Hcp Alloys Research Articles

Grain growth competition and formation of grain boundaries during solidification of hcp alloys

Grain growth competition during directional solidification of a polycrystal with hexagonal (hcp) symmetry (Mg-1wt%Gd alloy) is studied by phase-field modeling, exploring the effect of the temperature gradient G on the resulting grain boundary (GB) orientation selection. Results show that selection mechanisms and scaling laws derived for cubic (fcc, bcc) crystals also apply to hcp materials (within their basal plane), provided a re-estimation of fitting parameters and re-scaling to account for the sixfold symmetry. While grain growth competition remains stochastic with rare events of unexpected elimination or side-branching along the developing GBs, we also confirm an overall transition from a geometrical limit to a favorably oriented grain limit behavior with an increase of thermal gradient within the dendritic regime, and the progressive alignment of dendrites and GBs toward the temperature gradient direction with an increase of G during the dendritic-to-cellular morphological transition. Comparisons with original thin-sample directional solidification experiments show a qualitative agreement with PF results, yet with notable discrepancies, which nonetheless can be explained based on the stochastic variability of selected GB orientations, and the statistically limited experimental sample size. Overall, our results extend the understanding of GB formation and grain growth competition during solidification of hcp materials, and the effect of thermal conditions, nonetheless concluding on the challenges of extending the current studies to three dimensions, and the need for much broader (statistically significant) data sets of GB orientation selected under well-identified solidification conditions.

Twinning induced spatial stress gradients: Local versus global stress states in hexagonal close-packed materials

Length scale dependent microstructural heterogeneities serve as effective pathways in engineering materials for providing simultaneous strength-ductility enhancement. In this regard, hexagonal close-packed (hcp) materials that exhibit a combination of slip and multiple twinning modes potentially act as ideal candidates that generate heterogeneous microstructures. However, such an inhomogeneous distribution of crystallographic defects also results in build-up of spatially heterogeneous local stress gradients that can be distinct from globally applied stress state. In particular, stress fields arising at the vicinity of deformation twins and due to their interaction with grain interfaces often act as precursors to damage nucleation in most hcp metals and alloys. Hence, assessment of such local stresses and their overall impact on plasticity becomes necessary in order to understand the relationship between twinning and fracture in hcp materials. The current work utilizes commercially pure titanium (cp-Ti) as a model material to investigate the impact of twinning induced stress gradients on the local mechanical response. By means of correlative multiscale structural characterization and local stress gradient measurements, we establish a definitive relationship between applied stress vis-à-vis local stress on the local plasticity behavior ahead of a {112¯2} compression twin-grain boundary intersection in cp-Ti. Additionally, the role of twin interfacial structure for tension and compression twinning modes are experimentally determined and their corresponding impact on the local stress fields and associated twin migration mechanisms is assessed.

The complexity of [formula omitted] twin interface structure and energetics in HCP materials

The quest for a deeper understanding of the mechanical properties of HCP alloys has been an ongoing effort. Due to the lack of a sufficient number of slip systems, twin nucleation and migration originate as a substantial plastic deformation mechanism in HCP metals. The paper uncovers the energetics and atomistic mechanism of the {112¯1}〈112¯6〉 which is one of the most prominent twin modes in HCP materials that has not been fully understood. In view of corrugated {112¯1}interfaces, the positioning of lattice/motif atoms needs to be clarified. First, we establish the initial twin boundary (TB) structure unambiguously via the lattice offsets resulting in volume invariancy and correct relative positioning of the twin variants. Secondly, the TB migration mechanism is determined from a crystallographic (zero net shuffle, Σs→=0) and energetic standpoints (minimum energy path). The Generalized Planar Fault Energy (GPFE) curves are calculated for Ti, Hf, Mg, and Zr. Such GPFE curves for this twin mode are calculated for the first time, and point to unusually high unstable energies (300-850 mJ/m2), and low migration barriers (<60 mJ/m2) for twinning accompanied with large shuffle magnitudes, |s→|, comparable to the Burger's vector (|s→|≅be>0.44Å).

