Bcc Phase Research Articles

Role of aging temperature on thermal stability of Co-free Cr0.8FeMn1.3Ni1.3 high-entropy alloy: Decomposition and embrittlement at intermediate temperatures

This study prepared a reduced activation single-phase Co-free Cr0.8FeMn1.3Ni1.3 high-entropy alloy (HEA) through arc melting, which exhibits great potential as a radiation-resistant material. The microstructural evolution and mechanical property changes of the alloy after prolonged aging at temperatures of 300, 500, and 700 °C were systematically studied using scanning and transmission electron microscopy (SEM/TEM/STEM) and electron probe microanalysis (EPMA) techniques. The results reveal that the alloy forms a single-phase FCC solid solution following cold deformation and subsequent recrystallization annealing. After prolonged aging at 300 °C for 1500 h, the microstructure of the alloy remains stable, maintaining a single-phase structure with minimal changes in mechanical properties. After aging at 500 °C for 1500 h, the alloy undergoes complex phase decomposition, giving rise to tetragonal σ-FeCr and L10-NiMn phases, along with minor amounts of Cr-rich BCC and Fe-rich FCC phases. During this stage, the interfaces between σ-FeCr and L10-NiMn phases undergo amorphization, leading to room-temperature embrittlement of the alloy. Following aging at 700 °C for 1500 h, the alloy precipitates the Cr-rich BCC phase, forming a dual-phase microstructure, resulting in increased tensile strength and reduced ductility. This study discloses the thermal stability of Cr0.8FeMn1.3Ni1.3 HEA through extended aging at 300 °C and 500–700 °C for up to 300 h, simultaneously providing reference values for evaluating the stability of other HEA systems. The occurrence of complex phase decomposition significantly impacts the mechanical properties of HEAs, necessitating cautious evaluation of their application potential.

Study on the microstructure and wear resistance of AlCrCuFeNiTix high entropy alloys for surfacing welding

In order to investigate the effect of Ti element content on the microstructure and wear resistance of AlCrCuFeNiTix (x = 0, 0.3, 0.6, 0.9) high entropy surfacing alloy, a melting electrode gas shielded welding technology was used to prepare AlCrCuFeNiTix high entropy surfacing alloy on the surface of carbon steel plates. The microstructure, phase composition, and wear resistance of the alloy were analyzed. The results show that the phase composition of the surfacing alloy becomes a BCC+FCC solid solution phase, with a much higher content of BCC phase than FCC phase. The microstructure consists of disordered BCC phase rich in Fe and Cr, ordered BCC phase rich in Al and Ni, and FCC phase rich in Cu. The microstructure exhibits typical dendritic (DR) and interdendritic (ID) structures. With the increase of Ti element, hard Fe2Ti phase precipitates in the interdendritic zone, and the micro hardness of the alloy shows an increasing trend. The maximum hardness can reach 636 HV, which is 2.4 times that of the base material. With the increase of Ti element, the friction coefficient of the alloy shows a trend of first decreasing and then increasing, and the wear amount first decreases and then increases. When Ti is 0.6, the wear resistance of the high entropy surfacing alloy reaches its best.

Effect of Ti content on achieving development of mechanical properties and wear resistance of CoCrFeNiMoTix high entropy alloy

To achieve the synergy among density, strength, ductility, wear performance and thermal stability, a series of newly developed CoCrFeNiMoTix (x = 2 and 16.666 at. %) high entropy alloys (HEAs) via multiple phases nano crystalline structure were successfully fabricated using mechanical alloying (MA) following by spark plasma sintering (SPS). The results demonstrated that the SPSed Ti2-HEA and Ti16.6-HEA have a multiple phases structure consisting of FCC structure (Ni-type as a major phase), two types of BCC structure (Mo-type and Ti-type), topologically close packed (TCP) phases (laves, sigma (σ) and R phases) and geometrically close packed (GCP) phase (L12 precipitates). The micro hardness, strength and specific strength were increased from 711 HV, 1770 MPa and 197 MPa.Cm3/g for the Ti2-HEA to 817 HV, 1830 MPa and 236 MPa.Cm3/g for the Ti16.6-HEA, respectively, which were affected by more percentages of precipitates, intermetallic and BCC phases. Besides, fracture strain, coefficient friction, wear rate and the surface roughness were decreased from 16%, 0.19, 4.9*10−4 mm3 N−1 m−1 and 54.2 for the Ti2-HEA to 14.5%, 0.17, 2.7*10−4 mm3 N−1 m−1 and 31.6 for the Ti16.6-HEA. The ultimate shear strength (USS) values at 300 °C, 400 °C, 500 °C and 600 °C were increased from 960 MPa, 820 MP, 646 MPa and 24 MPa for the Ti2-HEA to 1180 MPa, 1010 MP, 802 MPa and 94 MPa for the Ti16.6-HEA, respectively. The Ti16.6-HEA illustrated higher thermal stability at all temperatures, where the USS slightly was decreased from 300 °C to 500 °C with a sudden drop at 600 °C.

