Showing papers on "Laves phase published in 2017"

Laves-phase formation criterion for high-entropy alloys

Abstract: In this study, we have analysed Laves-phase formation in high-entropy alloys (HEAs). For that purpose, the AlCrxNbTiV and AlxCrNbTiVZr (x = 0, 0.5, 1, 1.5) alloys were produced and examined. It was found that the AlNbTiV and AlCr0.5NbTiV alloys had single-phase body-centred cubic structure, while the other alloys contained Laves phase. Analysis has demonstrated that Laves-phase formation in the produced and in the other HEAs, which are predominantly composed of Al and the elements of 4–6 groups and tend to form body-centred cubic solid solutions, can be predicted by the atomic size mismatch, δr, and the Allen electronegativity difference, ΔχAllen, parameters. It was shown that Laves-phase formation is observed when δr > 5.0% and ΔχAllen > 7.0%.

Effects of thermal cycles on the microstructure evolution of Inconel 718 during selective laser melting process

Abstract: A staircase Inconel 718 block was fabricated to investigate the effects of the thermal cycles on the microstructure evolution in the selective laser melting (SLM) part using optical scope (OM), scanning electron microscope (SEM), and electron backscatter diffraction (EBSD). The laser beam scanning strategy was clearly shown in the part under OM, including laser scanning pattern and hatch spacing. The Y-plane (side surface) was characterized by elongated colonies of cellular dendrites with an average cell spacing of 0.511 ∼ 0.845 μm. In addition, Laves phase was observed in the inter-layers and inter-cellular regions. Under the continuing effects of the thermal cycles, the fraction of the Laves-phase showed a significant drop with their morphology changing from coarse and interconnected particles to discrete Laves phase. This is attributed to the reheating process as Laves phase can be dissolved at a proper heat treatment. In terms of the width of the cellular dendrites, the longer the thermal cycle period is, the coarser the elongated grains are. Due to the preferred orientation of the crystal, the Y-plane and Z-plane had a strong texture of and , respectively. With the repeating thermal cycle period elongating, the maximum intensity of the texture, together with the fraction of larger grains and the high misorientation angles, increased. At the same height, there was no significant changes with the grains size, but the fraction of the high misorientation angles (>15°) increased with the subsequent building of the part. Moreover, the area fraction of the porosity was below 0.2%, with no remarkable effects found from the thermal cycles and the build height.

Formation and effect of topologically close-packed phases in nickel-base superalloys

Abstract: The formation of topologically close-packed (TCP) phases in nickel-base superalloys is an issue of increasing importance as alloys are designed with higher refractory element contents to meet the r...

First-principles prediction of high-entropy-alloy stability

Abstract: High entropy alloys (HEAs) are multicomponent compounds whose high configurational entropy allows them to solidify into a single phase, with a simple crystal lattice structure. Some HEAs exhibit desirable properties, such as high specific strength, ductility, and corrosion resistance, while challenging the scientist to make confident predictions in the face of multiple competing phases. We demonstrate phase stability in the multicomponent alloy system of Cr–Mo–Nb–V, for which some of its binary subsystems are subject to phase separation and complex intermetallic-phase formation. Our first-principles calculation of free energy predicts that the configurational entropy stabilizes a single body-centered cubic (BCC) phase from T = 1700 K up to melting, while precipitation of a complex intermetallic is favored at lower temperatures. We form the compound experimentally and confirm that it develops as a single BCC phase from the melt, but that it transforms reversibly at lower temperatures. High entropy alloys with four species or more can be single phase solid solutions but can equally phase separate, making their design difficult. A collaboration led by Michael Widom at Carnegie Mellon University used free energy calculations paired with hybrid Monte Carlo molecular dynamics simulations to predict the separation of a Cr–Mo–Nb–V alloy from a stable single phase body-centered cubic solid solution at high temperature to a separated intermetallic Laves phase at lower temperatures. By synthesizing the alloy and annealing it at different temperatures, the authors confirmed the phase separation and showed that it was reversible. Combining quantum-mechanical total energy calculation with statistical mechanics to predict free energies is therefore an effective avenue that may be applied to many problems in alloy design.

