Intermetallic Titanium Aluminides Research Articles

Grindability study of intermetallic titanium aluminide using various abrasive wheels

Nickel (Ni) and Titanium (Ti) based superalloys satisfy the properties of a material suitable for high-performance applications. Titanium aluminide (γTiAl) is a classic example that finds wide application in aerospace and automobile industries. Even though γTiAl is having attractive mechanical and thermal properties, a widespread use of this material is restricted due their poor machinability and high procuring and processing cost. γTiAl during machining often have adverse impact on tool life and surface quality. At room temperature the material exhibits high brittleness and hardness, which along with a low ductility, lower thermal conductivity and fracture toughness, high temperature dependent strength and chemical reactivity with many tool materials. This often makes its machining a challenging task and the material is often considered under the category of difficult to machine material. Majority of the present TiAl components used in aerospace industry undergoes grinding operation. A wider investigation is necessary to understand the grindability of this material, as the variation of composition of the grinding wheel have significant influence on the machining. A consistent and economical machining is essential for the wide recognition of this promising material for wider industrial application. The paper investigates the grindability of intermetallic γTiAl using various abrasive wheels.

Laser Powder Bed Fusion of Intermetallic Titanium Aluminide Alloys Using a Novel Process Chamber Heating System: A Study on Feasibility and Microstructural Optimization for Creep Performance

A laser powder bed fusion process operating at elevated temperatures is introduced capable of fabricating crack-free and dense intermetallic titanium aluminide alloy specimens as well as demonstrator components using a base plate heating up to 900 °C and a unique heating system of the uppermost powder bed layer up to 1200 °C. Two so-called 4th generation alloys, TNM and TNM+, were used for this study. The microstructure and its evolution during subsequent heat treatments were investigated and explained by employing scanning electron microscopy, hardness testing, X-ray diffraction, differential scanning calorimetry and thermodynamic equilibrium calculation. Selected specimens were subjected to creep tests at 750 °C. The microstructures after processing consist of extraordinarily fine lamellar γ-TiAl/α2-Ti3Al-colonies with globular γ and βo-TiAl grains for both the TNM and TNM+ alloy, exhibiting a microstructure gradient from the last consolidated powder layer down to the starting layer due to cellular reaction, which increases the amount of globular γ and βo at the boundaries of the γ/α2-colonies. During annealing in proximity to the γ-solvus temperature, banded microstructures might form, as the α-grain size is only partially controlled by heterogeneously distributed γ/β-phase, which stems from the process-related Al loss. Additionally, the occurrence of thermally-induced porosity is investigated. Optimizing the microstructure to a homogenized, almost fully lamellar microstructure, involved annealing in the β-single phase field region and led to improved creep properties. Finally, TNM demonstrator components with complex geometries, such as aero engine blades and turbocharger turbine wheels, are fabricated by employing the novel laser powder bed fusion process.

Influence of Two‐Step Heat Treatments on Microstructure and Mechanical Properties of a β‐Solidifying Titanium Aluminide Alloy Fabricated via Electron Beam Powder Bed Fusion

Additive manufacturing technologies, particularly electron beam powder bed fusion (PBF‐EB/M), are becoming increasingly important for the processing of intermetallic titanium aluminides. This study presents the effects of hot isostatic pressing (HIP) and subsequent two‐step heat treatments on the microstructure and mechanical properties of the TNM‐B1 alloy (Ti–43.5Al–4Nb–1Mo–0.1B) fabricated via PBF‐EB/M. Adequate solution heat treatment temperatures allow the adjustment of fully lamellar (FL) and nearly lamellar (NL‐β) microstructures. The specimens are characterized by optical microscopy and scanning electron microscopy (SEM), X‐ray computed tomography (CT), X‐ray diffraction (XRD), and electron backscatter diffraction (EBSD). The mechanical properties at ambient temperatures are evaluated via tensile testing and subsequent fractography. While lack‐of‐fusion defects are the main causes of failure in the as‐built condition, the mechanical properties in the heat‐treated conditions are predominantly controlled by the microstructure. The highest ultimate tensile strength is achieved after HIP due to the elimination of lack‐of‐fusion defects. The results reveal challenges originating from the PBF‐EB/M process, for example, local variations in chemical composition due to aluminum evaporation, which in turn affect the microstructures after heat treatment. For designing suitable heat treatment strategies, particular attention should therefore be paid to the microstructural characteristics associated with additive manufacturing.

