2Cr 2Nb Research Articles

Structure and mechanical properties of the AlCrxNbTiV (x = 0, 0.5, 1, 1.5) high entropy alloys

The AlCrxNbTiV (x = 0, 0.5, 1, 1.5) high entropy alloys were produced by vacuum arc melting. The crystal structure, microstructure, density and compression mechanical properties at 22°C–1000 °C of the AlCrxNbTiV alloys after homogenization annealing at 1200 °C for 24 h are reported. After homogenization the AlNbTiV and AlCr0.5NbTiV alloys consist from single bcc solid solution phase, while the AlCrNbTiV and AlCr1.5NbTiV alloys contain bcc phase and respectively 13% and 35% of hexagonal (C14) Laves phase. The density of alloys increases with increase of Cr content from 5590 kg m−3 of the AlNbTiV alloy to 5900 kg m−3 of the AlCr1.5NbTiV alloy. The strength of the AlCrxNbTiV alloys increases with increase of Cr concentration at temperatures from room to 800 °C. The yield strength of the AlNbTiV and AlCr1.5NbTiV alloys is 1000 MPa and 1700 MPa respectively at room temperature and 560 MPa and 970 MPa at 800 °C. Strengthening of the AlCrxNbTiV alloys with increase of Cr content is accompanied by decrease of ductility. Phase composition of the alloys was affected by deformation at 800 °C and 1000 °C. In the AlNbTiV and AlCr0.5NbTiV alloys Nb2Al-type phase precipitated predominantly on grain boundaries, while in the AlCrNbTiV and AlCr1.5NbTiV alloys additional amounts of Laves phase appeared inside grains of bcc matrix. The experimentally observed phase composition of the AlCrxNbTiV alloys is compared with results of thermodynamical modeling of equilibrium phases in the alloys. The effect of phase composition of the alloys on mechanical properties is discussed and possibilities for properties optimization of the Al–Cr–Nb–Ti–V high entropy alloys are suggested.

Dissimilar joining of Ti3Al-based alloy to Ni-based superalloy by arc welding technology using gradient filler alloys

Abstract In this study, Ti–Al–Nb, Ti–Ni–Nb and Ni–Cr–Nb system alloys were designed and incorporated in order to construct a gradient structure at the surface of the joined Ti3Al base material. And the Ti3Al-based alloy and Ni-based superalloy were successfully joined together using gas tungsten arc (GTA) welding technology. The microstructure evolution, mechanical properties and fractured behaviors of the joints were investigated. The gradient structure remarkably decreased the formation tendency of brittle phases within the joints compared with a single filler alloy and thus improved the joint strength effectively. The average room-temperature tensile strength of the Ti3Al/In718 dissimilar joint reached 353 MPa, and the strength value at 873 K was 245 MPa. At the Ti–Ni–Nb/Ni–Cr–Nb interface, some Ni3(Nb, Ti) + (Nb, Ti)Cr2 and TiNi3 phases were detected in the Ti–Ni–Nb matrix. It was believed that their presence decreased the room-temperature strength of the Ti–Ni–Nb alloy but improved its high-temperature strength.

Microstructure characterization, mechanical properties and toughening mechanism of TiB2-containing conventional cast TiAl-based alloy

Effects of TiB2 addition on microstructure and mechanical properties of Ti–48Al–2Cr–2Nb+(0.72,1.62)wt% TiB2 alloys fabricated by the induction skull melting (ISM) process were investigated. Results showed that the TiB2-induced microstructure was characterized by randomly orientated fully lamellar colonies and both the colony size and lamellae spacing were refined (100μm and ~185nm, respectively) by TiB2 addition. The borides were identified to be TiB2 with plate, needle and block morphologies, determined by different growth stages during solidification. At room temperature and 700°C, the TiB2-containing alloys exhibit non-deteriorated fracture toughness and superior tensile properties than that of the as-cast and heat-treated matrix alloys. Furthermore, the fracture toughness anisotropy was eliminated due to the randomly orientated lamellar microstructure induced by TiB2 addition. The fine TiB2 particles with special morphology (plate and needle) and the easy-to-deform ligament bridges induced by the refined microstructure can account for the notable fracture toughness of the studied TiB2-containing alloys. The main toughening mechanism was analyzed and discussed in light of the microstructure characterization, size and morphology of borides and the deformed ligament bridges.

