Twins In Ti Research Articles

Subsequent growth and sequential twinning induced by the interaction of {332} twins in Ti[sbnd]15Mo alloy

The mechanisms associated with the interaction of {332} twins are investigated in metastable Ti15Mo alloy. There are 12 possible twin-twin junctions (TTJs) induced by {332} twins that can be classified into 5 types according to the crystallographic relation. Twin-twin interactions are accompanied with the formation of twin-twin boundaries (TTBs) on the acute side and obtuse side. Experimental results based on transmission electron microscopy (TEM) and procession electron diffraction (PED) show that subsequent growth of the TTBs is asymmetric with the twin thickening preferred only on one side for each type. Besides, it is observed that the twin-twin interaction could stimulate sequential {332} twinning inside the primary twin with a specific twin variant promoted. To account for the subsequent growth of the TTBs and the preferred selection of the sequential twinning variant associated with twin-twin interactions, the elastic energy analysis of the resultant dislocations at TTBs indicates that the growth of the TTBs is energetically favorable only on one side. Displacement gradient based analysis shows that the active sequential twin variant could maximumly accommodate the accumulated strain induced by the twin-twin interaction. This work is useful for predicting the preferential growth of the interfaces and the associated accommodation mechanisms induced by the interactions of localized shear bands.

The hardening effect of deformation twinning based on visco-plastic self consistent model and a multi-scale grain refinement prediction model during machining of titanium alloy

Deformation twins act as one of the deformation characteristics of hexagonal close packed (HCP) materials and occur during machining of Ti–6Al–4V alloy. To clarify the hardening mechanisms of deformation twins in Ti–6Al–4V machined surface, work-hardening effects induced by grain refinement in meso scale as well as deformation twins in micro scale were investigated. A 2D cutting finite element (FE) model was established to predict grain refinement in multi-scales and its hardening behavior during machining of Ti–6Al–4V. A visco-plastic self consistent model (VPSC) coupled with predominant twin reorientation (PTR) scheme was also used to simulate stress variation with considering the activations of twin systems. The results show that different grain refinement degrees in meso scale are observed with the increasing of cutting speeds (100–500 m/min). {101¯1} compression nano twins generate at higher cutting speeds, and the twin volume fraction is relatively small (20%). The hardening behaviors of grain refinement in meso scale as well as nano twins predicted with FE model and VPSC model are similar, and the generation of nano twins cause inhomogeneous hardening in Ti–6Al–4V machined surface. Nano-indentation tests further prove the accuracy of the prediction results for the hardening effect induced by nano twins in Ti–6Al–4V machined surface. The results provide two explanations and quantitative evaluations for the hardening effects of deformation twins from the macroscopic stress and the microscopic deformation of slip and twinning modes.

Unexpected de-twinning of strongly-textured Ti mediated by local stress

• An in-situ study coupling EBSD was carried out on twinning behaviors of Ti in Ti-Al layered metal. • The unexpected de-twinning of { 10 1 ¯ 2 } 〈 10 1 ¯ 1 ¯ 〉 extension twins was observed. • The local Schmid factor (LSF) that considers local stress, and the displacement gradient tensors of twinning provided valuable insights into the twinning behaviors of Ti in Ti-Al layered metal. The local stress tailored by a layered structure is believed to have a crucial influence on the mechanical properties of layered metals, but the correlation between local stress and deformation mechanism is still ambiguous. In the present work, a quantitative analysis of the twinning behavior of Ti in Ti-Al layered metal was conducted by means of electron backscatter diffraction (EBSD) coupled with in-situ tensile tests. Upon the uniaxial tensile loading, an unconventional de-twinning of { 10 1 ¯ 2 } 〈 10 1 ¯ 1 ¯ 〉 extension twin in Ti was observed instead of the expected { 11 2 ¯ 2 } 〈 11 2 ¯ 3 ¯ 〉 contraction twinning. This observation could be attributed to the multiaxial local stress in Ti induced by the layered structure and to differences in the local deformation accommodation among twinning modes. The local Schmid factor (LSF) that considers local stress, and the displacement gradient tensors of twinning provided insights into the twinning behaviors of Ti in Ti-Al layered metal.

