From Face-centered Cubic Research Articles

Orientation effects on shock-induced plastic deformation in FeNiCoCu high entropy alloy

FeNiCoCu high-entropy alloys (HEAs) demonstrate promising potential for widespread use in structural and functional applications. However, a thorough understanding of dynamic deformation processes in FeNiCoCu HEA is limited due to technological constraints in detecting real-time microstructural developments at the atomic level. This study examines the shock-induced plastic deformations in the equiatomic FeNiCoCu HEA, focusing on crystallographic orientation and particle velocity, using nonequilibrium molecular dynamics simulations. We obtained the P−V/V0, P−T, P−Up, and Us−Up Hugoniot relations and evaluated their anisotropy. The shock velocity, stress, and shear stress exhibit orientation dependence due to the differences in the plastic deformation mechanism. For shock loading along [100] orientations, dislocation dominates at lower shock intensities. However, a phase transition from face-centered-cubic (FCC) to body-centered-cubic becomes the primary plastic deformation at high shock intensity. For shock loading along [112¯] and [111] orientations, the generation of disordered structures and dislocation activities is revealed to play an important role in the development of localized plastic deformation. Moreover, the competition of disordered and hexagonal-close-packed (HCP) atoms is observed. The transition from FCC to disordered atoms provides nucleation sites for dislocations, and the slip of dislocations around disordered atoms leads to the formation of HCP structures. These findings are very helpful for learning the dynamic deformation behavior of FeNCoCu HEA.

Micro‐Ring Morphology of Ag7NCs and Light‐Induced Reversible Interconversion of FCC Ag14NCs via Cu2+ ions‐Mediated Particle‐Assisted Reversible Interconversion

AbstractDespite the remarkable progress made on intercluster conversion in atomically precise metal nanoclusters (MNCs) and their self‐organization to develop microscopic molecular architecture with well‐defined size and shape, achieving light‐induced reversible structural transformation and the development of micro‐ring self‐assembly in MNCs have, so far, remained elusive. The present investigation touches on these two long‐standing quests by showcasing a new route, light‐induced Particle‐Assisted Reversible Interconversion (PARI) for the reversible transformation from Face Centered Cubic (FCC) Ag14NCs to Ag7NCs. Our studies reveal that the lack of plasmonic silver nanoparticles (AgNPs) in the system results in the formation of Ag7NCs with metallic kernels having centrosymmetric crystal packing. The molecular self‐organization of Ag7NCs through various non‐covalent interactions such as C−H⋅⋅⋅O, C−H⋅⋅⋅H−C, and C−H⋅⋅⋅π leads to the formation of micro‐ring morphology, a unique molecular architecture in MNCs. The in situ generated AgNPs due to the acceleration of the reaction kinetics by Cu2+ ions facilitate the growth of Ag14NCs with FCC metallic kernel. These two structural units of AgNCs show light‐induced reversible structural transformation which is also associated with the reversible tuning of their spectroscopic and morphological signatures. This PARI‐guided interconversion strategy put forward a most appropriate example of a structure–property relationship in MNCs.

Micro-Ring Morphology of Ag7NCs and Light-Induced Reversible Interconversion of FCC Ag14NCs via Cu2+ ions-Mediated Particle-Assisted Reversible Interconversion.

Despite the remarkable progress made on intercluster conversion in atomically precise metal nanoclusters (MNCs) and their self-organization to develop microscopic molecular architecture with well-defined size and shape, achieving light-induced reversible structural transformation and the development of micro-ring self-assembly in MNCs have, so far, remained elusive. The present investigation touches on these two long-standing quests by showcasing a new route, light-induced Particle-Assisted Reversible Interconversion (PARI) for the reversible transformation from Face Centered Cubic (FCC) Ag14NCs to Ag7NCs. Our studies reveal that the lack of plasmonic silver nanoparticles (AgNPs) in the system results in the formation of Ag7NCs with metallic kernels having centrosymmetric crystal packing. The molecular self-organization of Ag7NCs through various non-covalent interactions such as C-H⋅⋅⋅O, C-H⋅⋅⋅H-C, and C-H⋅⋅⋅π leads to the formation of micro-ring morphology, a unique molecular architecture in MNCs. The in situ generated AgNPs due to the acceleration of the reaction kinetics by Cu2+ ions facilitate the growth of Ag14NCs with FCC metallic kernel. These two structural units of AgNCs show light-induced reversible structural transformation which is also associated with the reversible tuning of their spectroscopic and morphological signatures. This PARI-guided interconversion strategy put forward a most appropriate example of a structure-property relationship in MNCs.

