Bcc Crystal Structure Research Articles

Experimental and simulation study of microstructure and magnetic properties of (AlFeMnNi)1−x Nd x

ABSTRACT The high-entropy alloy (Al25Fe30Mn25Ni20)1−x Nd x (all in at.%) (AlFeMnNi)1−x Nd x ) was prepared by vacuum arc melting (AM) and casting method (CM). X-Ray Diffraction (XRD) and Scanning Electron Microscopy, transmission electron microscope and Integrated Energy-Dispersive X-ray spectroscopy were used to study the structure of our system. The (Al25Fe30Mn25Ni20)1−x Nd x crystallises in a Face-centred cube and centred cubic (BCC), depending on the x (x = 0.00, x = 0.01 at.% Nd). The increase of Neodymium (0.01 at.% Nd) content results in the formation of BCC structure. The chemical composition analysis of FCC and BCC crystal structures, their lattice constants demonstrate that the formation of a single BCC solid solution between 760°C and 1480°C. The Korringa–Kohn–Rostoker Method and Green's function combined with coherent potential approximation were used to study the magnetoelectronic properties of (Al25Fe30Mn25Ni20)1−x Nd x . We have obtained the value optimised of lattice parameter 2.9 Å and it is near to that obtained by experimental value using High Score programme and results XRD experimental.

A concise analytical framework for describing asymmetric yield behavior based on the concept of shape functions

A fully analytical framework of yield criterion is constructed under non-associated flow rule to describe asymmetric yielding and anisotropic hardening based on the concept of shape functions. For the yield function, eight parameters are identified by employing the corresponding tensile and compressive hardening functions along the four directions, i.e., uniaxial directions along 0°, 45°, 90° in relation to the rolling direction and equi-biaxial direction. This function can describe continuous non-uniform evolution of yield loci during anisotropic hardening. Besides, a new analytical plastic potential function is also developed to predict the asymmetric R-values along uniaxial tension and compression. Subsequently, a simpler curvature control method is proposed to control the curvature of yield surface, especially suitable for materials with large anisotropy. The effectiveness and accuracy of the proposed criterion are validated with HCP, FCC and BCC crystal structure materials for predicting asymmetric uniaxial yield stress curves, R-value curves, and non-uniform evolution of yield loci. The validations indicate that this new criterion has concise mathematical structure and great capability of asymmetry description. Eventually, an extension method for the yield criterion from 2D to 3D is discussed in the reasonability and incompressibility.

Effect of Re and Al additions on the microstructure and mechanical properties of Nb-18Ti-12W alloy

Microstructure and mechanical properties at 25–1200 °C of new refractory alloys, Nb-18Ti-12W (NTW), Nb-18Ti-12W-5Re (NTW-R) and Nb-18Ti-12W-5Re-1Al (NTW-RA), all compositions are in atomic percent, are reported. The alloys are developed as an alternative to a denser, more expensive and difficult-to-process refractory alloy WC-3009. After production by arc melting and hot isostatic pressing at 1400 °C, 207 MPa for 3 h, the alloys have a BCC crystal structure with the average grain size of ∼150–200 μm and contain low volume fraction of (Ti,N)-rich precipitates near grain boundaries. NTW shows excellent malleability at all studied temperatures. Its yield stress at 25 °C, 1000 °C and 1200 °C is 874 MPa, 353 MPa and 202 MPa, respectively. The NTW-R and NTW-RA alloys are stronger at all the studied temperatures; however, they lack in compression ductility at 25 °C. Extensive deformation twinning and formation of intergranular voids precede the fracture of these two alloys at 25 °C.