The tensile deformation behavior of laser repaired Inconel 718 with a non-uniform microstructure

Abstract: Laser repairing technology, based on laser additive manufacturing technology, has been used in repairing damaged components for saving time and economic cost. Due to the existence of laser deposited zone, the microstructure and mechanical properties of laser repaired (LRed) components are different from original wrought parts. In this paper, the tensile deformation behavior of LRed Inconel 718 was compared with that of wrought Inconel 718 by using digital image correlation (DIC) method. The microstructure and the failure mechanism of LRed Inconel 718 were also investigated. The results showed that the LRed Inconel 718 sample could be divided into two parts: laser deposited zone (LDZ) and substrate zone (SZ). The laser deposited zone consisted of columnar dendrite growing epitaxially along the deposition direction, where the γ″ phases gathered around the interdendritic Laves phases. However, the microstructure of the substrate zone was equiaxed grain with the uniform distribution of δ and γ″ phases. It means that the microstructure of the LRed Inconel 718 was non-uniform. The deformation compatibility of the LRed Inconel 718 was inferior to that of the wrought Inconel 718 because of the existence of the Laves phases and the heterogeneous distribution of the γ″ phases. As a result, strain concentrated in the laser deposited zone resulting in that most plastic deformation was clustered in this region. The hard and brittle Laves phases slipped and broke up into smaller parts due to local stress concentration. Microscopic holes formed at the interface of the Laves phases and the γ matrix and combined together to cause the fracture of the LRed Inconel 718.

Synthesis of a single phase of high-entropy Laves intermetallics in the Ti–Zr–V–Cr–Ni equiatomic alloy

Abstract: The high-entropy Ti–Zr–V–Cr–Ni (20 at% each) alloy consisting of all five hydride-forming elements was successfully synthesised by the conventional melting and casting as well as by the melt-spinning technique. The as-cast alloy consists entirely of the micron size hexagonal Laves Phase of C14 type; whereas, the melt-spun ribbon exhibits the evolution of nanocrystalline Laves phase. There was no evidence of any amorphous or any other metastable phases in the present processing condition. This is the first report of synthesising a single phase of high-entropy complex intermetallic compound in the equiatomic quinary alloy system. The detailed characterisation by X-ray diffraction, scanning and transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed the existence of a single-phase multi-component hexagonal C14-type Laves phase in all the as-cast, melt-spun and annealed alloys. The lattice parameter a = 5.08 A and c = 8.41 A was determined from the annealed material (anneal...

Effects of heat treatment on microstructures and tensile properties of IN718/TiC nanocomposite fabricated by selective laser melting

Abstract: Selective laser melting (SLM) is a powder-bed fusion additive manufacturing process that fabricates metallic parts in a layer-by-layer manner. IN718 is a nickel-based austenitic (γ) superalloy with broad applications in aerospace, chemical, and nuclear industries. In this study, fully-dense IN718-matrix composite reinforced by TiC nanoparticles were fabricated in-situ by SLM. As-built and heattreated IN718/TiC nanocomposite was compared against pure IN718 in terms of microstructures and tensile properties. Columnar and cellular microstructure morphologies in as-built pure IN718 and IN718/TiC nanocomposite were demolished by heat treatment. Microstructure refining and grain boundary dislocation pinning effect of TiC nanoparticles improved the tensile strength of as-built IN718/TiC nanocomposite. In both as-built IN718/TiC and pure IN718, γ and Laves phase are the major constituents while the other precipitates were suppressed by rapid solidification in SLM. Heat treatment dissolved Laves precipitates and grew γ'/γ'', δ, and MC/ M23C6 carbides that strengthened both pure IN718 and IN718/TiC nanocomposite. In heat-treated IN718/TiC, the TiC nanoparticles and M23C6 carbides degenerated from TiC counteracted the deficiency of δ precipitates. Therefore, heat-treated IN718/TiC nanocomposite had similar tensile strength but lower ductility than heat-treated pure IN718.