Time resolved x-ray diffraction using the flexible imaging diffraction diagnostic for laser experiments (FIDDLE) at the National Ignition Facility (NIF): Preliminary assessment of diffraction precision.

The Flexible Imaging Diffraction Diagnostic for Laser Experiments (FIDDLE) is a new diagnostic at the National Ignition Facility (NIF) designed to observe in situ solid-solid phase changes at high pressures using time resolved x-ray diffraction. FIDDLE currently incorporates five Icarus ultrafast x-ray imager sensors that take 2ns snapshots and can be tuned to collect X-rays for tens of ns. The platform utilizes the laser power at NIF for both the laser drive and the generation of 10keV X-rays for ∼10ns using a Ge backlighter foil. We aim to use FIDDLE to observe diffraction at different times during compression to probe the kinetics of phase changes. Pb undergoes two solid-solid phase transitions during ramp compression: from face centered cubic (FCC) to hexagonal close packed (HCP) and HCP to body centered cubic (BCC). Results will be reported on some of the first shots using the FIDDLE diagnostic at NIF on ramp compressed Pb to a peak pressure of ∼110GPa and a single undriven CeO2 calibration shot. A discussion of the uncertainties in the observed diffraction is included.

Dilution induced variation of microstructure and mechanical properties on Co-Cr-Fe-Ni high entropy alloy coatings prepared by laser cladding

The remarkable properties of some series of high entropy alloys (HEAs) have garnered extensive interest in the development of HEA coatings. In this study, we prepared non-equiatomic Co-Cr-Fe-Ni HEA coatings using laser cladding method with Co-Cr-Fe-Ni cable-type welding wire (CTWW) as the filling material and investigated the effect of dilution rate on the microstructure evolution of coatings. By changing the wire feeding speed, a large dilution rate range was obtained which is from over 80 % to below 20 % and a variation of the coating's microstructure and mechanical properties was discovered within this large dilution rate range. All of the coatings are dense and uniform, with good metallurgical bonding. A pivotal discovery was made when the dilution rate decreased from approximately 35 %–40 % to below this threshold. The coating transitions from face centered cubic (FCC) and body centered cubic (BCC) dual-phase to FCC single-phase, accompanied by a precipitous drop in hardness from approximately 425 HV to below 175 HV. The interlaced distribution and dislocation pile-up of the layered BCC eutectic structure within the FCC phase are the main reasons for this dramatic change. Therefore, controlling the dilution rate is an effective way to regulate the performance of Co-Cr-Fe-Ni HEA coatings.

Crack growth and fracture mechanics of CuCrFeNiCo high-entropy alloy during tension testing

In this work, the crack growth and fracture mechanics of CuCrFeNiCo high-entropy alloy (HEA) during tension process are studied through molecular dynamics simulation method. The single-crystalline, nanocrystalline, and twinned-nanocrystalline CuCrFeNiCo HEA samples with an initial crack are prepared. The influences of boundary conditions, crack length and crystallographic orientation are considered in detail. The results indicate that the phase transition from face-centered cubic (FCC) structure into hexagonal close-packed (HCP) structure and the appearance of Shockley dislocations are the majority in all samples. The dislocations appear most densely in the twinned-nanocrstalline sample and most sparsely in the single-crystalline sample. The growth of the initial crack combined with the formation and expansion of new cracks along the grain boundaries (GBs) is the determining factor in the fracture mechanics of the CuCrFeNiCo HEA samples. The deformation capacity of the samples with free boundary conditions along the y-axis is better and the plastic deformation process is longer than the samples with periodic boundary conditions along the y-axis. The tensile strength values of the CuCrFeNiCo HEA samples change significantly in the range from 2.61 GPa to 7.75 GPa when changing the simulation conditions. The von Mises stress in the grains is markedly lower than that in the GBs.

