Ti 48Al 2Cr Research Articles

Defect-free laser powder bed fusion of Ti–48Al–2Cr–2Nb with a high temperature inductive preheating system

In the industrial panorama, laser powder bed fusion (LPBF) systems enable the near net shaping of metal powders into complex geometries with unique design features. This makes the technology appealing for many industrial applications, which require high performance materials combined with lightweight design, lattice structures and organic forms. However, many of the alloys that would be ideal for the realisation of these functional components are classified as difficult to weld due to their cracking sensitivity. γ-TiAl alloys are currently processed via electron beam melting (EBM) to produce components for energy generation applications. The EBM process provides crack-free processing thanks to the preheating stages between layers, but lacks geometrical precision. The use of LPBF could provide the means for higher precision, and therefore an easier post-processing stage. However, industrial LPBF systems employ resistive heating elements underneath the base plate which do not commonly reach the high temperatures required for the processing of γ-TiAl alloys. Thus, elevated temperature preheating of the build part and control over the cooling rate after the deposition process is concluded are amongst the features which require further investigations. In this work, the design and implementation of a novel inductive high temperature LPBF system to process Ti–48Al–2Cr–2Nb is presented. Specimens were built with preheating at 800 °C and the cooling rate at the end of the build was controlled at 5 °C min−1. Crack formation was suppressed and apparent density in excess of 99% was achieved.

Impact of β-phase in TiAl alloys on mechanical properties after high temperature air exposure

In this paper, the influence of high temperature air exposure on the tensile properties at room temperature and on the fatigue strength at high temperature of two TiAl alloys, is studied. The alloys, Ti–48Al–2Cr–2Nb (Ti-48-2-2), and Ti–44Al–4Nb–1Mo-0.1B (TNM-B1), both exhibit near-γ microstructures, but the TNM-B1 alloy contains significant amounts of β phase. Air exposure at high temperature (650–700 °C, 500 h) induces significant loss of ductility, and decrease of fatigue strength at high temperature, in the case of the β-containing TNM-B1 alloy, but not for the Ti-48-2-2 alloy. This indicates a potential influence of the presence of β phase for embrittlement. Because embrittlement is believed to be the consequence of microstructure modifications in the sub-surface, first attempts have been performed to investigate these local phenomena. For this purpose, SEM, FIB, STEM-EDX and ACOM techniques have been employed. Destabilization of β and, to a lesser extent, of γ, into α2, has been observed in the sub-surface of the TNM-B1 alloy. We interpret this as a consequence of oxygen diffusion during the high temperature air exposure, which would promote formation of α2 phase in which oxygen solubility is high, at the expenses of the β and γ phases of lower oxygen solubility.

Phase equilibria in the Ti–Al–Cr system at 1000 °C

The phase equilibria in the Ti–Al and Ti–Cr binary systems and the isothermal section of the Ti–Al–Cr ternary system at 1000 °C were constructed from results obtained by studying diffusion multiples and quenched alloys with a scanning electron microscope in backscattered electron mode (SEM-BSE), a scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (SEM-EDX) and an electron probe microanalysis (EPMA). Seven three-phase regions and the corresponding single- or two-phase regions are observed in this work and compared to previous experimental and calculated results. The regions of the L12, β (Ti) and β (Cr) phases are larger than those in the prior calculated results. The three-phase regions are also different with the previous reported experimental results.

Effect of TiB2 addition on microstructure and fluidity of cast TiAl alloy

Effects of TiB2 addition on Microstructure and Fluidity of Ti–48Al–2Cr–2Nb + xTiB2 (x = 0.18, 0.36, 0.54, 0.72, 0.9, 1.8 wt%) alloys fabricated by vacuum non-consumable tungsten arc melting were investigated. Results showed that with adequate TiB2 addition (0.72% TiB2 and more), the as-cast microstructure transformed from coarse columnar grains to fine equiaxed grains, and the grain size was reduced from 800 μm to 200 μm. The morphology of borides transformed from flake to block and needlelike, resulted from the dissolution and re-precipitation of original TiB2 powders. The fluidity of TiAl alloy was improved by the addition of TiB2, and the T4822–0.72TiB2 alloy showed the largest filling length of 266.5 mm, 17.4% higher than that of the matrix alloy. The enhancement of fluidity and the cessation mechanism of melt flow for the TiB2-containing TiAl alloy were analyzed and discussed in light of the microstructure evolution, borides and solidification characterization.

