Gamma-based Titanium Aluminides Research Articles

Surface transformation of Ti–45Al–2Nb–2Mn–1B titanium aluminide by electron beam melting

Microstructure and phase evolution on the surface of Ti–45Al–2Nb–2Mn–1B (at.%) gamma based titanium aluminide was investigated by a series of electron beam melting with different beam energies and scanning speeds. X-Ray Diffraction (XRD), Glow Discharge Spectroscopy (GDS), Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) were performed to characterize the phase modification and morphology after the EBM treatment. At beam energies of 250 W and scanning speed of 16 mm s − 1, the lamellar structure of Ti–45Al–2Nb–2Mn–1B transformed into a dendritic structure composed of initial α 2 (Ti 3Al) dendrites and an interdendritic phase of the γ (TiAl). While at higher energies of 350 W and lower beam speeds of 7 mm s − 1, mainly B2 and α 2 (Ti 3Al) phases with higher titanium formed on the surface. All Phase transformations increased the hardness of the surface to a maximum of 600 HV if compared to 330 HV for untreated material. Lower energies and higher speeds induced cracks in the surface layers, while higher energies and lower speeds produced hard surface layers without cracking.

Structural and mechanical evaluation of the effect of oxygen boost diffusion on a gamma based titanium aluminide of Ti–45Al–2Nb–2Mn–1B

Air oxidation, vacuum heating and a subsequent oxidation (DT treatment) of Ti–45Al–2Nb–2Mn–1B (at.%) titanium aluminide resulted in several oxide layers and a hard Ti 3Al as an interlayer between the substrate and the oxide layers. Surface characterization was carried out using X-Ray Diffraction (XRD), Glow Discharge Spectroscopy (GDS), electron microscopy, microhardness tests, ball-on-disk tests and profilometry. The compositions of oxides were TiO 2 and Al 2O 3 at the surface layers after final air oxidation and Ti 3Al interlayer was enriched with oxygen. Multi step treatment improved the hardness of treated surfaces significantly in comparison with that of the sample thermally oxidized only at 800 °C (TO treatment). The hard Ti 3Al interlayer containing soluble oxygen supported the top oxide layers of the DT treated sample against indentation. Such supporting layer did not form between the soft substrate materials and the oxide layers of TO treated materials. The mean value and variation of friction coefficient on multi step treated surfaces were less than that of the untreated material. The friction behavior of worn surfaces on the multi step treated material against steel and WC-Co ball sliders was rather smoother than that of TO treated samples. Top surface layers of TO treated material were removed at very shorter sliding distances and lower loads than those of DT treated surfaces against both steel and tungsten carbide sliders.

A multi-step process of oxygen diffusion to improve the wear performance of a gamma-based titanium aluminide

A gamma-based titanium aluminide with the composition of Ti–45Al–2Nb–2Mn–1B (at%) was surface treated through a process in which thermal oxidation in air and diffusion were carried out in several steps to investigate the effect of oxide forming elements and diffusion of oxygen on the wear performance of surface layers. The microstructure and phases on the surface were characterized using optical and electron microscopy, X-ray diffraction (XRD) and glow discharge spectroscopy (GDS). The constituents of the top surface layers were several layers of oxide compounds of TiO 2 and Al 2O 3, beneath which a small amount of nitride compounds of TiN and Ti 2AlN were appeared. After diffusion of oxygen a layer of Ti 3Al with diffused oxygen appeared as a hard layer under the oxide compound layer. The ball on disk wear tests showed the dominant adhesive wear on the untreated surfaces. The treated surfaces were in most cases hard enough to be worn by the steel ball counterface and in fact the material of steel was transferred to the surface. The wear of steel counterface was also due to the ploughing action of the projected angular crystals of TiO 2 and Al 2O 3 on the surface. The Ti 3Al layer should also have played a major role in resistance to wear after the removal of the compound layer as well as supporting the top layer during the wear. The wear resistance and load bearing capacity of the treated samples improved up to two orders of magnitudes higher than those of untreated samples.

The Effect of Two Surface Treatments on the Tribological Behavior of Gamma-Based Titanium Aluminides

DC glow discharge is most widely used in plasma thermochemical processes such as diffusive ion nitriding/carburizing. Gamma-based titanium aluminides were plasma nitrided or air oxidized to compare the surface modifications arising from these treatments. The screened samples were investigated to compare the effect of these treatments. The phases and microstructures formed within the treated surface layers were characterized using X-ray diffraction (XRD), glow discharge spectroscopy (GDS), and optical and scanning electron microscopy. The compound layers consisted of TiN and Ti2AlN in nitrided samples, and TiO2 and Al2O3 in oxidized samples. The wear resistance of the surface-treated samples increased considerably against ball bearing steel (AISI 52100). The overall effect of oxidizing treatment was as good as plasma nitriding.