Study on Diffusion Kinetics and Law of Chromium on the Surface of Low-Carbon Steel

Cr/low-carbon steel surface composites were prepared by aqueous solution co-deposition and high-temperature solid-state diffusion technology, and the macro rule of the solid-state diffusion of chromium on the surface of low-carbon steel was analyzed. The molecular dynamics (MD) method was used to simulate and calculate the diffusion process of the Cr/Fe interface, and the macro and micro diffusion mechanisms were analyzed. The results show that the diffusion of the chromium in iron is the combined action of the temperature, crystal structure and lattice distortion, and the diffusion coefficients of chromium in α-Fe and γ-Fe have little difference. The vacancy diffusion mechanism of the first adjacent transition is the main diffusion mode. In practice, chromium atoms diffuse along the grain boundaries of the low-carbon steel matrix and provide pinning at the grain boundaries to prevent grain growth. The simulation law is in good agreement with the experimental law. The variation law of the average diffusion coefficient of chromium atoms with temperature is obtained. The diffusion rate of chromium in the bcc crystal structure is obviously higher than that in the fcc crystal structure. In the same crystal structure, the diffusion coefficient of chromium increases with the increase in temperature. However, in the lattice transition temperature region, the diffusion coefficient of chromium gradually decreases with the increase in temperature until the end of the transformation.

Cobalt free refractory high entropy alloys for total joint arthroplasty: In-vitro wear, corrosion and cytocompatibility evaluation

CoCrMo alloys are extensively used for bearing surfaces in total joint arthroplasty (TJA); however, Co poisoning remains a concern. Therefore, we propose a Co free refractory metal based high entropy alloys (RHEAs) for use in TJA. In this work, TiMoNbTaW based RHEA was prepared by arc melting. The alloy was further modified by Cr addition for improvement in wear, corrosion and cytocompatibility. X-ray diffraction revealed single bcc crystal structure where lattice constant decreased upon Cr addition. The improved hardness, yield strength and a ten-fold enhancement in wear resistance with Cr addition is ascribed to its severely distorted bcc lattice. In-vitro corrosion in Hank’s salt solution (HBSS) suggests superior thermodynamic stability and passivation of both RHEAs. Enhanced pitting resistance was seen upon Cr addition. Further, electrochemical impedance spectroscopy (EIS) analysis of the surface film showed an increase in film resistance by an order of magnitude attributed to the presence of Cr2O3 on the TiMoNbTaWCr surface as confirmed by X-Ray photoelectron spectroscopy (XPS) analysis after anodic passivation. Meanwhile, the live/dead imaging and MTT assay indicated good cell viability and proliferation of RHEAs. However, an enhanced cell adhesion and growth on TiMoNbTaWCr RHEA proves its potential applicability as a bearing surface in TJA.

Unusual precipitation and its hardness enhancement in Fe-alloyed 60NiTi alloy

• An unusual precipitation phenomenon is observed in Fe-alloyed 60NiTi. • The unknown Ni-rich nano-size phase is formed due to the addition of Fe. • Fe alloying considerably increases the hardness of 60NiTi. In this work, the microstructure and hardness of Fe-alloyed 60NiTi alloys, (Ni 55 Ti 45 ) 100-x Fe x (x = 0.5, 1, 3, 5 at. %), were studied. The addition of Fe leads to an unusual precipitation phenomenon that significantly influences the microstructure of the studied alloys. In the Fe-alloyed samples, an unknown secondary phase ( X phase) precipitates with bcc crystal structure and nano-scale size. The X phase that evolves as thin plates in a densely dispersed manner in the microstructure brings about a hardening effect similar to Ni 4 Ti 3 . The hardness of 60NiTi is considerably increased owing to the microstructural evolutions resulting from the addition of Fe, with a maximum value of 717 HV at 5 at.% Fe content in the alloy.