Microstructures and mechanical properties of TiAl alloy fabricated by spark plasma sintering

This work deals with the consolidation of a TiAl alloy powder by spark plasma sintering (SPS). Pre-alloyed powder with a composition of Ti–48Al–2Cr–2Nb (at.%) was consolidated in a SPS furnace at temperatures between 1200[Formula: see text]C and 1325[Formula: see text]C and with a pressure of 50 MPa. The microstructures obtained after SPS depend on the sintering temperature. Tensile tests at room temperature were performed. The alloy SPSed at temperatures not less than 1250[Formula: see text]C exhibits good properties at room temperature.

Anodic characteristics and electrochemical machining of two typical γ-TiAl alloys and its quantitative dissolution model in NaNO3 solution

Electrochemical machining (ECM) is a promising machining method for lightweight γ-TiAl alloys, owing to several advantages over traditional methods. In this work, the electrochemical and ECM experiments of two typical γ-TiAl alloys (Ti–48Al–2Cr–2Nb and Ti–45Al–2Mn–2Nb+0.8 vol% TiB2 XD) in NaNO3 solution are carried out to obtain their anodic dissolution characteristics, and some special phenomena are analyzed in detail. The polarisation behaviour of anode is measured by polarisation curve and cyclic voltammetry. The composition and structure of the passive film on the surface of anode are determined by X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The results show that the passive film is mainly composed of TiO2 and Al2O3, and its structure consists of a loose outer film and a dense inner film. ECM flat experiments at different feed rates are also carried out, where it is found that the lamellar microstructure of the two alloys is fully exposed. For TiAl 4822, there are lamellar and non-lamellar regions on the surface of the workpieces, and the direction of the lamella in the different lamellar colonies is also different. For TiAl 45XD, however, some protruding needle-like and globular TiB2 exist on the processed surface, which is the main factor influencing the surface quality. Finally, a quantitative dissolution model is also proposed to characterise the dissolution behaviour of TiAl 4822 and TiAl 45XD in 20%NaNO3 solution.

Dynamic recrystallization behavior of the Ti–48Al–2Cr–2Nb alloy during isothermal hot deformation

The hot deformation behavior of the as-cast Ti–48Al–2Cr–2Nb alloy was investigated by isothermal compression tests at deformation temperatures ranging from 1000 °C to 1200 °C, and strain rates from 0.001 s−1 to 0.1 s−1. The single peak stress features common to all flow curves indicate that DRX is the dominating softening mechanism. The calculated values of the hot deformation activation energy Q and stress index n are 296.5 kJ mol−1 and 3.97, respectively. Based on this, the Arrhenius type constitutive equation was successfully established. The DRX critical condition model and relationship among DRX volume fractions, deformation temperatures and strain rates were obtained to optimize the process. Combined with microstructure analysis, it's concluded that 1200 °C/0.01s−1 is the optimization parameter. Besides, both DDRX and CDRX were observed in the γ phase evolution. The deformation mechanism from the inter-grain dislocation motion to the grain boundary migration and grain rotation was discussed.

Microstructure and mechanical properties of TiC/Ti matrix composites and Ti–48Al–2Cr–2Nb alloy joints brazed with Ti–28Ni eutectic filler alloy

Ti–48Al–2Cr–2Nb and TiC/Ti matrix composites were successfully joined using Ti–28Ni eutectic filler metal at different brazing temperatures. Microstructure and mechanical properties of the joints were systematically studied. The results showed that the joints all showed integral interfaces and the microstructures of the joints for 980°C/15min were detected as Ti-48Al-2Cr-2Nb/α2-Ti3Al+τ3-Al3NiTi2/α2-Ti3Al/Ti(s,s)+δ-Ti2Ni+TiC/Ti matrix composites. As brazing temperature increased, continuous Ti2Ni layer diminish and Ti2Ni phase became more discrete. However, α2-Ti3Al rich layer has thickened which deteriorates the properties of the joints. The highest shear strength achieved 469.5MPa as the joint brazed at 1010°C for 15min. The evolution of microstructures brazed with different temperatures was studied and its relationship with the shear strength was also revealed in details.

