Hcp Structure Research Articles

Twinning in Hexagonal Close-Packed Materials: The Role of Phase Transformation

Twinning is a major mechanism of plastic deformation in hexagonal close-packed (hcp) structures. However, a mechanistic understanding of twin nucleation and growth has yet to be established. This paper reviews the recent progress in the understanding of twinning in hcp materials—particularly the newly discovered phase transformation-mediated twinning mechanisms—in terms of crystallographical analysis, theoretical mechanics calculations, and numerical simulations. Moreover, the relationship between phase transformation-mediated twinning mechanisms and twinning dislocations are presented, forming a unified understanding of deformation twinning. Finally, this paper also reviews the recent studies on transformation twins that are formed in hcp martensite microstructures after various phase transformations, highlighting the critical role of the mechanical loading in engineering a transformation twin microstructure.

Insight into the FCC→HCP Transformation in Co-Rich Co-Cr-Fe-Mn-Ni High-Entropy Alloys

The existence of an HCP phase in FCC-type high-entropy alloys can improve the alloy’s mechanical properties. In many cases, an HCP phase is induced by deformation. In the present work, an FCC to HCP transition was detected during the cooling of Co1.5CrFeMnNi0.5 and Co1.75CrFeMnNi0.25 alloys. Therefore, arc-melted annealed CoxCrFeMnNi2−x (x = 0.25–1.75) alloys that were then subjected to long-term vacuuming were investigated using XRD, DSC, HT-XRD, thermodynamic calculation, and first-principle calculation. It was confirmed that the FCC to HCP transition occurred at ~450 °C during the cooling of the alloys with x ≥ 1.5. The volume fraction of the HCP phase increased with Co content. It was proven that the HCP phase was not stable above 600 °C. First-principle calculations further indicated that the HCP structure was more stable than the FCC structure for Co1.75CrFeMnNi0.25 alloy, and there was a likelihood of an FCC to HCP transition. Moreover, experimental tests confirmed that the microhardness of the Co1.75CrFeMnNi0.25 alloy reached 213 HV because it contained a substantial HCP phase. This value is much higher than those of other non-HCP-containing alloys, either in their as-cast states or after annealing. These results provide guidance for the design of FCC-type high-entropy alloys with desirable mechanical properties through HCP phase strengthening.

Sintering, Microstructure, and Mechanical Properties of TiTaNbZrHf High-Entropy Alloys Prepared by Cold Isostatic Pressing and Pressure-Less Sintering of Hydrides

A TiTaNbZrHf refractory high-entropy alloy (RHEA) was synthesized through a cold isostatic pressing and a pressure-less sintering process in a hydrogen atmosphere using a powder mixture of metal hydride prepared either by mechanical alloying (MA) or by rotating mixing. This study investigates how differences in powder particle sizes impact the RHEA's microstructure and mechanical properties. HCP (a = b = 3.198 Å, c = 5.061 Å) and BCC2 (a = b = c = 3.40 Å) phases were observed in the microstructure of coarse powder TiTaNbZrHf RHEAs at 1400 °C. In contrast, fine powder RHEAs were found to possess two-phase structures of HCP and BCC1 (a = b = c = 3.36 Å) with a higher hardness of 431 HV, compression strength of 1620 MPa, and a plasticity of >20%.

X-Ray Diffraction of Ramp-Compressed Silicon to 390GPa.

Silicon (Si) exhibits a rich collection of phase transitions under ambient-temperature isothermal and shock compression. This report describes insitu diffraction measurements of ramp-compressed Si between 40 and 389GPa. Angle-dispersive x-ray scattering reveals that Si assumes an hexagonal close-packed (hcp) structure between 40 and 93GPa and, at higher pressure, a face-centered cubic structure that persists to at least 389GPa, the highest pressure for which the crystal structure of Si has been investigated. The range of hcp stability extends to higher pressures and temperatures than predicted by theory.

