Hcp Crystal Structure Research Articles

Laser additive manufacturing of a CoCr-based medium-entropy alloy: Impressive stacking fault-induced nanotwinning strengthening through rapid solidification

The directed energy deposition (DED) technique was used to deposit a CoCrNiWC medium entropy alloy (MEA) through a robotic-assisted laser additive manufacturing (LAM) machine. The microstructural features, including solidification grains, structural defects, and phase transformations, along with uniaxial tensile behavior in different orientations, were comprehensively studied. Furthermore, a quantitative assessment was carried out to evaluate the contribution of strengthening mechanisms through apprising stacking fault constant of 11362 MPa nm for the developed MEA, enrolling the impact of FCC to HCP crystal structure transition for the cobalt matrix after rapid cooling LAM solidification in the solid-state board. In addition, the presence of numerous ordinary screw dislocations interacting with each other, stacking faults (SFs), nanotwins, and refined grains led to their hindering as the critical factor for strengthening. The SF intersections resulted in the formation of effective Lomer-Cottrell (LC) locks and nanotwins, which were barriers to slip and attributed to dislocation interactions. Accordingly, the rapid cooling solidification of the LAM process contributed to impressive mechanical properties of deposited CoCr-based MEA with remarkable yield and tensile strengths of ∼895 and 1410 MPa, confirming the direct contributions of statistically stored dislocations upon impact SF and nanotwining strengthening.

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.

Phase control of solid-solution RuIn nanoparticles and their catalytic properties.

The properties of solids could be largely affected by their crystal structures. We achieved, for the first time, the phase control of solid-solution RuIn nanoparticles (NPs) from face-centred cubic (fcc) to hexagonal close-packed (hcp) crystal structures by hydrogen heat treatment. The effect of the crystal structure of RuIn alloy NPs on the catalytic performance in the hydrogen evolution reaction (HER) was also investigated. In the hcp RuIn NPs, enhanced HER catalytic performance was observed compared to the fcc RuIn NPs and monometallic Ru NPs. The intrinsic electronic structures of the NPs were investigated by valence-band X-ray photoelectron spectroscopy (VB-XPS). The d-band centre of hcp RuIn NPs obtained from VB-XPS was deeper than that of fcc RuIn NPs and monometallic Ru NPs, which is considered to enable the hcp RuIn NPs to exhibit enhanced HER catalytic performance.

Rapid acquisition of digital fingerprints of Ti-6Al-4V macrotexture from machining force measurement data

Titanium alloys display anisotropic deformation properties due to the hexagonal close-packed (hcp) crystal structure of the α-phase. When subjected to localised deformation during machining, this behaviour influences fluctuations in the cutting force response of the material as the tool encounters grains of different orientations. In this research, cutting force signals acquired during face turning of Ti–6Al–4V possessing a lamellar α colony structure have been spatially mapped demonstrating the ability to identify microstructural features such as prior-β grain boundaries, grain boundary α, and α colonies. Measured cutting forces have been correlated to texture using orientation information acquired from large area EBSD analysis. A relationship between the misalignment of the crystallographic a slip vector with respect to the cutting direction and the passive cutting force response has been established, demonstrating a rise in cutting forces as this misalignment is increased. This novel approach to in-process materials evaluation offers manufacturers the potential of a powerful digital quality assurance tool, with the results presented here demonstrating the possibility for rapid characterisation of entire component surfaces, revealing microstructural features, and inferring the crystallographic orientation of macrotextured regions in Ti–6Al–4V.