Effect of nano Y2O3 addition on microstructure and room temperature tensile properties of Ti–48Al–2Cr–2Nb alloy

Ti–48Al–2Cr–2Nb alloy reinforced with ceramic particle Y2O3 was fabricated by induction skull melting (ISM) technique in a vacuum environment. The thermodynamic feasibility of the reactions between nano Y2O3 and molten alloy has been considered. The as-cast microstructure of Ti–48Al–2Cr–2Nb adding nanoY2O3 during melting was analyzed by OM, SEM, EDS and TEM. It is concluded that multiple reinforcements are synthesized and they show different shapes. Y2O3 grows in regular geometry shapes when the content of Y2O3 is about 0.1 at. % and grows in dendritic shapes when the content of Y2O3 is below 0.05 at.%. The work shows that adding nano Y2O3 could refine microstructure of TiAl and improve strength and elongation of TiAl alloy effectively within a certain range.

Design of High Temperature Ti–Al–Cr–V Alloys for Maximum Thermodynamic Stability Using Self-Organizing Maps

Data generated for the Ti–Al–Cr–V system of metallic alloys from our previous publication, where the composition of 102 alloys were computationally Pareto optimized with the objective of simultaneously maximizing the Young’s modulus and minimizing density for a range of temperatures, was the starting point of the current research, where compositions at different temperatures of these alloys were analyzed for phase stability in order to generate new data for compositions and volume fractions of stable phases at various temperatures. This resulted in a large dataset where a lot of data were still missing as all the phases are not stable at a given temperature for all the compositions. The concept of Self-Organizing Maps (SOM) was then applied to determine correlations between alloy compositions, stabilities of desired phases at various temperatures, associated Young’s moduli and densities, and the effect of the composition of phases on these properties. This work should help alloy designers to determine the required chemical composition of a new alloy with reference to the temperature of application of that alloy and see the effect of temperature and composition on stable phases and associated properties of alloys.

A comprehensive study on the fabrication and characterization of Ti–48Al–2Cr–2Nb preforms manufactured using electron beam melting

This feasibility study examines the additive manufacturing of Ti–48Al–2Cr–2Nb preforms in a quest to make more complex engine components from this promising but hard to machine, brittle material. The preforms are built using the electron beam melting technique; hot isostatic pressing (HIP) was performed as the only post-process. Microstructural characterizations were carried with scanning electron microscopy techniques, X-ray diffraction and X-ray computed tomography while mechanical properties were obtained via tensile testing. We exploited computational thermodynamic modeling (widely known as CALPHAD) approach to investigate the phase stability pertinent to Nb and Cr modified TiAl. Defect-free structures were obtained in the HIPed condition with mechanical behavior comparable to/better than traditionally manufactured counterparts. Equiaxed duplex microstructures were achieved in the as-built condition that were sustained post-HIP. The effect of the build conditions and phase transformation kinetics on the observed microstructures is discussed. The microstructure-mechanical behavior relationship is also discussed.

Investigation of the Fracture and Strength of a Compound with a Carbide Base of a Wear-Resistant Ion-Plasma Vacuum-Arc Ti–N, Ti–Al–N, Ti–Al–Cr–N, and Ti–Al–Ni–N Coatings by the Scratch Testing

Comparative studies of the fracture behavior and adhesion strength during scratch testing of a wear-resistant ion-plasma vacuum-arc Ti–N, Ti–Al–N, Ti–Al–Cr–N, and Ti–Al–Ni–N coatings are carried out by the scratch test. The type of failure and the adhesion strength characteristics of the coatings based on LC1, LC2, and LC3 are determined by their structure, by the H 3/E 2 and H/E parameter values, and by the values of compressive macrostresses.

