Ti 48Al 2Cr Research Articles

Wear behavior of low-cost, lightweight TiC/Ti–6Al–4V composite under fretting: Effectiveness of solid-film lubricant counterparts

The wear behavior of low-cost, lightweight 10 wt% titanium carbide (TiC)-particulate-reinforced Ti–6Al–4V matrix composite (TiC/Ti–6Al–4V) was examined under fretting at 296, 423, and 523 K in air. Bare 10 wt% TiC/Ti–6Al–4V hemispherical pins were used in contact with dispersed multiwalled carbon nanotubes (MWNTs), magnetron-sputtered diamond-like carbon/chromium (DLC/Cr), magnetron-sputtered graphite-like carbon/chromium (GLC/Cr), and magnetron-sputtered molybdenum disulfide/titanium (MoS 2/Ti) deposited on Ti–6Al–4V, Ti–48Al–2Cr–2Nb, and nickel-based superalloy 718. When TiC/Ti–6Al–4V was brought into contact with bare Ti–6Al–4V, bare Ti–48Al–2Cr–2Nb, and bare nickel-based superalloy 718, strong adhesion, severe galling, and severe wear occurred. However, when TiC/Ti–6Al–4V was brought into contact with MWNT, DLC/Cr, GLC/Cr, and MoS 2/Ti coatings, no galling occurred in the contact, and relatively minor wear was observed regardless of the coating. All the MWNT, DLC/Cr, GLC/Cr, and MoS 2/Ti coatings on Ti–6Al–4V were effective from 296 to 523 K, but the effectiveness of the MWNT, DLC/Cr, GLC/Cr, and MoS 2/Ti coatings decreased as temperature increased.

Effect of chromium addition on microstructure and creep strength of as-cast Ti–48Al alloy

Abstract The microstructure and creep behaviour of as-cast gamma titanium aluminide based alloys with nominal composition of Ti–48Al, Ti–48Al–2Cr and Ti–48Al–4Cr (at.%) were investigated. Constant load tensile creep tests were performed at temperature and stress of 800 °C and 150 MPa, respectively. Creep characteristics such as steady-state creep rate, time to 1% creep deformation, creep deformed microstructures and creep fracture was analysed. The effect of Cr addition on creep strength were analysed with respect to solid solution strengthening by Cr in γ phase, formation of β phase and comparison with the base binary Ti–48Al alloy.

〈c〉-Component plastic displacements in different microstructures of TiAl-base intermetallics

Three different microstructures of a near-gamma Ti–48Al–2Cr–2Nb–B (at.%) alloy, nearly lamellar (NL), equiaxed with a low alpha 2 phase volume fraction (ELA), and equiaxed with the high alpha 2 phase volume fraction (EHA) were investigated. The three microstructural types exhibited different creep behaviour at 973 K and 350 MPa. Interrupted creep tests were performed to assess the evolution of dislocation densities and deformation twinning with increasing creep strain. Results of the quantitative analysis suggest that deformation twinning represents a relevant contribution to the strain accumulation kinetics at high temperatures and makes NL and EHA microstructures less creep resistant but more ductile.

Bone tissue reaction to Ti–48Al–2Cr–2Nb (at.%) in a rodent model: a preliminary SEM study

A variety of metals have been used to replace the skeletal framework of human beings. Gamma titanium aluminide (gammaTiAl) has been recently developed as a prospective material for turbine applications. In this preliminary study, the potential of gammaTiAl as a biomaterial was evaluated using an in vivo rat model. Sprague-Dawley rats were implanted with gammaTiAl cylinders in the femur and observed for an experimental period lasting up to 180 days. The rats were sacrificed after periods of 45, 90 and 180 days. The femurs with the gammaTiAl implants were extracted and examined using scanning electron microscopy (SEM). Normal bone growth processes were observed as early as 45 days after gammaTiAl cylinder implantation. No signs of rejection of the implant metal were observed. In fact, a layered bone growth was observed on the implant metal surface. The bone-metal interface showed signs of tissue growth from original bone to the metal surface. gammaTiAl appears to elicit a normal bone tissue reaction and hence, has potential as a metallic implant material.

