Dual-phase Titanium Alloys Research Articles

A CPFEM based study to understand the void growth in high strength dual-phase titanium alloy (Ti-10V-2Fe-3Al)

High strength titanium alloys are generally used in widespread applications ranging over, but not limited to biomedical, aerospace, automotive, marine, oil and gas, and energy. Besides other manufacturing processes, forming is one of the common manufacturing process used to produce components out of these alloys. Forming processes generally involve significant plastic deformation of material under complex multiaxial loading conditions. Titanium alloys undergo considerable plastic deformation before failure while later is governed by the mechanisms of void nucleation, growth and coalescence. A number of titanium alloys used for high strength applications are multiphase alloys having α and β phases. It has been reported in the past that the voids tend to nucleate on the phase boundaries. This study is focused on understanding the growth of the nucleated voids at two selected locations in a dual phase titanium alloy (Ti-10V-2Fe-3Al); globular α phase (hexagonal closed pack, HCP) and at the interface of lamellar α and β phases (α - HCP and β – body centred cubic, BCC). This is one of the very few 3D representative volume element (RVE) study of void growth in single crystal titanium (HCP), carried out using crystal plasticity finite element modelling (CPFEM) at higher triaxialities (ranging 1/3-3) and the first one on the interface of bicrystals with different crystal symmetry. The effects of initial porosity, crystal orientation and the Lode parameter on void growth in single crystal (α-HCP) has been studied and it is found that they affects void growth considerably. An effort has been made to explain the physics behind it. In the second part, growth in a void at the interface of two distinct single crystals (α - HCP and β –BCC) was studied. The effects of Burgers orientation relationship (BOR) variant of the two phases, initial porosity, and phase boundary inclination (PBI) on void growth is investigated. It is found that the PBI has a very strong impact on the void growth. The effect of initial porosity is similar to the void growth in single crystals. Choice of BOR variant affected the void growth in moderate triaxialities.

Effect of strain distribution on the evolution of α phase and texture for dual-phase titanium alloy during multi-pass forging process

Abstract The evolution of microstructure and texture of Ti-6Al-4V alloy during the multi-pass forging process were investigated in the present work. By using the integrated approaches of X-ray diffraction (XRD) and electron backscatter diffraction (EBSD), the effect of strain distribution on texture evolution was discussed. Results show that the strain distribution has a significant effect on morphology and volume fraction of primary α phase. The microstructural homogeneity of ingot is enhanced after multi-pass forging. With the progressing of forging passes, the volume fraction of αp increases remarkably and the intensity of texture components decreases gradually. Besides, the waved-like type dislocations were found be activated preferentially during first pass forging process, and then + type dislocations were activated with the progressing of forging passes. These results ultimately contribute to understanding of the microstructure evolution behavior, favoring development of microstructure controlling technologies for dual-phase titanium alloys.

The role of β-titanium ligaments in the deformation of dual phase titanium alloys

Multiphase titanium alloys are critical materials in high value engineering components, for instance in aero engines. Microstructural complexity is exploited through interface engineering during mechanical processing to realise significant improvements in fatigue and fracture resistance and strength. In this work, we explore the role of select interfaces using in-situ micromechanical testing with concurrent observations from high angular resolution electron backscatter diffraction (HR-EBSD). Our results are supported with post mortem transmission electron microscopy (TEM). Using micro-pillar compression, we performed in-depth analysis of the role of select β-titanium (body centred cubic) ligaments which separate neighbouring α-titanium (hexagonal close packed) regions and inhibit the dislocation motion and impact strength during mechanical deformation. These results shed light on the strengthening mechanisms and those that can lead to strain localisation during fatigue and failure.

Effect of Temperature and Cooling Rates on the α+β Morphology of Ti-6Al-4V Alloy

Ti-6Al-4V is a dual phase (α+β) titanium alloy widely used in aerospace industry. Mechanical properties of a component strongly depend upon its microstructure and morphology. Desired mechanical properties can be achieved by the development of appropriate microstructure with the help of different heat treatments and deformation. In the present work, different microstructures obtained through different heat treatment processes are studied and presented. A variety of morphologies of α+β microstructure are obtained by heating above and below β transus temperature and cooling at different rates i.e. water quench, air cool and furnace cool. Heating above β transus and cooling with different rates results into lamellar structure whereas a duplex microstructure is obtained after heating below β transus and cooling with different rates. A lamellar structure with blocky acicular α is obtained after heating above β transus followed by furnace cool up to temperature below β transus and cooling with different rates. It is observed that cooling rate has a significant effect on hardness of heat treated sample. Faster cooling results in higher hardness.