Shock response of single crystal rhenium: Effect of crystallographic orientation

Rhenium, a unique engineering material with the second highest melting point of all metals, has significant application potential in harsh service environments, and thus evaluating its mechanical responses under shock loadings is of great importance. Through molecular dynamic simulations, we have systematically investigated the effect of crystallographic orientations on the mechanical responses of single crystalline rhenium under shock compression. We have obtained the P–V/V0, Up-P, P-T, and Us-Up Hugoniot relations, and evaluated their anisotropies. The particle velocity, Pzz, and shear stress show orientation dependent when tracing the shock profiles at different time, and the difference is particularly pronounced for shear stress, resulting from the atomic arrangement in the HCP crystal structure of rhenium. The plastic deformations mechanisms of rhenium single crystal under shock loadings are revealed as the emission of dislocations and the generation of disordered atoms, and the influence of shock intensity on the plastic deformation mechanism has been systematically studied. The results presented in this work not only shed light on the effect of crystallographic orientations on the mechanical responses of monocrystalline rhenium under shock compression, but also offers fundamental insights for understanding the shock induced plastic deformation mechanisms, which could provide a valuable supplement to the investigations of mechanical properties pertaining to rhenium.

Atmospheric plasma spray coating of antibacterial nano silver-hydroxyapatite on titanium surface for biomedical applications

ABSTRACT In this paper, the nanosilver particles were deposited onto the hydroxyapatite (HAP) microparticles and the obtained nanoAg-HAP particles were coated onto the Ti metal surface using the atmospheric plasma spray method. Then, the obtained nanoAg-HAP coating on Ti substrate was annealed at 700°C. The antibacterial activities of the nanoAg-HAP coatings on Ti were also tested against the microorganisms. The prepared nanoAg-HAP particles were examined using the optic microscopy, FE-SEM, HR-TEM, XRD and the UV-vis. absorption spectroscopy methods. The nanoAg particles exhibited crystalline spherical morphologies with the particle sizes below 30 nm. In the FE-SEM-EDS measurements, well distribution of nanoAg particles on the HAP microparticles were observed with Ag concentrations of 0.153% and 0.313%. The HR-TEM measurements showed that the nanoAg particles have hexagonal close packing (hcp) crystal structure with the d-spacing values of 0.23 nm (111), 0.21 nm (002) and 0.14 nm (022). After 700°C heat treatment, the prepared nanoAg-HAP coating on Ti metal showed 100% antibacterial activity against E. coli and S. epidermidis bacteria. As a result, the obtained nanoAg-HAP coated Ti metal can be utilized effectively as a self-disinfecting biomedical implant material in the orthopedic bone repairing of the body parts.

Solidification texture dependence of the anisotropy of mechanical properties and damping capacities of an AZ31 Mg-based alloy fabricated via wire-arc additive manufacturing

Wire-arc additive manufacturing (WAAM) offers a brand-new method for short process and rapid manufacturing of large magnesium alloy components, which can avoid the formability dilemma faced by Mg alloys due to their poor plasticity. In this study, the relationships between the solidification texture and the anisotropy of mechanical properties/damping capacities of an AZ31 Mg-based alloy fabricated by the WAAM process were studied. The results revealed that the crystallographic texture with the (0001) basal plane along the building direction was promoted by the preferential growth of dendrite arms along the <11–20> direction of the HCP crystal structure. The (0001) basal texture had a strong influence on the anisotropy of mechanical properties and damping capacities. Compared to the sample with a tensile axis perpendicular to the building direction, the sample with a tensile axis along the building direction displays a simultaneous increase in tensile strength and ductility, which is related to the smaller Schmid factor (SF) for basal <a> slip and better plasticity deformation via non-basal slip and twinning. However, the smaller SF for basal <a> slip also results in a smaller resolved shear stress factor for the breakaway stress during the damping response testing, resulting in lower damping capacities. This work is helpful for further optimizing the mechanical properties and damping capacities of WAAM-processed Mg-based alloys.

A comprehensive modeling approach for polymorph selection in Lennard-Jones crystallization.

