Hcp Phase Research Articles

Development of a machine-learning interatomic potential for uranium under the moment tensor potential framework

Uranium is attracting growing interest across fundamental science and nuclear research, where atomistic simulations remain challenging. In this study, we developed a novel uranium interatomic potential based on a machine-learning approach embedded in the moment tensor potential package. An active learning framework was utilized in the potential training, using density-functional theory plus Hubbard U (DFT+U) modified data. The potential accurately reproduced the lattice parameters, cohesive energy, elastic, vibrational, and thermodynamic properties of α-U compared to DFT+U calculations and experimental results. The basic properties of other phases, including β, γ, HCP, and FCC phases, were also validated. In addition, molecular dynamics simulations were used to reproduce the temperature-induced allotropic transformation from α-U to γ-U, which agrees with experimental observations. Our potential provides a computationally efficient means to study the physical behavior of uranium with nearly DFT accuracy.

Optimizing strength-ductility of laser powder bed fusion-fabricated Ti–6Al–4V via twinning and phase transformation dominated interface engineering

Titanium alloys produced by laser powder bed fusion (LPBF) are known to possess fine microstructures and multi-scale interfaces. Here, we introduce a high density of α'/β, α/β phase interfaces, {10 1‾1} twin boundaries, and basal stacking faults (BSFs) in a Ti–6Al–4V alloy through LPBF and annealing. The LPBF-fabricated alloy consists of fine acicular α′ martensite with numerous {10 1‾1} twins and BSFs, which results in ultrahigh strength (>1300 MPa), but very low ductility (<5%) due to the massive interface strengthening. Subsequent annealing treatments decrease the strength while increasing the ductility, as the α′ martensite partially or fully decomposes into a lamellar (α+β) structure. The 955 °C-annealed alloy possesses a good strength-ductility combination (yield strength of 1000 MPa, tensile strength of 1078 MPa, and total elongation of 20%). During the stages of uniform plastic strain (up to ∼12%), deformation occurs by dislocation slips, leading to significant dislocation accumulations around the α/β interfaces. At later stages (∼20% strain), when necking occurs, a high density of fcc-γ bands is observed in the hcp-α phase, indicating the onset of deformation-induced martensitic transformation (DIMT) from hcp-α to fcc-γ. The occurrence of DIMT may be attributed to the decreased stacking fault energy and the cohesive energy difference between the hcp and fcc phases, due to the segregation of Al elements in the hcp-α phase. Low-loss electron energy loss spectroscopy reveals that the deformation-induced fcc phase is not Ti-hydride, but a new allotrope of Ti.

Influence of Al and Mo on microstructure and tribological behaviors of Co-based superalloy

In this study, three Co-based superalloys with different Al and Mo concentrations were prepared by powder metallurgy (PM) method to investigate their influences on the microstructure and tribological properties. Replacing Al with Mo in the alloy has been found to decrease its hardness and yield strength, but increases its compressive ductility at room temperature. With the increase of Mo content, the formation of the HCP phase from the original FCC solid solution phase was promoted, and the microstructure of the alloy became relatively coarse. All superalloys showed decreased friction coefficient during sliding wear at 800 °C. The alloy with the highest content of Mo and lowest content of Al exhibited the greatest wear resistance at both room temperature and 800 °C. Formation of a continuous compact glaze layer rich in Cr2O3 and CoMoO4 oxides results into a significant reduction in both friction coefficient and specific wear resistance, and the synergistic effect of high temperature wear-induced oxides is the dominant contribution rather than the transformation of HCP from the original FCC solid solution phase at high temperature.

Effects of V Addition on the Deformation Mechanism and Mechanical Properties of Non-Equiatomic CoCrNi Medium-Entropy Alloys.

Equiatomic CoCrNi medium-entropy alloys exhibit superior strength and ductility. In this work, a non-equiatomic CoCrNi alloy with low stacking fault energy was designed, and different fractions of V were added to control the stacking fault energy and lattice distortion. Mechanical properties were evaluated by tensile tests, and deformation microstructures were characterized by transmission electron microscope (TEM). The main deformation mechanisms of CoCrNiV alloy with low V content are dislocation slip, stacking faults, and deformation-induced HCP phase transformation, while the dominant deformation patterns of CoCrNiV alloy with high V contents are dislocation slip and stacking faults. The yield strength increases dramatically when the V content is high, and the strain-hardening behavior changes non-monotonically with increasing the V content. V addition increases the stacking fault energy (SFE) and lattice distortion. The lower strain-hardening rate of 6V alloy than that of 2V alloy is dominated by the SFE. The higher strain-hardening rate of 10V alloy than that of 6V alloy is dominated by the lattice distortion. The effects of V addition on the SFE, lattice distortion, and strain-hardening behavior are discussed.