Microstructure and mechanical properties of consolidated gamma titanium aluminides

Microsized γ-TiAl powders with composition of Ti–50Al (at.-%) and Ti–48Al–2Cr–2Nb (at.-%) were consolidated using a rapid consolidation technique called plasma pressure compaction (P2C®) to obtain samples with near gamma, duplex and fully lamellar microstructure. The consolidated samples exhibited minimal grain growth during consolidation, thereby retaining superior mechanical properties. The mechanical properties varied significantly with microstructure type, grain size and alloy composition. Samples with a duplex microstructure and average grain size of 5 μm exhibited the best set of mechanical properties with flexure strength, ductility, elastic modulus and fracture toughness as high as 1238 MPa, 2·3%, 154·58 GPa and 17·95 MPa m1/2 respectively. Exposure in air at temperatures >800°C, had an adverse effect on the flexure strength of the consolidated samples.

Multi-component nitride coatings derived from Ti–Al–Cr–Si–V target in RF magnetron sputter

The nitride coatings comprised of various constituents Ti, Al, Cr, Si, V were deposited on mild steel by RF magnetron sputtering process. The Ti–Al–Cr–Si–V multi-component target introduced in this study was fabricated by conventional metallurgical method with atomic ratio of each selected element at 1:1:1:1:1. Different coatings were fabricated under various working pressure by adjusting nitrogen flux from 0 to 30 sccm during sputtering. Compositions of the target and the sputtered films were measured by FE-EPMA, and both of them were near equi-molar. According to XRD patterns, amorphous structures were revealed for the metallic and nitride 1 ( N = 46.2 at.%) films. The face centered cubic phases were exhibited by nitride 2 and nitride 3 with nitrogen contents around 58 at.%. Two different surface morphologies were investigated by the AFM, and they were consistent with the nanostructure observed in the XRD pattern. In addition, microhardness of the nitride coatings was measured by nanoindentation, and hardness higher than 30 GPa was exhibited in both nitride 2 and nitride 3. The microhardness test provided evidence that the multi-component nitride could be a potential candidate coating for tool steel.

TEM observations of twin intersections in a Ti–47Al–2Cr–2Nb–0.1Y alloy compressed at room temperature

Twin intersections in TiAl alloy compressed at room temperature were investigated by high-resolution electron microscopy (HREM). One incident twin (TI) was observed to divide into three thin twin bands and leave several superlattice intrinsic stacking faults (SISFs) in the barrier twin (TB). It was a process related to the dislocation dissociation: (1/2)[1 1 2] → (1/6)[1 1 2] + SISF + (1/6)[1 1 2] + SESF + (1/6)[1 1 2]. The nucleation of type-I twin intersection was also observed. The incident twin was found to be a bundle of SISFs, instead of a perfect twin lath. In addition, a triangular striated structure around type-I twin intersection was examined by HREM. The formation mechanism of this triangular striated structure and the related dislocation reactions were discussed.

Deformation-induced γ ↔ α 2 phase transformation in TiAl alloy compressed at room temperature

Deformation-induced γ ↔ α 2 phase transformation in a Ti–47Al–2Cr–2Nb–0.2Y (at.%) alloy compressed at room temperature was investigated by high-resolution transmission electron microscopy (HREM) and energy dispersive spectrum (EDS). The deformation-induced (DI) γ → α 2 phase transformation occurred in a twin intersection region. On the other hand, the deformation-induced α 2 → γ transformation nucleated at the stacking faults of α 2 phase. The composition analysis of the γ and α 2 laths by EDS suggested that composition of some laths deviated much from their equilibrium values. These composition deviations promoted the deformation-induced γ ↔ α 2 phase transformation to occur; EDS results also suggested that there was no composition difference between the DI-γ plate and the primary γ phase. Based on the HREM and EDS experimental results, the mechanism of the deformation-induced γ ↔ α 2 phase transformation has been discussed.

Oxidation resistant coatings in combination with thermal barrier coatings on γ-TiAl alloys for high temperature applications

