Advancement of Compositional and Microstructural Design of Intermetallic γ-TiAl Based Alloys Determined by Atom Probe Tomography

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

Neutrons and synchrotron radiation in engineering materials science: from fundamentals to applications

Retaining its proven concept, the second edition of this ready reference specifically addresses the need of materials engineers for reliable, detailed information on modern material characterization methods. As such, it provides a systematic overview of the increasingly important field of characterization of engineering materials with the help of neutrons and synchrotron radiation. The first part introduces readers to the fundamentals of structure-property relationships in materials and the radiation sources suitable for materials characterization. The second part then focuses on such characterization techniques as diffraction and scattering methods, as well as direct imaging and tomography. The third part presents new and emerging methods of materials characterization in the field of 3D characterization techniques like three-dimensional X-ray diffraction microscopy. The fourth and final part is a collection of examples that demonstrate the application of the methods introduced in the first parts to problems in materials science. With thoroughly revised and updated chapters and now containing about 20% new material, this is the must-have, in-depth resource on this highly relevant topic

Thermal Expansion and Other Thermodynamic Properties of <i>α</i>₂-Ti₃Al and <i>γ</i>-TiAl Intermetallic Phases from First Principles Methods

Anisotropic thermal expansion coefficients of tetragonal &#947; -TiAl and hexagonal &#945; 2 -Ti3Al phases were calculated using first principles methods. Two approaches with different computational costs and degrees of freedom were proposed. The predicted values were compared with available experimental data showing that for &#947; -TiAl, the more computational demanding method with decoupled impact of volume and temperature effects on the cell shape leads to significantly better results than that with only ground-state optimised unit cell geometry. In the case of the &#945; 2 -Ti3Al phase, both approaches yielded comparable results. Additionally, heat capacity and bulk modulus were evaluated as functions of temperature for both phases, and were fitted to provide an analytical formula which can be further used

In-situ High-energy X-ray Diffraction on an Intermetallic β\beta-stabilised γ\gamma-TiAl Based AlloyIn-situ Beugungsexperimente mit hochenergetischer Röntgenstrahlung an einer intermetallischen, β\beta-stabilisierten γ\gamma-TiAl Legierung

In-situ high energy X-ray diffraction is a powerful technique to investigate the structure of a material in a dynamic setup. In the present ork, the technique was used to study an intermetallic TNM alloy with an increased content of β-stabilising alloying elements Nb and Mo. In a dilatometer setup placed in the synchrotron beam, heating experiments close to thermodynamic equilibrium conditions were performed on homogenised specimens. The experiments offered a deeper insight into the phase transformation behaviour of the investigated type of multi-phase alloy

Impact of Alloying on Stacking Fault Energies in γ-TiAl

Microstructure and mechanical properties are key parameters influencing the performance of structural multi-phase alloys such as those based on intermetallic TiAl compounds. There, the main constituent, a γ -TiAl phase, is derived from a face-centered cubic structure. Consequently, the dissociation of dislocations and generation of stacking faults (SFs) are important factors contributing to the overall deformation behavior, as well as mechanical properties, such as tensile/creep strength and, most importantly, fracture elongation below the brittle-to-ductile transition temperature. In this work, SFs on the { 111 ) plane in γ -TiAl are revisited by means of ab initio calculations, finding their energies in agreement with previous reports. Subsequently, stacking fault energies are evaluated for eight ternary additions, namely group IVB–VIB elements, together with Ti off-stoichiometry. It is found that the energies of superlattice intrinsic SFs, anti-phase boundaries (APBs), as well as complex SFs decrease by 20–40% with respect to values in stoichiometric γ -TiAl once an alloying element X is present in the fault plane having thus a composition of Ti-50Al-12.5X. In addition, Mo, Ti and V stabilize the APB on the (111) plane, which is intrinsically unstable at 0 K in stoichiometric γ -TiAl

Effect of carbon addition on solidification behavior, phase evolution and creep properties of an intermetallic β\beta-stabilized γ\gamma-TiAl based alloy

Improving mechanical properties of advanced intermetallic multi-phase γ-TiAl based alloys, such as the Ti-43.5Al-4Nb-1Mo-0.1B alloy (in at.%), termed TNM alloy, is limited by compositional and microstructural adaptations. A common possibility to further improve strength and creep behavior of such β-solidifying TiAl alloys is e.g. alloying with β-stabilizing substitutional solid solution hardening elements Nb, Mo, Ta, W as well as the addition of interstitial hardening elements C and N which are also carbide and nitride forming elements. Carbon is known to be a strong α-stabilizer and, therefore, alloying with C is accompanied by a change of phase evolution. The preservation of the solidification pathway via the β-phase, which is needed to obtain grain refinement, minimum segregation and an almost texture-free solidification microstructure, in combination with an enhanced content of C, requires a certain amount of β-stabilizing elements, e.g. Mo. In the present study, the solidification pathway, C-solubility and phase evolution of C-containing TNM variants are investigated. Finally, the creep behavior of a refined TNM alloy with 1.5 at.% Mo and 0.5 at.% C is compared with that exhibiting a nominal Ti-43.5Al-4Nb-1Mo-0.1B alloy composition

Hot-working behavior of an advanced intermetallic multi-phase γ\gamma-TiAl based alloy

New high-performance engine concepts for aerospace and automotive application enforce the development of lightweight intermetallic γ-TiAl based alloys with increased high-temperature capability above 750 °C. Besides an increased creep resistance, the alloy system must exhibit sufficient hot-workability. However, the majority of current high-creep resistant γ-TiAl based alloys suffer from poor workability, whereby grain refinement and microstructure control during hot-working are key factors to ensure a final microstructure with sufficient ductility and tolerance against brittle failure below the brittle-to-ductile transition temperature. Therefore, a new and advanced β-solidifying γ-TiAl based alloy, a so-called TNM alloy with a composition of Ti–43Al–4Nb–1Mo–0.1B (at%) and minor additions of C and Si, is investigated by means of uniaxial compressive hot-deformation tests performed with a Gleeble 3500 simulator within a temperature range of 1150–1300 °C and a strain rate regime of 0.005–0.5 s−1 up to a true deformation of 0.9. The occurring mechanisms during hot-working were decoded by ensuing constitutive modeling of the flow curves by a novel phase field region-specific surface fitting approach via a hyperbolic-sine law as well as by evaluation through processing maps combined with microstructural post-analysis to determine a safe hot-working window of the refined TNM alloy. Complementary, in situ high energy X-ray diffraction experiments in combination with an adapted quenching and deformation dilatometer were conducted for a deeper insight about the deformation behavior of the alloy, i.e. phase fractions and texture evolution as well as temperature uncertainties arising during isothermal and non-isothermal compression. It was found that the presence of β-phase and the contribution of particle stimulated nucleation of ζ-Ti5Si3 silicides and h-type carbides Ti2AlC enhance the dynamic recrystallization behavior during deformation within the (α+β) phase field region, leading to refined and nearly texture-free α/α2-grains. In conclusion, robust deformation parameters for the refinement of critical microstructural defects could be defined for the investigated multi-phase γ-TiAl based alloy