The effects of machined workpiece surface integrity on the fatigue life of γ-titanium aluminide

Tension–tension tests on turned, electro-chemical machined (ECM) and electro-discharge textured (EDT) specimens made from Ti–45Al–2Nb–2Mn+0.8 vol% TiB₂ alloy, showed the turned specimens to have a higher fatigue strength 475 MPa. It is likely that this was due to the presence of highly compressive surface residual stresses caused by the turning operation.     

On the high-speed Single Point Incremental Forming of titanium alloys

Single Point Incremental Forming processes show some limitations related to both dimensional accuracy and process slowness. The process slowness is here overcome by introducing the high speed forming, which allows a reduction to less than 1 min of execution time of target components made in Titanium alloys. The paper is aimed at analyzing the influence of the feed increasing on the material quality in order to investigate if the development of high speed machines could be a suitable solution to implement more extensively the Single Point Incremental Forming technique in practice.     

Selective laser melting of a beta-solidifying TNM-B1 titanium aluminide alloy

The interest for a wider range of useable materials for the technology of selective laser melting is growing. In this work we describe a new way to optimize the process parameters for selective laser melting of a beta solidifying titanium aluminide. This kind of material has so far not been processed successfully by this method. The new approach is easy to conduct and well transferable to other materials. It is based on the fact that the parts generated from selective laser melting can be described by an addition of multiple single tracks. Multiple types of single track experiments are performed and in combination with knowledge from laser welding tests optimized parameter combinations are derived. Compact samples are built with the optimized process parameters and characterized in terms of microstructure, phase composition and mechanical properties. With this technique the generation of a TNMB1 titanium aluminide alloy sample with a density greater than 99% could be achieved. The mechanical properties are comparable with material produced by conventional techniques.     

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 Ti₃Al 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 TiO₂ and Al₂O₃ at the surface layers after final air oxidation and Ti₃Al 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 Ti₃Al 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.     

Temperature changes and loads during hot-die forging of a gamma titanium–aluminide alloy

Non-isothermal forging is a non-steady state deformation process since temperatures of die, workpiece and environment are different from each other. The workpiece temperature decreases during the process by heat transfer to the dies and the environment. On the other hand, deformation heating can increase the workpiece temperature. The deformation loads depend not just on the initial temperature of the workpiece and the strain rate, but also on the duration of the deformation. This paper presents the results of a study that involved both physical experiments and finite element simulation of the non-isothermal deformation of a gamma titanium–aluminide.     

Crystallographic and morphological relationships between β phase and the Widmanstätten and allotriomorphic α phase at special β grain boundaries in an α/β titanium alloy

In the present study, the relationship between the crystallographic orientations and growth directions of grain boundary-allotriomorphic-α (GB α) and secondary Widmanstätten α laths growing from the GB α at grain boundaries separating β grains with specific misorientations has been examined. These relationships have been determined using a variety of characterization techniques, including scanning electron microscopy, orientation imaging microscopy, transmission electron microscopy (TEM) and a dual-beam focused ion beam instrument to provide site-selected TEM foils. Two very interesting cases, one in which the two adjacent β grains are rotated mutually by approximately 10.5° about a common 1 1 0 direction and the other in which the two β grains are in a twin relationship, i.e. a 60° rotation about a common 1 1 1 direction, have been studied. It was discovered that the α laths growing into two adjacent β grains from the common grain boundary may have the same orientation in both grains, while they may have either large (88.8°) or small (28.8°) angular differences in growth directions in the two adjacent β grains, depending on the relative misorientation of the β grains. The growth directions of the α laths growing from such boundaries are explained on the basis of the Burgers orientation relationship between the Widmanstätten α and the β phases and the interfacial structure proposed previously by various workers.     