Microstructural evolution and duplex structure-enhanced high-temperature mechanical properties of Al-doped VCoNi medium entropy alloy

The microstructures and mechanical properties of 7 at% Al-added VCoNi medium entropy alloy are studied and compared with those of the undoped alloy at various temperatures. Doping Al promotes the formation of a microstructure composed of the FCC matrix and the BCC-L21 phase, which remains stable until the matrix gradually transforms into the HCP-к and tetragonal-σ phases after 600 °C. Compared with the VCoNi alloy, the differences involve that the Al addition suppresses the strain-induced FCC-HCP transformation at 25 °C, and delays the eutectoid reaction at 700 °C. Corresponding to the microstructure evolution, the doped alloy preserves higher strength and strain hardening capacity until 600 °C due to the high-content harder BCC phase but comparable values at 700 and 800 °C with the relatively less HCP phase. Besides, the added Al can also reduce the susceptibility to the serration flow. The results reveal that the (VCoNi)93Al7 alloy has high potentials for intermediate-temperature applications.

Energy landscape in NiCoCr-based middle-entropy alloys

NiCoCr middle-entropy alloy is known for its exceptional strength at both low and elevated operating temperatures. Mechanical properties of NiCoCr-based alloys are affected by certain features of the energy landscape, such as the energy difference between the hcp and fcc phases (which is known to correlate with the stacking fault energy in the fcc phase) and curvature of the energy surface. We compute formation energies in the Ni-Co-Cr ternary and related quaternary systems and investigate dependencies of the relative energies on composition. Such computed composition-structure-property relations can be useful for tuning composition and designing next-generation alloys with improved strength.

Microstructure evolution and properties of Fe-Ni-Cr-Co-Mo-W high-entropy alloy coatings by plasma surface alloying technology

The purpose of this study is to develop a new type of high-entropy alloy (HEA) coating with high temperature oxidation resistance. In this paper, the Fe-Ni-Cr-Co-Mo-W HEA coatings were prepared by plasma surface alloying technology with different holding temperatures (1150 °C, 1200 °C, 1250 °C, 1300 °C). The microstructure morphology evolution and properties of Fe-Ni-Cr-Co-Mo-W HEA coatings were systematically studied. The results indicate that all the Fe-Ni-Cr-Co-Mo-W HEA coatings are composed of a deposition layer and diffusion layer, forming gradient structure HEA coating. The element gradient distribution in the diffusion layer achieves the metallurgical bonding effect. The deposition layer conforms to the definition of HEAs based on the composition or configuration entropy, and forms the surface HEAs. The phase of Fe-Ni-Cr-Co-Mo-W HEA coatings is all composed of HCP phase + FCC phase. The higher the holding temperature, the greater the supply of alloying elements are deposited by sputtering, and the higher the density of surface vacancy formation, which promotes the diffusion of atoms and the growth of grain structure. The surface morphology of the Fe-Ni-Cr-Co-Mo-W HEA coating changes from the pinecone-like large particle dense accumulation morphology to flat and compact plane, and then to fine particles compact accumulation morphology. The properties of the 1300 °C Fe-Ni-Cr-Co-Mo-W HEA coating is the best, the abrasive resistance and high temperature oxidation resistance are 3.66 and 5.5 times higher than those of the matrix respectively. The results indicate that the Fe-Ni-Cr-Co-Mo-W HEA coating with gradient structure and metallurgical bonding has a good application prospect.