Rapid electron backscatter diffraction mapping: Painting by numbers

Microstructure characterisation has been greatly enhanced through the use of electron backscatter diffraction (EBSD), where rich maps are generated through analysis of the crystal phase and orientation in the scanning electron microscope (SEM). Conventional EBSD analysis involves raster scanning of the electron beam and serial analysis of each diffraction pattern in turn. For grain shape, crystallographic texture, and microstructure analysis this can be inefficient. In this work, we present Rapid EBSD, a data fusion approach combining forescatter electron (FSE) imaging with static sparse sampling of EBSD patterns. We segment the FSE image into regions of similar colour (i.e. phase and crystal orientation) and then collect representative EBSD data for each segmented region. This enables microstructural assessment to be performed at the spatial resolution of the (fast) FSE imaging whilst including orientation and phase information from EBSD analysis of representative points. We demonstrate the Rapid EBSD technique on samples of a cobalt based superalloy and a strained dual phase titanium alloy, comparing the results with conventional analysis. Rapid EBSD is advantageous for assessing grain size distributions in time-limited experiments.

Stress Wave and Residual Stress Characteristics of TC17 Titanium Alloy Subjected to Warm Laser Shock Peening

Compared with Laser Shock Peening (LSP), Warm Laser Shock Peening (WLSP) can obtain higher residual stress, hardness, dislocation density, and finer grain. In this paper, the dual phase titanium alloy TC17 is studied. WLSP tests of TC17 specimen under different temperatures are carried out. The surface residual stress of the specimen is measured by XRD method, and residual stress distribution of the specimen is also numerically analyzed. It is found that the numerical results are in good agreement with the experimental results. Numerical results further show that the peak residual stress (−340 to −480 MPa) of the specimen increases with increased temperature. The stress wave gradually decreases along the thickness direction during the impact process, but it abruptly increases at the bottom of the specimen because of reflection. The equivalent plastic strain of the specimen increases with increased temperature. The dynamic yield strength (−1831.1 to −1468 MPa) of the material is calculated by deducing, which decreases with increased temperature. When the temperature is higher than 750 °C, residual stress hole phenomenon occurs, and the residual stress decreases with a further increment of the temperature. In addition, the influence of laser power, impact times, and overlapping rate on residual stress is also studied.

Microstructural effects on strain rate and dwell sensitivity in dual-phase titanium alloys

In this study, stress relaxation tests are performed to determine and compare the strain rate sensitivity of different α−β titanium alloy microstructures using discrete dislocation plasticity (DDP) and crystal plasticity finite element (CPFE) simulations. The anisotropic α and β phase properties of alloy Ti-6242 are explicitly included in both the thermally-activated DDP and CPFE models together with direct dislocation penetration across material-interfaces in the DDP model. Equiaxed pure α, colony, Widmanstatten and basketweave microstructures are simulated together with an analysis of the effect of α grain size and dislocation penetration on rate sensitivity. It is demonstrated that alloy morphology and texture significantly influence microstructural material rate sensitivity in agreement with experimental evidence in the literature, whereas dislocation penetration is found not to be as significant as previously considered for small deformations. The mechanistic cause of these effects is argued to be changes in dislocation mean free path and the total propensity for plastic slip in the specimen. Comparing DDP results with corresponding CPFE simulations, it is shown that discrete aspects of slip and hardening mechanisms have to be accounted for to capture experimentally observed rate sensitivity. Finally, the dwell sensitivity in a polycrystalline dual-phase titanium alloy specimen is shown to be strongly dependent on its microstructure.

Continuous Dynamic Recrystallization in Dual-Phase Titanium Alloy in Superplasticity

A new class of superplastic titanium alloy, Ti–4.5Al–2.5Cr–1.2Fe–0.1C–0.3Cu–0.3Ni, was deformed at 1073 K with strain rates of 1×10−4–1×10−1 s−1, and microstructures in the condition between superplastic regions II and III (= 1×10−2 s−1) were observed using scanning electron microscope and electron back-scattered diffraction. Continuous dynamic recrystallization was observed, resulting in grain refinement both in α and β phases. The grain size decreased significantly in α phase at the early stage of the deformation and in β phase at the later stage. In the recrystallized microstructure, the major sub-boundaries formed perpendicularly to slip directions <11−20> in α phase and parallel to slip planes {110} in β phase, which might be caused by the difference in the symmetry of the crystal structures.