Stacking faults in Zr(Fe, Cr) 2 Laves structured secondary phase particle in Zircaloy-4 alloy

Abstract: Stacking faults (SFs) in secondary phase particles (SPPs), which generally crystallize in the Laves phase in Zircaloy-4 (Zr-4) alloy, have been frequently observed by researchers. However, few investigations on the nano-scale structure of SFs have been carried out. In the present study, an SF containing C14 structured SPP, which located at grain boundaries (GBs) in the α-Zr matrix, was chosen to be investigated, for its particular substructure as well as location, aiming to reveal the nature of the SFs in the SPPs in Zr-4 alloy. It was indicated that the SFs in the C14 structured SPP actually existed in the local C36 structured Laves phase, for their similarities in crystallography. The C14 → C36 phase transformation, which was driven by synchroshearing among the (0001) basal planes, was the formation mechanism of the SFs in the SPPs. By analyzing the strained regions near the SPP, a model for understanding the driving force of the synchroshear was proposed: the interaction between SPP and GB resulted in the Zener pinning effect, leading to the shearing parallel to the (0001) basal planes of the C14 structured SPP, and the synchroshear was therefore activated.

Microstructure evolution and fracture mechanism of a novel 9Cr tempered martensite ferritic steel during short-term creep

Abstract: In this work, the microstructure evolution and fracture mechanism of a novel 9% chromium tempered martensite ferritic steel G115 were investigated over the temperature range of 625–675 °C using uniaxial creep tests. The creep curves consist of a primary transient stage followed by an apparent secondary stage, and an accelerated tertiary creep regime. The relationship between the minimum creep rate and the applied stress followed Norton's power law. Based on the EBSD analysis, there were no obvious textural features formed after creep deformation, and with the increase in creep time, the number of subgrains slightly increased, and then sharply increased, indicating dynamic recrystallization (DRX) occurs after creep deformation. In addition, three types of precipitates can be observed after creep deformation: W-rich Laves phase, Nb-rich MX, and Cu-rich precipitates. The Nb-rich MX with a square shape and Cu-rich precipitates with an ellipsoidal shape remain very stable. However, the W-rich Laves phases distributed mainly on the grain boundaries have rod-like, chain-like, and bulky shape, which are coarsened significantly. Representative fractographs of the G115 steel after creep deformation exhibit significant necking with an elliptical shape. A dense array of deep and equiaxed dimples appear in the central region under the tested creep conditions. Ductile fracturing is the dominant fracture mechanism during short-term creep deformation.

C14 Laves Phase Metal Hydride Alloys for Ni/MH Batteries Applications

Abstract: C14 Laves phase alloys play a significant role in improving the performance of nickel/metal hydride batteries, which currently dominate the 1.2 V consumer-type rechargeable battery market and those for hybrid electric vehicles. In the current study, the properties of C14 Laves phase based metal hydride alloys are reviewed in relation to their electrochemical applications. Various preparation methods and failure mechanisms of the C14 Laves phase based metal hydride alloys, and the influence of all elements on the electrochemical performance, are discussed. The contributions of some commonly used constituting elements are compared to performance requirements. The importance of stoichiometry and its impact on electrochemical properties is also included. At the end, a discussion section addresses historical hurdles, previous trials, and future directions for implementing C14 Laves phase based metal hydride alloys in commercial nickel/metal hydride batteries.

