Hexagonal Close-packed Phase Research Articles

Molecular dynamic simulations of the martensitic transformation for the dual-phase structure and dislocation activities in Ti80 alloys

Titanium alloy (Ti alloy) has been widely used in various industry areas and attracted extensive attentions due to their excellent properties. In this work, the martensitic transformation (MT) and complex dislocation activities are thoroughly simulated and analyzed for the commercial Ti80 alloy in molecular dynamic (MD) simulations. One specific interatomic potential is developed for Ti80 alloy within the framework of embedded atom method (EAM), which is capable of describing the related physical properties and the plastic deformation mechanism for this alloy. The stabilities of hexagonal close-packed (hcp) and body-centered cubic (bcc) phase boundary and the martensitic transformation mechanism is fully investigated in MD simulations. The orientation relationship between hcp and bcc phases obeys the Burgers relationship. Complex dislocation activities are reproduced in the simulation of the compression of the polycrystalline configuration, including the formation of dislocation wall, (112‾2) compression twin boundaries and dislocation cross slip mechanisms. The MD simulation results are well agreement with our experimental observations and provides an in-depth understanding of the deformation mechanisms of the Ti80 alloy and related Ti alloys. The revealed deformation mechanism and the newly-developed interatomic potential should be a useful guide to the development of the new series of high-performance Ti alloys in the future.

Pressure-driven structural transition in CoNi-based multi-principal element alloys

Pressure-driven phase transition in metals has been a hot topic because it is an effective means to induce fresh phase, benefit of tuning the properties of materials. Herein, CoNiFe, CoNiCr, and CoNiV multi-principal element alloys (MPEAs) were investigated by an in situ high-pressure x-ray diffraction technique. It is found that the pressure-induced phase transition from face-centered cubic to hexagonal close-packed phase occurs at 15.60, 13.84, and 8.20 GPa, respectively. The atomic size misfit of CoNiFe, CoNiCr, and CoNiV MPEAs is estimated to be 0.653%, 2.077%, and 3.013%, respectively, illustrating that the lattice distortion degree is increasing. The increase in lattice distortion can decrease the initial phase-transition-pressure because lattice distortion could reduce the strain to nucleate Shockley partial dislocation, which promotes the formation of a stacking fault (SF) stack of three atomic layers with hcp stacking. However, the quantitative calculation of stacking fault probability α as a function of pressure demonstrates that the probability of SF formation gradually increases in order of CoNiFe, CoNiCr, and CoNiV, which is in line with the critical pressure of phase transition decreasing orderly. Furthermore, the first peak in the pair distribution function curve after entirely decompression not fully reverts to its initial state, proving the densification of MPEAs under pressure. These findings provide an innovative light for understanding pressure-induced phase transitions in MPEAs.

Microstructural study of keyhole TIG welded nickel-based superalloy G27

The weld fusion zone (FZ) microstructure obtained after keyhole tungsten inert gas welding and post-weld solution heat treatments (PWSHTs) of a new nickel (Ni)-based superalloy called G27 is studied, and the grain growth behavior in the base material (BM) during PWSHTs is characterized. Microsegregation-induced interdendritic microconstituents in the FZ of as-welded G27 are identified by analytical (scanning) transmission electron microscopy ((S)TEM) as niobium (Nb)-rich MC carbides, Nb-rich Laves eutectic constituents, γ’ and η phases. The Laves eutectics are generally considered brittle and can adversely affect the mechanical properties of the weldment; thus, an hour PWSHTs were performed at 954 °C–1060 °C to eliminate the γ/Laves eutectics. PWSHT up to 1010 °C results in only partial removal of Laves eutectics with an excessive formation of η phase surrounding the Laves phase. Complete dissolution of Laves eutectics with no η phase formation is achieved after a PWSHT is performed at 1060 °C. Relative to INCONEL® alloy 718, the complete elimination of the γ/Laves eutectic constituents in the FZ of G27 through a PWSHT was proven to be achieved without causing excessive grain growth in the BM, which could be due to the pinning effect of the fine molybdenum (Mo)-rich precipitates, that are formed during solution heat treatment and identified as hexagonal close-packed phase particles through extensive (S)TEM analyses.

