Phase Transformation In Ti Research Articles

Pore formation mechanism and intermetallic phase transformation in Ti–Al alloy during reactive sintering

The mechanism of pore formation associated with intermetallic reaction is studied in sintered Ti50Al50 alloy with two different heating rates i.e. 1 °C/min and 10 °C/min. The heating rate significantly influences the pore parameters and the intermetallic reactions. The pore size and porosity increase with the extent of exothermic reactions during sintering. The intermetallic Ti2Al5 phase forms by a low-temperature solid-state diffusion of Al at the surface of Ti skeletons, while TiAl3 and TiAl2 intermetallic phases form at a high temperature owing to Ti2Al5 phase reaction with Al and Ti, respectively. A considerable change in thermal and volume expansion take place in the alloy heated with 10 °C/min compared to 1 °C/min. Therefore, it is found that the heating rate during sintering greatly influences the porous properties of Ti50Al50 by controlling the degree of exothermic reactions.

The role of deviatoric stress and dislocations on the α to ω phase transformation in Ti

Under extreme conditions, α-Ti becomes unstable and transforms either into β-Ti at high temperature or into ω-Ti at high pressure. In what concerns the α to ω phase transformation (PT), there has been a wide range of experimentally reported transition pressures from approximately 2 to 15 GPa at room temperature. Deviatoric stresses and internal defects are often assumed to be the root cause of this variation. In this study, these postulates are revisited using both continuum mechanics and molecular dynamics (MD) simulations. First, a simple continuum model, assuming linear elasticity and isotropic plasticity, is developed to describe the effects of applied stress and dislocations on the stability of an ω nucleus in an infinite α domain. Second, a new MD simulation method is developed to generate an ω nucleus in the α domain utilizing the displacement field identified from the topological analysis. Results from MD simulations show that despite the fact that phase diagrams typically delineate the limits between two phases in terms of only P and T, deviatoric stress promotes the α to ω phase transformation by reducing the critical radius above which an ω nucleus is stable. Furthermore, the required deviatoric stress to nucleate and stabilize a nanoscale ω nucleus is likely emanating from the internal stress of defects such as dislocations. The MD-informed micromechanics models are used to identify favorable configurations where dislocations help favor the α to ω transformation. These configurations show that the interaction with a basal or prismatic dislocation reduces the critical radius of a ω nucleus by about 10 or 16 %, respectively. In addition, prismatic edge dislocations are found to promote the growth of ω nucleus when interacting with the (1¯1¯20)α//(0001)ω interface. Importantly, a simple model of the arrival of dislocation at an ω nucleus suggests that PT does not necessarily require a pile-up to be present but could alternatively be mediated by a constant rapid flow of dislocations.

Effect of milling energy on phase transformation of Ti-Ni powders during mechanical alloying

ABSTRACT The effect of milling energy on the phase evolution of mechanically alloyed Ti–Ni powders is reported. Alloys were produced by low-energy and high-energy mechanical alloying (LEMA and HEMA, respectively). Powders milled for different times were characterised by X-ray diffraction and scanning electron microscopy. The phase transformations of powders milled by LEMA and HEMA before amorphisation are presented. During LEMA, TiNi, TiNi3, Ti3Ni4, and Ti2Ni were formed at 50 and 100 h. At these times, the results were consistent with the Ti–Ni equilibrium phase diagram. Up to 7.27 wt% of TiNi with a crystallite size of 32.63 ± 0.01 nm appeared at 50–75 h. During HEMA, iron contamination inhibited the formation of the TiNi phase at short milling times, and up to 47.9% of Fe0.2Ni4.8Ti5 was generated. These results indicate that amorphisation and heat treatment are not the only methods of promoting phase transformation in Ti–Ni alloys.

A novel sequential mechanism associated with stress-induced β→ω→β+α phase transformation in Ti–6Mo-3.5Cr–1Zr titanium alloy

A novel metastable β-type Ti–6Mo-3.5Cr–1Zr alloy is designed using d-electrons theory to introduce diverse phase transformations. Stress-induced ω (SIω) phase transformation dominates the initial deformation, while the SIω bands are transformed to α+β phase as the strain increases. The sequential mechanism associated with β→SIω→α+β is first reported in titanium alloys.

