₃Al phase and an orthorhombic body-centred phase, whose crystal structure has not yet been definitely identified. On the coating, a thin protective alumina scale formed. Applying this intermetallic layer as bond coat, thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on γ-TiAl based TNB samples using electron-beam physical vapour deposition. The results of cyclic oxidation testing (1 h at elevated temperature, 10 min. cooling at ambient temperature) revealed a TBC lifetime of more than 1000 h of cyclic exposure to air at 1000 °C. The ceramic topcoat exhibited an excellent adhesion to the thermally grown alumina scale which contained fine ZrO₂ precipitates.     

Evaluation of the mechanical properties and environmental resistance of René 125 and X-40 superalloys coated with controlled composition reaction-sintered CoNiCrAlY

The application of MCrAlY-type coatings to high temperature turbine components is primarily provided by electron beam physical vapor deposition (PVD) processing. The process of coating with controlled composition reaction-sintered (CCRS) CoNiCrAlY was developed as a cost-effective alternative using only conventional equipment and processing techniques.The CCRS method involves a two-stage process, the first step consisting of the application of the modifier (CoNiCrY powder) onto the cleaned prepared surface in an appropriate slurry bisque. This is followed by pack processing in a controlled activity aluminum pack where reaction sintering occurs to form the final CoNiCrAlY composition.The CCRS CoNiCrAlY coating was applied to both nickel- and cobalt-base superalloys, René 125 and X-40, and to a limited number of mechanically alloyed MA754 specimens. Mechanical properties evaluated at ambient and elevated temperatures include tensile strength, strain tolerance and cycle fatigue. In addition, burner rig tests were conducted at 870°C (1600°F) and 927°C (1700°F) for hot corrosion resistance and at 1149°C (2100°F) for oxidation resistance. For each of the tests, specimens coated with PVD CoCrAlY, PVD NiCrAlY and CODEP B were evaluated under similar conditions in order to compare the performance of these coatings currently widely used.Generally, the results showed that the CCRS CoNiCrAlY coated specimens were equivalent, and in some cases superior, to the PVD and CODEP B coatings. The combination of 15–17 wt.% Al in the CoNiCrAlY coating relative to the PVD coating with 8–13 wt.% Al and the reaction sintering method produced a highly protective coating characterized by multiphase microstructure, ductility and inexpensive processing.     

Improving long term oxidation protection for γ‐TiAl substrates

In previous work, a thermal spray multilayer system consisting of Zirconia (ZrO₂) and MCrAlY top coat showed promising results regarding the oxidation behavior of the Gamma Titanium Aluminides substrates tested, which encouraged further research activities. Diffusion of substrate material was successfully inhibited by a ceramic Zirconia coating. A building up of a dense and stable oxide layer could be achieved by additional application of an MCrAlY top coat, leading to improved oxidation resistance and thus showing feasibility. In this work the main focus for development was put on enhancing adhesion and lowering residual stresses of the coatings in order to allow long term and cyclic testing without delamination taking place. Being a very brittle material, Gamma Titanium Aluminides require special surface treatment to enable roughening which is crucial for a strong mechanical bond between substrate and coating. Alternatives to conventional grit blasting as a standard preparation method were investigated. These were micro‐abrasive blasting and blasting at elevated temperature (≈300–550°C) to allow a more ductile behavior. The paper will highlight the implications by means of these measures and will also show the present development status of the multilayer system.     

