Cold-pressing and vacuum arc melting of γ-TiAl based alloys

Beta (β) solidifying γ-TiAl intermetallic alloys of nominal composition Ti-48Al, Ti-48Al-2Nb, Ti-48Al-2Nb-0.7Cr alloys have been cold pressed and vacuum arc melted. The Al loss was due to compaction method used prior to the melting technique, since it was evident after compaction that Al particles migrated to the surface in contact with the die facets after cold pressing. Electron backscatter diffraction (EBSD)-orientation mapping demonstrated that the α-precipitation from the parent β-phase follows the Blackburn orientation relationship (BOR). Microstructural characterization of the alloys was studied by scanning electron microscopy (SEM) equipped with energy dispersion spectroscopy (EDS) for micro-analysis. X-ray diffraction (XRD) technique was used to detect phase compositions.     

Microstructures and Tensile Properties of Isothermally-forgedTi-47Al-2Nb-1Cr-1V and Ti-47Al-2V-1Cr Alloys

Microstructures and room-temperature tensile properties of isothermally-forged γ-base (γ + α2)alloys in Ti-Al-Nb-Cr-V system with different heat treatments were investigated. The results show that the microstructures of Ti-47Al-2Nb-1Cr-1V and Ti-47Al-2V-1Cr (at. pct) alloys are mainly determined by heat treating temperature in the (cr + 7) tWo-phase field, and the joint additions of Nb, Cr and V in the Ti-47Al alloy afFect Ta significantIy. The microstructure of Ti-47Al alloy with additions of Nb, Cr and V (1~2 at. pct) can be dupIex or nearly-lamellar by a suitable heat treatment after isothermal forging at 1000℃ for over 50% plastic strains.Therefore its tensile properties can be improved at room temperature.     

Effect of sputtered TiAlCr coatings on oxidation and hot corrosion resistance of Ti-24Al-14Nb-3V

The effect of sputtered Ti-50Al-10Cr and Ti-50Al-20Cr coatings on both isothermal and cyclic oxidation resistance at 800 similar to 900 degreesC and hot corrosion resistance at 850 degreesC of Ti-24Al-14Nb-3V was investigated. Results indicated that Ti-24Al-14Nb-3V alloys exhibited poor oxidation resistance due to the formation of Al2O3+TiO2+AlNbO4 mixed scales in air at 800 similar to 900 degreesC and poor hot corrosion resistance due to the spallation of scales formed in Na2SO4+K2SO4 melts at 850 degreesC. Both Ti-50Al-10Cr and Ti-50Al-20Cr coatings remarkably improved the oxidation and hot corrosion resistance of Ti-24Al-14Nb-3V alloy.     

Compression Properties of Ti-Al-X Alloys in Relation to Site Occupancies of Alloying Elements in TiAl and Ti3Al

1-IntroductionGammaTiAla1loyswithpotelltialforengineer-ingapplicationsinvariablycofltainasmallamountofa2-Ti3Alphase.Severalrepresentativetypesofmi-crostructurecanbeobtainedthroughbothalloyingandprocessing,resultinginsignificantvariationinmechanicalproperties[1].Extensiveresearchinthelastdecadehasbeendirectedatfindingthecorre-lationbetweenthemicrostructuralparametersandproperties;theunderstandingofthefundamentalmechanismofalloyinghasreceivedlessattention.Theimportanceofthislatterissueisbestil…     

Toughening effects of Mo and Nb addition on impact toughness and crack resistance of titanium alloys

Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb alloys were prepared to investigate the toughening effects of β sta-bilizers Mo and Nb on impact toughness and crack resistance of titanium alloys.Instrumented Charpy impact tests showed that the total impact absorbed energy of Ti-6Al-2Mo and Ti-6Al-3Nb (～64J) were two times higher than that ofTi-6Al (～30J),indicating the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.Analysis of load-displacement curves revealed the similar crack initiation energy of Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb (15.4J,16.1J and 15.0J,respectively).However,the higher crack propagation energy of Ti-6Al-2Mo and Ti-6Al-3Nb (46.7J and 48.3J,respectively) were about three times higher than that of Ti-6Al (14.4J),indicating the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Post-mortem analysis of impact samples demonstrated that the increased dislocation density and deformation twinning were mainly responsible for the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Due to the invisibility of dislocation activation and deformation twinning during the Charpy impact process,a mathematical model has been proposed to evaluate the effects of Al,Mo and Nb elements on dislocation mobility based on the Yu Rui-huang electron theory.Addition of Mo and Nb elements significantly improved the dislocation mobility in Ti-6Al-2Mo and Ti-6Al-3Nb compared to that in Ti-6Al alloy.Therefore,more dislocations were activated in Ti-6Al-2Mo and Ti-6Al-3Nb which supplied the larger plastic deformation under impact loading.A dislocation-based model also has been proposed to interpret the nucleation and propagation of deformation twinning under the impact loading.Dislocation pileup at α/β interfaces provided potential sites for nucleation of deformation twinning in Ti-6Al-2Mo and Ti-6Al-3Nb.Furthermore,deformation twinning facilitated the dislocation motion in α grains with hard orientations.The increased dislocation mobility and deformation twinning were responsible for the stronger crack resistance as well as the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.     

