Ti 48Al 2Cr 2Nb Alloy Research Articles

Anisotropy of microstructure features and tensile properties of Ti–48Al–2Cr–2Nb alloy produced by electron beam powder bed fusion

A Ti–48Al–2Cr–2Nb alloy was produced by electron beam powder bed fusion. The effect of the volume energy density parameter on the forming quality was investigated, and the tissue characteristics, hardness distribution, and mechanical properties of the alloy were analyzed. The results showed that the quality was better when the volume energy density parameter was 21.5–22.5 J/mm3. The microstructure of the alloy has the phenomenon of changing with the forming height: with an increase in the forming height, the forming part produces the phenomenon of strip microstructure along the construction direction, and the regional microstructure variability occurs under the cross section of the same height. The average hardness of the alloy reached 375.2 HV at the bottom, 381.8 HV in the middle, and 389.6 HV at the top, and the standard errors of the hardness values were 12.40, 10.49, and 7.12 HV, respectively, with the microhardness showing a gradual increase along the molding direction, and the homogeneity was gradually better. The transverse tensile strength at room temperature reached 526 MPa, the yield strength reached 461 MPa, and the elongation at break reached 1.24%, while the longitudinal tensile strength reached 486 MPa, the yield strength reached 414 MPa, and the elongation at break reached 1.02%.

In situ tailoring of precipitation behavior and improving mechanical properties of (Ti2AlC+Y2O3) reinforced TiAl alloy produced by directed energy deposition

A (Ti2AlC + Y2O3) synergistically reinforced Ti–48Al–2Cr–2Nb alloy was prepared by directed energy deposition with in situ tailoring of precipitate formation by induction preheating and thermal cycle. The influence of C and Y2O3 coaddition on the microstructure and high-temperature compressive properties was systematically investigated. The results indicated that Ti2AlC was formed by adding elemental C, and the Ti2AlC phase was mainly distributed in two forms: long rod-shaped Ti2AlC with a micron scale that distributed uniformly in the matrix, and lenticular Ti2AlC particles with nanometer size that distributed along the (α2/γ) lamellar interface. The Y2O3 particles were randomly distributed in the matrix. Columnar grains in vertical cross section along the building direction were completely eliminated by adding C and Y2O3. The high temperature compressive strength increased from 597 MPa to 758 MPa at 850 °C by adding C and Y2O3. Finally, the precipitation mechanism of nano-Ti2AlC was discussed in detail. This work provided a valuable reference for the preparation of high-performance TiAl alloys by in situ tailoring of the precipitates during the DED process.

The study on interaction mechanisms between Ti48Al2Cr2Nb alloy and Y2O3 ceramic crucibles with good thermal shock resistance: Assessing a novel, cost-effective approach to the preparation of high-performance TiAl alloys

Enhancing the thermal shock resistance of Y2O3 ceramic crucibles, coupled with advancements in melting and casting technologies to reduce the adverse influence of interfacial reactions, is essential for producing low-energy consumption, high-quality titanium-aluminum (TiAl) alloys. In this study, the Y2O3 ceramic crucible with good thermal shock resistance was developed through the optimization of particle size distribution and sintering holding time. The interaction mechanisms between Y2O3 ceramics and TiAl alloys under extreme conditions were investigated, and strategies for enhancing the mechanical properties of TiAl alloys through the integration of Y2O3 crucibles with centrifugal casting were discussed. The results indicated that introducing large particles of 1–3 mm under sintering conditions of 2023 K and 6 h effectively enhances the thermal shock resistance of Y2O3 crucible. The interaction between Y2O3 and TiAl alloys primarily involved physical erosion (spalling/dispersion), dissolution of separated particles, and surface modification of interface layer particles (reaction). Physical erosion played an important role in this dynamic interaction, with the extent of erosion intensifying through continuous use of the crucible, while electromagnetic forces ensured the uniform distribution of inclusions within the TiAl alloy matrix. Moreover, the application of centrifugal casting technology substantially refines the grain size and reduces the inclusion count, thereby significantly enhancing the mechanical properties of TiAl alloy.

