Face-centered Cubic Phase Research Articles

Eutectic Reaction and Microstructure Stability in CoCrFeNiNbx High-Entropy Alloys

Seven arc-melted and then annealed CoCrFeNiNbx (x = 0.3–0.6) alloys are experimentally and thermodynamically investigated in the present work. All the as-cast and 1000 °C annealed CoCrFeNiNbx alloys are composed of face-centered cubic (FCC) and C14 Laves phases. Nb content in the C14 phase stays at around 24.5 at.%, and the Liquid → FCC + C14 eutectic reaction occurred at around 10.8 at.% Nb in a narrow temperature range. It is found that the microstructure in the CoCrFeNiNbx alloys is dramatically affected by the cooling rate and annealing treatment. The C14 phase easily spheroidizes and coarsens under high temperature, which indicates that the interface energy between FCC and C14 is very large. Moreover, the solubility of Nb in the FCC phase decreases with decreasing temperature. After annealing at 800 °C, a needle-like nano Mg3Cd-type τ phase precipitates from the pro-eutectic FCC phase and increases alloy hardness for ~100 HV. This should be a method to strengthen alloys.

In-situ synthesized age-hardenable high-entropy composites with superior wear resistance

This work reports a new approach to in-situ synthesize high-entropy composite (HEC) with high age-hardening ability and superior wear resistance. The as-cast FeNiCrMoTiC composite consists of a face-centred cubic (FCC) solid solution matrix with in-situ formed randomly-distributed carbides and FCC/intermetallics eutectic structures. Aging at 800 °C for 96 h effectively increases the hardness and compressive yield strength of the composite due to the precipitation strengthening of intermetallics. The peak-aged composite thus shows significantly enhanced wear resistance, even higher than the high-chromium cast iron (HCCI) with much higher hardness. In contrast to the severe delamination in the HCCI, the peak-aged HEC shows moderate abrasive wear and minor delamination under sliding friction due to its unique microstructure. The in-situ formed carbides, eutectic structures and precipitates effectively hardens the soft FCC matrix of the HEC and thus decreases the material loss caused by abrasive wear. The relatively lower cracking susceptibility of the finer reinforced particles in HEC can also prevent severe brittle delamination. In addition, propagation of micro-cracks from the brittle particles is inhibited by the ductile FCC matrix, which suppresses the delamination behaviour. During the wear test, the spalled carbides or intermetallics were welded onto the surface of the FCC phase, which further strengthened the matrix and thus decreased the abrasive wear. The developed composite also exhibited superior oxidation and corrosion resistance, indicating a strong application potential in extreme environments associated with abrasion, corrosion and oxidation.

Ultrahigh specific hardness of Co-Ni-V-Al medium entropy alloy thin films

The high-performance alloy films deposited at low-cost substrates could be a cost-effective way to incorporate these materials of superior properties in harsh environment. Here, we report successful synthesis of body-centered cubic (BCC) dominated dual-phase Co-Ni-V-Al medium-entropy alloy (MEA) thin films prepared by magnetron sputtering at 298 K and 573 K, respectively, different from the equilibrium solidification products of face-centered cubic (FCC) matrix and B2 nanoprecipitates with 6% volume fraction. The 573 K-film exhibits an average hardness and compressional yield strength of 16.8 GPa and 5.3 GPa, and its specific hardness (hardness/density) is 2.249 GPa·cm3·g−1, to be the highest value among all reported high entropy alloys (HEAs). In addition, the films with a thickness of 950 ± 50 nm have an average particle size and surface roughness of 45.7 ± 9.8 nm and 1.53 ± 0.05 nm for 298 K-films and 46.5 ± 10.4 nm and 1.46 ± 0.05 nm for 573 K-films, respectively. The high hardness and yield strength of the 573 K-films are attributed to the tighter bonding of particles, the smaller grain size of both FCC and BCC phase, and collaborative deformation between BCC and FCC phase, suggesting that these HEA films could be beneficial for light-weight and high-strength material industrial applications.

