Ti 48Al 2Cr Research Articles

Textures of rectangular extrusions and their effects on the mechanical properties of thermo-mechanically treated, lamellar microstructure, Ti–47Al–2Cr–2Nb–0.15B

Textures of α2 and γ phases in Ti–47Al–2Cr–2Nb–0.15B (at.%) rectangular billet, extruded below its α transus temperature, were evaluated and their relation to the mechanical properties of TMTL microstructure was investigated. Strong α2 phase texture {112¯0}<101¯0> was found after extrusion and it evolved into {101¯0}<112¯0> recrystallization texture via oriented growth during α solution treatment. The α/α2 phase texture changes α grain growth behavior at Tα, such that it is slow at first but becomes rapid when the solution time exceeds a critical value, creating a new method of obtaining fine-grained TMTL microstructure. Mechanical properties of the TMTL microstructure were shown to be closely related to the Kearns texture factor of the α2 phase along the loading direction. The center and edge of as-extruded billet have distinct γ phase textures, dominated by β fiber (center) and featuring multiple texture components (edge), respectively. In the highly strained edge region, the presence of C-twin, Y, and Z texture components suggests the occurrence of mechanical twinning and shear band deformation, while B/G auxiliary component was found to act as a transition band from deformation to cubic recrystallization texture, in support of the oriented nucleation hypothesis for the formation of cubic texture.

Interface microstructure evolution and bonding strength of TC11/γ-TiAl bi-materials fabricated by laser powder deposition

Laser powder deposition was applied to fabricate the Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (wt%)/Ti–47Al–2Cr–2Nb–0.2W–0.15B (at%) bi-material system. The as-deposited TC11 alloy shows a basket-wave-like morphology while the as-deposited γ-TiAl alloy consists of fully α2/γ lamellar microstructures. Regarding the thermal mismatch between TC11 and γ-TiAl during processing, the interface microstructure evolution was concerned. The transformation pathway was illustrated. It is found that the content changes of Al elements and β-stabilizers Mo, Cr, and Nb are responsible for the evolution of microstructures at the interface. The fracture surfaces are located at the γ-TiAl side. The bi-material shows a brittle-fracture manner, with the ultimate tensile strength of 560 MPa.

Microstructure characteristic for high temperature deformation of powder metallurgy Ti–47Al–2Cr–0.2Mo alloy

Hot compression tests of a powder metallurgy (P/M) Ti–47Al–2Cr–0.2Mo (at. pct) alloy were carried out on a Gleeble-3500 simulator at the temperatures ranging from 1000°C to 1150°C with low strain rates ranging from 1×10−3s−1 to 1s−1. Electron back scattered diffraction (EBSD), scanning electron microscope (SEM) and transmission electron microscope (TEM) were employed to investigate the microstructure characteristic and nucleation mechanisms of dynamic recrystallization. The stress–strain curves show the typical characteristic of working hardening and flow softening. The working hardening is attributed to the dislocation movement. The flow softening is attributed to the dynamic recrystallization (DRX). The number of β phase decreases with increasing of deformation temperature and decreasing of strain rate. The ratio of dynamic recrystallization grain increases with the increasing of temperature and decreasing of strain rate. High temperature deformation mechanism of powder metallurgy Ti–47Al–2Cr–0.2Mo alloy mainly refers to twinning, dislocations motion, bending and reorientation of lamellae.

Selective electron beam melting of Ti–48Al–2Nb–2Cr: Microstructure and aluminium loss

In the present paper a broad parameter window for processing Ti–48Al–2Cr–2Nb by selective electron beam melting is investigated. Data for the aluminium loss during the process in relation to used beam parameters and process strategies for its minimisation are presented. Al losses as low as 0.5 at% were achieved. Using different beam parameters a wide range of microstructures from lamellar to massively transformed γ could be realized. This finding in principle gives the opportunity to locally adjust the microstructure of net or near-net-shape parts from TiAl during the process.