Computational predictions of the polymorphic outcomes of a crystallization process, referred to as polymorph selection, can accelerate the process development for manufacturing solid products with targeted properties. Polymorph selection requires understanding the interplay between the thermodynamic and kinetic factors that drive nucleation. Moreover, post-nucleation events, such as crystal growth and polymorphic transformation, can affect the resulting crystal structures. Here, the nucleation kinetics of the Lennard-Jones (LJ) fluid from the melt is investigated with a focus on the competition between FCC and HCP crystal structures. Both molecular dynamics (MD) simulations and 2D free energy calculations reveal that polymorph selection occurs not during nucleation but when the cluster sizes exceed the critical cluster size. This result contrasts with the classical nucleation mechanism, where each polymorph is assumed to nucleate independently as an ideal bulk-like cluster, comprised only of its given structure. Using the 2D free energy surface and the MD simulation-derived diffusion coefficients, a structure-dependent nucleation rate is estimated, which agrees with the rate obtained from brute force MD simulations. Furthermore, a comprehensive population balance modeling (PBM) approach for polymorph selection is proposed. The PBM combines the calculated nucleation rate with post-nucleation kinetics while accounting for the structural changes of the clusters after nucleation. When applied to the LJ system, the PBM predicts with high accuracy the polymorphic distribution found in a population of crystals generated from MD simulations. Due to the non-classical nucleation mechanism of the LJ system, post-nucleation kinetic events are crucial in determining the structures of the grown crystals.

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.

Estimation of the Magnetic Signature of Ferromagnetic Nanoparticles by Earth’s Magnetic Field

Envisioning the next generation electrified chemical reactors heated by induction that will be able to provide feedback on the material properties online, allowing early diagnosis of potential problems, authors in this paper study the magnetic behavior of supported cobalt (catalytic) nanoparticles, with both face-centered cubic (fcc) and hexagonal close pack (hcp) crystal structure, when the Earth magnetic field is applied. The investigation and corresponding simulations have been performed with finite element analysis. The magpar software has been used, allowing simulation of the hysteresis loop for each ferromagnetic sample. The influence of the next neighbor distance and the impact of the number of the particles on the hysteresis loop are studied. The magnetizations of each cobalt-based sample, along with the hysteresis loop have been calculated by simulations and validated by experiments with satisfactory agreement.. Simulations indicate that the number of the particles (different size, under the same total mass) does not affect the hysteresis loop of the material, while the next neighbor distance, has a significant influence. The objective of the present research paper is to develop a novel, versatile, low cost, in situ method for simulating and evaluating magnetic fields generated from heterogeneous catalysts targeting to real-time remote monitoring diagnostics of the catalytic process.

Formation of Ti2AlNx MAX phase by “Hydride Cycle” and SHS methods

In the present work a new approach is proposed for Ti2AlNx MAX phase synthesis using highly efficient methods: self-propagating high-temperature synthesis (SHS) and Hydride Cycle (HC). Two ways of synthesis were developed, which differ in the use of different precursors and methods for the forming of Ti2AlNx MAX phase. The synthesis in Way 1 was carried out according to the following scheme: step 1− synthesis of a solid solution of N2 in Ti (TiN0.18) and titanium hydridonitride (TiN0.18H1.34) by SHS method. Step 2: synthesis of Ti2AlNx Max phase by HC method according to the reaction: 2TiN0.18H1.34 +Al → Ti2AlN0.25+ H2↑. The synthesis in Way 2 follows the scheme: step 1 – synthesis of Ti2Al intermetallic compound by HC method according to the reaction: 2TiH2 ​+ ​Al→ Ti2Al + H2↑. Step 2: synthesis of Ti2AlNx Max phase by SHS method according to reaction: Ti2Al ​+ ​N2 → Ti2AlN0.63. The peculiarities of structure and formation processes were studied by step-by-step analysis of the intermediate products using XRF, DTA, and chemical analysis methods. The synthesized Ti2AlN0.63 and Ti2AlN0.25 compounds were identified as single-phase MAX phases with hcp crystal structure and the same lattice parameters a = 0.29892 nm; c = 1.36540 nm, c/a = 4.57. It has been established that in both cases the formation of Ti2AlN0.25 and Ti2AlN0.67 MAX phases proceeded according to solid-phase mechanism. Taking into account all the advantages of SHS and HC methods, the developed ways for the synthesis of Ti2AlNx MAX phases can be recommended as fundamentally new, resource-saving, economical, and “low cost” technologies.

