Hexagonal Close-packed Titanium Research Articles

Grand canonically optimized grain boundary phases in hexagonal close-packed titanium

Grain boundaries (GBs) profoundly influence the properties and performance of materials, emphasizing the importance of understanding the GB structure and phase behavior. As recent computational studies have demonstrated the existence of multiple GB phases associated with varying the atomic density at the interface, we introduce a validated, open-source GRand canonical Interface Predictor (GRIP) tool that automates high-throughput, grand canonical optimization of GB structures. While previous studies of GB phases have almost exclusively focused on cubic systems, we demonstrate the utility of GRIP in an application to hexagonal close-packed titanium. We perform a systematic high-throughput exploration of tilt GBs in titanium and discover previously unreported structures and phase transitions. In low-angle boundaries, we demonstrate a coupling between point defect absorption and the change in the GB dislocation network topology due to GB phase transformations, which has important implications for the accommodation of radiation-induced defects.

Atomistic Investigation of Shock-Induced Amorphization within Micro-shear Bands in Hexagonal Close-Packed Titanium

Atomistic Investigation of Shock-Induced Amorphization within Micro-shear Bands in Hexagonal Close-Packed Titanium

Sequential transmutation of prismatic dislocations during [formula omitted] twin-slip interaction in titanium

Atomistic simulations are performed to investigate the interaction between prismatic dislocations and {112¯2} twin boundary in hexagonal close-packed titanium. Very unusual and interesting dislocation transmutation is observed. When a (101¯0)13[1¯21¯0] dislocation interacts with the twin boundary, it is first transmuted into a (011¯2)13[21¯1¯0] dislocation in the twin. But the (011¯2) is not a common slip plane and the transmuted dislocation is unstable which is then further transmuted into a prismatic dislocation (011¯0)13[21¯1¯0] in the twin with a lower line energy. This behavior, along with the transmutations of other prismatic dislocations, can be understood from the perspective of lattice correspondence in deformation twinning.

Study on the formation mechanism of δ-titanium hydride by rotation electron diffraction

Rotation electron diffraction method was used to study the structures and crystallographic orientations of the face centered tetragonal structured γ-titanium hydride and the face centered cubic structured δ-titanium hydride by collecting the diffraction information of the hydrides and the matrix at the same time. It was found that the FCC001 δ-hydride is the result of phase transformation caused by continuing to charge hydrogen into γ-hydride. Therefore, we got the complete map of the paths in which hexagonal close-packed titanium transforms to stable δ-titanium hydrides with the increase of hydrogen atoms.

Influence of Nonmetallic Interstitials on the Phase Transformation between FCC and HCP Titanium: A Density Functional Theory Study

In addition to the common stable and metastable phases in titanium alloys, the face-centered cubic phase was recently observed under various conditions; however, its formation remains largely unclarified. In this work, the effect of nonmetallic interstitial atoms O, N, C and B on the formation of the face-centered cubic phase of titanium was investigated with the density functional theory. The results indicate that the occupancy of O, N, C and B on the octahedral interstitial sites reduces the energy gap between the hexagonal-close-packed (HCP) and face-centered cubic (FCC) phases, thus assisting the formation of FCC-Ti under elevated temperature or plastic deformation. Such a gap further decreases with the increase in the interstitial content, which is consistent with the experimental observation of FCC-Ti under high interstitial content. The relative stability of the interstitial-containing HCP-Ti and FCC-Ti was studied against the physical and chemical origins, e.g., the lattice distortion and the electronic bonding. Interstitial O, N, C and B also reduce the stacking fault energy, thus further benefiting the formation of FCC-Ti.

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.

Asymmetric [formula omitted] twin boundary and migration mechanism in hexagonal close-packed titanium

Classical twinning theory predicted that a simple shear was able to create a perfect, twinned structure without the need of atomic shuffles for (112¯1)[112¯6¯] twinning mode in hexagonal close-packed (HCP) metals, and the elementary twinning dislocation should be a two-layer zonal. However, it was revealed in the literature that this particular twinning mode could not have mirror symmetry between the parent and twin, and shuffles should be involved during twin boundary (TB) migration. These conflicting reports indicate that the (112¯1)[112¯6¯] twinning mechanism has not been completely resolved, and what configuration of twinning dislocation mediates twin growth is not well understood either. In this work, atomistic simulation is conducted to further understand the twinning mechanism in titanium. Novel structural analyses are performed to reveal how the parent lattice is transformed to the twin lattice. The results show that the mirror symmetry of a perfect, unrelaxed TB breaks down after relaxation, because of the very high repulsive force between the atom pairs on mirrored positions with a spacing less than half of the lattice constant. As a result, the stacking sequence of the twin differs from that of the parent after relaxation. During twin growth, each {11¯00} prismatic plane of the parent splits into two layers and reorganize into the prismatic planes of the twin, such that the twin prismatic are no longer aligned with the parent prismatic and the mirror symmetry breaks down. This process is accomplished by atoms shuffling in the opposite direction that is perpendicular to the twinning shear. The atomic shuffles spread over multiple consecutive twinning planes to minimize the shuffling magnitude on each twinning plane. Thus, the actual TB is no longer a single twinning plane, but a multi-layer interface zone. Careful examination shows that no well-defined twinning dislocation core can be identified, although single-layer steps can still be observed.