Atomistic simulation of FCC and HCP Ni-Gd core–shell nanosystem

This research is focused on simulations of Gd, Ni, and Ni-Gd core–shell magnetic nanosystem using VAMPIRE software and the Landau-Lifshitz-Gilbert-Heun approach. The generation of temperature dependent magnetization curves allows the estimation of the corresponding Curie temperature. Simulations at various Ni and Gd particle diameters provided critical values below which the Tc value differs from the theoretical bulk Tc value, being 5 and 7 nm for Ni and Gd, respectively. It was demonstrated that both FCC and HCP structure configurations yield undistinguishable behaviors at large particle sizes. After fitting, the values of the phenomenological shift exponent and the microscopic length of Ni and Gd corresponded to spherical nanosystem. The Tc values have been tuned by varying the core diameter of Ni-Gd core–shell nanosystem. Moreover, the antiferromagnetic coupling was seen to play a significant role in the M-T curves and spin distributions of cores and shells. The hysteresis loops produced coercivity field values driven by inter-domain coupling, as well as anisotropy decreases caused by Ni% increments and rising temperatures. These magnetic properties disclosed here are promising for future biomedical and spintronic applications.

Investigation of the wear transition in CoCrMo alloys after heat treatment to produce an HCP structure

This study reports that the wear rate of the heat-treated CoCrMo alloy shows a sudden transition of more than 5-fold when the load/contact pressure increases from 45 N (3.6 GPa) to 50 N (3.7 GPa). Heat treatment was used to change the structure of the commercially available CoCrMo from the initial face-centred cubic (γ-fcc) with a small amount of hexagonal close-packed (ε-hcp) to predominantly hcp ε-phase with dispersed nanoscale precipitates of σ-phase. PeakForce QNM was used to quantitatively map the local mechanical properties of the surface at the nanoscale. High-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscope (STEM), energy-dispersive X-ray spectroscopy (EDX), and precession electron diffraction (PED) integrated with the TEM is used to characterize the structure and chemical composition of the worn surface and tribofilm. The possible reason for wear transition was discussed.

Structures of elemental potassium at terapascal pressures

We investigate the pressure-temperature phase diagram of elemental potassium (K) up to multiterapascal (TPa) pressures using ab initio random structure searching (AIRSS), discovering eight structural phase transitions beyond the already known double hexagonal close-packed (dhcp) structure. Starting at 1.15 TPa, K transitions from the dhcp structure and passes through a variety of close-packed hexagonal and trigonal phases (dhcp $\ensuremath{\rightarrow}P{6}_{3}/mmc\ensuremath{\rightarrow}P3m1\ensuremath{\rightarrow}P\overline{6}m2)$, which differ only in their stacking sequence along the $c$ axis. At 2.55 TPa, K adopts the hexagonal close-packed (hcp) structure, and at 7.21 TPa transitions into a complex orthorhombic phase of $Fddd$ symmetry with 64 atoms in its (conventional) unit cell, before assuming an $Ibam$ structure at 8.60 TPa, and, eventually, the face centered cubic (fcc) structure at 23.6 TPa, which persists well into the petapascal (PPa) regime. Further, we calculate the full pressure-temperature dependence of the melting line of K through extensive molecular dynamics simulations. We study the evolution of the bonding topology of K with pressure, finding that K passes through two quasi-molecular phases featuring diatomic pairs (the $Fddd$ and $Ibam$ structures), before ultimately becoming a high pressure electride (HPE) in the fcc phase. The electron-phonon and superconducting properties of K at these extreme pressures are also investigated, where we find the critical temperature ${T}_{c}$ rises to a maximum of between 7.65 K and 15.70 K in the hcp phase, before eventually decreasing to essentially zero in the fcc phase. Our results fully elucidate the structural and electronic behavior of K under the most extreme conditions, and provide new case studies for multi-TPa dynamic compression experiments.

Effect of Porosity and Heat Treatment on Mechanical Properties of Additive Manufactured CoCrMo Alloys.

To minimize the stress shielding effect of metallic biomaterials in mimicking bone, the body-centered cubic (bcc) unit cell-based porous CoCrMo alloys with different, designed volume porosities of 20, 40, 60, and 80% were produced via a selective laser melting (SLM) process. A heat treatment process consisting of solution annealing and aging was applied to increase the volume fraction of an ε-hexagonal close-packed (hcp) structure for better mechanical response and stability. In the present study, we investigated the impact of different, designed volume porosities on the compressive mechanical properties in as-built and heat-treated CoCrMo alloys. The elastic modulus and yield strength in both conditions were dramatically decreased with increasing designed volume porosity. The elastic modulus and yield strength of the CoCrMo alloys with a designed volume porosity of 80% exhibited the closest match to those of bone tissue. Different strengthening mechanisms were quantified to determine their contributing roles to the measured yield strength in both conditions. The experimental results of the relative elastic modulus and yield strength were compared to the analytical and simulation modeling analyses. The Gibson-Ashby theoretical model was established to predict the deformation behaviors of the lattice CoCrMo structures.

