Addition Of Ta Research Articles

Identification of interdendritic phases in Ni[sbnd]30Cr weld metal with additions of tantalum and molybdenum using electron diffraction pattern and high-resolution scanning transmission electron microscopy image analysis

Ta-rich γ/MC eutectic is the predominant interdendritic constituent that forms upon solidification in Ni30Cr weld metal with Ta and Mo additions. In addition, a very small amount of sigma phase (σ) forms on weld solidification. These terminal solidification constituents were investigated in detail by a combination of various methods in transmission electron microscopy (TEM): selected area electron diffraction (SAED) analysis and associated SAED pattern simulation, high-angle annular dark-field (HAADF) STEM imaging and associated image simulation, as well as energy-dispersive X-ray spectroscopy (STEM-EDS). Ta-rich γ/MC and tetragonal sigma (σ) phase were observed based on acquisition, analysis and simulation of six SAED patterns by specimen tilt into six defined zone axis orientations. The presence of sigma (σ) phase was further validated and its atomic configuration was revealed by high-resolution HAADF-STEM imaging in combination with HAADF-STEM image simulation experiments.

Laves phase strengthening in ultrafine-grained Co–Cr–Ta micropillars under uniaxial compression at modest temperature

The substantial reduction in strength has limited the widespread usage of Co–Cr based alloys at elevated temperatures. Here, we designed and fabricated an ultrafine-grained Co–28Cr–3Ta alloy which could retain over 80% of its room temperature yield strength at 300 °C, two times that of its equivalent without addition of Ta. Such high thermal stability was achieved via a processing route which precipitated nanoscale Laves phases. The results demonstrate that Orowan strengthening operates as the dominant mechanism in the alloy. The dislocation loops formed from interactions between dislocations and precipitates could effectively prevent the dislocation gliding at modest temperature.

Effect of alloying additions and presence of water vapour on short-term air oxidation behaviour of cast Ni-base superalloys

Ni-14Cr-9Co (wt.%) based alloys with additions of Al, Ti and Ta were exposed in dry and wet (+20 vol.%H2O) air at 950 °C for up to 100 h and characterized with GD-OES and SEM/EDX. Ni-14Cr-9Co-alloy exhibited initially Cr2O3-scale formation followed by breakaway Ni-oxidation. 3%Al addition to Ni-14Cr-9Co stabilized the Cr2O3-scale and largely suppressed Ni-oxide formation. Ni-14Cr-9Co-3Al-5Ti formed a fast growing Cr2O3-scale attributed to p-type Ti-doping, but Ni-oxide formation was completely inhibited. 5%Ta addition to Ni-14Cr-9Co-3Al promoted formation of a thin alumina scale. Addition of H2O to the test gas increased growth of Ni-rich oxides and shortened time to breakaway for the Al-free alloy.

Effect of Mo, Ta, V and Zr on a duplex bcc+orthorhombic refractory complex concentrated alloy using diffusion couples

A scanning method based on diffusion couples has been set up to experimentally explore phase diagrams and to observe the stable phases in quinary alloys. This method enables to realize a fast, experimental and reliable scanning of composition and then determine promising alloy compositions for potential structural application. Aiming at developing promising refractory complex concentrated alloys (RCCAs), the method was applied on the Ti–Nb–Al–Si system with an initial two phase, body-centered cubic (bcc) + orthorhombic (O), microstructure and focuses on the effect of Mo, Ta, V and Zr additions on this microstructure. The evolution of the orthorhombic phase fraction according to the content of additional elements was quantified, as well as the maximum concentration of those elements in order to stabilize the duplex microstructure, and maintain it above 800 °C. In such framework, it appears that Mo, Ta and V show the most promising effects on the Ti–Nb–Al–Si system. The comparisons between experimental results and calculated phase diagrams using current thermodynamic database showed that these calculations applied to RCCAs are good qualitative tools but are not accurate enough to be used as predictive means.