Effect of Nitrogen Partial Pressure on the Structural, Mechanical, and Electrical Properties of (CrHfNbTaTiVZr)N Coatings Deposited by Reactive Magnetron Sputtering

Multi-element (CrHfNbTaTiVZr)N coatings were prepared through the magnetron sputtering of an equimolar CrHfNbTaTiVZr alloy target. This study determined the influences of N2-to-total (N2 + Ar) ratios (RN) on the composition, structure, mechanical properties, and electrical performance of the coatings. Coating thickness decreased from 898 nm to 128 nm with increasing RN from 0% to 100%. The alloy coating has bundles of fibrous structures with remarkable void boundaries. The coating changed from amorphous phase to face-centered cubic (FCC) phase with (111) preferred orientation, then to FCC phase with (200) preferred orientation, and finally to near-amorphous phase as RN increased from 0% to 100%. The microstructure of the nitride coatings transformed from a columnar structure with rough faceted tops and void boundaries into a dense and small structure with smooth domed tops. The grain size of the nitride coatings also decreased with RN. Accordingly, the electrical performance at high RN was poor. The nitride coating deposited at RN = 60% had the highest hardness of 16.6 GPa and the lowest friction coefficient of 0.52, owing to structural densification and grain refinement.

Constitutive modeling and finite element analysis of metastable medium entropy alloy

In this study, the constitutive model of metastable ferrous Fe60Co15Ni15Cr10 (at%) medium entropy alloy (FeMEA) was constructed based on deformation-induced martensitic transformation (DIMT) behavior under cryogenic temperature. The crystal structure of the FeMEA gradually transformed from a single face-centered cubic (FCC) phase to a body-centered cubic (BCC) phase during deformation. The critical initiation stress criterion and the volumetric fraction evolution model were employed to describe the DIMT kinetics during uniaxial tensile testing. The flow stress model of the FCC phase as a function of the applied strain was developed using the dislocation-based hardening model, and the BCC phase behavior was evaluated based on the macroscopic behavior of the multiphase material and phase transformation kinetics. The decline in the dislocation mean free path for the FCC phase was associated with the grain refinement of the FCC phase due to the transformed BCC islands. To verify its applicability, the model was compared to the experimental data describing phase transformation kinetics and the mechanical behavior.

Effect of Co on phase stability and mechanical behavior of CoxCrFeNiMnAl0.3 high entropy alloys with micro/nano hierarchical structure

Co-Cr-Fe-Ni-Mn high entropy alloys (HEAs) have attracted extensive attention due to their broad cryogenic engineering application potential, reasonable control of alloying elements can significantly improve its mechanical performance. In this work, the effects of Co content on the phase stability and mechanical behavior CoxCrFeNiMnAl0.3 HEAs were investigated. It is found that with the decrease of the Co content, the stability of the face-centered cubic (FCC) solid solution phase in the alloy decreases, resulting in the precipitation of the BCC (body-centered cubic) phase from the FCC phase, thereby obtaining the FCC + BCC dual-phase structure. During annealing, the precipitated BCC phase can effectively suppress the grain growth and recrystallization of the FCC solid solution, forming an ultrafine-grained microstructure. In addition, the Ni-Al type nano-B2 phase can form in the BCC phase under the influence of the negative formation enthalpy during solidification. This micro/nano hierarchical structure can effectively improve the mechanical properties of the alloys. Co0.25CrFeNiMnAl0.3 maintains relatively excellent tensile yield strength (486 MPa), the tensile elongation remains above 20 %. This study regulates the formation of the BCC phase and B2 phase by reducing the content of Co, which provides a new idea for the preparation of low-cost HEAs with excellent mechanical properties.

Effect of copper on microstructural evolution and mechanical properties of laser-welded CoCrFeNi high entropy alloy

In this work, laser welding of CoCrFeNi and CoCrCuFeNi HEA was carried out. X-ray diffraction, electron backscattered diffraction, mechanical property and microstructural analyses were performed. In CoCrFeNi alloy, a face centred cubic (FCC) structure was observed in both base metal and laser-welded conditions. The addition of Cu in CoCrFeNi alloy is resulted in two FCC phases and has increased the tensile strength from 593 to 666 MPa. The fusion zone of both the welds shows coarser columnar grain and lower hardness compared to that of base metals. Post-welding results show 53% decrease in tensile strength in the alloy containing Cu (CoCrFeNiCu). Whereas only a marginal decrease in strength is observed in CoCrFeNi alloy.