Friction Welding of Electron Beam Melted γ-TiAl Alloy Ti–48Al–2Cr–2Nb

In the current work, rotary friction welding of electron beam melted (EBM) γ-TiAl alloy Ti–48Al–2Cr–2Nb (at%) was investigated. Welding experiments were conducted using cylindrical bars of 12 mm diameter and 70 mm height in as-fabricated and heat treated (1275 °C/2 h/furnace cooling) conditions. No cracking problems were encountered during friction welding of as-fabricated EBM samples. However, in as-welded condition, the joints did not perform well in tensile tests and failures were observed to invariably occur at the weld interface. Friction welded joints produced in heat treated EBM samples were also found to suffer from the same problem. In both the cases, the weld region showed fine, fully lamellar γ + α2 microstructure and relatively high hardness. In order to overcome the problem, a post-weld heat treatment (PWHT) was carried out at 1275 °C/2 h/furnace cooling. After the PWHT, the weld region showed a more desirable duplex microstructure consisting of equiaxed primary γ and relatively coarser lamellar γ + α2. The PWHTed joints showed satisfactory tensile properties, and no failures were observed at the weld interface.

Hot Corrosion Behavior of SiO2–Al2O3–Glass Composite Coating on Ti–47Al–2Cr–2Nb Alloy: Diffusion Barrier for S and Cl

Type I hot corrosion behavior of SiO2–Al2O3–glass composite coating based on Ti–47Al–2Cr–2Nb substrate was investigated in the mixture salt of 25 wt%NaCl + 75 wt%Na2SO4 at 850 °C. The results showed that there was a bidirectional ion exchange between composite coating and the film of mixed salts, and the sodium ion in the molten salts penetrated into the glass matrix of composite coating, while the potassium ion in the glass matrix dissolved into the molten salts. A decrease in hot corrosion rate was achieved for the coated alloy in comparison with the bared substrate due to the composite coating acting as a diffusion barrier to sulfur and chlorine and preventing the molten salts from diffusing to the coating/alloy interface during the hot corrosion exposure. Additionally, the composite coating decreased the oxygen partial pressure at the coating/alloy interface and promoted the selective oxidation of Al to form a protective Al2O3 layer.

Thermally oxidized electron beam melted γ-TiAl: In vitro wear, corrosion, and biocompatibility properties

Abstract

Study of Selective Laser Melting of intermetallic TiAl powder using integral analysis

Abstract Selective laser melting (SLM) of intermetallic Ti–48Al–2Cr–2Nb powder is studied using different and complementary methods: (a) experimental parametric analysis (variation of laser power, beam scanning velocity, etc.); (b) metallography including X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM); (c) optical monitoring by infra-red (IR) camera, high speed pyrometer and (d) mathematical modelling. The advantage of the present approach is based on parallel application of diverse methods of analysis using the same SLM set-up and feedstock powder. Also the novelty is related to the preheating of the entire manufacturing module up to 450 °C which is applied before and during the manufacturing of the samples. XRD analysis shows domination of the α2-Ti3Al phase in the initial powder, heat treated powder before SLM and in the developed samples. Some minor peaks of γ-TiAl are identified in SLM parts. EDS analysis confirms the effect of Al evaporation, which intensifies with laser energy input. IR-camera is used for optical monitoring of a single track formation. The geometry of the thermal emission field for different processing parameters is compared with the track geometry. The intensification of hydrodynamic instabilities accompanied by material rejection from the zone of powder consolidation with beam scanning velocity is found. For the first time the evaporation of Al for elevated laser energy input is clearly detected by the IR-camera. Mathematical modelling shows that the same elementary volume of SLM sample can be remelted several times during its fabrication depending on the SLM processing parameters and beam scanning strategy. The maximum values of calculated cooling rates are in the order of 106 °C/s. For the first time the thermo-cycling during SLM parts fabrication is confirmed experimentally by pyrometer measurements.

