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Ti-6Al-4V alloy has limitations in terms of globularization when using additive manufacturing (AM), which requires a final forging step. To optimize this approach, this study investigated the effect of processing variables on the dynamic/static globularization of AM-processed Ti-6Al-4V alloy. Eight double-cone specimens were prepared with different processing variables such as effective strain, solution treatment, hot-forging temperature, and subsequent annealing. Microstructural evolutions were quantitatively characterized to interpret the kinetics of globularization based on the aforementioned variables. In particular, the combination of solution treatment, low-temperature (1073 K) forging, and subsequent annealing significantly accelerated the overall globularization. Such an accelerating effect stemmed from the reduced path for boundary splitting in fine α′ martensitic laths, which was induced via solution treatment. This accelerating effect disappeared at a high temperature (1223 K), which implies the necessity of optimizing the thermomechanical route to exploit martensitic laths.

Direct energy deposition (DED) is a highly applicable additive manufacturing (AM) method and, therefore, widely employed in industrial repair-based applications to fabricate defect-free and high degree precision components. To obtain high-quality products by using DED, it is necessary to understand the influence of the process parameters on the product quality. The optimization of such processing parameters provides several advantages such as minimization of the loss of material and time. However, the optimization of the complex relationship between the process parameters-geometry-properties of the fabricated sample is difficult to realize and requires significant experimentation. Herein, a computational model based on artificial neural networks was developed to optimize process parameters for DED-processed Ti-6Al-4V alloy. The model was developed to estimate the density and build height (the actual build height realized after fabrication) of the sample as a function of power, scan speed, powder feed rate, and layer thickness. The optimum model with high-accuracy predictions was employed to construct the process maps for the DED-processed Ti-6Al-4V. The relative importance indices of process parameters on the build height and density were investigated. Further, the effect of power and scan speed on the microstructure of Ti-6Al-4V alloy was discussed. Finally, based on the obtained results, the optimum fabrication conditions for the DED-processed Ti-6Al-4V alloy were determined.

The yield strength of commercially pure (CP) Ti of ASTM grade 4, the strongest among all the CP-Ti grades, is too low for structural applications that require high-strength materials. Here, we demonstrate the strengthening of grade-4 CP Ti by cryogenic-temperature rolling (CTR), which enables deformation twinning in grade-4 CP Ti to achieve twinning-induced grain refinement. CTR activated 11 2 - 2 twinning and 10 1 - 2 twinning, which are the most common twinning systems in pure Ti, whereas room-temperature rolling (RTR) did not activate any twinning system. CTR with imposing an area reduction of just 30% significantly increased the yield strength of the CP Ti to 946 MPa, which is not achievable through typical processes performed at or above room temperature and is comparable to that of commercial Ti-6Al-4V. The significant increase in strength was due to microstructural strengthening caused by twinning-induced grain refinement, combined with dislocation accumulation. In contrast to RTR, CTR greatly increased the stress concentration at grain boundaries (GBs), which caused the unusual activation of twinning in the grade-4 CP Ti by facilitating twin nucleation at GBs. The stress concentration increased because CTR activated the slip to a lesser extent compared to RTR, thereby reducing the strain compatibility between neighboring grains. These results will contribute to development of ultrahigh-strength CP Ti and may thereby extend its use to structural applications that require high-strength materials.

Commercially pure titanium (CP-Ti) has seen major applications in biomedical equipment. The use of selective laser melting (SLM) has become more common concerning the fabrication of CP-Ti components with a complicated shape in biomedical implants, where high strength and ductility are required. However, SLM manufactured CP-Ti often exhibits high strength and low ductility, as well as mechanical anisotropy because SLM process typically results in the formation of long columnar grains comprising of fine acicular α′ martensite. Heat treatment must be preceded to transform the acicular α′ to equiaxed α grain. This study demonstrated that annealing at 650 °C of SLM CP-Ti can create an equiaxed structure, resulting in a weakened texture. The formation of equiaxed grains occurred at two types of microstructural features, acicular α′ and irregular massive (α′m) martensite, which exhibited different sizes, morphologies, and contained different types of substructures under SLM as-fabricated condition. The formation mechanism of equiaxed grains in the α′ region is dominated by the coalescence of acicular α′ of the same variant and the dissolution of α′ into the matrix, whereas the formation mechanism in the α′m region is primarily dominated by the growth of the preexisting subgrains. A small number of equiaxed grains with new orientations were formed in the α′m region. The majority of equiaxed grains were formed in the acicular α′ region and inherited the grain orientations of the preexisting α′ or matrix. Therefore, a similar but weakened texture was inherited from the microstructure of the SLM CP-Ti after annealing.

