Dual-phase Titanium Alloys Research Articles

The microstructure evolution history and the underlying mechanism of an α/β dual phase titanium alloy during elastic-plastic deformation process

In this paper, in situ scanning electron microscope tensile test was carried out on the Ti-4.5Mo-5.1Al-1.8Zr-1.1Sn-2.5Cr-2.9Zn alloy, and the mechanism of grain rotation, shape change and micro-strain redistribution in macroscopic elastic deformation (stage I), macroscopic elastic-plastic transition (stage II) and macroscopic plastic deformation (stage III) stages were quantitatively investigated by electron backscatter diffraction and digital image correlation. In stage I, it was found that 10.1% of the viewing zone underwent grain rotation or shape change, and the micro-strain was mainly distributed in primary α phase (αp) and their intersection. The result of the separately designed elastic loading-unloading tensile test showed that the rotated αp no longer returned to the primary orientation. In stage II, the tensile plastic deformation regions reached 52.9%, and grains in βT regions continued to rotate and began to change shape. In stage III, 84.6% of the viewing zone got into the tensile plastic deformation state. Combined with geometrically necessary dislocation density distribution, it was found that some of the regions in low strain state (0 ≤ ε<1%) actually underwent complex tensile plastic deformation. In addition, the maximum micro-strain of the viewing zone was transferred to βT regions.

Effect of Third-Stage Heat Treatments on Microstructure and Properties of Dual-Phase Titanium Alloy.

Two-phase TC21 titanium alloy samples were solution-treated at 990 °C (β phase zone) and cooled by furnace cooling (FC), air cooling (AC), and water quenching (WQ), respectively. The second solution stage treatment was carried out at 900 °C (α + β phase zone), then aging treatment was performed at 590 °C. The influence of the size and quantity of the α phase on the properties of the sample were studied. The experimental results showed as the cooling rate increased after the first solution stage treatment, wherein the thickness of primary layer α gradually decreased, and the tensile strength and yield strength gradually increased. After the second solution stage treatment, the tensile properties of samples increased due to the quantity of layers α increased. The aging treatment promoted the precipitation of the dispersed α phase and further improved the tensile strength. After the third solution stage treatments, the FC samples with more β-phase had the best comprehensive mechanical properties.

Surface topography in machining Ti alloys for biomedical applications: correlative microscopy approach for qualitative and quantitative analysis

V-free Ti-6Al-7Nb alloy may be an interesting candidate as a substitute to the traditional Ti-6Al-4V alloy on the development of biomedical components. The inspection of surface integrity through digital microscopy techniques shows strong potential for comparative analysis and optimization of manufacturing processes. This work deals with the comparative analysis of turned surfaces of dual-phase (α+β) titanium alloys: Ti-6Al-4V and Ti-6Al-7Nb, under different cutting conditions. Digital image processing and analysis technique has been used to evaluate the volume fraction of phases and their distribution. An innovative methodology for digitizing the surface topography was applied, based on the association of modern microscopy techniques with digital image processing-correlative microscopy. The global outcomes show that Ti-6Al-4V samples presented better homogeneity, with a mean β volume fraction of about 17%, compared to 11% of Ti-6Al-7Nb samples. The combination of higher feed rate and lower velocity produce rougher topography for both alloys, while the topographic formation obtained by the combination of lower feed rate and higher velocity seems smoother. In addition, Ti-6Al-4V alloy presents rougher topography in comparison topography of Ti-6Al-7Nb, under all conditions, probably due to the different phase distribution. The correlative microscopy allowed a correspondence between the cutting conditions and the microstructural properties of both Ti-6Al-4V and Ti-6Al-7Nb alloys, through the analysis of the machined surface.