The modified generalized Tresca yield condition with application to mode I perfectly plastic crack problems

Basic properties of a newly defined yield criterion are explored and then applied to plane stress, mode I, perfectly plastic crack problems. This yield condition began as a perturbation of the traditional Tresca yield condition. In a previous study, a relationship was found for a yield criterion that spanned the Tresca to the von Mises yield criteria in the principal stress plane. In this study, a distinct but related yield criterion is used for those cases where the yield criterion lies on or outside the von Mises yield condition in the principal stress plane. For these cases, the associated mathematics becomes more complicated than those cases where the yield condition lies within or on the von Mises yield condition in the principal stress plane. The perfectly plastic mode I crack problem is solved for this modified version of the generalized Tresca yield condition. It is found that the maximum normal stress for this yield condition is higher than that for the equivalent mode I crack problem under the von Mises yield condition. Material parameters associated with the generalized Tresca yield condition are found for both the BCC and FCC crystal structures based on models proposed previously in the literature.

Microstructure evolution and compressive property variation of AlxCoCrFeNi high entropy alloys produced by directional solidification

The AlxCoCrFeNi (molar radio, x = 0.3, 0.6, 0.9 and 1.2) high entropy alloys (HEAs) were directionally solidified at the withdrawal rate of 150 μm/s. The effects of Al content on microstructure evolution, element segregation as well as compressive property were investigated. It was found that the increase of Al content resulted in the formation of BCC crystal structure. Meanwhile, the microstructure of AlxCoCrFeNi HEAs transformed from dendrite to equiaxed dendrite. The Al0.3CoCrFeNi HEA formed a single FCC solid solution phase, the Al0.6CoCrFeNi HEA consisted of CoFeCr-enriched FCC solid solution phase + NiAl-enriched BCC solid solution phase, the Al0.9CoCrFeNi and Al1.2CoCrFeNi HEAs were composed of NiAl-enriched B2 solid solution phase + CoFeCr-enriched BCC solid solution phase. Morphology of solid-liquid interface was coarse dendrite or typical dendrite. As the Al content increased, the compressive strain of directionally solidified AlxCoCrFeNi HEAs decreased from 21.26% to 18.05%, and the ultimate compressive strength increased first and then decreased. The improvement of strength results from crystal structure transition and solution strengthening. As the Al contents increased from 0.6 to 0.9, the crystal structure of AlxCoCrFeNi HEAs transformed from CoFeCr-enriched FCC structure + NiAl-enriched BCC structure to CoFeCr-enriched BCCstructure + NiAl-enriched B2 structure. The strain of Al0.3CoCrFeNi HEA was higher than others, and Al0.9CoCrFeNi HEA exhibited the maximum ultimate compressive strength of 2114.57 MPa.

APT based reliable quantification of Cr-rich phase separation and its relationship with mechanical properties in thermally aged Fe-20 at.% Cr alloy

In this work, thermal aging (at 773 K from 1 h to 1000 h) induced change in micro-hardness is directly correlated with underlying microstructural evolution at nano-scale for high-Cr binary ferritic Fe-20 at.% Cr alloy. The reason for hardening of the alloy is attributed to the decomposition of ferrite phase into the ultrafine Cr-rich α/ phase in the Fe-rich matrix. Extremely small spatial dimension (diameter < 5 nm), identical bcc crystal structure and similar lattice parameter of α/ with matrix pose extreme challenge in their characterization and reliable quantification. A combinatorial methodology utilizing atom probe tomography (APT), transmission electron microscopy (TEM) and small angle neutron scattering (SANS) is followed for reliable quantification of α/ precipitates (diameter > 2 nm) with reduced uncertainty for samples aged for 50 h onwards. In addition, Cr-clusters (diameter < 2 nm) are quantified for samples below 50 h of aging using APT and SANS. Such reliable quantification is utilized to estimate the change in micro-hardness of the alloy through various theoretical dispersion strengthening models. The current work demonstrates that selection of suitable hardening model (Friedel–Kroupa–Hirsch model (with fixed barrier strength of 0.2), Orowan model (with variable barrier strength value between 0.047 and 0.146)) along with reliable quantification of microstructural evolution (α/ here), can successfully estimate the experimentally observed mechanical response of the alloy.