Deformation mechanism finite element analysis and die geometry optimization of magnesium alloys by equal channel angular processing

Magnesium alloys are one of the highly promising structural metals. hcp structure makes it less plastic at room temperature, which greatly limits the development of magnesium alloys. The problem of poor plastic deformation of magnesium alloy can be solved by grain refinement methods, and equal channel angle pressing is one of the more effective methods in grain refinement. In this paper, two-dimensional dynamic simulation of equal channel angle pressing of magnesium alloy is carried out by using finite element software DEFORM F2™. The deformation of magnesium alloy with different of die angles and die corner angles was analyzed. The results show that in the main deformation zone, when the die angles are different, the deformation of the workpiece in the horizontal direction is very uniform. However, in the longitudinal direction of the workpiece, the larger the die angle is, the more uniform the workpiece deformation is. The die corner angle has no significant effect on the uniformity of workpiece deformation in the longitudinal direction, but it has an effect on the dead zone area and workpiece warpage. The dead zone area and workpiece warpage increase with the increase of die angle.

Effect of aging treatment on microstructure and tensile properties of Ti–4Al–6Mo–2V–5Cr–2Zr

Microstructure of novel high-strength metastable β titanium alloy, Ti–4Al–6Mo–2V–5Cr–2Zr, is extremely sensitive to both aging temperature and time length, which directly regulates their mechanical properties. Therefore, it is particularly important to clarify the roles of aging treatment in both microstructure and mechanical properties of Ti–4Al–6Mo–2V–5Cr–2Zr. Through varying the aging temperature from 520 to 640 °C and time from 2 h to 8 h, effects of aging technological parameters on the microstructure and tensile properties of Ti–4Al–6Mo–2V–5Cr–2Zr was systematically investigated. It is noted that the secondary α phase (αs) inside the β matrix coarsens as a function of the aging temperature and time length, and the shape of αs phase changes from fine needle to short rod. Moreover, lower aging temperature and longer aging time result in a higher volume fraction of αs phase. Combined with tensile test results, it is evident that volume fraction of αs phase leads to high strength. The volume fraction of αs phase in the alloy after aging at 520 °C for 6 h reaches 61.9% along with the highest ultimate tensile strength of 1367 MPa. In addition, the strengthening mechanism of reinforced phase αs was clarified with both SEM and TEM observations. Results show that the αs with the internal lattice distortion and HCP structure effectively prevent the dislocation slip. And the αs phase arranged in a triangular shape has a profound strengthening effect.

Effect of grain boundary atomic density and temperature on symmetric tilt grain boundaries in tungsten: An atomistic study

The grain boundaries (GBs) play an important role in absorbing the point defects (interstitials and vacancies) generated by irradiation, and thus have significant effects on the resistance of materials to irradiation damage. In this work, considering the change of GB atomic density caused by absorbing point defects and irradiation temperature, 19 <110> symmetric tilt grain boundaries (STGBs) in tungsten were systematically studied with different GB atomic densities and different temperatures, based on the Molecular Dynamics simulations. The GBE difference caused by the change of GB atomic density, ΔGBE[n], are different for different STGBs. Our calculations show that the STGBs with large ΔGBE[n] are generally poor sinks for interstitials, while those with small ΔGBE[n] are generally strong sinks. The STGBs acting as strong sinks are generally those high-angle and high-GBE GBs. High temperature may lead to the formation of GB complexions containing local close-packed hexagonal (HCP) or face-centered cubic (FCC) structures at some special STGBs, which is not only related to the preferred lattice misorientations, but also associated with the symmetry of STGBs. These GB complexion transitions may reduce the GBEs of these special STGBs. Compared with GB atomic density, high temperature may significantly further promote the formation of local HCP or FCC structures at these special STGBs. Furthermore, a new GB structure analysis method based on the stacking of effective interstices was proposed to assess the absorption capacity of the 19 STGBs to interstitials. It was found that the absorption capacity of GBs to interstitials is negatively related to effective interstice density, while positively correlated with effective interstice size. This effective interstice analysis method offers a new way to assess the absorption capacity of GBs to interstitials, which is valuable in materials designing of irradiation resistance.