Effects of the phase interface on initial spallation damage nucleation and evolution in dual phase titanium alloy

The Ti-6Al-4V dual phase alloy samples were dynamic loaded by one-stage light gas gun experiment and samples with initial spallation were softly recovered. During the loading experiment, the velocity of free surface particles was measured by photonic doppler velocimetry (PDV). The effect of α/β phase interface on nucleation, growth, and coalescence of dynamic damage in Ti-6Al-4V were investigated by 2-D or 3-D testing techniques, such as optical microscopy(OM), x-ray computer tomography(XRCT), and electron backscattered diffraction(EBSD). The results showed that the majority of voids were nucleated within α phases, rather than on the α/β phase interface as predicted by quasi-static damage theory. Due to the effects of reflection and transmission of shock wave at the phase interface, a tensile pulse would be formed within α phase when the shock wave transmit from α phase with high impedance to β phase with low impedance. When this tensile pulse was large enough, voids would be formed within α phase. The analyses of OM and XRCT indicated that the voids at the beginning of nucleation were nearly spherical, then grew up along the direction of 45° with the shock loading direction, and finally the rod-shaped voids were formed. Besides, the voids were not randomly nucleated within α phase, and the EBSD analysis showed that the voids were mainly nucleated at grain boundary triple points which composed of grains with large difference of Taylor Factor(TF) value within α phase. This is because the difference of plastic deformation capacity of this grains is larger, and it is easier to produce stress concentration. Thus these sites became the prior nucleation position of voids.

Slip transfer across phase boundaries in dual phase titanium alloys and the effect on strain rate sensitivity

Dislocation transmission through α/β phase boundaries in titanium alloys is studied using integrated crystal plasticity (CP) and discrete dislocation plasticity (DDP) modelling techniques, combined with experimental micro-pillar compression test results. Direct dislocation transmission together with the nucleation of new dislocations ahead of a pile-up at an α/β interface, termed indirect slip transfer, are both assessed and their role in controlling microstructure-dependent strain rate sensitivity considered. A critical shear stress criterion for direct slip transfer across an α/β interface in Ti-6242 has been established by capturing the local slip penetration through the phase boundary using CP and DDP comparisons with experimental two phase micro-pillar compression. The competition between direct and indirect slip transfer has been investigated using a single Frank-Read source DDP model. Direct slip transfer is found to occur only under specific conditions which have been quantified. The strain rate sensitivity of dual phase titanium alloys is demonstrated to depend on average pile-up size which is significantly influenced by α/β morphology.

The relationship between accumulation of GN dislocation near the α-β interfaces and the yielding conditions of β phase in dual-phase titanium alloys

The relationship between accumulation of GN dislocation near the α-β interfaces and the yielding conditions of β phase in dual-phase titanium alloys

Study on stress-wave propagation and residual stress distribution of Ti-17 titanium alloy by laser shock peening

Laser shock peening has advantages of less restriction, better reinforcement, stronger controllability, and so on, which could greatly improve the fatigue properties of the material compared with traditional processes. In this paper, dual phase titanium alloy Ti-17 has been studied, and numerical simulation has been taken to analyze the stress-wave propagation rules in Ti-17 titanium alloy plates with different thicknesses. In addition, the distribution of residual stress which is obtained by experiments is used to verify the accuracy of the above rules. Furthermore, the variation rules of fatigue characteristics of plates with different thicknesses under the same laser shock peening parameters are analyzed. The results show that due to the reflection of the stress-wave, the final residual stress and fracture toughness of the specimen decrease, resulting in an increase of the crack propagation rate and crack length. Since thinner specimens have a higher frequency of stress-wave reflection, they show greater effects.

Effect of microstructure on the elasto-viscoplastic deformation of dual phase titanium structures

The present study is devoted to the creation of a process–structure–property database for dual phase titanium alloys, through a synthetic microstructure generation method and a mesh-free fast Fourier transform based micromechanical model that operates on a discretized image of the microstructure. A sensitivity analysis is performed as a precursor to determine the statistically representative volume element size for creating 3D synthetic microstructures based on additively manufactured Ti–6Al–4V characteristics, which are further modified to expand the database for features of interest, e.g., lath thickness. Sets of titanium hardening parameters are extracted from literature, and The relative effect of the chosen microstructural features is quantified through comparisons of average and local field distributions.

The effect of the beta phase on the micromechanical response of dual-phase titanium alloys

This paper investigates the role of beta phase on the micro-mechanical behaviour of dual-phase titanium alloys, with particular emphasis on the phenomenon of cold dwell fatigue, which occurs in such alloys under room temperature conditions. A strain gradient crystal plasticity model is developed and calibrated against micro-pillar compression test data for a dual-phase alpha-beta specimen. The effects of key microstructural variables, such as relative beta lath orientation, on the micromechanical response of idealised alpha-beta colony microstructures are shown to be consistent with previously-published test data. A polycrystal study on the effects of the calibrated alpha-beta crystal plasticity model on the local micromechanical variables controlling cold dwell fatigue is presented. The presence of the alpha-beta phase is predicted to increase dwell fatigue resistance compared to a pure alpha phase microstructure.