Titanium aluminide alloys based on γ-TiAl are considered of growing interest for high temperature applications due to their attractive properties. To extend the service temperatures above 750 °C, the oxidation behaviour has to be improved predominantly by protective layers. In the present study environmental and thermal protection coatings on gamma titanium aluminides were investigated. Nitride and metallic overlay coatings based on Ti–Al–Cr–Y–N and Ti–Al–Cr, respectively, were produced by magnetron sputtering techniques. Thermal barrier coatings (TBCs) of partially yttria stabilized zirconia were deposited onto Ti–45Al–8Nb, either pre-oxidized or coated with protective layers, applying electron beam physical vapour deposition (EB-PVD). Cyclic oxidation tests were performed at 900 °C and 950 °C in air. The nitride coating exhibited poor oxidation resistance when exposed at 900 °C providing no protection for γ-TiAl. The oxidation behaviour of the Ti–Al–Cr coating was reasonable at both exposure temperatures. During prolonged exposure the coating was depleted in chromium, resulting in the breakdown of the protective alumina scale. EB-PVD zirconia coatings deposited on γ-TiAl exhibited promising lifetime, particularly when specimens were coated with Ti–Al–Cr. The adherence of the TBC on the thermally grown oxide scales was excellent; failure observed was associated with spallation of the oxide scale. At 950 °C, TBCs on specimens coated with Ti–Al–Cr spalled after less than 200 thermal cycles caused by severe oxidation of γ-TiAl and reactions between the zirconia coatings and the thermally grown oxides.

Low-cycle fatigue and deformation substructures in an engineering TiAl alloy

The cyclic stress–strain (CSS) behaviour of a Ti–48Al–2Cr–2Nb (atomic%) cast alloy is investigated at 25 °C and 750 °C. Meanwhile post-mortem TEM observations are carried out in order to get insights into the deformation mechanisms governing this behaviour. It has been shown that, at 25 °C, the pronounced hardening observed on CSS curves at high total-strain-amplitude Δ ɛ t values is related to a deformation mode mainly governed by twinning. Meanwhile the moderate hardening observed at intermediate Δ ɛ t is associated to ordinary dislocation glide and the subsequent formation of a vein-like structure at intermediate Δ ɛ t values. In contrast, at 750 °C the CSS behaviour is characterised by a rapid saturation of the stress amplitude, regardless of the applied strain amplitude, while the deformation structure analysis suggests a high activation of dislocation climb which is confirmed by in situ TEM ageing showing shrinkage of prismatic loops. Finally, the influence of parameters such as strain rate and strain ratio on CSS behaviour is also examined.

Sintering of a Gamma Ti-Al Alloy

Gamma-TiAl alloys are potential replacements for nickel alloys and conventional titanium alloys in the cooler sections of turbine engines, as well as in orbital platform vehicles. The combination of high specific stiffness and good oxidation resistance at intermediate temperatures can provide significant weight savings. However, they have a limited plasticity at room temperature and the tendency to brittle fracture. Powder metallurgy is a near net shape process that allows the parts production with complex geometry at low costs. An improved plasticity of the Ti-Al alloys is received by adding alloying elements and by microstructure modification. An alloy of two-phase structure Ti–48Al–2Cr–2Nb (at.%) was investigated using the blended elemental technique. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering at 1500 °C, in vacuum. It was shown that the samples presented a two-phase structure consisting of lamellar colonies of alternating layers of gamma and α2 phase.

The effect of grain boundary normal on predicting microcrack nucleation using fracture initiation parameters in duplex TiAl

In a prior study of a duplex Ti–48Al–2Cr–2Nb, Simkin et al. [B.A. Simkin, M.A. Crimp, T.R. Bieler, Scripta Mater. 49 (2003) 149], 11 cracked and 11 intact boundaries between γ grains were analyzed to determine why some boundaries that were similarly stressed cracked, and others did not. This led to a geometrically based fracture initiation parameter that considered the tensile stress axis and the deformation systems in both grains. In order to further examine the physical basis of this fracture initiation parameter, we have measured the grain boundary orientation of the previously studied boundaries. In contrast to the expectation that incorporation of the grain boundary orientation into this parameter would improve the ability to discriminate between cracked and intact boundaries, the orientation of the grain boundary normal was found to be statistically less significant than the orientation of the Burgers vectors of the most highly stressed twinning systems, quantified by | b ˆ tw ⋅ t ˆ | , where t ˆ describes the tensile axis direction. Grain boundary microcracking was found to be most likely when b ˆ tw was in the range of 30–50° from the tensile axis. Combining the Schmid factor with | b ˆ tw ⋅ t ˆ | and the effect of ordinary dislocation slip accommodation provides a simple and effective predictor of the propensity for microcrack nucleation.

