Pure Fcc Research Articles

Triple junction excess energy in polycrystalline metals

The energetics of triple lines are often negligible in polycrystalline systems, but may play a significant role in the finest nanocrystals, and in fact lower the excess defect energies of those polycrystals. This paper develops a methodology to assess polycrystalline average grain boundary and triple junction excess energies for pure fcc metals Ni, Cu, Al, Pd, Pt, Ag and Au using embedded atom method potentials. It is found that there are correlations between the triple line energy and physical quantities such as grain boundary and dislocation line energy, but with a negative sign indicating that triple junctions reduce intergranular excess energy per area on average. The relationship with grain boundary energy is of order ∼−4.5 × 10−10 m, and the triple junction energy is about −1/12 of the dislocation line energy. Despite their low energy, triple junctions can significantly affect total system energy due to their high density in the finest nanocrystals; for example, 6-nm Pd nanocrystals have an effective intergranular energy of ∼0.83 J/m2 (compared with the large grain size limit of 0.93 J/m2), translating to a measurable bulk excess enthalpy of ∼6 kJ/mol. Such excess enthalpy is experimentally assessable, and the present framework can be used to measure triple junction energies. For instance, re-analyzing data of Lu and Sun (Phil. Mag., 1997) we obtain grain boundary and triple junction energies of 0.33 J/m2 and −3.0 × 10−10 J/m respectively for Selenium nanocrystals, which can be compared with modeled values of 0.76 J/m2 and −1.02 × 10−10 J/m by using our method with a published bond-order potential for Se.

Structural, dielectric and Magnetic properties of Samarium doped (Ni–Zn) based Spinel Ferrite (Ni0.5Zn0.5SmxFe2-xO4) nanomaterials

Samarium doped Ni-Zn ferrites (Ni0.5Zn0.5SmxFe2-xO4) were synthesized using sol-gel auto-combustion technique with Sm3+ doping concentrations varying as x = 0.00, 0.05, 0.10, 0.15, and 0.20. The prepared nanomaterials were examined using X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), complex impedance spectroscopy (CIS) and vibrating sample magnetometer (VSM). The XRD examination verified the pure FCC spinel phase of all samples with no impurity peak. The FTIR analysis for spinel molecules formation identified two frequency bands at the octa and tetrahedral sites. The TEM analysis confirmed the shape, size and distribution of prepared nanoparticles. The different dielectric properties were analyzed with a frequency range of 1 MHz–3 GHz and the impact of doping was also studied simultaneously. Magnetic nature of prepared materials was investigated at room using the VSM. It is found that the prepared nanomaterials are suitable for high energy storage as well as magnetic applications.

Hydrazine metal complexes as a single source in the mechanochemical preparation of metallic magnetic nanoparticles and investigation of their magnetic hyperthermia properties

Ferromagnetic metallic cobalt and nickel nanoparticles were synthesized by intramolecular metal-hydrazine ligand oxidation reduction reaction in basic media and the solid state. Nickel nanoparticles Ni1 and Ni2 are obtained from two complexes, Ni(N2H4)2(C2O4) and Ni(N2H4)2(HCO2)2, respectively. X-ray powder diffraction (XRD) and energy-dispersive X-ray (EDX) data show pure fcc crystal structures of nanoparticles. Scanning electron microscopy (SEM) images revealed both Ni1 and Ni2 nanoparticles have agglomerated sphere morphologies. The crystallite size estimated by using the Scherer method for Ni1 and Ni2 were 18.7 and 6.2 nm. Vibrating sample magnetometer (VSM) data show the coercivity of metallic nickel samples Ni1 and Ni2 are 50 and 153 Oe, respectively. Both nanoparticles cause an increase in the temperature of water under applied alternative magnetic field and showed specific loss power (SLP) of 70 W/g and 104.8 W/g, respectively. The same method is used in preparation of cobalt nanoparticles from two complexes, Co(N2H4)2(C2O4) and Co(N2H4)2(HCO2)2. XRD analysis data show both nanoparticles have hcp crystal structure. SEM images show nanosheet structure with sheet thickness of 10 to 30 nm. Coercivity of Co1 and Co2 nanoparticles obtained by VSM and Hc are 182 and 171 Oe, respectively. In spite of high coercivity in both Co1 and Co2 nanoparticles, the magnetic hyperthermia heating effects are not observed in practice and specific loss power (SLP) is zero.