Fabrication and Characterization of the Newly Developed Superalloys Based on Inconel 740

The chemical composition of standard Inconel 740 superalloy was modified by changes in the Al/Ti ratio (0.7, 1.5, 3.4) and addition of Ta (2.0, 3.0, 4.0%). Remelted Inconel 740 (A0) and nine variants with various chemical compositions were fabricated by lost-wax casting. The microstructure, microsegregation, phase transformation temperatures, thermal expansion coefficients and hardness of the superalloys were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy, differential scanning calorimetry, dilatometry and Vickers measurements. Typical dendritic microstructure was revealed with microsegregation of the alloying elements. Segregation coefficient ki for Ti, Nb and Ta did not exceed unity, and so precipitates enriched mainly in these elements were found in interdendritic spaces. The Nb-rich blocky precipitates, MC carbides, MN nitrides, oxides, and fine γ’ was in all modified castings. Presence of other microstructural features, such as Ti-rich needles, eutectic γ-γ’ islands, small Al-rich and Cr-rich precipitates depended on the casting composition. The lowest solidus and liquidus temperatures were observed in superalloys with a high Al/Ti ratio. Consequently, in A7–A9 variants, the solidification range did not exceed 100 °C. In the A0 variant the difference between liquidus and solidus temperature was 138 °C. Hardness of all modified superalloys was at least 50% higher than for the remelted Inconel 740 (209 HV10).

Development of new γ′-strengthened Co-based superalloys with low mass density, high solvus temperature and high temperature strength

In this work we report new γ′ strengthened cobalt base superalloys that have low mass densities in the range of ~ 8.3–8.5 gm/cm3, high solvus up to ~ 1128 °C and high specific 0.2% proof strength of ~ 90 MPa.gm-1.cm3 at temperatures up to 770 °C. The basic alloy composition is Co–10Al–5Cr–2Ta that exhibits γ/γ′ microstructure on heat treatment. Electron diffraction and elemental partitioning across γ/γ′ indicate the γ′ has an ordered L12 structure with a stoichiometry Co3(Al,Ta,Cr) coherently embedded in disordered face-centered-cubic (fcc) γ-Co matrix. The γ′ structure is primarily stabilized by Ta addition. The γ′ volume fraction and the solvus temperature of the base alloy was measured to be 14% and 870 °C respectively. These values are significantly enhanced by the addition of 30at.% Ni (35% and 975 °C) and together with 4at.% Ti attaining the values of 63% and 1128 °C, respectively. Atom Probe measurements reveal both Ni and Ti strongly partition to γ′ precipitates enhancing the stability of the γ' while Cr partitions to the γ phase. At 900 °C after 200 h of aging, the 30at.% Ni added alloy without Ti reveals formation of coarsened γ′ at the grain boundaries, directionally coarsened γ′ with wavy γ/γ′ interface and coagulated rod shape γ′ inside the grains. Addition of Ti (2 and 4 at. %), however, suppresses the formation of these features up to 1000 h of aging at 900 °C. The feasibility of tuning the properties of the present alloys by alloying additions makes them attractive for their further development.

Effect of Ta addition on the fuzz formation of additively manufactured W-based materials

As a divertor plasma-facing material, W will experience high-flux plasma irradiation. In particular, severe surface morphology change like fuzz formation can be induced by He plasma irradiation. In this study, fuzz formation on additively manufactured W and W–Ta was investigated. Rolled W, laser-powder-bed-fused (LPBFed) W and W–Ta were exposed to high flux (∼1023 m−2 s−1) He plasma in the linear plasma generator Magnum-PSI with ion energy 12–13 eV at 1273 K. The mean thickness of the fuzz at grain interiors of rolled W, LPBFed W and W–Ta was measured as 0.37 μm, 0.71 μm and 0.23 μm, respectively. The fuzz suppression in LPBFed W–Ta can be attributed to the synergetic effect of solid-solution, dislocation, and secondary-phase nanoparticles. Abnormally grown fuzz was observed near the pre-existing cracks of LPBFed W, while no such structure was found in LPBFed W–Ta. It is found that dislocations play a crucial role in inhibiting fuzz growth. This is confirmed by the difference in fuzz structure in rolled W and LPBFed W, where rolled W has a much greater dislocation density compared to LPBFed W. This work suggests that the fuzz growth kinetics may be tuned by tailoring the microstructures using the LPBF technique.