Oxidation behaviour of TiAlYN/CrN and CrAlYN/CrN nanoscale multilayer coatings with Al2O3 topcoat deposited on γ-TiAl alloys

Abstract

TiAlYN/CrN and CrAlYN/CrN nanoscale multilayer coatings were deposited on γ-TiAl specimens using magnetron sputtering techniques. The nitride layers were manufactured by unbalanced magnetron sputtering (UBM) and high power impulse magnetron sputtering (HIPIMS). The CrAlYN/CrN coatings had an oxy-nitride overcoat. On some of the coated samples an additional alumina topcoat was deposited. The oxidation behaviour of the different coatings was investigated at 750 and 850°C performing quasi-isothermal oxidation tests in laboratory air. Mass change data were measured during exposure up to failure or the maximum exposure length of 2500 h. When exposed to air at 750°C, the Ti-based nitride films exhibited higher oxidation resistance than the Ti – 45Al –8Nb substrate material. The alumina topcoat enhanced the oxidation protection of this coating system, acting as diffusion barrier to oxygen penetration. At 850°C, the TiAlYN/CrN films exhibited poor stability and rapidly oxidised, and therefore were not applicable for long-term protective coatings on γ-TiAl alloys. The beneficial effect of the additional Al₂O₃ layer was less pronounced at this exposure temperature. The Cr-based nitride films exhibited high oxidation resistance during exposure at 850°C. HIPIMS deposition improved the oxidation behaviour of the CrAlYN/CrN nanoscale multilayer coatings in comparison to UBM coatings. For these coatings, the decomposed and partially oxidised nitride films were an effective barrier to oxygen inward diffusion. The alumina topcoat did not significantly increase the oxidation resistance of the γ-TiAl alloy coated with Cr-based nitride films.     

Thermal Treatment Optimization of Electrodeposited Hydroxyapatite Coatings on Ti6Al4V Substrate

Thermal behavior of electrodeposited hydroxyapatite (HAP) coating on a titanium alloy (Ti6Al4V) is investigated in order to optimize the heat treatment conditions for this prosthetic material. The synthesized coatings are annealed in air atmosphere at 400, 600, 800, and 1000 °C, and then characterized by X‐ray diffraction (XRD) and selected area electron diffraction (SAED) for structure and phases analysis. Scanning and transmission electron microscopy associated to energy dispersive X‐ray microanalysis (SEM‐EDXS and STEM) are used for morphology and composition analysis. The results show that when the electrodeposited coating is annealed at temperatures greater than 600 °C, a well‐crystallized HAP is obtained with a notable change of its morphology. However, at these temperatures the surface of Ti6Al4V alloy (uncoated zones of the implant) is deteriorated by the formation of a thick surface oxide layer. Therefore, we limit the heat treatment temperature for the electrodeposited coatings on a Ti6Al4V alloy at 550 °C. At this optimized temperature it is demonstrated that the link between the coating and the substrate is improved and the crystallinity of the coating is controlled which make it well bioactive.     

TEM Studies of High Temperature Corrosion Behaviour of TiAl Intermetallics with Surface Modifications

The oxidation/sulphidation behaviour of a Ti‐46.7Al‐1.9W‐0.5Si alloy with a TiAl₃ diffusion coating was studied in an environment of H₂/H₂S/H₂O at 850^oC. The kinetic results demonstrate that the TiAl₃ coating significantly increased the high temperature corrosion resistance of Ti‐46.7Al‐1.9W‐0.5Si. The SEM, EDX, XRD and TEM analysis reveals that the formation of an Al₂O₃ scale on the surface of the TiAl₃‐coated sample was responsible for the enhancement of the corroison resistance. The Ti‐46.7Al‐1.9W‐0.5Si alloy was also modified by Nb ion implantation. The Nb ion implanted and as received sampels were subjected to cyclic oxidation in an open air at 800^oC. The Nb ion implantation not only increased the oxidation resistance but also substantially improved the adhesion of scale to the substrate.     

Self-lubricating CrAlVN coatings for turning of Ti6Al4V: oxidation and wear behavior

The titanium alloy Ti6Al4V enables significant performance increases in various branches of industry. Nevertheless, it is difficult to machine, because of its material properties. Due to the low thermal conductivity of titanium, the heat generated while turning Ti6Al4V mainly flows into the tool leading to high temperature loads. In addition, the comparatively low Young's modulus and high yield strength contributes to high mechanical stresses during machining. Temperature active, self-lubricating physical vapor deposition hard coatings appear to be suitable for reducing friction and tool wear during turning of Ti6Al4V compared to the most commonly used uncoated carbide tools. The ability of the coating to form lubricating oxide phases at high temperatures is crucial for this purpose. This paper investigates the oxidation and diffusion behavior of vanadium doped chromium aluminium nitride (CrAlN) coatings after heat treatment at ϑ = 600 °C, ϑ = 700 °C und ϑ = 800 °C in atmosphere as well as the resulting coating properties. The wear behavior of certain coating variants while turning of Ti6Al4V is analyzed.     