Hot deformation characteristics and microstructure evolution of Ti–5Al–3Mo–1.5V alloy

A detailed investigation was conducted to study the concurrent effect of temperature and strain rate on the microstructure evolution in Ti–5Al–3Mo–1.5V dual-phase Titanium alloy. By applying varying strain rates from $$10^{-3} $$ to 10 $$\hbox {s}^{-1}$$ between 1098 and 1298 K at an interval of 50 K, isothermal compression characteristics and microstructural changes were recorded. The sizes of globules, concentrated predominantly at the lamellar kinks, were found to be inversely proportional to the strain rate. Further, a dynamic material model was employed to assess and plot the processing map displaying the safe hot working regime. The apparent hot-working activation energy in the $$\alpha +\beta $$ and $$\beta $$ phase field was 636 kJ/mol and 379 kJ/mol, respectively. A higher activation energy than the self-diffusion threshold of the $$\alpha +\beta $$ and $$\beta $$ field was attributed to lamellae breakup and dynamic recrystallization in the respective phase fields. The microstructure analysis and identified softening mechanisms further helped in concluding the safe hot working regime to be 1248 K and 10 $$^{-3}$$ $$\hbox {s}^{-1}$$ .

Deformation behavior of Ti-6Al-4V microstructures under uniaxial loading: Equiaxed Vs. transformed-β microstructures

The dual-phase titanium alloy Ti-6Al-4V can be thermomechanically treated to produce a variety of microstructures to obtain desired mechanical properties. The extreme microstructure morphologies were developed by heat treatment of mill annealed microstructure to equiaxed, and transformed-β microstructures (lamellar) of coarser α-lath and α′-laths (martensite). The uniaxial tensile test shows the highest elongation in the equiaxed microstructure followed by coarse α-lath lamellar, while the α′-lath morphology has the least elongation. The higher ductility in the equiaxed microstructure is due to smaller slip length compared to coarse α-lath lamellar. On the other hand, the poor ductility of the α′-lath is due to premature crack initiation. Both equiaxed and coarse α-lath lamellar microstructures mostly show prismatic and pyramidal slip <a>/ <c + a>. In addition to this, though less prevalent, these microstructures exhibit twinning as the other deformation mechanism, which is uncommon in Ti-6Al-4V. The twin boundary interaction with the grain boundary led to the damage nucleation, causing the intra-grain crack. In the coarse lath lamellar (furnace cooled) morphology, the crack was mostly observed at the junction of α-colonies as well at the α-layer grain boundary/α-colony interface due to strain localization. However, in the lamellar (water quenched), the primary α′-lath shows the void nucleation at the junction of the primary and secondary-α′ interface, which coalesce to form microcrack and further grow instantly to fracture. The void nucleation is generally observed in a basal orientation along the primary α′-lath, oriented 45° to the loading axis, and having dominant pyramidal slip (<a>/<c + a>). Thus, the deformation mechanisms slip, twin, and fracture depend on the microstructure morphology in Ti-6Al-4V.

Numerical simulations of friction stir welding of dual phase titanium alloy for aerospace applications

Dual phase titanium alloy is mainly used in aircraft industries due to low density, greater strength and superior thermal properties. However, titanium alloy is highly reactive to all materials and easy immersion of interstitial constituent leads to material embrittlement, so it is difficult to process and weld. So experimental investigation of friction stir welding (FSW) process is expensive. To overcome these, in the present work a finite element simulation has been developed in ABAQUS software for the FSW process to comprehend the process physics and to predict the mechanical properties of FSW component. The simulation has been carried out by changing the parameters and the performed simulation results such as stress strain plots, percentage elongation, yield and ultimate tensile strength has been verified and calibrated with experiment results. The FSW welding parameters have been optimized using Genetic algorithm to get better weld quality. The fitness function for genetic algorithm is evaluated using regression analysis. The optimized value of rotational and transverse speed to achieve maximum yield and ultimate tensile strength are 1000 rpm and 4 in./min respectively. The optimized results have also been compared with the Taguchi method.

