Addition Of Refractory Elements Research Articles

Effect of refractory element addition on high temperature oxidation process of complex concentrated alloy coatings

The sluggish diffusion effect of complex concentrated alloys (CCAs) affords them considerable application potential in the field of high temperatures. In this study, the role of refractory elements in the high temperature oxidation process of denary complex concentrated alloy coatings (CCACs, namely NiCoCrFeAl0.8Si0.1(MoNbTaW)x, where x is 0.025, 0.05, 0.1 and 0.15) prepared via laser cladding was investigated. Four groups of the CCACs with different refractory contents were subjected to 100 h of high-temperature oxidation experiments, and the oxidation products and microscopic characteristics of the samples were observed via XRD and SEM, respectively. Atomic-scale characterisation and analysis of the oxidised samples were performed using TEM and XPS technology to explore the diffusion behaviour of elements in the high-temperature process. Results indicate that the solid solution limit of the CCACs is obtained by regulating the topological instability of the lattice matrix via the addition of refractory elements. Consequently, when 0.1 > x > 0.025, the coating is a body-centred cubic (BCC) + face-centred cubic (FCC); when x = 0.1, the coating maintains a single-phase FCC structure; and when x > 0.1, the coating changes to an FCC + BCC duplex structure. The x0.1 group exhibits the smallest relative strain among all coatings, which is conducive to maintaining the structural stability of the face-centred cubic at high temperatures and delaying atomic migration within the lattice. Furthermore, the oxidation kinetics curve of the CCACs with a single- phase FCC exhibits low Kp values. In a single- FCC lattice, the atom diffusion rate through volume slows down and grain boundary diffusion decreases. Considering the Weibull modulus for the hardness statistical value after coating oxidation, the modulus of single-FCC phase CCACs is also higher than others. This study has the potential to contribute to the application of high-temperature thermal protection for CCACs.

Improving mechanical properties of (Co1.5FeNi)88.5Ti6Al4R1.5 (R = Hf, W, Nb, Ta, Mo, V) multi-component high-entropy alloys via multi-stage strain hardening strengthening

In this study, (Co1.5FeNi)88.5Ti6Al4R1.5 (R = Hf, W, Nb, Ta, Mo, V) high entropy alloys were fabricated by vacuum arc melting, followed by cold rolling and aging treatment. The effects of refractory elements on the phase structure, microstructure and mechanical properties of high entropy alloys (HEAs) were systematically studied by means of XRD, SEM and TEM. The results show that the phase composition of all alloys is the face-centered-cubic (FCC) + L12 dual-phase structure, and the addition of refractory elements has not changed the phase structure of the alloys. The tensile results show that the yield strength and elongation of the matrix HEA are 750 MPa and 18 %, respectively. The strength of HEAs is improved variously by adding refractory elements. The HEAs with Ta element have the best comprehensive mechanical properties, exhibit excellent strength-ductility combination, with the yield strength and elongation of 950 MPa and 20 %, respectively, and the fracture mechanism changes from complete ductile fracture to mixed brittle-ductile fracture mode. In addition, the HEAs have multi-stage strain-hardening behavior, which has higher strength when the strain-hardening curve has larger fluctuations, which indicates that the HEAs can improve the strength-plasticity matching relationship by a multi-stage strain hardening behavior.

Microstructure and tensile behavior of powder metallurgy FeCrAl accident tolerant fuel cladding

Defect-free seamless FeCrAl cladding tubes with 0.3 mm wall thickness have been successfully developed via full-scale powder metallurgy (PM) manufacturing routes, providing a cost neutral replacement of Zircaloy-2 tubes with enhanced accident tolerant fuel. Microstructure and tensile properties at room temperature and 315 °C were evaluated in the tubing of two yttrium-free FeCrAl alloy compositions PM-C26M and Ferritic Alloy – Sandvik Material Technology (FA-SMT) that differ in Cr, Al, Mo and minor addition of refractory elements. The powder metallurgy FeCrAl tubes reveal finer grain size than the smallest achievable grain size by cast/wrought tube fabrication process, low retained strain, and tensile properties superior to Zircaloy-2 cladding tubes. <101> fiber texture along the tube axial direction was observed. In-situ neutron diffraction during tensile loading shows qualitatively similar trend of intergranular load transfer during elastoplastic deformation in PM-C26M and FA-SMT, while FA-SMT indicates higher dislocation density and PM-C26M reveals more intensive <101> texture evolution along loading direction. Precipitates in FA-SMT are inferred to share load from the matrix, while such load sharing is not evident in PM-C26M. Compared to texture free ferritic steel data in the literature, the <101> fiber texture in the FeCrAl tubes seems to have little effect on the grain-level tensile deformation behavior including elastic anisotropy and plastic anisotropy.