Deformation-Induced Phase Transformations in Gold Nanoribbons with the 4H Phase.

The mechanical stability of metallic nanomaterials has been intensively studied due to their unique structures and promising applications. Although extensive investigations have been carried out on the deformation behaviors of metallic nanomaterials, the atomic-scale deformation mechanism of metallic nanomaterials with unconventional hexagonal structures remains unclear because of the lack of direct experimental observation. Here, we conduct an atomic-resolution in situ tensile-straining transmission electron microscopy investigation on the deformation mechanism of gold nanoribbons with the 4H (hexagonal) phase. Our results reveal that plastic deformation in the 4H gold nanoribbons comprises three stages, in which both full and partial dislocations are involved. At the early deformation stage, plastic deformation is governed by full dislocation activities. Partial dislocations are subsequently activated in regions that have undergone full dislocation gliding, leading to phase transformation from the 4H phase to the face-centered cubic (FCC) phase. At the last stage of the deformation process, the volume fraction of the FCC phase increases, and full dislocation activities in the FCC regions also play an important role.

Tailoring magnetic property and corrosion resistance of FeCoNiCuAl high-entropy alloy with Ce additive

The FeCoNiCuAl high-entropy alloy (HEA) has good magnetic and –mechanical properties which can be finely tuned by the fraction, composition, and distribution of the BCC and FCC phases. In this paper, we report the magnetic properties, corrosion resistance, and microstructure of the FeCoNiCuAlCex (0 ≤ x ≤ 0.09) HEAs. The results show that Ce addition is beneficial to the remanence (Br), coercivity (Hc), and hysteresis losses (Pu), but detrimental to the maximum permeability (μm) and maximum flux density (Bm). When x = 0.09, the Br, Hc, and Pu of the HEAs are improved by about 91%, 64% and 91%, respectively. The FeCoNiCuAl HEA has good corrosion resistance, comparable to 304 stainless steels. The addition of Ce improved the Icorr of the HEAs. X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM) results show that the FeCoNiCuAl HEAs are composed of a face-centered cubic (FCC) phase and a body-centered cubic (BCC) phase, with the Cu-rich nano-precipitates embedded in the BCC phase. With the addition of Ce, the nano-precipitates become even smaller and more uniform. Meanwhile, the FCC phase becomes Cu-rich and its magnetism changes from ferromagnetic to non-ferromagnetic. The electronic density of state (DOS) for the alloys calculated by density functional theory (DFT) is discussed.

Effect of Additive Elements (x = Cr, Mn, Zn, Sn) on the Phase Evolution and Thermodynamic Complexity of AlCuSiFe-x High Entropy Alloys Fabricated via Powder Metallurgy

In this study, equimolar AlCuSiFe-x (x = Cr, Mn, Zn, Sn) HEAs were fabricated by mechanical alloying (MA) and spark plasma sintering methods (SPS). The MA was performed for 45 h followed by densification of powder compacts at 650 °C. The results revealed the formation of dual face-centered cubic (FCC) and body-centered cubic (BCC) structures in AlCuSiFe-x (x = Zn, Sn) while a single BCC solid solution was noticed in AlCuSiFe-x (x = Cr, Mn). After SPS treatment, AlCuSiFeSn alloy contained FCC with CuxSny while AlCuSiFe–Zn changed to FCC + BCC structure. Similarly, AlCuSiFeCr and AlCuSiFeMn showed the formation of BCC + FCC with additional σ- and µ-phases in the HEA matrix. The calculated thermodynamic parameters of HEAs also supported the formation of different solid-solution phases in each of the above HEAs. It was found that HEAs with the additive elements Sn and Zn tend to have major FCC phases, while those with Cr and Mn give rise to major BCC with brittle σ- and µ-phase, which further improves their mechanical strength. Graphic Abstract

Thin films made by reactive sputtering of high entropy alloy FeCoNiCuGe: Optical, electrical and structural properties