The formation of ordered clusters in Ti–7Al and Ti–6Al–4V

TiAl alloys can suffer from a chemical decomposition on ageing around 500°C or on air cooling. At long ageing times this results in the formation of α2 (Ti3Al) precipitates. At reduced times or elevated temperatures, diffuse electron or neutron diffraction peaks can be observed, which has been termed ‘short range ordering’ (SRO). Here, we present correlative transmission electron microscopy (TEM) and atom probe tomography (APT) results showing that the reaction proceeds through the formation of ordered Al-rich precipitate clusters. Notably, Al–Al clustering could be observed well before the appearance of distinct precipitates in the TEM. In addition, the V-containing α phase of Ti–6Al–4V formed ordered clusters much faster than in binary Ti–7Al. This implies that the ternary addition of β stabilisers exacerbates the problem of α2 precipitate formation in commercial dual phase titanium alloys.

Dual phase titanium alloy hot forging process design: experiments and numerical modeling

Titanium alloys are considered desirable materials when both good mechanical properties and weight reduction are required at the same time. This class of materials is widely used in those fields (aeronautics, aerospace) in which common steels and light-weight materials, e.g., aluminum alloys, are not able to satisfy all operative service conditions. During the last decade, forging of titanium alloys has attracted greater attention from both industrial and scientific/academic researchers because of their potential in providing a near net shaped part with minimal need for machining. In this paper, a numerical model of the forging sequences for a Ti-6Al-4V titanium alloy aerospace component is presented. The model was tested and validated against experimental forgings. The model is then applied to predict loads final microstructure and defects of an aeronautical component. In addition to metal flow and die stresses, microstructural transformations (α and β phases) are considered for the determination of proper process parameters. It is found that transformation from α/β to β phase during forging and reverse transformations in post-forge cooling needs to be considered in the computational model for reasonable prediction of forging loads and product properties.

Metallurgical Evolutions in Hot Forging of Dual Phase Titanium Alloys: Numerical Simulation and Experiments

Titanium forging has been encountering a growing interest in the scientific and industrial communities because of the distinct advantages it provides with respect to machining, in terms of both mechanical properties of the product and material waste, thus significantly reducing the Buy to Fly ratio. In the paper, a numerical FE model, based on a tri-coupled approach and able to predict the microstructural evolutions of the workpiece during the process, is developed and set up. Calculated results are compared to experiments for a few industrial case studies. The final phases distribution in the forged parts is experimentally measured and compared to the FE model output finding satisfying overlapping.

The integrated influence on hot deformation of dual-phase titanium alloys incorporating dynamic recrystallization evolution and α/β phase transformation

The integrated influence of dynamic recrystallization evolution (DRX) and α/β phase transformation on hot deformation of dual-phase titanium alloys has been investigated in the present paper. Kocks–Mecking relation for dislocation density variation has been developed. Corresponding constitutive relationship and work-hardening behavior have been characterized under validation by isothermal compression of Ti55511 titanium alloy. α/β phase transformation determines the work-hardening basis and DRX evolution influences the softening effect of work-hardening behavior.

Dynamic softening behavior of TC18 titanium alloy during hot deformation

Abstract Dynamic softening behavior of TC18 titanium alloy have been investigated in this paper based on isothermal compression experiment. Strain-rate sensitivity coefficient m and deformation activation energy Qdef under different deformation conditions have been calculated through Arrhenius constitutive model. The dependence of strain-rate sensitivity and activation energy on deformation parameters has been analyzed from the perspective of internal variables. A function of deformation temperature has been proposed to estimate the self-diffusion activation energy Qself for dual-phase titanium alloys. The dependence of DRX/DRV on deformation parameters has been demonstrated by novel methods. From comparisons between self-diffusion and deformation activation energy, deformation undergoing DRX has been identified with low strain rates and elevated temperatures. Finally, microstructure observation has been taken carried out to verify the applicability of the new methods. Meanwhile, Poliak–Jonas criterion and power dissipation efficiency distribution has been employed to discuss the dependence of softening behavior on deformation condition.

Phase evolution in hot forging of dual phase titanium alloys: Experiments and numerical analysis

Modern aeronautical and aerospace industries must face the demanding challenge of reducing operational consumption and production costs coming from materials and labor. Current trend of engineering is oriented to meet both requirements increasing the use of materials characterized by high specific resistance as titanium alloys. Hot forging can be used to reduce the production costs of titanium components: forging in closed dies of billets or semi-finished forms, at different temperatures above or below the β-transus temperature, allows the production of complex shapes with limited amount of edge trim removal and machining rework after forging. Unfortunately, as far as Ti–6Al–4V titanium alloy is regarded, several material peculiarities have to be properly taken into account in order to produce defect free Ti–6Al–4V alloy components. In the paper, an experimental and numerical campaign, focused on a typical case study for aeronautical engines, is carried out. The aim of this research is to investigate the process mechanics and the causes for the final microstructure observed through micrographic analysis. Once set-up and tested against experimental loads, the model was used to predict phases distributions after both forging and cool down.