Quantitative Characterization of Heat Treatment Response in a Single Crystal Nickel-Based Superalloy

To study quantitatively the effect of heat treatment on the microstructure, composition and mechanical property in a new single crystal nickel-based superalloy for industrial gas turbine (IGT) applications, the eutectic fraction, carbide fraction, and the fraction, size, shape and distribution of the γ ́ phase was characterized by quantitative metallographic method, the evolution of chemical composition and hardness between core and inter dendrite was tested through EMPA and nanoindentation. The experimental results indicate that: The eutectic fraction decreases from (0.52±0.08) % to (0.03±0.01) %. The carbides fraction decreases from (0.23±0.04) % to (0.12±0.03) %, and Feret ratio decreases from 3.21±2.54 to 2.14±0.98. The γ ́ fraction increases from (55.66±4.18) % to (73.78±3.24) % in core dendritic region, from (64.82±1.44) % to (70.11±3.10) % in inter dendritic region. The γ ́-size is 406±111(nm) in core dendritic region and 918±384(nm) in inter dendritic region before heat treatment, 359±69(nm) in core dendritic region and 361±57(nm) in inter dendritic region after heat treatment. The γ ́-cuboidal degree is 1.08±0.20 in core dendritic region and 1.14±0.23 in inter dendritic region before heat treatment, 1.08±0.19 in core dendritic region and 1.02±0.14 in inter dendritic region after heat treatment. The solidification segregation coefficient of main segregation elements, such as Re, W, Hf, Ta, Al, and Mo, is closer to 1, with an average decrease of 27% after heat treatment. The hardness and modulus increase in core and inter dendritic, and their inhomogeneity is reduced between cores and inter dendritic. The improvement of properties result from the improvement of size uniformity and cuboidal degree of γ ́, and the reduction of carbides and eutectic through element homogeneity during heat treatment. The solidification segregation coefficient of main segregation elements, such as Re, W, Hf, Ta, Al, and Mo, is closer to 1, with an average decrease of 27% after heat treatment. With the addition of refractory elements, some elements partition to the dendrite core, while other elements tend to accumulate in the interdendritic liquid and then solidify as the interdendritic and eutectic regions during solidification. The hardness and modulus increase in core and inter dendritic, and their inhomogeneity is reduced between cores and inter dendritic. The improvement of properties result from the improvement of size uniformity and cuboidal degree of γ ́, and the reduction of carbides and eutectic through element homogeneity during heat treatment.

Microstructural evolution and creep mechanisms in Ni-based single crystal superalloys: A review

Nickel-based single crystal superalloys (Ni-SXs) are of great interests in aircraft industry due to their excellent high temperature creep strength deriving from the rigid gamma/gamma prime (γ/γ′) microstructure and the single crystal nature. The creep properties of Ni-SXs depend strongly on the degradation rate of the original rigid γ/γ′ microstructure with the dislocation movements in the microstructure. Since the applied temperature and stress strongly affect the dislocation movements, the different creep mechanisms are displayed to Ni-SXs during low temperature (∼750 °C), mid-temperature (∼950 °C) and high temperature (∼1100 °C) creep. The creep deformation during low temperature creep is mainly caused by the shearing of γ′ phase by a limited number of dislocations, while high temperature creep mainly occurs to the accumulation of creep strains with the increased dislocation activities in γ phase. To enhance the creep properties of alloy, a variety of strategies of the microstructural optimization need to be considered. One is to increase the volume fraction of γ′ phase to enhance the precipitation strengthening effects. But excessive amounts of γ′ phase are disadvantageous to low temperature strength of alloy. The coarsening of γ′ phase and the rafting process strongly correlates to dislocation movements. It is of importance to discuss the specific mechanisms of coarsening and rafting as well as their influence to the creep properties of alloy. The lattice misfit between the γ and γ′ phase plays an important role in influencing the coarsening of γ′ phase and the creep properties of alloy. A proper lattice misfit helps to form the cubic γ′ phase which is advantageous to alloy. But a too large lattice misfit can promote the coarsening of γ′ phase that damages the structural stability of alloy at high temperatures. The increased additions of refractory elements in alloy promote the formation of topologically close-packed (TCP) phase. TCP phase facilitates the nucleation and propagation of micro-cracks around it that damages the creep properties of alloy. This review aims to summarize some aspects of microstructural evolution during creep of alloy with considering their effects to creep properties. To guide the design of Ni-SXs in future, some perspectives about the microstructural optimization are properly provided.