Outstanding specific yield strength of a refractory high-entropy composite at an ultrahigh temperature of 2273 K

We reported on the mechanical properties and microstructural evolution of a W20Ta30Mo20C30 (at.%) refractory high-entropy composite (RHEC). The RHEC exhibited outstanding yield strength at both 2273 and 1873 K. Microstructural investigations revealed that the as-cast RHEC had a triple-phase structure consisting of FCC dendrites, HCP matrix, HCP-BCC eutectic structure, and FCC-BCC eutectoid structure, and exhibited high-density defects owing to the complex phase transformations during solidification. After annealing at 2273 K, the precipitation of the BCC phase from the FCC dendrites and the decomposition of the HCP phase into the FCC-BCC eutectoid structure was observed to significantly refine the grain sizes of all triple phases. After compression at 2273 K, the ceramic phases and solid solution precipitated out from each other, which helps to avoid persistent softening after the yielding of RHEC. Further analyses suggested that the dominant deformation mechanisms of the BCC phase and HCP phase are dislocation glide and transformation-induced plasticity; whereas those of the FCC phase are twinning- and transformation-induced plasticity. The outstanding yield strength of this RHEC at ultrahigh temperatures may originate from the high-content ceramic phases and the structural metastability of the multi-principal composition. These findings provide a novel strategy to design RHECs by alloying high-content nonmetallic elements, which contributes to further breaking through their performance limits at ultrahigh temperatures.

Investigation of microstructures and strengthening mechanisms in an N-doped Co-Cr-Mo alloy fabricated by laser powder bed fusion

ABSTRACT This study presented a comprehensive investigation into the microstructures, strengthening mechanisms and deformation behaviours of a novel N-doped Co-28Cr-6Mo (CCMN) alloy fabricated by laser powder bed fusion (LPBF). In addition to the well-known cellular structures, lattice defects including dislocations and stacking faults (SFs), the near-spherical shaped Cr2N precipitates with two typical size distributions were detected. Tensile test results revealed that the LPBF fabricated CCMN alloy demonstrated superior yield strength of 845 ± 49 MPa and elongation to fracture of 12.7 ± 1.9%. The grain boundaries (∼277 MPa), high density of dislocations (∼176–193 MPa), Cr2N precipitates (∼243 MPa) and SFs (∼131 MPa) are regarded as the dominate strengthening contributors. On the other hand, HCP phase triggered by strain induced martensite transformation (SIMT) and the Lomer-Cottrell locks (L-C locks) associated with the numerous SFs significantly enhanced the alloy strain hardening rate. More importantly, the formed Cr2N nanoprecipitates effectively suppressed the strain localisation and the premature failure along the HCP/FCC interfaces by deflecting the continuous growth of SFs, further contributing to the high ductility of the LPBF processed CCMN alloy. The present study is expected to shed light on the future development of N-doped high-performance cobalt-based alloy for the LPBF process.

A Comparison of Experimental and Ab Initio Structural Data on Fe under Extreme Conditions

Iron is the major element of the Earth’s core and the cores of Earth-like exoplanets. The crystal structure of iron, the major component of the Earth’s solid inner core (IC), is unknown under the high pressures (P) (3.3–3.6 Mbar) and temperatures (T) (5000–7000 K) and conditions of the IC and exoplanetary cores. Experimental and theoretical data on the phase diagram of iron at these extreme PT conditions are contradictory. Though some of the large-scale ab initio molecular dynamics (AIMD) simulations point to the stability of the body-centered cubic (bcc) phase, the latest experimental data are often interpreted as evidence for the stability of the hexagonal close-packed (hcp) phase. Applying large-scale AIMD, we computed the properties of iron phases at the experimental pressures and temperatures reported in the experimental papers. The use of large-scale AIMD is critical since the use of small bcc computational cells (less than approximately 1000 atoms) leads to the collapse of the bcc structure. Large-scale AIMD allowed us to compare the measured and computed coordination numbers as well as the measured and computed structural factors. This comparison, in turn, allowed us to suggest that the computed density, coordination number, and structural factors of the bcc phase are in agreement with those observed in experiments, which were previously assigned either to the liquid or hcp phase.