An evaluative approach to correlate machinability, microstructures, and material properties of gamma titanium aluminides

Several generations of gamma titanium aluminides have been developed over time, and they are nowadays commercially available. The differences in chemical composition, as well as the thermal treatments, greatly influence the properties of the alloys. This implies considerable effects on the production process performances. Benchmark trials were performed on three γ-TiAl alloys: Ti–48Al–2Cr–2Nb, Ti–43.5Al–4Nb–1Mo–0.1B, and Ti–45Al–2Nb–2Mn+0.8vol.% TiB2 XD, focusing on machinability and material characterization. The extremely dissimilar results obtained when turning and milling can be traced back to the different microstructures, as well as to the alloying elements, factors both affecting the mechanical and thermal material properties.

Development of an oxidation resistant glass–ceramic composite coating on Ti–47Al–2Cr–2Nb alloy

Three glass–ceramic composite coatings were prepared on Ti–47Al–2Cr–2Nb alloy by air spraying technique and subsequent firing. The aim of this work is to study the reactions between glass matrix and inclusions and their effects on the oxidation resistance of the glass–ceramic composite coating. The powders of alumina, quartz, or both were added into the aqueous solution of potassium silicate (ASPS) to form slurries used as the starting materials for the composite coatings. The coating formed from an ASPS-alumina slurry was porous, because the reaction between alumina and potassium silicate glass resulted in the formation of leucite (KAlSi2O6), consuming substantive glass phase and hindering the densification of the composite coating. Cracks were observed in the coating prepared from an ASPS-quartz slurry due to the larger volume shrinkage of the coating than that of the alloy. In contrast, an intact and dense SiO2–Al2O3-glass coating was successfully prepared from an ASPS-alumina–silica slurry. The oxidation behavior of the SiO2–Al2O3-glass composite coating on Ti–47Al–2Cr–2Nb alloy was studied at 900°C. The SiO2–Al2O3-glass composite coating acted as an oxygen diffusion barrier, and prevented the inward diffusion of the oxygen from the air to the coating/alloy interface, therefore, decreasing the oxidation rate of the Ti–47Al–2Cr–2Nb alloy significantly.

Constitution of the liquidus and solidus surfaces of the Al–Ti–Cr system

Abstract The solidus and liquidus projection of the ternary Al–Ti–Cr system in the compositional range from 45 to 75 at.% Al were analyzed. The primary crystallization fields of the αTi, β, γ-TiAl, ζ-Ti 2 Al 5 , e-TiAl 3 , (Al 8 Cr 5 ) h , Al 4 Cr, Al 11 Cr 2 and τ phases were determined based on the metallographic investigations of as-cast alloys combining X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The three-phase equilibria corresponding to the solidus projection for Al contents

High temperature oxidation behaviour of TiAl–Cr–Nb–Mo alloys

The oxidation behaviour of six different TiAl–Cr–Nb–Mo alloys was investigated by thermogravimetric method. Isothermal experiments were carried out at 900 °C and 1000 °C in air.The aim of this work was to evaluate the effect of aluminium and of other alloying elements on the oxidation behaviour of TiAl intermetallic alloys. The alloy's microstructure and composition as well as the composition distribution of the oxide scales were analysed by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-Ray Diffractometry (XRD). The study highlighted that the oxidation resistance of the considered TiAl-based alloys is strongly affected by its aluminium content, while it seems that an increase of Cr, Nb and Mo concentrations does not affect the oxidation behaviour of the alloy. Yttrium addition, at least in the tested quantity, has a detrimental effect since it promotes the formation of a TiO2–Al2O3 mixed oxide instead of a compact and thin alumina layer.

Effect of CrN addition on the structure, mechanical and thermal properties of Ti-Al-N coating

Optimization of Ti–Al–N coating by incorporation of fourth substantially alloying elements attracts extensive attention in order to meet the severe application requirements at elevated temperature. Here, we investigate the effect of CrN addition on structure, mechanical and thermal properties of Ti–Al–N coating with cubic structure. Alloying with CrN promotes the spinodal decomposition of Ti–Al–N to form Al-depleted and Al-enriched domains and also the formation of hexagonal AlN during thermal annealing. Cr-containing coating has an increase in hardness from the as-deposited value of ~34.4GPa to the maximum value of ~38.7GPa with Ta=900°C, whereas the hardness of Ti–Al–N coating only increases from ~31.2GPa to ~34.1GPa. Thermal annealing of Ti–Al–Cr–N coating results in the N-loss to form h-Cr2N with Ta=1000°C and finally Cr with Ta=1200°C. Also addition of CrN into Ti–Al–N coating retards the transformation of metastable an-TiO2 into stable r-TiO2 and thereby the growth of porous Ti-oxide rich sublayer, which is beneficial to its oxidation resistance. Only a thin scale of ~0.55μm for Cr-containing coating is shown after annealing in air at Ta=850°C for 10h, whereas Ti–Al–N coating is already completely oxidized. Consequently, Ti–Al–Cr–N coated inserts have a better machining performance than Ti–Al–N coated inserts during continuous turning.

