Ti 48Al 2Cr Research Articles

High mechanical functionalization of Ti–Al–Cr–Fe–C system alloy for next-generation aircraft applications through microstructural control

The relationship between microstructures and mechanical properties such as tensile and fatigue properties were systematically investigated by conducting several heat treatments on Ti–4.5Al–4Cr–0.5Fe–0.2C (KS EL-F) and Ti–4.5Al–2Cr–1Fe–0.1C (KS EL-Fmod) for next-generation aircraft applications. TiC and TiCr2 are formed in KS EL-F and KS EL-Fmod annealed at temperatures higher than β-transus and aged at 773 K, respectively. However, the precipitation trends of TiC and TiCr2 are considerably higher in the case of KS EL-F because the amounts of both these precipitates are drastically suppressed in KS EL-Fmod even when the heat treatments are identical. As-received KS EL-F and KS EL-Fmod exhibit excellent fatigue strength as compared with annealed Ti–6Al–4V. Although the presence of TiCr2 tends to decrease the fatigue strength, the aged KS EL-Fmod exhibits higher fatigue strength than the as-received KS EL-Fmod with very low TiCr2 amounts. Therefore, the aging treatment for KS EL-Fmod can be effective in improving the fatigue strength.

Electron beam melting of Ti–48Al–2Cr–2Nb alloy: Microstructure and mechanical properties investigation

Gas atomized Ti–48Al–2Cr–2Nb powders have been used as precursor material in order to evaluate additive manufacturing for the production of near-net-shape γ-TiAl specimens to be employed in the field of aero-engines. In particular electron beam melting (EBM) is used to realize a selective densification of metal powder by melting it in a layerwise manner following a CAD design. The microstructure, the residual porosity and the chemical composition of the samples have been investigated both immediately after EBM and after heat treatments. High homogeneity of the samples, very low pickup of impurities (oxygen and nitrogen) with respect to the starting powders have been observed and due to an extremely low level of internal defects, intrinsic to EBM process, the tensile properties of the EBM γ-TiAl appear very consistent with a small scatter.

Thermodynamic modeling of the Ti–Al–Cr ternary system

The Ti–Al–Cr ternary system is one of the most important systems to studying the titanium alloys. Some experimental data of this ternary system are available and a few partial thermodynamic assessments are reported. However, no full thermodynamic descriptions were published. In this study, the previous work on the Ti–Al–Cr system and its related binary systems are reviewed. Based on the thermodynamic descriptions of the Ti–Al, Ti–Cr and Al–Cr systems and the ternary experimental data in literature, the Ti–Al–Cr ternary system is assessed by means of the Calphad method. Several isothermal sections from 1073 K to 1573 K and some invariant reactions are calculated, which are in good agreement with the most of the experimental results.

Effects of heat shock at 800 °C on mechanical properties of γ-TiAl alloys

Effect of heat shock on the mechanical properties and the microstructure of TiAl alloys (Ti–47Al–2Cr–2Nb and Ti–45.3Al–2Cr–2Nb–0.1W–0.15B) with near fully lamellar structure were investigated. After heat shock process from room temperature to 800 °C for 500 cycles, the microstructure demonstrated that lamellar microstructure has been destructed by the presentation of some γ and α 2 block phases in lamellar structure, resulting in the ductility of as-polished Ti–47Al–2Cr–2Nb alloy decreased by about 70% and the ultimate tensile strength (UTS) reduced by about 25%, and the ductility of as-unpolished Ti–47Al–2Cr–2Nb alloy decreased by more than 70% and the ultimate tensile strength (UTS) was reduced by about 35%. The ductility of Ti–45.3Al–2Cr–2Nb–0.1W–0.15B alloy decreased by about 60% and the ultimate tensile strength (UTS) reduced by about 18% after heat shock, which was resulted from the appearance of small α 2 block phase at interfaces of lamellar colonies and microcracks at the interfaces of ribbon boride and lamellar structure.