Facilitation of detwinning through controlling crystal structure in Ti–Zr–Ni–Pd high temperature shape memory alloys

Ti–(Zr, Hf)–Ni alloys, in which Zr or Hf is added to Ti–Ni shape memory alloys, are expected to be excellent high temperature shape memory alloys because they have martensitic transformation temperatures above 100°C and high strength at elevated temperatures by utilizing H-phase precipitation. However, during reorientation of martensite variants, deformation stress increases with applied strain without showing a plateau region, making it difficult to obtain a large shape recovery strain. In the martensite phase of Ti–(Zr, Hf)–Ni alloys, thin and dense (001)B19′ compound twins are introduced during the martensitic transformation, and the detwinning of martensite variants becomes difficult to occur, resulting in the increase in deformation stress during the variant reorientation. Therefore, we propose a new alloy-design concept to control the crystal structure by adding a fourth element in order to facilitate detwinning. In this paper, we first considered the cause of (001)B19′ compound twinning in Ti–(Zr, Hf)–Ni alloys, and then selected Pd as the fourth element to change the crystal structure for preventing the formation of (001)B19′ compound twins. Experimental investigations of the effects of alloy composition on the crystal structure, transformation temperatures, and precipitate formation realized this alloy-design concept through presenting a candidate composition range of Ti–(15–20)Zr–49.7Pd and surrounding Ni-containing alloys for new high temperature shape memory alloys which readily undergo detwinning.

Facilitation of Detwinning Through Controlling Crystal Structure in Ti-Zr-Ni-Pd High Temperature Shape Memory Alloys

Ti-(Zr, Hf)-Ni alloys, in which Zr or Hf is added to Ti-Ni shape memory alloys, are expected to be excellent high temperature shape memory alloys because they have martensitic transformation temperatures above 100°C and high strength at elevated temperatures by utilizing H-phase precipitation. However, during reorientation of martensite variants, deformation stress increases with applied strain without showing a plateau region, making it difficult to obtain a large shape recovery strain. In the martensite phase of Ti-(Zr, Hf)-Ni alloys, thin and dense (001)B19′ compound twins are introduced during the martensitic transformation, and the detwinning of martensite variants becomes difficult to occur, resulting in the increase in deformation stress during the variant reorientation. Therefore, we propose a new alloy design concept to control the crystal structure by adding a fourth element in order to facilitate detwinning. In this paper, we first consider the cause of (001)B19′ compound twinning in Ti-(Zr, Hf)-Ni alloys, and then select Pd as the fourth element to change the crystal structure for preventing the formation of (001)B19′ compound twins. Experimental investigations of the effects of alloy composition on the crystal structure, transformation temperatures, and precipitate formation realized the concept with presenting a candidate composition range of Ti-(Zr, Hf)-Pd-based alloy systems for new high temperature shape memory alloys which readily undergo detwinning.

Three-dimensional atomic scale characterization of [formula omitted] twin boundaries in titanium

The {112¯2}<1123¯> compression twin can accommodate a considerable amount of strain under c-axis compression in Ti. However, unlike the tensile {101¯2}〈1¯011〉 twin, the structure of {112¯2}<1123¯> compression twins has not been completely characterized. Here, we apply a combined technique of HR-TEM characterization, topological analysis, and atomistic simulations to explore the facets that bound the {112¯2}<1123¯> twin in Ti. In addition to the currently known facets (CTB and B-Py), six new facets are observed and categorized for the first time from atomic-scale TEM observations along five crystallographic directions. The six new facets are (112¯0)//(112¯6), PrPr1, PyPy1, (211¯1)//(1¯21¯1), (11¯04)//(011¯1¯), and (011¯0)//(211¯4). Results from the topological and computational analysis are in reasonable agreement with and support the HRTEM observations. Specifically, (1) the observed facets align with low-index interfaces in both twin and matrix domains, (2) the facets with lower surface energies are found to form extended interfaces, and (3) high-surface-energy facets are found at the twin tip region and explained by the fact that the energy of the combined facet and facet junction configuration is energetically preferred in the twin tip region. These results not only provide a comprehensive understanding of the 3D structure of {112¯2}<1123¯> compression twins in Ti, but also validate the MD procedure and the Ti interatomic potential employed. This is extremely important for future study of {112¯2} twin mobility and interactions with other defects, both features that remain extremely challenging to capture in experiments.