Alloying effects of refractory elements in the dislocation of Ni-based single crystal superalloys

The alloying effects of W, Cr and Re in the [100] (010) edge dislocation cores (EDC) of Ni-based single crystal superalloys are investigated using first-principles based on the density functional theory (DFT). The binding energy, Mulliken orbital population, density of states, charge density and radial distribution functions are discussed, respectively. It is clearly demonstrated that the addition of refractory elements improves the stability of the EDC systems. In addition, they can form tougher bonds with their nearest neighbour (NN) Ni atoms, which enhance the mechanical properties of the Ni-based single crystal superalloys. Through comparative analysis, Cr-doped system has lower binding energy, and Cr atom has evident effect to improve the systemic stability. However, Re atom has the stronger alloying effect in Ni-based single crystal superalloys, much more effectively hindering dislocation motion than W and Cr atoms.

Effects of refractory elements on the structure and dynamics of molten Ni: An ab initio molecular dynamics study

The effects of minor refractory elements (Re, W and Ta) on the structure and dynamics of molten Ni are studied by ab initio molecular dynamics simulations. The results show that minor additions of refractory elements would not significantly change the structure of liquid Ni. However, the local structure around Ta is quite different from that around Re or W. In the liquid Ni alloys, the 15lm-type bond pairs dominate. As temperature increases, the numbers of 16lm- and 15lm-type bond pairs would decrease while those of 14lm- and 13lm-type bond pairs would increase. The Ni self-diffusion coefficient of pure Ni shows a temperature dependence described by the Arrhenius law. With the addition of Re and W, the Ni self-diffusion coefficients still exhibit an Arrhenius relationship. However, the self-diffusion of Ni deviates from the Arrhenius law with the addition of Ta.

Topologically Close-Packed Phase Precipitation in Ni-Based Superalloys

The addition of refractory elements is effective in improving the comprehensive performance of Ni-based superalloys, and meanwhile, the precipitation trend of TCP phases increases with the elements content due to the segregation of refractory elements during high temperature service. The precipitation of TCP phases obviously decreased the creep properties and creep rupture life of superalloys. The toughness and plastic of superalloys also dramatically deteriorated. The addition of Ruthenium (Ru) is shown to suppress the formation of TCP phases in Ni-based superalloys, resulting in much improved the creep resistance and the microstructural stability. The studies on TCP phases in Ni-based superalloys including crystallography and precipitation of TCP phases and the effect of alloy composition on TCP phases are reviewed.

Effect of The Refractory Element Additions on Microstructure and Mechanical Property of Two-phase Intermetallic Alloys Based on The Ni3Al-Ni3V Pseudo-binary Alloy System

ABSTRACTTwo-phase intermetallic alloys composed of geometrically close packed (GCP) Ni3Al (L12 phase) and Ni3V (D022 phase) have attractive mechanical properties at high temperature, and are therefore considered to be used as high temperature structural materials. In this study, the effect of Ta and Re addition on the microstructure and hardness of two-phase intermetallic alloys was investigated. The addition of Ta remarkably enhanced the hardness due to solid solution hardening of the constituent phases. On the other hand, the addition of Re retarded the formation of the two-phase microstructure, resulting in the lowest hardness in the solution treated condition. By aging at 1223 K, the Ni solid solution in the Re added alloy decomposed to Ni3Al and Ni3V, accompanied by precipitates of a Re-rich phase. Consequently, the hardness rapidly increased with increasing aging time. Simultaneous addition of Ta and Re induced very fine precipitates of a Re-rich phase after aging, and consequently resulted in a higher hardness than by the addition of Ta or Re alone.