The evolution of microstructure during the processing of gamma Ti−Al alloys

This paper describes titanium research at the Queen's University of Belfast, particularly focusing on the successful combination of experimental and computer modeling techniques. Experimental work combined x-ray diffraction, optical microscopy, and scanning-electron microscopy performed on a Ti-46Al-1.9Cr-3Nb alloy before and after various heat treatments. A phase-field model was developed to simulate the formation and evolution of lamellar microstructure in TiAl alloys. In addition, a model was developed to predict the correlation between alloy composition and microstructure and its tensile properties in gamma-based titanium aluminide alloys through the use of the artificial neural network.

Processing gamma-based TiAl sheet materials by cyclic cold roll bonding and annealing of elemental titanium and aluminum foils

Previous work by the authors investigated cold roll bonding followed by annealing as a simplistic method for processing Ti/TiAl 3 multi-layered composites from elemental titanium and aluminum foils. In this study, the Ti/TiAl 3 multi-layered composites were subjected to further processing at higher temperatures and increased cyclic cold rolling to promote the formation of gamma-based titanium aluminide (γ-TiAl). The resulting γ-TiAl phase was characterized using light microscopy equipped with digital image analysis, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and Vickers microhardness testing. The thermodynamics and kinetics of the diffusion reaction were investigated using differential scanning calorimetry (DSC) and enthalpy ( H), entropy ( S), and capacity ( C) thermodynamic calculations. The physical, chemical, mechanical, and microstructure changes that accompanied the phase transformation to γ-TiAl was discussed.

Joining of Gamma-Based Titanium Aluminides – A Review

The optimisation of joining technologies is essential to the application of advanced materials in the design of parts and devices. The development of intermetallic compounds, as structural materials, inevitably requires a new approach to join these compounds to themselves or to other materials. Among different intermetallic classes, titanium aluminides are one of the most studied. However, the industrial application is far from being proportional to the research, due to different problems, where joining processes have an important role. The present paper highlights the state of art on joining γ-TiAl alloys. A review is presented with special emphasis on solid-state diffusion bonding process, because it seems to be the most suitable technique to produce high quality joints of advanced materials. The influence of the bonding conditions on the physical and mechanical properties of the joints is highlighted and the introduction of single or multiple interlayers to assist in the bonding process is discussed. A novel approach developed by the authors to the solid-state diffusion bonding of γ-TiAl alloys using Ti/Al multilayer thin films as bonding materials is proposed. The improvement of the solid-state diffusion bonding will induce sound joints at lower temperatures or pressures.

Sliding wear of TiAl intermetallics against steel and ceramics of Al 2O 3, Si 3N 4 and WC/Co

Unlubricated wear tests have been performed with a ball-on-disc tribometer, in which, a disc sample of gamma-based titanium aluminide (TiAl) was sliding against an 8 mm diameter ball made of steel and ceramics of Al 2O 3, Si 3N 4 and WC/Co. Experimental results have shown that sliding wear property of the investigated titanium aluminide is strongly dependent on the counterface materials. Under the testing conditions, wear loss of TiAl in the contact pair of TiAl/Al 2O 3 is the highest, and that in the pair of TiAl/steel is the lowest. There is no clear difference in wear performance of TiAl when the counterface ball is made of WC or Si 3N 4. In addition to wear loss of TiAl, the ball slider was also worn and the amount of wear loss varied significantly with the materials for the ball. Ceramics of Si 3N 4 and Al 2O 3 suffered severe wear loss regardless of their high hardness and generally believed high wear resistance. In contrast, wears of WC and steel were found to be much smaller. From a tribological point of view, sliding contact of titanium aluminide intermetallics against Al 2O 3 and Si 3N 4 may not be optimal designs since these contact pairs can result in very high overall wear loss. Based on XRD, SEM and EDX analysis results, the mechanisms of TiAl wear loss in different contact pairs are discussed.

Characterization and tribological performance of oxide layers on a gamma based titanium aluminide

In this investigation, Ti–45Al–2Nb–2Mn–1B (at%) titanium aluminide was thermally oxidised at different temperatures of 700–950 °C and at different times. Most oxidation exposures resulted in the formation of a compound layer consisting of several oxide layers, a TiO 2 layer as the outermost layer on top of a layer of Al 2O 3 particles, a mixed layer of TiO 2 + Al 2O 3 underneath, and TiN and Ti 2AlN were found at the interface of the compound oxide layer and the matrix. The compound layers were wear tested using a ball-on-disc configuration against 10 mm diameter hard steel (AISI 52100) balls under dry sliding conditions at applied loads of 20, 50, 80 and 100 N. The oxide layers and worn surfaces were characterized using optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), glow discharge spectroscopy (GDS) and microhardness measurements. Steel was transferred to the oxidised surfaces without any considerable deformation of the surfaces. In general, the wear resistance of oxidised surfaces of titanium aluminide was several times higher than that of untreated surfaces.

