Hcp Metals Research Articles

EAM potentials for BCC, FCC and HCP metals with farther neighbor atoms

The modified analytic embedded atom method (EAM) theory considering farther neighbor atoms is improved. We not only adopt an end processing function and an enhanced smooth continuous condition of the pair potential, but also adjust model parameters of multi-body potential by fitting cohesion energies, mono-vacancy formation energies, Rose equation curves, energy differences between several structures, elastic parameters and the equilibrium conditions of crystals. The calculation results of structure energy differences misfit the experiment data for several metallic materials in the unimproved EAM, because its model parameters have not considered these differences. After the modification, the coincidence degree of the structure energy—the reduced distance r/r 1 from the present model with that from the Rose equation is greatly improved for Cr. The calculated results of structure stabilities are in good agreement with experiment data and other calculated results for all considered metals, significantly better than the unimproved model with farther neighbor atoms.

Estimation of surface energies of hexagonal close packed metals using computational intelligence technique

Surface phenomena such as corrosion, crystal growth, catalysis, adsorption and oxidation cannot be adequately comprehended without the full knowledge of surface energy of the concerned material. Despite these significances of surface energy, they are difficult to obtain experimentally and the few available ones are subjected to certain degree of inaccuracies due to extrapolation of surface tension to 0K. In order to cater for these difficulties, we have developed a model using computational intelligence technique on the platform of support vector regression (SVR) to establish a database of surface energies of hexagonal close packed metals (HCP). The SVR based-model was developed through training and testing SVR using fourteen experimental data of periodic metals. The developed model shows accuracy of 99.08% and 100% during training and testing phase, respectively, using test-set cross validation technique. The developed model was further used to obtain surface energies of HCP metals. The surface energies obtained from SVR-based model are closer to the experimental values than the results of the well-known existing theoretical models. The outstanding performance of this developed model in estimating surface energies of HCP metals with high degree of accuracy, in the presence of few experimental data, is a great achievement in the field of surface science because of its potential to circumvent experimental difficulties in determining surface energies of materials.

Shock wave compression and release of hexagonal-close-packed metal single crystals: Inelastic deformation of c-axis magnesium

To understand inelastic deformation mechanisms for shocked hexagonal-close-packed (hcp) metals, shock compression and release wave profiles, previously unavailable for hcp single crystals, were measured for c-axis magnesium crystals. The results show that the elastic-inelastic loading response is strongly time-dependent. Measured release wave profiles showed distinct peaked features, which are unusual for inelastic deformation during unloading of shocked metals. Numerical simulations show that pyramidal slip provides a reasonably good description of the inelastic loading response. However, {101¯2} twinning is needed to explain the unloading response. The results and analysis presented here provide insight into the relative roles of dislocation slip and deformation twinning in the response of shocked hcp metals.

Grain Breakup During Elevated Temperature Deformation of an HCP Metal

AbstractA combination of mechanical testing, EBSD and crystal plasticity finite element modeling were used to investigate the influence of temperature on the fragmentation of grains in a zirconium alloy. The results demonstrate that grains of Zircaloy-4 fragment more as the temperature rises. This trend can be explained by an increasing difference between the CRSS values for 〈c+a〉 slip and 〈a〉 slip as temperature rises. This change in relative slip activities with temperature is supported by experimental observations of macroscopic anisotropy and in-grain misorientation axes calculated from EBSD data, as well as plasticity modeling. By tracking the microstructural evolution during deformation, it is shown that the two major texture components fragment to different degrees under the action of prismatic slip. Grains in the$$ \left\langle {11\overline{2} 0} \right\rangle $$112¯0fiber are significantly more stable than those in the$$ \left\langle {10\overline{1} 0} \right\rangle $$101¯0fiber, which break up. Grains of the latter fiber fragment heterogeneously as portions of the grain rotate in opposite directions, and some do not rotate at all.