Hexagonal close-packed (hcp) alloys under dynamic impacts

Predictions on the mechanical behavior of metals and alloys with a hexagonal close-packed (HCP) lattice under dynamic influences in a wide range of temperatures are in demand for solving a wide range of applied problems. This article presents new results of numerical simulation showing the general similarity of the mechanical behavior of HCP titanium, zirconium, hafnium, and beryllium alloys under dynamic loadings in a wide range of temperatures. These alloys belong to the important isomechanical subgroup of HCP alloys. A model for numerical simulation of mechanical behavior of HCP alloys under dynamic loadings in a wide temperature range was proposed. The model takes into account the change in contributions to the flow stress from the mechanisms of twinning and dislocation slip in the considered subgroup of HCP alloys. A kinetic damage model was adopted to describe the damage evolution under complex stress conditions and under dynamic loading. Thus, it was possible to increase the accuracy of predicting the dynamic fracture under tensile loads including the spall fracture.

Inverse Pole Figures and the Anisotropy of the Properties of HCP Alloys

Inverse Pole Figures and the Anisotropy of the Properties of HCP Alloys

Stability and ordering of bcc and hcp TiAl+Mo phases: An ab initio study

Atomic ordering in bcc and hcp TiAl+Mo alloys near equiatomic TiAl composition is investigated by different ab initio tools. We show that small addition of Mo, about 5 at. %, is enough to make bcc alloys with more than 50 at. % of Ti stable with respect to the hcp alloys. Moreover, such alloying also leads to stabilizing the B2 ordered structure with respect to its B22 modification, which is the bcc-based ground state structure of binary TiAl. The site preference of Mo in the B2 and B19 ordered alloys is investigated by different methods: in the dilute limit, using the transfer energy formalism; in concentrated alloys, from the total energies of disordered and partially ordered alloys in the mean-field coherent potential approximation; and from Monte Carlo simulations. These methods produce consistent results for the B2 phase predicting a strong preference of Mo to Al sublattice. The site preference of Mo in the B19 phases varies from a weak preference for Al sites in the single impurity calculations to a quite strong preference for Ti sites in the mean-field approximation and finally to a strong Al preference in Monte Carlo simulations. Mo alloying dramatically increases the order–disorder transition temperatures in bcc and hcp Al-deficient Ti0.5Al0.5−xMox alloys.

Tribo-Induced Surface Deformation Mechanisms Govern Friction and Wear in Ultralight HCP and Duplex Mg-Li Alloys

Tribo-Induced Surface Deformation Mechanisms Govern Friction and Wear in Ultralight HCP and Duplex Mg-Li Alloys

An experimental investigation on the influence of elevated-temperature constrained groove pressing on the microstructure, mechanical properties and hardening behaviour of Ti6Al4V alloy