Electro-codeposition of Al2O3-Y2O3 composite thin film coatings and their high-temperature oxidation resistance on γ-TiAl alloy

In this paper, electro-codeposition based on electrophoretic deposition and electrolytic deposition was developed to prepare Al₂O₃-Y₂O₃ composite thin film coatings on a γ-TiAl based alloy. Scanning electron microscope observations showed that the Al₂O₃-Y₂O₃ composite coatings were very compact and consisted of uniform nano-particles after microwave sintering. Cyclic oxidation at 900 °C indicated that the Al₂O₃-Y₂O₃ composite thin film coatings improved the oxidation and scale spallation resistance of the γ-TiAl alloy significantly. The superior oxidation and spallation resistance of the coatings were attributed to the suppression of outward diffusion of Ti and Al and inward diffusion of O, the promoted selective oxidation of Al in the γ-TiAl alloy, and the improved adhesion of oxide scale induced by the Al₂O₃-Y₂O₃ composite thin film coatings.     

Dissociation of super-dislocations and the SISF energy in γ-TiAl based alloy with Nb-doping, as studied by HRTEM

The dissociation of super-dislocations in γ-TiAl in Nb-doped Ti-48 at.% Al-5 at.% Nb has been studied by high resolution transmission electron microscopy (HRTEM). The SISF energy in γ-TiAl in Ti-48 at.% Al-5 at.% Nb was calculated to be 35 mJ/m² according to the dissociation width. It is concluded that Nb addition may affect the electronic structure of TiAl and furthermore decrease SF energies in TiAl alloys, which is partly responsible for the strengthening effect of Nb-doping.     

Influence of different annealing processes under various atmospheres on the oxidation behaviour of γ-TiAl

Abstract

A detailed study of the oxidation behaviour of bare γ-TiAl based alloy Ti–45Al–8Nb under various conditions, such as different atmospheres, pressures, temperatures (900°C, 1000°C) and times (100–200 h) is presented. Under high vacuum conditions (10^–6 mbar) a continuous zone of α-₂Ti₃Al was formed at the surface with an oxygen-enriched phase on top. No oxide scale formation was obvious. During thermal treatment under Ar-atmosphere at low vacuum pressure (approximately 50 mbar) mainly nitrides (TiN, Ti₂AlN) and Al₂O₃ particles were formed at the surface with an α-₂layer below. Annealing γ-TiAl in hydrogen atmosphere (about 1040 mbar) led to the formation of a thick reaction zone. A TiO₂ layer was formed on top, followed by a mixed oxide scale. Beneath that scale a thick region with alumina, σ–Nb₂Al and α-₂Ti₃Al was observed.

Moreover, the oxidation behaviour of several thermally pre-treated samples was tested by cyclic oxidation at 900°C in air. The microstructure of the oxide scale formed after testing can be compared with that of non pre-treated material. γ-TiAl annealed under high vacuum conditions exhibits the lowest oxidation rate, while the mass gain of specimens pre-treated under Ar-atmosphere increased rapidly in the first cycles. All pre-treated specimens exceeded a lifetime of 600 cycles at minimum. The reference material failed after 520 cycles.     