De-twinning and Texture Change in an Extruded AM30 Magnesium Alloy during Compression along Normal Direction

Twinning and de-twinning are the salient deformation mechanisms in hexagonal close-packed (hcp) metals. The aim of this study was to examine and quantify the de-twinning process involving a reversible motion of twin boundaries in an extruded AM30 magnesium alloy after re-compression along the normal direction (ND) of pre-compressed samples along the extrusion direction (ED). {10 1 ¯ 2} extension twins were first introduced at a compressive strain of 3.7% along the ED. The subsequent compressive deformation along the ND induced a gradual shrinkage of twins with increasing cumulative true strain, and the complete de-twinning occurred at a strain of ∼7.7%. The twin width decreased linearly with increasing true strain. Texture measurements verified the rotation of c-axes of hcp unit cells towards the anti-compression direction due to {10 1 ¯ 2} extension twinning after compression along the ED, and a gradual return of c-axes to the initial orientation due to twin shrinking or de-twinning during the following compression along the ND. The {10 1 ¯ 2} twinning corresponded to the formation of new texture components C{ 1 ¯ 2 1 ¯ 0} and D{01 1 ¯ 0} and a decrease in the initial texture components A{0001} 1 ¯ 1 ¯ 0> and B{0001} 1 ¯ 0>, while the twin shrinking or de-twinning was characterized by a gradual vanishing of components C{ 1 ¯ 2 1 ¯ 0} and D{01 1 ¯ 0} and an increase in the components A{0001} 1 ¯ 1 ¯ 0> and B{0001} 1 ¯ 0>.

Modeling of Stacking Fault Energy in Hexagonal-Close-Packed Metals

The deformation of metals is known to be largely affected by their stacking fault energies (SFEs). In the review, we examine the theoretical background of three normally used models, supercell model, Ising model, and bond orientation model, for the calculation of SFE of hexagonal-close-packed (hcp) metals and their alloys. To predict the nature of slip in nanocrystalline metals, we further review the generalized stacking fault (GSF) energy curves in hcp metals and alloys. We conclude by discussing the outstanding challenges in the modeling of SFE and GSF energy for studying the mechanical properties of metals.

OS0119-314 純粋せん断試験によるhcp金属のすべり系のCRSSの評価

o estimate CRSSs of slip systems in hcp crystals, shear tests of magnesium single crystals were carried out at room temperature. In the shear test, basal slip occurred within shear deformation area, and CRSS of the basal slip was estimated as 0.35MPa. However, specimens for prismatic slip deformed with {101^^-2} twin. The test result shows that CRSS of prismatic slip in magnesium will be more than 90MPa. To estimate CRSS of the prismatic slip precisely, alternate specimens for shear test should be necessary.

Atomistic modeling to optimize composition and characterize structure of Ni-Zr-Mo metallic glasses.

An interatomic potential was constructed for the Ni-Zr-Mo ternary metal system with the newly proposed long-range empirical formulism, which has been verified to be applicable for fcc, hcp and bcc transition metals and their alloys. Applying the constructed potential, molecular dynamics simulations predict a hexagonal composition region within which metallic glass formation is energetically favored. Based on the simulation results, the driving force for amorphous phase formation is derived, and thus an optimized composition is pinpointed to Ni45Zr40Mo15, of which the metallic glass could be most stable or easiest to obtain. Further structural analysis indicates that the dominant interconnected clusters for Ni64Zr36-xMox MGs are 〈0, 0, 12, 0〉, 〈0, 1, 10, 2〉, 〈0, 2, 8, 2〉 and 〈0, 3, 6, 4〉. In addition, it is found that the appropriate addition of Mo content could not only make a more ordered structure with a higher atomic packing density and a lower energy state, but also improve the glass formation ability of Ni-Zr-Mo alloys. Moreover, inherent hierarchical atomic configurations for ternary Ni-Zr-Mo metallic glasses are clarified via the short-range, medium-range and further in the extended scale of the icosahedral network.

Reconfiguration of Point Defects in FCC and HCP Metals at Initial Stage of Recovery Process/ Rekonfiguracja Struktury Defektów Sieciowych W Początkowym Stadium Zdrowienia Metali O Sieci Rsc I Hz

Abstract The conducted studies regarded the analysis of change of structure of point defects occurring during initial stage of recovery of FCC (Al, Cu) and HCP (Ti, Mg and Zn) metals at temperature close to Th =0.5Tm. The changes in resistivity of the deformed and later recovered samples were measured. The recovery time was 1, 2, 3, 4 or 5 min. The observed changes were correlated with reorganization of arrangement of lattice defects during annealing.