Abstract Due to its high strength-to-weight ratio and remarkable resistance to corrosion, Ti6Al4V alloy finds its use in a wide range of fields such as defence, aerospace and biomedical applications. To further enhance its properties and widen its applications, this paper focuses upon the novel application of constrained groove pressing (CGP) on Ti6Al4V alloy at elevated temperatures as it is hard-to-deform at room temperature due to its HCP structure. Till date, CGP is reported mainly on FCC and BCC alloys which are relatively easier to deform as compared to HCP alloys and metals, especially at room temperature. In this work, CGP process has been successfully performed on Ti6Al4V at 750 °C and 550 °C. Mechanical and microstructural evolution in CGP processed (CGPed) material is characterized by tensile testing, microhardness testing, optical microscopy, SEM Fractography and XRD. Enhancement in mechanical properties at room temperature such as yield strength, ultimate tensile strength and microhardness is observed more for CGP at 550 °C as compared to CGP at 750 °C and as-received material. Further analysis of tensile test results revealed that CGPed specimens at 750 °C and 550 °C follow different hardening behaviour although hardening mechanism for both is governed by Voce hardening model. Optical microscopy revealed fine distribution of β-phase along the grain boundaries in CGPed specimens. Microstructural analysis showed evolution of grain structure due to dynamic recrystallization in CGPed specimen at 750 °C whereas work hardening and grain fragmentation was observed for CGP at 550 °C. As-received (AR) and CGPed Ti6Al4V were observed to follow the Hall Petch relationship. SEM Fractography was performed on broken tensile test specimens, revealing the change in mode of failure from ductile mode for AR to mixed ductile-brittle mode for CGPed specimen at 550 °C. No precipitates were observed in SEM images which confirmed strain hardening effect of CGP and inherent voids as the main reason of failure. XRD analysis confirmed bulk strain hardening in CGPed Ti6Al4V. Thus, CGP at elevated temperatures may effectively be used for tailoring the microstructure and mechanical properties of Ti6Al4V alloys for extending its application domain.

Diffusion behaviors and atomic mobilities in Mg–Sc hcp and bcc alloys: Investigation via single-phase and multi-phase diffusion couples

Diffusion behaviors in Mg–Sc hcp and bcc solid solutions between 773 and 873 K were investigated using both single-phase and multi-phase diffusion couple techniques. The EPMA detected composition-distance profiles were smoothed and fitted using the error function expansion (ERFEX). The interdiffusion coefficients were extracted using Sauer–Freise integral. The interdiffusion coefficients in hcp phase showed a slightly parabolic composition dependence at the Mg-rich part and the maximum value was around 2–3 at. % Sc. However, the interdiffusion coefficients in the bcc phase monotonously decreased with the increase of solubility of Sc. The determined inter- and impurity diffusion coefficients in the hcp Mg–Sc alloys were assessed to develop the atomic mobility database, and their validity was justified by reproducing the composition profiles and diffusion fluxes obtained in this diffusion couple experiment. Meanwhile, the development of bcc atomic mobility was realized via the Maclaurin approximation, extrapolation, and optimization. The results make up for the missing data of Mg–Sc diffusion kinetics.

Prediction of diffusion coefficients in fcc, bcc and hcp phases remained stable or metastable by the machine-learning methods

Diffusion coefficient play a crucial role in material designing, and physical phenomenon explaining during the material preparation and post-treatment. However, it is unavailable in some metallic systems. In this paper, based on basic physical properties (including atom properties, lattice parameters, melting temperature, elastic stiffness constant and etc.), the diffusion activate energy model were developed by machine-learning methods. First, the melting temperature (Tm) and elastic stiffness constant (Cij) models were built by machine-learning methods to fill the absent values in properties. Second, the diffusion activate energy (Q) model was built, and a hybrid features selection method was used to decrease features from 73 to 11 in the model. The Tm, Cij and Q models showed a good predictive ability and goodness of fit. Finally, features in the models were analyzed and compared with the parameters in various prior models. This work provides further understanding on the mechanism of the melting process, elastic deformation and diffusion process. Moreover, the models could be able to provide an easy and reliable method to obtain the diffusion coefficients in bcc, fcc, and hcp alloys when they are needed but unavailable.

Exploring the correlation between solvent diffusion and creep resistance of Mg-Ga HCP alloys from high throughput liquid-solid diffusion couple

The liquid-solid diffusion couple technique, supported by phenomenological analysis and nano-indentation tests, is proposed on account of the relatively low melting points of Mg to explore the diffusion mobility and creep deformation. The potential of this strategy is demonstrated in MgGa hcp alloys where Ga solute (i.e. impurity) and Mg solvent diffusions in hcp MgGa alloys were both unveiled. It was followed by mapping the compressive creep behavior via nanoindentation along the composition arrays within the same MgGa couple sample. The compressive creep resistance of MgGa hcp alloys increased with the Ga content, and this enhancement was similar to the one found in MgZn alloys and superior to the one reported in MgAl alloys though Al is a slower impurity diffuser in hcp-Mg than Zn and Ga. Thereby, the solvent diffusion and its variation with the composition, rather than the solute diffusion, was suggested to govern the creep properties at high temperatures and low stresses.