In vitro evaluation of human osteoblast adhesion to a thermally oxidized γ-TiAl intermetallic alloy of composition Ti–48Al–2Cr–2Nb (at.%)

Ti–48Al–2Cr–2Nb (at.%) (γ-TiAl), a gamma titanium aluminide alloy originally designed for aerospace applications, appears to have excellent potential as implant material. Thermal treatment of γ-TiAl renders this alloy extremely corrosion resistant in vitro, which could improve its biocompatibility. In this study, the surface oxides produced by thermal oxidation (at 500°C, and at 800°C for 1 h in air) on γ-TiAl were characterized by X-ray photoelectron spectroscopy (XPS). hFOB 1.19 cell adhesion on thermally oxidized γ-TiAl was examined in vitro by a hexosaminidase assay, scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) after 1, 7 and 14 days. Ti–6Al–4V surfaces were used for comparison. Hexosaminidase assay data and CLSM analysis of focal contacts and cytoskeleton organization showed no differences in cell attachment on autoclaved and both heat-treated γ-TiAl surfaces at the different time points. SEM images showed well organized multi-layers of differentiated cells adhered on thermally oxidized γ-TiAl surfaces at day 14. Unexpectedly, thermally oxidized Ti–6Al–4V surfaces oxidized at 800°C exhibited cytotoxic effects on hFOB 1.19 cells. Our results indicate that thermal oxidation of γ-TiAl seems to be a promising method to generate highly corrosion resistant and biocompatible surfaces for implant applications.     

Protective Effect of the Sputtered TiAlCrAg Coating on the High-Temperature Oxidation and Hot Corrosion Resistance of Ti-Al-Nb Alloy

The effect of a sputtered Ti-48AI-8Cr-2Ag (at. pct) coating on the oxidation resistance of the cast Ti-46.5AI-5Nb (at. pct) alloy was investigated in air at 1000-1100℃. Hot corrosion in molten 75 wt pct Na2SO4+25 wt pct K2SO4 was investigated at 900℃. The scale on the cast TiAINb tends to spall in air, while the scale on coating is very adherent. The sputtered Ti-48AI-8Cr-2Ag coating remarkably improved high temperature oxidation resistance of the cast Ti-46.5AI-5Nb alloy because of the formation of an adherent Al2O3 scale. Due to the inward diffusion of Cr, Kirkendall voids were found at the coating/substrate interface. TiAICrAg coating provided excellent hot corrosion resistance for TiAINb alloy in molten 75 wt pct Na2SO4+25 wt pct K2S04 at 900℃ due to the formation of a continuous Al2O3 scale.     

Transient oxidation of alumina forming Ti–Al–Ag-based alloys and coatings studied by SEM,AFM, XPS and LRS

Abstract

γ-TiAl based intermetallics possess poor oxidation properties at temperatures above approximately 700°C. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. Recently, this type of materials has therefore been proposed as oxidation resistant coatings for high strength TiAl alloys. In the present study, a number of cast Ti–Al–Ag alloys and magnetron sputtered Ti–Al–Ag coatings were investigated in relation to transient oxide formation in air at 800°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques, such as SEM, ESCA, AFM and LIOS-RS. The γ-TiAl–Ag alloys and coatings appear to form an α-Al₂O₃ oxide scale from the beginning of the oxidation process, in spite of the relatively low oxidation temperature of 800°C. The formation of metastable alumina oxides seems to be related to the presence of Ag-rich precipitates in the alloy matrix.     

Microstructure and oxidation behaviour of TiAl(Nb)/Ti<Subscript>2</Subscript>AlC composites fabricated by mechanical alloying and hot pressing

TiAl-based intermetallic matrix composites with dispersed Ti₂AlC particles and different amounts of Nb were successfully synthesized by mechanical alloying and hot pressing. The phase evolution of Ti–48 at%. Al elemental powder mixture milled for different times with hexane as a process control agent was investigated. It was found that after milling for 25 h, a Ti(Al) solid solution was formed; also with increase in the milling time to 50 h, an amorphous phase was detected. Formation of a supersaturated Ti(Al) solid solution after 75 h milling was achieved by crystallization of amorphous phase. Addition of Nb to system also exhibited a supersaturated Ti(Al,Nb) solid solution after milling for 75 h, implying that the Al and Nb elements were dissolved in the Ti lattice in a non-equilibrium state. Annealing of 75 h milled powders resulted in the formation of equilibrium TiAl intermetallic with Ti₂AlC phases that showed the carbon that originates from hexane, participated in the reaction to form Ti₂AlC during heating. Consolidation of milled powder with different amounts of Nb was performed by hot pressing at 1000°C for 1 h. Only the presence of γ-TiAl and Ti₂AlC was detected and no secondary phases were observed on the base of Nb. Displacement of γ-TiAl peaks with Nb addition implied that the Nb element was dissolved into TiAl matrix in the form of solid solution, causing the lattice tetragonality of TiAl to increase slightly. The values for density and porosity of samples indicated that condition of hot pressing process with temperature and pressure was adequate to consolidate almost fully densified samples. The isothermal oxidation test was carried out at 1000°C in air to assess the effect of Nb addition on the oxidation behaviour of TiAl/Ti₂AlC composites. The oxidation resistance of composites was improved with the increase in the Nb content due to the suppression of TiO₂ growth, the formation and stabilization of nitride in the oxide scale and better scale spallation resistance.     