Additively manufactured Ni-20Cr to V functionally graded material: Computational predictions and experimental verification of phase formations

A database for the Cr-Ni-V system was constructed by modeling the binary Cr-V and ternary Cr-Ni-V systems using the CALPHAD approach aided by density functional theory (DFT)-based first-principles calculations and ab initio molecular dynamics (AIMD) simulations. To validate this new database, a functionally graded material (FGM) using Ni-20Cr and elemental V was fabricated using directed energy deposition additive manufacturing (DED AM) and experimentally characterized. The deposited Ni-20Cr was pure fcc phase, while increasing the amount of V across the gradient resulted in the formation of sigma phase, followed by the bcc phase. The experimentally measured phase data was compared with computational predictions made using a Cr-Ni-V thermodynamic database from the literature as well as the database developed in the present work. The newly developed database was shown to better predict the experimentally observed phases due to its accurate modeling of binary systems within the database and the ternary liquid phase, which is critical for accurate Scheil calculations.

Tribological behavior of detonation sprayed CrFeNiAl0.3Ti0.3 high entropy alloy coatings in seawater environment

In this study, the CrFeNiAl0.3Ti0.3 high entropy alloy coating was deposited onto a SUS304 substrate steel by detonation spraying. The CrFeNiAl0.3Ti0.3 coating consists of FCC, BCC and B2 phases. It’s hardness, bonding strength and porosity are 576 HV, 86 MPa and 0.87%, respectively. In seawater, the tribological properties of the coating depends on the counterparts. The CrFeNiAl0.3Ti0.3/SiC sliding-pairs presents the best tribological properties in seawater due to the lubricious SiO2·nH2O and a dense tribo-layer. For AISI 440 C and Al2O3 counterparts, moderate abrasive wear occurs for the coating. For CuZn40 alloy counterpart, a soft copper-rich tribo-layer reduces the wear. For Ti6Al4V counterpart, many hard TiO2 and Al2O3 abrasion particles lead to the severe wear of the two contacting surfaces.

Effect of Milling Time on the Structure Stability of FeMnNiCrAl Non-equiatomic High-Entropy Alloy

A non-equiatomic Fe34.9Ni30.2Mn18.6Cr9.3Al7 high-entropy alloy (HEA) was synthesized by mechanical alloying using different milling times. To study the effect of milling time on the structure stability, X-ray diffraction (XRD), scanning electron microscopy and energy-dispersive X-ray spectroscopy were conducted. For comparison, an Al-free alloy (Fe37.5Ni32.5Mn20Cr10) was produced at 25 h milling. XRD indicated a single FCC phase alloy after 5 h milling time and a dual FCC and BCC phase at 25 h milling time. Clearly, it is found that Al addition is not necessarily the main factor leading to BCC phase formation as reported for similar HEAs produced by casting route. Increasing the milling time, the lattice strain increased reaching an average maximum value of 0.8% with an increase in d-spacing while the crystallite size was reduced to about 5.7 nm. A dual-phase structure formation was related to a decrease in the accumulated strain (ca.32%) confirming a strain-induced transformation.

