Showing papers on "Laves phase published in 2018"

Effect of Ti on microstructures and mechanical properties of high entropy alloys based on CoFeMnNi system

Abstract: In this study, the microstructures and mechanical properties of CoFeMnNiTix (where the molar ratio (x) is 0, 0.25, 0.5, and 0.75) high entropy alloys were examined. Both the CoFeMnNi and CoFeMnNiTi0.25 alloys exhibit a simple face-centered cubic (FCC) solid-solution structure, whereas the CoFeMnNiTi0.5 and CoFeMnNiTi0.75 alloys exhibit a dual-phase structure consisting of a FCC solid-solution and a Laves phase. The Laves phase is identified as a Fe2Ti type and has a hexagonal C14 structure. The volume fraction of the Laves phase increases with increasing the Ti content. The Ti content has an obvious effect on the mechanical properties of the alloys. The yield strength and hardness of the alloys improve but ductility simultaneously decreases as the Ti content increases. The CoFeMnNiTi0.5 alloy has a good balance of both strength and ductility, and is superior to other similar CoCrFeNi-based and CoCrCuFeNi-based alloys. The yield strength, fracture strength and fracture strain of the CoFeMnNiTi0.5 alloy can reach 803 MPa, 2300 MPa and 34%, respectively. In addition, nanoindention tests show that the nanohardness value of the Laves phase is almost two times of the FCC phase, and the elastic modulus of the Laves phase is higher than that of the FCC phase in the current alloy system. By measuring the first pop-in event, the deformation behaviors of FCC and the Laves phases of the CoFeMnNiTi0.5 alloy were investigated.

Absence of dynamic strain aging in an additively manufactured nickel-base superalloy.

Abstract: Dynamic strain aging (DSA), observed macroscopically as serrated plastic flow, has long been seen in nickel-base superalloys when plastically deformed at elevated temperatures. Here we report the absence of DSA in Inconel 625 made by additive manufacturing (AM) at temperatures and strain rates where DSA is present in its conventionally processed counterpart. This absence is attributed to the unique AM microstructure of finely dispersed secondary phases (carbides, N-rich phases, and Laves phase) and textured grains. Based on experimental observations, we propose a dislocation-arrest model to elucidate the criterion for DSA to occur or to be absent as a competition between dislocation pipe diffusion and carbide–carbon reactions. With in situ neutron diffraction studies of lattice strain evolution, our findings provide a new perspective for mesoscale understanding of dislocation–solute interactions and their impact on work-hardening behaviors in high-temperature alloys, and have important implications for tailoring thermomechanical properties by microstructure control via AM. Detrimental serrated plastic flow via dynamic strain aging (DSA) in conventionally processed nickel superalloys usually occurs during high temperature deformation. Here, the authors suppress DSA via a unique microstructure obtained using additive manufacturing and propose a new dislocation-arrest model in nickel superalloys.

Effect of microalloying with silicon on high temperature oxidation resistance of novel refractory high-entropy alloy Ta-Mo-Cr-Ti-Al

Abstract: The effect of 1 at.% Si addition to the refractory high-entropy alloy (HEA) Ta–Mo–Cr–Ti–Al on the high temperature oxidation resistance in air between 900 °C and 1100 °C was studied. Due to the formation of protective chromia-rich and alumina scales, the thermogravimetric curves for Ta–Mo–Cr–Ti–Al and Ta–Mo–Cr–Ti–Al–1Si showed small mass changes and low oxidation rates which are on the level of chromia-forming alloys. The oxide scales formed on both alloys at all temperatures are complex and consist of outermost TiO2, intermediate Al2O3, and (Cr, Ta, Ti)-rich oxide at the interface oxide/substrate. The Si addition had a slightly detrimental effect on the oxidation resistance at all temperatures primarily as a result of increased internal corrosion attack observed in the Si-containing HEA. Large Laves phase particles distinctly found in the Si-containing alloy were identified to be responsible for the more rapid internal corrosion.

Effect of processing parameters on microstructure of laser solid forming Inconel 718 superalloy

Abstract: The present work studies the influence of various parameters on the as-deposited Inconel 718 (IN718) superalloy using laser solid forming method. The microstructure and mechanical properties of IN718 superalloys were analysed. Fine and coarse columnar dendritic microstructure of as-deposited IN718 superalloy are respectively obtained at different laser power and laser beam diameter. The average widths of columnar grains of LSF IN718 superalloy enlarge with increasing laser power and laser beam diameter. The formation of Nb and Mo rich Laves phase in the inter-dendritic regions is found. The highest yield strength and ultimate tensile strength is about 729 MPa and 936 MPa, respectively.

A New Design of In Situ Ti(C,N) Reinforced Composite Coatings and Their Microstructures, Interfaces, and Wear Resistances