High‐Dissolution Mechanism of Ti–48Al–2Cr–2Nb Alloy in Electrochemical Machining from a Nonlinear Dynamic Perspective

High‐rate dissolution process of Ti–48Al–2Cr–2Nb alloy in electrochemical machining exhibits strong nonlinear dynamic characteristics, but the quantitative relationship between these nonlinear characteristics and surface topography, as well as the electro‐dissolution mechanism, is unknown. Therefore, the fractal and recursive features of dissolution behavior and quantitative characterization of the dissolved surface topography were investigated. Additionally, an electro‐dissolution mechanism model in electrochemical machining process was presented. Applying a higher potential of 11 V can enhance the stability of the electrochemical system, resulting in a smoother and uniform surface. The saturation correlation dimension, surface roughness, and integral area of lacunarity initially decrease before increasing with higher electrolytic pressure, while laminarity and determinism show an initial increase followed by a decrease. These five characteristic parameters reach their optimal values at an electrolyte pressure of 0.4 MPa, indicating that achieving a stable and deterministic electrochemical system is crucial for obtaining a uniform smooth surface. These nonlinear dynamic characteristic parameters can be considered as in‐situ monitoring parameters for surface quality in ECM. The anodic dissolution process comprises three stages: initial passive film dissolution and breakdown, rapid dissolving with surface coarsening, and stable dissolving with polishing.

Creep behavior and creep-oxidation interaction of selective electron beam melted Ti–48Al–2Cr–2Nb alloy at different temperatures

In this work, creep behavior and creep-oxidation interaction behavior of Ti–48Al–2Cr–2Nb alloy fabricated by selective electron beam melting at different temperatures were investigated in detail. This alloy exhibits a near-γ microstructure composed of coarse/fine mixed equiaxed grains. Creep test results show that creep behavior of this alloy is sensitive to temperature. This is mainly manifested in that creep life reduces significantly by 84.8 % as temperature increases from 800 °C to 850 °C. During creep, discontinuous dynamic recrystallization characterized by grain nucleation and growth and continuous dynamic recrystallization characterized by strain gradient and orientation accumulation occur simultaneously. The formation of mechanical twins plays an important role in stimulating dynamic recrystallization. Meanwhile, creep deformation and fracture mechanisms of this alloy at different temperatures were also discussed. On the other hand, oxidation test results reveal that the applied tensile stress has a positive effect on the oxidation behavior of this alloy: the growth of TiO2 oxide is inhibited and the oxidation rate is slowed down. However, under creep-oxidation interaction, the intergranular cracking of the oxide film occurs along with the necking of the alloy matrix during creep.

Modification of microstructure and mechanical properties for twin-wire directed energy deposition-arc fabricated Ti–48Al–2Cr–2Nb alloys via current regulation

Considering the advantages of low density and excellent high-temperature performance, titanium aluminide is identified as an ideal aerospace structural material. In recent years, twin-wire directed energy deposition-arc (TW-DED-arc), which possesses the characteristics of low cost but high deposition efficiency, has continuously attracted worldwide attention. However, obtaining equiaxed grains with small Al variation for titanium aluminide using TW-DED-arc method is still a key concern. In the present research, Ti–48Al–2Cr–2Nb alloy with equiaxed lamellar colonies and less than 1 at.% of Al content was successfully fabricated by regulating deposition current of TW-DED-arc. Also, the effect of deposition current on microstructure and mechanical properties was systematically investigated. The experimental results that focus on top and middle regions of as-fabricated TiAl-4822 alloys, indicate that increasing current favors in generation of equiaxed grains and γ lamellae, which leads to columnar to equiaxed transition and higher γ phase content, respectively. Also, elemental homogeneity can be significantly homogenized. As deposition current increases, tensile strength increases while degree of tensile anisotropy decreases significantly, benefiting from the equiaxed grains with satisfying size. Importantly, microstructure evolution, tensile anisotropy and fracture characteristics of TW-DED-arc fabricated TiAl-4822 alloys are discussed in detail. Generally, these findings provide an effective process method to optimize microstructure and mechanical properties of TW-DED-arc fabricated TiAl-4822 alloys.