Impact response of metastable dual-phase high-entropy alloy Cr10Mn30Fe50Co10

Plate impact experiments are conducted on a metastable dual-phase high-entropy alloy (HEA) Cr10Mn30Fe50Co10 with different matrix grain sizes and phase fractions. Free surface velocity histories are obtained along with microstructure characterizations. Postmortem samples are characterized with transmission electron microscopy and electron backscatter diffraction. Multiple deformation mechanisms are observed, including dislocation slip and stacking fault in the face-center cubic (FCC) phase, dislocation slip and deformation twinning in the hexagonal close-packed (HCP) phase, and the FCC to HCP transformation. For spallation, damage is ductile in nature. Despite different microstructures, micro-voids prefer to nucleate at high-angle grain boundaries within the FCC phase, leading to similar spall strength. Upon shock loading, a dynamic plastic deformation partitioning behavior among the two phases is observed; at high shock stress, denser dislocations in the HCP phase result in increased fractions of voids around the FCC/HCP phase interface and within the HCP phase during spallation damage.

High-coercivity SmFe12-based anisotropic sintered magnets by Cu addition

We achieved a record-high coercivity of 1.40 T in a SmFe12-based ThMn12-type anisotropic sintered magnet. This magnet was prepared by sintering jet-milled powder made from strip-cast alloys with a nominal composition of Sm8Fe73.5Ti8V8Al2Cu0.5, using the conventional process for anisotropic sintered magnet production. Detailed microstructural investigations conducted using scanning electron microscopy and transmission electron microscopy revealed that the formation of Fe-lean intergranular phases (with < 20 at.% of Fe) surrounding the fine SmFe12 phase grains resulted in the high coercivity. Nanobeam electron diffraction patterns showed that these Fe-lean intergranular phases (IGPs) exhibiting excellent wettability in this magnet are mainly composed of two phases: a hexagonal close-packed Mg-type Sm-based phase and a primitive cubic CsCl-type SmCu-based phase, which co-exist in the same intergranular region. Such Fe-lean IGPs minimize the intergranular exchange coupling in the SmFe12 main phase, which contributes to the high coercivity, as revealed by magneto-optical Kerr effect microscopy. Overall, this study provides detailed microstructure-coercivity correlations and insights for the future development of high-performance SmFe12-based sintered magnets.

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.

Toward a Quantitative Understanding of Crystal-Phase Engineering of Ru Nanocrystals

The crystal phase with a specific stacking sequence of atoms largely affects the catalytic performance of metal nanocrystals. Since the control of the phase at the same composition is extremely difficult, the phase-dependent performance of metal nanocrystals is studied rarely. Here, we show the synthesis of Ru nanocrystals with different percentages of face-centered cubic (FCC) and hexagonal close-packed (HCP) phases via kinetic control, further revealing a quantitative correlation between the phase percentage of Ru nanocrystals and the initial reduction rate of Ru(III) precursors. Specifically, we manipulate the single parameter-initial reduction rate by controlling the Ru(III) injection rate into the dropwise synthesis at a fixed reaction temperature and correlate the kinetic data with the Ru phase percentage analyzed by atomic-resolution electron microscopy and synchrotron X-ray scattering. Based on the quantitative analysis, the ranges of initial reduction rates of Ru precursors can be determined for synthesizing Ru nanocrystals with the percentages of unusual FCC phase from 9.0 to 55.1%. We demonstrate that a low initial reduction rate corresponds to the crystallization of the Ru HCP phase, while a high initial reduction rate favors the crystallization of the FCC lattice. Furthermore, we also systematically examine the catalytic performance of Ru nanocrystals with different phases.

Tuning the microstructure and strengthening mechanism of the CoCrFeNi high-entropy alloy via doping metalloid and interstitial elements

The synergistic strengthening and toughening of metalloid and interstitial elements were demonstrated for the first time in the CoCrFeNi high-entropy alloy (HEA) by the sequential addition of Si, B, and N elements. The addition of Si simultaneously enhances the strength and ductility of the CoCrFeNi HEA, further, the doping of B promotes the formation of Cr2B precipitates and the grain refinement, and the addition of N further refines the grain size and increases the lattice distortion and the degree of chemical short-range order (SRO). The strengthening effect to yield strength of the addition of Si, B, and N elements is well described by the strengthening model. Doping Si, B, and N elements can regulate the stacking fault energy (SFE), lattice distortion, and short-range order of the alloy, and consequently induce the multiple twins and nanoscale transformation from the face-centered cubic (FCC) to the hexagonal close-packed (HCP) phase upon room-temperature tension, which enhances the work-hardening capability of the alloy. Finally, a dual heterostructure of grain size and precipitate phase distribution is formed in CCFN-Si-B-N-700 HEA by regulating heat treatment. The yield strength of the current alloy is further enhanced up to 1.2 GPa, and the ductility is maintained at 13 %. This study not only provides new insights into the strengthening and toughening mechanism of metalloid and interstitial elements but also a new paradigm for the development of HEAs with excellent strength-ductility synergy.

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.