Laser cladding of titanium alloy coating on titanium aluminide alloy substrate

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Synthesis and high-temperature stability of titanium aluminide matrixin situ composites

A premixture of elemental powders of titanium and aluminum was supplied as a spray material for the direct fabrication of titanium aluminide matrixin situ composites by means of reactive low-pressure plasma spraying with a nitrogen and hydrogen mixed plasma gas. The aluminum content varied from 10 to 63 wt.% in the premixtures. The matrix of sprayed layers consisted of three kinds of titanium aluminides—Ti₃A1, TiAl, and TiAl₃—which begin to form on a low-carbon steel substrate immediately after deposition. The formation of nitrides, which act as a reinforcement, occurs both during the flight of liquid droplets and on the substrate. The nitrogen content is approximately 4 to 5 wt.% in the sprayed intermetallic matrix composites, regardless of the aluminum content of the premixtures. The kinds of titanium aluminides andin situ nitrides developed depend on the aluminum content of the premixtures. The homogeneity of the distribution of aluminum and titanium in sprayed intermetallic matrix composites has been improved by vacuum annealing. The predominant TiAl phase that formed in the sprayed intermetallic matrix composites with a Ti-36 wt.% AI premixture increases in quantity through annealing. Although some minor nitrides disappear through annealing, the principal reinforcement, Ti₂AlN, does not decompose, but increases in quantity. The hardness of sprayed intermetallic matrix composites varies with aluminum content of the premixtures, but is always greater than that of sprayed titanium aluminides containing no nitrides. Annealing does not reduce the hardness of sprayed intermetallic matrix composites. Sprayed and annealed intermetallic matrix composites with a Ti-36 wt.% Al premixture maintain their hardness of approximately 500 HV up to 800 K. Hence, reactive low-pressure plasma spraying offers a promising fabrication method for titanium aluminide matrixin situ composites, which are expected to excel in wear resistance applications at elevated temperatures.     

The effects of hydrogen on the deformation and fracture of β-titanium

The hydrogen-induced ductile–brittle transition in the BCC β-titanium alloy, Timetal® 21S, occurs abruptly at a critical hydrogen concentration that decreased with decreasing tensile test temperature. Mechanical property tests showed that solute hydrogen reduced the yield strength of ductile specimens and decreased the fracture stress of brittle specimens. To identify the operative mechanism a series of experiments were performed to test the applicability of the stress-induced hydride mechanism, the hydrogen-enhanced plasticity mechanism, and the decohesion mechanism of hydrogen embrittlement. The experiments showed that no hydrides were associated with the fracture process, indicating that the stress-induced hydride mechanism was not responsible for the observed sharp ductile–brittle transition. In situ straining experiments in a controlled environment transmission electron microscope showed that hydrogen enhanced the mobility of dislocations in both uncharged and hydrogen charged alloys, showing that the hydrogen-enhanced localized plasticity mechanism cannot account for the observed behavior. The experimental results are, however, fully consistent with the decohesion mechanism of hydrogen embrittlement.     

Performance of thermal barrier coatings on γ‐TiAl

The current development of new generation gamma titanium aluminides is expected to result in alloy chemistries and microstructures capable of operating at temperatures in excess of 850 °C. Under these conditions, environmental and thermal protection becomes a concern since oxidation might eventually limit the maximum service temperatures achievable. Therefore protective coatings are necessary to exploit the full potential of gamma titanium aluminides at moderately elevated temperatures; however, as yet no coating system tested has proven sufficient performance for long‐term use in automotive and aerospace applications. Thermal barrier coatings (TBCs), typically applied to nickel‐based alloys, offer the potential to increase the service temperature of components by lowering the metal surface temperature in combination with cooling systems. The paper is focussed on development of thermal barrier coatings for gamma titanium aluminides. Different coatings were used for oxidation protection and bond coat application. Substrate specimens were either pre‐oxidized or coated with PVD‐Al₂O₃, TiAlCrYN, or diffusion aluminides. Yttria‐stabilized zirconia TBCs were deposited applying electron‐beam physical vapour deposition. Cyclic and quasi‐isothermal oxidation tests were carried out at 900 °C in air. Post‐oxidation analysis of the coating systems was performed using scanning electron microscopy with energy‐dispersive X‐ray spectroscopy. Zirconia top coats offer a promising thermal protection concept to be applied on γ‐TiAl components. However, high oxidation resistance has to be supplied by protective coatings. Diffusion layers of the TiAl₃ aluminide provided excellent environmental protection because of the formation of a continuous alumina scale. No spallation of the thermal barrier coatings was observed on aluminized specimens during 1000 1‐h cycles and 3000 h of cyclic and isothermal oxidation testing, respectively.     