First-principles calculation of twin boundary energy and strength/embrittlement in hexagonal close-packed titanium

We performed first-principles calculations to determine the effects of 14 metallic solutes on twin boundary (TB) energy and strength/embrittlement in Ti alloys, with Ti {1 0 1¯ 2} and {1 0 1¯ 1} TBs as models. Twin formation energies were calculated to identify the effects of solutes on TB stability. Segregation energy and the associated segregation concentration and solubility energy were examined to identify the existence states of the solutes. The influences of solutes on TB strength were identified through strengthening/embrittling energy and its mechanical and chemical contributions. Tensile tests were performed using the “uniaxial-strain loading” and the “uniaxial-stress loading” methods to determine maximum TB strength.

A Multi-modal Data Merging Framework for Correlative Investigation of Strain Localization in Three Dimensions

A multi-modal data-merging framework that enables the reconstruction of slip bands in three dimensions over millimeter-scale fields of view is presented. The technique combines 3D electron back-scattered diffraction (EBSD) measurements with high-resolution digital image correlation (HR-DIC) information collected in the scanning electron microscope (SEM). A typical merging workflow involves the segmentation of features within the strain field (slip bands, deformation twins) and the microstructure (grains), alignment of datasets and the projection of slip bands into the 3D microstructure, using the knowledge of the local crystallographic orientation. This method is demonstrated in two materials: a face-centered cubic (FCC) nickel-base superalloy and hexagonal close-packed (HCP) titanium alloy.

Extreme Expansion and Reversible Hydrogen Solubility in h.c.p. Titanium Stabilized by Colossal Interstitial Alloying

Titanium and its alloys are very reactive and have a high affinity to interstitial elements. Titanium is one of the metals that forms very stable borides, carbides, nitrides, oxides, and even hydrides. The solubility of nitrogen and oxygen in the hexagonal crystal structure of (metallic) titanium is very high; on the other end limited quantities of boron, carbon, and hydrogen can be dissolved. In the present contribution, we report on a significant increase in the solid solution limit of hydrogen in hexagonal close-packed (h.c.p.) titanium, where the h.c.p. crystal lattice is stabilized by deliberate interstitial alloying with high quantities of nitrogen or oxygen atoms. The presence of nitrogen/oxygen prevents the hydrogen-induced transformation of the h.c.p. titanium lattice to a face-centered cubic (f.c.c.) titanium lattice that occurs if no oxygen or nitrogen is present. The hydrogen content that can be accommodated at room temperature in h.c.p. titanium is as high as an unprecedented 50 at. pct from about 0 at. pct and causes an anisotropic expansion of the hexagonal lattice. This finding showcases that there could be an unexploited potential for combining large quantities of interstitials in titanium-based lattices.

A half-shear-half-shuffle mechanism and the single-layer twinning dislocation for [formula omitted] mode in hexagonal close-packed titanium

Among the twinning modes in hexagonal close-packed (HCP) metals, the mechanism for {112¯2}〈112¯3¯〉 mode is particularly confusing and controversial. In the literature reports, there are three possible second invariant planes, i.e. the K2 planes for {112¯2}〈112¯3¯〉 twinning mode: {112¯4¯} which has been widely accepted and corresponds to a three-layer zonal twinning dislocation; {112¯2¯} that is deemed unfavorable; and (0002) which has only been observed in atomistic simulations and corresponds to a single-layer twinning dislocation. {112¯4¯} was predicted by classical twinning theory and the experimentally measured magnitude of twinning shear s in titanium and zirconium seemed to agree well with the prediction. However, {112¯4¯} has never been verified in simulations which show that (0002) should be the K2 plane. This conflict has not been resolved due to the lack of experimental observation of the structure of twinning dislocations. In this work, scanning transmission electron microscopy (STEM) observations are conducted to resolve the twin boundary structure in deformed pure titanium on the atomic scale, combined with atomistic simulations. Atomic resolution STEM unambiguously shows that the twinning dislocation only involves a single twinning plane and the K2 plane is (0002), which is consistent with the atomistic simulations. The STEM results also reveal a half-shear-half-shuffle process which is manifested by a unique twin boundary structure generated by the glide of single-layer twinning dislocations. To explain these results, the lattice correspondences of all three K2 planes are examined in great detail. In particular, shear and shuffle required in the lattice transformations are analyzed inside the framework of classical theory. These analyses explain well why (0002) is the more favorable K2 plane than {112¯4¯} and {112¯2¯}, and properly resolve the conflict between the prediction of the classical twinning theory and the simulation results.