Effect of Cr/Mn Addition in TiVNb on Hydrogen Sorption Properties: Thermodynamics and Phase Transition Study

High-entropy alloys (HEAs) are a promising class of materials that can grant remarkable functional performances for a large range of applications due to their highly tunable composition. Among these applications, recently, bcc HEAs capable of forming fcc hydrides have been proposed as high-capacity hydrogen storage materials with improved thermodynamics compared to classical metal hydrides. In this context, a single-phase bcc (TiVNb)0.90Cr0.05Mn0.05 HEA was prepared by arc melting to evaluate the effect of combined Cr/Mn addition in the ternary TiVNb. A thermodynamic destabilization of the fcc hydride phase was found in the HEA compared to the initial TiVNb. In situ neutron and synchrotron X-ray diffraction experiments put forward a fcc → bcc phase transition of the metallic subnetwork in the temperature range of 260–350 °C, whereas the H/D subnetwork underwent an order → disorder transition at 180 °C. The absorption/desorption cycling demonstrated very fast absorption kinetics at room temperature in less than 1 min with a remarkable total capacity (2.8 wt.%) without phase segregation. Therefore, the design strategy consisting of small additions of non-hydride-forming elements into refractory HEAs allows for materials with promising properties for solid-state hydrogen storage to be obtained.

Dual wire arc additive manufacturing of compositionally graded Alx-Co-Cr-Fe-Ni high-entropy alloy

Graded materials are promising alloys due to their improved strength and ductility. In this study, a functionally graded Alx-Co-Cr-Fe-Ni high entropy alloy with a variation of Al concentration along the building direction was produced in-situ by dual wire arc additive manufacturing for the first time, and the microstructure and mechanical properties of the deposited alloy were investigated. The results showed that the FCC + BCC dual-phase gradually transformed to a single BCC phase with increasing Al content, the precipitation temperature of the ordered B2 phase became high, and the precipitation temperature range of the disordered BCC phase became wider, which promoted the formation of the disordered BCC phase. With the change in phase ratio, the hardness gradually increased from 342 HV to 397 HV, and the bottom deposited alloy had an optimal combination with a compressive strength of 827.4 MPa and elongation of 42.3%. Compared to the bottom region, the top region shows much higher sliding wear resistance. The high-density dislocations and grain refinement result in better toughness and strength in the bottom region, while the segregation of Cr elements at the grain boundaries changes the properties of the grain boundaries and makes them more brittle in the top region. This investigation realizes a novel method to fabricate GHEAs, which can produce a more convenient and cost-effective gradient material with a complex composition.

Transitional and refractory elements-based high entropy alloy developed through mechanical alloying and microwave sintering: Tribological and corrosion responses

Over the last decade, High Entropy Alloys (HEAs) gained recognition in various sectors (nuclear, aerospace, marine, and biomedical) due to their excellent strength, thermal stability, biocompatibility, wear and corrosion characteristics. This study deals with the development of a novel HEA through a microwave-assisted powder metallurgy, which is an economical approach and overcome the limitations of conventional fabrication techniques. A series of multi-principal alloys are developed, and noted that the HEA is developed through 15 h of ball milling of metallic powders followed by compaction and microwave sintering at 1400 °C at controlled heating and cooling rates. Thus, developed alloys were further examined to record their mechanical properties, tribological, and corrosion responses. The microstructural and metallurgical changes were recorded using Field Emission Scanning Electron Microscopy (FESEM), X-Ray Diffraction (XRD), and X-Ray Photoelectron Spectroscopy (XPS). Finally, the tribological and corrosive responses were recorded using a multifunctional tribometer and a potentiostat (potentiodynamic polarization and electrochemical impedance spectroscopy (EIS)). It is observed that the 1400 °C sintered HEA exhibited superior mechanical, tribological, and corrosive properties. It is worth hypothesizing that the transformation of dual phases (BCC and FCC) to the single BCC phase was responsible for the exceptional properties. Hence, this affordable fabrication approach can be used to develop bulk HEAs to explore other functional characteristics in the future.