Lifetime of Thermal Barrier Coatings Deposited on γ-TiAl Based Alloys Using Intermetallic Ti–Al–Cr Bond Coats with Additions of Yttrium and Zirconium

Thermal barrier coatings (TBC) of yttria stabilized zirconia were deposited on the γ-TiAl based alloy Ti–45Al–8Nb (at.%) using electron-beam physical vapor deposition. The bond coats used were 10 μm thick intermetallic Ti–Al–Cr layers with additions of the reactive elements Y and Zr produced by magnetron sputtering. Cyclic oxidation tests at 900 and 1,000 °C in air revealed excellent oxidation resistance of the Ti–Al–Cr–Y bond coat associated with the precipitation of Y-rich particles in the thermally grown alumina scale as well as in the intermetallic layer. A less protective behavior was found with the zirconium containing bond coat. Lifetimes exceeding 1,000 1-h cycles were determined for both TBC systems at 900 °C. Edge chipping of the zirconia topcoat occurred at 1,000 °C. As observed by cross-sectional examination, a continuous alumina scale was still present on the samples with Ti–Al–Cr–Y bond coat, whereas the Ti–Al–Cr–Zr layer was severely degraded and a thick mixed oxide scale formed after 1,000 cycles at 1,000 °C.

Cobalt-doped Ti–48Al–2Cr–2Nb alloy fabricated by cold compaction and pressureless sintering

An addition of 1.5at% Co to Ti–48Al–2Cr–2Nb (in at%) transformed the alloy from essentially unsinterable to fully sinterable at 1300°C. This, together with a simple powder coating process developed recently, has allowed near-net shape fabrication of the alloy for the first time by cold compaction and pressureless sintering. The addition of Co results in the formation of an intermediate face centred cubic (fcc) CoAl2Ti phase prior to 1220°C during heating. It subsequently reacts with an α phase leading to the formation of a Co-containing, wettable sintering liquid through a two-step process, CoAl2Ti+α→Liquid at 1256.2°C and CoAl2Ti+α→γ-TiAl+Liquid at 1267.2°C, and therefore full densification of the alloy. Without Co, sintering of the Ti–48Al–2Cr–2Nb alloy powder at 1300°C is controlled by the slow self-diffusion of Ti and interdiffusion of Ti and Al according to the activation energy determined. Transmission electron microscopy (TEM) identified an fcc CoAl2Ti phase and a hexagonal close packed (hcp) Co-enriched Ti(Al, Co, Cr, Nb) phase in the final as-sintered Ti–48Al–2Cr–2Nb–1.5Co alloy. They both form during cooling at 1240°C through Liquid+α→CoAl2Ti+Ti (Al, Co, Cr, Nb). The tensile and compressive properties of the as-sintered Ti–48Al–2Cr–2Nb–1.5Co alloy were compared to the original General Electric (GE) Ti–48Al–2Cr–2Nb alloy fabricated by casting or metal injection moulding.

Cyclic oxidation behaviour of the titanium alloys Ti-6242 and Ti-17 with Ti–Al–Cr–Y coatings at 600 and 700 °C in air

Oxidation protective intermetallic Ti–Al–Cr–Y coatings of 10μm thickness were deposited on specimens of the near-α alloy Ti-6242 and the (α+β) alloy Ti-17 using magnetron sputtering. The oxidation behaviour of the coated samples was studied at 600°C and 700°C under cyclic oxidation conditions in air. The coatings exhibited very good adhesion to the substrate materials and provided effective oxidation protection to the titanium alloys at both temperatures. A thin alumina scale formed on top of the coatings which showed a multi-phase microstructure consisting of the Ti(Cr,Al)2 Laves phase and, dependent on exposure temperature and time, the γ-TiAl, r-TiAl2 and/or AlCr2 phases.

