A review of refractory high-entropy alloys

This work reviews recent progress in the alloy design,microstructure,and mechanical properties of refractory high-entropy alloys(RHEAs).What’s more,the underlying strengthening mechanisms and deformation behavior are discussed.Composed mainly of near-equimolar refractory elements,RHEAs have superior mechanical properties,especially at high temperatures.However,many of them have limited room-temperature ductility.Much work has been done to solve this trade-off,and some of the RHEAs have the potential to be used for high-temperature applications in the future.In addition to their mechanical properties,RHEAs have other attractive properties,such as biocompatibility and wear resistance,which are discussed.Finally,current problems and future suggestions for RHEAs are discussed.

Elastic-viscoplastic constitutive equations of K439B superalloy and thermal stress simulation during casting process

K439B nickel-based superalloy is a new type of high-temperature material.There is insufficient research on its constitutive equations and numerical modeling of thermal stress.Isothermal tensile experiments of K439B superalloy at different temperatures(20°C-1,000°C)and strain rates(1.33×10^(-3)s^(-1)-5.33×10^(-3)s^(-1))were performed by using a Gleeble-3800 simulator.The elastic moduli at different temperatures(20°C-650°C)were measured by resonance method.Subsequently,stress-strain curves were measured for K439B superalloy under different conditions.The elastic-viscoplastic constitutive equations were established and the correspongding parameters were solved by employing the Perzyna model.The verification results indicate that the calculated values of the constitutive equations are in good agreement with the experimental values.On this basis,the influence of process parameters on thermal stress was investigated by numerical simulation and orthogonal experimental design.The results of orthogonal experimental design reveal that the cooling mode of casting has a significant influence on the thermal stress,while pouring temperature and preheating temperature of shell mold have minimal impact.The distribution of physical fields under optimal process parameters,determined based on the orthogonal experimental design results,was simulated.The simulation results determine separately the specific positions with maximum values for effective stress,plastic strain,and displacement within the casting.The maximum stress is about 1,000.0 MPa,the plastic strain is about 0.135,and the displacement is about 1.47 mm.Moreover,the distribution states of thermal stress,strain,and displacement are closely related to the distribution of the temperature gradient and cooling rate in the casting.The research would provide a theoretical reference for exploring the stress-strain behavior and numerical modeling of the effective stress of the alloy during the casting process.

Microstructure and dry sliding wear behavior of laser clad AlCrNiSiTi multi-principal element alloy coatings

The approximately equimolar ratio A1CrNiSiTi multi-principal element alloy （MPEA） coatings were fab- ricated by laser cladding on Ti-6Al-4V （Ti64） alloy. Scanning electron microscopy （SEM）, equipped with an energy-dispersive spectroscopy （EDS）, and X-ray diffrac- tion （XRD） were used to characterize the microstructure and composition. Investigations show that the coatings consist of （Ti, Cr）5Si3 and NiA1 phases, formed by in situ reaction. The phase composition is initially explicated according to obtainable binary and ternary phase diagrams, and the formation Gibbs energy of TisSi3, VsSi3 and CrsSi3. Dry sliding reciprocating friction and wear tests of the A1CrNiSiTi coating and Ti64 alloy substrate without coating were evaluated. A surface mapping profiler was used to evaluate the wear volume. The worn surface was characterized by SEM-EDS. The hardness and wear resistance of the A1CrNiSiTi coating are well compared with that of the basal material （Ti64）. The main wear mechanism of the AICrNiSiTi coating is slightly adhesive transfer from GCrl5 counterpart, and a mixed layer com- posed of transferred materials and oxide is formed.

Microstructure and hardness of Cu-12% Fe composite at different drawing strains

Cu-12% Fe （in weight） composite was prepared by casting, pretreating, and cold drawing. The microstructure was observed and Vickers hardness was measured for the composite at various drawing strains. Cu and Fe grains could evolve into aligned filaments during the drawing process. X-ray diffraction （XRD） was used to analyze the orientation evolution during the drawing process. The axial direction of the filamentary structure has different preferred orientations from the radial directions. The strain of Fe grains linearly increases with an increase in the drawing strain up to 6.0, and deviates from the linear relation when the drawing strain is higher than 6.0. With an increase in the drawing strain, the microstructure scales of Fe filaments exponentially decrease. The density of the interface between Cu and Fe phases exponentially increases with an increase in the aspect ratio of Fe filaments. There is a similar Hall-Perch relationship between the hardness and Fe filament spacing. The refined microstructure from drawing deformation at drawing strains lower than 3.0 can induce a more significant hardening effect than that at drawing strains higher than 3.0.

Microstructural Characterization of Pure Titanium Treated by Laser Surface Treatment Under Different Processing Parameters

Advanced characterization techniques are utilized to investigate the effect of laser surface treatment on microstructural evolution of pure titanium（Ti）.The results show that there are three distinctly different types of microstructure from surface to substrate in Ti samples,including phase transformation and solidification microstructure in zone I（melting zone）;insufficient recrystallization grains with residual a martensitic plates in zone II（heat-affected zone,HAZ）;fully recrystallization microstructure in zone III（base metal,BM）.The hardness evolution profiles under different laser treatment parameters are similar.The highest hardness in MZ is ascribed to α plate,while the lowest hardness value in HAZ is due to the insufficiently recrystallized grains.The metallurgical process on the laser-modified Ti samples is systematically discussed in this work.

Influence Factors of Aluminum–Slag Interfacial Reaction Under Electric Field

The interfacial reaction between aluminum melt and molten slag under an electric field plays a significant role in aluminum electro-slag refining. Here we studied this interracial reaction within 680 and 820 ℃ under an electric field between 0 and 9 V. The evolution of aluminum composition was analyzed by inductively coupled plasma atomic emission spectroscopy. The dominant factor during the interfacial reaction was identified through orthogonal experiments, in which the slag-to-aluminum mass ratio, initial silicon concentration, electric voltage, reaction time, and temperature were selected as the influence factors. The greatest influence factor on the interracial reaction was found to be the reaction time. Also, single-factor experiments revealed that the reaction kinetic processes largely obeyed an irreversible kinetic model, and the silicon removal efficiency was enhanced by increasing the voltage and slag/metal ratio.