Deciphering the multiple deformation mechanisms responsible for sustained work hardening in a FeCrCoNi medium entropy alloy

Two important and desirable properties of materials for most structural applications are high tensile strength and ductility, which typically require high work hardening to delay necking. Here, we designed and tensile tested a face-centered cubic (fcc) Fe-Cr-Co-Ni medium-entropy alloy in which multiple deformation mechanisms are triggered during tensile loading at different temperatures to induce sustained work hardening. Our strategy involved control of the relative stabilities of the fcc, hcp (hexagonal close-packed), and bcc (body-centered cubic) phases in this quaternary system via high-throughput thermodynamic calculations. This alloy not only exhibits extensive deformation-induced nanotwinning at room temperature, but also displays a two-step sequential phase transformation [γ (fcc) → ε (hcp) martensite → α’ (bcc) martensite] at 77 K, which contrasts with the single-step phase transformation [γ → ε martensite] observed in many other fcc high/medium entropy alloys with a low stacking fault energy. The sequence of phase transformation at 77K was supported by first-principles density functional theory calculations. This work provides new templates for the design of alloys capable of multiple deformation mechanisms for sustained work hardening.

TRIP behavior of non-equiatomic Co40Cr20Ni15Fe15Mo10 high entropy alloys

Herein, the transformation-induced plasticity (TRIP) effect is investigated in a novel Co-rich high entropy alloy (HEA), Co40Cr20Ni15Fe15Mo10, by optimizing its constituent elements and processing conditions. The tensile properties of the HEA are shown to increase with strain rate, reaching a maximum tensile strength and elongation at a strain rate of 1 × 10−3 s−1, due to the formation of martensite. The X-ray diffraction patterns reveal that the as-annealed HEA has initially a single FCC structure, but transforms a dual FCC and HCP structure at the above-mentioned strain rate, with the main deformation mechanisms being dislocation slip and phase transformation. The tensile fracture surface of the Co-rich HEA shows a stepped interphase boundary between the FCC and HCP phases, thus suggesting failure along the brittle HCP structure.

Metastable Phase Formation in Electrodeposited Co-Rich Co-Cu and Co-Ni Alloys

In a previous work [El-Tahawy et al., J. Magn. Magn. Mater. 560, 169660 (2022)], we reported that from a sulfate type bath, hcp-Co can be electrodeposited at high pH and low current density and investigated the structure and magnetoresistance (MR) characteristics of such hcp-Co electrodeposits. Based on this earlier work, Co-rich Co-Cu and Co-Ni alloy electrodeposits were prepared under the same conditions by adding varying amounts of CuSO4 and NiSO4, respectively, to the CoSO4 bath. According to the results of detailed structural studies by various X-ray diffraction (XRD) geometries, in both the Co-Cu and Co-Ni systems an hcp phase formed exclusively up to about 2 at% of the alloying element. Above this concentration, a significant fcc phase fraction appeared in Co-Cu and a minor fcc fraction in Co-Ni up to about 8 at%. This means that the destabilization effect of Cu on hcp-Co is higher than that of Ni. Although the reduction of the stability of hcp-Co with increasing Cu and Ni content is a well-known phenomenon, a quantitative comparison of this effect in Co-Cu and Co-Ni alloys is missing from the literature. The measured lattice constants are analyzed in comparison with Vegard’s law for the Co-Cu and Co-Ni element pairs deduced from data previously reported for the hcp and fcc phases of all three pure elements. For Co-rich Co-Ni alloys, the concentration dependence of the lattice parameters was found to follow Vegard’s law for both the hcp and fcc phases due to the miscibility of the two components. For the Co-rich Co-Cu alloys, the data indicate a positive deviation from Vegard’s law for both the hcp and fcc phases in agreement with the known similar behavior of fcc Co-Cu alloys for the whole composition range. The positive deviation from Vegard’s law in the Co-Cu system is due to the excess mixing volume required for solid solution alloy formation of these immiscible elements in either phases. The MR data are discussed in the light of the observed phases and of the MR parameters reported in our previous work on the hcp and fcc phases of pure Co.