Effect of microalloying with silicon on high temperature oxidation resistance of novel refractory high-entropy alloy Ta-Mo-Cr-Ti-Al

Abstract

The effect of 1 at.% Si addition to the refractory high-entropy alloy (HEA) Ta–Mo–Cr–Ti–Al on the high temperature oxidation resistance in air between 900 °C and 1100 °C was studied. Due to the formation of protective chromia-rich and alumina scales, the thermogravimetric curves for Ta–Mo–Cr–Ti–Al and Ta–Mo–Cr–Ti–Al–1Si showed small mass changes and low oxidation rates which are on the level of chromia-forming alloys. The oxide scales formed on both alloys at all temperatures are complex and consist of outermost TiO₂, intermediate Al₂O₃, and (Cr, Ta, Ti)-rich oxide at the interface oxide/substrate. The Si addition had a slightly detrimental effect on the oxidation resistance at all temperatures primarily as a result of increased internal corrosion attack observed in the Si-containing HEA. Large Laves phase particles distinctly found in the Si-containing alloy were identified to be responsible for the more rapid internal corrosion.     

Study of high temperature mechanical behavior of the thermally oxidized Ti‐6Al‐4V alloy

The influence of oxidation of a Ti‐6Al‐4V alloy at 800 °C on its tensile properties at 600 °C has been studied. Specimens of this alloy were oxidized at 800 °C for 0.5, 1, 5, 10, 20 and 40 h. Tensile tests at 600 °C were carried out and the fracture surfaces were also examined. Oxidation of the specimens resulted in the formation of an oxide layer that spalled and another oxide layer that adhered to the substrate. Oxide formation increased with increase in duration of oxidation. In this investigation, density curves of the oxide layer as a function of duration of oxidation at 800 °C were used to identify a parabolic oxide growth law. The results of this study revealed coherence between the experimental data and calculations based on the Pilling‐Bedworth law. The mechanical strength of the Ti‐6Al‐4V alloy did not vary significantly with oxidation, but reduction in cross sectional area with increase in oxide layer thickness, as well as the slope of the stress‐strain curve decreased beyond the ultimate tensile strength. Fracture of the tensile tested specimens was predominantly ductile with microcavities. At certain regions of the oxide layer, brittle fracture with radial cracks was observed indicating intergranular fracture.     

Improvement on the Oxidation Resistance of a Ti3Al Based Alloy by Cr1-xAlxN (0≤x≤0.47) Coatings

Cr1-xAlxN coatings have been deposited on a Ti3Al based alloy by reactive sputtering method. The results of the isothermal oxidation test at 800-900 ℃ showed that Cr1-xAlxN coatings could remarkably reduce the oxidation rate of the alloy owing to the formation of Al2O3 Cr2O3 mixture oxide scale on the surface of the coatings. No spallation of the coatings or oxide scales took place during the cyclic oxidation at 800℃. Ti was observed to diffuse into the coatings, the diffusion distance of which was very short, and the diffusion ability of it was proportional to the Al content in the coatings. Compared to Ti, Nb can diffuse much more easily through the whole coatings and oxide scales.     