A novel strategy for developing α + β dual-phase titanium alloys with low Young’s modulus and high yield strength

In this study, a novel strategy for developing α + β dual-phase titanium alloys with low Young’s modulus and high yield strength was proposed, and a Ti-15Nb-5Zr-4Sn-1Fe alloy was developed through theoretical composition design and microstructure manipulation. After hot-rolling and subsequent annealing, a high volume fraction of ultrafine grained α phase embedded in metastable β-matrix was formed in the microstructure as intended. Consequently, this alloy exhibits both low Young’s modulus (61 GPa) and high yield strength (912 MPa). The experimental results prove that the proposed strategy is appropriate for developing titanium alloys with superior yield strength-to-modulus ratio than those of conventional metallic biomedical materials. Present study might shed light on the research and development of advanced biomedical titanium alloys with low Young’s modulus and high yield strength.

Modeling Ti–6Al–4V using crystal plasticity, calibrated with multi-scale experiments, to understand the effect of the orientation and morphology of the α and β phases on time dependent cyclic loading

Classically, crystal plasticity modeling has used a range of constitutive equations, in which the incorporation of additional physics-based relationships typically results in additional model parameters. These additional parameters need to be reliably calibrated, which often necessitates the use of a range of experimental data acquired at multiple length scales. In this work, a crystal plasticity based finite element (CPFE) model for a dual-phase Titanium alloy, Ti–6Al–4V, is developed. The α and β phases of the microstructure are explicitly modeled. The model is calibrated using a systematic optimization routine and experimental data that consist of macroscopic stress-strain curves coupled with lattice strains on different crystallographic planes for the two phases. These experimental data were obtained from in situ high energy X-ray diffraction experiments for multiple material pedigrees, with varying crystallographic orientation distribution and β volume fractions. Depending on the thermomechanical-processing route and the heat treatment used to manufacture the alloy, Ti–6Al–4V can exist in a wide number of microstructural forms, which often results in the α and β phases either having well aligned slip systems (following the Burgers orientation relationship (BOR)) or possessing no alignment of the slip systems across the interphase boundary (not following the BOR). In this study, the fully-calibrated CPFE model is used to gain a comprehensive understanding of the deformation behavior of Ti–6Al–4V, specifically, the effect of microstructures that follow the BOR (or not) on time-dependent cyclic loading (including the effects of dwell hold times).

Study on crack growth and fatigue life in TC6 titanium alloy by integrated phase proportion and grain size modeling

In this paper, a theoretical model is developed to reveal the crack growth and fatigue life in polycrystalline alloys with dual-phase microstructure. The present quantitative analysis proves that the grain size and phase proportion can efficaciously affect the crack growth and fatigue life. The dislocations emitted from the crack tip and penetrated three types of grain boundaries (GBs) under cyclic shear stress. According the different boundaries, we obtained the calculation of three kinds of critical penetration stress (τcr) and dislocation stress (τ1). Moreover, it can be worked out the crack growth and fatigue life. As the results suggest that the fatigue crack growth reduces with decreases of grain size and increases of minor phases proportion (Xβ). Comparison of fatigue life prediction, we derived the fatigue fracture characteristics. In a word, the results offer awareness to improve the mechanical properties in dual-phase titanium alloy by optimizing the microstructures.

Tensile behavior of dual-phase titanium alloys under high-intensity proton beam exposure: Radiation-induced omega phase transformation in Ti-6Al-4V

A high-intensity proton beam exposure with 181 MeV energy has been conducted at Brookhaven Linac Isotope Producer facility on various material specimens for accelerator targetry applications, including titanium alloys as a beam window material. The radiation damage level of the analyzed capsule was 0.25 dpa at beam center region with an irradiation temperature around 120 °C. Tensile tests showed increased hardness and a large decrease in ductility for the dual α+β-phase Ti-6Al-4V Grade-5 and Grade-23 extra low interstitial alloys, with the near α-phase Ti-3Al-2.5V Grade-9 alloy still exhibiting uniform elongation of a few % after irradiation. Transmission Electron Microscope analyses on Ti-6Al-4V indicated clear evidence of a high-density of defect clusters with size less than 2 nm in each α-phase grain. The β-phase grains did not contain any visible defects such as loops or black dots, while the diffraction patterns clearly indicated ω-phase precipitation in an advanced formation stage. The radiation-induced ω-phase transformation in the β-phase could lead to greater loss of ductility in Ti-6Al-4V alloys in comparison with Ti-3Al-2.5V alloy with less β-phase.