Asymmetric {11-21}<11-2-6> Twin Boundary and Migration Mechanism in Hexagonal Close-Packed Titanium

Asymmetric {11-21}<11-2-6> Twin Boundary and Migration Mechanism in Hexagonal Close-Packed Titanium

Twinning path determined by broken symmetry: A revisit to deformation twinning in hexagonal close-packed titanium and zirconium

Deformation twinning is one of the major deformation mechanisms in crystals, which plays an important role in determining the mechanical properties of metals and alloys. One of the important issues to understand twinning mechanisms is the determination of the deformation path. However, due to a lack of theoretical tools, a fundamental relationship between symmetry breaking and the deformation path has not been established in materials science, which conceals the physical origin of deformation twinning. Utilizing a graph approach for deformation pathways, we show that twinning modes in hexagonal close-packed (hcp) titanium and zirconium are dictated by both the symmetry of hcp and the symmetry breaking associated with the bcc to hcp transformation. Our work not only opens another avenue to investigate the symmetry and symmetry breaking in hcp crystals, but also provides insight into the physical origin of crystalline defects.

Constitutive modeling of dynamic strain aging for HCP metals

Dynamic strain aging hugely affects the microstructural mechanical behavior of metallic materials when activated. It has been extensively reported in the literature that the dynamic strain aging causes significant gaps between model predictions and experimental measurements. Without considering this phenomenon, a constitutive model is unable to accurately capture the material behavior when it is active. The new constitutive model for hexagonal close-packed metals is developed in this work to predict the dynamic strain aging-induced hardening based on the probability function. The proposed model consists of three elements, i.e. dynamic strain aging-induced flow stress as well as thermal and athermal flow stresses. Physically motivated derivation of all the three elements is presented. The proposed model is applied to pure hexagonal close-packed titanium under quasi-static loading (ε˙=0.001/s and ε˙=0.1/s) to dynamic loading (ε˙=2200/s and ε˙=8000/s) across a broad range of temperatures (77 −1000 K).

Anisotropic elastic and thermodynamic properties of the HCP-Titanium and the FCC-Titanium structure under different pressures

Stress induced phase transformation from hexagonal close-packed titanium (HCP-Ti) to face-centered cubic titanium (FCC-Ti) is believed to be a reason for the pronounced work hardening of carbon nanotube-reinforced titanium (CNT/Ti) composites prepared by high-pressure torsion (HPT). Here, the correlation between the phase transformation from the HCP-Ti to the FCC-Ti structure in Ti and the improved mechanical properties of CNT/Ti composite is revealed by investigating the structural transformation mechanism, the stability, electronic properties, anisotropic elasticity and thermodynamics of the FCC-Ti and HCP-Ti crystals under pressure of 0–15GPa by means of first-principles calculations and comparing with the experimental findings. The results show that the formation enthalpies △HTi, the bulk modulus B, the shear modulus G and the Young's modulus E of the FCC-Ti and HCP-Ti structures gradually increase with increasing pressure, and the hybridization between the electronic orbitals of the atoms becomes stronger. The Young’s modulus of the cubic FCC-Ti structure shows strong anisotropy along the [0 1 0] and [1 1¯ 0] directions, while the HCP-Ti structure exhibits an obvious anisotropy of E in the (1 0 0) crystal plane. The thermodynamic stability of the HCP-Ti and FCC-Ti structures decreases under high pressure. The different relative stability of the two structures results in a high propensity of structural transformation from the HCP-Ti to the FCC-Ti structure. A large number of FCC-Ti structures are prepared, which can effectively improve the mechanical properties of CNT/Ti composites. Our results may help to better understand the phase transition from HCP-Ti to FCC-Ti under high pressure, and may reveal the structure-property relationship of CNT/Ti composites.

Interactions between hydrogen and the [formula omitted] twin boundary in hexagonal close-packed titanium

In hexagonal close-packed titanium, the interactions between the (112¯1) twin boundary (TB) and hydrogen solute atoms with several different concentrations are investigated by using first-principles calculations. The preferential occupation sites of hydrogen atoms in the (112¯1) TB region are searched and vary with the amount of hydrogen. Both the shift of the TB and the diffusion of hydrogen atoms, as well as the mutual effect on the movement of each other, are studied. The energy barriers of the TB shift increase with the hydrogen concentration. Additionally, the simulated tensile tests are applied for several systems with co-existing (112¯1) TB and hydrogen atoms, and different geometry transformation behaviors at different hydrogen concentrations are found under the increasing tensile strain.