Effect of initial microstructure and further thermomechanical processing on microstructural evolution in a Ti–47Al–2Cr alloy

In this paper, the development of microstructure in a chromium alloyed gamma alloy has been investigated. Wedge shaped specimens of a Ti–47Al–2Cr alloy with various starting microstructures, including lamellar, duplex and feathery structures, were subjected to isothermal hot forging. Microstructural and process variables that influence the resulting deformed microstructure were elucidated. The results indicated that in the case of initial feathery structure, the hot forging at 1050 °C leads to a very fine-grained structure with homogenous grain size distribution. Such a microstructure could not be achieved in the samples with the lamellar or duplex structures due to the anisotropic flow behavior of the starting structures.

Corrosion evaluation of Ti–48Al–2Cr–2Nb (at.%) in Ringer’s solution

The corrosion behavior of Ti–48Al–2Cr–2Nb (at.%) in Ringer’s solution was studied to evaluate its potential as a biocompatible material. Corrosion properties of Ti–6Al–4V were determined under the same conditions for comparison. Two electrochemical techniques, potentiodynamic anodic polarization and electrochemical impedance spectroscopy, were employed to test Ti–48Al–2Cr–2Nb and Ti–6Al–4V. Surface modifications to the samples were made by autoclaving and by oxidation in air at 500 °C and 800 °C. The results show excellent corrosion resistance for unmodified Ti–48Al–2Cr–2Nb, corroborated by the high values of polarization resistance and corrosion potential and low values of corrosion current and corrosion rate. Ti–48Al–2Cr–2Nb appears to possess corrosion characteristics similar to Ti–6Al–4V. Surface modification rendered the Ti–48Al–2Cr–2Nb material extremely corrosion resistant.

Effect of Coatings on the Creep and Oxidation Behavior of TiAl Alloys at 1173K in Air

The effects of coatings on the creep and oxidation behavior of Ti-50Al alloy were investigated at 1173K in air at a constant loading of 30MPa. The coating was formed by a two-step Cr/Al diffusion treatment and consisted of an outermost TiAl2 layer, an outer Al-rich γ layer, an intermediate γ, Laves and β mixture layer, and a Cr diffusion zone. Creep tests were also carried out with sole Cr or Al coated TiAl and also of uncoated TiAl. The oxide scales formed on the uncoated TiAl and the sole Cr coated specimens were a mixture of TiO2 and Al2O3, which displayed several exfoliations. Both the two-step Cr/Al coated TiAl and the sole Al coated specimens formed a protective Al2O3 layer and little oxide exfoliation was observed here. Significant cracks were observed in the sole Al coated TiAl, while no cracks were observed in the sole Cr coated TiAl; the two-step Cr/Al coated TiAl showed a number of cracks in the coatings. Low creep rates in the two-step Cr/Al coated TiAl could be due to the Laves phase with a hexagonal C14 structure in the intermediate, γ, β and Laves phase mixture, and the high creep rates of the sole Cr coated TiAl may originate in the major β phase component with a B2 structure in the γ, β, and Laves phase mixture.

Effects of exposure at 700 °C on RT tensile properties in a PM γ-TiAl alloy

Gas atomized powder compacts of a Ti–47Al–2Cr–2Nb alloy have been used to assess their mechanical performance after long-term exposure at potential service temperatures. Unexposed test specimens were prepared with different machining conditions including turning, grinding and electro-polishing and the results revealed that the room-temperature tensile properties were not really affected by the machining conditions. On the other hand, tensile test specimens, when exposed to a temperature of 700 °C up to 400 h, showed a pronounced reduction in tensile elongation irrespective of the machining procedure. A number of techniques including Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) experiments and surface roughness measurements, were used to identify the origins of such an embrittlement.

Modification of tribology and high-temperature behavior of Ti–48Al–2Cr–2Nb intermetallic alloy by laser cladding