The Role of Deviatoric Stress and Dislocations on the α to ω Phase Transformation in Ti

The Role of Deviatoric Stress and Dislocations on the α to ω Phase Transformation in Ti

Phase transformations in Ti–Nb–Zr–Ta–O beta titanium alloys with high oxygen and reduced Nb and Ta content

Phase transformations in Ti–Nb–Zr–Ta–O beta titanium alloys with high oxygen and reduced Nb and Ta content

Techniques for studying materials under extreme states of high energy density compression

The properties of materials under extreme conditions of pressure and density are of key interest to a number of fields, including planetary geophysics, materials science, and inertial confinement fusion. In geophysics, the equations of state of planetary materials, such as hydrogen and iron, under ultrahigh pressure and density provide a better understanding of their formation and interior structure [Celliers et al., “Insulator-metal transition in dense fluid deuterium,” Science 361, 677–682 (2018) and Smith et al., “Equation of state of iron under core conditions of large rocky exoplanets,” Nat. Astron. 2, 591–682 (2018)]. The processes of interest in these fields occur under conditions of high pressure (100 GPa–100 TPa), high temperature (>3000 K), and sometimes at high strain rates (>103 s−1) depending on the process. With the advent of high energy density (HED) facilities, such as the National Ignition Facility (NIF), Linear Coherent Light Source, Omega Laser Facility, and Z, these conditions are reachable and numerous experimental platforms have been developed. To measure compression under ultrahigh pressure, stepped targets are ramp-compressed and the sound velocity, measured by the velocity interferometer system for any reflector diagnostic technique, from which the stress-density of relevant materials is deduced at pulsed power [M. D. Knudson and M. P. Desjarlais, “High-precision shock wave measurements of deuterium: Evaluation of exchange-correlation functionals at the molecular-to-atomic transition,” Phys. Rev. Lett. 118, 035501 (2017)] and laser [Smith et al., “Equation of state of iron under core conditions of large rocky exoplanets,” Nat. Astron. 2, 591–682 (2018)] facilities. To measure strength under high pressure and strain rates, experimenters measure the growth of Rayleigh–Taylor instabilities using face-on radiography [Park et al., “Grain-size-independent plastic flow at ultrahigh pressures and strain rates,” Phys. Rev. Lett. 114, 065502 (2015)]. The crystal structure of materials under high compression is measured by dynamic x-ray diffraction [Rygg et al., “X-ray diffraction at the national ignition facility,” Rev. Sci. Instrum. 91, 043902 (2020) and McBride et al., “Phase transition lowering in dynamically compressed silicon,” Nat. Phys. 15, 89–94 (2019)]. Medium range material temperatures (a few thousand degrees) can be measured by extended x-ray absorption fine structure techniques, Yaakobi et al., “Extended x-ray absorption fine structure measurements of laser-shocked V and Ti and crystal phase transformation in Ti,” Phys. Rev. Lett. 92, 095504 (2004) and Ping et al., “Solid iron compressed up to 560 GPa,” Phys. Rev. Lett. 111, 065501 (2013), whereas more extreme temperatures are measured using x-ray Thomson scattering or pyrometry. This manuscript will review the scientific motivations, experimental techniques, and the regimes that can be probed for the study of materials under extreme HED conditions.

Effect of Irradiation with Si+ Ions on Phase Transformations in Ti–Al System during Thermal Annealing

The article deals with the effect of irradiation with Si+ ions on phase transformations in the Ti–Al system during thermal annealing. An aluminum film with a thickness of 500 nm was deposited on VT1-00 titanium samples by magnetron sputtering, followed by ion implantation. Samples before and after irradiation with Si ions were annealed in a vacuum of 10−4 Pa in the temperature range 600–1000 °C. It was established that ion implantation reduces the dissolution of Al in α-Ti with the formation of titanium silicides (TiSi2, Ti5Si3) and stabilizes aluminide phases Ti3Al rich in aluminum. As a result, a composite structure based on titanium silicide/aluminide was obtained on the surface of the sample synthesized by complex treatment: deposition, irradiation with Si+, and thermal annealing at the near-surface layers. The formation of the phase-structural state of the implanted layers is associated with the displacement of atoms of the crystal lattice, a result that is reflected in an increase in the size of the crystal lattice and a decrease in microdistortion of the lattice. The opposite effect is observed with increasing temperature. This fact is explained by the relaxation of unstable large grains with an excess of internal energies. At the annealing temperature of 900–1000 °C, a significant increase in microhardness was observed due to silicide phases.