Multi-scale crystal plasticity finite element simulations of the microstructural evolution and formation mechanism of adiabatic shear bands in dual-phase Ti20C alloy under complex dynamic loading

A dynamic compression test was performed on α + β dual-phase titanium alloy Ti20C using a split Hopkinson pressure bar. The formation of adiabatic shear bands generated during the compression process was studied by combining the proposed multi-scale crystal plasticity finite element method with experimental measurements. The complex local micro region load was progressively extracted from the simulation results of a macro model and applied to an established three-dimensional multi-grain microstructure model. Subsequently, the evolution histories of the grain shape, size, and orientation inside the adiabatic shear band were quantitatively simulated. The results corresponded closely to the experimental results obtained via transmission electron microscopy and precession electron diffraction. Furthermore, by calculating the grain rotation and temperature rise inside the adiabatic shear band, the microstructural softening and thermal softening effects of typical heavily-deformed α grains were successfully decoupled. The results revealed that the microstructural softening stress was triggered and then stabilized (in general) at a relatively high value. This indicated that the mechanical strength was lowered mainly by the grain orientation evolution or dynamic recrystallization occurring during early plastic deformation. Subsequently, thermal softening increased linearly and became the main softening mechanism. Noticeably, in the final stage, the thermal softening stress accounted for 78.4 % of the total softening stress due to the sharp temperature increase, which inevitably leads to the stress collapse and potential failure of the alloy.

Strain rate sensitivity, microstructure variations, and stress-assisted β → α′′ phase transformation investigation on the mechanical behavior of dual-phase titanium alloys

In this study, the microstructures of three widely used dual-phase titanium alloys: Ti-6Al-4V, Ti-6Al-2Sn-4Zr-2Mo, and Ti-6Al-2Sn-4Zr-6Mo are characterized using backscatter electron imaging and the deformation behaviors in their individual α and β phases are investigated using in situ high-energy X-ray diffraction during uniaxial tension experiments. This is accompanied by calculation of the strain rate sensitivity (SRS) in these alloys, as a function of applied loading. The results of the calculated SRS and the mechanical response (in terms of lattice strains and hardening behaviors) are rationalized based on the local microstructure, composition, and texture of the alloy. A stress-induced β (BCC) to α" (orthorhombic) phase transformation is observed during the in-situ diffraction experiment, which permitted analysis of the lattice strain evolution during the phase transformation. Results from this study provide insights into the relationship between microstructure, composition, applied loading, and the deformation behavior, including effects on loading with time holds. Specifically, the results show that the microstructure, and not necessarily the chemical composition (Mo content), along with the applied loading have a dominant role in the observed SRS variation in titanium alloys. Also, the evolution of the α" (orthorhombic) phase is observed for the first time during deformation of an industrially relevant titanium alloy, Ti-6246.

In-situ investigation via high energy X-ray diffraction of stress-induced(0002)α→(110)β transformation in a Ti-5.5Mo-7.2Al-4.5Zr-2.6Sn-2.1Cr alloy