Influence of surface ultrasonic impact treatment on texture evolution and elastic properties in the volume of Zr1Nb alloy

The study is devoted to examination of the features of influence of ultrasonic impact treatment (UIT) on the evolution of texture and the changes of elastic parameters in the volume of specimens of Zr1Nb alloy with preliminary-formed strong crystallographic texture. In this case the bulk-wave ultrasonic velocities (vij) in the orthogonal directions in the different sections of the specimens were measured by the automatic US apparatus in initial state (after high-temperature rolling) and after UIT. The density (ρ) of the specimens was also measured in initial and deformed states. Based on the obtained results orientation distribution coefficients (Wij0) were calculated, complete pole figures were plotted, relative intensity of diffraction lines of the basal plane (ΔІ(0001)) and Kearns parameters (fi) were determined. The extreme values of Young (E) and shear (G) moduli, Poisson ratio (η) and their anisotropy were also calculated for different sections of the treated specimens. It is shown that UIT in applied regime changes the texture and elastic parameters in all specimen volume. Pole figures (0001) after UIT keep the view typical for rolled HCP alloys with ratio c/a < 1.633. The clear qualitative correlation between the texture and elastic parameters of the specimens in initial state and after UIT in their different sections is defined. The revealed features indicate that the evolution of crystallographic texture of Zr1Nb alloy specimens is a result of the depth nonuniformity of plastic deformation accumulated during the UIT process. The results are in agreement with available literature data. The effectiveness of applied US method and equipment for layer-by-layer analysis of the texture and elastic characteristics in the volume of spatially inhomogeneous materials subjected to directional surface thermomechanical treatment is shown.

Evolution of microstructure and mechanical properties of Ti6Al4V alloy by multiple passes of constrained groove pressing at elevated temperature

Constrained groove pressing (CGP) is one of the best available sheet material processing technique for grain refinement and improving material properties without changing the original dimensions of the specimen. In this work, CGP is successfully performed on 10 mm thick mill-annealed Ti6Al4V plates at a temperature of 550 °C up to three passes. Microstructure studies reveal that grain size decreased from 18.81 μm in as received (AR) sample to 9.24 μm after third pass. XRD studies confirm the lattice strain and grain refinement phenomenon by peak shifting and broadening. Work hardening and grain refinement lead to increase in microhardness from 346 HV in AR to 392.1 HV, 365.9 HV and 371.4 HV in one pass, two pass and three pass CGP processed (CGPed) Ti6Al4V respectively. CGP leads to improvement of YS from 964.75 MPa in AR to 1085.01 MPa, 1068.11 MPa and 1078.63 MPa in one, two and three pass CGPed Ti6Al4V respectively. Similarly, UTS is observed to improve from 1014.12 MPa in AR to 1118.26 MPa, 1015.21 MPa, 1098.05 MPa in one, two and three pass CGPed Ti6Al4V respectively. SEM fractography reveals change in the mode of fracture of Ti6Al4V from ductile to ductile-brittle mode before and after the CGP process. Work hardening behavior of CGPed Ti6Al4V changes from stage III and IV to purely stage III along with initial instantaneous hardening rate increases after the first pass and decreases for the subsequent passes. CGP on Ti6Al4V alloy at 550 °C is found to have a significant effect on the stabilization of its microstructure and mechanical properties which can be useful for extending CGP for HCP alloys and their applications in aerospace and defense sector.