Thermal stability and oxidation resistance of non-equimolar ratio AlTiVCrNb refractory high-entropy alloys

In this work, we investigated the thermal stability, oxidation resistance and high temperature mechanical properties of refractory high entropy alloys Ti75Al10V5Cr5Nb5, Ti67Al12V7Cr7Nb7 and Ti55Al15V10Cr10Nb10(at.%). It is found that the second phase would be precipitated and occurs order-disorder transformation in refractory high entropy alloys at high temperature. The HCP phase produced orderly B2 phase and decreased the orderly B2 phase precipitation with the increasement of temperature in Ti67Al12V7Cr7Nb7 and Ti55Al15V10Cr10Nb10. According to the oxidation kinetics of high entropy alloys after 100h oxidation at 800 °C, the oxidation index factor n is between 1.1 and 1.2 and the degree of oxidation is between oxidized and inoxidized, all three refractory high entropy alloys formed oxides with rutile structure, the Ti67Al12V7Cr7Nb7 being less oxidation resistant due to the generation of Nb2O5 which is less stable at high temperature. Ti67Al12V7Cr7Nb7 has a yield strength of 522 MPa with 50% elongation at 650 °C. Ti75Al10V5Cr5Nb5 and Ti67Al12V7Cr7Nb7 have lower strength but 150% deformation without fracture when stretched at 750 °C. The multi-component intermetallic compound nanoparticles formed by the second phase and ordered phase together in the refractory high entropy alloys have an Orowan strengthening effect, to improve the mechanical properties at high temperature since it effectively prevents dislocation movement and stores dislocations.

Formation of core-shell-like structure in β-solidified TiAl alloy and its effect on hot workability

Understanding how phase evolves from its parent phase is critical to control the transforming microstructures and thus material performance. TiAl alloys exhibit lightweight and excellent elevated temperature properties, making them suitable for high-temperature applications. However, it is difficult to further improve the microstructure and properties of TiAl alloys through hot working, as the lamellar colonies in the original microstructure cannot be completely broken up. These remnant colonies are hard to eliminate and their presence reduces the microstructural homogeneity and properties of the alloys. Here, we propose a method for complete fragmentation of the lamellar structure in the β-solidified TiAl alloys by introducing a core-shell-like structure before hot working. The core-shell-like structure with a (α2+γ) lamellar colony in the core and an α2 matrix in the shell is obtained. The lamellar decomposition dominated by non-classical L10 (γ) to hcp (α) phase transformation is essential to ensure the formation of the core-shell-like structure, while the effects of factors such as defects and β0 grains are also further discussed in the thesis. In addition, the core-shell-like structure obtained in the β-solidified TiAl alloys before hot working can enhance the hot workability.

Carbon-Doped Nickle Via a Fast Decarbonization Route for Enhanced Hydrogen Oxidation Reaction in Alkaline Media.

Nickel (Ni) based materials with non-metal heteroatom doping are competitive substitutes for platinum group catalyst toward alkaline hydrogen oxidation reaction (HOR). However, the incorporation of non-metal atom into the lattice of conventional fcc phase Ni can easily trigger a structural phase transformation, forming hcp phase nonmetallic intermetallic compounds. Such tangle phenomenon makes it difficult to uncover the relationship between HOR catalytic activity and doping effect on fcc phase Ni. Herein, taking trace carbon doped Ni (C-Ni) nanoparticles as an example, a new nonmetal doped Ni nanoparticles synthesized by a simple fast decarbonization route using Ni3 C as precursor is presented, which provides an ideal platform to study the structure-activity relationship between alkaline HOR performance and non-metal doping effect toward fcc phase Ni. The obtained C-Ni exhibits an enhanced alkaline HOR catalytic activity compared with pure Ni, approaching to commercial Pt/C. X-ray absorption spectroscopy confirms that the trace carbon doping can modulate the electronic structure of conventional fcc phase nickel. Besides, theoretical calculations suggest that the introducing of C atoms can effectively regulate the d-band center of Ni atoms, resulting in the optimized hydrogen absorption, thereby improving the HOR activity.

Phase transformation behavior of aluminum under high hydrostatic pressure: A molecular dynamics study

The atomic mechanism of phase transformation under high pressure has been the focus of researches. In the present study, the roles of hydrostatic pressure (P) on phase transformation temperature and microstructural evolution in aluminum (Al) during cyclic thermal loading are studied from molecular dynamics simulations. The FCC → Liquid → FCC, FCC → liquid → FCC + BCC, FCC → BCC → Liquid → BCC and BCC → Liquid → BCC transformation pathways were observed under 0 ≤ P ≤ 14 GPa, 15 ≤ P ≤ 21 GPa, 22 ≤ P ≤ 25 GPa and 26 ≤ P ≤ 100 GPa, respectively, during cyclic thermal loading. It should be noted that the melting temperature predicted by the used potential at high P is obviously lower than experimental results. The FCC → BCC transformation involves nucleation and growth of BCC phase. The nucleation of BCC phase undergoes by the continuous deformation of FCC lattice, corresponding to the Bain transformation path. The growth of BCC phase is completed by the atomic diffusion. With increasing P, the 111<112̅> twin and {111} stacking fault in FCC phase evolve to five-fold twin and HCP phase, respectively. The microstructure of BCC phase is dominated by the {112}<110> twin. Insight obtained in this work paves the way for understanding the atomic mechanism of high pressure phase transformation.

