Showing papers on "Laves phase published in 2012"

Effect of Nb addition on the microstructure and properties of AlCoCrFeNi high-entropy alloy

Abstract: The microstructures and properties of the AlCoCrFeNbxNi high-entropy alloys (HEAs) were investigated. Two phases were found in the prepared AlCoCrFeNbxNi HEAs: one is body-centered-cubic (BCC) solid solution phase; the other is the Laves phase of (CoCr)Nb type. The microstructures of the alloy series vary from hypoeutectic to hypereutectic, and the compressive yield strength and Vickers hardness have an approximately linear increase with increasing Nb content. The residual magnetization (Mr) reaches a maximum for AlCoCrFeNb0.1Ni alloy, which is 6.106 emu/g. The factor of Ω, which is defined as entropy of mixing times 1000 over enthalpy of mixing, well predicts the phase formation for the multicomponents alloys.

Microstructures and mechanical properties of multiprincipal component CoCrFeNiTix alloys

Abstract: The purpose of this study is to investigate the effects of Ti addition on the microstructures and mechanical properties of multiprincipal component CoCrFeNiTi x ( x values in molar ratio, x =0, 0.3, and 0.5) alloys. The CoCrFeNi quaternary alloy displayed a crystalline structure constructed by a simple face-centered cubic solid solution, whereas a plate-like structure consisting of a mixture of (Ni, Ti)-rich R phase and (Cr, Fe)-rich σ phase was observed within the face-centered cubic matrix of a CoCrFeNiTi 0.3 alloy. In a CoCrFeNiTi 0.5 alloy, an face-centered cubic matrix, a (Ti, Co)-rich Laves phase, and R+σ mixed phases were discovered. The compressive strength of the alloys rose by approximately 75% after the addition of Ti. Alloys with high levels of Ti content had high yield stress values and low ductility values. The solid-solution strengthening of the face-centered cubic matrix and the secondary-phase hardening were the two main factors that strengthened the alloy. The CoCrFeNiTi 0.3 alloy exhibited a compressive strength of 1529 MPa and a fracture strain of 0.60; this indicates that this material shows potential for the development of a ductile, high-strength alloy.

Irradiation-Induced Formation of Nanocrystallites with C15 Laves Phase Structure in bcc Iron

Abstract: A three-dimensional periodic structure is proposed for self-interstitial clusters in body-centered-cubic metals, as opposed to the conventional two-dimensional loop morphology. The underlying crystal structure corresponds to the $C15$ Laves phase. Using density functional theory and interatomic potential calculations, we demonstrate that in $\ensuremath{\alpha}$-iron these $C15$ aggregates are highly stable and immobile and that they exhibit large antiferromagnetic moments. They form directly in displacement cascades, and they can grow by capturing self-interstitials. They thus constitute an important new element to account for when predicting the microstructural evolution of iron base materials under irradiation.

Structural and electronic properties of the Laves phase based on rare earth type BaM2 (M = Rh, Pd, Pt)

Abstract: We will presented the study of structural, electronic and elastic properties of Laves phase based on rare earth type BaM2 (M = Rh, Pd, Pt). For this, we used the method of linear augmented plane wave (FP-LAPW) based on density functional theory (DFT). We studied the structural properties, we calculated the formation energy to prove the existence of these compounds experimentally and the cohesive energy to determine the energy required to disassemble into its component parts, then the electronic properties. And finally, we calculated the elastic constants.

Laves phase evolution in a modified P911 heat resistant steel during creep at 923 K

Abstract: The evolution of Fe2(W, Mo) Laves phase in a 3%Co modified P911 heat resistant steel was examined during creep tests at 923 K. The tempered martensite lath structure evolved after heat treatment was characterized by dispersion of MX carbonitrides and M23C6 carbides. Appearance of Laves phase particles was recorded after a creep strain of 1%. The mean size of Laves phase particles increased from 190 to 265 nm with increasing strain to 18%. The Laves phase particles were spaced on various boundaries including low-angle boundaries (LABs) of laths/subgrains, but most of these particles (about 90%) were located on high-angle boundaries (HABs) at all strains studied. The size of Laves phase particles located on HABs was larger and their coarsening kinetics was faster than those precipitated on LABs. It is assumed that the evolution of Laves phase during creep is controlled by the grain boundary diffusion of tungsten and molybdenum.