Cultivating a comprehensive understanding of microstructural attributes and wear mechanisms in FeCrCoNiAlTix high-entropy alloy coatings on TC6 substrates through laser cladding fabrication

Titanium effectively improves the hardness of FeCrCoNiAl high-entropy alloy (HEA) coatings. In this study, FeCrCoNiAlTix (x = 0, 0.3, 0.5, 0.8, and 1.0) high-entropy alloy coatings were successfully prepared on TC6 titanium alloy substrates via laser cladding(LC). Through the preparation of LC, the effects on the microstructure, phase composition and fretting wear behavior of coatings with different Ti molar ratios were investigated in detail. The results showed that the FeCrCoNiAlTix (x = 0, 0.3) HEA coatings consisted of face-centered cubic (FCC) and body-centered cubic (BCC) phases. With increasing Ti content of the FeCrCoNiAlTix (x = 0.5, 0.8, and 1.0) HEA coatings, a new eutectic BCC2 hard phase appeared. The FCC phase was the continuous in the microstructure, whereas the BCC and BCC2 phases formed a hard skeleton. The hardness of the FeCrCoNiAlTix HEA coatings increased with increasing Ti content, and the hardness of the Ti1.0 HEA coating reached a maximum of 848 HV0.1. The FeCrCoNiAlTix (x = 0,0.3,0.5,0.8, and 1.0) HEA coatings effectively improved the resistance of TC6 alloys to fretting by fatigue, oxidative, adhesive, and abrasive wear.

Strengthening mechanisms of femtosecond laser fabricated multi-phase ZrNbMoTaW body-centered cubic refractory high entropy alloy with enhanced hardness

The present work reports a new fabrication method for refractory high-entropy alloy (RHEA) based on femtosecond laser. The strengthening mechanisms in ZrNbMoTaW body centered cubic RHEA prepared via femtosecond laser fabricating are discussed comprehensively. By analyzing the mechanical properties of femtosecond laser fabricated ZrNbMoTaW RHEAs with different processing parameters, the optimal femtosecond laser fabricating window of ZrNbMoTaW RHEAs with enhanced microhardness and low porosity was obtained. The grain morphology, phase structure, elemental distribution, local misorientation and lattice parameters of ZrNbMoTaW RHEA at optimized processing parameters were investigated. The instantaneous fabricating temperature was measured to analyze the effect of femtosecond laser on powder melting. The results show that the microstructure of ZrNbMoTaW RHEA consists of homogeneous BCC1 and BCC2 phase distribution with refined grains (sizeave = 2.1 μm). Based on the numerical deduction of RHEA with enhanced hardness, contributions to the multiple strengthening mechanisms are given to establish the relationship between the microstructure and hardness of femtosecond laser fabricating ZrNbMoTaW RHEAs, including frictional stress (16.4%), dislocation hardening (17.9%), grain boundary strengthening (9.9%) and solid solution strengthening (55.8%), respectively. Adequate solid solution effect, high dislocation density and low grain size endow ZrNbMoTaW RHEA with high hardness (Vickersvalue = 565.7 HV). These suggest that femtosecond laser is influential in governing the mutual solubility of multiple elements and introduction of strengthening factors. The revealed strengthening mechanisms can serve as a basis for design and the improvement of mechanical properties in NbMoTaW-based RHEAs.

Microstructure and Dry-Sliding Wear Resistance of CoCrFeNiMoTix High Entropy Alloy Coatings Produced by Laser Cladding

The new-type CoCrFeNiMoTix high-entropy alloy coatings were successfully devised and prepared on Q235 steel using laser cladding. Influence of Ti content on their microstructure and wear-resistance was studied systematically; the relevant mechanisms were deeply revealed. The CoCrFeNiMoTix coatings consisted of NiTi, FCC, and BCC phases, and with the increasing of Ti content, contents of BCC phase and FCC phase gradually increased and decreased, respectively. The CoCrFeNiMoTi0.75 coating had the highest hardness (950 HV), which was about 6.5 times higher than the substrate (Q235 steel, 150 HV). According to Archard law, metal materials’ wear resistance is generally proportional to hardness; thus, the CoCrFeNiMoTi0.75 high entropy alloy coating with the highest hardness showed the best wear resistance, exhibiting a wear mechanism of slight abrasive wear.