The deformation and load partitioning mechanisms among phases are critical to the detailed understanding of heterogeneous stress distributions at the submicron scale during the process of elastic-plastic transformation in dual-phase (α+β) titanium alloys. Therefore, in this work, a Ti-5.5Mo-7.2Al-4.5Zr-2.6Sn-2.1Cr titanium alloy was investigated via in-situ high energy X-ray diffraction. We examined the evolution of the lattice strain, peak intensity, and full width at half maximum under uniaxial tensile loading at room temperature. The results indicated that the stress-induced (0002)α→(110)β transformation can be preferentially initiated for lattice strains of 702 × 10−6–1500 × 10−6 and 592 × 10−6–1355 × 10−6 during the elastic loading stage of the {0002}α and {110}β reflections, respectively. In addition, various dog-bone-shaped specimens with interrupted strain amplitudes (0.3%, 3%, 5% and 8.5%) were characterized via transmission electron microscopy. The corresponding selected area electron diffraction pattern confirmed the occurrence of a (0002)α//(110)β Burgers orientation relationship when the alloy was subjected to a strain of ε = 0.3%. Furthermore, the results revealed that the anisotropic non-linear response of specific reflections (i.e., the (0002)α and (110)β reflections) will cause a phase transformation.

Investigations on ductility improvement and reloading yielding during stress relaxation of dual phase Ti–6Al–4V titanium alloy

Transient stress relaxation test can be used to estimate activation volume for plastic deformation. The analysis is complicated due to the non-constancy of mobile dislocation density during relaxation. The reloading yielding post stress relaxation has a direct correlation with the change in mobile dislocation density during relaxation. However, this feature is ignored in the past studies. In the present work, the reloading yielding phenomenon in dual phase (α+β) titanium alloy (Ti–6Al–4V) is investigated using phenomenological stress relaxation model. Based on indirect experimental evidences, it is shown that the reloading yielding is due to regeneration of new dislocations and not due to unpinning of sessile dislocations, as assumed previously. The improvement of ductility due to stress relaxation follows a similar trend reported in literature. The ductility improvement is analyzed using both microstructure changes and thermal gradient. Fractographs of failed samples were investigated to understand the fracture characteristics and quantitative analyses were performed using image processing to correlate the microstructure mechanisms responsible for enhanced ductility. Thermal history at select locations were recorded using thermocouple and were used to analyze the contribution of thermal gradient in ductility. The necessity of a comprehensive approach to model ductility improvement in non-monotonic stress relaxation test is explored.

Data Driven Tools and Methods for Microtexture Classification and Dwell Fatigue Life Prediction in Dual Phase Titanium Alloys

Microtexture has been linked to large reductions in cold dwell fatigue (CDF) life of specific dual phase titanium alloy aeroengine components. A recently completed Metals Affordability Initiative (MAI) funded program led by Pratt &amp; Whitney (P&amp;W) and includes ATI Forged Products, Boeing, GE Aviation, Rolls Royce (RR), Arconic, Titanium Metals Corporation (TIMET), PCC-Wyman Gordon (PCC-WG), Scientific Forming Technologies (SFTC), Materials Resources LLC (MRL) and The Ohio State University (OSU) has developed improved techniques for the characterization of microtexured regions (MTR) in titanium billet and forgings, and integrated computational materials engineering (ICME). These methods are aimed at developing and integrating process and property modeling tools for the prediction of microtexture and fatigue life in titanium components. These characterization and fatigue life prediction tools have near-term application off ramps that will enable use for process and product development and quality control. Key results for two widely used alloys, Ti-6242 and Ti-64, will be reviewed in this paper.

Recognition and Quantification of Dual Phase Titanium Alloy Microstructures Using Convolutional Neural Networks

Recent advances in machine learning and image recognition tools/methods are being used to address fundamental challenges in materials engineering, such as the automated extraction of statistical information from dual phase titanium alloy microstructure images to support rapid engineering decision making. Initially, this work was performed by extracting dense layer outputs from a pretrained convolutional neural network (CNN), running the high dimensional image vectors through a principal component analysis, and fitting a logistic regression model for image classification. Kfold cross validation results reported a mean validation accuracy of 83% over 19 different material pedigrees. Furthermore, it was shown that fine-tuning the pre-trained network was able to improve image classification accuracy by nearly 10% over the baseline. These image classification models were then used to determine and justify statistically equivalent representative volume elements (SERVE). Lastly, a convolutional neural network was trained and validated to make quantitative predictions from a synthetic and real, two-phase image datasets. This paper explores the application of convolutional neural networks for microstructure analysis in the context of aerospace engineering and material quality.