Size-Dependent Fracture Characteristics of Intermetallic Alloys

BackgroundLightweight alloys such as intermetallic titanium aluminide (TiAl) alloys are poised to be a potential candidate for replacing heavier nickel based super alloys in an aero engine. However, before an industry wide implementation is possible, it is indispensable to develop physically accurate computational material models which account for essential deformation and fracture mechanisms. This assists the virtual prototyping required for the new product development using TiAl components.ObjectiveThe objective of this work is to determine the effect of size of tested specimens on their fracture energy and provide a physically motivated scaling law.MethodsIn this work, the quasi-brittle behavior of TiAl alloys is experimentally and numerically investigated. A total number of 29 geometrically identical TiAl specimens of three different sizes are tested in a three-point bending setup. Since the final abrupt failure of each specimen is preceded by plasticity, a theoretical and numerical framework which accounts for both elastic and plastic work densities is applied in simulations.ResultsThe fracture energy density for each tested size is calculated numerically which is found to be lower for larger volumes, thereby, confirming the size effect in intermetallic TiAl alloys. A novel size effect law is proposed which is based on two physically motivated coefficients.ConclusionsThe work concludes with the quantitative knowledge of the size-dependent fracture energy of intermetallic alloys and an empirical scaling law to predict the same. Excellent predictive capability of the proposed law is successfully established with data of various quasi-brittle materials from literature.

Reclamation of intermetallic titanium aluminide aero-engine components using directed energy deposition technology

Titanium Aluminide (TiAl) alloys are intermetallics that offer low density, high melting point, good oxidation and corrosion resistance compared to Ni-based superalloys. As a result, these alloys are used in aero-engine parts such as turbine blades, fuel injectors, radial diffusers, divergent flaps, and more. During operation, aero-engine components are subjected to high thermal loading in an oxidizing and corrosive environment, which results in wear and other material damage. Replacement of the entire component may not be desirable due to long lead time and expense. In such cases, repair and refurbishing may be the best option for the reclamation of TiAl parts. Unfortunately, approved repair technology is not currently available for TiAl based components. Additive Manufacturing (AM) based Directed Energy Deposition (DED) may serve as an option to help repair and restore expensive aero-engine parts. In this work, a review of efforts to utilize the DED technique to repair damaged TiAl-based aerospace parts locally is conducted. Replacing the entire TiAl part is not advisable as it is expensive. DED is a promising technique used to produce, repair, rework, and overhaul (MRO) damaged parts. Considering the high-quality standard of the aircraft industry, DED repaired TiAl parts to be certified for their future use in the aircraft is very important. However, there are no standards for the certification of TiAl repaired parts is reported. Case studies reveal that DED is under consideration for repair of TiAl parts. Hybrid technology comprising machining, repair and finishing capability in a single machine is an attractive implementation strategy to improve repair efficacies. The review shows that the investigations into development and applications of DED-based repairing techniques are limited, which suggests that further investigations are very much needed.

Quasi-static and dynamic fracture toughness of a γ-TiAl alloy: Measurement techniques, fractography and interpretation

Intermetallic titanium aluminides (TiAl) offer an enormous potential for high temperature applications due to their high specific strength, creep and oxidation resistance. However, their use for structural components is limited by their low ductility and fracture toughness. During service, along with a high number of load cycles, the material may also be subjected to unexpected impact loads, which requires therefore knowledge about dynamic material parameters for a damage-tolerant component design. Here we present a feasibility study to determine the fracture toughness of a third generation TiAl-alloy over a wide range of loading rates. Quasi-static crack resistance curves (R-curves) were determined using the direct current potential drop (DCPD) technique. The dynamic experiments were performed with a drop tower with impact velocities ranging from 1 to 10 m/s on pre-cracked SENB specimens. To exclude the influence of inertial forces, the measurements were carried out with strain gauges applied in the elastic near-field of the crack tip. Despite a critical consideration of possible systematic errors of this measurement method, it was found that the dynamic fracture initiation toughness slightly increases up to loading rates of K̇I = 106 MPa√m/s. The fracture behaviour of the material and possible reasons for the toughness increase are discussed based on comprehensive fractographic investigations as well as crack path analyses within the microstructure.