Films were reactively sputtered from a high entropy alloy (HEA) FeCoNiCuGe target in an Ar/O2 plasma. The case of zero O2 gas flow yielded HEA films of FeCoNiCuGe. These as-deposited HEA films have a face centered cubic (FCC) structure, showing a texture with columnar grains containing many planar defects. The residual electrical resistivity of the films is around 225 μΩcm and the temperature dependence of the resistivity is metal-like. The temperature coefficient of resistivity is small (4.5 ppm/K). The Hall coefficient is positive while the Seebeck coefficient is negative. This is interpreted as arising from an electronic structure having both holes and electrons at the Fermi level as indicated by band structure calculations. The HEA FCC structure is unstable upon annealing in forming gas and showed demixing. Annealing in O2 also yielded inhomogeneous oxides, with a thick layer of CuO growing on the surface. The cases of reactive sputtering with an oxygen flow yielded oxides that are either nanocrystalline or amorphous dependent upon the sputter conditions. These can be classified as high entropy oxides (HEO) and have an optical bandgap around 1.9 eV and high transmission in the infrared region. The amorphous HEO has an electrical conduction interpreted as due to variable range hopping described by the Efros–Shklovskii theory. The HEO films were reduced in forming gas. For the amorphous HEO film, the reduction at 300–500 °C yielded hexagonal Ni5Ge2 and an FCC phase. For the nanocrystalline HEO, reduction resulted in creation of FCC and body centered cubic HEA metal phases.

Intermediate state of hexagonal close-packed structure to face-centered cubic structure transformation: Direct evidence for basal-type face-centered cubic phase via partial dislocation in zirconium

• Intermediate state of B-type FCC zirconium phase was firstly reported. • B-type FCC phase obey the path of HCP→Intermediate state→FCC. • Intermediate state is direct evidence of FCC phase via partial dislocation. An intermediate state of basal-type (B-type) face-centered cubic (FCC) zirconium phase is firstly reported in as-solidified pure zirconium by means of transmission electron microscopy (TEM) technique. The B-type hexagonal close-packed (HCP) to FCC phase transformation can be realized by following path: HCP→Intermediate state→FCC and each stage contained Shockley partial dislocations with Burgers vector of 1 / 6 [ 0 1 ¯ 10 ] . The observed intermediate state is direct evidence of the B-type FCC zirconium phase via partial dislocation.

Structural investigations on Ge2Sb2Te5 thin films using polarised Raman studies

Ge-Sb-Te (GST) based materials are the main source of ingredient for Phase change memory (PCM) applications. This ternary compound has the capacity to shift faster between crystalline as well as amorphous state when a suitable electrical or optical pulse is applied on it. Its crystalline phase has two states viz. face centered cubic (FCC) which is actually the metastable state and hexagonal phase which is primarily more stable state as compared to the FCC state. However, only FCC phase is suitable for PCM applications, but this study emphasizes the study of stable hexagonal phase also. The need to understand the stable hexagonal phase arises to further understand the useful metastable FCC stage. In this work, local anatomical studies have been carried out using X-ray diffraction technique (XRD) along with Raman spectroscopy that includes polarization studies on Ge2Sb2Te5 thin films in amorphous, FCC and hexagonal states. This work provides information about the symmetry of vibrational states using inelastic Raman scattering studies in polarization mode having vertical and horizontal geometries showing different Raman scattering intensities corresponding to parallel (VV-vertical vertical) and perpendicular (VH-vertical horizontal) scattering configurations. It is expected that interpretations from these studies will be helpful to understand the molecular structure and orientation of various flawed structural states such as coordination and orientation defects.

Phase constitution, surface chemistry and corrosion behavior of electrodeposited MnFeCoNiCu high entropy alloy-graphene oxide composite coatings