Oxidation resistance of γ-TiAl based alloy Ti–45Al–8Nb coated with intermetallic Ti–Al–Cr–Y layers and EB-PVD zirconia topcoats at 950 °C in air

Samples of a high niobium bearing γ-TiAl based alloy were coated with intermetallic Ti–52Al–15Cr–0.4Y (in at.%) layers produced by magnetron sputtering. On some of the specimens thermal barrier coatings of 7wt.% yttria partially stabilised zirconia were deposited using electron-beam physical vapour deposition. The oxidation behaviour of the coated Ti–45Al–8Nb (at.%) alloy with and without thermal protection was investigated at 950°C under cyclic oxidation conditions in air. The 20μm thick Ti–Al–Cr–Y layer exhibited excellent oxidation resistance at this exposure temperature associated with the formation of a thin continuous alumina scale. Lifetimes exceeding 1000cycles of 1h dwell time at 950°C were determined for the TBC system consisting of zirconia topcoat and Ti–Al–Cr–Y bond coat.

SPS synthesis and consolidation of TiAl alloys from elemental powders: Microstructure evolution

Some works have studied the sintering of TiAl by means of Spark Plasma Sintering (SPS) and it is believed that it is possible to obtain very fine microstructures and very interesting mechanical properties. As raw materials, normally pre-alloyed powders have been used, obtained by atomization or mechanical alloying. However, few works have studied the consolidation of elemental powders. This work presents the synthesis and densification of TiAl-based alloys by means of Spark Plasma Sintering (SPS) using elemental powders as raw materials. The selected composition was Ti–48Al–2Cr–2Nb. The densification process, the evolution of the microstructure with the temperature and the diffusion of the elements (including heavy metals Cr and Nb) are presented. In addition, SPS products are compared with TiAl alloys from elemental powders obtained by conventional methods.

Surface recrystallization of a gamma-TiAl alloy induced by shot peening and subsequent annealing treatments

Fine-grained recrystallization microstructure induced by shot peening and subsequent annealing treatment was achieved in the surface of Ti–48Al–2Cr–2Nb alloy. Annealing treatment at 900°C for 6–12h and the lamellar orientation angle of close to 45° are the most favorable conditions for recrystallization. The rotation, dissolution and invasion of the lamellae fragments into the adjacent normal lamellae under the driving force of residual deformation energy with phase boundary bulging mechanism is considered to be the most possible microscopic approach of the recrystallization.

Preparation and oxidation behavior of SiO2–Al2O3–glass composite coating on Ti–47Al–2Cr–2Nb alloy

A SiO2–Al2O3–glass composite coating was prepared on Ti–47Al–2Cr–2Nb alloy by air spray technique and subsequent firing. The Ti–47Al–2Cr–2Nb alloy with and without composite coating were oxidized in air for 100h at 800°C and 900°C, respectively. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and electron probe microanalysis (EPMA) technique have been employed to study the microstructures of the specimens before and after oxidation tests, X-ray diffraction (XRD) was used to analyze the oxide phase. For the bare Ti–47Al–2Cr–2Nb alloy, a typical layered structure oxide scale formed, the outer layer is TiO2, the intermediate one is an Al2O3-rich layer, the inner one is a mixture of TiO2 and Al2O3, a small amount of dissolved Cr and Nb ion being preferentially segregated in the inner layer. The composite coating provides good oxidation resistance for the Ti–47Al–2Cr–2Nb alloy. An interfacial reaction of the composite coating and the substrate occurred, forming an Al2O3/Ti5Si3/Al2O3/Z-phase Ti5Al3O2 layered structure in the interfacial zone. The composite coating/Al2O3 bilayer act as oxygen diffusion barrier and benefit oxidation resistance of the Ti–47Al–2Cr–2Nb alloy. Al2O3 and quartz were instable in the glass phase; Al2O3 and quartz inclusions partially dissolved into the glass phase, and some quartz particles transformed into cristobalite.