Modelling tensile properties of gamma-based titanium aluminides using artificial neural network

A model was developed for the prediction of the correlation between alloy composition and microstructure and its tensile properties in gamma-based titanium aluminide alloys through the use of artificial neural network (ANN). The inputs of the neural network were alloy composition, microstructure type and work (test) temperature. The outputs of the model were four important tensile properties: ultimate strength, elongation, reduction of area, and elastic modulus. The model was based on feed-forward neural networks, trained with data collected from various sources of literature. A good performance of the network was achieved, and some explanation of the predicted outputs from a metallurgical point of view was offered. The model can be used for prediction of tensile properties of gamma-based titanium aluminides at various working temperatures. It can also be used to optimise processing parameters to obtain desirable tensile properties. A graphical user interface (GUI) was developed for easy use of the model.

The structural and mechanical property characterisation of thermal oxidation treated $gamma;-based titanium aluminide

A gamma-based titanium aluminide (Ti–48Al–2Cr–2Nb–1B) has been thermal oxidation (TO) treated in an air-circulating furnace. The microstructure, composition and mechanical properties (hardness and elastic modulus) of the TO-treated material were characterised using a variety of techniques including X-ray diffraction, glow discharge spectroscopy, scanning electron microscopes, cross-sectional transmission electron microscopy, microindentation and nanoindentation tests. The load bearing capacities of the oxide layers were evaluated using both static microindentation tests and progressive scratch tests. It has been found that when TO treated at 850 °C for more than 8 h, a hard multi-layered oxide scale consisting of TiO 2 , Al 2 O 3 , Ti 2 AlN and TiN is formed on the titanium aluminide surface. The oxide scale is supported by a diffusion zone with a smooth hardness gradient, resulting in a high load bearing capacity. Based on the experimental results and systematic analyses, the structure of the oxide layer, the mechanical properties (hardness and elastic modulus) and the load bearing capacity of the TO-treated material are discussed.

Physical aspects of hot-working gamma-based titanium aluminides

Titanium aluminide alloys containing relatively large Nb additions and subjected to precipitation hardening exhibit attractive thermo-physical properties, which extend the service range of conventional TiAl alloys. In order to use these materials at their full potential, structural and chemical consolidation is required. To this end, the feasibility of wrought processing of high Nb containing alloys was systematically investigated. The major areas of the study involve: (i) high-temperature deformation mechanisms, (ii) primary ingot break-down, (iii) secondary processing, (iv) texture evolution, and (v) mechanical properties.

The structural and mechanical property characterisation of thermal oxidation treated γ-based titanium aluminide

A gamma-based titanium aluminide (Ti–48Al–2Cr–2Nb–1B) has been thermal oxidation (TO) treated in an air-circulating furnace. The microstructure, composition and mechanical properties (hardness and elastic modulus) of the TO-treated material were characterised using a variety of techniques including X-ray diffraction, glow discharge spectroscopy, scanning electron microscopes, cross-sectional transmission electron microscopy, microindentation and nanoindentation tests. The load bearing capacities of the oxide layers were evaluated using both static microindentation tests and progressive scratch tests. It has been found that when TO treated at 850 °C for more than 8 h, a hard multi-layered oxide scale consisting of TiO 2, Al 2O 3, Ti 2AlN and TiN is formed on the titanium aluminide surface. The oxide scale is supported by a diffusion zone with a smooth hardness gradient, resulting in a high load bearing capacity. Based on the experimental results and systematic analyses, the structure of the oxide layer, the mechanical properties (hardness and elastic modulus) and the load bearing capacity of the TO-treated material are discussed.

Work-hardening and recovery mechanisms in gamma-based titanium aluminides

The work-hardening mechanisms in two-phase γ-titanium aluminide alloys were characterized in terms of the glide obstacles determining the velocity and slip path of dislocations, utilizing transmission electron microscopy (TEM) observations and thermodynamic-glide parameters. There was clear evidence that short-range obstacles in the form of dislocation debris and dipoles were produced during plastic deformation at room temperature. These dislocation obstacles contributed significantly to work hardening. The observed strong strain hardening arose from long-range elastic dislocation interactions and the production of dipole and debris defects. The thermal stability of these deformation-induced defects was assessed by isothermal and isochronal annealing. The results indicated that the dipole and debris defects were relatively unstable upon annealing at moderately high temperatures, which led to significant recovery of work hardening.