The present work investigates the thermal stability and mechanical properties of a Co–20Cr–15W–10Ni (wt%) alloy with a bimodal grain (BG) structure. The BG structure consisting of fine grains (FGs) and coarse grains (CGs) is thermally stable under high-temperature exposure treatments of 760 °C for 100 h and 870 °C for 100–1000 h. The size of both FGs and CGs remains no significant changes after thermal exposure treatments. The microstructural stability is associated with the slow kinetics of grain growth and the pinning of carbides. The thermal stability enables to maintain the BG structures, leading to the same mechanical properties as the sample without thermal exposure treatment. In particular, the BG alloy samples after thermal exposure treatment exhibit superior mechanical properties of both high strength and high ductility compared to the unimodal grain (UG) structured ones. The BG structure of the alloy samples after thermal exposure is capable of avoiding severe loss of ductility and retaining high strength. More specifically, the ductility of the BG alloy samples after thermal exposure treatments of 870 °C for 500–1000 h is ten times higher (44.6% vs. 3.5% and 52.6% vs. 5.0%) than that of the UG ones. The finding in the present work may give new insights into high-temperature applications of the Co–20Cr–15W–10Ni alloy and other metallic materials with a BG structure.

The influence of pre-strain on the formation of bimodal grain structures and tensile properties of a Co–20Cr–15W–10Ni alloy was investigated. The bimodal grain structures consist of fine grains (FGs; 2–3 μm in diameter) and coarse grains (CGs; 8–16 μm in diameter), which can be manipulated by changing the pre-strain (ɛ = 0.3–0.7) and annealing temperatures (1000–1100 °C). High pre-strain applied in the samples can intensify the plasticity heterogeneity through increasing the total dislocation density and the local volumes of high-density dislocations. This can essentially result in finer FGs, a higher FG volume fraction, and overall grain refinement in the samples after annealing. High-temperature essentially increases both the size and volume fraction of CGs, leading to an increase in the average grain size. The tensile test suggests that the bimodal grain structured samples exhibited both high strength and ductility, yield strengths of 621–877 MPa and ultimate tensile strengths of 1187–1367 MPa with uniform elongations of 55.0%–71.4%. The superior strength-ductility combination of the samples arises from the specific deformation mechanisms of the bimodal grain structures. The tensile properties strongly depend on the size ratio and volume fraction of FGs/CGs in addition to the average grain size in the bimodal grain structures. The grain structures can be modified via changing the pre-strain and annealing temperature.

The crack formation mechanisms of Ti–48Al–2Cr–2Nb single tracks processed by laser powder bed fusion were extensively investigated in a wide range of laser powers and scan speeds. The crack patterns were categorized by their directionalities, which were parallel (longitudinal crack) and/or perpendicular (transverse crack) to the scan direction. For the representative process conditions of the keyhole, transition, and conduction modes, cracking behaviors were characterized by combining the fractography and the microstructural analysis. Further, thermal-mechanical finite element method simulations were performed to predict the distribution of temperatures and thermal stresses during the melt pool formation. On the basis of the combined results, the cracks formed in keyhole, transition, and conduction modes were clarified as a solidification crack and/or a thermal crack. In addition, the formation of these cracks was thoroughly understood in terms of thermal stresses and microstructural factors that affect the crack susceptibility. Finally, comprehensive mechanisms responsible for cracking of Ti–48Al–2Cr–2Nb single tracks under laser powder bed fusion were proposed for different process conditions (the keyhole, transition and conduction modes).

We controlled the twin structure in pure Ti by deforming it at cryogenic temperature (∼77 K). Numerous thin individual twins were formed in the grains, thereby enabling strong twinning-induced grain refinement. The processed material had an ultrafine-grained microstructure, which greatly increased its strength. Notably, the ductility did not decrease significantly, despite the considerable strengthening, leading to superior tensile properties. This remarkable phenomenon occurred because the twin structure and the relatively small number of dislocations in the processed material enabled high resistance to necking instability. Our finding offers a new microstructural engineering method to achieve excellent tensile properties in pure Ti.

Ultrafine-grained (UFG) structure is beneficial for overcoming the strength-ductility trade-off and enhancing the superplasticity of two-phase Ti alloys. Recently, it has been demonstrated that com...