Phase equilibria at 1473 K in the ternary Al–Cr–Ti system

The isothermal section at 1473K of the Al–Cr–Ti system was experimentally studied in the compositional region below 75at.% Al using X-ray diffraction, scanning electron microscopy, electron probe micro-analysis and electron backscatter diffraction. The formation of the continuous β phase solid solution was confirmed, three tie-triangles τ+ε-TiAl3+ζ-Ti2Al5, τ+γ-TiAl+β and τ+β+(Al8Cr5)h were determined, and several tie-lines were measured. The homogeneity ranges of the τ, γ-TiAl and (Al8Cr5)h phases and the ternary extension of the C14 Laves phase (γ-Ti(Al,Cr)2) into the Al–Cr–Ti system were also investigated.

Effect of stress orientation on microstructural evolution during creep of near-lamellar Ti–47Al–2Cr–2Nb

Microstructural evolution was studied in a near-lamellar two phase (α2+γ) Ti–47Al–2Cr–2Nb alloy under high temperature creep and exposure conditions. The aim of this study was to probe the role of stress orientation, with respect to lamellar plates, on microstructural changes during primary creep. Creep testing was complemented with SEM and TEM based microstructural characterization. It was observed that retention of excess α2 resulted in an unstable microstructure. Under stress and temperature, excess α2 was lost and Cr-rich precipitates formed. Depending on stress orientation, the sequence of precipitates formed was different. α2 loss was accompanied by formation of the non-equilibrium C14 Laves phase when lamellar plates were oriented parallel to the stress axis. In contrast, α2 loss did not result in formation of the C14 phase in perpendicular samples. It was concluded that C14 formed preferentially in certain test orientations because of its effectiveness in relieving residual stresses in α2 that arose from lattice misfit and modulus mismatch.

High temperature deformation behaviors of polysynthetically twinned (PST) Ti–47Al–2Cr–2Nb alloy

The characteristics of deformation and fracture of polysynthetically twinned (PST) Ti–47Al–2Cr–2Nb alloy at 800°C had been identified by scanning and transmission electron microscopy. The tensile properties of the fully laminated Ti–47Al–2Cr–2Nb showed anisotropy for three different orientations (N, P and I orientation) at elevated temperature. Some differences in fracture modes between three oriented-lamellae at 800°C had been observed, which were mainly due to the normal stress and shear stress. Also, it was found that the α2-lamellae decomposed and the γ-lamellae coarsened during high temperature deformation, but similar tendency was more obvious in hard orientation. At last, the deformation modes had been identified by transmission electron microscopy. For the hard orientation loaded PST Ti–47Al–2Cr–2Nb samples (N and P orientation) all variants of γ-lamellae deformed only by hard dislocation slip and hard twinning. While the γ-lamellae deformed more profusely for soft orientation loaded PST Ti–47Al–2Cr–2Nb sample (I orientation). It was observed that a majority of γ-variant lamellae deformed by the soft modes of parallel twinning and by soft ordinary dislocation glide and hard modes of twinning as well as hard dislocation activity had been observed for the remainder of the variants of γ-lamellae in the soft orientation loaded PST-TiAl.

The sintering densification, microstructure and mechanical properties of gamma Ti–48Al–2Cr–2Nb alloy with a small addition of copper

Titanium aluminide based alloys are difficult to consolidate by pressureless sintering. A systematic study has been made of the effect of small additions of copper on the sintering densification, microstructure and mechanical properties of gamma Ti–48Al–2Cr–2Nb (in at% throughout). The density of the Ti–48Al–2Cr–2Nb alloy sintered at 1375°C for 120min increased consistently from 74% theoretical density (TD) without Cu to >98%TD with an addition of up to 2at%Cu. The enabling effect of a small addition of Cu is due to the formation of a wetting liquid, supported by dilatometry, differential scanning calorimetry (DSC) and Thermal-Calc analyses. Rapid sintering shrinkage occurred around 1251°C and 1370°C corresponding to liquid formation. Transmission electron microscopy (TEM) identified the formation of a Cu- and Cr-enriched hexagonal close-packed Ti(Al, Cr, Cu, Nb) phase with a=0.520Å and c=0.803Å. Its presence increased with increasing Cu content. The compressive, tensile and flexure properties of the as-sintered Ti–48Al–2Cr–2Nb–2Cu alloy were assessed. The tensile properties are similar or superior to those of the Ti–48Al–2Cr–2Nb alloy fabricated by metal injection moulding and sintering while the compression strength is about twice that of the as-cast Ti–48Al–2Cr–2Nb.