48Al 2Cr 2Nb Research Articles

In-situ synthesized a dual-scale Ti2AlC reinforced TiAl composites with superior mechanical properties

To enhance the mechanical properties and service temperature of TiAl alloy, the dual-scale Ti2AlC particles reinforced TiAl composites were successfully designed and fabricated by spark plasma sintering (SPS), utilizing pre-alloyed Ti–48Al–2Nb–2Cr powder and multi-walled carbon nanotubes. The TiAl composites exhibited a fully lamellar structure, with micro-laminated Ti2AlC particles formed at grain boundaries and nano-laminated Ti2AlC particles precipitated at the interfaces of α2 and γ lamellae by adding 0.5 wt% multi-walled carbon nanotubes. The TiAl composites exhibited excellent mechanical properties at room temperature and elevated temperatures. The ultrahigh tensile strength of TiAl composite was 599.6 MPa at 800 °C which was improved by 28.3 % compared with TiAl matrix, while the fracture strain remained 4.2 % without sacrifice. These superior mechanical properties were mainly attributed to the refinement strengthening, solid solution strengthening, and the formation of dual-scale Ti2AlC particles. Moreover, the dual-scale Ti2AlC particles reinforced α2 and γ lamellar interfaces and lamellar colony boundaries resulting in improvement of strength and toughness simultaneously. The TiAl composite was reinforced by dual-scale Ti2AlC particles, which expected to further break the property limitation of TiAl alloy and expanded its application at high temperature.

Superior high-temperature strength-ductility of TiB2–Ti2AlN/TiAl composite with core-shell microstructure

Herein, a novel core-shell structured TiAl composite consisting of nanoparticles “shell” and a near fully lamellar “core” was designed using a Ti–48Al–2Nb–2Cr alloy and Boron nitride nanoplates. By tailoring microstructure and nanoparticles strengthening, the TiAl composite exhibited a superior combination of strength and ductility at high temperature, which achieved an ultimate tensile strength of 514.2 MPa and an outstanding fracture strain of 18.8 % at 850 °C.

Emissivity measurements conducted on intermetallic γ-TiAl-based alloys for aeronautical applications

The directional spectral emissivity of a Ti–48Al–2Nb–2Cr alloy (in at.%), 4822 alloy, and a Ti-43.5Al–4Nb–1Mo-0.1B alloy (in at.%),TNM alloy, used in the aeronautical industry, are measured between 150 and 850 °C. The differences in the emissivity values between both alloys at the lowest temperatures, indicates that the βo phase, only present in TNM, exhibit higher emissivity values. By numerical integration of the measured data, the total directional and hemispherical emissivity have been calculated. At 850 °C the total hemispherical emissivity in vacuum are nearly identical with 0.274 ± 0.006 for the 4822 alloy and 0.273 ± 0.007 for the TNM alloy. The lower emissivity change with temperature measured in TNM alloys is related with the deconvolution of βo phase by diffusion processes. Afterwards, near-normal spectral emissivity measurements are performed in both alloys during isothermal oxidation treatments at 750 °C and 850 °C for 120 h. The emissivity data reveal that the TNM alloy exhibits higher oxidation resistance especially at 750 °C. In parallel, microstructural characterization has been performed before the measurements, after the directional emissivity measurements prior to oxidation and after isothermal oxidations. The formed oxide scale is composed of four layers that coincide with those reported in the literature: an outer layer of TiO2 contiguous with a layer of Al2O3, followed by a TiO2/Al2O3 mixed layer and finally a thin layer of Nb-rich nitride. This mixed layer governs the interferential part of the alloys’ emissivity spectra, which, in combination with the background, determines the overall radiative behavior of the alloys under service conditions.