Effect of tantalum on short-term creep of a 12%Cr-3%Co-0.07%Ta martensitic steel for steam blades

9-12%Cr martensitic steels are prospective materials for elements of boilers, tubes, pipes, heaters and steam blades for fossil power plants, which are able to work at ultra-supercritical parameters of steam. Decreasing N content and increasing B content together with increasing Cr content required the optimization of ferrite-stabilizing and austenite-stabilizing elements to avoid the formation of δ-ferrite at high temperatures. Addition of 3-4%Co, 0.01%B, 0.80%Cu and 0.07%Ta as austenite-stabilizing elements can compensate increasing Cr content up to 12% at decreasing N content to 0.003% and provides the δ-ferrite in the amount less than 10%. Two Co-containing 12%Cr steels were developed and investigated. The main difference between these steels is Ta addition; one of the 12%Cr steel studied contains 0.07%Ta, other 12%Cr steel is Ta-free one. Creep tests at 650°C and the applied stresses ranging 180-80 MPa with a step of 20 MPa for both Co-containing 12%Cr steels were carried out. Addition of Ta in the Co-containing 12%Cr steel provide the increment in the creep time to rupture at both high and low applied stresses. This increment in the creep properties due to Ta addition was explained by decrease in coarsening rate of Ta-rich MX carbonitrides during creep at 650°C.

Effects of Ta on the high temperature oxidation behavior of IN617 alloy in air

Effects of Ta on the oxidation behavior of IN617 alloy were investigated by isothermal oxidation tests in air at 900 °C and 1000 °C. The oxidation resistance was improved with Ta content increasing. The Ta addition induced the formation of TaO2 at both temperatures. Ta was found to impede the out-diffusion of Ti resulting in the formation of TiO2 at the interface between outer Cr2O3 layer and metal substrate. TiO2 and TaO2 filled the voids inside Cr2O3 layer, which was the reason for the improved density of Cr2O3 scale in Ta-added alloys. Moreover, Ta inhibited the coarsening of inner Al2O3 oxides.

A new single crystal high entropy alloy with excellent high-temperature tensile property

A new single crystal high entropy alloy is developed and produced using Bridgman directional-solidification and seed-crystal method. The microstructure exhibits the directional dendrite morphology and is mainly composed of the FCC matrix and the L12 ordered precipitates. Its high-temperature yield strengths are far higher than those of the first-generation single crystal superalloys, and close to those of the second- generation ones. The solid-solution strengthening effect from the high content of W and Mo, as well as the precipitation strengthening effect from the combined addition of Al, Ti, Ta, and Nb, should be responsible for the excellent high-temperature strength of the investigated alloy.

Influence of Ta content on martensitic transformation and shape memory properties of Cu–Al–Ni–Mn shape memory alloy

This study presents the effect of Ta addition on martensitic transformation behavior and shape memory properties of Cu82.6−xAl12.4Ni4Mn1Tax (x = 0, 1, 2 and 3 wt%) high-temperature shape memory alloys. The results indicated that martensite structure, phase components, transformation characteristics and shape recovery ratios of the Cu82.6−xAl12.4Ni4Mn1Tax alloys remarkably changed with the addition of Ta element. The alloys display dual-phase martensite structure ( $$ \beta_{1}^{'} + \gamma_{1}^{'} $$ ) for x = 0 and 1, whereas those for x = 2 and 3 have only the $$ \beta_{1}^{'} $$ martensite phase. It is seen that the shape memory properties of the alloys improve considerably as a result of the addition of Ta and the highest shape recovery ratio of 87.5% is achieved with 3 wt% Ta-added alloy.

Effects of Cr on the properties of multicomponent cobalt-based superalloys with ultra high γ’ volume fraction

Abstract Chromium is known to confer oxidation resistance on Co-based superalloys. Herein we studied the influence of Cr concentration on the properties of a Co-based superalloy, Co–30Ni–11Al–2Ti-5.5W-2.5Ta-0.1B (at.%), with high γ ’ -volume fractions (up to 98%). Additions of 4, 8, or 12 at.% Cr decreased the γ ’ -solvus temperature and the γ ’ -volume fraction, while also reducing the mean radius of γ ’ -particles. Synchrotron X-ray diffraction and atom-probe tomography confirmed that Cr reduces the lattice parameter misfit by altering the partitioning behavior of W between the γ- and γ ’ -phases. Concomitantly, an increase in Cr content significantly improved the oxidation resistance of the alloy. The results indicate that 4 at.% Cr is sufficient to decrease the oxidation rate by two orders of magnitude. A creep study at 850 ∘C revealed that Cr does not affect adversely the creep performance in high γ ’ -volume fraction Co-based superalloys, but the tendency toward γ ’ rafting changes with Cr concentration. Compared to Ni3Al intermetallic compounds, the alloys with high γ ’ -volume fraction studied exhibited a significantly higher creep resistance. This may be attributed to the increase in antiphase boundary energy caused by the addition of Cr, Ti, or Ta, and significant solid-solution hardening in the Co-based intermetallic γ ’ -phase.