Microstructure and mechanical properties of high niobium containing TiAl alloys elaborated by spark plasma sintering

Spark Plasma Sintering (SPS) is used to process TNB alloys. Pre-alloyed powders (Ti–46Al–9Nb) were sintered between 975 °C and 1300 °C under 100 MPa. The microstructures of the products are determined by X-ray diffraction and by scanning and transmission electron microscopy. Their mechanical properties are evaluated by tensile tests performed at room temperature and creep experiments at 700 °C under 300 MPa stress. Depending on the sintering temperature, two-phased, duplex and lamellar microstructures have been obtained. A quasi-α 2 phase has been observed, which is in fact an α 2 phase in which Nb atoms have substituted Ti atoms. The TNB alloys consolidated by SPS are stronger than the alloys previously elaborated by SPS using Ti–47Al–2Cr–2Nb and Ti–44Al–2Cr–2Nb–1B powders but they exhibit a lower ductility. Moreover, some specimens broke prematurely due to Fe inclusions.

High-performance wear-resistant ion-plasma coatings based on five-component nitrides for a hard-alloy cutting tool operating under constant loads

Studies are made of multicomponent coatings obtained in the system Ti–Al–Cr–Zr–Nb–N by the Arc-PVD method on vacuum-type ion-plasma unit Bulat – NNV 6.6-I1. The coatings were obtained in a medium of reactive gaseous nitrogen. The study results showed that the coatings exhibit high values for hardness and durability when used under continuous cutting conditions.

High temperature deformability and microstructural evolution of Ti–47Al–2Cr–0.2Mo alloy

High temperature deformability and microstructural evolution of Ti–47Al–2Cr–0.2Mo alloy were studied by using a Gleeble-1500 simulated machine at temperature from 1050 °C to 1150 °C with strain rate from 2.5 s −1 to 7.5 s −1 and engineering compressive strain from 20% to 50%. The results indicate that the largest engineering compressive strain of the alloy increases with decreasing strain rate and increasing deformation temperature. It has been shown that the strain rate affects the largest engineering compressive strain of the alloy effectively. High temperature deformation mechanism of Ti–47Al–2Cr–0.2Mo alloy mainly refers to the bending and fracturing of the lamellar structures and dynamic recrystallization (DRX). Electron back scattered diffraction (EBSD) and transmission electron microscopic (TEM) results reveal the DRX mostly occurring at grains boundary and interlamellar spacing at a strain rate of 7.5 s −1 is much finer than that at a strain rate of 2.5 s −1 at 1150 °C.

Oxidation of Powder Metallurgy (PM) Ti–48%Al–2%Cr–2%Nb–(0–1%)W Alloys Between 800 and 1000°C in Air

Three powder metallurgy (PM) TiAl alloys with a fully lamellar structure were oxidized isothermally and cyclically between 800 and 1000°C in air in order to find the effect of W on the oxidation behavior of Ti–48Al–2Cr–2Nb alloys. The alloys oxidized parabolically during isothermal oxidation. Tungsten improved the isothermal and cyclic oxidation resistance. The oxide scales consisted primarily of an outer TiO2 layer, an intermediate Al2O3-rich layer, and an inner TiO2-rich layer. The alloying elements of Cr, Nb, and W tended to segregate in the lower part of the scale owing to their thermodynamic nobility. In the vicinity of the scale/matrix interface, TiN and Ti2AlN coexisted.

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.%) (gamma-TiAl), a gamma titanium aluminide alloy originally designed for aerospace applications, appears to have excellent potential as implant material. Thermal treatment of gamma-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 degrees C, and at 800 degrees C for 1 h in air) on gamma-TiAl were characterized by X-ray photoelectron spectroscopy (XPS). hFOB 1.19 cell adhesion on thermally oxidized gamma-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 gamma-TiAl surfaces at the different time points. SEM images showed well organized multi-layers of differentiated cells adhered on thermally oxidized gamma-TiAl surfaces at day 14. Unexpectedly, thermally oxidized Ti-6Al-4V surfaces oxidized at 800 degrees C exhibited cytotoxic effects on hFOB 1.19 cells. Our results indicate that thermal oxidation of gamma-TiAl seems to be a promising method to generate highly corrosion resistant and biocompatible surfaces for implant applications.