Abstract: Here, a unique combination of a novel carbon-nitrogen source (g-C3N4) with different mole ratios of Ti/g-C3N4 has been utilized to fabricate iron matrix composite coatings by a synchronized powder feeding plasma transferred arc (PTA) cladding technology. The results show that submicron Ti(C,N) particles are successfully fabricated in situ on a Q235 low carbon steel substrate to reinforce the iron matrix composite coatings and exhibit dense microstructures and good metallurgical bonding between the coating and the substrate. The microstructure of the coating consists of an α-Fe matrix and Ti(C,N) particles when the mole ratio of Ti/g-C3N4 is no more than 5:1. The microhardness and wear resistance of the coating gradually improve with increasing abundance of the in-situ-synthesized Ti(C,N) particles. Interestingly, for a Ti/g-C3N4 mole ratio of 6:1, a fine lamellar eutectic Laves phase (Fe2Ti) appears, and this phase further improves the microhardness and wear resistance of the coating. The microhardness of the coating is 3.5 times greater than that of the Q235 substrate, and the wear resistance is enhanced 7.66 times over that of the substrate. The Ti(C,N)/Fe2Ti and Fe2Ti/α-Fe interfaces are very clean, and the crystallographic orientation relationships between the phases are analyzed by high-resolution transmission electron microscopy (HRTEM) and an edge-to-edge matching model. The theoretical predictions and the experimental results are in good agreement. Furthermore, based on the present study, for the solidification process near equilibrium, smaller interatomic spacing misfits and interplanar spacing d-value mismatches contribute to the formation of crystallographic orientation relationships between phases during the PTA cladding process. The existence of orientation relationships is beneficial for improving the properties of the coatings. This work not only expands the application fields of g-C3N4 but also provides a new idea for the preparation of Ti(C,N) particle-reinforced composite coatings.

Study on the effect of ageing on laves phase evolution and their effect on mechanical properties of P92 steel

Abstract: P92 steel is candidate material for application in reactor pressure vessels in nuclear power plants. In present investigation, Laves phase evolution (at 650 °C) with varying ageing time (upto 3000 h) in P92 steel and their effect on mechanical properties have been investigated. During thermal ageing, the microstructure analysis showed the evolution of Laves phase that degrades the strength of P92 steel. The formation of Laves phase was observed after the thermal ageing of 720 h and it showed higher coarsening rate in ageing time range of 720 h −1440 h. The Laves phase formation was also confirmed by the XRD analysis, and line mapping. The strength and ductility decreased as a result of deprivation of solid solution strengthening and formation of Laves phase. The hardness of P92 steel was also affected by ageing time but less pronounced as compared to strength. Charpy toughness was also reduced continuously with increase in ageing time as a result of thermal straining of particles and Laves phase formation.

Microstructural and Microhardness Evolution from Homogenization and Hot Isostatic Pressing on Selective Laser Melted Inconel 718: Structure, Texture, and Phases

Abstract: In this work, the microstructure, texture, phases, and microhardness of 45° printed (with respect to the build direction) homogenized, and hot isostatically pressed (HIP) cylindrical IN718 specimens are investigated. Phase morphology, grain size, microhardness, and crystallographic texture at the bottom of each specimen differ from those of the top due to changes in cooling rate. High cooling rates during the printing process generated a columnar grain structure parallel to the building direction in the as-printed condition with a texture transition from (001) orientation at the bottom of the specimen to (111) orientation towards the specimen top based on EBSD analysis. A mixed columnar and equiaxed grain structure associated with about a 15% reduction in texture is achieved after homogenization treatment. HIP treatment caused significant grain coarsening, and engendered equiaxed grains with an average diameter of 154.8 µm. These treatments promoted the growth of δ-phase (Ni3Nb) and MC-type brittle (Ti, Nb)C carbides at grain boundaries. Laves phase (Fe2Nb) was also observed in the as-printed and homogenized specimens. Ostwald ripening of (Ti, Nb)C carbides caused excessive grain growth at the bottom of the HIPed IN718 specimens, while smaller grains were observed at their top. Microhardness in the as-fabricated specimens was 236.9 HV and increased in the homogenized specimens by 19.3% to 282.6 HV due to more even distribution of secondary precipitates, and the nucleation of smaller grains. A 36.1% reduction in microhardness to 180.5 HV was found in the HIPed condition due to γ ″ phase dissolution and differences in grain morphology.

Influence of microwave hybrid heating on the sliding wear behaviour of HVOF sprayed CoMoCrSi coating

Abstract: CoMoCrSi superalloy powder (Tribaloy-T400) consists of intermetallic laves phase and primary eutectic phase of Co-rich solid solution. Processing of Tribaloy-T400 powder is carried out through high-energy ball milling (HEBM) technique to obtain a higher volume fraction of intermetallic laves phases. The feedstock is sprayed using high-velocity-oxy-fuel (HVOF) process on titanium grade-15 substrate. The coating microstructure is homogenized by microwave hybrid heating technique. Characterization of feedstock, as-sprayed and microwave fused coatings is done by using Scanning Electron Microscopy (SEM), Energy dispersive spectroscopy (EDS) and x-ray Diffraction (XRD). Porosity, surface roughness, microhardness, and bond strength are measured. Adhesive wear behavior of the coatings under the dry sliding condition is evaluated at an applied load of l0 and 20 N and temperature of 200, 400 and 600 °C Fused coating exhibit higher wear resistance than the as-sprayed coatings and substrate. The hard intermetallic laves phases which are amorphous (bulk metallic glass) in nature strengthen the coating at high temperatures.

High Electron Density on Ru in Intermetallic YRu2: The Application to Catalyst for Ammonia Synthesis

Abstract: Ruthenium is the most effective catalyst reported to date for ammonia synthesis under mild conditions, especially when an electron promoter is used. However, electron donation from the promoter has not been sufficient because the promoter contacts with Ru only through its surface. Here, we report a Laves phase intermetallic bulk catalyst, YRu2, which has higher electron density on Ru. This is derived from large electron transfer from Y to Ru, which is first confirmed by X-ray absorption fine structure measurements and theoretical calculations. In addition, YRu2 has high hydrogen solubilities leading to suppression of hydrogen poisoning, a common drawback of Ru-based catalysts. Consequently, YRu2 exhibits higher catalytic activity for ammonia synthesis over 300 times that with pure ruthenium. The present results suggest a simple concept for ammonia synthesis: Laves phase intermetallic compounds of Ru and more electropositive metals are more efficient catalysts than pure Ru because of the large electron pro...