We used selective laser melting (SLM) to produce a three-dimensional specimen of pure Ti and found that an equiaxed grain structure and weak crystallographic texture could be achieved simultaneously via subsequent heat treatment. These traits have never been attained simultaneously in pure Ti prepared by traditional methods such as casting and rolling. This remarkable achievement was possible because recrystallization occurred during heat treatment without plastic deformation; such deformation introduces stored energy that drives recrystallization but inevitably causes a strong texture to develop. The occurrence of recrystallization was attributed to unique features of the SLM process including a very high cooling rate and repetitive layer stacking. These features generated considerable stored energy by affecting solidification and the β → α phase transformation. Steep in-grain orientation gradients and a fine lath structure also contributed to the activation of recrystallization by facilitating recrystallization nucleation. The heat-treated specimen showed tensile properties with significantly reduced anisotropy. This finding will provide new strategies for developing isotropic metallic materials and may introduce new applications of SLM.

A nearly fully dense β stabilized γ-TiAl alloy was additively manufactured by electron beam melting. The as-fabricated specimen exhibited a fine structure consisting of α2/γ colonies ( 50%) in the sample at 800 °C. Annealing and subsequent stabilization treatments resulted in α2/γ colonies (

Ti-2Al-9.2Mo-2Fe is a low-cost β titanium alloy with well-balanced strength and ductility, but hot working of this alloy is complex and unfamiliar. Understanding the nonlinear relationships among the strain, strain rate, temperature, and flow stress of this alloy is essential to optimize the hot working process. In this study, a deep neural network (DNN) model was developed to correlate flow stress with a wide range of strains (0.025–0.6), strain rates (0.01–10 s−1) and temperatures (750–1000 °C). The model, which was tested with 96 unseen datasets, showed better performance than existing models, with a correlation coefficient of 0.999. The processing map constructed using the DNN model was effective in predicting the microstructural evolution of the alloy. Moreover, it led to the optimization of hot-working conditions to avoid the formation of brittle precipitates (temperatures of 820–1000 °C and strain rates of 0.01–0.1 s−1).

In this study, the deformation mechanism of a metastable Ti-4Al-4Fe-0.25Si-0.1O alloy was investigated via ex-situ electron backscattering diffraction (EBSD) and field-emission transmission electron microscopy (FE-TEM). A stress-induced α′-martensitic transformation occurred from the retained β phase. New variants of α′-martensite were nucleated and grew in the transformed α′-martensite under increasing levels of applied stress. Microstructural observations revealed that the orthorhombic nanodomains were sheared in the prior α′-martensite; these sheared nanodomains contributed to the formation of different variants of α′-martensite. This additional martensitic transformation resulted in an excellent combination of strength (ultimate tensile strength; UTS: 1340 MPa) and ductility (30%).

The rolling texture formed in the conventional cold rolling process of commercially pure titanium (CP-Ti) for producing a metal sheet significantly limits the potential applications of CP-Ti sheets in various industrial sectors by impairing the formability. Here, we report that by exploiting a twinning-induced crystallographic texture modification, the rolling texture can be weakened and dispersed effectively, leading to a simultaneous improvement in the formability and yield strength. A two-stage cold rolling process was designed with intermediate annealing at a late stage of the conventional cold rolling process to generate deformation twins. The intermediate annealing drove the activation of $$\{11\bar{2}2\}$$ { 11 2 ¯ 2 } twin and $$\{11\bar{2}2\}$$ { 11 2 ¯ 2 } – $$\{10\bar{1}2\}$$ { 10 1 ¯ 2 } double twin in the second stage of the rolling process by removing the internal reaction stress developed in the first stage of the rolling process through recrystallization, and the crystallographic feature of the $$\{11\bar{2}2\}$$ { 11 2 ¯ 2 } twinned region, i.e., $$\{11\bar{2}2\}$$ { 11 2 ¯ 2 } twin texture, was effective for type slips and $$\{10\bar{1}2\}$$ { 10 1 ¯ 2 } twinning to accommodate a through-thickness strain as well as for reducing the planar anisotropy. This enhanced thinning capability and reduced planar anisotropy in the $$\{11\bar{2}2\}$$ { 11 2 ¯ 2 } twin texture led to an improvement of the formability. We demonstrated the feasibility of the suggested two-stage cold rolling process with ASTM grade 2 CP-Ti.

