Phase Transformation In Titanium Research Articles

Microstructure refinement induced by elastic compressive stress in Ti-6Al-4V alloy

The β→α transformation is a crucial and fundamental phase transformation in titanium and its alloys. This work investigated the effect of elastic compressive stress (ECS) on the microstructure and mechanical property during cooling from β phase region to α + β phase region in Ti-6Al-4V alloy. Results indicated that ECS in this process results in significant refinement and increased aspect ratios of α lamellae (αL) and α colonies (αC). These changes are attributed to the enhanced phase transformation driving force to promote nucleation of α lamellae. The ECS-cooling induced alloy exhibits higher hardness than the stress-free counterpart, mainly resulting from the refined microstructure. This work provides a potential method to tailor the microstructure and improve the mechanical properties of Ti alloys.

Molecular dynamics simulation of the behavior of titanium under high-speed deformation

We present molecular dynamics simulation to study the α–ω phase transformation in titanium under different conditions simulating high-energy impacts. We employed the interatomic potential developed within the N-body method, which predicts the stability of the ω phase and the stacking fault energy in the α phase in excellent agreement with the experimental and theoretical data. The latter is crucial for the correct description of the deformation mechanisms. The dependence of the beginning and mechanism of the α–ω transition process on loading conditions are derived. In particular, at the uniaxial compression along the [0001] direction at 300 K, the ω phase is localized in deformation bands within the α phase, and the α–ω transition is observed at a pressure of more than 3 GPa. With this type of deformation, the residual inclusions of the α phase remain in the ω phase volume. A similar deformation at a temperature of 700 K does not lead to the formation of the ω phase. Meanwhile, at the hydrostatic compression, the α–ω transition is restrained and at a pressure of 20 GPa is not observed. In the case of anisotropic three-axis deformation along the α–ω transition pathways proposed by Trinkle et al at a constant pressure of 20 GPa, the transition mechanism includes the formation of dislocations, followed by the transformation of the regions between the dislocations into the ω phase. The simulation results demonstrate good agreement with the experimental data and confirm the applicability of the employed interatomic potential for simulating the deformation of titanium.

In-situ Experiments to Capture the Evolution of Microstructure During Phase Transformation of Titanium Under Dynamic Loading

Under sufficient stresses, such as during dynamic loading, titanium experiences a phase transformation from hcp alpha phase to hexagonal omega phase. Omega phase is often retained in the microstructure after unloading, and has a strong influence on subsequent mechanical properties. Simulations suggest there are multiple pathways and underlying mechanisms for this transformation. Due to the incredibly short timescales involved, experimental measurements for model validation have been difficult. However, new capabilities at the Advanced Photon Source have enabled diffraction measurements during plate impact experiments to study the evolution of titanium during transformation. These high-rate data allow us to probe the mechanism and kinetics of phase transformations in new ways. Recent results will be presented and compared to post-mortem characterization of soft-recovered shocked specimens. Comparisons are made with previous tests where material was shock-loaded and soft recovered for microstructural analysis. Together these techniques create a consistent picture of material behavior during the shock-induced ff–! phase transformation in titanium.

Complexion-mediated martensitic phase transformation in Titanium

The most efficient way to tune microstructures and mechanical properties of metallic alloys lies in designing and using athermal phase transformations. Examples are shape memory alloys and high strength steels, which together stand for 1,500 million tons annual production. In these materials, martensite formation and mechanical twinning are tuned via composition adjustment for realizing complex microstructures and beneficial mechanical properties. Here we report a new phase transformation that has the potential to widen the application window of Ti alloys, the most important structural material in aerospace design, by nanostructuring them via complexion-mediated transformation. This is a reversible martensitic transformation mechanism that leads to a final nanolaminate structure of α″ (orthorhombic) martensite bounded with planar complexions of athermal ω (a–ω, hexagonal). Both phases are crystallographically related to the parent β (BCC) matrix. As expected from a planar complexion, the a–ω is stable only at the hetero-interface.