Microstructure segmentation using multi-angle polarized light microscopy

Grain size and shape are important microstructure attributes affected by material processing and contributing to material properties. We develop automated tools to segment multi-angle polarized light microscopy (PLM) images of low symmetry metals such that statistics of grain size and shape can be automatically measured. To demonstrate the tools, we apply PLM to alpha‑titanium, an hcp crystal structure metal with axisymmetric optical properties and a good candidate for the technique. The segmentation routine uses a region growing algorithm based on similarity of the intensity variance of neighboring pixels for three sample rotations, and grain dilation that checks the proximity of pixels to grain boundaries and triple points. For relatively fine-grained Ti, the segmentation is accurate for 97.5% of the grain boundaries in the microstructure when compared to EBSD results, and the accuracy is expected to improve for larger grained materials. The technique does a good job of defining twin features in deformed materials that, due to twin size and optical limitations, exhibit substantial intensity gradients making segmentation challenging. • Automated multi-angle polarized light microstructure segmentation. • Segmentation of twinned titanium microstructures. • Better than 97.5% accurate compared to EBSD. • Large microstructure area segmentation.

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.

Microstructural Development of Ti-6Al-4V Alloy via Powder Metallurgy and Laser Powder Bed Fusion

A detailed study was carried out to gain a better understanding of the microstructural differences between Ti-6Al-4V parts fabricated via the conventional powder metallurgy (PM) and the laser powder bed fusion (L-PBF) 3D printing routes. The parts were compared in terms of the constituent phases in the microstructure and their effects on the micro- and nano-hardness. In L-PBF parts, the microstructure has a single phase of martensitic α′ with hcp crystal structure and acicular laths morphology, transformed from prior parent phase β formed upon solidification of the melt pool. However, for the sintered parts via powder metallurgy, two phases of α and β are noticeable and the microstructure is composed of α grains and α + β Lamellae. The microhardness of L-PBF processed Ti-6Al-4V samples is remarkably higher than that of the PM samples but, surprisingly, the nano-hardness of the bulk martensitic phase α′ (6.3 GPa) is almost the same as α (i.e., 6.2 GPa) in PM samples. This confirms the rapid cooling of the β phase does not have any effect on the hardening of the bulk martensitic hcp α′. The high microhardness of L-PBF parts is due to the fine lath morphology of α′, with a large concentration of low angle boundaries of α′. Furthermore, it is revealed that for the α phase in PM samples, a higher level of vanadium concentration lowers the nano-hardness of the α phase. In addition, as expected, the compacting pressure and sintering temperature during the PM process led to variations in the porosity level as well as the microstructural morphology of the fabricated specimens, which will in turn have a significant effect on the mechanical properties.

Ductilization of a diffusion-bonded heterostructured AZ31/GW103K/AZ31 alloy by interfacial reinforcement

Heterostructured materials are an emerging class of materials with superior performances that are unattainable by their conventional homogeneous counterparts, and have recently attracted extensive attentions from the materials community. However, due to the hcp crystal structure, the conventional processing methods of heterostructured materials are difficult to be applied in Mg alloys. To overcome the negative effects, namely poor formability and easy oxidation, in Mg alloys, diffusion bonding under an ultra-vacuum condition was employed to prepare multi-layer heterostructured materials with different alloys, i.e. AZ31 (Mg–3Al–1Zn wt. %) and GW103 K (Mg-10Gd-3Y-0.4Zr wt. %) alloys. Moreover, the reinforcement of interface was further improved by interaction of alloying elements during post-annealing. The new formed interfacial phase was found to be the main reason for the reinforcement of interface. Without a decrease of strength, the ductility of post-annealed samples was increased to more than twice of the diffusion bonded ones. Based on high resolution TEM observation, the crystal lattice of the interfacial phase was determined as a fcc structure with a lattice parameter of a = 7.9 Å.