Microstructure and type IV cracking behavior of HAZ in P92 steel weldment

Abstract: The creep rupture behavior and microstructure changes of W strengthened P92 steel weld joints have been investigated at 873 K, 898 K and 923 K. The joints were prepared by submerged arc welding (SAW). The results showed that low ductility type IV fracture took place more easily at higher temperature and lower stress. There would be a critical Larson–Miller parameter (LMP) of 35.5 and a critical applied stress of 120 MPa for type IV fracture. The critical stress was independent of creep temperature. Type IV cracks occurred in the fine grained heat affected zone (FGHAZ), corresponding to the maximum heating temperature just above A c3 , which showed the fine equiaxed microstructure without lath structure and the lowest hardness due to the instability of microstructure. An increased number density of Laves phases precipitated on grain boundaries in FGHAZ compared with other zones of weldment during creep, while the coarsening of M 23 C 6 carbides was not very significant in W strengthened P92 steel. The fracture location in FGHAZ exhibited the most severe creep damage among the various zones of weldment and many cavities formed at the grain boundaries during creep. It was considered that the coarse Laves phase at the grain boundaries acted as the preferential cavity nucleation sites. We believe that the degradation of lath substructure and fast formation of Laves phase may be the main metallurgical factors for the type IV cracking.

Laves Phase Control in Inconel 718 Weldments

Abstract: The detrimental laves formation in fusion zone during welding of Inconel 718 is controlled with compound current pulsing technique along with helium shielding gas. Also solid solution filler wire is used to minimize the niobium segregation. Welds were produced in 2mm thick sheets by GTA welding process and subjected to the characterization techniques. The results show, refined fusion zone microstructure, reduced amount of laves phase, minimum niobium segregation and softer fusion zone in the as welded condition.

Solidification Behaviour of Ti–Cu–Fe–Co–Ni High Entropy Alloys

Abstract: For the first time, we report here that the development of the novel Ti–Cu–Fe–Co–Ni high entropy alloys (HEAs) via vacuum arc melting technique using non consumable tungsten electrode under high purity Ar atmosphere on a water-cooled copper hearth. Ti–Cu–Fe–Co–Ni multicomponent alloys with varying Ti/Cu (x) molar ratio (x = 1/3, 3/7, 3/5, 9/11, 1, 11/9 and 3/2) have been prepared through the tailoring of microstructure to get understanding of the phase formation and the microstructural evolution of these multicomponent HEAs. X-ray diffraction and scanning electron microscopy coupled with energy dispersive spectroscopic results confirm the presence of (Cu)ss, (Co)ss and (β-Ti)ss dendrites with ultrafine eutectic between cubic (Cu)ss and Laves phase (Ti2Co type). The solidification pathways of novel alloys are critically discussed as follows. For x = 9/11, 1, 11/9 and 3/2; firstly, (β-Ti)ss dendrite is formed from the liquid, followed by the liquid phase separation between the cobalt-rich solid solution (Co)ss and copper-rich solid solution (Cu)ss and finally, the remaining liquid undergoes eutectic reaction between copper solid solution (Cu)ss and the Laves phase (Ti2Co Type), whereas for x = 1/3, 3/7 and 3/5; (β-Ti)ss dendrite is formed first from the liquid and then remaining liquid undergoes the liquid phase separation resulting two different dendrites of (Cu)ss and (Co)ss phases. Detailed thermodynamic calculations have been carried to rationalize the formation of stable solid solution phases of these newly developed multicomponent Ti–Cu–Fe–Co–Ni HEAs.