Unveiling the roles of initial phase constituents and phase metastability in hydrogen embrittlement of TRIP‐assisted VCrCoFeNi medium‐entropy alloys

Medium-entropy alloys (MEAs) that exhibit transformation-induced plasticity (TRIP) from face-centered cubic (FCC) to body-centered cubic (BCC) are considered promising for liquid hydrogen environments due to their remarkable cryogenic strength. Nonetheless, studies on hydrogen embrittlement (HE) in BCC-TRIP MEAs have not been conducted, although the TRIP effect and consequent BCC martensite usually deteriorate HE susceptibility. In these alloys, initial as-quenched martensite alters hydrogen diffusion and trap behavior, and deformation-induced martensitic transformation (DIMT) provides preferred crack propagation sites, which critically affects HE susceptibility. Therefore, this study aims to investigate the HE behavior of BCC-TRIP MEAs by designing four V10Cr10Co30Fe50–xNix (x = 0, 1, 2, and 3 at%) MEAs, adjusting both the initial phase constituent and phase metastability. A decreased Ni content leads to a reduced fraction and mechanical stability of FCC, which in turn increases HE susceptibility, as determined through electrochemical hydrogen pre-charging and slow-strain rate tests The permeation test and thermal desorption analysis reveal that the hydrogen diffusivity and content are affected by initial BCC fraction, interconnectivity of BCC, and refined FCC. As these initial phase constituents differ between the alloys with FCC- and BCC- dominant initial phase, microstructural factors affecting HE are unveiled discretely among these alloy groups by correlation of hydrogen-induced crack behavior with hydrogen diffusion and trap behavior. In alloys with an FCC-dominant initial phase, the initial BCC fraction and DIMT initiation rate emerge as critical factors, rather than the extent of DIMT. For BCC-dominant alloys, the primary contributor is an increase in the initial BCC fraction, rather than the extent or rate of DIMT. The unraveled roles of microstructural factors provide insights into designing HE-resistant BCC-TRIP MEAs.

Synergy effect of multi-strengthening mechanisms in CoNiCr-based high-entropy superalloy at cryogenic temperature

The evolution and formation mechanism of microstructures in a high-entropy CoNiCr-based superalloy induced by cold-drawing deformation at cryogenic temperature were investigated. The improvement of strength while maintaining the ductility during cryogenic-drawing deformation in the CoNiCr-based high-entropy superalloy originated from the activation of multiple deformation mechanisms including deformation twins, dislocations, 9R phase and hexagonal close-packed (HCP) phase transformation. Two dominant contributions have been suggested for the increase in strength, namely, nanotwins strengthening and the existence of 9R and HCP phase. In this paper, cryogenic-deformation-induced phase transformation from face-centered cubic (FCC) to HCP in the CoNiCr-based high-entropy superalloy was found, and the atomic-scale mechanism of FCC-9R-Twin/HCP transformation was revealed. A new crystallographic model was established to explain the FCC→9R→Twin/HCP transformation. The phase transformation is significant for the comprehensive understanding of the deformation behavior of this CoNiCr-based high-entropy superalloy at cryogenic temperature, and for guiding the design of high-entropy superalloy with advanced mechanical properties.