Effect of Ta addition on solidification characteristics of CoCrFeNiTax eutectic high entropy alloys

Effect of Ta addition on solidification characteristics of CoCrFeNiTax (x = 0.1–0.7) eutectic high entropy alloys were investigated. With increasing Ta content, CoCrFeNiTax alloys changed from hypoeutectic alloys with primary γ phase to eutectic alloy (x = 0.43) and to hypereutectic alloys with primary Laves phase. Consequently, Ta content in primary phase significantly increased, while the content of Ta element in γ/Laves eutectic phases slightly decreased. Meanwhile, the solidification path and solidification characteristic temperatures of CoCrFeNiTax alloys both agreed with typical binary eutectic phase diagram, and obvious microsegregation of Ta element was only found in CoCrFeNiTa0.1 alloy. Furthermore, the contents of Co, Cr, Fe and Ni elements in primary phases and distribution behaviors of Co, Cr, Fe and Ni elements were both different in CoCrFeNiTax alloys, which indicated that CoCrFeNiTax alloys cannot be simplified as (CoCrFeNi)–Ta pseudo binary system when analyzing distribution behaviors of Co, Cr, Fe and Ni elements.

The Oxidation Resistance of Nb-Si-Based Alloys at Intermediate and High Temperatures

The oxidation behavior of three Nb-Si-based alloys were evaluated at intermediate (800 °C) and high (1250 °C) temperatures for 100 h in air. At 800 °C, the Nb-24Ti-15Si-13Cr-2Al-2Hf (at. %) alloy suffered from catastrophic pest oxidation. This pest phenomenon was suppressed by the addition of Sn. However, Ta addition protected the Nb-Si-based alloys from pest oxidation for a short time. At 1250 °C, Sn could enhance the oxidation resistance of Nb-Si-based alloys due to the formation of a Sn-rich layer. In addition, the oxidation mechanisms of Nb-Si-based alloys at intermediate and high temperatures were discussed.

The high-temperature performance of Ti-46.5Al-%xTa (x = 0.8, 4 and 8 at.%) alloys produced using SPS

In this study, γ-TiAl based alloys of composition Ti-46.5Al-%xTa (x = 0.8, 4 and 8 at.%) were fabricated by mechanical alloying and consolidated by spark plasma sintering. The main objective of the study was to evaluate the influence Ta contents had on strengthening effect of the γ-TiAl based alloy. X-ray diffractometry (XRD) at room and high temperatures and scanning electron microscopy (SEM) analyses were used to characterise the sintered alloys and fractured surfaces. Compression tests measured mechanical properties from 25 to 1000 °C. The results show that the addition of 0.8 at.% Ta increased the compressive strength of the γ-TiAl alloy from 980 N.mm−2 (at 25 °C) to 1280 N.mm−2 (at 850 °C). At 1000 °C, the strength of the alloys increased with increasing Ta additions. At 850 °C and higher, the mode of failure of all the sintered alloys was by intergranular fracture.

Colossal permittivity characteristics and mechanism of (Sr, Ta) co-doped TiO2 ceramics

Colossal permittivity (CP) materials have great application prospects in electronic devices. However, obtaining a giant dielectric constant, low dielectric loss, and good temperature stability in one phase simultaneously is a challenge. In this work, the colossal permittivity characteristics along with the related mechanism of (Sr, Ta) co-doped TiO2 ceramics were investigated. According to the dielectric analysis, the dielectric properties are significantly improved when Sr and Ta co-doped, where the addition of Sr is responsible for the reduced tan δ, and the addition of Ta is responsible for the improved e′. The enhanced dielectric constant (> 105) with relatively low dielectric loss is obtained in a large substitution range from x = 1 to 7%, and the good frequency and temperature stability are observed in these samples. In addition, the optimum dielectric properties are obtained at x = 0.05, where e′ = 1.86 × 105, tan δ = 0.15. XPS analysis confirms the existence of reduced Ti3+ ions and oxygen vacancies, which may form the highly stable defect clusters. The defect clusters are responsible for the improved dielectric properties in (Sr, Ta) co-doped TiO2 ceramics. Meanwhile, the interfacial polarization is mentioned to explain CP phenomenon of TiO2-based ceramics. This research is conducive to the development and application of TiO2-based new CP materials.