Effect of growth rate on microstructure and tensile properties of Ti–45Al–2Cr–2Nb prepared by electromagnetic cold crucible directional solidification

Abstract Ti–45Al–2Cr–2Nb alloy was directionally solidified at different growth rates varying from 0.2 to 1.2 mm/min by applying an electromagnetic cold crucible directional solidification technique. It was determined that well-aligned α2 (Ti3Al)/γ(TiAl) lamellar structures, B2 phase and blocky γ phase were generated in columnar grains. The interlamellar spacing (λ) decreases with the increasing growth rate (V) according to the relationship λ ∝ V− 0.48, but the volume fraction of B2 is increased as growth rate is increasing. Results of uniaxial tensile tests show that the B2 phase and the blocky γ phase have significant influence on tensile failure when they are presented in the matrix of α2/γ lamellar structures, because they are usually employed to act as cracking sources during the tension process.

Investigation on dynamic properties and failure mechanisms of Ti–47Al–2Cr–2Nb alloy under uniaxial dynamic compression at a temperature range of 288 K–773 K

Uniaxial compression tests were performed on Ti–47Al–2Cr–2Nb alloy at a temperature range of 288 K–773 K. The mechanical response and microstructure of Ti–47Al–2Cr–2Nb alloy after uniaxial compression were systematically analyzed. Investigation on mechanical properties show that under dynamic compression, the yield strength and ductility of Ti–47Al–2Cr–2Nb alloy remain almost same at all loading temperatures, while the strain hardening rate of the Ti–47Al–2Cr–2Nb alloy present an obvious declining trend with the increased loading temperatures, resulting in the decreasing of ultimate compressive strength. Failure analysis reveals that under quasi-static compression, the failure mechanism of Ti–47Al–2Cr–2Nb alloy is micro-porous coalescence failure in α2 phases. Under dynamic compression at a temperature range of 288–773 K, shear failure is the main failure mode of Ti–47Al–2Cr–2Nb alloy. It is interesting to find that at a temperature range of 288 K–373 K, the failure mechanism of Ti–47Al–2Cr–2Nb alloy is brittle shear failure owing to the rapid propagation of micro-cracks along the lamella interface. When the loading temperature increases over 473 K, adiabatic shear band (ASB) is observed, indicating that the failure mechanism of Ti–47Al–2Cr–2Nb alloy transfers from brittle shear failure to adiabatic shear failure.

Oxidation response of three Nb–Cr–Mo–Si–B alloys in air

The oxidation behavior in air of three alloys from the Nb–Cr–Mo–Si–B system has been evaluated in the temperature range of 700–1400°C. The alloy compositions in atomic percent are Nb-25Cr-15Mo-20Si-15B (2015), Nb-25Cr-15Mo-20Si-10B (2010), and Nb-25Cr-15Mo-15Si-15B (1515). Increased oxidation resistance has been obtained for the 2015 alloy at high temperatures. Low temperature pest oxidation is not observed for the 2010 and 2015 alloy and only minimal pest products are obtained in the 1515 alloy. Analysis indicates the oxide scale includes Nb2O5 (monoclinic), CrNbO4, and SiO2. As-cast microstructural phases include Si-rich C14-NbCr2 Laves phase, (Nb, Mo)5Si3 silicide and (Nb, Mo)3Si silicide.

Effects of laser shock peening on microstructure and residual stress evolution in Ti–45Al–2Cr–2Nb–0.2B alloy

Laser peening is a novel surface treatment technique for improving the mechanical properties of metal parts' surface. To investigate and evaluate the effect of laser peening on the microstructural changes, texture evolution, and residual stress distribution changes, laser peening experiment was undertaken using Nd:YAG laser system with the pulse–width of 20ns and max pulse–energy of 9J. Micro-hardness measurements of the untreated and treated specimens were carried out with Vickers indenter. Depth-resolved characterization of the residual stresses and strains was achieved using X-ray diffraction. The structure–texture–phase–stress combined analysis was performed based on the X-ray diffraction patterns using the whole pattern fitting method. The relationship between laser peening processing parameters, microstructure, texture, residual stress distribution and hardness are presented and discussed. The results showed that laser peening could improve microstructures and properties of TiAl alloy.