Impact of Microstrucutre on Dwell Fatigue in Dual-Phase Titanium Alloys

Dual phase titanium alloys, such as Ti-6242, experience a significant reduction in fatigue lifetime when the peak load is held at each cycle. This type of sustained peak loading, also known as dwell fatigue, mimics the long periods of high mean stress experienced by titanium fan and compressor components during takeoff and cruise. The reduction in fatigue lifetime is known as the dwell debit, and is attributed to the phenomenon of load shedding. Both local microstructure and temperature are known to impact load shedding and thereby the macroscopic response of Ti-6242 when subject to dwell fatigue, but the underlying mechanisms are still under active investigation. This study utilized electron backscatter diffraction (EBSD) and digital image correlation (DIC) to characterize the role of local microstructure and temperature on load shedding during dwell fatigue. EBSD was used to determine local orientation and texture information, and DIC provided information about the heterogeneity of the strain distribution and plastic strain accumulation. Ex-situ tests were performed to investigate the link between the deformation of local microstructures and macroscopic damage. The resultant strain fields and orientation maps were statistically analyzed to provide quantitative insights into the impact of local microstructure on load shedding during dwell fatigue.

Modeling Strain Localization in Microtextured Regions in a Titanium Alloy: Ti–6Al–4V

Large and highly textured regions, referred to as macrozones or microtextured regions, with sizes up to several orders of magnitude larger than those of the individual grains, are found in dual-phase titanium alloys as a consequence of the manufacturing process route. These macrozones have been shown to play a critical role in the failure of titanium alloys, specifically being linked to crack initiation and propagation during cyclic loading. Modeling microstructures containing macrozones using continuum-level formulations to describe the elastic–plastic deformation at the grain scale, i.e., crystal plasticity, poses computational challenges due to the large size of the macrozones, which in turn prevents the use of modeling approaches to understand their deformation behavior. In this work, a crystal plasticity-based modeling approach is implemented to model macrozones in Ti–6Al–4V. Further, to overcome the large computational expense associated with modeling microstructures containing macrozones, a modeling strategy is introduced based on a crystal plasticity description for the macrozone with a reduced-order model for the surrounding aggregate combining anisotropic elasticity and J2 plasticity, based on crystal plasticity-based training data. This modeling approach provides a grain-level description of deformation within macrozones using elastic–plastic continuum simulations, which has often been overlooked. Finally, the reduced-order model is used to investigate the strain localization within the microstructure and the effect of varying the misorientation tolerance on the localization of plastic strain within the macrozones.

Effect of Solution Treatment on the Microstructure, Micromechanical Properties, and Kinetic Parameters of the β → α Phase Transformation during Continuous Cooling of Ti-6Al-4V Titanium Alloy

The aim of this study is to examine the effect of solution treatment temperature (STT) on the microstructure, the micromechanical properties, and the kinetic parameters of the β → α phase transformation during continuous cooling of the dual phase titanium alloy Ti-6Al-4V. Increasing the STT from 1050 to 1200 °C delays the formation of the α phase during cooling and increases the value of its activation energy. The microstructural analysis reveals the emergence of αW platelets from protuberances on the αGB/αW interface. The investigation of the morphology of the αW platelets reveals the presence of ledges on their longest side showing a sharp extremity. The micromechanical properties determined by nanoindentation and microhardness tests are almost insensitive to the cooling rate but are strongly affected by the STT; the higher the STT, the lower the overall microhardness of the Ti-6Al-4V alloy. In addition, the STT affects the microhardness and the Young’s modulus of both α and β phases differently; when the STT increases, the microhardness and the Young’s modulus of the α phase decrease, whereas those of the β phase increases.

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.