Effect of Long-Term Service on Microstructure and Mechanical Properties of Martensitic 9% Cr Steel

Abstract: The paper presents the results of research on the X10CrMoVNbN9-1 (T91) steel after long-term service. The material for testing was taken from a pipe section of a boiler superheater coil serviced for around 105,000 h at the temperature of 540 °C, at the pressure of 12.5 MPa. A quantitative analysis including the measurement of mean diameter of subgrains and precipitates as well as the density of dislocations of the examined steel was performed by means of TEM. The microscopic tests of T91 steel were complemented with the results of tests on mechanical properties which included also the short creep tests. After service, the investigated steel was characterized by a retained lath microstructure of tempered martensite with fine subgrain and quite large density of dislocations as well as numerous precipitates. In the microstructure, apart from the particles of M23C6 and MX (VX, NbC, V-wings), the precipitates of Laves phase and single particles of Z phase were revealed. It has been shown that the extent of degradation of the T91 steel microstructure was minor, which resulted from its low temperature of service. Performed tests of mechanical properties showed that these properties fulfilled the minimum requirements for this steel in the as-received condition. A favorable influence of fine precipitates of Laves phase on mechanical properties was observed. Moreover, an insignificant influence of single precipitates of Z phase on the creep resistance of the examined steel was stated.

Influence of Laves phase on creep strength of modified 9Cr-1Mo steel

Abstract: The influence of Laves phase in the microstructure on the creep strength of a modified 9Cr-1Mo tempered ferritic steel, F91, is investigated in this research. Through additional aging heat treatment up to 5000 h, a pronounced increase of Mo-rich Laves phase is found around Cr-rich carbide M23C6, which is pinned on the prior-austenite grain boundaries and martensitic lath boundaries. This secondary precipitation hardening provided by the Laves phase is rapidly offset by its coarsening into large clusters during long-term thermal exposure and under creep conditions, leading to earlier creep rupture than the pristine material. Uniaxial constant-load creep tests are conducted on both aged and non-aged coupons under the same creep conditions. The creep rupture behavior of both materials is rationalized with the assistance of a deformation-mechanism-based true-stress creep model that was developed in the previous work. The effects of Laves phase on the creep strength of F91 are related to its influence on the well-recognized existing deformation mechanisms: dislocation glide, dislocation climb, and grain boundary sliding.

High-entropy alloys: Interrelations between electron concentration, phase composition, lattice parameter, and properties

Abstract: An analysis of more than 200 high-entropy alloys (HEA) allowed us to find interrelations between the electron concentration, phase composition, lattice parameter, and properties of solid solutions with bcc and fcc crystal lattices. Main conditions for the appearance of high-entropy chemical compounds, such as Laves, σ, and μ phases were determined. The necessary condition for the formation of 100% high-entropy σ phase is the formation of σ phase in two-component alloys for different combinations of elements, which are components of the HEA, and the electron concentration should be 6.7–7.3 electrons per atom. To form a 100% high-entropy Laves phase, the following conditions should be fulfilled: the total negative enthalpy of mixing of alloy is about –7 kJ/mol and less; the difference between the atom sizes in a pair is more than 12%; the enthalpy of the mixing of two present elements is less than –30 kJ/mol; and the average electron concentration is 6–7 electrons per atom. It was shown that the ratios of lattice parameters of solid-solution HEA, which were experimentally determined, to the lattice parameter of the most refractory metal in the HEA determine the value of the modulus of elasticity.

Precipitates and Particles Coarsening of 9Cr-1.7W-0.4Mo-Co Ferritic Heat-Resistant Steel after Isothermal Aging.

Abstract: The precipitates obtained by EPE technology from the 9Cr-1.7W-0.4Mo-Co ferritic heat-resistant steel subject to isothermal aging were investigated using SEM, TEM and XRD. The particle size distribution and the coarsening kinetics of M23C6 with duration of isothermal aging were also analyzed with or without consideration of Laves phase. The results show that the isolated dislocations were detected in delta ferrite interior, and the precipitates on delta ferrite and martensite boundaries are obviously larger than other locations. Fe2W-Laves phase can only be found as duration of aging time to 2000 h, and is preferential to form adjacent to M23C6 particles. The small M23C6 particles firstly coarsen, but the large M23C6 are relatively stable during short aging. The total coarsening rate of M23C6 precipitates is 9.75 × 10−28m3s−1, and the coarsening of M23C6 depends on the formation of Laves phase.