Twin boundary activated α → ω phase transformation in titanium under shock compression

The role of grain boundary structures on the shock response of hexagonal-close-packed (hcp) metals is little understood. We use molecular dynamics simulations to investigate deformation mechanisms in shock compressed Ti bicrystals with three types of grain boundary (GB) microstructure. Our results show the shock response of phase Ti polycrystals are influenced by the GB characteristics, i.e., elastic shock wave induced inelastic deformation occurs on both sides of the {101¯2} coherent twin boundaries (CTBs) but only on one-side of the symmetric incoherent twin boundaries (ITB) or {12¯10} tilt grain boundaries regions. In particular, we find that the elastic shock wave can readily trigger the α→ω transformation at {101¯2} CTBs but not the other two GBs, and the α→ω transformation at CTBs leads to considerable wave attenuation (i.e., the elastic precursor decay). Combined with first principle calculations, we find that CTBs can facilitate the overcoming of the energy barrier for the α→ω transformation. Our findings have the potential to influence interface engineering and materials design under extreme conditions.

Characteristics of the allotropic phase transformation in titanium processed by high-pressure torsion using different rotation speeds

An investigation was initiated to examine the effect of rotation speed on the allotropic α to ω-phase transformation in titanium. A grade 2 commercial purity titanium was processed by high-pressure torsion (HPT) at room temperature up to a maximum of 10 turns using a pressure of 5.0GPa and different rotation speeds from 0.5 to 2rpm. It is shown that the allotropic phase transformation occurs during HPT at different rotation speeds but the volume fraction of the ω-phase decreases when the rotation speed is increased. The results indicate that the hardness and strength are decreased by processing the specimens at higher rotation speeds. Additional annealing for 15min at 423 and 473K after HPT processing is also effective in changing the volume fractions of the ω-phase and producing an ω to α reverse phase transformation. The results are interpreted in terms of the temperature rise during HPT processing.

Texture Evolution and Phase Transformation in Titanium Investigated by In-Situ Neutron Diffraction

Time-of-flight neutron diffraction was used to study in-situ texture evolution and the α → β phase transformation in cold-drawn titanium upon continuous heating. The texture changes in the α phase at elevated temperatures upon recrystallization are presented. For the first time, a transient β texture was observed during the α → β transformation, as indicated by the initial rise and the final drop of the {110} β reflection intensity. This unusual observation is explained in terms of competitive growth between inter- and intragranular β allotriomorphs.

Stress, Texture and Phase Transformation in Titanium Thin Films

{111} fiber textured face centered cubic (fcc) titanium has been found to coexist with the {0002} fiber textured hexagonal close packed (hcp) titanium in polycrystalline titanium (Ti) thin films (thickness: 144 nm to 720 nm) deposited on Si (100) substrate by magnetron sputtering. X-ray diffraction investigation confirms that relative phase fraction of such metastable fcc Ti phase decreases with increasing film thickness indicating thickness dependent fcc-hcp phase transformation of titanium. Texture development in hcp Ti phase was due to film microstructure (thickness effect) rather than the phase trans-formation. Diffraction stress analysis (by d-sin2 method) indicates that fcc to hcp phase transformation is also accompanied by the reduction of compressive stress in the hcp Ti phase with increasing film thickness. Strain energy calculations for both phases of titanium indicate that fcc Ti is a more stable phase compared to hcp Ti at relatively low film thickness (144 nm to 432 nm). It has been concluded that film stress favours fcc to hcp phase transformation towards the higher film thickness. Reverse transformation (hcp to fcc) occurs towards the lower film thickness.

Enhanced densification of metal powders by transformation-mismatch plasticity

The densification of titanium powders is investigated in uniaxial die pressing experiments carried out isothermally at 980°C (in the β-field of titanium) and during thermal cycling between 860 and 980°C (about the α/ β phase transformation of titanium). Thermal cycling is found to enhance densification kinetics through the emergence of transformation-mismatch plasticity (the mechanism responsible for transformation superplasticity) as a densification mechanism. The isothermal hot-pressing data compare favorably with existing models of powder densification, and these models are successfully adapted to the case of transformation-mismatch plasticity during thermal cycling. Similar conclusions are reached for the densification of titanium powders containing 1, 5, or 10 vol.% ZrO 2 particles. However, the addition of ZrO 2 hinders densification by dissolving in the titanium matrix during the hot-pressing procedure.

Study of the irradiation-induced α → ω phase transformation in titanium: Kinetics and mechanism

Abstract An atomic-crystallographic mechanism is proposed for the α → ω transformation during irradiation. The mechanism is based on the propagation of a localized displacement wave of the [100]α close-packed rows. From the phonon dispersion curves along (010)α, the displacement wave is decomposed into two transverse optical phonons and one transverse acoustic phonon. The proposed mechanism accounts for the ω morphology and can explain the presence of (210)α planes of diffuse intensity in reciprocal space. The kinetic of the transformation was followed by in-situ length and electrical resistance measurements. An analytical model based on the proposed mechanism enabled us to fit the experimental curves. Finally the kinetic results were compared with those obtained previously for the transformation occurring under an applied static pressure.