Polymorphic phase transition in CoCrNi medium-entropy alloy under impact loadings

Polymorphic phase transition in metallic materials under high pressure is a critical aspect of dynamic properties and has been attracting a great interest. Despite the extensive researches have been made on understanding of this phase transition in traditional single-principal element alloys, little is known about the phase transition in recently emergent multi-principal medium and high entropy alloys, especially compressed under high strain rates. In this work, based on molecular dynamic simulations, three impact loading strategies with distinct loading paths, such as single-shock, double-shock and ramp-wave loading are carried out on the single crystalline CoCrNi medium-entropy alloy (MEA) to investigate the phase transition under high strain-rate compression. Careful characterizations show that the phase transition of CoCrNi MEA is loading-path dependent, as evidenced by the significant differences in macroscopic pressure evolution and microscopic structural phase transition among the samples under various thermodynamic paths. An intriguing pressure “overshoot” is found and demonstrated as the characteristic of the critical structural phase transition from face-centered cubic (FCC) structure to hexagonal-close-packed (HCP) structure mediated by body-centered cubic (BCC) like clusters. We show that such loading-path dependence is attributed to the strain rate and temperature rise in the loading process, which control the evolution of microstructure and deformation field. The inherent correlation between the atomistic process of phase transition and loading strategies results in polymorphic phase transition under high strain rates. These findings shed new light on the nature of impact phase transition of multi-principal alloys.

Induced phase transformation in ionizable colloidal nanoparticles

Acid–base equilibria directly influence the functionality and behavior of particles in a system. Due to the ionizing effects of acid–base functional groups, particles will undergo charge exchange. The degree of ionization and their intermolecular and electrostatic interactions are controlled by varying the pH and salt concentration of the solution in a system. Although the pH can be tuned in experiments, it is hard to model this effect using simulations or theoretical approaches. This is due to the difficulty in treating charge regulation and capturing the cooperative effects in a colloidal suspension with Coulombic interaction. In this work, we analyze a suspension of ionizable colloidal particles via molecular dynamics (MD) simulations, along with Monte Carlo simulations for charge regulation (MC-CR) and derive a phase diagram of the system as a function of pH. It is observed that as pH increases, particles functionalized with acid groups change their arrangement from face-centered cubic (FCC) packing to a disordered state. We attribute these transitions to an increase in the degree of charge polydispersity arising from an increase in pH. Our work shows that charge regulation leads to amorphous solids in colloids when the mean nanoparticle charge is sufficiently high.Graphical abstract

D-d Orbital coupling induced by crystal-phase engineering assists acetonitrile electroreduction to ethylamine

The d-d orbital coupling induced by crystal-phase engineering can effectively adjust the electronic structure of electrocatalysts, thus showing significant catalytic performance, while it has been rarely explored in electrochemical acetonitrile reduction reaction (ARR) to date. Herein, we successfully realize the structural transformation of PdCu metallic aerogels (MAs) from face-centered cubic (FCC) to body-centered cubic (BCC) through annealing treatment. Specifically, the BCC PdCu MAs exhibit excellent ARR performance with high ethylamine selectivity of 90.91%, Faradaic efficiency of 88.60%, yield rate of 316.0 mmol h−1 g−1Pd+Cu and long-term stability for consecutive electrolysis within 20 h at −0.55 V vs. reversible hydrogen electrode, outperforming than those of FCC PdCu MAs. Under the membrane electrode assembly system, BCC PdCu MAs also demonstrate excellent ethylamine yield rate of 389.5 mmol h−1 g−1Pd+Cu. Density functional theory calculation reveals that the d-d orbital coupling in BCC PdCu MAs results in an evident correlation effect for the interaction of Pd and Cu sites, which boosts up the Cu sites electronic activities to enhance ARR performance. Our work opens a new route to develop efficient ARR electrocatalysts from the perspective of crystalline structure transformation.