Deformation behavior and phase transformation of nanotwinned Al/Ti multilayers

Nanotwinned Al/Ti multilayers have exhibited size-dependent microstructure evolution and high strength. However, their deformation mechanisms are less well understood. In this work, we investigated the deformation mechanisms of nanotwinned Al/Ti multilayers with FCC/HCP layer interfaces by using in situ micropillar compression tests. Nanotwinned Al/Ti multilayers exhibit compressive strength up to 2.4 GPa and good work hardening capability. Post-compression TEM analyses reveal high-density stacking faults and the HCP-to-FCC phase transformations in Ti. Molecular dynamics simulations elucidate the mechanisms of deformation induced phase transformation in Ti and the influence of collective movement of partial dislocations on the deformability of Al/Ti multilayers.

Interfacial phenomena during ultrasonic welding of ultra-low-carbon steel and pure Ti

Changes in bonding strength and interfacial formation during the ultrasonic welding (USW) of steel and Ti were investigated. It was found that when the temperature of the joint was increased above 600 °C during USW, the bonding strength increased drastically and resulted in base-metal fracture. Interfacial, fracture, compositional, and crystallographical analyses revealed that phase transformation in Ti from α (hexagonal close-packed: HCP) to β (body-centered cubic: BCC) led to improved deformability at elevated temperatures, which promotes extensive bonding formation through the elimination of gaps near the bonding interface. Through this process, joints with good bonding and strength can be obtained.

Interfacial Phenomena During Ultrasonic Welding of Ultra-Low-Carbon Steel and Pure Ti

Changes in bonding strength and interfacial formation during the ultrasonic welding (USW) of steel and Ti were investigated. It was found that when the temperature of the joint was increased above 600 oC during USW, the bonding strength increased drastically and resulted in base-metal fracture. Interfacial, fracture, compositional, and crystallographical analyses revealed that phase transformation in Ti from 𝛼 (hexagonal close-packed: HCP) to β (body-centered cubic: BCC) led to improved deformability at elevated temperatures, which promotes extensive bonding formation through the elimination of gaps near the bonding interface. Through this process, joints with good bonding and strength can be obtained.

Electropulsing-Induced α to β Phase Transformation of Ti–6Al–4V

AbstractElectrically assisted forming (EAF) has been increasingly utilized as an effective auxiliary processing technology to improve the formability of hard-to-deform metals. Previous works have revealed that the phase transformation of titanium alloys subjected to electropulsing treatment (EPT) can occur at a lower temperature and in a remarkably shorter time compared with those subjected to the traditional heating treatment (THT). However, an in-depth experimental verification and further analysis is still missing so far. Therefore, to characterize the specific effects of EPT on α → β transformation process, both EPT and THT experiments were conducted on Ti–6Al–4V sheet specimens. After that, a calculation method based on the analysis of optical microscopic (OM) metallographs was developed to characterize the amount of phase transformation in EPT and THT. According to the results, it was found that the pulse current can significantly reduce the phase transus temperature and accelerate the transformation process in EPT compared with that in THT. Furthermore, the specific effects of EPT on transus temperature and phase transformation rate were investigated in detail. Based on that, the transformation kinetics of the electropulsing-induced α → β phase transformation was also analyzed using the Johnson–Mehl–Avrami model. It is revealed that the activation energies of both nucleation and growth of phase transformation are reduced by electric current. Hence, the phase transformation can start at a lower temperature and with a higher rate in EPT. The mechanism behind the effects was also discussed in detail in the present work.