The role of stacking fault tetrahedra on void swelling in irradiated copper

A long-standing and critical issue in the field of irradiated structural materials is that void swelling is significantly higher in face-centered cubic-structured (fcc) materials (1% dpa−1) as compared to that of body-centered cubic-structured (bcc) materials (0.2% dpa−1). Despite extensive research in this area, the underlying mechanism of the difference in swelling resistance between these two types of materials is not yet fully understood. Here, by combining atomistic simulations and STEM imaging, we find stacking fault tetrahedra (SFTs) are the primary cause of the high swelling rate in pure fcc copper. We reveal that SFTs in fcc copper are not neutral sinks, different from the conventional knowledge. On the contrary, they are highly biased compared to other types of sinks because of the SFT-point defect interaction mechanism. SFTs show strong absorption of mobile self-interstitial atoms (SIAs) from the faces and vertices, and weak absorption of mobile vacancies from the edges. We compare the predicted swelling rates with experimental findings under varying conditions, demonstrating the distinct contributions of each type of sink. These findings will contribute to understanding the swelling of irradiated structural materials, which may facilitate the design of materials with high swelling resistance.

Optical Observations of Sputtered Au-nanostructures and Characterizations

o develop novel optoelectronic devices, controlling and predetermined absorption is necessary. In the current work, the gold nano-islands were sputtered onto quartz surface substrates using a DC sputtering unit with an optimized chamber. The effects of the sputtering time (10, 15, and 20 seconds) on the characteristics of the golden layers deposited on quartz substrates were studied. Au nanofilms were investigated by XRD (X-ray diffraction), UV-Vis (ultraviolet-visible light) diffractometer, and AFM (atomic force microscopy) techniques. The thicknesses of the three films prepared at different sputtering times (10, 15, and 20 seconds) were calculated using the theoretical deposition formula method. The average layer thickness, and hence the size of the golden crystallites, rose from 6.8 to 13.6 nm when the sputtering period was increased. The X-ray spectra revealed a very distinguish peak at (111), pointing that the gold single crystal has fully oriented along [111] and the gold film has a pure crystalline Fcc structure. When deposition times are lengthened, the color of the resulting films changes from blue to green. Nano-films have seen important changes in their surface shape and roughness. The structural layer of Au's surface is remarkably semi-spherical, giving it the appearance of spherolytic and hummock-like. Analyzing the UV-VIS spectra of the precipitated structures using Tauc's paradigm revealed the optical energy gap (non-zero Eg in the range of 2.36 to 2.38 eV) that is associated with the nanostructure's semiconducting properties. In addition, as the sputtering duration increases from 10 to 20 seconds, the wavelengths at which peak values of the surface plasmon resonance occur shift from 610 to 650 nm, and the widths of the peaks rise. The AFM images of the ultra-thin Au layers showed smooth surfaces whose roughness decreases from 1.82 to 0.673 nm with increasing sputtering time from 10 to 20 nm. Solar cells can take advantage of the higher absorption in the blue spectrum region by using a set of depositing parameters and prescribed thicknesses. In addition, the formation of nano-islands can also be used as nucleation sites (seed layer) to promote the growth of various nanostructures to obtain a good aspect ratio and improve the performance of various optoelectronic devices and gas sensors.

Study of Urbach energy and Kramers–Kronig on Mn and Zn doped NiFe2O4 ferrite nanopowder for the determination of structural and optical characteristics

MxNi1-xFe2O4 spinel ferrite (M = Mn, Zn, and x = 0, 0.05) has been successfully synthesized by co-precipitation technique with hydrazine hydrate reduction agent (instead of NaOH) and Ethylene glycol surfactant. The XRD spectra of the samples illustrated high crystallinity. The structural characterization of pure and doped fcc NiFe2O4 were calculated by Scherrer, Modified Scherrer, Williamson–Hall, and SSP methods. In comparison of several methods, the Scherrer method is unreasonable method and W–H method has an acceptable range and can calculate both < L > and strain without restriction. The specific surface area in Zn-doped increased, demonstrate increment of adsorption properties in Ni ferrite structure. TEM images revealed the shape of grains is spherical, cubic, and irregular, with a grain size in the range of 35–65 nm. Hysteresis loops illustrated the magnetic behavior of samples. From the reflectance data, the band gap energies were estimated at 1.984, 1.954, and 1.973 eV for un-doped, Mn, and Zn-doped NiFe2O4 respectively (red shift). The almost low value of Urbach energy for pure, Mn, and Zn -doped NiFe2O4 indicates low structural disorder, which can approve the high crystallinity of samples. Direct band gap energy (Eg), refractive index, and extinction coefficient were estimated by the Kramers–Kronig method with linear optical evaluations. The Eg by K-K method is in good agreement with the Eg by Kubelka–Munk function.