Phase Relations in TiAl+Nb System

The microstructures and phases of ternary TiAl+Nb alloys containing 50-60 at.-%Al, 0-21 at.-%Nb have been studied using transmission electron microscopy (TEM) and X-ray powder diffraction. The phases present in the alloys and their distribution were found to be a sensitive function of composition. The phase relations between γ-TiAl and γ1 (a new ordered ternary intermetallic compound based on γ-TiAl) were determined. Essentially single γ phase was determined for alloys with relativety low Nb content (≤10 at.-%Nb). the γ1 phase was determined to exist in the composition range containing higher Nb contents (15-21 at.-%Nb). Between γ and γ1 phases, with intermediate Nb contents there is a transitional phase γ1 (a superstructure of γ-TiAl). As for the influence of Al concentration on the phase relations. the γ1 phase was inclined to form in the alloys with relatively high Al contents. The ordering transformation of γ, to γ1 is a continuous ordering process and the transition may be second order.     

Ti-45Al-10Nb合金的高温氧化行为

研究了Ti—45A1—10Nb(原子分数，％)合金在800-960℃在氧气和空气中的氧化行为．结果表明，该合金具有较好的高温抗氧化性能，其氧化增重速率与铁基耐热不锈钢相似或略优．与Ti—50A1合金相反，Ti-45A1-10Nb合金在空气中的氧化增重速率明显低于在纯氧中的氧化增重速率．X射线衍射与能谱分析表明，Ti-45Al一10Nb合金在氧气中的氧化产物主要有TiO2和Al2O3，但在空气中的氧化产物中有TiN相．这是降低氧化速率的主要原因，同时，合金元素Nb稳定了氧化层中的TiN相，因而提高了合金在空气中的抗氧化性能．     

Processing TiAl-Based Alloy by Elemental Powder Metallurgy

1. IntroductionTiAl based alloy has long been considered as apromising candidate for high temperature applications, because it has a high specific strength and goodhigh temperattire properties[1-31. The main methods for the preparation of TiAl based alloys includecasting and powder metallurgy. In general, thermomechanical treatment (such as forging and extrusion)and subsequellt heat treatment should be conductedfor cajst TiAl-based alloys in order to refine the coarsemicrostructure[4,5], whe…     

Modification of Microstructure for Improved Oxidation Resistance in γ-Based Ti-Al-X Alloys

The effect of microstructural refinement on the oxidation behavior of γ + laves Ti-45Al-15Cr and γ + σ Ti-40Al-27Nb was investigated. A fine, uniform γ + laves microstructure in Ti-45Al-15Cr resulted in protective alumina scale formation over most of the sample surface at 800°C and 1000°C in air. In contrast, the oxidation resistance of this same alloy was severely degraded by a coarse γ + laves microstructure. The oxidation behavior of Ti-40Al-27Nb at 1200°C in air was poor. Refinement of the γ + σ microstructure in this alloy was not effective in improving oxidation resistance. The mechanistic reasons for these effects, and the implications for improved oxidation resistance through microstructural refinement are discussed.     

Oxidation behaviour of TiAlYN/CrN and CrAlYN/CrN nanoscale multilayer coatings with Al2O3 topcoat deposited on γ-TiAl alloys