In Situ Alloying of Fe-Cr-Co Permanent Magnet by Selective Laser Melting of Elemental Iron, Chromium and Cobalt Mixed Powders

Fe-25Cr-15Co (wt.%) permanent magnets were fabricated via selective laser melting (SLM) and in situ alloying from a blend of Fe, Cr and Co elemental powders. Under the optimal laser scanning process, the as-built Fe-25Cr-15Co alloy has a homogeneous composition distribution without defects such as holes or un-melted particles, and presents a single α phase with the bcc crystal structure. The density of as-built samples was 7.705 g/cm3 (the relative density is 99.32%). The preferred magnetic properties of the sample in the isotropic state were obtained as Hc = 22.84 kA/m, Br = 0.86 T and (BH)max = 7.98 kJ/m3. The hardness and yield strength of Fe-25Cr-15Co permanent magnets are above 331.5 HV and 800 MPa, respectively. The results of this study verified the feasibility of fabricating Fe-Cr-Co permanent magnets by SLM in situ alloying and can be extended to a wide range of applications that require complex shapes with variable magnetic circuit characteristics or gradient structures.

Microstructure characterization of Co–Cr–Mo–xTi alloys developed by micro-plasma based additive manufacturing for knee implants

This paper reports on effects of adding 2, 4 and 6 wt.% Ti to Co–Cr–Mo by micro-plasma based additive manufacturing (MPBAM) process on density, porosity, microstructure, phase formation, inter-diffusion zones, microhardness, and wear characteristics of the resultant alloy with an objective to develop better material for knee implant applications. Bulk and relative density found to decrease, and porosity increase with increase in Ti % to Co–Cr–Mo alloy. Co–Cr–Mo–4Ti alloy showed more pores and their uniform distribution. Microstructures of Co–Cr–Mo–2Ti and Co–Cr–Mo–4Ti alloys are porous and crack-free. Phase analysis of Co–Cr–Mo–4Ti revealed presence of α-Co, ε-Co, and β-titanium phases (having FCC, HCP, and BCC crystal structure respectively), inter-metallic CoTi2, and lamellar chromium carbides i.e. Cr7C3 and Cr23C6. It is confirmed by the phase mapping also. Inverse pole figure maps did not show any preferential orientation of grains and revealed presence of ε-Co phase matrix with traces of grains of chromium carbides and CoTi2 phases. Increasing Ti% in Co–Cr–Mo alloy increased formation of β-Ti and CoTi2 phases which have less hardness than the carbide phases therefore average microhardness of Co–Cr–Mo–2Ti alloy is found as the highest followed by Co–Cr–Mo–4Ti alloy. Coefficient of friction, specific wear rate, and wear volume increase with increase in Ti% in Co–Cr–Mo alloys due to decrease in microhardness and increase in porosity. It also increased ploughing and delamination in the worn track. This study found Co–Cr–Mo–4Ti as a better knee implant material due to its lesser density, uniform porous structure, absence of cracks, moderate microhardness, and wear characteristics.

Effects of Nb Additions and Heat Treatments on the Microstructure, Hardness and Wear Resistance of CuNiCrSiCoTiNbx High-Entropy Alloys.

In this research, a set of CuNiCrSiCoTi (H-0Nb), CuNiCrSiCoTiNb0.5 (H-0.5Nb) and CuNiCrSiCoTiNb1 (H-1Nb) high-entropy alloys (HEAs) were melted in a vacuum induction furnace. The effects of Nb additions on the microstructure, hardness, and wear behavior of these HEAs (compared with a CuBe commercial alloy) in the as-cast (AC) condition, and after solution (SHT) and aging (AT) heat treatments, were investigated using X-ray diffraction, optical microscopy, and electron microscopy. A ball-on-disc configuration tribometer was used to study wear behavior. XRD and SEM results showed that an increase in Nb additions and modification by heat treatment (HT) favored the formation of BCC and FCC crystal structures (CS), dendritic regions, and the precipitation of phases that promoted microstructure refinement during solidification. Increases in hardness of HEA systems were recorded after heat treatment and Nb additions. Maximum hardness values were recorded for the H-1Nb alloy with measured increases from 107.53 HRB (AC) to 112.98 HRB, and from 1104 HV to 1230 HV (aged for 60 min). However, the increase in hardness caused by Nb additions did not contribute to wear resistance response. This can be attributed to a high distribution of precipitated phases rich in high-hardness NiSiTi and CrSi. Finally, the H-0Nb alloy exhibited the best wear resistance behavior in the aged condition of 30 min, with a material loss of 0.92 mm3.