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.

Minimum Free-Energy Shapes of Ag Nanocrystals: Vacuum vs Solution.

We use two variants of replica-exchange molecular dynamics (MD) simulations, parallel tempering MD and partial replica exchange MD, to probe the minimum free-energy shapes of Ag nanocrystals containing 100-200 atoms in a vacuum, ethylene glycol (EG) solvent, and EG solvent with a PVP polymer containing 100 repeat units. Our simulations reveal a shape intermediate between a Dh and an Ih, a Dh-Ih, that has distinct structural signatures and magic sizes. We find several prominent features associated with entropy: pure FCC nanocrystals are less common than FCC crystals containing stacking faults, and crystals with the minimum potential energy are not always preferred over the range of relevant temperatures. The shapes of the nanocrystals in solution are influenced by the chemical identities of the solution-phase molecules. Comparing Ag nanocrystal shapes in EG to those in an EG+PVP solution, we find more icosahedra in EG and more decahedra in EG+PVP across all of the nanocrystal sizes probed in this study. At certain critical sizes, nanocrystal shapes can change dramatically with the addition and removal of a single atom or with a change in temperature at a fixed size. The information in our study could be useful in efforts to devise processing routes to achieve selective nanocrystal shapes.

Effect of non-rare-earth element enrichment on in-the-column secondary electron image contrast of Nd-rich phases in Nd-Fe-B sintered magnets

In-the-column secondary electron (SE) imaging has demonstrated an important role in identifying various Nd-rich phases in Nd-Fe-B sintered magnets at a microscale field of view. It is well known that modifying the microstructure of Nd-Fe-B sintered magnets by adding non-rear-earth (NRE) elements dissolved into the intergranular Nd-rich phases during post-sinter annealing can effectively increase the magnet coercivity. However, the effect of NRE element enrichment on in-the-column SE image contrast of Nd-rich phases has not received much attention. In this study, we have combined a focused ion/electron dual beam system (FIB/SEM) and a scanning/transmission electron microscope (S/TEM) to investigate the in-the-column SE image contrast, structure, and element distribution of intergranular Nd-rich phases in commercial heavy-rare-earth (HRE) free Nd-Fe-B sintered magnets. In-lens SE imaging revealed that Cu, Co, or Al-enriched neodymium oxides with fcc, Ia3¯-type, or amorphous structures are brighter than the Nd2Fe14B matrix at an accelerating voltage of 2 kV and a working distance of 4 mm. Bright stripes inside the intergranular fcc NdOx were also visible in high-resolution through-the-column SE imaging, reflecting the nanoscale composition variation with Co or Cu enrichment. Contrarily, at the same imaging conditions, the nearly pure fcc NdOx and Ia3¯-Nd2O3 exhibit darker than the Nd2Fe14B matrix in the in-lens SE image. It has been demonstrated that the in-lens SE image contrast of Nd-rich phases with respect to the Nd2Fe14B matrix remains constant by systematically adjusting the accelerating voltage from 2 kV to 5 kV and the working distance from 4 mm to 6 mm. Our research results suggest that low-voltage in-the-column SE imaging could be used to track element diffusion in triple junction and grain boundary Nd-rich phases of Nd-Fe-B sintered magnets during post-sinter annealing by in-situ MEMS heating in SEM.

Investigation of the applicability of Cu–Fe–Mn–Ni based high entropy and compositionally complex alloys as metal matrix composites for cobalt free hot-pressed diamond tools