Experimental investigation of phase equilibria in the Zr–Nb–Cr system at 1573 K and 1373 K

Phase equilibria in the Zr–Nb–Cr system at 1573 K and 1373 K were characterized by X-ray diffraction (XRD) and electron probe microanalysis (EPMA) coupled with wavelength-dispersive spectroscopy (WDS). Three single-phase regions and two two-phase regions were identified in the isothermal sections of 1573 K and 1373 K. The single-phase regions are BCC (Cr), C15 and BCC (Zr, Nb), and the two-phase regions are BCC (Cr)+C15 and BCC (Zr, Nb)+C15. With the temperature decreasing from 1573 K to 1373 K, the BCC (Zr, Nb) phase region narrows slightly, while the C15 phase region shrinks noticeably in the Nb-rich direction. The problems concerning sample equilibration and weight loss in the experiments were discussed, and the C15 single-phase region was explored based on the site occupancy modelled by the compound energy formalism (CEF) coupled with first-principles calculations.

Thermodynamic modeling of Cr–Nb and Zr–Cr with extension to the ternary Zr–Nb–Cr system

The total energies of Laves phases in the Cr–Nb and Zr–Cr systems have been calculated by the pseudo-potential VASP code with a full relaxation of all structural parameters. The special quasirandom structures (SQSs) have been constructed and their total energies have been calculated by the VASP code to predict the enthalpies of mixing for bcc and hcp solid solution phases. The phonon calculations for the C14 and C15 Laves phases have been performed to analyze the phase stability at elevated temperatures. The experimental study on the Zr–Cr system has been carried out at different temperatures to determine the phase boundaries. Based on these results, thermodynamic models of Cr–Nb and Zr–Cr with extension to the ternary Zr–Nb–Cr systems have been developed in this work by using the CALPHAD approach.

Grain refinement by trace TiB2 addition in conventional cast TiAl-based alloy

Effects of trace TiB2 addition on the solidification microstructure of Ti–48Al–2Cr–2Nb+(0.18, 0.54, 0.9) wt.% TiB2 alloys were investigated by arc melting and electromagnetic levitation melting in this paper. Results showed that the matrix alloy generated a large-grained (~800μm) fully lamellar microstructure, and the grain size decreased with increasing TiB2 additions. With 0.18wt.% TiB2 addition, the grains remained fully lamellar, at about 700μm in diameter. For more TiB2 addition (0.54wt.%), grain size decreased to an average value of 200μm. The grain refinement could be explained by constitutional undercooling and α phase nucleation on the β laths of secondary TiB2/β borides. With 0.9wt.% TiB2, the alloy melt produced primary TiB2 crystals during solidification to nucleate β grains. The β grains in turn nucleated and were transformed to α grains with the generation of secondary TiB2 flakes. This resulted in equiaxed, refined (~200μm) lamellar grains without element segregation. The elimination of element segregation could be attributed to the fine equiaxed dendrites, corresponding to the result of electromagnetic levitation melting quenching experiments. With equivalent B additions, the grain refinement effect was equal, except for the cavity defects.

Efeito do Tratamento Térmico de Solubilização e Estabilização Convencional e Alternativo na Microestrutura de uma Junta Soldada com Aço Inox 347