Recent Developments in the Design and Processing of Gamma-Based Titanium Aluminide Alloys

Recent Developments in the Design and Processing of Gamma-Based Titanium Aluminide Alloys

A probabilistic framework for the modeling of fatigue in cast lamellar gamma-based titanium aluminides

This paper presents a probabilistic framework for the modeling of fatigue damage in three gamma-based titanium aluminides: Ti–45Al–2Mn–2Nb + 0.8 vol% TiB 2, Ti–47Al–2Mn–2Nb + 0.8 vol% TiB 2 and Ti–48Al–2Cr–2Nb (all compositions quoted in atomic percent unless stated otherwise). This includes: empirical stress life and fracture mechanics approaches to the estimation of material reliability or the risk of failure. Empirical reliability functions are obtained initially from multiple stress-life experiments designed to identify the statistical distributions that best describe the measured variabilities in fatigue life. Fracture mechanics-based reliability functions are also derived using statistical distributions that characterize the measured variabilities in fatigue crack growth data obtained from fatigue crack growth experiments in the long crack regime. The anomalous behavior of short cracks is then discussed before assessing the implications of the current work for the design of engineering structures and components from gamma-based titanium aluminides.

Tribological performance of plasma nitrided gamma based titanium aluminides

Ti–48Al–2Nb–2Mn (at.%) alloy was plasma nitrided to investigate the effect of this thermochemical surface modification on the tribological behaviour of gamma based titanium aluminides. The effects of nitriding temperature, nitriding time and H 2/N 2 gas ratio were investigated. Characterisation of the microstructure and phases formed within the nitrided surface layers was performed using optical and electron microscopy, X-ray diffraction (XRD) and glow discharge spectroscopy (GDS). The wear resistance of the plasma nitrided samples increased considerably against ball bearing steel (AISI 52100). Experimental results indicated that time controls the thickness and nitriding temperature, and H 2/N 2 ratio is responsible for the crystal structure of the nitrided layers. The nitrided compound layers consist of TiN and Ti 2AlN phases. The diffusion zone, in most cases comprises of recrystallised gamma grains beneath the compound layer.

Surface properties of a γ-based titanium aluminide at elevated temperatures

Nano indentation tests have been performed to investigate the mechanical properties of a gamma based titanium aluminides (Ti–48Al–2Nb–2Cr–B) at temperatures ranging from 25 to 400 °C. Indention and nano-scratch test have also been carried out after the samples were cooled down to room temperature to evaluate the effect of higher temperature exposure on the surface properties of the aluminide. The results show that the surface hardness and elastic modulus of the investigated intermetallics are higher at elevated temperatures as compared to those measured at room temperature. XPS analysis reveals that mixed oxides of TiO 2, Al 2O 3 and Nb 2O 5 have formed on the sample surface during the high temperature indentation tests. The Ti:Al ratio obtained from the integrated peak area of Ti 2p and Al 2p spectra decreases with increasing temperature and reaches a minimum value at 400 °C, indicating surface enrichment of aluminium oxide. The oxide layer formed at the higher temperature improves room temperature abrasive wear resistance of the titanium aluminide, as proved by the nano scratch test.

Ductile layer toughening of brittle intermetallic composites

This paper presents the results of a combined experimental and theoretical study of the ductile layer toughening of brittle intermetallics. Following a brief description of small- and large-scale bridging models for the estimation of the crack-tip shielding due to small- and large-scale bridging, the effects of ductile layer toughening are examined in a gamma-based titanium aluminide alloy (Ti-48Al) reinforced with TiNb strips; MoSi2 reinforced with Nb layers, and NiAl reinforced with V layers with different thicknesses. In all cases, the measured resistance-curves are well predicted by existing small- and large-scale bridging models. The intrinsic specimen-independent toughness values are also shown to increase with increasing layer thickness, in the case of the NiAl/V composites. Finally, the general implications of the results are discussed for the toughening of brittle matrix composites under monotonic and cyclic loading.

A probabilistic framework for the modeling of fatigue in a lamellar XD TM gamma titanium aluminide alloy

This paper presents a probabilistic framework for the modeling of fatigue damage in a gamma titanium aluminide alloy (Ti–45Al–2Mn–2Nb +0.8 vol% TiB 2). This includes: empirical stress-life and fracture mechanics approaches to the estimation of material reliability or the risk of failure. Empirical reliability functions are obtained initially from multiple stress-life experiments designed to identify the statistical distributions that best describe the measured variabilities in fatigue life. Fracture mechanics-based reliability functions are also derived using statistical distributions that characterize the measured variabilities in fatigue crack growth data obtained from fatigue crack growth experiments in the long crack regime. The anomalous behavior of short cracks is then discussed before assessing the implications of the current work for the design of engineering structures and components from gamma-based titanium aluminides.