Nanoindentation analysis of the mechanical behavior of Zr-based metallic glasses with Sn, Ta and W additions

Zr 57Cu 20Al 10Ti 8Ni 5 and modified composition by adding Sn, W or Ta are studied using standard mechanical test and nanoindentation. Addition of refractory elements with a Sn micro-addition increases clearly the Young's modulus and the hardness of basic BMG. However, Sn reduces plasticity. Moreover these experiments allowed, in confine plasticity conditions, estimating an apparent activation volume associated to a plastic deformation (≈150 Å 3).

Low cycle fatigue of superalloys

Abstract Low cycle fatigue (LCF) of a fourth-generation single-crystal (SC) Ni-base superalloy TMS-138 was studied by comparison with typical third-generation (TMS-75) and second-generation (CMSX-4) Ni-base SC superalloys. TMS-138 exhibits improved LCF behaviour under a condition of the R ratio of 0 at temperatures of 1073 K and 1173 K. The addition of refractory elements resulted in a remarkable improvement of the LCF properties compared to those of the reference superalloys due to the different microstructure developed in TMS-138.

Deformation microstructures after low-cycle fatigue in a fourth-generation Ni-base SC superalloy TMS-138

The deformation microstructures of a fourth-generation single-crystal (SC) Ni-base superalloy TMS-138 after low-cycle fatigue (LCF) have been studied. For comparison, a typical third-generation (TMS-75), and a second-generation (CMSX-4) Ni-base SC superalloy were also examined. The TMS-138 exhibited an excellent LCF behavior under a condition of R ratio of 0 at temperatures of 1073 K and 1173 K. The addition of refractory elements resulted in a remarkable improvement of LCF properties due to the different microstructure developed in TMS-138 as compared to the reference superalloys. The formation of a large number of stacking faults (SFs) in the matrix at 1073 K and the more completed rafts and smaller interfacial dislocation spacing at 1373 K were observed and discussed in relation to the performance of TMS-138 under a specific condition of R = 0. This work suggests that a fourth-generation SC superalloy TMS-138 is a promising candidate for applications in aeroengines.

Effect of Refractory Element Additions on the Properties of Sintered Stainless Steels

This paper studies the change in properties of powder metallurgical 316L stainless steel when pure vanadium, molybdenum and tungsten (3% vol.) are added. The objective is to study the possibility of nitrogen pick-up by these refractory elements better than the chromium from plain steel. All materials have been manufactured mixing the powders with a 0.6% (wt.) of wax. Compaction was carried out at 700 MPa, and green density was evaluated. Finally, sintering was done (for 30 min) in 25%N 2 -75%H 2 at 1230 °C. Evaluated properties include sintering density, hardness, and corrosion (through salt spray test), and a broad microstructural study (optical and scanning electron microscopy) was also carried out.

Nanocrystallization behaviour of FeSiBCu(NbX) alloys

The crystallization of amorphous Fe 73.5Si 13.5B 9Cu 1Nb 1X 2 (X = Zr, Nb, Mo, V) was investigated by combined use of differential scanning calorimetry, X-ray diffraction and electron microscopy techniques. The addition of refractory elements raises thermal stability of the amorphous alloys in the order V < Mo < Nb < Zr. The nanocrystallization reaction exhibits a low rate kinetic, as denotes the value close to 1 of the Avrami index. The volume fraction of the nanophase for the same annealing conditions decreases in the order V > Mo > Nb > Zr but a maximum value of about 65% is found for all cases. Refractory element alloying controls the mean grain size of the Fe,Si nanocrystals, growing up to a value in the range 10–18 nm for the alloys in the sequence V > Mo > Nb > Zr.

Observations on the structure, hardness and adhesion properties of a selection of multicomponent refractory element nitride coatings

In recent years, the introduction of titanium-based nitride and carbide coatings has led to a marked improvement in the tribological behaviour and service life of many engineering components such as cutting tools and metal forming dies. Although TiN performs satisfactorily, increasing demands from manufacturing industries have driven the development of thin coatings to optimised performance. Coatings are being sought with, inter alia, improved hardness and wear resistance. In this context, attention has been focussed on a number of advanced multi-component nitride coating systems based on, or incorporating, refractory element additions. Thus the use and incorporation of refractory and semi-refractory elements such as Al, Zr, Cr or V promises to provide coatings with higher hardness and improved wear resistance due to the formation of novel complex structures. In the present investigation, a wide selection of single and multi-component coatings (TiN, CrN, ZrN, TiAlN, TiAlVN, TiCrN, TiZrN and TiCN) have been deposited onto high speed steel substrates, using the technique of unbalanced magnetron sputtering. This paper presents data on the structure, hardness and adhesion properties of these coatings. The implication of these results is then discussed.