Machine learning guided BCC or FCC phase prediction in high entropy alloys

High entropy alloys （HEAs） have excellent properties because they can form simple solid solution (SS) phases, including body-centered cubic (BCC) phase, face-centered cubic (FCC) phase, or FCC + BCC phase, so phase prediction is the first step in alloy design. In current research, machine learning (ML) approach had been widely used to guide the discovery and design of materials. The prediction of HEAs phase structure based on machine learning (ML) is a hot topic. In this work, five ML algorithms were utilized to predict HEAs for SS and amorphous (AM) phases based on 399 collected data sets, including 120 BCC alloys, 87 FCC alloys, 82 BCC + FCC alloys and 110 a.m. alloys. To enhance the model's accuracy, grid search and K-fold cross validation were used to optimize performance. Valence electron concentration (VEC) and ΔHmix exhibit high importance in prediction in compared to other parameters. The results show that the random forest can effectively distinguish BCC phase, FCC phase, mixed solid solution phase (FCC + BCC) and AM, with an accuracy is 0.87. After that, the CoCrFeNiAlx (x = 0, 0.5, 1) system alloys were characterized by XRD and SEM-EDS. The experimental results validated that the phase structure of CoCrFeNiAlx alloys changed from FCC to BCC + FCC and BCC with the increase of Al content, which is consistent with the ML prediction.

Effect of ultrasonic treatment on comprehensive properties of (Ti32.5V27.5Zr7.5Nb32.5)0.91Ni0.09 high entropy hydrogen storage alloy

This study verified the effect of ultrasonic treatment (hereinafter referred to as UST) on the comprehensive properties of (Ti32.5V27.5Zr7.5Nb32.5)0.91Ni0.09 high entropy alloy (HEA). The UST refines the interdendritic of (Ti32.5V27.5Zr7.5Nb32.5)0.91Ni0.09 HEA and reduces the content of C14 Laves phase. The increase of BCC/C14 Laves value has an effect on the diffusion of hydrogen atoms, this factor makes the hydrogen absorption amount appear the law of the sample before UST > the sample after UST at low and medium temperature, and the sample before UST < the sample after UST at high temperature. The UST makes the BCC phase at the dendrite become coarse. This factor prolongs the distance for hydrogen atoms to detach from the BCC phase, making the removal of hydrogen atoms difficult.

Oxidation behavior of lightweight Al2NbTi3V2Zr high entropy alloy at 1000°C

AbstractLightweight high entropy alloy (LWHEA) has great potential in aeroengine hot component applications. Anticorrosion property of LWHEA at high temperatures is one of the key determining factors. In this study, an LWHEA containing Al, Nb, Ti, V, and Zr elements with a molar ratio of 2: 1: 3: 2: 1 was designed and prepared by low energy (LE) and high energy (HE) mechanical milling followed by vacuum hot pressing. The sintered LE alloy consisted of a simple cubic matrix and α intermetallic while the BCC matrix along with α and β intermetallic phases were observed in HE alloy. The oxidation of LE and HE at 1000°C for 48 h was mainly concentrated on α and β phases, and the resultant oxides were ZrO2 and TiO2. Weight gain curve of HE sample within oxidation time of 48 h followed the parabolic law while a two‐stage parabola was found in LE bulk. The steady‐state oxidation of α precipitate was dominant within 12 h and showed a preferential oxidation mode. High temperature induced simple cubic matrix to stable BCC phase transformation and thus LE alloy with longer oxidation time and HE exhibited a similar uniform oxidation. The oxidation rate of the alloy is related to the content of α and β phases.

Pitting corrosion mechanism of BCC+FCC dual-phase structured laser cladding FeCoCrNiAl0.5Ti0.5 HEAs coating

This work elaborated the microstructure and corrosion behaviors of laser cladding (LC) FeCoCrNiAl0.5Ti0.5 high-entropy alloys (HEAs) coating on AISI 1045 steel substrate. It revealed that the LC FeCoCrNiAl0.5Ti0.5 HEAs coatings were composed of dendritic region (DR) and interdendritic region (IR). Furthermore, DR was mainly composed of BCC phase while IR was composed of FCC phase. There was also the transformation of BCC to L21 structure detected due to the addition of Ti element. Due to the uniform microstructure and addition of corrosion-resistant elements (such as Cr), the dual-phase HEAs coatings exhibited pretty good corrosion resistance. A dense passive film would form during the corrosion process to protect the inner structure. It was demonstrated that the corrosion rate of the coating was only 32.8% of that of the substrate. However, the pores formed during the LC process on the surface might be the initial site for pitting corrosion.