Effect of Ta on microstructural evolution of NiCrAlYSi coated Ni-base single crystal superalloys

The effect of Ta on microstructural evolution of NiCrAlYSi coated Ni-base single crystal superalloys at 1000 °C and 1100 °C has been investigated. Interdiffusion occurs between the NiCrAlYSi coating and the substrate, leading to the formation of inter-diffusion zone (IDZ) and secondary reaction zone (SRZ). After exposure at 1000 °C for 100 h, the initial γ/γ′ transforms to γ′ phases in the IDZ, and SRZ with σ and μ topologically close-packed (TCP) phases is formed primarily due to the inward diffusion of Al. Different behavior of SRZ formation in the alloys with various Ta content can be observed after exposure at 1000 °C, which highlights the potency of Ta addition in promoting SRZ formation. This may be ascribed to the fact that Ta promotes the precipitation of TCP phases. After exposure at 1100 °C for 100 h, the initial γ/γ′ transforms to γ layer and coarse γ′ phases in the IDZ, and no SRZ can be found. The formation of IDZ is significantly suppressed by Ta addition, which may be related to the retardation of interdiffusion between the coating and the substrate due to the increased γ′ volume fraction. Furthermore, γ′ rafting parallel to and/or perpendicular to the coating/substrate interface in the substrate beneath IDZ are found in the alloys after exposure at 1000 °C and 1100 °C. Microstructural observation shows that Ta has a potent influence on the development of perpendicular γ′ rafting. The mechanism of parallel γ′ rafting and perpendicular γ′ rafting has been discussed, respectively.

Synthesis, microstructure, mechanical properties and luminescence of a ceramics Gd(NbxTa1−x)O4

Ceramic solid solutions GdNbx[Formula: see text]O4 ([Formula: see text]–1) were synthesized by sol–gel. Morphological particularities of microstructure were studied, strength characteristics (Young’s modulus) and stress intensity factor for mode I [Formula: see text] were evaluated. The latter is a criterion of crack resistance of the material. Luminescent properties of solid solutions GdNbx[Formula: see text]O4 were researched under UV exciting radiation. A comparatively small addition of Ta ([Formula: see text]) was shown to increase luminescence intensities of centers [Formula: see text]–O−. An addition of Ta to GdNbO4 leads to creation of solid solution [Formula: see text][Formula: see text]O4. Intensity of these centers is 3–6.5 times larger than that of GdNbO4 and GdTaO4.

Effect of Nanocrystalline Nickel Powder and Co, Mo, Ta, and Al Additions on Isothermal Oxidation Behavior of Ni–17Cr Alloy

In this paper, the effect of nanocrystalline nickel powder developed via high-energy ball milling process and the addition of Co, Mo, Ta, and Al on the isothermal oxidation behavior of Ni–17Cr binary alloy developed by spark plasma sintering system was investigated. The oxidation behavior of the sintered alloys was studied in air at 1100 °C for exposure period up to 312 h. The oxidation rate and kinetics show that the addition of Al and nanocrystalline nickel enhances the oxidation resistance of the binary alloy followed by Mo, Co, and Ta in their reducing order. The formation of the chromia scale was common to all the sintered alloys investigated as confirmed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy analysis.

The effect of Ta additions on the oxidation resistance of SPS-produced TiAl alloys

The influence of the environment on the properties of TiAl alloys is a concern for elevated temperature applications. One element that stands out in improving the resistance of TiAl alloys against high-temperature oxidation is Nb. Nb promotes the activities of Al and favors the formation of a more protective Al2O3 surface oxide which subsequently stymies oxygen diffusion to the alloy. Ta is an equal replacement of Nb. Thus, this study aims to evaluate the effect of 0.8, 4, and 8 at. % Ta additions on the oxidation resistance of spark plasma sintered Ti-46.5Al (at. %) alloy. Isothermal oxidation tests were carried out for 360 and 100 h at 1123 and 1273 K respectively. Surface oxides examination include scanning electron microscopy with energy-dispersive X-ray spectroscopy and X-ray diffractometry. The results show that 4 and 8 at. % Ta additions significantly support the formation of an interconnected, non-porous Al2O3 layer at the metal-oxide interface. The continuous non-porous Al2O3 layer serves as a diffusion barrier which leads to superior oxidation resistance of the TiAl alloys.