First-principles calculation of phase equilibrium of V-Nb, V-Ta, and Nb-Ta alloys

Abstract: In this paper, we report the calculated phase diagrams of V-Nb, V-Ta, and Nb-Ta alloys computed by combining the total energies of 40–50 configurations for each system (obtained using density functional theory) with the cluster expansion and Monte Carlo techniques. For V-Nb alloys, the phase diagram computed with conventional cluster expansion shows a miscibility gap with consolute temperature T_c=1250 K. Including the constituent strain to the cluster expansion Hamiltonian does not alter the consolute temperature significantly, although it appears to influence the solubility of V- and Nb-rich alloys. The phonon contribution to the free energy lowers T_c to 950 K (about 25%). Our calculations thus predicts an appreciable miscibility gap for V-Nb alloys. For bcc V-Ta alloy, this calculation predicts a miscibility gap with T_c=1100 K. For this alloy, both the constituent strain and phonon contributions are found to be significant. The constituent strain increases the miscibility gap while the phonon entropy counteracts the effect of the constituent strain. In V-Ta alloys, an ordering transition occurs at 1583 K from bcc solid solution phase to the V_(2)Ta Laves phase due to the dominant chemical interaction associated with the relatively large electronegativity difference. Since the current cluster expansion ignores the V_(2)Ta phase, the associated chemical interaction appears to manifest in making the solid solution phase remain stable down to 1100 K. For the size-matched Nb-Ta alloys, our calculation predicts complete miscibility in agreement with experiment.

Effect of Rare Earth Elements and Alloy Composition on Hydrogenation Properties and Crystal Structures of Hydrides in Mg2–xRExNi4

Abstract: The effect of the rare earth elements and alloy composition on the hydrogenation properties and crystal structures of hydrides in Mg2–xRExNi4 (RE = La, Pr, Nd, Sm, and Gd; x = 0.6 and 1.0) was investigated. All Mg2–xRExNi4 alloys had a C15b Laves phase before hydrogenation. Mg1.4RE0.6Ni4 (RE = Pr, Sm, and Gd) alloys were hydrogenated through one plateau to form Mg1.4RE0.6Ni4H∼3.6 while maintaining the C15b structure. Mg1.0RE1.0Ni4 (RE = La, Pr, and Nd) alloys were hydrogenated to ∼1.0 H/M proceeding through two plateaus, and Mg1.0RE1.0Ni4 (RE = Sm and Gd) alloys were hydrogenated to 0.6–0.7 H/M through one plateau. Mg1.0RE1.0Ni4 alloys initially transformed into Mg1.0RE1.0Ni4H∼4 with an orthorhombic structure. In addition it was experimentally confirmed that Mg1.0RE1.0Ni4H∼4 with La, Pr, and Nd transformed into Mg1.0RE1.0Ni4H∼6 with a C15b structure, while no formation of Mg1.0RE1.0Ni4H∼6 (RE = Sm and Gd) was observed at 40 MPa at 250 K. Theoretical calculations suggest that Mg1.0RE1.0Ni4H∼4 with Sm and G...

Studying Plasticity in Hard and Soft Nb–Co Intermetallics†

Abstract: It is shown that micropillar compression can be used to study the plastic flow behavior of both a hard, brittle cubic Laves phase, NbCo2, and a related monoclinic compound, Nb2Co7, which is known to be softer, but is also brittle and has, so far, only been prepared in a fine-grained form. Flow in the Laves phase occurs at a shear stress of 3.1 GPa on the {111} slip system, consistent with previous observations and estimates from hardness measurement. Micropillar compression and electron microscopy of single crystals of Nb2Co7 indicates that flow takes place parallel to the (001) plane, with and type Burgers vectors, at a critical resolved shear stress between 0.1 and 0.5 GPa. It is shown that this is much less than predicted by hardness measurements, consistent with the idea that the hardness of plastically anisotropic materials is determined by flow on the hardest operating slip system.

Cd4Cu7As, the first representative of a fully ordered, orthorhombically distorted MgCu2 Laves phase.

Abstract: The ternary Laves phase Cd4Cu7As is the first intermetallic compound in the system Cu–Cd–As and a representative of a new substitution variant for Laves phases. It crystallizes orthorhombically in the space group Pnnm (No. 58) with lattice parameters a = 9.8833(7) A; b = 7.1251(3) A; c = 5.0895(4) A. All sites are fully occupied within the standard deviations. The structure can be described as typical Laves phase, where Cu and As are forming vertex-linked tetrahedra and Cd adopts the structure motive of a distorted diamond network. Cd4Cu7As was prepared from stoichiometric mixtures of the elements in a solid state reaction at 1000 °C. Magnetic measurements are showing a Pauli paramagnetic behavior. During our systematical investigations within the ternary phase triangle Cd–Cu–As the cubic C15-type Laves phase Cd4Cu6.9(1)As1.1(1) was structurally characterized. It crystallizes cubic in the space group Fd3m with lattice parameter a = 7.0779(8) A. Typically for quasi-binary Laves phases Cu and As are both o...