MnFeCoNiCu high entropy alloy (HEA) coatings with and without graphene oxide (GO) incorporation was deposited onto a mild steel substrate by the electrodeposition method. Coating morphology, phase constitution, corrosion properties, and coatings surface chemistry before and after corrosion in a 3.5 wt% NaCl solution was investigated. A predominantly body-centered cubic (BCC) phase with a minor fraction of face-centered cubic (FCC) phase was obtained in the pristine coating. Whereas a nearly equal volume fraction of FCC and BCC phases formed in coatings with increasing GO content. Corrosion characteristics of the MnFeCoNiCu HEA coatings with different GO content were studied. All the GO containing coatings exhibited higher corrosion resistance than the coating without GO in 3.5 wt% NaCl solution. The corrosion resistance of the coatings increased with increasing GO content. The surface chemistry of the coatings before and after exposure was studied by X-ray photoelectron spectroscopy (XPS). These results showed that the GO incorporation into the MnFeCoNiCu matrix enhanced the formation of stable oxides, which can reduce ionic diffusion, thus improving the corrosion resistance of the MnFeCoNiCu HEA-GO coatings when compared to the MnFeCoNiCu HEA coating without GO. • MnFeCoNiCu high entropy alloy-Graphene oxide (GO) composite coatings were electrodeposited. • Phase fraction, microstructure, and corrosion behavior of the coatings changed with changes in the GO volume fraction. • GO incorporation enhanced stable oxide formation which enhanced corrosion resistance.

Low neutron cross-section FeCrVTiNi based high-entropy alloys: design, additive manufacturing and characterization

The development of high-entropy alloys (HEAs) based on the novel alloying concept of multi-principal components presents opportunities for achieving new materials with desired properties for increasingly demanding applications. In this study, a low neutron cross-section FeCrVTiNi-based HEA was developed for potential nuclear applications. A face-centred cubic (FCC) HEA with the nominal composition of FeCr0.4V0.3Ti0.2Ni1.3 is proposed based on the empirical thermodynamic models and the CALculation of PHAse diagrams (CALPHAD) calculation. Verifications of the predictions were performed, including the additive manufacturing of the proposal material and a range of microstructural characterizations and mechanical property tests. Consistent with the prediction, the as-fabricated HEA consists of a dominant FCC phase and minor Ni3Ti precipitates. Moreover, significant chemical segregation in the alloy, as predicted by the CALPHAD modelling, was observed experimentally in the produced dendritic microstructure showing the enrichment of Ni and Ti elements in the interdendritic regions and the segregation of Cr and V elements in the dendritic cores. Heterogenous mechanical properties, including microhardness and tensile strengths, were observed along the building direction of the additively manufactured HEA. The various solid solution strengthening effects, due to the chemical segregation (in particular Cr and V elements) during solidification, are identified as significant contributing factors to the observed mechanical heterogeneity. Our study provides useful knowledge for the design and additive manufacturing of compositionally complex HEAs and their composition-microstructure-mechanical property correlation.

Gradient nanostructure, phase transformation, amorphization and enhanced strength-plasticity synergy of pure titanium manufactured by ultrasonic surface rolling

In this study, ultrasonic surface rolling process (USRP) was used to fabricate a gradient nanostructured commercial pure titanium. The site-specific microstructure, refining mechanisms and mechanical properties were investigated by high-resolution transmission electron microscopy, electron backscatter diffraction and tensile test. The results show that the surface layer exhibited multi-gradient features along the depth, including grain size gradient, deformation twin gradient, strain gradient, orientation gradient and hardness gradient. The gradient structure consisting of equiaxed nanograin layer, elongated lamellar layer, deformation twinning profuse layer, and coarse-grained layer along the depth direction. The formation mechanisms of gradient nanostructure are thoroughly elucidated, and the synergistic effect of twin and dislocation plays an important role in grain refinement. In addition, amorphous bands (generated by localized dislocations or phase transformation triggered crystalline to amorphous transition) and nano-thick lamellar with face-centered cubic (FCC) structure (<0001> HCP //<001> FCC , {1 1 ¯ 00} HCP //{2 2 ¯ 0} FCC and {11 2 ¯ 0} HCP //{220} FCC ) were formed in the nanograin layer, and the phase transformation mechanisms were elucidated. The HCP to FCC phase transformation and amorphization play a role in refining grain. Tensile test results suggested that the gradient nanostructure improved the strength (yield strength and ultimate tensile strength improved from 342 MPa and 526 MPa to 412 MPa and 598 MPa, respectively) while maintaining enough plasticity. The strength-plasticity synergy was attributed to the coordinated deformation of the gradient nanostructure and the coarse-grained core.