Structure and pressure dependence of the Fermi surface of lithium

We report studies of the Fermi surface (FS) of isotopically pure polycrystalline $^{7}\mathrm{Li}$ from ambient pressure to 4.7 GPa. Shubnikov--de Haas (SdH) oscillations at 300 mK in external magnetic fields up to 35 T are measured to map the spherical parts of the FS of lithium. Our ambient pressure data show that the principal SdH frequencies consist of three distinct peaks at $41.25\ifmmode\pm\else\textpm\fi{}0.05, 41.65\ifmmode\pm\else\textpm\fi{}0.05$, and $42.05\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{kT}$, which are theoretically consistent with the presence of two crystal structure domains exhibiting nearly spherical FS. The size of the spherical parts of the FS is compatible with bcc and fcc crystal structures. The measured frequencies at 41.25 and 42.65 kT present direct quantitative evidence for the spherical deformation of the FS in fcc Li. Our high-pressure data show that while the FS of Li deforms under compression, it remains mostly spherical up to 4.7 GPa and the pressure dependence of the SdH frequency is consistent with the theoretically calculated pressure dependence in the fcc structure. Finally, we find that the electron effective mass does not deviate under pressure significantly from its ambient pressure value.

Modified extended Finnis Sinclair potential for cubic crystal metal

Unlike the modified embedded atom method (MEAM), which requires consideration of the bond angles, the embedded atom method (EAM) and Finnis Sinclair (FS) potential have achieved a wide range of applications in molecular dynamics simulations of amorphous alloys and disordered systems. In this paper, the potential functions of 10 cubic crystalline metals are reconstructed using the extended Finnis Sinclair (EFS) potential to improve the FS potential in terms of defects, surface energy and thermal properties. First of all, for the BCC crystal structure and FCC crystal structure, the fitted physical quantity expressions suitable for EAM potential and FS potential are deduced, including pressure, bulk modulus, shear elastic modulus elastic constant, vacancy formation energy and surface energy. Then, based on these physical quantities and the Rose equation of state, the least-squares method is used to fit the new potential parameters. Finally, using the current potential, the cohesive energy, elastic constants (C11, C12 and C44), vacancy formation energy, self-interstitial formation energy, surface energy, coefficient of thermal expansion, specific heat capacity and the melting point of 10 kinds of cubic crystals are calculated by molecular dynamics simulation. The reliability of the current potential is verified by comparing with other potential functions, experimental values and first-principles calculations. The results show that the current potential can well reproduce the characteristics of physical quantities such as lattice constant, cohesive energy and elastic constants (C11, C12 and C44). Compared with the previous EFS potential, the current potential is closer to the Rose equation of state. The calculated surface energy is more accurate, and the performance of defects and thermal properties shows the reliability of the current potential function, and these improvements are due to the improvement of the fitting method.

A unified fracture criterion considering stress state dependent transition of failure mechanisms in bcc steels at –196 °C

The fracture properties of a high-strength steel with a body-centered cubic (bcc) crystal structure have been characterized at –196 °C by performing tensile tests with different specimen geometries, three-point bending tests using Charpy specimens, and fracture mechanics tests, covering a broad range of stress states under quasi-static conditions. Both strength and ductility of the bcc steel are significantly increased when the temperature is decreased from room temperature to –196 °C. Enormous plasticity occurs in the material during tensile tests using various specimens at –196 °C, while macroscopic brittle fracture takes place in high triaxiality scenarios. A stress state dependence of ductile to brittle transition properties is observed, as the failure mechanisms at –196 °C change from cleavage fracture to shear failure with decreasing stress triaxiality. A unified stress-state-dependent fracture criterion, which considers the transition of failure mechanisms, is proposed to describe the fracture properties of similar bcc materials at cryogenic temperatures. The threshold triaxiality at which the transition of failure mechanisms takes place is a material property that is determined by the strain hardening capacity and fracture strength. In addition, a probabilistic formulation relying on the extreme value distribution has been incorporated into the model to render the statistical nature of cleavage fracture.