An ab initio study of structural phase transitions of crystalline aluminium under ultrahigh pressures based on ensemble theory

We recently put forward a direct integral approach to solve the partition function with ultrahigh efficiency and precision so that the rigorous ensemble theory can be applied to study the phase behaviors of realistic condensed-matter materials. In this work, the approach is used to investigate the structural phase transitions of crystalline aluminium up to 600GPa at room temperature and the obtained equation of state is in an excellent agreement with recent experiments within 4% deviations. For the disputed transition pressures of the FCC→HCP→BCC, our calculated results are 194 and 361GPa, which solely coincide with those measured by one of the experiments (Dewaele et al., 2018) within 2% and 0.3% differences respectively but deviate 13% and 5% from the others. In addition, our computations show that the axial ratio of the stable HCP structure is 1.62 and there exists discontinuities in the equation of state at the phase boundaries accompanied with subtle volume changes of −0.67% and −0.90%, all of which exactly support the observations from the experiment by Dewaele et al.

Synthesis of Co-Ni Alloy Particles with the Structure of a Solid Substitution Solution by Precipitation in a Supercritical Carbon Dioxide.

Mixed Co-Ni bimetallic systems with the structure of a solid substitution solution have been synthesized using the supercritical antisolvent precipitation (SAS) method, which uses supercritical CO2 as an antisolvent. The systems obtained have been characterized in detail using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Fourier-transform infrared (FTIR) spectroscopy, and magnetostatic measurements. It has been found that Co-enriched systems have a defective hexagonal close-packed (hcp) structure, which was described by a model which embedded cubic fragments of packaging into a hexagonal close-packed (hcp) structure. It has been shown that an increase in water content at the precipitation stage leads to a decrease in the size of cubic fragments and a more uniform distribution of them in Co-enriched systems. It has also been shown that mixed systems have the greatest coercivity in the line of samples. Ni-enriched bimetallic systems have a cubic close-packed (ccp) structure with modified crystal lattice parameters.

Using classical density functional theory to determine crystal-fluid surface tensions.

Classical density functional theory is used to determine the fluid-solid surface tensions for low-index faces of crystals of hard spheres and Lennard-Jones particles. The calculations make use of the recently introduced explicitly stable fundamental measure theory model for hard spheres, and we show that this gives state-of-the-art accuracy compared to simulation. For the Lennard-Jones system, results are presented for both solid-liquid and solid-vapor interfaces, and in both cases the FCC results compare favorably with existing results from the literature. We find that the BCC crystal has significantly lower solid-liquid surface tension than the FCC structure. For the solid-vapor interface, our results indicate that the BCC phase is unstable with respect to transition to the HCP structure, in agreement with various zero-temperature results in the literature.

Theory of transformation-mediated twinning.

High-density and nanosized deformation twins in face-centered cubic (fcc) materials can effectively improve the combination of strength and ductility. However, the microscopic dislocation mechanisms enabling a high twinnability remain elusive. Twinning usually occurs via continuous nucleation and gliding of twinning partial dislocations on consecutive close-packed atomic planes. Here we unveil a completely different twinning mechanism being active in metastable fcc materials. The transformation-mediated twinning (TMT) is featured by a preceding displacive transformation from the fcc phase to the hexagonal close-packed (hcp) one, followed by a second-step transformation from the hcp phase to the fcc twin. The nucleation of the intermediate hcp phase is driven by the thermodynamic instability and the negative stacking fault energy of the metastable fcc phase. The intermediate hcp structure is characterized by the easy slips of Shockley partial dislocations on the basal planes, which leads to both fcc and fcc twin platelets during deformation, creating more twin boundaries and further enhancing the prosperity of twins. The disclosed fundamental understanding of the complex dislocation mechanism of deformation twinning in metastable alloys paves the road to design novel materials with outstanding mechanical properties.

Effect of the Preparation Conditions on the Magnetic Coercivity of CoPt Alloy Nanowires

In this paper, 3 µm length and 200 nm diameter CoPt nanowire arrays (NWs) with different Co contents were prepared by electrodeposition at a controlled potential from an aqueous hexachloroplatinate solution. The synthesis occurred at two different solution pH values (2.5 and 5.5) in an electrochemical bath free of additives, as well as with saccharin as an organic additive. A complete morphological, compositional, structural and magnetic characterization of the as-prepared nanowires has been carried out. The results show that, by controlling the electrodeposition conditions, the Co content of the alloy can be tuned from 16% to 92%. The crystalline structure of the as-deposited compounds can also be controlled, with the obtained data showing that the face-centered cubic (fcc) crystalline structure changes into a hexagonal close-packed (hcp) structure when saccharin is used as an organic additive during the electrodeposition. The changes in the alloy’s composition and crystalline structure strongly influence the magnetic properties of the NW’s arrays.