Due to the rising demand and carcinogenic effect of cobalt, alternative metal matrixes need to be developed for hot-pressed diamond tools. Due to this reason High-Entropy alloys without cobalt were calculated via phase fraction diagrams. Three alloys of the Al–Cu–Fe–Mn–Ni system Al30Cu30Fe5Mn25Ni10, Al11.25Cu35Fe5Mn20Ni28.75 and Al5Cu20Fe25Mn25Ni25 were chosen due to their different crystal structures ranging from pure bcc, eutectic fcc-bcc to pure fcc crystal structure. Cr5Cu20Fe25Mn25Ni25 was chosen to verify the change of one element on the consolidation properties. The alloys were mechanically alloyed and hot-pressed at 800 °C for 3 min without and at 900 °C for 3 min with diamonds. Porosity increased with the fraction of bcc solid solution in the investigated alloys of the Al–Cu–Fe–Mn–Ni system. Samples consisting of Cr5Cu20Fe25Mn25Ni25 showed the lowest porosity, which was attributed to precipitation of a second copper-rich fcc solid solution around the remaining pores. At a process temperature of 800 °C and 3 min isothermal hold the samples featured a porosity of only 2.72%. Within the XRD patterns and SEM images of the hot-pressed samples with diamonds no graphitization or formation of carbides could be observed. Therefore, Cr5Cu20Fe25Mn25Ni25 was identified as a promising cobalt free metal-matrix candidate for diamond tools.

Structure Refinement and Bauschinger Effect in fcc and hcp Metals

Although the Bauschinger effect has been investigated in some detail in various materials, the number of articles on the effect of grain size is extremely limited, and in current nanostructured materials it is practically absent. Since such materials are considered as promising for structural applications, it is important to understand their mechanical behavior under conditions of changing the direction of deformation, and, therefore, it is necessary to study the Bauschinger effect and its dependence on grain size. The Bauschinger effect was investigated by a single exemplary method for tensile compression of commercially pure hcp titanium and fcc copper, with different grain sizes in the range from hundreds of microns to hundreds of nanometers. The change in grain size was performed by structure refinement by the method of severe plastic deformation using equal-channel angular pressing and subsequent annealing. It has been established that, in both materials, the Bauschinger effect increases with a decrease in grain size, the degree of permanent strain and the duration of exposure between forward and reverse deformation. The signs of the Bauschinger parameter in copper and titanium are opposite. The relationship between the Bauschinger effect and the nature of strain hardening in titanium and softening in copper in the ultrafine-grained state is discussed.

Ab initio studies on structural and thermodynamic properties of magnetic Fe

The present work systematically investigates the total energy, phonon spectra, and thermodynamic properties of different polymorphs of pure Fe, i.e., FCC, BCC, and HCP, with the ab initio approach, considering various magnetic configurations. In general, the calculated energy vs. volume curves and phonon spectra agree well with previous calculations and the experimental data. In addition, their thermodynamic properties are estimated by the quasiharmonic approximation (QHA). Specifically, a superposition approach based on the latest Zentropy theory was utilized to predict magnetic transition temperatures and thermodynamic properties of pure Fe. With the ensemble of the partition function considering the multiplicity of each magnetic microstate, the current work successfully reproduced the Curie/Néel temperature and the Schottky anomaly of heat capacity in FCC, BCC, and HCP Fe purely based on the ab initio input, which exhibits good agreement with the experimental data and CALPHAD modeling.

Similarities and trends in adsorbate induced reconstruction – Structure and stability of FCC iron and cobalt surface carbides

Thin FCC (100) iron and cobalt carbide films were prepared on Cu(100) to study the connection between their structure, electronic properties and stability. We present the first detailed, real space experimental confirmation of the C-induced clock reconstruction on the FCC(100) surfaces of iron and cobalt. Both Fe and Co surface carbides show p4g (2 × 2) surface reconstruction with tetracoordinated square planar carbon and pure FCC (100) metal layers underneath. Combining tip-sample distance dependent STM imaging with theoretical calculations we present different imaging modes of Fe2C. Using a combination of angle-resolved x-ray photoelectron spectroscopy (AR-XPS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and theoretical calculations we provide detailed electronic and structural models for Fe2C and Co2C p4g (2 × 2) surface carbides and other 2D Fe2X interstitial compound systems. In various Fe2X (X = B, C, N, O) surface compounds moving to the right in the periodic table with increasing electrons the reconstruction becomes less favorable, while iron carbide shows the highest stability.