A First-principles analysis of Structural, Electronic, Elastic, and vacant properties of BCC, FCC and HCP tungsten at different electronic temperatures

Since tungsten (W) was considered as the most promising plasma facing materials (PFMs) in fusion reactors, there has been extensive research on the physical performance of W-PFMs. It is found that under the extreme conditions in a fusion reactor, W-PFMs should be in a nonequilibrium state of high electronic temperature and low ionic temperature. This leads to the possibility of non-thermal phase transitions, where the crystal structure of the tungsten material may change from body-centered cubic (bcc) phase to hexagonal close-packed (hcp) phase or face-centered cubic (fcc) phase. Consequently, it is necessary to investigate the relevant physical properties of hcp-W and fcc-W under the electron-excited state. In this work, the fundamental physical properties, including atomic structures, electronic structures, elastic constants, and vacancy formation energies, of bcc-W, hcp-W and fcc-W, were theoretically calculated at various electronic temperatures. The mechanical stability of these three phases was also systematically analyzed under varying electronic temperatures. The results of this research are expected to provide a certain guidance in the optimization of W-PFMs in future fusion reactors.

A high-entropy alloy syntactic foam with exceptional cryogenic and dynamic properties

Metal foams are in great demand in extreme service environments with cryogenic temperatures and high strain rates due to their unique properties. However, the reduced ductility and impact toughness under such conditions limit their applications. In this work, we tested the quasi-static and dynamic compression properties of CoCrFeMnNi high entropy alloy syntactic foam at liquid nitrogen temperature (−196 °C). We found that it exhibits ultra-high strengths and energy absorption capacity, especially a superior resistance to embrittlement at cryogenic temperature. Microstructural characterizations reveal that the foam matrix has a high twinning activity and shear localization propensity in cryogenic environments. Dense deformation twins and multiple shear bands intersected, forming a weave-like microstructure that can disperse the deformation and benefits energy absorption. Deformation twins can also strengthen the matrix and delay the crack nucleation and growth. While under dynamic loading, an FCC to HCP phase transformation was activated, forming a nano-laminated dual-phase (NLDP) FCC/HCP structure in the matrix, leading to further strengthening and toughening. Deformation twinning and HCP phase transformation act as additional plastic deformation mechanisms along with stacking faults and shear bands to offset the shortcomings caused by limited dislocation movements at low temperatures and high strain rate dynamic loading, enabling the high strength and energy absorption of the syntactic foam.

Temperature dependence of the deformation behavior and mechanical response of CoCrNi medium-entropy alloy: Experiment and simulation

Tailoring the microstructure of metallic materials by controlling the fabrication process is crucial to achieving superior mechanical properties. Herein, the mechanism of modifying the microstructure and mechanical properties of medium-entropy CoCrNi alloys was investigated by equal-channel angular pressing (ECAP) at room temperature (RT) and cryogenic temperature (CT) and post-deformation annealing (PDA) and was analyzed by molecular dynamics simulations. It was found that temperature influences the deformation mechanism of the alloy, dislocations, stacking faults, deformation twins, and FCC-HCP phase transitions, which assume different roles at different temperatures. The dislocation density of the ECAP-CT sample is significantly higher than that of the ECAP-RT sample; however, the ECAP-RT sample exhibits unique deformation features of kink bands and hierarchical twin structure, resulting in dynamic grain refinement effects. In addition, the HCP phase strength increment caused by annealing-induced HCP phase transformation is higher in the ECAP-RT sample than in the ECAP-CT sample, thus leading to a substantial increase in strength after PDA. The present study exhibits the strengthening mechanisms and provides a generic approach to obtaining desirable mechanical properties of alloys.