Este trabalho estuda a microestrutura formada na zona termicamente afetada (ZTA) e na zona fundida (ZF) de uma junta soldada pelo processo TIG automático no passe de raiz, processo plasma nos passes de enchimento e soldagem SAW para os passes de acabamento em junta de topo com chanfro “V” simples, usando o aço inoxidável austenítico AISI 347. Foram estudadas as condições: como soldada; após tratamento térmico convencional de solubilização a 1060 °C e estabilização a 900 °C; e após tratamento térmico alternativo, onde a peça foi solubilizada a 1060 °C e mantida no forno que foi resfriado até 900 °C para a estabilização. Os resultados mostram que, na condição como soldado houve crescimento de grão na ZTA e a ZF apresentou 13% de ferrita �� com diferentes morfologias, tendo sido observada a precipitação de carbetos de Cr e de Nb. Após o tratamento térmico convencional de solubilização e estabilização, foi observada uma diminuição significativa do teor de ferrita �� na ZTA e na ZF, tendo sido observados carbetos de Nb e Cr, distribuídos de forma mais dispersa e com tamanhos menores, que contribuem para minimizar a possibilidade de ocorrer o fenômeno da sensitização. Após tratamento térmico alternativo de solubilização e estabilização, a microestrutura da ZTA e ZF foi similar à obtida no tratamento convencional, sendo que o teor de ferrita δ na ZF foi ainda mais reduzido, de 4,5%. Foi concluído que o tratamento térmico alternativo de solubilização e estabilização foi tão eficaz quanto o tratamento térmico convencional para a adequação da microestrutura da junta soldada, com uma significativa vantagem do tratamento térmico proposto em reduzir o tempo e o custo desta operação.

Microstructure Characterization and Fracture Toughness of Laves Phase-Based Cr–Nb–Ti Alloys

Three Laves phase-based alloys with nominal compositions of Cr2Nb–xTi (x = 20, 30, 40, in at%) have been prepared through vacuum non-consumable arc melting. The results show that the microstructures of Cr2Nb-(20, 30) Ti alloys are composed of the primary Laves phase C15–Cr2(Nb,Ti) and bcc solid solution phase, while the microstructure of Cr2Nb–40Ti alloy is developed with the eutectic phases C15–Cr2(Nb,Ti)/bcc solid solution. The measured fracture toughness of ternary Laves phase C15–Cr2(Nb,Ti) is about 3.0 MPa m1/2, much larger than 1.4 MPa m1/2 for binary Laves phase Cr2Nb. Meanwhile, the fracture toughness of Cr2Nb–xTi (x = 20, 30, 40) alloys increases with increasing Ti content and reaches 10.6 MPa m1/2 in Cr2Nb–40Ti alloy. The eutectic microstructure and addition of Ti in Cr2Nb are found to be effective in toughening Laves phase-based alloys.

The temperature effect of hot isostatic pressing of γ-TiAl based alloy castings on phase composition and structure

Isothermal sections of the Ti–Al–Nb–Cr and Ti–Al–Nb–Mo systems in a range of temperatures from 1100 °C to 1400 °C as regard to hot isostatic pressing process of two characteristic alloys on the basis of γ-TiAl (48-2-2 type and TNM) have been calculated. Their phase composition including mass fractions of different phases (α, β, γ) is shown to depend strongly on the hot isostatic pressing temperature. Taken TNM alloy as an example, it is shown that the experimental data confirm the calculation results as a whole.

The solidification path related columnar-to-equiaxed transition in Ti–Al alloys

Columnar-to-Equiaxed Transition (CET) of binary Ti–Al alloys and multi-component Ti–48Al–2Cr–2Nb alloys is studied using Bridgman solidification technique. The effect of aluminum concentration and growth rate on CET is determined. It is found in Ti–46Al and Ti–50Al alloy ingots equiaxed grains develop ahead of the moving solid–liquid interface with a growth rate of 500 μm/s; microstructures in Ti–49Al alloy stay columnar dendrites with the same growth rate. CET in Ti–Al alloys are not only influenced by growth rate, but also by the solidification path that is related to alloying composition. CET in Ti–Al alloys is predicted using the dendritic growth model based on the criterion of growth at marginal stability. According to the calculated results and directionally solidified microstructures, values of the nucleation undercooling for α and β phases are given. The growth rates to avoid CET in Ti–48Al–2Cr–2Nb alloy are suggested.

Use of the Fe–Cr–Nb–B Systems With Low Curie Temperature as Mediators in Magnetic Hyperthermia