Microstructure and Mechanical Properties of Two-Phase Cr-Cr 2 Ta Alloys

Abstract: The microstructure and fracture toughness values of a series of binary Cr-Cr2Ta alloys have been studied in the as-cast condition and after long-term annealing of up to 500 hours at 1573 K (1300 °C). With increasing annealing time, both the Cr(Ta) solid solution and the Cr2Ta Laves phases progressively approached equilibrium compositions. This observation was accompanied by the transformation of the Laves phase from the hexagonal C14 to the cubic C15 polytype, along with a small fraction of an intermediate hexagonal polytype. The overall hardness values of the alloys increased with the Laves phase fraction, which was accompanied by a decrease in the room-temperature fracture toughness values. The fracture surface of the three-point bend specimens revealed brittle cleavage-type fracture in all the alloys.

Orientation relationships between icosahedral clusters in hexagonal MgZn2 and monoclinic Mg4Zn7 phases in Mg-Zn(-Y) alloys

Abstract: Intermetallic precipitates formed in heat-treated and aged Mg-Zn and Mg-Zn-Y alloys have been investigated via electron microscopy. Coarse spheroidal precipitates formed on deformation twin boundaries contained domains belonging to either the MgZn2 hexagonal Laves phase or the monoclinic Mg4Zn7 phase. Both phases are structurally related to the quasi-crystalline phase formed in Mg-Zn-Y alloys, containing icosahedrally coordinated zinc atoms arranged as a series of broad rhombohedral units. This rhombohedral arrangement was also visible in intragranular precipitates where local regions with the structures of hexagonal MgZn2 and Mg4Zn7 were found. The orientation adopted by the MgZn2 and Mg4Zn7 phases in twin-boundary and intragranular precipitates was such that the icosahedral clusters were aligned similarly. These results highlight the close structural similarities between the precipitates of the Mg-Zn-Y alloy system.

AB initio study of C14 laves phases in Fe-based systems

Abstract: where X stands for Si, Cr, Mo, W, Ta) were investigated using the pseudopotential VASP (Vienna Ab initio Simulation Package) code employing the PAW-PBE (Projector Augmented Wave - Perdew-Burke-Ernzerhof) pseudopotentials. Full relaxation was performed for all structures studied including the reference states of elemental constituents and the equilibrium structure parameters as well as bulk moduli were found. The structure parameters of experimentally found structures were very well reproduced by our calculations. It was also found that the lattice parameters and volumes of the unit cell decrease with increasing molar fraction of iron. Thermodynamic analysis shows that the Fe2X configurations of Laves phases are more stable than the X2Fe ones. Some of the X2Fe configurations are even unstable with respect to the weighted average of the Laves phases of elemental constituents. Our calculations predict the stability of Fe2Ta. On the other hand, Fe2Mo and Fe2W are slightly unstable (3.19 and 0.68 kJ.mol-1, respectively) and hypothetical structures Fe2Cr and Fe2Si are found unstable as well.

Microstructure and solidification behaviour characterisation of phosphorus and boron doped IN718 superalloy

Abstract: The additions of 0·022–0·025 mass-% phosphorus and 0·010–0·012 mass-% boron could enhance the creep properties and increase the service temperature of IN718 superalloy markedly. However, the as cast microstructure and solidification behaviour of modified IN718 with P and B additions are not completely recognised. In the present paper, differential scanning calorimeter (DSC) technique accompanied by scanning electron microscope and electron probe microanalysis characterisation is applied to understand the effects of P and B additions. It is found that P and B additions may promote the formation of big blocky Laves phase and result in the existence of B bearing phase enriched in Nb and Mo. In the meantime, another endothermic peak appearing before the Laves phase peak in the DSC heating curve is related to the melting of the B bearing phase. After the thermodynamics calculation, this new endothermic peak detected may be involved with the melting of M3B2. Furthermore, the endothermic peak appears bef...