In order to improve the tribology and high-temperature oxidation properties of the Ti–48Al–2Cr–2Nb intermetallic alloy simultaneously, mixed NiCr–Cr 3C 2 precursor powders had been investigated for laser cladding treatment to modify wear and high-temperature oxidation resistance of the material. The alloy samples were pre-placed with NiCr–80, 50 and 20%Cr 3C 2 (wt.%), respectively, and laser treated at the same parameters, i.e., laser output power 2.8 kW, beam scanning speed 2.0 mm/s, beam dimension 1 mm × 18 mm. The treated samples underwent tests of microhardness, wear and high-temperature oxidation. The results showed that laser cladding with different constitution of mixed precursor NiCr–Cr 3C 2 powders improved surface hardness in all cases. Laser cladding with NiCr–50%Cr 3C 2 resulted in the best modification of tribology and high-temperature oxidation behavior. X-ray diffraction (XRD), optical microscope (OM), scanning electron microscopy (SEM) and energy-dispersive spectrometer (EDS) analyses indicated that the formation of reinforced Cr 7C 3, TiC and both continuous and dense Al 2O 3, Cr 2O 3 oxide scales were supposed to be responsible for the modification of the relevant properties. As a result, the present work had laid beneficial surface engineering foundation for TiAl alloy applied as future light weight and high-temperature structural candidate materials.

Phase equilibria among α-Fe(Al, Cr, Ti), liquid and TiC and the formation of TiC in Fe 3Al-based alloys

In the context of the development of high-strength Fe 3Al-based alloys, phase equilibria among α-Fe(Al, Cr, Ti), liquid and TiC phases in the Fe–Al–Cr–Ti–C quinary system and the formation of TiC were determined. A pseudo-eutectic trough (L ⇔ α + L + TiC) exists at 1470 °C at around Fe–26Al–5Cr–2Ti–1.7C on the vertical section between Fe–26Al–5Cr (α) and Ti–46C (TiC) in at.%. Large faceted TiC precipitates form from the melt after the formation of primary α phase even in hypoeutectic alloys. The TiC formation is thought to be due to the composition change of the liquid towards the hypereutectic compositions by solidification of the primary α. In order to remove the faceted TiC, which are unfavourable for strengthening the material, two different processing routes have been successfully tested: (i) solidification with an increased rate to reduce the composition variation of the liquid during solidification, and (ii) unidirectional solidification to separate the light TiC precipitates from the melt.

Cyclic deformation mechanisms in a cast gamma titanium aluminide alloy

The present study tackles the issue of the identification of the deformation mechanisms governing the cyclic stress–strain behaviour of a cast Ti–48Al–2Cr–2Nb (numbers indicate at.%) with a nearly fully lamellar microstructure. At room temperature, this behaviour and the corresponding deformation mechanisms are shown to be strongly dependent on the applied strain range. Indeed, at low strain range, where almost no hardening is noticed, deformation occurs by motion of long and straight ordinary dislocations. The moderate hardening observed at intermediate values of the strain range is associated with the formation of a vein-like structure due to the progressive tangling of ordinary dislocations. Finally, at higher strain-range values, twinning, by delaying the formation of this vein-like structure, induces a more pronounced cyclic strain hardening. At high temperature (750 °C), the material exhibits a rapid saturation of the stress amplitude, regardless of the applied strain range. Transmission electron microscopy indicates that twinning is no longer operative at this temperature, but that dislocation climb is activated.

Microstructure, wear and high-temperature oxidation resistance of laser clad Ti5Si3/γ/TiSi composite coatings on γ-TiAl intermetallic alloy

Mixed NiCr–Si precursor powders on γ-TiAl intermetallic alloy (Ti–48Al–2Cr–2Nb) have been investigated for laser cladding treatment to modify the tribology and high-temperature oxidation properties of the material simultaneously. The alloy samples are pre-placed with NiCr–50% and NiCr–40%Si (wt.%), respectively, and laser treated at the same parameters, i.e., laser output power 2.8 kW, beam scanning speed 2.0 mm/s, beam dimension 1×18 mm. The treated samples underwent tests of microhardness, full dry sliding wear and high-temperature oxidation. The results showed that laser cladding with different constitution of mixed precursor NiCr–Si powders improve the surface hardness in all cases. Laser cladding with NiCr–40% Si precursor mixed powders results in the better modification of tribology and high-temperature oxidation behavior. However, with NiCr–50%Si precursor powders shows decreased wear resistance compared to the original TiAl alloy due to the existence of a large amount of hard and brittle Ti5Si3 and TiSi compounds. Optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive spectrometer (EDS) analyses indicated that the formation of the reinforced primary Ti5Si3, γ/TiSi eutectics and the refinement of the microstructure and both the continuous and dense Al2O3, SiO2 hybrid oxide scales are supposed to be responsible for the modification of the relevant properties. Laser cladding with NiCr–Si mixed powders is anticipated to be a promising wear- and oxidation-resisting surface modification technique for TiAl intermetallic alloy.