The effect of molybdenum (Mo) concentrations on the mechanical and magnetic properties of electrodeposited Co rich ternary CoMoW thin films from citrate electrolytic bath

Cobalt–Molybdenum-Tungsten (CoMoW) alloy thin films were prepared through an induced electroplating route from a citrate bath on the surface of the copper electrode at the controlled value of pH 8. The CoMoW thin films have been prepared by varying the Mo concentrations like 0.1, 0.2, 0.3 and 0.4 M at a deposition time of 30 minutes over a plating current potential of 40 mA / cm2. The electrodeposited CoMoW coatings have been investigated with the help of Field Emission Scanning Electron Microscopy (FESEM), powder crystal X-ray diffraction (XRD), Electrochemical studies (impedance and polarization) and Vibrating Sample Magnetometer (VSM) to reveal its respective microstructure-based information, mechanical and soft magnetic nature of the synthesized CoMoW thin layers. The CoMoW thin films of an HCP crystal structure have been attained. The induced electroplated condition such as Mo concentration has a significant impact on the crystal structure system, surface morphology, and soft magnetic performances. The crystalline size of the CoMoW thin layers has varied from 22.66 nm to 42.87 nm. The synthesized CoMoW thin layers were smooth, without cracks and had uniform morphology. All the electroplated CoMoW films have the highest Co content along with low Mo content (Co content gradually decreased while increasing the Mo content in the deposits) and thickness varied from 10 to 20 μm. Through the electrochemical investigation studies, it is concluded that the corrosion rate of CoMoW thin films was slightly increased by increasing the Mo content and the corrosion resistance varied from 80.2 KΩ to 92.7 KΩ. The CoMoW thin alloy films with higher Co content exhibited a lower coercivity value of 3.69 Oe and the saturation magnetization of 49.049 emu /cm2.

Gaseous nitriding of Co-10 at% and -15 at% Cr alloys at 400 °C and 450 °C

Co-10 at% and -15 at% Cr with the stable hexagonal close packed (hcp) crystal structure at room temperature have been subjected to gaseous nitriding treatment using nitriding temperatures of 400 °C and 450 °C far below the austenite start temperature. Using X-ray and electron backscatter diffraction (EBSD), structural changes as a result of nitrogen incorporation into the initial hcp lattice can be divided into the following steps: (i) expansion of the lattice due to N uptake yielding hcp (hcpexp) at nitriding temperature of 400 °C, (ii) gradual transformation of hcpexp into an expanded face-centered cubic phase (fccexp) by increasing time at 400 °C/ or temperature to 450 °C (mixture of two phases), (iii) development of stable nitrided layer containing fccexp using moderate nitriding times up to 3 h at 450 °C and (iv) decomposition of fccexp layer into CrN nitrides and less expanded austenite and the reverse transformation from fcc to hcp at near-surface regions at 450 °C after prolonged nitriding time of 24 h. Due to occurrence of step (iv), the square-root treatment-time dependence of the nitrided layer thickness breaks down. Nitrogen-depth profiles obtained for Co-15 at% Cr by glow-discharge optical emission spectroscopy (GDOES) along with the Non-Rutherford MeV proton backscattering spectrometry reveal a nitrogen supersaturation of around 20 at% at surface-adjacent areas in hcpexp. A relatively large unit volume expansion has been recognized for hcpexp with a change of c/a ratio of around 0.6%. Hardness-depth profiles obtained for nitrided Co-10 at% and -15 at% Cr alloys using nanoindentation evidence an increase in hardness inside the nitrided layer, compared to the untreated core.