Ti–48Al–2Cr–2Nb alloys prepared by electron beam selective melting additive manufacturing: Microstructural and tensile properties

Here, an electron beam selective melting (EBM) technique was employed to shape Ti–48Al–2Cr–2Nb alloy to combat its intrinsic brittleness and insufficient hot workability. Through process optimization, high-density samples (4.2327 g/cm3) were produced. First, the mechanism of interaction between phase transformation and microstructural evolution of EBM-formed Ti–48Al–2Cr–2Nb alloy was investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscattering diffraction (EBSD). And then, the mechanism of interaction between tensile properties and microstructural evolution of EBM-formed Ti–48Al–2Cr–2Nb alloy was discussed. Based on the results, the microstructure consists of coarse equiaxed grains and fine lamellar grains, which gradually coarsen from upper to bottom. As the thickness of the layer increases from upper to bottom, the content of the B2 phase increases continuously, with a predominant component of TiAl and a small amount of Ti3Al and B2 phases. The majority of the γ-TiAl phase does not exhibit significant texture, while the α2-Ti3Al phase shows a texture of (0001) with 19.32 times the random intensity, and its c-axis is parallel to the deposition direction. Benefiting from its unique structure features, the maximum room temperature tensile strength of as-deposited Ti–48Al–2Nb–2Cr alloy reaches 854.6 MPa with an elongation of over 2%. And the maximum high-temperature tensile strength of EBM-formed Ti–48Al–2Nb–2Cr alloy reaches 707 MPa at 650 °C with an elongation over 3.5%. According to these results, the mechanical properties of our EBM Ti–48Al–2Cr–2Nb alloy are superior to those previously reported.

Influence of oxidation protective SiAl coatings on the tensile and fatigue behavior of Ti–48Al–2Cr–2Nb at elevated temperatures

This study considers the influence of novel oxidation protective SiAl-coatings on the tensile and fatigue behavior of the γ-Titanium Aluminide alloy Ti–48Al–2Nb–2Cr. The SiAl-coatings were applied by pack cementation (PC) and magnetron sputtering (MS) processes. After coating, the specimens were pre-oxidized for 300 h at 850 °C. As reference, bare untreated and bare pre-oxidized specimens were tested. In ambient temperature tensile tests, pre-oxidation reduced both ultimate tensile strength and elongation. In comparison, PC-coated specimens showed higher ultimate tensile strength and elongation indicating that the coatings can effectively prevent embrittlement due to surface oxidation. The fatigue lifetimes of PC-coated, MS-coated and pre-oxidized specimens reduced drastically at higher stress amplitudes. Damage observations indicate that pre-oxidizing facilitates early crack initiation in the embrittled surface layer above a certain transition stress amplitude. Both coating types can attenuate embrittlement of the surface layer. However, the coatings themselves tend to initiate early cracks above a transition stress amplitude leading to a lifetime behavior similar to pre-oxidized specimens. Below the respective transition stress amplitude, the lifetimes of both coated specimen types reach almost those of bare untreated specimens. The results suggest that the benefits of the coatings in the form of reduced oxidation and embrittlement outweigh the slight fatigue life penalty at service relevant low stress amplitudes.

The influence of micro-nano Ti2AlC on the hot deformation behavior of TiAl alloys

To strengthen TiAl alloys, micro-nano Ti2AlC precipitates were fabricated in situ by spark plasma sintering using Ti–48Al–2Nb–2Cr pre-alloyed powder and multilayer graphene oxide. The effect of micro-nano Ti2AlC precipitation on the hot deformation behavior of Ti–48Al–2Nb–2Cr alloy was systematically studied at 1100, 1150, and 1200 °C under strain rates of 0.01, 0.1, and 1 s−1. The fully lamellar structure was transformed to a nearly lamellar structure when the graphene content was increased. In addition, the lamellar colony size significantly decreased and the number of Ti2AlC particles increased with increasing graphene content. The hot deformation results show that the flow stress of the Ti2AlC–TiAl composites first increased and then decreased with increasing graphene content, reaching the maximum with the addition of 0.3 wt% graphene. Micro-nano Ti2AlC particles hindered dislocation slip, leading to a strengthening effect. However, Ti2AlC particles promoted dynamic recrystallization through grain refinement and dislocation pile-up, resulting in a softening effect. The strengthening and softening effects of the Ti2AlC–TiAl composites depend on the Ti2AlC particle content.