The influence of refractory element additions on the magnetic properties and on the crystallization behaviour of nanocrystalline soft magnetic Fe-B-Si-Cu alloys

The influence of additions of the refractory elements R = V, Mo, W, Ta, Nb on the magnetic properties and on the microstructure of annealed amorphous Fe (76.5− x) Si 15.5 B 7 Cu 1 R x ( x=1−3) was investigated. Crystallization temperature, grain size, and coercivity of the formed Fe 3Si phase are controlled by the amount and the type of refractory element. The onset of crystallization is shifted to higher annealing temperatures while the particle size is lowered by the same order. This effect is correlated with the atomic diameter in the following order V (0.264 nm); Mo (0.272 nm); W (0.274 nm); Ta (0.286 nm); Nb (0.286 nm). The differences in atomic diameter between iron and the refractory elements stabilize the amorphous phase and slow the kinetics down by limiting diffusion in the alloys. The behavioyr of the coercivity is correlated to the grain size but can not be described generally by a simple power law. The saturation magnetostriction fits well with a linear dependence on the volume fraction of the formed Fe 3Si phase but not with a linear mixture model.

The role of microstructure on oxidation of Ni-Cr-Al base alloys at 1023 and 1123 K in air

The oxidation behavior at 1023 and 1123 K in air has been followed for several Ni-Cr-Al ternary alloys and with small additions of refractory elements Nb, Mo, and Ta. Prior to testing, the alloys were characterized by microscopic and microanalytic methods. Microstructural observations have shown that the formation of oxide layers depends on the original microstructure. Discontinuous thermogravimetric tests were used to relate the behavior of different chromium, aluminum, and refractory-element additions, and eventually to service conditions in similar environments in which general oxidation takes place. The addition of refractory elements generally impaired the oxidation resistance.

The influence of additions of refractory elements on the sulphidation behaviour of cobalt-based alloys

The sulphidation of a number of Co-Cr-Al-Y-X type experimental alloys, where X is Ti, Hf, V, Nb, Mo or W at levels of 1.5, 5 or 10%, has been studied. Experiments were conducted isothermally in a bioxidant sulphidizing/oxidizing environment ( p S 2 = 10 −6 atm) (10 −1 Pa) and p O 2 = 10 −23 atm (10 −18 Pa) at 750°C for periods of up to 240 h. Their performance was compared with that of four model alloys generic to these Co-Cr-Al-Y-K types, namely Co-5Al-1Y, Co-25-Cr, Co-25Cr-1Y, and Co-25Cr-3.5Al-1Y, and two commercial alloys, HK40(L) and Airesist 213. It has been established that generally such additions are beneficial, with a substantial reduction in material wastage. The composition and morphology of the scales is described. The significance of the results is considered in the light of theory concerning the design and development of sulphidation resistant alloys.

Some Characteristics of Automotive Exhaust Valve Alloys

AbstractThe PH stainless steels and the nickel‐base superalloys, can be evaluated for exhaust valve applications by considering their metallurgical, environmental and high temperature strength properties. The PH stainless steels are characterized by their good sulfidation and high temperature strength. Good PbO corrosion resistance is achieved with the low silicon, nickel containing alloys. Stable alloys show the greatest high temperature strength which can be improved further by a solution and age treatment. Aging below the optimum temperature of 760°C results in grain boundary sensitization and low impact properties while higher temperatures produce more of the discontinuous phase. The addition of refractory elements can be detrimental to the oxidation resistance of these alloys.The highest elevated temperature strength and best PbO and oxidation resistance is achieved with the nickel‐base superalloys. These alloys are completely stable and highest strength at elevated temperatures is achieved with a solution and age treatment. These alloys show lower sulfidation corrosion resistance relative to the PH stainless steels, however this can be improved with higher chromium contents.