Characterization of a novel AlLiMgTiZr-type light high entropy alloy

Al11Li10Mg23Ti21Zr35 was designed as a novel Light High Entropy Alloy (LHEA) considering terms and parameters of respective phase formation rules. The alloy was vacuum melted utilizing high-purity elements, then suction cast and homogenized. The experimentally measured density of the ingot was 4.43 g/cm3 compared to the theoretical density of 4.02 g/cm3. Chemical composition, microstructural properties, phase structure, compressive mechanical properties, and microhardness were characterized. Approximate chemical homogeneity was observed in the microstructure and the Hexagonal Close-Packed (HCP) solid solution phase with c/a = 1.544 and AlZr2 intermetallic compounds were identified. High specific hardness (230 Vickers. cm3/g) was achieved satisfying among all LHEAs introduced up to now. Different hardness values were measured at various points of the microstructure connected to the presence of AlZr2 intermetallic. The lattice distortion of alloys' crystal structures was analyzed accurately, and the distorted solid solution structure consists of elements with different atomic radii, proposed as the dominant mechanism (contributed more than 80%) among grain boundary, dislocation strengthening and solid solution strengthening mechanisms. The alloy's ductility was fair due to the HCP lattice and intermetallic compounds.

Microstructure and tensile properties of metastable Fe50Mn30Co10Cr10 high-entropy alloy prepared via powder metallurgy

With the characterization of microstructure and tensile properties, the feasibility of preparing Fe50Mn30Co10Cr10 high-entropy alloys (HEAs) via powder metallurgy was explored, and the effect of sintering temperature and deformation temperature on the microstructure and tensile properties of the alloy was investigated. The alloys sintered at different temperatures possessed a dual-phase structure with face-centered cubic (FCC) and hexagonal close-packed (HCP). The average grain size, HCP phase fraction, and twinning boundary fraction increased with the sintering temperature. The best combination of strength and ductility was obtained in the alloy sintered at 1000 °C, which exhibited a strongly temperature-dependent mechanical behavior, i.e., when the deformation temperature decreased from 298 to 77 K, the yield strength and ultimate tensile strength increased from ∼287 to ∼490 MPa and from ∼745 to ∼1107 MPa, respectively, with only a tiny loss of uniform elongation; this is mainly attributed to the deformation mode of the alloy varied with deformation temperature. During tensile deformation at 298 K, dislocation slip, phase transformation, detwinning of annealing twins, and mechanical twinning were the dominant mechanisms, while no mechanical twinning was activated at 77 K. The excellent combination of strength and ductility for the alloy at 77 K relative to 298 K is mainly attributed to a more significant TRIP effect. Powder metallurgy can be used as a promising way for manufacturing metastable high entropy alloys with excellent tensile properties.

The effect of annealing on mechanical properties of selective laser melted CoCrNi medium-entropy alloy

The mechanical properties were evaluated in the selective laser melted CoCrNi medium-entropy alloy after the annealing at 800 °C. The as-selective laser melted sample delivered the yield strength of 820 MPa, the ultimate tensile strength of 1090 MPa and the fracture strain of 34%, While corresponding value for annealed sample at 800 °C for 4 h was 630 MPa, 870 MPa and 42%, respectively. High density dislocation induced high strength of as-selective laser melted sample, and higher ratio of the low-angle grain boundaries in sample annealed at 800 °C for 1 h impeded the dislocation movement and improved the strength. Moreover, high density dislocation and phase transformation from a face-centered cubic phase to a hexagonal close-packed phase in better recrystallization sample annealed at 800 °C for 4 h synergistically induced strength-ductility trade-off.

Effects of rare earth elements on AlCoCrFeNi2.1 eutectic high-entropy alloy

In order to explore the effect of rare earth elements (REEs) on the eutectic high-entropy alloys (EHEAs), 0.2 atomic per cent of Sc, Y, and La elements were added to the AlCoCrFeNi2.1(REE0) EHEA in this work. The phase composition, element distribution, electrochemical properties and mechanical properties of the alloys were analysed. The results showed that the AlCoCrFeNi2.1-based HEAs are still composed of Co, Cr, Fe (face-centred cubic)-rich phase and Al, Ni (face-centred cubic)-rich phase. After Sc, Y, La were added, Ni2Sc, Al2Y, Al2La and other intermetallics were formed in the AlCoCrFeNi2.1 alloy. In addition, it was proved that Y element can promote the formation of hexagonal close-packed (HCP) phase. AlCoCrFeNi2.1Y0.2(Y0.2) alloy has excellent corrosion resistance. AlCoCrFeNi2.1Sc0.2(Sc0.2) alloy has the best mechanical properties. HIGHLIGHTS With the addition of rare earth elements, the face-centred cubic phase and the B2 phase still exist in the alloy. In addition, there are intermetallics in the alloys. Combined with first-principles calculations, it was determined that the Laves phases in AlCoCrFeNi2.1Y0.2 alloy may be Al2Y, Al3Y or AlY. The Laves phase in the AlCoCrFeNi2.1La0.2 alloy may be Al2La, Al3La or LaNi5. The addition of Y element can promote the formation of HCP phase in AlCoCrFeNi2.1 alloy. The generated HCP phase occupies the position of the original B2 phase. Y element can also improve corrosion resistance. The Vickers hardness of AlCoCrFeNi2.1Sc0.2 alloy is 20% higher than that of AlCoCrFeNi2.1 alloy, while the compressive strength of 1897 MPa and the compressive deformation of 26% are retained.