Strain direction dependency of deformation mechanisms in an HCP-Ti crystalline by molecular dynamics simulations

In this work, effects of uniaxial tensile directions on the deformation mechanisms of a hexagonal close-packed (HCP) titanium crystalline were investigated by molecular dynamics simulations. Three uniaxial tensile directions, namely the [21-1-0], [011-0] and [0001] directions, were studied. When the tensile loading was along the [21-1-0] direction, the parent HCP phase transformed firstly into the body-centered cubic (BCC) phase following the Pitsch-Schrader orientation relationship (OR), and then transformed either into the face-centered cubic (FCC) phase following the Bain path or back to the HCP phase following different variants of the Pitsch-Schrader OR. The new-forming and matrix HCP structures were in a {101-1} twinning relationship with each other. The newly formed FCC phase was in a prismatic-type (P-type) OR with the HCP matrix and in a basal-type (B-type) relationship with the new-forming HCP structure at individual contacting interface. The FCC/HCP interfaces in the P-type OR was immobile while that in the B-type OR propagated by the slip of Shockley partial dislocations. Both FCC/HCP interfaces in the P-type and B-type ORs were observed under high-resolution transmission electron microscope. With the tensile loading along the [011-0] direction, deformation mechanism of the system was dominated by the slip and dissociation of prismatic <a> dislocations. The system stretched along the [0001] direction deformed mainly through the slip and dissociation of pyramidal <c + a> dislocations, as well as through the activation of {101-2} twinning by a pure shuffle mechanism. The present investigation can provide clues in designing titanium alloys with both high strength and good plasticity.

Influence of simple metals on the stability of [formula omitted] basal screw dislocations in hexagonal titanium alloys

Basal slip acts as a secondary deformation mode in hexagonal close-packed titanium and becomes one of the primary mechanisms in titanium alloyed with simple metals. As these solute elements also lead to a pronounced reduction of the energy of the basal stacking fault, one can hypothesize that they promote basal dissociation of dislocations which can then easily glide in the basal planes. Here, we verify the validity of this hypothesis using ab initio calculations to model the interaction of a screw dislocation with indium (In) and tin (Sn). These calculations confirm that these simple metals are attracted by the stacking fault existing in the dislocation core when it is dissociated in a basal plane, but this interaction is not strong enough to stabilize a planar configuration, even for a high solute concentration in the core. Energy barrier calculations reveal that basal slip, in the presence of In and Sn, proceeds without any planar dissociation, with the dislocation being spread in pyramidal and prismatic planes during basal slip like in pure Ti. The corresponding energy barrier is higher in presence of solute atoms, showing that In and Sn do not ease basal slip but increase the corresponding lattice friction. This strengthening of basal slip by solute atoms is discussed in view of available experimental data.

Oxygen-dislocation interaction in titanium from first principles

The interaction between screw dislocations and oxygen interstitial atoms is studied with ab initio calculations in hexagonal close-packed titanium. Our calculations evidence a strong repulsion when the solute atoms are located in the dislocation glide plane, leading to spontaneous cross-slip, which allows the dislocation to bypass the atomic obstacle. This avoidance process explains several experimental observations in titanium in presence of oxygen: (1) a larger lattice friction against screw dislocation motion, (2) a reduction of the dislocation glide distance in prismatic planes and (3) an enhancement of cross-slip in pyramidal planes.

Gradient nanostructure evolution and phase transformation of α phase in Ti-6Al-4V alloy induced by ultrasonic surface rolling process

The gradient nanostructure evolution and the mechanism governing this evolution of α phase in Ti-6Al-4V alloy induced by ultrasonic surface rolling process were investigated. A gradient nanostructure consisting of a roughly equiaxed nanograin layer, an elongated nano-lamellar layer, an elongated ultrafine lamellar layer, a refined grain layer, and a low-strain coarse-grained layer was formed with a thickness of more than 400 µm. The formation of gradient nanostructure of α phase was dominated by complex dislocation activities in hcp grains without twins occurring, supplemented by hexagonal close-packed (hcp) titanium (Ti) to face-centered cubic (fcc) Ti phase transformation. During the microstructural evolution, the coarse hcp-Ti grains were first elongated into lamellae. Then, these sub-micron lamellae were gradually transformed into roughly equiaxed nanograins via two deformation modes of longitudinal splitting and transverse breakdown, accompanied by dynamic recovery. The fcc-Ti grains were deformed mainly via twin-twin intersections and twin-dislocation interactions, accompanied by longitudinal splitting and transverse breakdown, resulted in refinement of the micron-scale fcc-Ti grains to roughly equiaxed nanograins. The interaction of hcp and fcc phases influenced and synergistically promoted the microstructural evolution process. In addition, the microhardness improvement in the surface layer of Ti-6Al-4V alloy was attributed to the increase of dislocation density, grain refinement and the occurrence of deformation twinning in fcc-Ti grains.