Ti–6Al–4V alloy is a typical 3D printing metal, and its application has been expanded to various fields owing to its excellent characteristics such as high specific strength, high corrosion resistance, and biocompatibility. In particular, direct energy deposition (DED) has been actively explored in the fields of deposition and the repair of large titanium parts. However, owing to the complicated thermal history of the DED process, the microstructures of the fusion zone (FZ), heat-affected zone (HAZ), and base metal (BM) are different, which results in variations of their mechanical characteristics. Therefore, the process reliability needs to be optimized. In this study, the microstructure and hardness of each region were investigated with respect to various DED process parameters. An artificial neural network (ANN) model was used to correlate the measured characteristics of the FZ, HAZ, and BM of Ti–6Al–4V components with the process parameters. The variation in the mechanical characteristics between the FZ, HAZ, and BM was minimized through post-heat treatment. Heat treatment carried out at 950 °C for 1 h revealed that the microstructure and hardness values throughout the component were homogeneous.

Step rolling can be used to mass-produce ultrafine-grained (UFG) Ti-6Al-4V sheets. This study clarified the effect of subsequent annealing on the tensile properties of step-rolled Ti-6Al-4V at room temperature (RT) and elevated temperature. The step-rolled alloy retained its UFG structure after subsequent annealing at 500–600 °C. The RT ductility of the step-rolled alloy increased regardless of annealing temperature, but strengthening was only attained by annealing at 500 °C. In contrast, subsequent annealing rarely improved the elevated-temperature tensile properties. The step-rolled Ti-6Al-4V alloy without the annealing showed the highest elongation to failure of 960% at 700 °C and a strain rate of 10−3 s−1. The ambivalent effect of annealing on RT and elevated-temperature tensile properties is a result of microstructural features, such as dislocation tangles, subgrains, phases, and continuous dynamic recrystallization.

The isothermal compression tests were carried out to study the hot deformation behavior and microstructure evolution of Ti–19Al–22Mo alloy. The samples were deformed in the temperature range from 1100 to 1250 °C with an interval of 50 °C, strain rate ranging from 0.01 to 1 s−1 and the height reduction of 50% using Gleeble-3800 thermal–mechanical simulator. By using this experimental data an artificial neural network (ANN) model was developed and evaluated with unseen data. Further, the developed ANN model was used to predict flow stress correction from adiabatic heating at finer intervals of strain rates and temperatures. The predicted isothermal flow stress values were utilized to construct processing maps for Ti–19Al–22Mo alloy at true strain of 0.4 and 0.6. The maximum efficiency was noticed at 1100 °C with the strain rate of 0.01 s−1 associated with dynamic recrystallization and dynamic recovery. The deformation conditions of the instability domains in processing map showed wedge cracking and flow localization. Using the processing maps safe working parameters for hot deformation of Ti–19Al–22Mo alloy was identified.

A high-performance magneto-mechano-triboelectric nanogenerator (MMTEG) was demonstrated by introducing accelerated water-soluble nano-bullet modified nanostructures to convert a gentle magnetic field into electric energy for powering an indoor wireless positioning system. NaCl salt nanoparticles were accelerated by an aerosol deposition (AD) process to collide on a perfluoroalkoxy (PFA) film with a high kinetic energy for the formation of a complicated nanomorphology on the triboelectric active surface. Under an alternating current (AC) magnetic field of 7 Oe, the MMTEG generated an open-circuit peak-to-peak voltage (Vpp) and a short-circuit current of 708 V and 277 μA, respectively. The harvesting device also presented a maximum peak power of 21.8 mW as well as a continuous AC output power of 4.8 mW (4.8 mJ per second). A self-powered indoor IoT positioning system was constructed by integrating the MMTEG, a power managing circuit, a storage element, and an IoT Bluetooth beacon. The electric energy from the MMTEG device enabled continuous operation of a beacon device, and we successfully confirmed the accurate location of the installed wireless positioning system, subsequently resulting in transmission of our indoor position to the main monitoring computer. Lastly, the MMTEG generated an open-circuit Vpp and a short-circuit current of 330 V and 23 μA, respectively, near a 60 Hz power cable connected to home appliances, which were large enough to turn on 108 blue light emitting diodes (LEDs).

