After almost three decades of intensive fundamental research and development activities, intermetallic titanium aluminides based on the ordered γ‐TiAl phase have found applications in automotive and ...aircraft engine industry. The advantages of this class of innovative high‐temperature materials are their low density and their good strength and creep properties up to 750 °C as well as their good oxidation and burn resistance. Advanced TiAl alloys are complex multi‐phase alloys which can be processed by ingot or powder metallurgy as well as precision casting methods. Each process leads to specific microstructures which can be altered and optimized by thermo‐mechanical processing and/or subsequent heat treatments. The background of these heat treatments is at least twofold, i.e., concurrent increase of ductility at room temperature and creep strength at elevated temperature. This review gives a general survey of engineering γ‐TiAl based alloys, but concentrates on β‐solidifying γ‐TiAl based alloys which show excellent hot‐workability and balanced mechanical properties when subjected to adapted heat treatments. The content of this paper comprises alloy design strategies, progress in processing, evolution of microstructure, mechanical properties as well as application‐oriented aspects, but also shows how sophisticated ex situ and in situ methods can be employed to establish phase diagrams and to investigate the evolution of the micro‐ and nanostructure during hot‐working and subsequent heat treatments.
Development and processing of high‐temperature materials is the key to technological progress in engineering areas where materials have to meet extreme requirements. After almost three decades of intensive fundamental research and development activities, intermetallic titanium aluminides based on the ordered γ‐TiAl phase have found applications in automotive and aircraft engine industry. This review gives a general survey of engineering γ‐TiAl based alloys, but concentrates on β‐solidifying γ‐TiAl based alloys, e.g., TNM™ alloys, which show excellent hot‐workability and balanced mechanical properties when subjected to adapted heat treatments.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
After more than 30 years of fundamental research and development activities intermetallic titanium aluminides based on the ordered γ-TiAl phase have found applications in aerospace and automotive ...industries. The advantages of this class of innovative high-temperature lightweight materials are their low density, their good strength and creep properties, as well as their oxidation resistance up to 750 °C. A drawback, however, is their limited ductility at room temperature, which is reflected by a low plastic strain at fracture. Advanced engineering TiAl alloys, such as the β-solidifying so-called TNM alloy with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in atomic percent), are complex multi-phase materials which can be processed by ingot or powder metallurgy, precision casting methods as well as additive manufacturing. Each production process leads to specific microstructures which can be altered and optimised by thermomechanical processing and/or subsequent heat treatments, whereby the knowledge of the occurring solidification processes and phase transformation sequences is essential. Therefore, thermodynamic calculations were conducted to predict the phase fraction diagrams. After experimental verification, these phase diagrams provided the base for the development of heat treatments to adjust balanced mechanical properties. To determine the influence of deformation and kinetic aspects, sophisticated ex- and in situ methods have been employed. Finally, the application of TiAl alloys in aerospace is reported.
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BFBNIB, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
A change in the mechanical properties of a carbide-free bainitic steel was observed during prolonged holding at austempering temperature after termination of the bainitic transformation. To determine ...the origin of the property change, the microstructure was investigated by correlative electron microscopy. Although the retained austenite content remains the same during prolonged holding, its morphology changes from thin films separating the individual bainitic sub-units to a more globular structure. Since films of austenite contain a higher C concentration, the blocky austenite becomes gradually enriched in C during this morphology change. The more homogeneous distribution of the C after prolonged austempering leads to higher deformability as a result of a more pronounced TRIP effect.
•Higher deformability after prolonged austempering of carbide-free bainite.•Microstructure-property relationship revealed by correlative electron microscopy.•Change in austenite morphology.•Spherodization of film austenite; C enrichment & homogenization of blocky austenite.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The equiatomic high-entropy alloy (HEA), CrMnFeCoNi, has recently been shown to be microstructurally unstable, resulting in a multi-phase microstructure after intermediate-temperature annealing ...treatments. The decomposition occurs rapidly in the nanocrystalline (NC) state and after longer annealing times in coarse-grained states. To characterize the mechanical properties of differently annealed NC states containing multiple phases, nanoindentation was used. The results revealed besides drastic changes in hardness, also for the first time significant changes in the Young's modulus and strain rate sensitivity. Nanoindentation of NC HEAs is, therefore, a useful complementary screening tool with high potential as a high throughput approach to detect phase decomposition, which can also be used to qualitatively predict the long-term stability of single-phase HEAs.
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•Nanoindentation as a high throughput, screening tool to probe decomposition in HEAs•Qualitative characterization of phase formation by Young's modulus analysis•Reduction in experimental preparation time compared to advanced microstructural characterization techniques (TEM, APT)•Increased strain-rate sensitivity in UFG but also in CG state•Strong scatter in Young's modulus in CG and SX state due to elastic anisotropy
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Thermally activated deformation mechanisms in three different W-Re alloys were investigated by performing high temperature nanoindentation experiments up to 800 °C. With increasing Re content the ...athermal hardness increases, while the temperature-dependent thermal contribution is strongly decreased. This results in a reduced strain rate sensitivity for W-Re alloys compared to pure W. The origin of this effect is a reduction of the Peierls potential due to Re, manifesting in an increased activation volume at lower temperatures. This gives rise to a solid solution softening effect, while at high-temperature application the mechanical behavior is governed by dislocation-dislocation interaction and solution strengthening.