Friction properties of SLM formed Ti48Al2Cr2Nb alloy

Friction properties of SLM formed Ti48Al2Cr2Nb alloy

Effect of applied load on tribological behaviour of Ti–48Al–2Cr–2Nb alloy under oil-lubricated condition

With their low weights and excellent mechanical properties, TiAl alloys are expected to be applied in pistons to achieve energy savings and emissions reductions in internal combustion engines. The tribological behaviours of the Ti–48Al–2Cr–2Nb alloy against the QT600-3 material at different applied loads under oil-lubricated conditions were investigated in this study. The results showed that as the load increased, the friction coefficient and wear-scar volume significantly decreased. The wear surface of the TiAl alloy was analysed using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The wear mechanism of the TiAl alloy under oil lubrication conditions was mainly the combined effect of adhesive wear and oxidation wear, different from the wear mechanism under dry-friction conditions. Furthermore, by observing the cross-sectional morphology of the TiAl alloy after focused ion beam, it was found that the QT600-3 wear debris had compacted and accumulated on the surface of the TiAl alloy, preventing further contact between the TiAl alloy and the QT600-3. This study has the potential to accelerate the application of the Ti–48Al–2Cr–2Nb alloy in lightweight pistons.

Simultaneously enhanced tensile strength and ductility of nano-Y2O3-reinforced TiAl alloy prepared by directed energy deposition

The effect of Y2O3 nanoparticles on microstructure and high-temperature mechanical properties of Ti–48Al–2Cr–2Nb alloy prepared by directed energy deposition was investigated. Cracks in the TiAl alloy sample were eliminated by a new directed energy deposition method with a preheating unit. The results indicated that the microstructure of TiAl alloy without Y2O3 nanoparticles addition was composed primarily of (α2/γ) lamellar colony. While the equiaxed γ phase was generated within the (α2/γ) lamellar colony and on the colony boundaries, and the microstructure exhibited a duplex structure due to the 0.5 wt% nano-Y2O3 addition, and the grain size was decreased from 183 μm to 25 μm. The ultimate tensile strength and fracture elongation tested at 750 °C were enhanced simultaneously from 615.1 to 805.7 MPa and from 5.56% to 7.40%, respectively, by 0.5 wt% Y2O3 nanoparticles addition. Finally, the strengthening mechanism and microstructure refinement mechanism were discussed in detail. This investigation provides a valuable reference for TiAl alloy fabrication and widespread engineering applications.

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.

Microstructure evolution and precipitation behavior in a Y2O3-bearing TiAl alloy during creep

The paper studies the microstructure stability of a Y2O3-bearing Ti–48Al–2Cr–2Nb alloy during creep. Tensile creep tests were performed at 800–850 °C under 100 or 275 MPa. Compared to thermal exposure, the low creep stress promotes the microstructure degeneration of lamellar colonies, mainly including α2-lath dissolution, γ-lath coarsening and βo phase precipitation. The high creep stress further accelerates the α2 → γ transformation, but goes against the precipitation of βo grains inside α2 laths. Similarly, more Cr-rich precipitates with the composition of Ti(Al, Cr)2 are observed in creep samples, which are identified as the C14-Laves phase. It has a {0001}C14//{111}γ and <101¯0>C14//<11¯0>γ orientation relationship with the γ phase. The high imposed stress also facilitates the precipitation behavior. It should be noted that there are a few C14-Laves phases precipitating along the Y2O3 interface by heterogeneous nucleation. The high strain energy around Y2O3 particles, especially under the high creep stress, contributes to the precipitation of C14-Laves phase not only at the Y2O3 interface, but also in the γ matrix.