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 α₂ (Ti₃Al) 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 α₂ (Ti₃Al) 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.     

Process and properties of hydride formed on gamma titanium aluminide during cathodic charging

A cathodic charging procedure was used to study the process of the formation of the hydride layer on gamma titanium aluminides. This electrolytic process was carried out at constant current densities of 1 and 2 A/m² for 24 h of charging in a 1N sulfuric acid solution. The hydride layer formed as a result of the charging process was observed using scanning electron microscopy. Nanohardness and microhardness of this hydride layer were also measured. Results show that the hydride forms initially as isolated islands and becomes continuous with increased charging time. The hydride layer is quite brittle and its degree of porosity increases from the metal surface outward. The thickness of the hydride layer also increases with charging current density. This is confirmed by the hardness measurements. EDS signals show the presence of the constitutive elements of gamma titanium aluminide in the hydride. ICPS analysis of the electrolyte indicates increasing metal content with increasing time of exposure probably as a result of the brittle hydride flaking off and falling into the electrolyte during the charging process.     

Internal friction of γ-TiAl alloys at high temperature

Intermetallic γ-TiAl based alloys of engineering interest with respect to high temperature applications are two-phase alloys consisting of γ-TiAl (ordered face-centered tetragonal structure) and ₂-Ti₃Al (ordered hexagonal structure). For this investigation a γ-TiAl based alloy with a composition of Ti-46.4 at%Al-4 at% (Cr, Nb, Ta, B) was studied utilizing a low frequency subresonance apparatus at frequencies between 0.01 and 10 Hz. Above 1000 K, the damping increases strongly up to Q⁻¹=0.2. The frequency and temperature dependence of the high-temperature background was analysed by applying a Maxwell rheological model for viscoelastic relaxation including a distribution of relaxation times. The activation enthalpy of H=3.9 eV agrees well with that from creep experiments (H=3.6 eV) carried out at temperatures between 973 and 1073 K at 200 MPa. The results are discussed in terms of diffusion controlled dislocation climb.     

Creep feed grinding of gamma titanium aluminide and burn resistant titanium alloys using SiC abrasive

Following a brief introduction to titanium alloys and their machinability, the cutting performance of a gamma titanium aluminide intermetallic (γ-TiAl) alloy: Ti–45Al–8Nb–0.2C wt% and a burn resistant titanium (BuRTi) alloy: Ti–25V–15Cr–2Al–0.2C wt%, is compared with creep feed grinding using SiC abrasive. The work utilised 2 separate L9 Taguchi fractional factorial arrays. Typically G-ratios were a factor of 10× greater for γ-TiAl than BuRTi, with on average 10% lower maximum power and 25% lower maximum specific energy for the γ-TiAl alloy. A combination of a moderately high wheel speed: 35 m/s, low depth of cut: 1.25 mm and low feed rate: 150 mm/min, produced the lowest average workpiece surface roughness (Ra1.4 μm). Workpiece surface integrity evaluation indicated that with lower operating parameter levels, in particular a wheel speed of 15 m/s, surfaces free of burn and cracks could be produced, while at higher wheel speeds: 35 m/s, extensive workpiece surface burn was evident, with the γ-TiAl alloy suffering extensive cracking. Microhardness measurements showed in some instances slightly increased workpiece surface hardness of around 50–60HK_0.025 for the BuRTi alloy and 200HK_0.025 for the γ-TiAl material over respective bulk hardness values of 375HK_0.025 and 400HK_0.025.     