On the fatigue ductile-brittle transition of EA4T steel for high-speed railway axle

EA4T is a widely used high-speed railway axle steel with BCC crystal structure. It has the characteristics of ductile-brittle transition under single impact load, but whether it has the ductile-brittle transition under cyclic load has not been investigated. For this reason, the rotating bending fatigue tests (stress ratio of R = −1) for EA4T steel were carried out at RT, −40 °C, −80 °C and −120 °C, respectively. For all temperatures tested, except −120 °C, the fatigue strength increases with the drop of temperature. When the temperature is −120 °C, the fatigue strength at lower stress level was higher than that at −80 °C, whereas, the fatigue strength at higher stress level was lower than that at −80 °C. Based on the above, the mechanism of low temperature strengthening of fatigue performance was explained. When the temperature was higher than TFDBT (fatigue ductile-brittle transition temperature), lower temperature leads to increased flow stress, and then, the higher fatigue performance. When the temperature was lower than TFDBT, the longitudinal tensile stress near the crack-tip increased and the location of the maximum normal stress (σyymax) was closer to the crack tip with the drop of temperature, resulting in σyy⩾σf was satisfied at the characteristic distance, and then, cleavage fracture occurred. In addition, the FATT50 (Fracture Appearance Transition Temperature) of EA4T was −35.46 °C determined by impact test, which was higher than the TFDBT. Because the strain rate under impact load is higher, which results in higher yield stress and strain hardening exponent under impact load.

Enhancement of critical current density and strong vortex pinning in high entropy alloy superconductor Ta[formula omitted]Nb[formula omitted]Hf[formula omitted]Zr[formula omitted]Ti[formula omitted] synthesized by spark plasma sintering

We synthesized the high entropy alloy (HEA) Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 compound by planetary ball milling and the spark plasma sintering (SPS) method. The sintered HEA sample had a BCC crystal structure with small amount of secondary phases. The superconducting phase transition is observed in the electrical resistivity (Tc∼ 7.8 K) and magnetic susceptibility (Tc∼ 7.6 K) measurements, and the Tc was similar to that of an arc-melted sample. The zero-temperature limit of the upper critical magnetic fields Hc2(0)= 10.5 T and coherence length ξ = 5.66 nm were slightly decreased or comparable to the arc-melted HEA sample (Hc2(0)= 12.05 T and ξ= 5.23 nm). The field-dependent isothermal magnetization hysteresis M(H) exhibited the typical type- 2 superconducting behavior with magnetic flux avalanches in the low magnetic field region. The calculated critical current densities were significantly increased by 286 % (Jc= 32,606 A cm−2 at 2 K) ∼ 687 % (Jc= 73,200 A cm−2 at 4 K) compared to the arc-melted HEA sample (Jc= 11,401 A cm−2 at T= 2 K). The enhanced Jc of the sintered HEA sample is caused by enhancement of the pinning force due to point pinning as well as surface pinning effects. The vortex relaxation measurements showed stable remanent magnetization for over 10,000 s after magnetic fields were turned off, which is unlike conventional sueprconducting vortex relaxation behavior. The strong pinning force in the SPS sintered Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 compound is beneficial for practical applications because of the significantly enhanced critical current density Jc with stable remanent magnetization Mrem(t).