Effects of tungsten alloying and fluorination on the oxidation behavior of intermetallic titanium aluminides for aerospace applications

Current limitations to a wider use of intermetallic TiAl alloys in aircraft and automotive engines arise from an insufficient oxidation resistance at temperatures above approximately 800 °C. In this paper, the high temperature oxidation behavior of three engineering γ-TiAl-based alloys at 900 °C in air is reported. The performance of the TNM alloy (Ti-43.5Al–4Nb–1Mo-0.1B), the 4822 alloy (Ti–48Al–2Cr–2Nb), and the Nb-free IRIS alloy (Ti–48Al–2W-0.08B) is compared (all chemical compositions are given in at.%). During testing in air non-protective mixed oxide scales developed on all untreated samples, but with different compositions and thicknesses. These different oxide layers are characterized and their formation mechanisms are discussed. The presence of W in the IRIS alloy leads to a better oxidation behavior compared to untreated TNM and 4822. This behavior was changed in the direction of a protective alumina layer formation via the so-called “fluorine effect”. The above-mentioned alloys were treated with fluorine via a liquid phase process by evenly spraying a fluorine containing polymer on all faces of the specimens. The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated specimens. Due to the fluorination all treated test coupons exhibited slow oxidation kinetics. The results of isothermal as well as thermocyclic exposure tests are presented and discussed in the view of the chemical composition and processing conditioned microstructure of the three investigated γ-TiAl-based alloys.

Influence of Nb on Ti diffusion in γ-TiAl intermetallics studied by mechanical spectroscopy

The development of intermetallic titanium aluminides has been driven by the aeronautic and aerospace industries because of the excellent mechanical properties and low density of γ-TiAl based alloys. Up to now, several generations of γ-TiAl based alloys were developed with increasing complexity of the alloy systems. Nb is one of the most important alloying elements in γ-TiAl alloys and although it is considered as a slow diffuser, its influence has not been fully quantified yet. In this work we demonstrate, through mechanical spectroscopy measurements conducted on several γ-TiAl based alloys with different Nb content, that Nb impedes the diffusion of Ti atoms in the α2-Ti3Al phase. Internal friction measurements show a relaxation peak P(α2), which is associated with short distance diffusion of Ti atoms in the α2 phase, involving stress-induced rotation of dipoles Al-VTi-Al, whose activation energy is dependent on the Nb content. The increase of the activation energy is quantified as ΔEa(Ti)= 0.037 eV × at% Nb, being attributed to the next-neighbor interaction of Nb atoms with the local configuration of Ti-VTi. This mechanism also produces a further broadening of the relaxation peak, which is attributed to the near-next-neighbor interactions for high Nb contents. Finally, an atomic model for the mechanism responsible for this relaxation is proposed allowing to explain the observed experimental behavior.

Surface burn behavior in creep-feed deep grinding of gamma titanium aluminide intermetallics: characterization, mechanism, and effects

This article presents the surface burn behavior when creep-feed grinding gamma titanium aluminide (γ-TiAl) intermetallic. First, the main characteristics of burnt surface morphology were detected. Subsequently, surface burn mechanism was revealed by analyzing compositional changes in the surface layer via X-ray photoelectron spectroscopy. Lastly, the effects of grinding temperature on grinding force and surface integrity were discussed. Finding showed that a high grinding temperature would lead to a heavy color, from light brown to hyacinthine, on the burnt surface. The burning process was dominated by oxidation of Ti and Al. Ti was generally in the state of TiO2 on the ground surface, whereas lower valence oxide of Ti was found on the subsurface. This oxide layer could produce a reduced ratio of the tangential grinding force to the normal one. Elevated temperature would produce grinding and quenching cracks on a burnt surface, and tensile residual stress might be developed if the temperature is extremely high. Surface morphology and topography characteristics indicated that, unlike conventional titanium alloys (e.g., Ti-6Al-4V), a marginal physical reaction occurred on the workpiece surface during the burning process of γ-TiAl intermetallics due to poor material ductility.

Designing advanced intermetallic titanium aluminide alloys for additive manufacturing

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.

A study on hot-working as alternative post-processing method for titanium aluminides built by laser powder bed fusion and electron beam melting