Effect of strain rate on mechanical properties of HCP/FCC dual-phase CoCrFeNiNb0.5 high-entropy alloy

By using split-Hopkinson pressure bar, scanning electron microscope and transmission electron microscope, the influence of strain rate on mechanical behavior of the as-cast CoCrFeNiNb0.5 high-entropy alloy (HEA) consisting of hexagonal close-packed (HCP) pro-eutectic phase and eutectic structure (HCP laminate and face-centered cubic (FCC) laminate) was investigated. As the strain rate increases from 1×10−4 to 6×103 s−1, it is found that yield strength does not increase evidently and that fracture strain drops drastically. Quasi-static deformation mechanism of HCP laminate is the multiplication of tangled dislocation. However, the dynamic deformation mechanism of HCP laminate is shearing. Shearing may lead to the formation of the micro-crack at low compression strain of 0.1. Since the number of the micro-crack increases rapidly with the increase of dynamic compression strain, the avalanche fracture of HEA, which is caused by the covalence of micro-crack evolved from the shearing of HCP laminate, occurred, contributing to the dynamic compressive embrittlement.

Effect of heat treatment regime on microstructure and phase evolution of AlMo0.5NbTa0.5TiZr refractory high entropy alloy

This paper reports the effect of heat treatment process on the ensuing microstructure evolution, and hardness variation of a cast and homogenized AlMo0.5NbTa0.5TiZr refractory high-entropy alloy (RHEA). Heat treatments were carried out on the homogenized samples at 1000, 1100, and 1200 °C for 10, 24, and 48 h. It was revealed that the phase decomposition in the alloy took place in a much shorter time than that reported in the literature. The significant tendency of Zr and Ti in respect of separation from the solid solution was found as the main reason for the formation of multi-phase structure and causing thermal instability. The phase decomposition occurred though the formation of Zr-rich needle-like phase constituents and Ti-rich globular ones. The semi-stable Zr-rich phase constituents are believed to form on B2 lamellas, and during their growth, the crystal structure gradually transforms from body-centered cubic (BCC) to a hexagonal close-packed (HCP) configuration. The electron back-scattered diffraction (EBSD) analyses confirmed that the HCP phase fraction increased significantly with increase in annealing temperature and/or time. A remarkable increase of 78% in Vickers hardness was measured for the sample heat treated at 1100 °C for 24 h compared to that of the homogenized state. It turned out that the morphology, size, interface coherency, and volume fraction of the needle-like precipitates played the leading role in influencing the alloy’s hardness. A comprehensive mechanism has been proposed for the phase decomposition of the alloy, based on the microstructure and chemical composition variations.

Revealing the Crystal Phase-Activity Relationship on NiRu Alloy Nanoparticles Encapsulated in N-Doped Carbon towards Efficient Hydrogen Evolution Reaction.

Adjusting the crystal phase of a metal alloy is an important method to optimize catalytic performance. However, detailed understanding about the phase-property relationship for the hydrogen evolution reaction (HER) is still limited. In this work, the crystal phase-activity relationship of NiRu nanoparticles is studied employing N-doped carbon shell coated NiRu nanoparticles with different phase contents. It is found that the NiRu@NC (mix) with both face-centred cubic (fcc) and thermodynamically unstable hexagonal close-packed (hcp) phase NiRu give the best HER performance. Further activity studies demonstrate that hcp NiRu has better HER performance, and NiRu@NC (mix) with rich (∼70 %) hcp phase presented outstanding performance with an overpotential of only 27 mV @ 10 mA ⋅ cm-2 . The high HER activity of NiRu@NC (mix) is attributed to the formation of hcp phase. This finding indicates that the regulation of crystal structure can provide a new strategy for optimizing HER activity.