Extra strengthening and Bauschinger effect in gradient high-entropy alloy: A molecular dynamics study

The gradient nano-grained (GNG) structures achieve a combination of high strength and ductility compared with homogeneous nano-grained structures. In order to study the mechanical behavior of GNG with extremely small grain size, the systematic tension, compression and cyclic deformation behavior of gradient and homogeneous nano-grained CoCrFeMnNi high-entropy alloy (G-HEA and H-HEA) are studied by the molecular dynamics (MD) simulation. The simulation results show that G-HEA and H-HEA have multiple plastic deformation mechanisms during uniaxial and cyclic loading, such as dislocation slip, deformation twinning, phase transformation from face-centered cubic (FCC) to hexagonal close-packed (HCP), and grain boundary plastic behavior. During uniaxial loading, the heterogeneous deformation makes the yield stress of G-HEA higher than that calculated by the rule of mixture (ROM), indicating an extra strengthening in GNG material. However, the extra strengthening of G-HEA is weakened with increased strain due to the dislocation without piled up in front of the grain boundary and relaxed heterogeneous deformation. During cyclic loading, the sessile dislocations promote the reverse movement of dislocations, resulting in the Bauschinger effect. However, the sessile dislocation density of G-HEA is close to that calculated by the ROM, resulting in the Bauschinger effect of G-HEA close to that calculated by the ROM. Therefore, as an alternative perspective, the MD simulation confirms that dislocation pinning in front of grain boundaries is an important source of extra strengthening for gradient structured materials as revealed by experiments. The results extend the understanding of the deformation mechanism of CoCrFeMnNi GNG HEA with extremely small grain size under tension, compression and cyclic loading.

Supercritical CO2 Mediated Multi‐scale Structural Engineering in PdCu/C for Boosting Electrocatalytic Formic Acid Oxidation

AbstractNowadays, PdCu alloy nanocatalyst with excellent performance in electrocatalytic formic acid oxidation reaction (FAOR) is believed to have great potential in application of direct formic acid fuel cells. Structural engineering has shown great success in achieving PdCu alloys with high catalytic performance, while achievement of multi‐scale structure engineering is still a great challenge. In this work, we found that supercritical carbon dioxide (SC CO2) could lead to multi‐scale structure engineering in PdCu/C nanocatalysts, including surface defect engineering, phase engineering, morphology engineering and substrate structure engineering. With the assistance of SC CO2, amorphous phase in surface, the transformation from face‐centered cubic (FCC) to body‐centered cubic (BCC) phase, the morphology of 2D nanoflakes and the curved carbon as substrate all contribute to the ultrahigh mass activity for electrocatalytic FAOR as 3624.3 mA/mgPd in PdCu/C nanocatalysts, which is the highest value in PdCu alloy reported up to now. Therefore, this work not only displays the great potential of SC CO2 in multi‐scale structure engineering, but also provides new inspiration of material design to achieve nanocatalysts with ultrahigh catalytic performance.

On the low-cycle fatigue behavior of a multi-phase high entropy alloy with enhanced plasticity

A multi-phase non-equiatomic FeCrNiMnCo high entropy alloy (HEA) was fabricated using vacuum induction melting. Thermo-mechanical treatments consisting of cold rolling and annealing at 750 °C and 850 °C were employed to improve the mechanical properties of the HEA in focus. Tensile experiments revealed that yield strength and ultimate tensile strength levels can be enhanced significantly after thermo-mechanical processing (TMP). At the same time, ductility remains at an adequate level. Strain-controlled low-cycle fatigue (LCF) experiments were carried out in order to assess the mechanical properties of this HEA under cyclic loading conditions. At the same strain amplitude, the stress levels of the processed samples were considerably higher than that of the as-received counterpart. Similarly, fatigue lives of the former could surpass the base condition at the strain amplitudes of 0.2% and 0.4%; however, at the higher strain amplitudes, cyclic softening was observed. Electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) results revealed that phase transformation from face-centered cubic (FCC) to body-centered cubic (BCC/B2) took place at a higher occurrence with increasing strain amplitude (0.2% to 0.6%). Furthermore, transmission electron microscopy (TEM) studies confirm that upon tensile deformation additional plasticity mechanisms, i.e., deformation twinning and phase transformation, contribute to the overall mechanical behavior of the multi-phase HEA.