Microstructure and mechanical properties of micro-nano Ti2AlC-reinforced TiAl composites

In order to improve the mechanical properties of TiAl alloys, micro-nano Ti2AlC-reinforced TiAl composites were successfully fabricated by means of spark plasma sintering (SPS) using the ball-milled multilayer graphene oxide and Ti–48Al–2Nb–2Cr pre-alloyed powders. The micro-nano Ti2AlC phase precipitated at the interface between α2-Ti3Al and γ-TiAl phases utilizing the reaction of TiAl and high-activity graphene. Fully lamellar structure was obtained after sintering above 1300 °C, and Ti2AlC phase characteristic were depended on sintering temperature and graphene content. The compressive strength and fracture strain of TiAl-0.5G alloy were improved at room temperature and elevated temperature, which was improved by 23.61% and 5.03% compared to the TiAl-0G alloy at room temperature. The tribological properties of TiAl composites were significantly improved by micro-nano Ti2AlC at the room temperature, and the average friction coefficient of TiAl-0.5G alloy is 0.217 compared with TiAl-0G alloy is 0.312, and the wear mechanism is ploughing wear. The micro-nano Ti2AlC improves the strength and oxidation resistance of TiAl composites at 650 °C, which is detrimental to the wear resistance due to the lower ductility and the third wear by the loose oxide particles at elevated temperature.

Preparation of low-oxygen-containing Ti–48Al–2Cr–2Nb alloy powder by direct reduction of oxides

A high-purity Ti–48Al–2Nb–2Cr alloy powder with an oxygen content as low as 0.0572 wt.% and a particle size of <150 μm was produced from a mixture of TiO2, Al2O3, Nb2O5, and Cr2O3 powders through reduction with magnesium and deoxidation with calcium. The phase and composition of the products were analyzed. The final product mainly included γ-TiAl and minor α2-Ti3Al phases, and Ti, Al, Cr, and Nb were homogenously distributed in the powder with a mole ratio of 49.73:43.51:2.05:1.98. The reduction and deoxidation mechanisms were investigated by thermodynamic modeling using the HSC Chemistry software and Pandat software based on the Ti alloy database.

Microstructure control and mechanical properties of directionally solidified large size TiAl alloy by electromagnetic confinement

Ti–48Al–2Nb–2Cr alloys with a diameter of 30 mm were prepared by electromagnetic confinement directional solidification with Ti–43Al–3Si seed under different pulling rates. The macro/microstructure evolution and mechanical properties of the directionally solidified Ti–48Al–2Nb–2Cr alloys were investigated. With increasing pulling rates, the grain sizes of the directionally solidified alloys increase to the maximum at the pulling rate of 15 μm/s and then decrease. During the process of directional solidification, the primary phase of the alloy transforms from the α phase into β phase with the increase of pulling rates. At the pulling rate of 5 μm/s, the equiaxed grains with fully lamellar microstructure are formed, and some B2 phases and massive γ phases are found at the grain boundary. The well aligned α2/γ lamellar orientation is obtained at the pulling rate of 15 μm/s, while the inclined lamellar orientation is obtained when the pulling rate increases to 20 μm/s. The interlamellar spacing (λ) decreases with increasing pulling rate (V) according to the relationship λ=6966V−0.563 and r2=0.986. For the DS samples, the nanoindentation hardness of the lamellae and the interlamellar spacing satisfy the relationship of HN=53.83d−0.309 and r22=0.96. The room-temperature tensile strength of directionally solidified alloy reaches the maximum value at the pulling rate of 15 μm/s. The fracture changes from interlamellar to translamellar mode with increasing pulling rate. The synergistic effect of nanotwins and dislocations contributes to the relatively high room-temperature tensile strength and elongation during the deformation.

In Situ Investigation of the Rapid Solidification Behavior of Intermetallic γ‐TiAl‐Based Alloys Using High‐Energy X‐Ray Diffraction

Representing an attractive new processing method, additive manufacturing can be used to manufacture parts made of γ‐TiAl‐based alloys for high‐temperature applications. However, in terms of nucleation during rapid solidification and subsequent solid‐state phase transformations, the process is not yet fully understood, and research is still going on. This article focuses on a setup to study solidification processes during laser melting via in situ high‐energy X‐ray diffraction at a synchrotron radiation source. To create conditions similar to those encountered in powder bed‐based additive manufacturing processes, such as electron beam melting or selective laser melting, a thin platelet is laser‐melted on its upper edge. Phase transitions are measured simultaneously via high‐energy X‐ray diffraction in transmission geometry. The use of a thin platelet instead of the usual powder bed precludes the unfavorable contribution of solid phases from surrounding powder particles to the diffraction signal. First results of the in situ high‐energy X‐ray diffraction experiment on a Ti–48Al–2Nb–2Cr (in at%) alloy prove the applicability of the used setup for an accurate tracing of phase transformations upon rapid solidification.