Formation mechanism of hierarchical twins in the CoCrNi medium entropy alloy

• The competitive mechanisms between primary/conjugate slips and twins are clarified. • The particularity of mechanism of hierarchical twins in CoCrNi MEA is unraveled. • The hierarchical twinning behavior depending on strain rate and deformation temperature is revealed. • The twinning map help predict the formation of hierarchical twins as a reference for experiments. The three-dimensional hierarchical twin network has been proved to be the source of the excellent strength-ductility combination in the CoCrNi medium entropy alloy. Revealing the formation mechanism of hierarchical twins, however, remains a challenge using either the post-mortem or the in-situ microstructural characterization. In this study, the atomistic formation mechanism of hierarchical twins was investigated using molecular dynamics simulations, with special focus on the effects of strain rate and deformation temperature. Compared to the primary twin boundaries kink-driven hierarchical twinning tendency in pure FCC metals, the chemical inhomogeneity in CoCrNi can reduce the necessary kink height to trigger conjugate twins (CTWs), fascinating the formation of twin networks. At room temperature, the plastic deformation is dominated by primary twins (PTWs) and conjugate slips at a relatively lower strain rate (e.g., 5×10 7 s −1 ). The hierarchical twins can be activated in cases of deforming at a higher strain rate (e.g., 2×10 8 s −1 ). Further increasing the strain rate (e.g., 1×10 10 s −1 ) leads to the phase-transformation induced plasticity. At cryogenic temperatures, the hierarchical twins are promoted within a large range of strain rates (e.g., 5×10 7 -1×10 10 s −1 ). A higher temperature leads to the synergy of CTWs and primary slips at a lower strain rate, but hierarchical twins at a higher strain rate. On this basis, a qualitative comparison and scalable trends between experiments and simulations were revealed. The present study would not only provide the basic understanding for the twinning behavior found experimentally, but also contribute to the design of medium/high entropy alloys with excellent mechanical performances by tuning microstructures.

Effects of undercooling on atomic crystallization behaviors and growth mechanisms of pure metals

The atomic crystallization behaviors at the crystal–melt interfaces in a broad range of undercoolings are investigated by molecular dynamics simulations for two representative pure metals, FCC Cu and BCC Ta. Results show that the atomic transformation displacements against temperature for both metals have the same trend, i.e., increasing significantly as temperature goes up at small undercooling and keeping invariant at large undercooling. By classifying the interfacial atomic attachment behaviors into ballistic and diffusive motions based on the displacement analysis, it is found that the crystal growth of both metals involves many ballistic attachments, and a small increment of diffusive attachments at the Ta interface leads to a significant energy barrier for crystallization comparing to that of Cu. The temperature effects on the interfacial structures and atomic dynamics to attach onto the crystal are also studied in detail, and their correlations with the different growth mechanisms at low and deep undercoolings are disclosed. Finally, the crystallization rate is proved to be dominated by the atomic transformation displacement and interfacial atomic movement rate for either metal, rather than the atomic thermal velocity or liquid diffusion coefficient.

Effect of Co-doping on the structure, magnetic and electronic properties of Heusler alloys Mn2Fe1-xCoxGa (x = 0–1)

A series of Mn2Fe1-xCoxGa (x = 0–1) Heusler alloys was synthesized by melt-spinning method. The crystal structure, magnetic properties and electronic structure of them were investigated experimentally and theoretically. In undoped Mn2FeGa, a pure FCC phase is identified. The substitution of Co for Fe in Mn2Fe1-xCoxGa tends to stabilize the BCC Heusler phase, with the help of melt-spinning technique, single BCC Heusler phase was obtained within the range of × = 0.5–1.0. The FCC Mn2Fe1-xCoxGa are antiferromagnetic and their Néel temperatures TN decrease with Co-doping. But in BCC Mn2Fe1-xCoxGa, a ferromagnetic character is observed and the saturated magnetization Ms at 5 K increases rapidly with increasing Co content. First-principles calculations suggest that Co-doping can make the formation energy of Mn2Fe1-xCoxGa BCC phase more negative and increase the phase stability, which agrees well with the appearance of the BCC phase in the XRD pattern when Co content is high. This effect can be explained from the change of electronic structure with Co-doping. The calculated total spin moments of Mn2Fe1-xCoxGa increase from 1.04μB / f.u. for × = 0 to 2.01 μB / f.u. for × = 1, following the Slater-Pauling curve of M = Z −24. The Ms at 5 K agrees well with the theoretical results when × = 0.75–1.0. Calculations also suggest that Mn2Fe1-xCoxGa alloys all have 100% or quite high spin polarization ratio. All this makes them promising candidates for spintronic applications when Co content is high.