Phenomenon of ultra-fast tracer diffusion of Co in HCP high entropy alloys

An experimental-ab initio approach is applied to investigate solute diffusion in hexagonal high-entropy alloys. The radioisotope 57Co is selected to probe the diffusion behaviour in a series of HCP alloys, from equiatomic binary (HfZr) and ternary (HfTiZr) to quinary Al5Sc20Hf25Ti25Zr25 at.% and Al15Sc10Hf25Ti25Zr25 at.% high-entropy alloys. Diffusion of Co in the present alloys is found to be strongly anisotropic and exceptionally fast (by 5 to 10 orders of magnitude faster than self-diffusion) which can be explained in terms of a dissociative diffusion mechanism. The origin of ultra-fast diffusion of Co in the given HCP high-entropy alloys and the impact of the multi-element environment on the interstitial positions are analyzed in the framework of ab initio computations. Our results show that solute diffusion can be used as a tool to probe potential short range order in HCP multi-principal element alloys on very short time scales with respect to the jump rates of matrix atoms.

A Review of Hydride Precipitates in Titanium and Zirconium Alloys: Precipitation, Dissolution and Crystallographic Orientation Relationships

This work proposes a review of recent results on the formation and dissolution of hydrides in HCP alloys (Ti and Zr alloys) correlated to the nature of crystallographic hydride phases and their ORs. The crystallographic coherence observed between the surface hydride layer and the substrate is very important for many applications as for biomaterials devices. Five particular orientation relationships (OR) were identified between titanium/zirconium hydride precipitates and the oc-Ti and a-Zr substrates. In addition, the nature of hydrides have a large implication on the ductility, the strain hardening, and the local plastic strain accommodation in the Ti alloys. Our studies using XDR, TEM and SEM-EBSD have been demonstrating that the nature of the hydride phase precipitates depends on the hydrogen content. DSC has been used to obtain the hydride dissolution and precipitation energy values at the bulk scale, whose difference can be associated to misfit dislocations. Local in-situ TEM dissolution observations show the depinning of part of misfit dislocations during dissolution process. Hydride reprecipitation is thus possible only if hydrogen is not driven away during heating by misfit dislocations depinning.

Interaction of {11[formula omitted]2} twin variants in hexagonal close-packed titanium

As multiple {112¯ 2} twin variants are often formed during deformation in hexagonal close-packed (hcp) titanium, the twin-twin interaction structure has a profound influence on mechanical properties. In this paper, the twin-twin interaction structures of the {112¯2} contraction twin in cold-rolled commercial purity titanium were studied by using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Formation of the {112¯2} twin variants was found to deviate the rank of Schmid factor, and the non-Schmid behavior was explained by the high-angle grain boundary nucleation mechanism. All the observed twin-twin pairs manifested a quilted-looking structure, which consists of the incoming twins being arrested by the obstacle twins. Furthermore, the quilted-looking {112¯2} twin-twin boundary was revealed by TEM and high resolution TEM observations. De-twinning, lattice rotation and curved twin boundary were observed in the obstacle twin due to the twin-twin reaction with the impinging twin. A twin-twin interaction mechanism for the {112¯2} twin variants was proposed in terms of the dislocation dissociation, which will enrich the understanding for the propagation of twins and twinning-induced hardening in hcp metals and alloys.

Size effects of nano-spaced basal stacking faults on the strength and deformation mechanisms of nanocrystalline pure hcp metals

The size effects of nano-spaced basal stacking faults (SFs) on the tensile strength and deformation mechanisms of nanocrystalline pure cobalt and magnesium have been investigated by a series of large-scale 2D columnar and 3D molecular dynamics simulations. Unlike the strengthening effect of basal SFs on Mg alloys, the nano-spaced basal SFs are observed to have no strengthening effect on the nanocrystalline pure cobalt and magnesium from MD simulations. These observations could be attributed to the following two reasons: (i) Lots of new basal SFs are formed before (for cobalt) or simultaneously with (for magnesium) the other deformation mechanisms (i.e. the formation of twins and the < c + a > edge dislocations) during the tensile deformation; (ii) In hcp alloys, the segregation of alloy elements and impurities at typical interfaces, such as SFs, can stablilise them for enhancing the interactions with dislocation and thus elevating the strength. Without such segregation in pure hcp metals, the < c + a > edge dislocations can cut through the basal SFs although the interactions between the < c + a > dislocations and the pre-existing SFs/newly formed SFs are observed. The nano-spaced basal SFs are also found to have no restriction effect on the formation of deformation twins.