A new method to elucidate fracture mechanism and microstructure evolution in titanium during dissimilar friction stir welding of aluminum and titanium

In the friction stir welding (FSW) of dissimilar materials, the weld nugget exhibits composite properties and is composed of hard particles (high-strength material) distributed in a soft matrix material. The distribution of these particles influences the properties of the weld. Therefore, it is useful to characterize the deformation and fragmentation of the high-strength material from which they originate. In the current study, FSW of aluminum (Al) to titanium (Ti) was performed and a new technique was introduced to remove Al from the post-weld sample to characterize the deformation and fragmentation of Ti in the weld nugget. The post-weld sample showed that Ti particles were inhomogeneously distributed. It was understood that the plastic deformation of the Ti depends on its location of the weld. A detailed investigation revealed a number of fracture zones in titanium. A systematic analysis revealed the evolution of twins in Ti in the thermos-mechanically affected zone (TMAZ), whereas no-twins and recrystallized grains were noticed on the Ti flake that was consolidated in the weld nugget. The X-ray computed tomography (XCT) showed that Ti flakes were subjected to deformation and bending. Furthermore, it was noticed that the width of the flakes was not identical along their length and that the flakes were broken at the middle region into two parts. It can be concluded that Ti, at the interface and as flakes, was subjected to severe deformation at a lower homologous temperature that leads to the formation of adiabatic shear bands (ASBs). The local evolution of temperature within the ASBs was much higher than the general weld temperature, leading to the recrystallization of Ti at the interface. Moreover, ASBs are prone to crack nucleation and propagation; hence, particles with different morphology and sizes were noticed in the nugget zone. The distribution of particles was inhomogeneous due to variation in the particle size and complex mechanical mixing. The distribution of particles varied with location across the weld nugget. A homogeneous mechanical mixing was noticed at the center of the weld nugget due to complex material flow and distribution of finer particles. With this understanding of the deformation and fracture mechanism of titanium during dissimilar FSW of Al to Ti, it is possible to engineer the developed composites and/or weld nuggets of dissimilar materials to achieve the required combination of mechanical properties.

Interactions between 〈a〉 dislocations and three-dimensional [formula omitted] twin in Ti

〈a〉dislocations on basal or prismatic planes and {112¯2} compression twins are commonly activated in deformed Titanium (Ti). In the present work, their interactions are investigated by both crystallographic analysis and atomistic simulations. For a three-dimensional {112¯2} twin, we firstly analyze seven possible twin boundaries (TBs) bonding two low index planes in matrix and twin. Next, we focus on the two lower energy boundaries, {112¯2}M/T∥{112¯2}T/M coherent twin boundary (CTB) and {1¯21¯1}M/T∥{12¯11¯}T/M. Depending on dislocation character and boundary type, we define four types of interactions between 〈a〉 dislocations and these TBs. Further, we predict possible dislocation reactions on/across TBs using crystallographic analysis according to the deformation compatibility and the change in elastic energy, such as twinning/detwinning of the primary twin, slip transmission and secondary twinning, for each type of interaction. Molecular dynamics (MD) simulations are then conducted for all interactions under pre-selected loadings in order to explore the dynamic process associated with each of these interactions and examine the predicted reactions. MD simulations predict that the interaction between 〈a〉 dislocations and some facets can lead to the formation of secondary twins and 〈a〉 dislocations on basal or prismatic planes in twins, and reveal the possibility of forming 〈c〉 and 〈c+a〉 dislocations in twins. Moreover, some of the possible reactions take place on lateral TBs other than CTBs.

Serendipitous discovery of the faint solar twin Inti 1

Methods. We determine the atmospheric parameters and differential abundances using high-resolution ($R \approx 50 000$), high signal-to-noise (S/N $\approx$ 110 - 240 per pixel) Keck HIRES spectra for our solar twin candidate, the previously known solar twin HD 45184, and the Sun. Results. For the bright solar twin HD 45184, we found $T_{\rm{eff}} = 5864 \pm 9$ K, log $g = 4.45 \pm 0.03$ dex, $v_{t} = 1.11 \pm 0.02$ $\rm{km\ {s}}^{-1}$, and [Fe/H]$ = 0.04 \pm 0.01$ dex, which are in good agreement with previous works. The star Inti 1 has atmospheric parameters $T_{\rm{eff}} = 5837 \pm 11$ K, log $g = 4.42 \pm 0.03$ dex, $v_{t} = 1.04 \pm 0.02$ $\rm{km\ {s}}^{-1}$, and [Fe/H]$ = 0.07 \pm 0.01$ dex that are higher than solar. The age and mass of the solar twin HD 45184 (3 Gyr and 1.05 $\rm{M_{\odot}}$) and the faint solar twin Inti 1 (4 Gyr and 1.04 $\rm{M_{\odot}}$) were estimated using isochrones. The differential analysis shows that HD 45184 presents an abundance pattern that is similar to typical nearby solar twins; this means this star has an enhanced refractory relative to volatile elements, while Inti 1 has an abundance pattern closer to solar, albeit somewhat enhanced in refractories. The abundance pattern of HD 45184 and Inti 1 could be reproduced by adding $\approx 3.5\ \rm{M_{\oplus}}$ and $\approx 1.5\ \rm{M_{\oplus}}$ of Earth-like material to the convective zone of the Sun. Conclusions. The star Inti 1 is a faint solar twin, therefore, it could be used to calibrate the zero points of different photometric systems. The distant solar twin Inti 1 has an abundance pattern similar to the Sun with only a minor enhancement in the refractory elements. It would be important to analyze other distant solar twins to verify whether they share the Sun's abundance pattern or if they are enhanced in refractories, as is the case in the majority of nearby solar twins.