First-principles study on the solute-induced low diffusion and self-trapping of helium in fcc iron

The addition of alloying elements plays an essential role in the behaviors of helium (He) produced by transmutation in metal alloys. The effects of solutes (Ni, Cr, Ti, P, Si, and C) on the behavior of He and He–He pairs in face-centered cubic (fcc) iron were investigated via first-principles calculations based on density functional theory (DFT). For the interactions between solutes and He, we found that Ti, P, Si, and C were more potent in attracting He than Ni and Cr in fcc iron. We determined the most stable configuration for the He–He pair, which is the substitutional–tetrahedral He pair with a binding energy of 1.60 eV. In considering the effect of solutes on the stability of the He–He pair, we proposed a unique definition of binding energy. By applying this definition, we suggest that Ti and P can weaken He self-trapping, Cr and C are beneficial for He self-trapping, and Ni is similar to the matrix Fe itself. For the diffusion of He, which is the necessary process of forming the He bubble, we determined that the most stable interstitial (int) He is in a tetrahedral (tetra) site and can migrate with the energy barrier of 0.16 eV in pure fcc iron. We also found that Ti and Si can increase the barrier to 0.18 and 0.20 eV; on the contrary, Cr and P decrease the barrier to 0.10 and 0.06 eV, respectively. Summarizing the calculations, we conclude that Ti decreases while Cr increases the diffusion and self-trapping of He in fcc iron.

Can every substance exist as an amorphous solid?

Many substances can exist as amorphous solids, but it is an open question whether or not all substances can be put into the amorphous form. It has been proposed that the demonstration of pure amorphous argon is the most convincing evidence for the universality of the amorphous state. However, theoretical studies have shown that a non-crystalline structure made by quenching liquid argon is not a solid, for it crystallizes immediately. Here, we demonstrate that amorphous argon model containing triple-shell icosahedral clusters can exist for more than one day below 7 K, supporting the universality of the amorphous state. Our findings open new perspectives not only for making amorphous solids of substances such as pure noble gases and fcc metals which have not yet been put into the amorphous form but also for enhancing the stability of existing amorphous solids.

Degradation of tensile mechanical properties of two AlxCoCrFeNi (x=0.3 and 0.4) high-entropy alloys exposed to liquid lead-bismuth eutectic at 350 and 500°C

Tensile mechanical properties of two AlxCoCrFeNi (x = 0.3 and 0.4) high-entropy alloys (HEAs) have been tested in liquid lead-bismuth eutectic (LBE) at 350 and 500 °C. The results show that LBE has a mild effect on the mechanical properties of Al0.3CoCrFeNi HEA which is a pure FCC structure, but some traces of liquid metal embrittlement (LME), such as cleavage, are detected at 350 °C and a tendency of intergranular (IG) cracking at 500 °C is also discovered. The mechanical properties of Al0.4CoCrFeNi HEA are strongly temperature-dependent. At 350 °C, the degradation effect of LBE is weak, while after the testing temperature increases to 500 °C, the mechanical properties are severely deteriorated even without the help of LBE. The mechanism responsible for this severe degradation phenomenon can be attributed to a network of thin (Ni, Al)-rich precipitates existing at the grain boundaries of the Al0.4CoCrFeNi HEA. As temperature increases, the precipitate/matrix interphase boundaries (IBs) become weak and can be perfectly wetted by LBE, leading to prevalent IG cracking facets on the fracture surface.

Role of local chemical fluctuations in the shock dynamics of medium entropy alloy CoCrNi

In this work, molecular dynamics simulations are conducted to investigate the shock responses and corresponding deformation mechanisms in single crystalline (SC) and nanocrystalline (NC) microstructure of the medium entropy alloy (MEA) CoCrNi. The effects of lattice distortion (LD) and chemical short-range order (CSRO) on the shock wave propagation, defect evolution, and the cavitation process are explored to distinguish the unique shock properties of MEA. The results reveal an anomalous anisotropy in the Hugoniot elastic limit different from that seen in pure FCC metals since LD reduces the barrier for Shockley partial (SP) formation but increases the resistance for SP propagation. With sufficient dislocations nucleated in the first shock compression stage, LD aids in the formation of nanotwins by slowing down dislocation propagation in the following release and tension stages. However, because a higher degree of CSRO increases the average intrinsic stacking fault energy above that of the random material, more stacking faults annihilate in the release stage, reducing the chances for nanotwinning. We show that voids prefer to nucleate at Ni segregation sites (with high CSRO) due to the large hydrostatic tensile strain created by the lattice mismatch between the neighboring Ni and CoCr regions, and moreover, the nucleation event favors the grain boundary during spallation in NCs.