Magnetic hyperthermia uses the therapeutic heat given by the magnetic particles (MPs) in the alternating magnetic field for the healing of various cancers. The low Curie temperature particles are intelligent tools for the malignant cell destruction. These magnetic systems are used in the magnetic hyperthermia due to their special magnetic properties as high magnetization and magnetic permeability/susceptibility and low Curie temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> within the range of 42 °C-46 °C. Capability of the Fe-Cr-Nb-B systems to heat uniformly in the hyperthermia range, a complex region with multiple metastases (two spherical metastatic tumors) surrounded by a cubic healthy tissue was the main purpose of this paper. The temperature inside as well as outside the region was computed by finite element method. A significant blood vessel (BV) is located at different distances from tumors. The cooling effect produced by blood flowing in a BV was considered. The presence of the BV induces the thermal gradients within tumors. Their values depend on the tumor-BV distance. The frequency and amplitude of magnetic field and MP concentration are optimized to get the temperature therapeutic range 40 °C-42 °C within tumors as liver, lung, and breast. The heating is automatically stopped when the temperature reaches T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 42 °C within tumors. Low particle concentrations inserted in the small injection sites are able to heat the tumor to the therapeutic range.

In vitro cytotoxicity of Fe–Cr–Nb–B magnetic nanoparticles under high frequency electromagnetic field

Abstract The heating potential, cytotoxicity, and efficiency of Fe 68.2 Cr 11.5 Nb 0.3 B 20 magnetic nanoparticles (MNPs), as such or coated with a chitosan layer, to decrease the cell viability in a cancer cell culture model by using high frequency alternating magnetic fields (AMF) have been studied. The specific absorption rate varied from 215 W/g for chitosan-free MNPs to about 190 W/g for chitosan-coated ones, and an equilibrium temperature of 46 °C was reached when chitosan-coated MNPs were subjected to AMF. The chitosan-free Fe 68.2 Cr 11.5 Nb 0.3 B 20 MNPs proved a good biocompatibility and low cytotoxicity in all testing conditions, while the chitosan-coated ones induced strong tumoricidal effects when a cell–particle simultaneous co-incubation approach was used. In high frequency AMF, the particle-mediated heat treatment has proved to be a critical cause for decreasing in vitro the viability of a cancer cell line.

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.

Corrosion properties of Fe–Cr–Nb–B amorphous alloys and coatings

In the present work, we report on the corrosion properties of the Fe60Cr8Nb8B24 (at.%) alloy produced using pure and commercial materials in the following conditions: amorphous ribbons, partially crystallized ribbons and coatings produced by spray deposition and powder flame spraying process, in this case LVOF (low velocity oxygen fuel). The amorphous ribbons showed excellent corrosion resistance with formation of a stable passive film that ensured a very large passivation plateau. The (LVOF) coatings presented high fraction of amorphous phase with a layered structure, high porosity (16.2%) and low oxidation level (~0.1%). The spray formed coatings presented crystalline structure with low porosity (1.9%) and low oxidation level (~0.1%). The coatings showed higher corrosion current densities (up to two orders of magnitude) compared to the amorphous ribbons of the same composition for all pH. This deterioration in the corrosion properties were found to be impaired by the presence of crystalline phases.

Microstructure and oxidation behavior of a SiC–Al2O3–glass composite coating on Ti–47Al–2Cr–2Nb alloy

A SiC–Al2O3–glass composite coating was prepared on Ti–47Al–2Cr–2Nb alloy by a slurry made from aqueous solution of potassium silicate and powders of SiC and Al2O3. The oxidation behavior of the specimens was studied at 800°C, 900°C and 1000°C. The composite coating prevented the inward diffusion of oxygen from air to the coating/alloy interface, therefore, decreasing the oxidation rate of the alloy remarkably. Besides, the composite coating exhibited good thermal cyclic resistance on Ti–47Al–2Cr–2Nb alloy. The microstructure changes of the composite coating, including oxidation of the SiC inclusions and the coating/alloy interfacial reaction, were investigated.

Effect of boron doping on microstructure of directionally solidified Ti–46Al–2Cr–2Nb alloy

In this paper, Ti–46Al–2Cr–2Nb and Ti–46Al–2Cr–2Nb–0·2B alloys were directionally solidified under different cooling rates. Based on the comparison, the effects of minor B doping on the primary dendrite arm and lamellar spacing were investigated. TiB was formed during solidification before the formation of the lamellar structure. The primary dendrite arm spacing was slightly reduced by TiB when the α phase was solidified as the primary phase. However, the lamellar spacing significantly increased in the Ti–46Al–2Cr–2Nb–0·2B alloy because TiB reduced the undercooling temperature required for the formation of the γ phase.