Study of dry sliding wear behaviour of Ti-48Al-2Nb-2Cr alloy

ABSTRACT The aim of this research was to study the tribological behaviour of Ti–48Al–2Nb–2Cr alloy under dry sliding reciprocating conditions. A reciprocating wear machine was employed to evaluate the typical abrasion resistance of the alloy. Infrastructure assessment of worn surfaces was performed by scanning electron microscopy (SEM) analysis equipped with Energy Dispersive X-Ray Spectroscopy (EDS). The average surface roughness of initial specimen was 32 nm. According to the SEM observations of worn surfaces and debris, the dominant wear mechanism under defined tribosystem was identified to be abrasive and delamination wear. The weight loss measurements showed that by increasing the applied load from 10 to 40 N, the weight loss was significantly increased from 5.4 to 18.5 µg after passing 1000 m slip distance.

Microstructure refinement assisted by α-recrystallization in a peritectic TiAl alloy

Fine-grained nearly-lamellar (FGNL) microstructures and ultrafine α2/γ lamellar microstructure were successfully obtained by means of α-recrystallization in a Ti–48Al–2Nb–2Cr alloy. The dynamic recrystallization of α phases and subsequent cooling control contributed to microstructure refinements, which were governed by small-strain α-deformation. Lamellar grain refinement is due to dominating continuous dynamic recrystallization (CDRX). Lamellar structure refinement is mainly achieved by the increase in γ lamellae precipitation sites by α-deformation and fast cooling. Finally, the microstructure evolution and control mechanisms were illustrated, suggesting a promising method for improving the mechanical properties of TiAl alloy.

Microstructure and oxidation resistance of Si modified aluminide coating on TiAl based alloys

Two γ-TiAl based alloys of Ti–48Al–2Nb–2Cr (at-%) and Ti–45Al–2Nb–2Mn–1B (at-%) were used as substrate materials. The coatings’ deposition process was prepared using the slurry method. After the deposition of water based slurry containing Al and Si powders on the substrates, diffusion treatment has been carried out at 1000°C for 6 h in Ar atmosphere. The structure of the silicon modified aluminide coatings is as follows: (a) an outer layer consisting of a TiAl3 matrix and Ti7Al5Si12 grains dispersed within this matrix, (b) a middle layer composed of a TiAl3 matrix and columnar grains of Ti5Si3 and (c) an inner layer as TiAl2 phase. According to the energy dispersive spectroscopy analysis results, it can be concluded that Cr had a high tendency to move towards the Si enriched compounds. The results of isothermal oxidation test of 1000°C/200 h showed that the formation of Si modified aluminide coatings on γ-TiAl alloys by slurry method can greatly improve their high temperature oxidation resistance.

Centrifugal SHS of cast Ti–Al–Nb–Cr alloys

The technique of centrifugal SHS was used to fabricate an alloy with a composition close that of commercially available alloy 48Ti–48Al–2Nb–2Cr by using Ti–Al–Nb2O5–Cr2O3–CaO2 thermite mixtures. The synthesized alloy exhibited a uniform structure and a composition close to that of commercial prototype. Its main drawback is the presence of undesired admixtures of O, N, P, Ca, C, and Si. Further efforts should be oriented on diminishing a contamination level.