Structure Evolution, Elastic and Electronic Properties of Pt‐Doped Ti Alloy under Pressure

The hexagonal closest packed (hcp) to face‐centered cube (fcc) phase transformation of Ti(Pt) alloy with phase transformation pressure of ≈36.8 GPa at 0 K is found using the first‐principle method. The relatively low phase transformation pressure of hcp to fcc structure for Ti(Pt) alloy may be caused by the foregoing tetragonal isostructural transition of fcc Ti(Pt) alloy at ≈32 GPa. Accompanied with the tetragonal isostructural transition, the unit‐cell parameters (a, c) and volume of fcc Ti(Pt) alloy are discontinuous. The axial ratio (c/a) changes from ≈0.471 to ≈0.338; a axis has anomalously negative compressibility, c axis has positive compressibility. Anomalous elastic properties between 32 and 40 GPa, such as the extremely irregular elastic stability factor F and elastic anisotropy index AU, indicate the weak mechanical stability of fcc Ti(Pt) alloy in the phase transformation region. The total density of states, Mulliken population analysis, and difference of charge density indicate that the Pt–Ti interatomic interactions weaken with increasing pressure.

Solid phase transformation of Ti–6.6Al–3.4Mo alloy induced by electroshocking treatment

The effect of high-density electroshocking on the solid phase transformation of Ti–6.6Al–3.4Mo alloy was studied in this paper. The microstructure at the same position of the specimens before and after electroshocking treatment with different parameters was characterized by the optical microscopy and scanning electron microscopy. The experimental results reveal that the refined and spheroidized primary α phase can be obtained under the appropriate treatment condition. Meanwhile, the fine orthorhombic α″ phase at submicron was discovered by the X-ray diffraction and transmission electron microscopy in the transformation products. A physical model was established to analyze non-uniform current distribution at microscale and to explain the spheroidization mechanism. The phase transition process was interpreted from the point of view of thermodynamics.

Size dependent strengthening in high strength nanotwinned Al/Ti multilayers

Mechanical behavior of metallic multilayers has been intensively investigated. Here we report on the study of magnetron-sputtered highly textured Al/Ti multilayer films with various individual layer thicknesses (h = 1–90 nm). The hardness of Al/Ti multilayers increases monotonically with decreasing layer thickness without softening and exceeds 7 GPa, making it one of the strongest light-weight multilayer systems reported to date. High-resolution transmission electron microscopy and X-ray diffraction pole figure analyses confirm the formation of high-density nanotwins and 9R phases in Al layers. The density of nanotwins and stacking faults scales inversely with individual layer thickness. In addition, there is an HCP-to-FCC phase transformation of Ti when h ≤ 4.5 nm. The high strength of Al/Ti multilayers primarily originates from incoherent layer interfaces, high-density twin boundaries, as well as stacking faults.

Strain-induced α-to-β phase transformation during hot compression in Ti−5Al−5Mo−5V−1Cr−1Fe alloy

Abstract The dynamic phase transformation of Ti−5Al−5Mo−5V−1Cr−1Fe alloy during hot compression below the β transus temperature was investigated. Strain-induced α-to-β transformation is observed in the samples compressed at 0−100 K below the β transus temperature. The deformation stored energy by compression provides a significant driving force for the α-to-β phase transformation. The re-distribution of the solute elements induced by defects during deformation promotes the occurrence of dynamic transformation. Orientation dependence for the α-to-β phase transformation promotion is observed between {100}-orientated grains and {111}-orientated grains. Incomplete recovery in {111}-orientated grains would create a large amount of diffusion channels, which is in favor of the α-to-β transformation. The effects of reduction ratio and strain rate on the dynamic phase transformation were also investigated.

Numerical methods for microstructural evolutions in laser additive manufacturing

Different methods are developed for simulation of microstructural evolution in metals and alloys subject to Laser Additive Manufacturing. The Monte Carlo (MC) method is combined with a new proposed two scales strategy for simulation of the solidification and the subsequent solid-state phase transformation in Ti–6Al–4V.The Cellular Automaton (CA) method shows its higher efficiency in comparison with MC method. Moore neighbor type with energy barrier is recommended for the CA simulation of grain growth of Al 6061. The phase field (PF) model shows that different temperature gradients lead to different columnar grains in different layers of Ti–32wt.%Nb. The computed structures are related to experimental observations.