The γ-TiAl alloys fabricated by additive manufacturing have gathered significant attention in recent years due to their unique microstructural features and properties. However, the oxidation kinetics of additively manufactured γ-TiAl alloys have not been studied thoroughly. Herein, the TNM-B1 alloy was fabricated with the help of electron beam melting (EBM) for the first time and tested for 100 h over a temperature range of 800 °C-1000 °C; this was aimed at investigating its isothermal oxidation kinetics. Subsequently, the exposed samples were systematically analyzed to study the oxide scales, which was achieved with the help of X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Spallation was observed at 900 °C and 1000 °C after an exposure of 30 h and 5 h, respectively. The results showed that the oxide comprises alternative layers of Al2O3, TiO2, and mixture phases. The oxidation resistance of the EBM-processed TNM-B1 alloy was observed to be superior to those of the conventionally processed TNM-B1 and other γ-TiAl alloys. Finally, the significance of a fine fully lamellar microstructure on the oxidation kinetics of TNM-B1 alloy was explained.

Recently, the hybrid method has been developed in which wire and arc additive manufacturing (WAAM) use to produce the near net shape preform for the single-step hot forging process. The hybrid method overcomes the defects and anisotropic properties of WAAM processed preform and produce the net shape of the component with better mechanical properties. This study investigates the hot deformation behavior and mechanical properties of WAAM Ti-6Al-4V alloy containing widmantatten microstructure (0.34 – 0.48 µm) produced by the hybrid method. Hot deformation tests were conducted in the temperature range 700–1000 °C and strain rate range 0.01 s−1 – 10 s−1 up to the height reduction of 60%, using the thermal-mechanical simulator gleeble-3800. The artificial neural network model (ANN) has been developed to correct the non-isothermal flow curve at finer intervals of experimental conditions. The microstructural studies were carried out at various regions using a developed processing map. The microstructures show an instability region at a high strain rate and lower temperature, associated with flow localization and lamellae kinking. At the same time, the high efficiency and stable regions are related to dynamic recrystallization in the temperature range 900–950 °C at a strain rate below 1 s−1. The self-deformation activation energy in the α + β and β phase regions was 308.7 kJ/mol and 493.2 kJ/mol, respectively. The forged sample at 920 °C and strain 0.6, 0.8, and 0.9 show high strength, elongation, and weak texture compared to the received and stress relieved sample.

A bimodal grain structure consisting of fine grains (d ~ 1–2 µm) and coarse grains (d ~ 10–20 µm) was achieved in the biomedical Co–20Cr–15W–10Ni alloy via medium cold rolling (area reduction of 50%) followed by short-term annealing (15 min) at relatively low temperatures (950–1100 °C). The medium cold rolling induced a heterogeneous microstructure featuring the coexistence of severely deformed regions and weakly deformed domains. With short-term annealing at low temperatures, fine and coarse grains were preferentially recrystallized in severely deformed regions and weakly deformed domains, respectively, resulting a bimodal grain size distribution. During tension, dislocations were more rapidly generated in the fine grains, thus increasing the strain hardening, while they glided longer on the {111} planes in the coarse grains, contributing a high ductility. The combination of these two factors provided well-balanced strength–ductility behavior with an ultimate tensile strength of 1278 MPa, a yield strength of 787 MPa, and an elongation to fracture of 53%, making the alloy suitable for surgical implant and stent applications where the strength and ductility are both important to ensure mechanical reliability in a human body.

The effects of Cr and Fe addition on the mechanical properties of Ti–6Al–4V alloys prepared by direct energy deposition were investigated. As the Cr and Fe concentrations were increased from 0 to 2 mass%, the tensile strength increased because of the fine-grained equiaxed prior β phase and martensite. An excellent combination of strength and ductility was obtained in these alloys. When the Cr and Fe concentrations were increased to 4 mass%, extremely fine-grained martensitic structures with poor ductility were obtained. In addition, Fe-added Ti–6Al–4V resulted in a partially melted Ti–6Al–4V powder because of the large difference between the melting temperatures of the Fe eutectic phase (Ti–33Fe) and the Ti–6Al–4V powder, which induced the formation of a thick liquid layer surrounding Ti–6Al–4V. The ductility of Fe-added Ti–6Al–4V was thus poorer than that of Cr-added Ti–6Al–4V.