Nucleation of the α / γ Massive Phase Transformation in Titanium- 48at% Aluminum

Massive phase transformations exhibit a change in crystal structure without a change in composition. Nucleation and growth of this difTUsionless transformation has been studied in numerous systems including Cu-Zn, Ag-Zn and Ag-Al. There is general agreement that heterogeneous grain boundary nucleation occurs with a rational orientation relationship with at least one of the parent grains, although growth of the massive product can often occur with a partially coherent interface.The typical Ti-48at%Al microstructure is a lamellar structure of hexagonal α2 (DO19 superlattice) and the ordered tetragonal γ phase (Ll0 with c/a = 1.01). Water or ice brine quenching of the high temperature disordered α phase results in a partial massive transformation of a into γ. Electron beam melting has been used to further suppress the equilibrium transformations. However, the current study uses rapid solidification by electromagnetic levitation and twin anvil splat quenching (cooling rates > 106 K/s) to capture earlier stages of the transformation.

Variant selection in α → β → α phase transformation of cold rolled titanium sheet

It is well known that {alpha} {yields} {beta} {yields} {alpha} phase transformation in titanium proceeds with a definite orientation relationship between parent and product phases, i.e., the Burgers` relationship, namely, {l_brace}110{r_brace}{sub {beta}} {parallel} (0001){sub {alpha}} and {sub {beta}} {parallel} {sub {alpha}}. Because of the crystal symmetry, starting with a single component of texture in {alpha} phase, a total of 72 reorientations are possible with equal probability after the double {alpha} {yields} {beta} {yields} {alpha} transformation. According to this assumption, it can be deduced that repeated cycling through the transformation sequence should result in a random texture. In fact, this is, however, not often the case. Some orientation variants are preferentially selected during cyclic {alpha} {yields} {beta} {yields} {alpha} phase transformation in titanium sheet. This has been called variant selection. Although many investigations have been made towards understanding the mechanism involved in the variant selection taking place in cubic crystal metals, there is limited work dealing with the phase transformation in hexagonal close packed (HCP) metals like {alpha}-titanium and its alloys. It is therefore necessary to obtain detailed information on this subject.

Phase transformation in titanium induced by laser heating at high pressure

Phase transformation in titanium induced by laser heating at high pressure

Detection of new co phase morphologies in β-III Ti using transmission electron microscopy

The to phase transformation in titanium and other alloys has been extensively studied1. This phase has an hep structure with three atoms per cell located at (000), (1/3, 2/3, 1/2) and (2/3, 1/3, 1/2). ‘Isothermal’ ω phase precipitates are oriented in the bec β matrix such that [lll]β ‖ [0001]ω and [110]β ‖ω. In fact four varients of such an ω phase exist oriented in all four [111] directions. Isothermal ω phase has been studied as a displacement controlled growth as distinct from another type of ω phase (as quenched) which is produced by a dif fusionless transformation resulting from a very rapid cooling rate.Using transmission electron microscopy (TEM) and small angle neutron scattering (SANS) we have detected and identified two previously unreported morphologies in β-III Ti (Ti-11.0 Mo-6.0 Zr-4.5 Sn-0.15 O2). Fig. 1 shows a TEM micrograph of a conventional ‘isothermal’ ω precipitate (obtained by annealing β-III Ti 20 hours at 400°C) imaged in bright field with its diffraction pattern showing hep spots as well as the brighter spots of the bec matrix. The long axis of the precipitates is oriented along a [111] direction.

Phase transformation in titanium in shock waves

Phase transformation in titanium in shock waves

Thermodynamic Study of the α → β Phase Transformation in Titanium by a Pulse Heating Method.

Measurements of the temperature and energy of the α → β transformation, and the electrical resistivity near and at the transformation point of titanium by means of a subsecond duration pulse heating technique are described. The results yield 1166 K for the transformation temperature and 4170 J · mol-1 for the transformation energy. Electrical resistivity is found to decrease by about 9 percent during the α → β transformation. Estimated inaccuracies of the measured properties are: 7 K for the transformation temperature, 5 percent for the transformation energy, and 2 percent for the electrical resistivity.