Ir–Re binary alloys under extreme conditions and their electrocatalytic activity in methanol oxidation

The formation of the hcp-Ir0.70Re0.30 alloy from the single-source precursor (NH4)2[Ir0.70Re0.30Cl6] upon heating in hydrogen atmosphere can be associated with the formation of two intermediates: a crystalline iridium-based intermediate and a fcc-structured alloy. Ir–Re alloys show lower thermal expansion coefficients and smaller compressibility in comparison with individual metals. The high-temperature high-pressure treatment of hcp-Ir0.70Re0.30 alloy enable us to probe the Ir–Re pressure dependent phase diagram. The miscibility gap between hcp and fcc alloys slightly shifts towards the rhenium side below 4 GPa. Above 4 GPa, the miscibility gap does not drift with pressure and narrows with compression. The electrocatalytic activity of Ir–Re alloys has been tested for methanol oxidation in acidic water solution. Ir–Re alloys show higher electrocatalytic activity in comparison with pure Ir and Re, which makes them perspective candidates for fuel cells application. The highest electrocatalytic activity has been obtained for the two-phase Ir0.85Re0.15 composition.

Anisotropic hydrogen diffusion in \u03b1-Zr and Zircaloy predicted by accelerated kinetic Monte Carlo simulations

This report presents an accelerated kinetic Monte Carlo (KMC) method to compute the diffusivity of hydrogen in hcp metals and alloys, considering both thermally activated hopping and quantum tunneling. The acceleration is achieved by replacing regular KMC jumps in trapping energy basins formed by neighboring tetrahedral interstitial sites, with analytical solutions for basin exiting time and probability. Parameterized by density functional theory (DFT) calculations, the accelerated KMC method is shown to be capable of efficiently calculating hydrogen diffusivity in α-Zr and Zircaloy, without altering the kinetics of long-range diffusion. Above room temperature, hydrogen diffusion in α-Zr and Zircaloy is dominated by thermal hopping, with negligible contribution from quantum tunneling. The diffusivity predicted by this DFT + KMC approach agrees well with that from previous independent experiments and theories, without using any data fitting. The diffusivity along <c> is found to be slightly higher than that along <a>, with the anisotropy saturated at about 1.20 at high temperatures, resolving contradictory results in previous experiments. Demonstrated using hydrogen diffusion in α-Zr, the same method can be extended for on-lattice diffusion in hcp metals, or systems with similar trapping basins.

An interatomic potential for simulation of Zr-Nb system

We report a new attempt to study properties of Zr-Nb structural alloys. For this purpose we constructed an angular-dependent many-body interatomic potential. The potential functions were fitted towards the ab initio data computed for a large set of reference structures. The fitting procedure is described, and its accuracy is discussed. We show that the structure and properties of all Nb and Zr phases existing in the Zr-Nb binary system are reproduced with good accuracy. The interatomic potential is appropriate for study of the high-pressure hexagonal ω-phase of Zr. We also estimated characteristics of the point defects in α-Zr, β-Zr and Nb; results are proven to correlate with the existing experimental and theoretical data. In case of α-Zr the model reveals anisotropy of the vacancy diffusion, in agreement with previous calculations and experiments. The potential provides an opportunity for simulation of Zr-Nb alloys based on α-Zr and β-Zr. This conclusion is illustrated by the results obtained for the alloys with different niobium concentrations: up to 7% in case of hcp alloys and up to 50% for bcc alloys.