Abstract

TiAlYN/CrN and CrAlYN/CrN nanoscale multilayer coatings were deposited on γ-TiAl specimens using magnetron sputtering techniques. The nitride layers were manufactured by unbalanced magnetron sputtering (UBM) and high power impulse magnetron sputtering (HIPIMS). The CrAlYN/CrN coatings had an oxy-nitride overcoat. On some of the coated samples an additional alumina topcoat was deposited. The oxidation behaviour of the different coatings was investigated at 750 and 850°C performing quasi-isothermal oxidation tests in laboratory air. Mass change data were measured during exposure up to failure or the maximum exposure length of 2500 h. When exposed to air at 750°C, the Ti-based nitride films exhibited higher oxidation resistance than the Ti – 45Al –8Nb substrate material. The alumina topcoat enhanced the oxidation protection of this coating system, acting as diffusion barrier to oxygen penetration. At 850°C, the TiAlYN/CrN films exhibited poor stability and rapidly oxidised, and therefore were not applicable for long-term protective coatings on γ-TiAl alloys. The beneficial effect of the additional Al₂O₃ layer was less pronounced at this exposure temperature. The Cr-based nitride films exhibited high oxidation resistance during exposure at 850°C. HIPIMS deposition improved the oxidation behaviour of the CrAlYN/CrN nanoscale multilayer coatings in comparison to UBM coatings. For these coatings, the decomposed and partially oxidised nitride films were an effective barrier to oxygen inward diffusion. The alumina topcoat did not significantly increase the oxidation resistance of the γ-TiAl alloy coated with Cr-based nitride films.     

Influence of noble metals alloying additions on the corrosion behaviour of titanium in a fluoride-containing environment

Titanium alloys exhibit excellent corrosion resistance in most aqueous media due to the formation of a stable oxide film, and some of these alloys (particularly Ti-6Al-7Nb) have been chosen for surgical and odontological implants for their resistance and biocompatibility. Treatment with fluorides (F⁻) is known to be the main method for preventing plaque formation and dental caries. Toothpastes, mouthwashes, and prophylactic gels can contain from 200 to 20,000 ppm F⁻ and can affect the corrosion behaviour of titanium alloy devices present in the oral cavity. In this work, the electrochemical corrosion behaviour of Ti-1M alloys (M = Ag, Au, Pd, Pt) was assessed in artificial saliva of pH = 3.0 containing 910 ppm F⁻ (0.05 M NaF) through open circuit potential, E_OC, and electrochemical impedance spectroscopy (EIS) measurements. The corrosion behaviour of the Ti-6Al-7Nb commercial alloy was also evaluated for comparison. E _OC measurements show an active behaviour for all the titanium alloys in fluoridated acidified saliva due to the presence of significant concentrations of HF and HF₂ ⁻ species that dissolve the spontaneous air-formed oxide film giving rise to surface activation. However, an increase in stability of the passive oxide layer and consequently a decrease in surface activation is observed for the Ti-1M alloys. This behaviour is confirmed by EIS measurements. In fact, the Ti-6Al-7Nb alloy exhibits lower impedance values as compared with Ti-1M alloys, the highest values being measured for the Ti-1Au alloy. The experimental results show that the corrosion resistance of the studied Ti-1M alloys is similar to or better than that of Ti-6Al-7Nb alloy currently used as biomaterial, suggesting their potential for dental applications.     

Influences of Alloying Elements W, Mo, Cr and Nb on Retained Beta Phase in 47Al Based near γ-TiAl Alloys

The influences of alloying elements W, Mo, Cr, and Nb on retained β phase in 47Al based near γ-TiAI alloys have been studied.The results reveal that the amount of retained β phase is increased by the addition of Cr, Mo, W in rising rank, although the distribution of β phase in Cr-bearing alloys is different from that of Mo- or W-bearing alloys. For Nb-doped alloys, no retained β was found even when 5 at. pct Nb was added. The as-cast microstructural features and the distribution of the β phase in the different alloy families were compared and interpreted in terms of the different segregation behaviour of these elements in Ti.     

XPS characterization of surface modified titanium alloys for use as biomaterials

Three different Ti alloys of biomedical interest have been studied by means of X-ray photoelectron spectroscopy (XPS) to determine their surface chemical composition in both as-received condition and after oxidation at 750 °C in air for different times. Compositions of the investigated alloys were, in wt.%, Ti-7Nb-6Al, Ti-13Nb-13Zr and Ti-15Zr-4Nb. XPS analyses showed a behaviour of the Ti-7Nb-6Al alloy different from that of the two TiNbZr alloys, evidencing the role of the chemical composition of the alloys on the oxidation mechanisms. The oxidation process generates an aluminium-oxide rich surface on the Ti-7Nb-6Al, while in the case of the TiNbZr alloys a titanium-oxide rich layer is formed. The effect of the heat treatment on the contribution of the minority elements at the surface is also discussed.