Microstructural design and deformation behavior of a TRIP/TWIP tri-phase heterogeneous high-entropy alloy

In this study, a tri-phase heterogeneous high-entropy alloy (HEA) (Al9(Fe50Mn30Co10Cr10)91, at. %) exhibiting transformation- and twinning-induced plasticity (TRIP-TWIP) that overcomes the strength-ductility trade-off. The current HEA was systematically investigated the microstructure and mechanical properties under different thermo-mechanical processing (TMP). The HEA had a hierarchical microstructure of face-centered cubic (FCC), body-centered cubic (BCC), and a small content of hexagonal close-packed (HCP) phases after TMP. As regards the mechanical properties, after introducing recrystallized zone of ∼14.5 vol%, the HEA showed a strength of 1.15 GPa with total elongation of 10%. When the recrystallized fraction increased to ∼69.9 vol%, the HEA exhibited a combination of yield strength ∼489 MPa and tensile strength ∼704 MPa, while a satisfactory uniform elongation of 37% was maintained. The deformation activated multiple strengthening mechanisms, including dislocation slip, stacking faults, TRIP-TWIP effects, etc. led to a significant increase of work hardening rate in the intermediate stage of the strain. Additionally, the phase distribution and recrystallized grains morphology were characterized and the relationship between their results and the strength-ductility synergy were discussed. These findings provide a new method to tune the mechanical behavior of HEAs.

Grain Refinement and Phase Transformation of Pure Titanium Foils During Asymmetrical Rolling

Ultrafine‐grained microstructure and face center cubic (FCC) phase are produced in the pure titanium foils through asymmetrical rolling (ASR) to explore the mechanical properties of pure titanium foils with different thicknesses. Herein, the grain refinement process and phase transformation behavior are systematically investigated. The grain refinement process can be described as: 1) coarse equiaxed grains are elongated along RD due to the compressive strain and shear strain caused by the ASR process. 2) elongated grains are split into thin laths and subgrains by dislocation walls under greater strain. 3) thin laths and subgrains are further refined and ultrafine‐grained microstructures are obtained. In addition, hexagonal close‐packed (HCP)‐FCC phase transformation is also discovered during the ASR process. The orientation relationship between the HCP and FCC phases is determined as [0001]HCP//[001]FCC, (100)HCP//(20)FCC and the phase transformation is caused by Shockley partial dislocation with Burgers vector 1/6[110] gliding on each (100) plane. Uniaxial tensile tests on heat‐treated pure titanium foils of different thicknesses show a size effect on tensile properties. As the thickness decreases, tensile strength and elongation are reduced, with fewer/single‐layer equiaxed grains at the section of the specimen.

Phase stability and possible superconductivity of new 4d and 5d transition metal high-entropy alloys

We report the synthesis of new equimolar high-entropy alloys formed from five or six 4d/5d transition metals that are each from a different Group of the Periodic Table. These include MoReRuRhPt and MoReRuIrPt, which appear to crystallise as single hexagonal close-packed phases and so are isostructural and isoelectronic to the high-pressure phase of the well-known high-entropy alloy CrMnFeCoNi. Other compositions crystallise as mixed phases, which we rationalise using the atomic size difference, δ, and the electronegativity difference, Δχ, of the constituent elements. Finally, we find signs of superconductivity in eight of these new systems.

Facile manipulation of mechanical properties of Ti-6Al-4V through composition tailoring in laser powder bed fusion

Ti-6Al-4 V alloy when processed by laser powder bed fusion (LPBF) is a useful material which can be used for the manufacture of complex 3D components for aerospace and medical applications. LPBF fabricated Ti-6Al-4 V typically shows high tensile strength (>1200 MPa) but poor ductility (<10%), explained by the characteristic microstructures that form under high cooling rates and multiple thermal cycles. Here, the composition of the feedstock Ti-6Al-4 V powder was modified via the addition of commercially pure Ti (CP Ti) to achieve improved ductility of LPBF parts through an in-situ modified composition, while maintaining good strength. The LPBF printed Ti alloy (with nominal composition Ti-3Al-2 V) showed a tensile strength of ∼1000 MPa, with improved ductility (∼13%), comparable to wrought Ti-6Al-4 V. These properties are thought to be explained by the decreased c/a ratio (where c and a are the lattice parameters of the titanium hexagonal close-packed phase) resulting in enhanced dislocation slip. Good relative density was also achieved using the modified composition under the same processing parameters. Some non-uniform regions were seen in the developed alloys, as well as evidence of increased martensite α′ thickness. It is proposed that with optimized parameters, further enhancements may be achieved. This work has shown that microstructural and mechanical properties can be facilely manipulated with proper CP Ti addition.