Splitting phenomenon in the precipitation evolution in an Fe–Ni–Al–Ti–Cr stainless steel

The evolution of precipitation and mechanical properties of an Fe–Ni–Al–Ti–Cr stainless steel was studied during ageing at 525 °C. Atom probe tomography and transmission electron microscopy were applied to follow the microstructural evolution. An initial hardening reaction, which is remarkable in terms of extent, is reported to be caused by the formation of complex multi-component clusters. They are composed mainly of Ni, Al and Ti. After ageing to peak hardness (3 h), splitting of these clusters into spherical and elongated particles was observed. Based on the chemical composition, the spherical precipitates were identified to be of type NiAl B2, and the elongated particles were associated with the η-phase (Ni 3(Ti, Al)). Βoth types of precipitates contribute to the strength of the material.

On the shock response of cubic metals

The response of four cubic metals to shock loading is reviewed in order to understand the effects of microstructure on continuum response. Experiments are described that link defect generation and storage mechanisms at the mesoscale to observations in the bulk. Four materials were reviewed; these were fcc nickel, the ordered fcc intermetallic Ni3Al, the bcc metal tantalum, and two alloys based on the intermetallic phase TiAl; Ti–46.5Al–2Cr–2Nb and Ti–48Al–2Cr–2Nb–1B. The experiments described are in two groups: first, equation of state and shear strength measurements using Manganin stress gauges and, second, postshock microstructural examinations and measurement of changes in mechanical properties. The behaviors described are linked through the description of time dependent plasticity mechanisms to the final states achieved. Recovered targets displayed dislocation microstructures illustrating processes active during the shock-loading process. Reloading of previously shock-prestrained samples illustrated shock strengthening for the fcc metals Ni and Ni3Al while showing no such effect for bcc Ta and for the intermetallic TiAl. This difference in effective shock hardening has been related, on the one hand, to the fact that bcc metals have fewer available slip systems that can operate than fcc crystals and to the observation that the lower symmetry materials (Ta and TiAl) both possess high Peierls stress and thus have higher resistances to defect motion in the lattice under shock-loading conditions. These behaviors, compared between these four materials, illustrate the role of defect generation, transport, storage, and interaction in determining the response of materials to shock prestraining.

Effect of cyclic heat treatment parameters on the grain refinement of Ti–48Al–2Cr–2Nb alloy

This work presents the influence of individual parameters of a cyclic heat treatment, i.e. upper cycle temperature, heating and cooling rates between room temperature and an upper temperature and number of cycles, on the grain refinement of a Ti–48Al–2Cr–2Nb alloy. The grain size as determined by the value of the average plane section diameter and indices describing its distribution (standard deviation) is sufficient to define grain refinement when the refinement obtained as a result of the treatment concerns the whole section of a sample. In the event when the refinement process only takes place partially or locally it becomes necessary to present distributions of the grain plane section area as a function of the frequency of occurrence and area fraction. The paper presents a possible mechanism of grain refinement of Ti–48Al–2Cr–2Nb alloy. The use of cyclic heat treatment provides an increase in the mechanical parameters of the alloy.

Long-term creep and creep rupture characteristics of TiAl-base intermetallics

The present study investigates creep in three TiAl-base intermetallic alloys, Ti–48Al–2Cr–2Nb–1B, Ti–46Al–7Nb–0.6Cr–0.2Ni–0.1Si and Ti–46Al–2W–0.5Si–0.7B (all at.%), in the range of rupture times up to 25 000 h. Creep behaviour of the individual alloys at 1023 K is assessed and compared and reasons for creep strength variations are discussed. Results suggest that dislocation processes contribute to the creep strain accumulation even at creep rates as low as 7 × 10 −10 s −1. Finally the applicability of Monkman–Grant-type relationships is critically analyzed. It is shown that these relationships stem from a general functional identity that is valid for any continuous creep curve. The role of the creep curve shape factor S is highlighted.