Formation of Diffusion Aluminide Coatings on γ-TiAl Alloy with In-Pack and Out-Pack Processes

In this investigation, Ti–48Al–2Nb–2Cr (at.%) alloy was aluminized by two kinds of pack cementation processes, in-pack and out-pack processes. In both methods, pack powder mixture consisting of 10 wt% Al, 5 wt% AlCl3, and balance Al2O3 was used to aluminize γ-TiAl at 1000 °C for 6 h. The coated samples were characterized by XRD and SEM. The results indicated that a single-layered TiAl3 coating by thickness of approximately 100 μm was formed by the use of in-pack method, but a two-layered aluminide coating including an outer thin layer of TiAl3 with thickness of approximately 2 μm and also an inner layer of TiAl2 with thickness of approximately 10 μm was formed by out-pack process. In both methods, aluminide coatings with uniform structure free from any microcracks were formed on the surface of the substrates. The results showed that the flux of active aluminum atoms on the substrate surface in out-pack method is to be expected to be much slower than in-pack method.

Stability of lamellar microstructures in a Ti–48Al–2Nb–2Cr alloy during heat treatment and its application to lamellae alignment as a quasi-seed

The stability of lamellar microstructures in the Ti–48Al–2Nb–2Cr alloy during heat treatment depends more on heating rate and less on holding time and cooling rate. When the heating rate is higher than 61 °C/min, the original lamellae orientation at room temperature can still maintain unchanged even though the lamellae are transformed to single α grains at a temperature close to melting point. Therefore, the high-temperature α grains, which are transformed from the α2/γ lamellae within a quasi-seed, are successfully employed to align the lamellar microstructure by directional solidification.

Effects of test orientation on fracture and fatigue crack growth behavior of third generation as-cast Ti–48Al–2Nb–2Cr

The effects of changes in test orientation and load ratio on the room temperature fracture and fatigue crack growth behavior of as-cast Ti–48Al–2Nb–2Cr titanium aluminide was investigated to determine the presence of any anisotropy in mechanical properties. As-cast samples were tested in the longitudinal and transverse directions to the casting direction at room temperature in air. Load ratios ranging from R = 0.1 to R = 0.9 were used in the fatigue tests in order to determine its effects on the threshold for fatigue cracking, the Paris law slope, and fatigue crack instability toughness, Kc, in addition to determining both notched and fatigue-precracked values for toughness. Optical metallography and SEM fractography were used to document the effects of orientation on the fracture path and morphology. Significant effects of changes in load ratio were obtained on the fatigue threshold and Paris law slope, while its effects on Kc and the effects of sample orientation were found to be minimal. These are rationalized by considering microstructural effects on the properties measured and are compared to similar materials processed via different techniques.

Vacuum brazing of TiAl-based intermetallics with Ti–Zr–Cu–Ni–Co amorphous alloy as filler metal

An amorphous Ti41.7–Zr26.7–Cu14.7–Ni13.8–Co3.1 (wt%) ribbon fabricated by melt spinning was used as filler to vacuum braze Ti–48Al–2Nb–2Cr (at%) intermetallics. The influences of brazing temperature and time on the microstructure and strength of the joints were investigated. It is found that intermetallic phases of Ti3Al and γ-Ti2Cu/Ti2Ni form in the brazed joints. The tensile strength of the joint first increases and then decreases with the increase of the brazing temperature in the range of 900–1050 °C and the brazing time varying from 3 to 15 min. The maximum tensile strength at room temperature is 316 MPa when the joint is brazed at 950 °C for 5 min. Cleavage facets are widely observed on all of the fracture surfaces of the brazed joints. The fracture path varies with the brazing condition and cracks prefer to initiate at locations with relatively high content of γ-Ti2Cu/Ti2Ni phases and propagate through them.

Microstructures and fracture toughness of Ti–(43–48)Al–2Cr–2Nb prepared by electromagnetic cold crucible directional solidification

In the present paper, the microstructures of directionally solidified Ti–(43–48)Al–2Cr–2Nb alloys prepared by electromagnetic cold crucible directional solidification technique were studied in detail. The results show that with the decrease of Al content, the interlamellar spacing of α2/γ decreases, but the volume fraction of B2 phase increases. In addition, the fracture toughness of these alloys with different lamellar orientation was investigated by single edge notched beam three-point bending test (SENB). It is revealed that B2 phase has greatly impacted on the fracture toughness. When the laminates are perpendicular to the loading stress, the increasing of B2 phase leads to a lower fracture toughness value. On the contrary, B2 phase becomes barriers for crack propagations and increases fracture toughness value when the laminates are parallel to the loading stress.

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.