Mechanical twinning and dislocation slip multilayered deformation microstructures in β-type Ti–Mo base alloy

A 10-layer Ti–10Mo–1Fe/Ti–10Mo–3Fe alternating stack material was manufactured by hot-roll bonding and heat treatment. The interfaces between layers could not be detected except for transition region in Fe concentration. However, the interfaces prevented {3 3 2} twinning in the Ti–10Mo–1Fe region from expanding throughout Ti–10Mo–3Fe region which was deformed by dislocation slips despite having the same crystallographic character. The artificially multilayered twinning/dislocation slip deformation microstructures provide a novel approach for controlling the balance between uniform elongation and yield strength in β-type titanium alloys.

On the correlation between macrozones and twinning in Ti–6Al–4V at very high strain rates

The optimization of Ti–6Al–4V structures for impact loading events requires understanding of the effects of macrozones (or micro-texture) on the high strain rate behavior of Ti–6Al–4V. In this paper, electron backscatter diffraction analysis of the pre- and post-deformation microstructures from a Taylor impact specimen shows that macrozones have a significant effect on the activation of deformation twins in Ti–6Al–4V at very high strain rates.

{1 1 2¯ 4} deformation twinning in pure Ti during dynamic plastic deformation

Abstract The type of deformation twins in purity titanium after dynamic plastic deformation (DPD) and quasi-static compression (QSC) at room temperature is investigated by electron backscatter diffraction techniques, respectively. DPD with a high strain rate (4.5–5 × 10 2 /s) activates more deformation twins in Ti. Four types of twins have been identified in DPD samples, while three types of twins in QSC samples. The { 1 1 2 ¯ 4 } twinning is only observed in DPD samples, associating with the { 1 1 2 ¯ 2 } twinning. The misorientations of { 1 1 2 ¯ 2 }–{ 1 1 2 ¯ 4 } variant pairs are 12–13°, suggesting they follow the same misorientation profile ( 0 1 1 ¯ 0 ) plane and rotated direction.

Formation of zigzag-shaped {1 1 2}〈1 1 1〉 β mechanical twins in Ti–24.5 Nb–0.7 Ta–2 Zr–1.4 O alloy

Zigzag-shaped {112} mechanical twins are formed in a cold-compressed Ti-24.5 Nb-0.7 Ta-2 Zr-1.4 O beta alloy. The formation process involves a 1/2 screw dislocation dissociation, movement of three partials on adjacent {112} planes to form microtwins, constriction of blocked extended dislocation to form a new screw dislocation, as well as cross-slip of such dislocation onto another {112} plane in repetitive sequences. Obstacles to extended dislocation and appropriate stacking fault energy of the 13 phase should be the key factors for the formation of such twins. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Faceted interfacial structure of [formula omitted] twins in Ti formed during equal channel angular pressing

The boundary structure of {101¯1} deformation twins formed in commercially pure titanium during an initial pass of equal channel angular pressing processing has been studied by transmission electron microscopy (TEM) and high-resolution TEM. A new twinning mode with a twinning dislocation of b3/3 has been identified using a dichromatic diagram method. The new mode of the {101¯1} twin has substantially higher interface energy and accommodates more shear strain during deformation than the conventional b4 twinning mode. The deviation angle between the habit trace of the twin boundary and the theoretical twinning plane, K1, is dependent upon the density of twin dislocations.