Decoupling the roles of constituent phases in the strengthening of hydrogenated nanocrystalline dual-phase high-entropy alloys

Nanocrystalline (NC) dual-phase Al0.7CoCrFeNi HEAs containing face-centered cubic (FCC) and body-centered cubic (BCC) microstructural phases were fabricated by high-pressure torsion (HPT). The influences of hydrogen on the thermal desorption and nanoindentation responses of NC HEA were compared with the coarse-grained alloy. The plastic zone size and indentation size effects were carefully considered to identify the distinct contributions of the constituent phases to the hardness and its variation with hydrogen charging. Results show that the FCC phase is susceptible to a larger degree of hydrogen-induced hardening than the BCC phase. Such difference is negated in the NC samples. These results are discussed in terms of the distinct responses of FCC and BCC HEA phases to hydrogen and the governing deformation mechanisms in coarse grained and NC samples.

Enhancement in mechanical properties through an FCC-to-HCP phase transformation in an Fe-17.5Mn-10Co-12.5Cr-5Ni-5Si (in at%) medium-entropy alloy

The present work focuses on developing a low-cost medium entropy alloy (MEA) with desirable mechanical properties according to the benchmark CoCrFeMnNi high entropy alloy (MEA). By adjusting the ratio of each component and adding silicon to the system, the Fe50Mn17.5Cr12.5Co10Ni5Si5 MEA with a single face-centered cubic (FCC) phase was developed. After homogenization, hot rolling, cold rolling, and annealing, fully recrystallized MEA specimens with grain sizes ranging from 10 µm to 149 µm were used for tensile tests. The microstructure of the elongated MEAs showed a ɛ-martensite transformation from the FCC phase to the hexagonal close-packed (HCP) phase, indicating the stacking fault energy (SFE) of the MEA was significantly reduced. The room-temperature deformed MEA showed improved mechanical properties in yield strength and tensile strength than the CoCrFeMnNi MEA. Meanwhile, the volume fraction of the HCP phase in cryogenic-deformed MEA is much larger than that in room-temperature deformed MEA; its yield strength was increased by two times, while the tensile strength exceeded the level of 1 GPa.

Effect of Mo addition on structures and properties of FeCoNiCrMn high entropy alloy film by direct current magnetron sputtering

FeCoNiCrMnMox high-entropy alloy (HEA) films were prepared by direct current (DC) magnetron co-sputtering. The influence of the Mo content on the microstructures and mechanical properties of the HEA films were systematically studied by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), Vickers hardness test and tribological test. The addition of Mo results in the formation of a denser film with refined grain size, and promotes the transformation from a face-centered cubic (FCC) phase to a mixture of FCC and body-centered cubic (BCC) phases. The hardness of the films increases from 8.5 GPa (x = 0) to 12 GPa (x = 1) and the friction coefficient decreases from 0.5 (x = 0) to 0.3 (x = 1), which greatly enhance the damage tolerance of the film. The improved mechanical and tribological properties of the HEA films are attributed to the formation of the hard BCC phase and grain refinement at high contents of Mo. The FeCoNiCrMnMox HEA films are suitable candidates for structural application as protective coatings.

Microstructure and hardness of solution treated and cold-rolled Co75Cr25 alloy

In this study, the effects of solution treatment and plastic deformation on the microstructure and hardness of a Co75Cr25 alloy were investigated. The microstructures of the solution-treated and deformed specimens were investigated through X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The results indicated that the Co75Cr25 alloy cooled by air (AC) during solution treatment has a single hexagonal close-packed (HCP) phase structure, while the alloy quenched by water (WQ) has a dual phase structure with the mixture of HCP phase and face-centered cubic (FCC) phase. Fast cooling rate inhibited the FCC to HCP martensitic phase transformation and led to more FCC phase retained at room temperature. During subsequent cold rolling (CR), stress-induced HCP to FCC martensitic transformation further increased the fraction of FCC phase in the WQ-CR specimen, through gliding of Shockley partial dislocations. Besides, stress-induced 101¯2 and 101¯1 twins were also widely observed in the WQ-CR specimen. Both martensitic transformation and twinning refined the microstructure of the WQ-CR specimen. However, for the AC-CR specimen, the deformation did not introduce obvious martensitic phase transformation. Some 101¯2and 112¯1 twins were activated to accommodate deformation. The hardness values of specimens after cold rolling were higher than those of the specimens before cold rolling. Deformation induced grain refinement played a very important role for improving the hardness.