Oxidation behaviour of an intermetallic Ti–Al–Cr–Zr bond coat on a γ–TiAl based TNB alloy with 7YSZ thermal barrier coating

Intermetallic titanium aluminide alloys are attractive light-weight materials for high temperature applications in automotive and aero engines. The development of γ-TiAl alloys over the past decades has led to their successful commercial application as low pressure turbine blades. The operating temperatures of γ-TiAl based alloys are limited by deterioration in strength and creep resistance at elevated temperatures as well as poor oxidation behaviour above 800 °C. Since improvement in oxidation behaviour of γ-TiAl based alloys without impairing their mechanical properties represents a major challenge, intermetallic protective coatings have aroused increasing interest in the last years.In this work, a 10 μm thick intermetallic Ti–46Al–36Cr–4Zr (in at.-%) coating was applied on a TNB alloy using magnetron sputtering. This layer provided excellent oxidation protection up to 1000 °C. Microstructural changes in this coating during the high temperature exposure were extensively investigated using scanning and transmission electron microscopy. The coating developed a three-phase microstructure consisting of the hexagonal Laves-phase Ti(Cr,Al)2, the tetragonal Cr2Al phase and the cubic τ-TiAl3 phase. After long-term exposure the three-phase microstructure changed to a two-phase microstructure of the hexagonal α2-Ti3Al phase and an orthorhombic body-centred phase, whose crystal structure has not yet been definitely identified. On the coating, a thin protective alumina scale formed. Applying this intermetallic layer as bond coat, thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on γ-TiAl based TNB samples using electron-beam physical vapour deposition. The results of cyclic oxidation testing (1 h at elevated temperature, 10 min. cooling at ambient temperature) revealed a TBC lifetime of more than 1000 h of cyclic exposure to air at 1000 °C. The ceramic topcoat exhibited an excellent adhesion to the thermally grown alumina scale which contained fine ZrO2 precipitates.

A Combined Electromagnetic Levitation Melting, Counter‐Gravity Casting, and Mold Preheating Furnace for Producing TiAl Alloy

In this work, the authors describe the development of TiAl castings over a wide range of approaches. To overcome casting defects and cracks that appear in TiAl castings, a novel furnace is designed and constructed. The design combines induction skull melting, counter‐gravity casting, and mold heating, which facilitates both filling and microstructure formation via a controllable process. This aim is to improve shaping capabilities and microstructural control for TiAl castings. Melting and casting experiments on TiAl alloys with a nominal composition corresponding to Ti–48Al–2Cr–2Nb (at%) are carried out and discussed. X‐ray examinations indicate that the shaping of the TiAl components dose not contain macro casting defects, validating the advantages of this technique. The results are of interest to researchers devoted to technical innovations and modifications for TiAl casting at the industrial scale.

Interactions between Exogenous Magnesia Inclusions with Endogenous Inclusions in a High Alloyed Steel Melt

Interactions between exogenous MgO particles and endogenous inclusions on the surface of a high alloyed steel melt have been investigated in situ in a high temperature confocal laser scanning microscope. Exogenous magnesia particle forms immediately a liquid Mg–Si–Al–Ti–Cr phase surrounding the solid particle. The real attraction forces between the liquid layer surrounding MgO particles and endogenous Al2O3 inclusions are determined in the range of 6 × 10−18 N to 1 × 10−16 N. In contrast to earlier investigations with exogenous Al2O3 particles, lower real attraction forces between exogenous and endogenous particle pairs have been observed in comparison to endogenous inclusions pairs.

Investigation on Behavior of Elastoplastic Deformation for Ti–48Al–2Cr–2Nb Alloy by Micro‐Indentation and FEM‐Reverse Algorithm

The Young's modulus, microhardness, and plastic properties of Ti–48Al–2Cr–2Nb alloy were determined using the micro‐indentation technique. Oliver–Pharr method was used to calculate Young's modulus and microhardness. The indentation load was inversely correlated to Young's modulus and microhardness. The decreased Young's modulus was associated with indentation damage, while decreasing hardness was due to indentation size effect. The plastic properties were determined using proposed FEM‐reverse algorithm, which combine finite element method and Matlab GA optimization tools. We used uniaxial compression test to verify the plastic properties calculated from the indentation tests, and it was found that the stress–strain plots predicted by FEM‐reverse algorithm was quite similar to the test results.