Strengthening of CoCrNi medium entropy alloy with gadolinium additions

A series of (CoCrNi)100–xGdx (x = 0/0.1/0.3/0.5/1.0) medium entropy alloys (MEAs) was fabricated by high-vacuum arc melting. The effects of rare-earth element, Gd, additions on the evolution of microstructures and mechanical properties of CoCrNi MEAs were studied. The phase and microstructure were examined using X-ray diffraction, scanning electron microscope and transmission electron microscope. The tensile properties were measured by material testing system (MTS) and the hardness was measured by both Vickers indenter and nanoindentation. As the Gd concentration increased, the phase transformed from face-centered cubic (FCC) to hexagonal close-packed (HCP) precipitates in FCC matrix. Compared to the single phase CoCrNi MEA with the yield strength of 400 MPa and fracture strength of 880 MPa, CoCrNi99.5Gd0.5 MEA exhibited a much higher yield strength of 751 MPa and ultimate tensile strength of 1216 MPa, while the ductility still maintained a high value of ∼55%. The enhanced mechanical properties were ascribed to solid solution, precipitation and grain refinement strengthening mechanisms. The relations among compositions, microstructures and mechanical properties of (CoCrNi)100–xGdx MEAs were discussed.

Multiscale-phase-driven strength-ductility synergy in Fe3Cr2CoNiAlx high entropy alloys

Fe3Cr2CoNiAlx (x = 0.6, 0.7, 0.8, 1.0, 1.1, 1.2) high entropy alloys (HEAs) with dual-scales B2 precipitated were prepared. The microstructure changed from face-centered-cubic (FCC) and body-centered-cubic (BCC) to a single BCC structure with the increasing of Al content. By controlling the concentration of Al element in the BCC-based HEAs, the structure can be tuned to two coherent BCC phases: medium B2 (m-B2) precipitates with an average length of 200–300 nm and smaller size B2 (s-B2) precipitates with an average diameter of 10–50 nm. The compression tests indicated that the Fe3Cr2CoNiAl1.2 HEA exhibited excellent compression properties, which yield strength and fracture strain is 1464 MPa and 20%, respectively. Furthermore, the strengthening mechanism has been discussed, and it is determined that the main strengthening mechanism was precipitation strengthening. This work provides a useful mean by tailoring multi-scale phase to achieve excellent properties in BCC-based high entropy alloys.

Extraordinary combination of strength and ductility in an additively manufactured Fe-based medium entropy alloy through in situ formed η-nanoprecipitate and heterogeneous microstructure

The current study investigated the mechanical properties of Fe65Ni15Co8Mn8Ti3Si (at%) medium entropy alloy (MEA), printed by laser-based direct energy deposition (DED) in the room and liquid nitrogen environments. The as-printed microstructure contains various levels of heterogeneities, including different grain sizes, dual-phase microstructure, in situ formed η nanoprecipitate, cellular structure, and elemental segregation generated during the DED process. The impact of each observed heterogeneity on the mechanical properties and the predominant deformation mechanisms were examined. The corresponding MEA revealed a high ultimate tensile strength (UTS) of ∼1.02 GPa, total elongation (TE) of ∼46%, and exceptional cryogenic mechanical properties with a UTS and TE of 1.83 GPa and 40%, respectively. According to microstructural observations, the deformation-induced phase transformation from face-centered cubic (FCC) to body-centered cubic (BCC) is the predominant strengthening mechanism at both room and liquid nitrogen temperatures in addition to solid-solution strengthening and dislocation-mediated plasticity. The non-shearable η nanoprecipitates also enhanced the mechanical properties through precipitation/stacking fault strengthening. Hetero-deformation-induced (HDI) strengthening also occurred in the presence of dual-phase microstructure, cellular microstructure, and elemental segregation in the FCC phase. This upgraded synergy of tensile strength and ductility at liquid nitrogen temperature can be explained through the phase transformation and additional activation of twinned martensite formation, which results in two-step strain-hardening behavior. The presented results expand possibilities for developing DED-processed ferrous HEAs/MEAs to overcome the strength and ductility trade-off at room and liquid nitrogen temperatures.