Improvement on high-temperature tensile strength and ductility of selective electron beam melting TiAl alloys via small-strain forging

In the present work, a Ti–48Al–2Cr–2Nb alloy prepared by selective electron beam melting (SEBM) and post-treated by forging at a small strain was investigated. The results show that the microstructure of the as-built TiAl alloys exhibited an alternating layer of coarse single γ grains and fine γ+α2 grains configuration along the building direction. After forging at a small strain, the fraction of the coarsely grained region decreases, and substantial fine sub-grains form in the fine-grained region. Additionally, fractions of the α2 phase and α2/γ lamellar colonies increase. These factors lead to an increase in the tensile strength of alloys. Meanwhile, the low-angle grain boundaries (LAGBs) in fine sub-grains and the substantial mobile dislocations in coarse γ grains are favorable for improving the ductility of alloys. The formation of deformation twins in the γ grains facilitates the simultaneous improvement of strength and ductility. In addition, the anisotropy of tensile properties is reduced after small-strain forging, which can be attributed to the reduction in fraction of the coarse γ grains, and the increase in the fractions of the α2 phase and α2/γ lamellar structure. Therefore, small-strain forging was verified as an effective post-treatment method for the improvement of tensile strength and ductility of SEBM prepared TiAl alloys at high temperatures.

Pollution-free grain refinement of cast TiAl alloys by vacuum mechanical vibration solidification

The implementation of the well-known methods for grain refinement of cast TiAl alloys, such as boron refiner, leads to compositional pollution and deterioration of specific types of performances. The development of pollution-free grain refinement methods is of crucial importance in aviation industries. Along these lines, in this work, a pollution-free grain refinement method was developed using the vacuum mechanical vibration (VMV) solidification technique. More specifically, a significant microstructure refinement of a typical Ti–48Al–2Cr–2Nb alloy was achieved, with the average grain size refined from the initial value of 141 μm–72 μm. As a result, the yield strength and compressive strength were both increased, almost without the sacrifice of the peak strain, which nearly overcome the strength-ductility trade-off with simple processing.

Improvement in the high temperature mechanical properties of additively manufactured Ti–48Al–2Cr–2Nb alloy using heat treatment

Gamma Ti–48Al–2Cr–2Nb (Ti4822) alloy, which is manufactured by an electron beam melting (EBM) process, has excellent ductility but poor high-temperature strength, because it has a near gamma structure. The objective of this study was to control the microstructure of Ti4822 by considering various forms of heat treatment to improve the mechanical properties at room and high temperatures. The heat treatment conditions were varied to create nearly lamellar (NL) and fully-lamellar (FL) structures. First, the EBM-built NL-Ti4822 exhibited a lamellar structure with an average layer thickness of about 92 nm and equiaxed γ-phase particles with a size of several tens of micrometers. On the other hand, the EBM-built FL-Ti4822 had an average lamellar thickness of about 347.2 nm, but in this case the equiaxed γ phase was not observed. The results of the room- and high-temperature compression test showed that the mechanical properties of the two heat-treated materials were significantly superior to those of the as-built Ti4822 across all temperature ranges. Moreover, both materials were highly ductile at room and high temperature despite having a lamellar structure, and exhibited intensified yield stress anomaly and dynamic recrystallization as the fraction of lamellar structure increased. Based on the above results obtained by subjecting the materials to different heat treatment methods, the effects of the microstructures of the two materials on the mechanical properties and deformation mechanisms at room and high temperatures were investigated.

Investigation on Hot Deformation and Processing Parameter Optimization of Ti48Al2Cr2Nb Alloy with Bimodal Grain Size Distribution Fabricated by Powder Metallurgy

Investigation on Hot Deformation and Processing Parameter Optimization of Ti48Al2Cr2Nb Alloy with Bimodal Grain Size Distribution Fabricated by Powder Metallurgy

Creep properties of Ti–48Al–2Cr–2Nb alloy having similarly oriented lamellae with fine lamellar spacing

The tensile creep behavior of suction-cast and heat-treated Ti–48Al–2Cr–2Nb (at. pct., SC4822) alloy featuring aligned lamellar microstructure with fine lamellae was investigated. The alloy showed excellent creep performance in the temperature range of 775–825 °C under applied stresses varying from 200 to 350 MPa. The creep properties of SC4822 alloy greatly outperformed that of several other typical TiAl alloys with randomly oriented lamellae. The stress exponent (n) for SC4822 alloy was calculated to be 5.1 at 800 °C under different stresses, whereas the apparent activation energy (QC) was found to be 397.18 kJ/mol under 300 MPa at different temperatures. This suggested that creep deformation was dominantly controlled by dislocation climb, as confirmed by subsequent microstructure observation. Besides, dynamic recrystallization, twinning and slipping were also found to contribute to the deformation of creep.