Intermetallic Titanium Aluminides (TiAl) have been designed for high-temperature lightweight applications. Powder-bed additive manufacturing (AM) processes such as laser powder bed fusion (LPBF) or electron beam melting (EBM) allow near-net-shape production of TiAl components. However, TiAl parts produced by LPBF or EBM do not reach the structural integrity and mechanical properties of forged parts. The main post-processing step of TiAl parts made by AM is hot-isostatic pressing (HIP), which has several disadvantages such as a long process time and an undesired coarsening of the microstructure. High-performance TiAl components such as turbine blades are thus still produced by isothermal forging of preforms made by investment casting and hot isostatic pressing. Due to the low workability of TiAl alloys, often more than one deformation step is required, and the tooling costs and the low material yield make this process chain very expensive. This paper explores the feasibility of combining AM with isothermal forming in order to replace HIP by thermomechanical post-processing. The hot forming behavior of the Ti-43.5Al-4Nb-1Mo-0.1B (at. %) TNM-B1 alloy, manufactured by EBM and LPBF, is analyzed concerning the evolution of the voids and grain sizes and compared to hot isostatic pressing. It is shown that the fine microstructure produced by AM yields a much lower flow stress and faster globularization kinetics in comparison to conventional cast and HIPed material. While HIP experiments are shown to significantly coarsen the microstructure, isothermal hot working is shown to convert the AM microstructure to a dense and refined state. Moderate hot working with total strains of ∼1 of AM pre-forms may thus serve as an alternative process chain to conventional large strains forging of cast pre-forms and to AM + HIP in the series production of high-performance TiAl components.

Reinforcing effect of laminate structure on the fracture toughness of Al3Ti intermetallic

Metal/intermetallic laminate composites can improve the mechanical properties of intermetallic materials using metal layers. In recent years, titanium aluminide intermetallics have received increasing attention due to their excellent performance properties, such as high melting point, high specific strength and stiffness, and good corrosion resistance. However, the low fracture toughness of Al3Ti alloys at room temperature has greatly limited their application, and fiber or particle reinforcement has not shown a significant toughening effect. Research into the reinforcing effects of the interface and near-interface zone on the fracture behavior of Al3Ti is lacking. Ti/Al3Ti metal/intermetallic laminate composite was synthesized from titanium and aluminum foils using vacuum hot-pressed sintering technology. The microstructure of the prepared material was analyzed by scanning electron microscope and electron backscattered diffraction. Results illustrate that both Ti and Al3Ti were single-phase and there was a noticeable stress concentration on the interface. To obtain indentation and cracks, loads were applied to different locations of the composite by a microhardness tester. The growth path of the cracks was then observed under microscope, showing that crack propagation was prevented by the interface between the Ti and Al3Ti layers, and the cracks that propagated parallel to the laminate shifted to the interface. Fracture toughness of the different areas, including Al3Ti layers, interface, and near-interface zone, were measured by the indentation fracture method. The fracture toughness at and near the interface was 1.7 and 2 times that of the Al3Ti layers, respectively. Results indicate that crack blunting and crack front convolution by the laminate structure was primarily responsible for increased toughness.

Wear of silicon carbide wheel during grinding of intermetallic titanium aluminide

Wear of silicon carbide wheel during grinding of intermetallic titanium aluminide

Metallography of Intermetallic Titanium Aluminides – the (Additive) Manufacturing Makes the Difference

Abstract The global economic network, increasing mobility and wealth have a significant impact on energy consumption and environmental degradation, creating a serious social and political pressure on climate protection issues and a sustainable use of limited natural resources. In this context, a variety of programs are launched worldwide on a political and scientific / technical level to reduce aviation as well as automobile emissions. To meet these requirements, apart from new and improved design and lightweight construction concepts, high-temperature lightweight structural materials and their processing technologies play a key role. Due to their high specific (creep) strength and low density, intermetallic titanium aluminides have a particularly great potential, which is already being used industrially. While in the last decades, predominantly ingot metallurgical processes have been developed for the production of pre-material, which have subsequently been processed by casting and hot-working, the introduction of powder-based manufacturing technologies (e. g. additive manufacturing), with the availability of high-quality alloy powder, opens up new ways of material processing and component design. The basis of this work is the process-adapted 4th generation TNM-alloy, which was developed at the Chair of Physical Metallurgy and Metallic Materials. Due to its reactivity, manufacturing methods used are electron beam melting and laser powder-bed fusion as well as spark plasma sintering. Furthermore, high demands are placed on the production of the powder, in particular with regard to its purity. The chemical composition of the project alloy is designed and optimized so that it is “resistant” to the characteristics of the different manufacturing processes and their physical conditions. The starting powders and the manufactured specimens are subjected to a comprehensive characterization involving microstructural investigations on several length scales as well as the examination of the mechanical properties. Moreover, in order to further optimize the mechanical properties at elevated temperatures, it is an essential goal to develop suitable heat treatments. This work will show how conventional and high-resolution metallography can be used to combine innovative alloys with new processing technologies.