Advancing strength and counteracting embrittlement by displacive transformation in heterogeneous high-entropy alloys containing sigma phase

Metallic alloy design for room temperature applications typically aims at avoiding undesired brittle intermetallic phases. In transition metal alloys, the sigma phase is particularly known as a harmful phase leading to serious embrittlement. Here, we develop a novel strategy that utilizes displacive transformation and heterogeneous structures to mitigate the embrittlement of sigma phase particles in high-entropy alloys (HEAs). A careful study of the deformation behavior reveals that the displacive transformation from face-centered cubic (FCC) to hexagonal close packed (HCP) phase can effectively suppress the propagation of microcracks originated in these brittle sigma particles (310±52 nm) and contributes to high work hardening behavior during tensile deformation. This is achieved by tuning the stacking fault energy of the FCC matrix by reducing the Ni content to promote transformation induce plasticity (TRIP) around the sigma phase in a non-equiatomic Fe34Mn20Co20Cr20Ni6 (at. %) HEA. Such TRIP effect can be optimized in various heterogeneous structures with bimodal grain sizes via simple cold-rolling (∼60%) and subsequent annealing (30 min at 700 or 800 °C). The heterogeneously structured HEAs containing brittle sigma particles exhibit ultimate tensile strengths as high as ∼1.2 GPa while maintaining a ductility up to ∼50%. This is mainly attributed to the transformation induced stress-relaxation around the regions containing brittle sigma particles. The insights provide a new design strategy of combining TRIP effect and heterogeneous structures for developing strong and ductile alloys.

Microstructures and properties of Crx-FeNiCu0.5Ti0.5 high-entropy alloys for corrosion resistance

In this study, a novel high entropy alloy (HEA) of Crx-FeNiCu0.5Ti0.5 (in mole ratio, x = 0, 0.3, 0.5, 0.7, 0.9 and 1.2) was designed and prepared by arc melting to investigate its microstructure and corrosion resistant behavior. The phase analysis manifested that the microstructures transformed from face-centered cubic (FCC) + intermetallic compounds (IMs) of Cr-free alloy to FCC + IMs + body-centered cubic (BCC) phase with the increase of Cr molar ratio. The potentiodynamic polarization curves indicated that FeCr0.7NiCu0.5Ti0.5 alloy showed good corrosion performance with a corrosion potential of −0.181 V and a self-corrosion current density of 5.455 × 10−7 A/cm2 in 0.5 M H2SO4 solution. The chemical composition from XPS test displayed that Cr2O3, TiO2 and Fe-oxides on the sample surface form different strength passive films, reducing ionic diffusion and improving corrosion resistant, which was the key anti-corrosive mechanism.

Unusual deformation-induced martensitic transformation in Fe-Co-Ni-Cr-Mn high entropy alloy thin films

Deformation-induced martensitic transformation in Fe-Co-Ni-Cr-Mn high entropy alloy (HEA) thin film with nanocolumnar growth feature are revealed to readily occur at intermediate deposition power rather than low and high deposition power ranges. The high probability of deformation-induced martensitic transformation from face-centered cubic (FCC) to hexagonal close-packed (HCP) phase in 150 W-film results in low nanoindentation hardness and compressive yield strength, in comparison with 60 W- and 210 W-films. Dependence of the martensitic transformation probability has been explained in terms of the competition between grain size effect and nanocolumn cohesion effect on the phase metastability. The increased grain size with deposition power favors the mechanical instability of FCC parent phase caused by the weakened slip obstruction, while the enhanced nanocolumn cohesion with deposition power hinders the formation of HCP martensite due to the increased activation energy for martensitic transformation. The grain size effect is dominated over the cohesion between nanocolumn at low deposition power range, while column cohesion effect is dominant at high deposition power range, which gives rise to the crossover of FCC-to-HCP with deposition power. This work might provide a deeper understanding in phase metastability of sputtered films, for instance, the effect of deposition power on deformation-induced martensitic transformation.