Synthesis of Topped ZnSeO4 QDs and their Evaluation as Chemical Nanosensors for Anthracene, Benzo (A) Pyrene, Pyrene and Pyridine

Synthesis and characterization of hexamine capped ZnSeO4 QDs (ZnSeO4-Hex) by heating up method (HU) was achieved. These, of two crystallite sizes, denoted QDsS1 and QDsS2; with crystallite diameters of 8.6 nm and 14.0 nm respectively. X-ray diffraction (XRD) pattern bared hexagonal close packed (hcp) crystal structure. Band gap for QDsS1 was 5.85 eV and for QDsS2 3.8 to 4.3 eV. Hexamine (C6H10N4) cap on ZnSeO4 QDs was elucidated by Fourier Transform Infrared Spectroscopy (FTIR) and Gas Chromatography-Mass Spectroscopy (GC-MS) results. Transmission Electron Microscopy (TEM) images revealed polycrystallites of different orientation, showing crystal grains separated by tilted grain boundary folds. These QDs were tested as optical chemical nano-sensors for carcinogenic organic pollutants: Anthracene (ANTH), Benzo (a) pyrene (BaP), pyrene (PRN) and pyridine (py). Results revealed that, when the organic pollutants interacted with the QDs, they caused characteristic changes in the way these nanoparticles interacted with characteristic fluorescence and absorbance spectrum

Mechanism of stress- and thermal-induced fct → hcp → fcc crystal structure change in a TiAl-based alloy compressed at elevated temperature

The mechanism of stress- and thermal-induced fct → hcp → fcc crystal structure change in a TiAl-based alloy was investigated via experiments and molecular dynamics simulations. According to the experimental results, laths occurring in γ-phase grains were identified as γ-phase twins, and increasing strain promoted the γ → α2 (fct → hcp) phase transformation when the alloy was deformed at 1150 °C. A large number of ultrafine γ lamellae precipitated when deformed at 1200 °C, which was attributed to the thermal-mechanical coupling. The atomic mechanisms of the α2(α) → γlamella phase transformations were investigated using molecular dynamics simulations. The results showed that the generalised stacking fault energy along different directions (<1‾010>) exhibited remarkable anisotropy on the basal plane (0001) of the hcp crystal structure, regardless of the ordering or disordering phase. The disordering of the hcp crystal structure significantly affected the formation of the γ lamellae. Regardless of the ideal stoichiometric or off-stoichiometric composition, the stress-induced hcp → fcc crystal structure transformation occurred more easily in the disordered phases. Grain-boundary nucleation dominated the precipitation of the ultrafine γ lamellae. In addition, intragranular nucleation of γ lamellae was observed, which was mainly attributed to the activation and movement of the Shockley partial dislocation loop with a Burgers vector of 1/6<1‾010> whose formation was not directly related to the dissociation of the superpartial dislocation 1/6<112‾0>.

Unveiling the interplay of deformation mechanisms in a metastable high entropy alloy with tuned composition using synchrotron X-ray diffraction

The mechanical behavior of a metastable high entropy alloy (HEA) with composition Fe39-Mn20-Co20-Cr15-Si5-1Al at% is investigated. The deformation mechanisms contributing to its mechanical properties are unveiled, by performing uniaxial tensile tests in situ with synchrotron X-ray diffraction. Three distinct deformation regimes are detected. The initial elastic and early dislocation-based plasticity deformation is followed by a moderate work hardening rate regime, which is associated with the deformation-induced phase formation of ε-martensite, with hexagonal close-packed (hcp) crystal structure as well as slip in the parent austenitic phase. During the third deformation regime, the phase transformation continues to occur in combination with 2 modes of hcp twinning as well as slip in the unfavorably oriented grains for twinning. The interplay of all these co-existing deformation mechanisms can be evidenced by synchrotron X-ray diffraction.