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•Constant indentation strain rate and jump tests were performed up to 800 °C.•Temperature- and strain rate-sensitivity below 0.2 Tm are strongly decreased by Re.•Reduced Peierls potential manifests as an increased activation volume below 400 °C.•Activation energy is reduced by Re at low temperatures and not affected > 400 °C.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Advanced intermetallic alloys based on the γ-TiAl phase have become widely regarded as most promising candidates to replace heavier Ni-base superalloys as materials for high-temperature structural ...components, due to their facilitating properties of high creep and oxidation resistance in combination with a low density. Particularly, recently developed alloying concepts based on a β-solidification pathway, such as the so-called TNM alloy, which are already incorporated in aircraft engines, have emerged offering the advantage of being processible using near-conventional methods and the option to attain balanced mechanical properties via subsequent heat-treatment. Development trends for the improvement of alloying concepts, especially dealing with issues regarding alloying element distribution, nano-scale phase characterization, phase stability, and phase formation mechanisms demand the utilization of high-resolution techniques, mainly due to the multi-phase nature of advanced TiAl alloys. Atom probe tomography (APT) offers unique possibilities of characterizing chemical compositions with a high spatial resolution and has, therefore, been widely used in recent years with the aim of understanding the materials constitution and appearing basic phenomena on the atomic scale and applying these findings to alloy development. This review, thus, aims at summarizing scientific works regarding the application of atom probe tomography towards the understanding and further development of intermetallic TiAl alloys.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Intermetallic γ-TiAl based alloys are innovative lightweight structural high-temperature materials used in aerospace and automotive applications due to already established industrial-scale processing ...routes, like casting and hot-working, i.e., forging. A promising alternative method of production, regarding manufacturing of near net-shape components, goes over the powder metallurgy route, more precisely by densification of TiAl powder via spark plasma sintering. In this study, gas atomized powder from the 4th generation TNM alloy, Ti-43.5Al-4Nb-1Mo-0.1B (in at.%), was densified and the microstructure was investigated by means of electron microscopy and X-ray diffraction. The sintered microstructure exhibits lamellar α
-Ti
Al /γ-TiAl colonies surrounded by globular γ- and ordered β
-TiAl phase. The coarse lamellar spacing stems from the low cooling rate after densification at sintering temperature. Against this background, subsequent heat treatments were designed to decrease the lamellar widths by a factor of ten. Accompanying, tensile tests and creep experiments at different temperatures revealed that the modified almost fully lamellar microstructure is enhanced in strength and creep resistance, where a small volume fraction of globular γ-phase provides ductility at ambient temperatures.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Lightweight intermetallic γ-TiAl based alloys are innovative high-temperature structural materials. So far, these alloys are in use as turbine blades or turbocharger turbine wheels in advanced ...aerospace and automotive engines, where they are produced by means of investment casting as well as wrought processing, e.g. hot-forging. Through the development of powder-based additive manufacturing processes within the last decade, a real paradigm shift for future component production as well as their design and materials properties was created. While so-called proven alloy systems are presently used worldwide for additive manufacturing, the approach of this work is the development of novel process-adapted γ-TiAl based alloys, which on the one hand fulfill the specific requirements of additive manufacturing and on the other hand provide excellent high temperature properties after a suitable heat treatment. Based on the concept of an engineering γ-TiAl based alloy, i.e. the so-called TNM alloy, two alloys are presented. Due to the chemical reactivity of titanium aluminide alloys, electron beam melting processes come into consideration as production methods using optimized manufacturing parameters, providing dense components with only small variations in the Al content between the individual powder layers, which is a decisive factor for the subsequent heat treatment above the γ solvus temperature. The additively produced samples show a fine equiaxed microstructure, whereas the heat-treated samples exhibit a fully lamellar α₂/γ microstructure with an excellent creep resistance. In summary, the adaptation of the additive manufacturing parameters in combination with innovative alloys and subsequent heat treatments are the basis for producing reliable high-performance TiAl components in the near future.
•Development of novel process adapted γ-TiAl based alloys for electron beam melting.•Manufacturing the alloys with optimized parameters and a subsequent heat treatment.•The fully lamellar microstructure shows no banded or columnar phase distribution.•The microstructure of the heat-treated samples provides excellent creep resistance.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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•Establishing the impact of EBM parameters on TiAl microstructure and their mechanical response.•Correlation of the experimental results with numeric simulation, regarding the Al ...evaporation.•Discovery of a parameter-dependent solidification structure, masked by two solid-state transitions.
Additive manufacturing of lightweight intermetallic γ-TiAl based alloys combines process-related freedom of design with material-specific excellent high-temperature properties. Nevertheless, where locally melting the powder by an electron beam, there is a risk that Al evaporates due to its high vapor pressure, causing compositional and microstructural variations. This work investigates the impact of different process parameters on the total and local Al-content as well as the resulting as-built and heat-treated microstructure in a complex multiphase Ti-44.8Al-4.1Nb-0.7W-1.1Zr-0.4Si-0.5C-0.1B (at.%) alloy. The examinations applied are complementary, employing electron microscopy, X-ray spectroscopy and diffraction experiments with synchrotron X-ray radiation, supported by numerical simulations. The mechanical anisotropy of the heat-treated microstructure was analyzed by micro-hardness measurements. The results demonstrate that the amount of γ-TiAl phase decreases with increasing energy input of the electron beam in the as-built and heat-treated microstructure owing to the total and local loss of Al. Besides, the investigations of the crystal orientations within the multiphase alloy reveal a preferred orientation of the γ phase at high energy inputs. This follows from the fact that the preferred γ orientation is inherited through directional solidification of the β phase. The obtained process-microstructure-property relationships show that tailor-made material properties of additively manufactured γ-TiAl components are achievable.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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