Transient liquid phase bonding of Gamma Met PX: microstructure and mechanical properties

An investigation of microstructural development and structure–property relationships of transient liquid phase (TLP) bonded current generation γ-TiAl alloy, known as Gamma Met PX, is presented. This joining technique employed a composite interlayer consisting of a non-melting constituent (TiAl alloy) and a liquid forming constituent (copper). The microstructures of the bonds, identified using light microscopy and scanning electron microscopy, are correlated with room temperature mechanical performance. These studies suggested that joints can retain properties similar (i.e. >90%) to that of the bulk material when employing a suitable composite interlayer, bonding conditions and post-bond heat treatment. Additionally, comparisons are drawn between wide gap TLP bonding of an earlier generation γ-TiAl alloy, Ti–48Al–2Cr–2Nb (at.-%), and wide gap TLP bonding of Gamma Met PX.

Microstructure and mechanical properties of gamma TiAl based alloys produced by combustion synthesis + compaction route

Microstructure and mechanical properties of γ-TiAl alloy with the chemical composition Ti–48Al–2Cr–2Nb (at.%) have been investigated. The alloy was prepared in one step using a combustion synthesis + compaction process, where the synthesis and the shaping take place at the same time. Two different combustion routes were used: forced self-propagating high-temperature synthesis + compaction (FSHS + C) and thermal explosion + compaction (TE + C). Fully lamellar microstructure was obtained in both processing routes after the appropriate homogenisation thermal treatment. Nevertheless, TE + C route showed coarser lamellar colonies. A remarkable oxygen content reduction was achieved in samples synthesised by FSHS + C route. Tensile properties (especially UTS) were found to be in the same range as other alloys obtained by conventional processing routes, however, low ductility was achieved. Good creep and fatigue properties were obtained. All fracture surfaces showed a brittle fracture mechanism. Finally, it was found out that the studied processing routes showed promising results as an alternative manufacturing technology for γ-TiAl based alloys.

On the influence of coating and oxidation on the mechanical properties of a γ-TiAl based alloy

The use of γ-TiAl based alloys in high-temperature applications requires an effective protection against oxidation and concomitant loss of ductility. Two coatings, Al 2Au and Cr 0.45Al 0.53Y 0.02N (CrAlYN), are tested for their oxidation protection in air and their influence on the mechanical properties of a Ti–47Al–2Cr–0.2Si alloy. Both coatings significantly improve the oxidation resistance of the investigated γ-TiAl. The plastic strain in the γ-TiAl outer fiber during four-point-bending tests at room temperature is reduced by the deposition with Al 2Au and CrAlYN. After 168 h oxidation at 800 °C uncoated and Al 2Au coated γ-TiAl samples crack without plastic deformation due to the oxide layers and interdiffusion zones formed. Contrary, a CrAlYN protected γ-TiAl four-point-bending specimen still exhibits a plastic strain of 0.12% after oxidation at 800 °C for 672 h, as a thin and dense oxide layer forms. We conclude that CrAlYN effectively retards oxidation and interdiffusion of γ-TiAl and hence the commonly observed deterioration of mechanical properties by oxidation and interdiffusion is significantly reduced.