Effect of high-density current electropulsing on corrosion cracking of titanium aluminide intermetallic

The effect of electropulsing on the corrosion cracking of titanium aluminide produced by self-propagating high-temperature synthesis has been investigated. The electropulsing treatment led to improved corrosion resistance in sodium fluoride solution and also eliminated corrosion cracking at the α₂/γ interface during corrosion in a solution of nitric and hydrofluoric acids. This behavior was attributed to thermal and athermal effects resulting from electropulsing and leading to the interaction of conduction electrons with the defect structure. The effect of magnetic field accompanying electropusling on depinning of dislocations also has been discussed. Support for this is provided on the basis of X-ray diffraction analysis and microhardness testing.     

Surface protection of titanium by Ti5Si3 silicide layer prepared by combination of vapour phase siliconizing and heat treatment

In this work we proposed a new method for preparation of protective Ti₅Si₃ silicide layer. The proposed method consisted of two steps: (1) silicon electron beam evaporation and (2) heat treatment. The silicide-modified titanium was subjected to structural examination, hardness profiling and isothermal oxidation tests. The investigations revealed that the proposed surface treatment produces a layer with a thickness of 1–2 μm and hardness of about 1500 HV. The isothermal oxidation tests showed that the layer is highly resistant to oxidation in air even at 900 °C. Its oxidation rate was comparable or even lower than that of some high-temperature γ-TiAl intermetallics oxidized under similar conditions. The oxidation mechanism was discussed in terms of the internal structure and chemical composition of the scales. It was demonstrated that the silicide layer could serve as excellent protection of titanium, Ti-based alloys and intermetallics against the high-temperature oxidation and wear.     

热变形对TC11-Ti2AlNb电子束焊接件力学性能的影响

本文应用纳米压痕和维氏硬度的方法表征了TC11/Ti2AlNb电子束焊接焊缝区域在不同状态下的硬度和弹性模量分布,结合组织的演变分析了微纳米尺度的力学的变化。结果表明：在TC11合金的热影响区,马氏体α"相的分解是显微硬度降低的主要原因;而在焊缝以及Ti2AlNb热影响区区域,相的析出导致了显微硬度的增加。通过热变形以及锻后热处理都能够提高焊接区域的弹性模量。相比较而言,焊接态的焊缝弹性模量只有92GPa;而在变形和热处理后,弹性模量的值达到了130GPa。通过拉伸实验结果分析,焊缝在变形及热处理后屈服强度得到了较大提高,这和焊缝区域硬度和弹性模量的变化趋势一致。     

An overview of power-law creep in polycrystalline β-titanium

A constitutive law for power-law creep of BCC β-Ti is developed, based on experimental data from eight independent studies. The present compilation adds more than twice as many data points as previous analyses, covers nine orders of magnitude in strain rate from 10⁻⁷ to 10² s⁻¹, and incorporates recent data for the shear modulus of β-Ti.     

Features of feathery γ structure in a near-γ TiAl alloy

This work characterizes the feathery-like structures produced in a Ti–46.8Al–1.7Cr–1.8Nb (at.%) alloy during rapid continuous cooling from the α domain. Their morphology and crystallography are described using different microscopy and orientation mapping techniques. These feathery-like structures are divided into many small domains, characterized by low-angle misorientations (rotated less than 15°) between the domains. The domains comprise multiple parallel γ lamellae and rare traces of α₂. These lamellae follow the Blackburn orientation relationship and have a {1 1 1}_γ habit plane. Two types of γ-feathery structures were identified according to their location and crystallography. The grain-boundary γ-feathery structures originate from lamellar structures that grow into a neighboring grain. Alternatively, the internal γ-feathery structures are located in the interior of prior α grains and show an average misorientation of 36° around one $〈100{〉}_{{\mathrm{\gamma }}_{\text{L}}}$ axis of the lamellar structure in which it is embedded. This paper describes these two γ-feathery structures in detail and discusses their development in light of the mechanisms available in the literature, particularly sympathetic nucleation.