Tuning crystal structure by composition in Al–Cr–Fe–Ni multi-principal component alloy films

Multi-principal component alloys (MCAs), especially high entropy alloys, provide us a novel platform for material design aiming to achieve certain or multifunctional properties by tuning crystal structure. However, tuning crystal structure by composition will take much more time and more cost compared with the traditional binary and ternary alloys. Here, we provide a more effective method to tune crystal structure of MCAs by composition. Using radio frequency magnetron sputtering, single-phase Al–Cr–Fe–Ni multi-principal component alloy films (MCFs) with different composition fluctuations were deposited on silicon substrates. The effects of composition on the crystal structure of Al–Cr–Fe–Ni MCFs were investigated in detail. The results indicate that Al and Cr tend to stabilize bcc crystal structure, while Ni and Fe tend to stabilize fcc crystal structure of Al–Cr–Fe–Ni MCFs. Factor analysis (FA) reveals the order of stabilizing effect of each principal element on the bcc structure of Al–Cr–Fe–Ni MCFs. Magnetic measurement shows a close relationship of structure and magnetic properties for Al–Cr–Fe–Ni MCFs. The superparamagnetism behaviors originated from the small grain size of the Al–Cr–Fe–Ni MCFs, while the antiferromagnetism stemmed from the lattice contraction induced by the fcc-bcc transiton. All the results show that MCFs deposited by magnetron sputtering is a effective platform to tune crystal structure and properties by composition, which will surely help us in new MCA design.

The microstructure and mechanical properties of the additive manufactured AlCoCrFeNi high entropy alloy

The equimolar AlCoCrFeNi high entropy alloy (HEA) was successfully fabricated by direct energy deposition (DED), and its phase composition, microstructure and mechanical properties were discussed systematically. The DED samples showed a duplex BCC crystal structure, which is consistent with the original powder state. The microstructure of the DED samples transferred from columnar grain structure into equiaxed dendritic structure along building direction from bottom to top. What's more, the crystallographic orientation in different positions of the DED samples showed an apparent difference. The columnar grain structure presented a strong texture in building direction at the bottom, while equiaxed dendritic structure exhibited random crystallographic orientations at the top. The equiaxed dendrites structure in the DED samples is comprised of dendrites (DR) and interdendrites (ID) regions due to elemental segregation. Meanwhile, the fine microstructure of DR and ID regions is discovered to be nano-sized spherical precipitates morphology and weave-like morphology, respectively. Furthermore, compared with AlCoCrFeNi HEA produced by conventional casting method, the mechanical properties of the DED samples display a superior combination of strength and ductility. Therefore, this research could provide an effective theoretical guidance for the preparation of high strength-ductility of HEAs.

Effect of Re on the Microstructure and Mechanical Properties of NbTiZr and TaTiZr Equiatomic Alloys

The microstructure, phase composition, and mechanical properties of NbTiZr, TaTiZr, Re0.3NbTiZr, and Re0.3TaTiZr are reported. The alloys were produced by vacuum arc melting and hot isostatically pressed (HIP’d) at 1400 °C for 3 h under 276 MPa hydrostatic pressure of high-purity argon prior to testing. NbTiZr had a single-phase BCC crystal structure, while TaTiZr had a Ti- and Zr-rich BCC matrix phase and Ta-rich nanometer-sized BCC precipitates, at volume fractions of 0.49 and 0.51, respectively. Re0.3NbTiZr consisted of a BCC matrix phase and Re-rich precipitates with a FCC crystal structure and the volume fraction of 0.14. The microstructure of Re0.3TaTiZr consisted of a Zr-rich BCC matrix phase and coarse, Re and Ta rich, BCC particles, which volume fraction was 0.47. NbTiZr and TaTiZr had a room temperature (RT) yield stress of 920 MPa and 1670 MPa, respectively. While, 10 at.% Re additions increased the RT yield stress to 1220 MPa in Re0.3NbTiZr and 1715 MPa in Re0.3TaTiZr. Re also considerably improved the RT ductility of TaTiZr, from about 2.5% to 10% of true strain. The positive strengthening effect from the Re additions was retained at high (800–1200 °C) temperatures.