Effect of heat treatment on the microstructure and creep properties of Ti–48Al–2Cr–2Nb alloy produced by selective electron beam melting

This study investigated the effect of heat treatment on the microstructure and creep behavior of Ti–48Al–2Cr–2Nb alloy prepared by selective electron beam melting. The microstructure of the as-built and annealed TiAl alloys exhibited near γ structure and fully (α2/γ) lamellar structures, respectively. The creep properties were evaluated on the as-built and annealed TiAl alloys under a 250–300 MPa creep stress at 800 °C. The results showed that the creep property was improved significantly by post-heat treatment. The creep rupture life of heat-treated TiAl alloy was 1.6, 2.2, and 2.4 times that of as-built TiAl alloy under the stress of 250, 275, and 300 MPa at 800 °C, respectively. The creep stress exponents of the as-built TiAl alloy and heat-treated alloy were calculated and they were 8.94 and 5.96, respectively. Characterizing the fracture surfaces and deformed microstructure after the creep tests, the as-built alloy exhibited ductile fracture characteristics with numerous dimples. Whereas the heat-treated TiAl alloy displayed brittle fracture characteristics. Finally, the creep fracture behavior was discussed in detail.

Twin-wire directed energy deposition-arc of Ti–48Al–2Cr–2Nb alloy: Feasibility, microstructure, and tensile property investigation

Due to low density and good high temperature performance, titanium aluminide has been continuously attractive for aerospace and automotive industries. In recent years, additive manufacturing of titanium aluminide has become a popular research area. The present work has first time fabricated commercial graded TiAl-4822 alloy using an innovative twin-wire directed energy deposition-arc (TW-DED-arc) process, which uses a unique twin-wire feeding method to produce the target alloy in-situ. According to experimental results, TW-DED-arc fabricated TiAl-4822 alloy possesses direct full density, homogeneous chemical composition, uniformed microstructure, and stable mechanical property. Also, the formation mechanism of layered microstructure consisting of alternative columnar and equiaxed grain regions has been investigated in detail. Even under as-fabricated condition, TW-DED-arc fabricated TiAl-4822 performs competitive tensile properties compared to the same alloy produced by casting and other additive manufacturing methods. Generally, successful TW-DED-arc buildup of TiAl-4822 provides a high-efficiency and low-cost route for the further development of titanium aluminide production.

Microstructure analysis and mechanical properties of electron beam powder bed fusion (PBF-EB)-manufactured γ-titanium aluminide (TiAl) at elevated temperatures

Abstract Additively manufactured γ-titanium aluminide has a high specific strength and temperature resistance. This opens new possibilities for future lightweight constructions for aerospace applications. The objective of this work was to characterize additively manufactured Ti–48Al–2Cr–2Nb alloy specimens, which were successfully manufactured by electron beam powder bed fusion. For microstructural characterization, the as-built state was investigated with light and scanning electron microscopy. In the electron backscatter diffraction analysis, the size and the orientation of the grains were observed. The pore size and distribution were examined in computer tomographic scans, which showed a near fully dense material with a relative density of &gt;99.9%. Furthermore, the hardness curve over the building height was examined in hardness mappings. Thereby, a strong decrease in hardness could be observed with an increase in part height. To evaluate the reliability of the manufactured alloy, quasi-static compression tests were carried out at temperatures up to 650 °C. Within these tests, a high compression strength (σ c,p,0.2,650 °C = 684 MPa) was determined, which implicated a potential substitution of nickel-based superalloy components in aerospace applications under compressive loads.

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.