Corrosion resistance investigation of the Ti–Al–Mo system supported by CALPHAD approach and key experiments

The corrosion resistance of Ti–Al–Mo alloys in seawater were investigated by thermodynamic calculations and electrochemical tests. Four as-cast alloys were designed based on thermodynamic calculations. The calculated results were verified by experimental determination. The Gibbs energy and electrochemical experiment results showed Ti55Al40Mo5 alloy has the most positive Gibbs energy and the best corrosion resistance. The bcc(Ti) phase has best corrosion resistance among AlTi, AlTi3 and bcc(Ti). The present work further proves the important role of phase diagram calculation in alloy design, and thermodynamic database provides important theoretical support for efficient development of new alloys.

Influence of the Gas Flow Rate on the Crack Formation of AlCoCrNi High-Entropy Metallic Film Fabricated Using Magnetron Sputtering

In the present study, the AlCoCrNi high-entropy metallic film was deposited on a Si wafer using a magnetron sputtering system. To capture the effects of the sputtering parameters on the microstructure and mechanical properties of the film, the flow rate of Ar gas injected into the chamber (5, 7, and 8 sccm) was controlled. All films were identified as being of BCC phase with compositions of near equiatomic proportions, regardless of the gas flow rates. Nano-scale clusters were observed on the surfaces of all films, and nano-cracks were found in the film deposited at the Ar gas flow rate of 8 sccm, unlike the films deposited at the gas flow rates of 5 and 7 sccm. Detailed microstructural analysis of film deposition at an Ar gas flow rate of 8 sccm indicated that the void boundaries contribute to the formation of nano-cracks. The nano-indentation results indicated that the Ar gas flow rate 5 sccm specimen, with the smallest cluster size at the topmost surface, showed the highest hardness (12.21 ± 1.05 GPa) and Young’s modulus (188.1 ± 11 GPa) values.

Aging temperature effect on the phase composition, microstructure, and selected mechanical properties of non-equiatomic TiZrNbTaMo/Mn HEAs targeted for biomedical implants

High entropy alloys (HEAs) have been developed for many applications and recognized for their superior strength, ductility, and corrosion resistance, attracting the attention for biomedical applications. This study aims to evaluate the effect of aging treatment at distinct temperatures on the phase composition, microstructure, and selected mechanical properties of non-equiatomic TiZrNbTaMo and TiZrMoNbTaMn HEAs for potential use as biomedical implants. The TiZrNbTaMn sample exhibited a single BCC phase, while the TiZrNbTaMo a major BCC and minor HCP phases. The microstructure of the TiZrNbTaMo sample was composed of BCC grains with some acicular precipitations of the HCP phase in the grain boundaries, having the aging temperature acted in the grain growth and secondary phase precipitation. Contrarily, the TiZrNbTaMn sample exhibited larger BCC grains permeated with a minor amount of acicular structures, which were entirely decomposed with the aging treatments. The semi-quantitative chemical analysis indicated that the phase composition was a result of elemental chemical segregation, having the refractory metals (Mo, Ta, and Nb) preferentially located in the inner region of the grains, while the other ones (Ti, Zr, and Mn) in the grain boundary. The aging treatment acted in the phase precipitation/dissolution oppositely on the samples through the atomic diffusion mechanism induced by the temperature. The elastic modulus and Vickers microhardness were sensitive to the chemical and phase composition of the samples, having the TiZrNbTaMo aged at 773 K achieved better performance than the commercial Ti-6Al-4 V alloy for use as biomedical implants.

Unveiling the effect of Ni on the formation and structure of Earth’s inner core

Ni is the second most abundant element in the Earth's core. Yet, its effects on the inner core's structure and formation process are usually disregarded because of its electronic and size similarity with Fe. Using ab initio molecular dynamics simulations, we find that the bcc phase can spontaneously crystallize in liquid Ni at temperatures above Fe's melting point at inner core pressures. The melting temperature of Ni is shown to be 700 to 800 K higher than that of Fe at 323 to 360 GPa. hcp, bcc, and liquid phase relations differ for Fe and Ni. Ni can be a bcc stabilizer for Fe at high temperatures and inner core pressures. A small amount of Ni can accelerate Fe's crystallization at core pressures. These results suggest that Ni may substantially impact the structure and formation process of the solid inner core.