Corrosion behavior of (Ti–Al–Cr–Si–V) xN y coatings on mild steels derived from RF magnetron sputtering

Multi-component metallic (MC) and multi-component nitride (MCN) coatings was deposited on mild steel substrates with the multi-component Ti–Al–Cr–Si–V target by adjusting nitrogen inlet in the sputtering process. Thickness of the coating was controlled around 1.7 μm. Corrosion resistance of the MC and MCN coatings was investigated by Tafel plot and potentiodynamic polarization tests in 3.5% NaCl electrolyte. Although MCN coating exhibited favorable hardness, the corrosion current of the MCN 3 coating ( i corr = 19.4 μA/cm 2) was higher than that of mild steel ( i corr = 16.4 μA/cm 2), indicating poor corrosion resistance and a porous structure derived from sputtering process. From the cross-sectional observation, numerous pinholes were found along the nano-columns in the MCN 3 coating. After introduction of 100 nm MC interlayer, these defects were suppressed and the calculated porosity rate was significantly decreased. The improvements on corrosion potential ( E corr = − 520 mV) and current ( i corr = 8.05 μA/cm 2) suggested that the MC/MCN coatings provided not only mechanical strength but also satisfactory anti-corrosion properties.

Centrifugal precision cast TiAl turbocharger wheel using ceramic mold

It is the potential candidate for gamma TiAl-based alloys that would replace Ni-based superalloys, which are being used in fabrication of the turbocharger wheel currently. Cast Ti–47Al–2Cr–2Nb alloys due to their specific performance requirement are now on the verge of a commercial application. A novel precision casting technique, which combines the ceramic mold casting with centrifugal casting, was described in this work. Multiple-section ceramic mold was designed and fabricated successfully. All the mold sections fit together to form a cavity and then open in many directions to eject the molded part. Microstructures and mechanical properties of cast Ti–47Al–2Cr–2Nb alloys under the action of centrifugal force are discussed.

Oxidation behaviour of gamma titanium aluminides with EB-PVD thermal barrier coatings exposed to air at 900 °C

The oxidation behaviour of two γ-TiAl-based alloys coated with thermal barrier coatings (TBCs) was investigated in air at 900 °C for up to 1000 h under quasi-isothermal conditions. Samples bare or with a 4 μm thick Ti–51Al–12Cr (at.%) protective layer were coated with yttria-stabilized zirconia using electron-beam physical vapour deposition. Prior to TBC deposition, the specimens were pre-oxidised in air and low-pressure oxygen. Thermal barrier coatings on samples of the alloy Ti–45Al–8Nb–0.2B–0.15C (at.%) did not spall off during the maximum exposure length, whereas failure occurred with pre-oxidised specimens of the alloy Ti–45Al–6Nb–1Cr–0.4W–0.2B–0.5C–0.2Si (at.%). Scanning electron micrographs revealed an outer oxide scale with a columnar structure grown by outward diffusion of Ti and Al cations. Below this columnar oxide layer, a broad porous zone formed consisting predominantly of titania. A discontinuous nitride layer was observed in the transition region between oxide scale and substrate. Spallation of the TBC was associated with cracking in the porous titania-rich layer. The zirconia top coat exhibited an excellent adherence to the thermally grown oxide scales. Pre-exposure in low-pressure oxygen did not improve the oxidation resistance. Post-oxidation microstructural examinations of specimens coated with Ti–Al–Cr revealed that the protective layer was severely deteriorated. On the degraded Ti–Al–Cr coating, an outer scale with columnar structure formed beneath the TBC.

Pulsed nanocrystalline plasma electrolytic carburising for corrosion protection of a γ-TiAl alloy: Part 1. Effect of frequency and duty cycle

Tafel polarization and electrochemical impedance spectroscopy were employed in Ringer's solution to test carburised Ti–48Al–2Cr–2Nb, treated by a novel method called pulsed nanocrystalline plasma electrolytic carburising. The results show excellent corrosion resistance for modified Ti–48Al–2Cr–2Nb. The effect of frequency and duty cycle of pulsed current was investigated. It was found that frequency and duty cycle of pulsed current affect the size and porosity of nanocrystalline carbides. This surface modification with controlled effective parameters can render the Ti–48Al–2Cr–2Nb material extremely corrosion resistant as a biomaterial.