Materials for future gas turbine applications

The complexity of adopting new or using existing advanced alloy systems in demanding environments where safety is paramount is highlighted. Of particular importance is the ability to define operative deformation and degradation mechanisms that may limit part life or functionality. With respect to the underlying fundamental material science there is a high degree of commonality between the aero and power generation sectors. The vast array of new experimental and modelling techniques that can be brought to bear on long standing technical challenges offers the potential for a renaissance in materials science. Of particular interest from an industrial context would be the ability to ‘design’ material microstructure and texture optimised for service use manufactured via the most energy and time efficient route. The requirements and potential for progress in this area is summarised with an emphasis on gas turbine and nuclear reactor applications for hcp metals.

Dislocation cross-slip controlled creep in Zircaloy-4 at high stresses

Abstract Uniaxial creep tests were performed on Zircaloy-4 sheet in the temperature range of 500–600 °C at high stresses (>10−3E), to uncover the rate-controlling mechanism. A stress exponent of 9.3–11 and a stress-dependent activation energy in the range of 220–242 kJ/mol were obtained from the steady state creep data. TEM analyses revealed an extensive presence of hexagonal screw dislocation network on the basal planes indicating recovery of screw dislocations by cross-slip to be the dominant mechanism. The creep data was therefore analyzed in the light of Friedel׳s cross slip model for HCP metals according to which the stress-dependency of the activation energy determined from the creep data was written in the form, U = ( 150 ± 4 ) + ( 2236 ± 124 τ ) kJ / mol The constriction energy of screw dislocations of 150 kJ/mol is in agreement with the values reported in the literature for zirconium and other HCP metals. Further analysis of the yield strength and the activation volume data obtained from stress relaxation tests in the temperature range 500–600 °C favors cross-slip of screw dislocations as the rate controlling mechanism over the test conditions.

Trends in Formic Acid Decomposition on Model Transition Metal Surfaces: A Density Functional Theory study

We present a first-principles, self-consistent periodic density functional theory (PW91-GGA) study of formic acid (HCOOH) decomposition on model (111) and (100) facets of eight fcc metals (Au, Ag, Cu, Pt, Pd, Ni, Ir, and Rh) and (0001) facets of four hcp (Co, Os, Ru, and Re) metals. The calculated binding energies of key formic acid decomposition intermediates including formate (HCOO), carboxyl (COOH), carbon monoxide (CO), water (H2O), carbon dioxide (CO2), hydroxyl (OH), carbon (C), oxygen (O), and hydrogen (H; H2) are presented. Using these energetics, we develop thermochemical potential energy diagrams for both the carboxyl-mediated and the formate-mediated dehydrogenation mechanisms on each surface. We evaluate the relative stability of COOH, HCOO, and other isomeric intermediates (i.e., CO + OH, CO2 + H, CO + O + H) on these surfaces. These results provide insights into formic acid decomposition selectivity (dehydrogenation versus dehydration), and in conjunction with calculated vibrational frequenc...

Microscopic properties of lithium, sodium, and magnesium battery anode materials related to possible dendrite growth.

Lithium and magnesium exhibit rather different properties as battery anode materials with respect to the phenomenon of dendrite formation which can lead to short-circuits in batteries. Diffusion processes are the key to understanding structure forming processes on surfaces. Therefore, we have determined adsorption energies and barriers for the self-diffusion on Li and Mg using periodic density functional theory calculations and contrasted the results to Na which is also regarded as a promising electrode material in batteries. According to our calculations, magnesium exhibits a tendency towards the growth of smooth surfaces as it exhibits lower diffusion barriers than lithium and sodium, and as an hcp metal it favors higher-coordinated configurations in contrast to the bcc metals Li and Na. These characteristic differences are expected to contribute to the unequal tendencies of these metals with respect to dendrite growth.

Defect clustering upon dislocation annihilation in α-titanium and α-zirconium with hexagonal close-packed structure

The annihilation of vacancy- and interstitial-type dislocation dipoles is investigated employing atomistic simulations and metadynamics in hexagonal close-packed (hcp) metals α-titanium and α-zirconium, with a variety of dipole heights, orientations and annealing temperatures. Molecular dynamics simulations reveal that depending on the dipole type, height, orientation, etc., dipolar configurations transform into specific reconstructed configurations at low temperature, while vacancy or interstitial clustering occurs at high temperature. The time of clustering and the lifetime of the resulting clusters are estimated through the search of the lowest-energy paths. Compared with previous knowledge on face-centered cubic (fcc) metals, the general processes of dislocation annihilation and point defect clustering are similar, however, due to the varied lattice symmetry: (1) the atomic structures of the reconstructed configurations at low temperature and the clusters at high temperature in hcp systems are different from those in fcc systems; and (2) in hcp systems the clustering process is shortened and the stability of the resulting clusters is enhanced compared with fcc systems.