Molecular dynamics simulation of phase transition and crack propagation in metastable high entropy alloy

Crack propagation mechanisms and phase transformation play a crucial role in improving the formability of metastable high-entropy alloys (HEA). In this study, a multi-component single-crystal Ta 0.5 HfZrTi metastable high-entropy alloy with a single-sided corner crack is investigated during the stretching process by Molecular dynamics. Based on the stress state during stretching, crack propagation is divided into two groups, phase transition at the crack tip and crack growth. We are investigating the phase transformation behavior of metastable high-entropy alloys under different stress states by tensile and shear deformation of defect-free single crystals. This paper studies the crack propagation process by creating cracks of different angular sizes on the single side of a single crystal. The results explain the atomic motion pattern of the phase transformation in metastable high-entropy alloys from BCC to HCP structure and the reasons for creating FCC structures during the phase transformation. Tensile simulations of single crystals with single-sided cracks explain the relationship between crack propagation and phase transition.

Variation in gold monolayer properties on interaction with DNA/RNA nucleobases useful for DNA sensing

Interfacing DNA/RNA nucleobases with gold nanoparticles (AuNPs) has been thoroughly investigated for several biomedical and diagnostic applications because of its numerous advantages. In this work, the interaction of DNA/RNA nucleobases with recently reported pristine gold monolayer in HCP structure, has been explored by utilizing the Ab-initio calculations. The binding characteristics between the two entities, the charge transfer, and the optical and electronic properties of the nucleobase–gold monolayer system, show distinct results for each nucleobase. The binding energies of the nucleobase-Au monolayer system vary as: G > A ∼ C > T ∼ U. The density of states of the nucleobase-gold monolayer system clearly reveals the modulation in electronic properties owing its origin to the presence of nucleobase atoms. The changes in optical properties (plasmon frequencies, reflection cofficient and refractive index), and tunneling current characteristic of gold monolayer are observed due to gold-nucleobase interactions. On adsorption of nucleobase on gold monolayer, additional peaks appear in the EELS spectra and imaginary part of dielectric function and the values of refractive index, static dielectric constant and reflectance increase. We observe higher tunnel current and enhanced negative resistance region for nucleobase adsorbed systems. The results indicate that pristine Au-monolayer in HCP structure is a good candidate for DNA/RNA sequencing devices. Results from our current study are helpful in understanding experimental findings and may be beneficial for advancements in biosensing technology.

Phase transformation path in Aluminum under ramp compression; simulation and experimental study

We present a framework based on non-equilibrium molecular dynamics (NEMD) to reproduce the phase transformation event of Aluminum under ramp compression loading. The simulated stress-density response, virtual x-ray diffraction patterns, and structure analysis are compared against the previously observed experimental laser-driven ramp compression in-situ x-ray diffraction data. The NEMD simulations show the solid–solid phase transitions are consistent to experimental observations with a close-packed face-centered cubic (fcc) (111), hexagonal close-packed (hcp) structure (002), and body-centered cubic bcc (110) planes remaining parallel. The atomic-level analysis of NEMD simulations identifiy the exact phase transformation pathway happening via Bain transformation while the previous in situ x-ray diffraction data did not provide sufficient information for deducing the exact phase transformation path.

Investigation of the Microstructures and Properties of B-Bearing High-Speed Alloy Steel

This work aims to research the influence of boron and quenching temperature on the microstructures and performances of boron-bearing high-speed alloy steel. The results showed that the hardness and wear resistance of boron-bearing high-speed alloy steel were improved after increasing the boron content. The volume fraction of boron-rich carbide gradually decreased, and the hardness increased significantly with the rise in quenching temperature. The highest comprehensive mechanical properties were obtained for samples quenched at 1040 °C. The TEM results showed the boron-rich carbide was M7(C, B)3 with an HCP structure, and the precipitated particles were M23(C, B)6 with an FCC structure after tempering. This work may help improve the wear resistance of materials in the field of surface coatings.