Crystallography and morphology of various interfaces in Ti–Ni, Ti–Pd and Ni–Mn–Ga shape memory alloys

Crystallography and morphology of various interfaces in Ti–Ni, Ti–Pd and Ni–Mn–Ga shape memory alloys are investigated by transmission electron microscopy. Neither ledge nor step structures are recognized at the irrational interface of the <0 1 1> B19′ type II twin in Ti–Ni martensite and the <1 2 1> B19 type II twin in Ti–Pd martensite in the two dimensional lattice image from the η 1 direction. The combination of plate variants of the 10 and the 14 M Ni–Mn–Ga martensites is subsequently discussed. There are four plate variants commonly designated as A, B, C and D, and three fundamental plate combinations can be identified in a given plate group, namely A: B (C: D) of type II, A: C (B: D) of type I and A: D (B: C) of compound twins in both the martensites. An interesting phenomenon is the migration of A: D boundary in the 10 M martensite during the observation. Finally, we present the atomic structures of {1 1 4} B2 Σ9 boundary in the B2 parent phase of Ti–Ni. The influence of heat treatment atmosphere on the boundary structures is also discussed.

Direct evidence of correlation between {2 0 1̄} B19 ′ and {1 1 4} B2 deformation twins in Ti–Ni shape memory alloy

The correlation between {2 0 1 ̄ } B19 ′ twinning in the deformed martensite and {1 1 4} B2 twinning in the reverse transformed parent phase in the Ti–Ni shape memory alloy is directly demonstrated by in situ heating conventional transmission electron microscopy (CTEM) and high resolution electron microscopy (HREM) observations.

Structural interpretation of the nucleation and growth of deformation twins in Zr and Ti—II. Tem study of twin morphology and defect reactions during twinning

A TEM study of mechanical twinning in Ti and Zr was performed. A large number of [0001]-dislocations which are rarely seen in the matrix, and a new type of stacking fault were found within deformation twins in both materials. Matrix dislocations were observed to actively interact with the tips of twin embryos and step ledges of growing twin lamellae. Twin embryos bounded by straight coherent and inclined semi-coherent boundaries were observed. These results as well as the morphological variation of twin lamellae during deformation do not agree with twinning dislocation theories but support a step-wise nucleation and growth mechanism proposed for deformation twinning in h.c.p. structures. Extended discussions are provided to address unanswered questions in the literature concerning deformation twinning.

Interfacial structure of {1011} twins and twinning dislocations in titanium

Abstract The interfacial structure of [1011] twins obtained using atomic-scale computer simulation by Serra, Pond and Bacon in 1991 is shown to be qualitatively consistent with high-resolution transmission electron micrographs of such twins in Ti obtained by Kasukabe, Yamada, Peng and Bursil in 1993. In addition, images of two twinning dislocations are analysed using a circuit mapping method to determine their Burgers vectors and to demonstrate that they correspond to defects studied previously using computer simulation by Serra, Bacon and Pond in 1988 and by Serra, Pond and Bacon in 1991.

Transmission electron microscopy examination and analysis of an anomalous stacking fault in h.c.p. metals

Abstract An anomalous stacking fault is reported that is observed within deformation twins in Ti and Zr. Their formation may be restricted to situations where deformation twinning takes place. An understanding of this process sheds new light on the twinning process. In this work the crystal structure of the stacking fault and its formation mechanism are investigated.

Deformation twinning in metals and ordered intermetallics-Ti and Ti-aluminides

The role of deformation twinning in the strength and ductility of metals and ordered interrnetallic alloys is examined on the basis of crystallography, energetics and kinetics of deformation twinning. A systematic analysis is made by taking Ti, T13Al, TiAl, and A13Ti as four model systems. In comparison with profuse twinning in Ti, the intrinsic difficulty of twinning in T13Al is rationalized in terms of the interchange shuffling mechanism. A fault (SISF) dragging mechanism based on the interaction torque explains the physical source for the low mobility of screw superdislocations in TiAI, which may lead to (1Ii) [1Ii] twin nucleation. In TiAI and A13Ti alloys, the twin-slip (ordinary) conjugate relationship makes an important contribution to the strain compatibility for high,temperature plasticity. Potentially beneficial alloying additions to promote twinning are discussed. (I) Research sponsored by the Division of Materials Sciences, Office of Basic Energy Sciences, U-SDepartment of Energy, under contract DE-ACo5-840R21400 with Martin Marietta Energy Systems, Inc. (2) Permanent address.Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931, U-S-Aloss JOURNAL DE PHYSIQUE III M 6