Improvement on the Oxidation Resistance of a Ti3Al Based Alloy by Cr1-xAlxN （0≤x≤0.47） Coatings

Cr1-xAlxN coatings have been deposited on a Ti3Al based alloy by reactive sputtering method. The results of the isothermal oxidation test at 800-900℃ showed that Cr1-xAlxN coatings could remarkably reduce the oxidation rate of the alloy owing to the formation of Al2O3＋Cr2O3 mixture oxide scale on the surface of the coatings. No spallation of the coatings or oxide scales took place during the cyclic oxidation at 800℃. Ti was observed to diffuse into the coatings, the diffusion distance of which was very short, and the diffusion ability of it was proportional to the AI content in the coatings. Compared to Ti, Nb can diffuse much more easily through the whole coatings and oxide scales.     

High‐Temperature Shape Memory Effect in Ti‐Ta Thin Films Sputter Deposited at Room Temperature

Ti‐Ta based alloys are potential high‐temperature shape memory materials with operation temperatures above 100 °C. In this study, the room temperature fabrication of Ti‐Ta thin films showing a reversible martensitic transformation and a high temperature shape memory effect above 200 °C is reported. In contrast to other shape memory thin films, no further heat treatment is necessary to obtain the functional properties. A disordered α″ martensite (orthorhombic) phase is formed in the as‐deposited co‐sputtered Ti₇₀Ta₃₀, Ti₆₈Ta₃₂ and Ti₆₇Ta₃₃ films, independent of the substrate. A Ti₇₀Ta₃₀ free‐standing film shows a reversible martensitic transformation, as confirmed by temperature–dependent XRD measurements during thermal cycling between 125 °C to 275 °C. Furthermore, a one‐way shape memory effect is qualitatively confirmed in this film. The observed properties of the Ti‐Ta thin films make them promising for applications on polymer substrates and especially in microsystem technologies.     

Thermally grown oxide scales on γ-TiAl coated with thermal protection systems

Abstract

Thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on gamma TiAl samples using electron-beam physical vapour deposition. The specimens were coated with intermetallic Ti –Al – Cr layers and CrAlYN/CrN nanoscale multilayer coatings. The lifetime of the TBC systems was determined performing cyclic oxidation tests in air at temperatures between 850 and 950–C. The TBC systems with Ti –Al – Cr and CrAlYN/CrN layers did not fail at 850 and 900–C during the maximum exposure time period of 1000 cycles of 1 h dwell time at high temperature. No spallation of the thermal barrier coatings was observed. As revealed by post-oxidation microstructural analysis, the protective coatings were severely degraded when exposed at 900–C, resulting in growth of mixed oxides on the substrate. Underneath the thermal barrier coating an outer oxide scale with a columnar structure was observed, consisting of rutile and α-Al₂O₃. Energy-dispersive X-ray spectroscopy analysis revealed zirconia and chromia being dissolved in the outer oxide scale. The columnar structure and the presence of zirconia indicated an effect of the TBC on the morphology of the outer oxide scale. The zirconia top coat exhibited an excellent adherence to this oxide scale formed on the protective layers when degraded, and at defects like cracks. When thermally cycled at 950–C, the TBC system on specimens coated with Ti –Al – Cr failed by spallation of the thermally grown mixed oxides, whereas the thermal barrier coating was well adherent to the outer oxide scale at this temperature, too.     

Production of FeCrAlY and CoCrAlY coatings by laser surface fusion and their oxidation behavior

In this paper the results of a study aimed at coating large areas with FeCrAlY and CoCrAlY alloys by laser surface fusion are described. Metallurgical characteristics and oxidation behavior of the coatings are discussed.The results indicate that fusion of the coating materials was complete and a metallurgical bond was formed between the coatings and the type 304 stainless steel (SS) substrate as a result of laser processing. Some dilution of the coating alloys, in particular the aluminum content, was noticed due to melting of a portion of the substrate. Some small cracks, observed in the CoCrAlY coating, were formed as a result of shrinkage during solidification.Oxidation studies were performed at 1000–1185 °C in air for various times. The coatings, although diluted, showed excellent oxidation resistance and protection, while the type 304 SS substrate deteriorated rapidly. The effect of the thermal and oxidation treatment on the microstructural and chemical stabilities of the coatings are discussed.