Response of Ti microstructure in mechanical and laser forming processes

Microstructural deformation mechanisms present during three different forming processes in commercially pure Ti were analysed. Room temperature mechanical forming, laser beam forming and a combination of these two processes were applied to thick metal plates in order to achieve the same final shape. An electron backscatter diffraction technique was used to study the plate microstructure before and after applying the forming processes. Substantial differences among the main deformation mechanisms were clearly detected. In pure mechanical forming at room temperature, mechanical twinning predominates in both compression and tensile areas. A dislocation slip mechanism inside the compression and tensile area is characteristic of the pure laser forming process. Forming processes which subsequently combine the laser and mechanical approaches result in a combination of twinning and dislocation mechanisms. The Schmid factor at an individual grain level, the local temperature and the strain rate are factors that determine which deformation mechanism will prevail at the microscopic level. The final microstructures obtained after the different forming processes were applied are discussed from the point of view of their influence on the performance of the resulting formed product. The observations suggest that phase transformation in Ti is an additional microstructural factor that has to be considered during laser forming.

Simulation Kinetics of Austenitic Phase Transformation in Ti+Nb Stabilized IF and Microalloyed Steels

In the present study, the influence of cooling rates (low to ultrafast) on diffusion controlled and displacive transformation of Ti-Nb IF and microalloyed steels has been thoroughly investigated. Mechanisms of nucleation and formation of non-equiaxed ferrite morphologies (i.e., acicular ferrite and bainitic ferrite) have been analyzed in details. The continuous cooling transformation behavior has been studied in a thermomechanical simulator (Gleeble 3800) using the cooling rates of 1-150 °C/s. On the basis of the dilatometric analysis of each cooling rate, continuous cooling transformation (CCT) diagrams have been constructed for both the steels to correlate the microstructural features at each cooling rate in different critical zones. In the case of the IF steel, massive ferrite grains along with granular bainite structures have been developed at cooling rates > 120 °C/s. On the other hand, a mixture of lath bainitic and lath martensite structures has been formed at a cooling rate of ~ 80 °C/s in the microalloyed steel. A strong dependence of the cooling rates and C content on the microstructures and mechanical properties has been established. The steel samples that were fast cooled to a mixture of bainite ferrite and martensite showed a significant improvement of impact toughness and hardness (157 J, for IF steel and 174 J for microalloyed steel) as compared to that of the as-received specimens (133 J for IF steel and 116 J for microalloyed steel). Thus, it can be concluded that the hardness and impact toughness properties are correlated well with the microstructural constituents as indicated by the CCT diagram. Transformation mechanisms and kinetics of austenitic transformation to different phase morphologies at various cooling rates have been discussed in details to correlate microstructural evolution and mechanical properties.

First-principles study of the α-ω phase transformation in Ti and Zr coupled to slip modes

We present first-principles density functional theory calculations to study the α-ω phase transformation in Ti and Zr and its coupling to slip modes of the two phases. We first investigate the relative energetics of all possible slip systems in the α and ω phases to predict the dominant slip system that is activated during a plastic deformation under an arbitrary load. Using this and the crystallographic orientation relationships between α and ω phases, we construct low energy α/ω interfaces and study the energetics of the slip system at the interface between α and ω to compare to the slip systems in the bulk phases. We find that for a particular crystallographic orientation relationship, where (basal)α∥(prismatic-II)ω, and [a]α∥[c]ω, the slip at the interface is preferred compared to its bulk counterparts. This implies that the plastically deformed α/ω phase with this orientation relationship prefers to retain the interface (or coexisting phases) than transforming back to the pure phase after unloading. This is consistent with the observation that the ω-phase is retained in samples loaded in flyer plate experiments or under high-pressure torsion. Furthermore, calculation of the energy barrier for α to ω phase transformation as a function of glide at the α/ω interface shows significant coupling between the α-ω phase transformation and slip modes in Ti and Zr.