A phase-field model of twinning and detwinning coupled with dislocation-based crystal plasticity for HCP metals

In this study, a phase-field model describing both twinning and detwinning that is coupled with a dislocation-based crystal plasticity model is constructed for HCP metals, particularly for magnesium. The driving force of twinning is assumed to be resolved shear stress acting on the twin system. The plastic anisotropy of dislocation mobility in an HCP crystal is described via the dislocation mean free path model, which is incorporated into the hardening law. These models are coupled through an order parameter and stress. Using the present model, several FE analyses on single-crystal magnesium are carried out. The obtained results show that the nominal stress–strain response, twinning and detwinning behavior and the consistent value of twin shear around twins are reproduced by this model. In addition, from the results of tensile simulation of a specimen including a number of twin nuclei, twin nucleation is suggested to occur so as not to increase the mechanical energy by the accumulation of elastic strain and dislocation.

Modeling Twin Clustering and Strain Localization in Hexagonal Close-Packed Metals

It has been suggested that microtextures related to deformation twinning can affect the stretch formability of hexagonal close-packed (hcp) metals. We study the evolution of strain localization and the formation of twin clusters in hcp polycrystals due to softening induced by twinning reorientation of the crystal lattice. We performed three-dimensional crystal plasticity finite element simulations of uniaxial and biaxial deformation using basal starting textures of various strengths. Weaker textures resulted in relatively homogeneous deformation, strong textured systems, however, showed significant shear band formation in biaxial tension. In this case, highly strained regions coincided very well with dense clusters of twins. Due to the scarcity of available slip systems, such shear localization due to twin clustering may lead to eventual crack initiation and failure. The spatial correlation of twins was analyzed and was found to be significantly affected by texture. The decay of the correlation function was well approximated by a power law, a characteristic of a wide range of phenomena associated with self-organized criticality.

Modelling of Texture Evolution and Grain Refinement on Complex SPD Paths

A computationally efficient procedure for modelling of microstructural changes on complex and spatially nonuniform deformation paths of severe plastic deformation (SPD) is presented. The analysis follows a two-step procedure. In the first step, motivated by saturation of material hardening at large accumulated strains, the steady-state kinematics of the process is generated for a non-hardening viscoplastic model by using the standard finite element method for a specified SPD scheme. In the second step, microstructural changes are investigated along the deformation-gradient trajectories determined in the first step for different initial locations of a material element. The aim of this study is to predict texture evolution and grain refinement in a non-conventional process of cold extrusion assisted by cyclic rotation of the die, called KOBO process, which leads to an ultra-fine grain structure. The texture evolution is calculated for fcc and hcp metals by applying crystal visco-plasticity combined with the self-consistent scale transition scheme. In parallel, by applying the simplified phenomenological model of microstructure evolution along the trajectories, grain refinement is modelled. The results are compared with available experimental data.

Reassessment of temperature increase and equivalent strain calculation during high-pressure torsion

The problems of temperature increase during high-pressure straining and equivalent strain calculation are reassessed on the basis of general considerations and experimental evidence. Temperature evolution is measured for some pure fcc and hcp metals (Cu, Al, Ni, Ti and Zr) and different regimes of HPT (pressure, strain accumulated and strain rate). The results obtained are compared with modelling and theoretical estimates. A simple model taking into account microstructure evolution during HPT is applied in order to explain the consequent "straining-hardening-softening". The heat release of plastic work is calculated on the basis of both the von Mises and Hencky equivalent strains giving some assessment on the applicability of these equations.

Analysis of Crystallographic Orientation Changes during Deformation of Magnesium Single Crystals

Magnesium single crystals of three di erent orientations were deformed by tension at room temperature to investigate the geometrical criterion resulting from the Schmid law for activation of basal and non-basal slip systems. Changes of crystallographic orientations of investigated single crystals were systematically measured during deformation and the geometrical criterion was examined in respect to the changes of crystallographic orientations during deformation. The geometrical criterion itself failed to explain lack of activity of non-basal slip systems in the deformed magnesium single crystals. Therefore the ratio of activation volumes of soft to hard slip systems is considered to be introduced for a better understanding of the deformation in hcp metals.

Erratum to: “Theoretical shear strength of BCC and HCP Metals”

Erratum to: “Theoretical shear strength of BCC and HCP Metals”