Selected Methods of Quantitative Phase Analysis of an Additively Manufactured TNM Titanium Aluminide Alloy

Abstract The properties of intermetallic titanium aluminides, such as the so-called TNM alloy, which is, owing to its high specific strength and low density, already used in the aviation and automotive sector in the temperature range from 600–800 °C, are highly dependent on the fractions and nature of the present phases. Reliable options of quantitative phase analysis are therefore indispensable. Against this background, samples of an additively manufactured TNM alloy with the composition Ti-42.1Al-4.1Nb-1.0Mo-0.1B (at. %) were metallographically analyzed and the phase fractions of the α2-, β0 and γ phases were quantitatively determined by image analysis, electron backscatter diffraction, and Rietveld analysis in the course of X-ray diffraction experiments performed in this work. With regard to the fractions of the α2 and γ phases, different results were obtained for the three methods applied which can be attributed to the characteristics of additive manufacturing and the quantitative phase analysis of multiphase samples.

Electroplated CBN superabrasive wheel for grinding intermetallic titanium aluminide

An experimental investigation is reported on the surface grinding of intermetallic gamma titanium aluminide using electroplated cubic boron nitride grinding wheel. In order to implement the wide acceptability of this material in various fields, the machinability investigation finds crucial especially during grinding. In the present work, the machinability of gamma titanium aluminide of two different compositions was investigated by analysing the normal and tangential force, surface integrity, chip morphology and the condition of grinding wheel after the experiments. The minor variation in the composition of titanium aluminide has influence on the generated cutting forces, the knowledge of which is essential for satisfactory use of this material in various applications. The observations from the machined surface have shown the presence of various surface defects and the analysis of chip morphology revealed the occurrence of high temperature during the grinding. The topography of the grinding wheel is also investigated after the experiments.

Electroplated CBN superabrasive wheel for grinding intermetallic titanium aluminide

Electroplated CBN superabrasive wheel for grinding intermetallic titanium aluminide

Wear of Silicon Carbide Wheel During Grinding of Intermetallic Titanium Aluminide

An experimental investigation is reported on the wear of silicon carbide wheel and its influence on the wheel topography and grinding behaviour during the surface grinding of high performance intermetallic gamma titianium aluminide. The radial wear of the wheel was characterised by initial transient state followed by a steady state wear regime and finally a steel rise in wear rate. The initial transient region progressed up to a radial wear of 40 μm. The normal and tangential forces remained approximately constant up to about 25 μm wheel wear and subsequently progressively increased to a maximum at 60 μm wear. The specific energy began approximately at about u = 140 J/mm3 and started progressing beyond radial wear of 25 μm and increased up to 600 J/mm3 at wear of 60 μm. There was high initial roughness value with the fresh wheel with a progressive decrease with continued grinding. There was noticeable increase in roughness value after a radial wear of 60 μm. With the progress of wear there was presence of surface burns, chatter marks and cracking. The presence of blue oxide discoloration was observed during the visual inspection signifying the generation of reasonably high temperature.

Computer Simulation of Plastic Deformation in Tial Alloys in the Presence of Chromium

Quantum-mechanical calculations were used to investigate shear rupture in intermetallic titanium aluminide (TiAl) alloys in the presence of vacancy or chromium dopant. The substitution of both Ti and Al atoms by Cr atoms in the γ-TiAl crystal lattice was considered. The simulation of shear was carried out in the (111) slip plane along two directions, namely the [110] and [11-2]. The decrease in the shear resistance of the defects present in the γ-TiAl lattice was estimated. It was shown that when chromium occupies a titanium vacancy, it can compensate for this defect by increasing the shear modulus for the {111} <110> slip system.

Multi-Scale Microstructural Characterization

Abstract A great number of applications and technical processes is limited by the materials’ maximum load bearing capacity, while the development of new materials is inextricably linked with the availability of appropriate characterization methods. This work aims to point out methods used and fields of research worked on, in the past years and currently, at the Department of Physical Metallurgy and Material Testing of the Montanuniversität Leoben. In this context, the study aims to present the use of classic metallography as well as the benefit of high-resolution characterization methods for a comprehensive understanding of the structure of metallic materials. The examples shown range from grain boundary etchings in high strength steels, atom probe measurements on automotive steels, and the use of scattering techniques to characterize intermetallic titanium aluminide alloys, up to cases of damage on galvanized structural steels. This work furthermore demonstrates the use of ab initio calculations on a high-alloy steel for the prediction of phase preferences of alloying elements.