The present study focused on estimating the complex nonlinear relationship between the composition and phase transformation temperatures of Ti–Ni–Pd shape memory alloys by artificial neural networks (ANN). The ANN models were developed by using the experimental data of Ti–Ni–Pd alloys. It was found that the predictions are in good agreement with the trained and unseen test data of existing alloys. The developed model was able to simulate new virtual alloys to quantitatively estimate the effect of Ti, Ni, and Pd on transformation temperatures. The transformation temperature behavior of these virtual alloys is validated by conducting new experiments on the Ti–rich thin film that was deposited using multi target sputtering equipment. The transformation behavior of the film was measured by varying the composition with the help of aging treatment. The predicted trend of transformational temperatures was explained with the help of experimental results.

Commonly used high-temperature near-alpha titanium alloys contain Al, Zr and Si as their alloying elements. Significant losses of mechanical properties and cleavage mode failures are evident due to the presence of Ti3Al and S2 types of zirconium silicides ((TiZr)6Si3) in these alloys. We developed a new alloy (Ti-6.5Al-3.0Sn-4.0Hf-0.2Nb-0.4Mo-0.4Si-0.1B) by replacing Zr with Hf to avoid the formation of zirconium silicides. Aging at 700 °C for five hours successfully eliminated the Ti3Al phase and substantially improved the room-temperature ductility. Tensile tests were carried out at room temperature and at 650 °C. The mechanical properties of the present alloy were significantly improved compared to those of existing high-temperature titanium alloys under testing at ambient and elevated temperatures.

Various titanium alloys (Ti-Al-Fe-Si, VT3-1, Ti-62222S, and Ti-64) were prepared to investigate the effect of the prior β grain size on the kinetics of α'-martensitic transformation. The prior β grain size was controlled using various solution treatments (740–960 °C). Electron backscattering diffraction, energy dispersive spectroscopy, and field-emission transmission electron microscopy techniques were used to determine the volume fraction, composition, and grain size of the phase constituents. The grain refinement of prior β inhibited the α'-martensitic transformation. Moreover, the modified Mo equivalent and transformation kinetics model including the prior β grain size were formulated.

Transformation-induced plasticity (TRIP) Ti alloys are promising structural materials that offer high strength and ductility. However, these alloys often include heavy, expensive, and high-melting-point β-stabilizing elements such as V, Nb, Mo, and W. Herein, an artificial neural network (ANN) was used to develop a Ti–Al–Fe–Mn-based TRIP alloy comprising lighter and/or cheaper elements. The ANN model was trained with 30 experimental tensile datasets for heat-treated (830–920 °C) Ti–4Al–2Fe–xMn (x = 0–4 wt%) alloys, and used to generate 400 tensile datasets with more finely tuned composition and temperature intervals. Based on the predicted data, an 883 °C-heat-treated Ti–4Al–2Fe–1.4Mn alloy was produced (conditions not used in the training datasets), which exhibited ultra-high specific strength (289 MPa·cm3/g) and high elongation (34%). Thus, the ANN approach successfully led to the development of a new alloy while minimizing the number of labor-intensive and time-consuming experiments.

Fine-grained materials possess great industrial significance as they have exceptional strength, ductility, and superplastic formability. However, the poor thermal stability of these materials leads to an excessive grain growth and property deterioration during welding. In this regard, friction stir welding (FSW) is considered as a potential candidate for the welding of fine-grained materials as it is a solid-state welding process with a lower process temperature. In this work, the microstructure development in the stir zone of a fine-grained Ti-6Al-4V alloy during FSW was investigated. The microstructure, misorientation distribution and crystallography of the base metal and stir zone were characterized using the electron backscatter diffraction technique to explain the microstructural evolution. The microstructure of different layers of stir zone composed of either fine α + β lamellar structure or fine equiaxed α and β grains or the mixture of both. The combination of heating/cooling induced α/β phase transformations and plastic deformation induced continuous dynamic recrystallization (CDRX) was responsible for such microstructural developments in the stir zone. Homogeneous misorientation increase in low angle boundaries was found as the dominant mechanism of CDRX in the stir zone of fine-grained Ti-6Al-4V during FSW. The hardness measurement confirmed the absence any plausible softening though there was a little grain coarsening in the top layer of the stir zone.

Gum metals feature properties such as ultrahigh strength, ultralow elastic modulus, superelasticity, and superplasticity. They are composed of elements from Groups 4 and 5 of the periodic table and exist when the valance electron concentration (\(\overline{e/a}\)) is 4.24; the bond order (\(\overline{\text{Bo}}\)) is 2.87; and the “d” electron-orbital energy level (\(\overline{\text{Md}}\)) is 2.45 eV. Typical compositions include Ti–23Nb–2Zr–0.7Ta–O and Ti–12Ta–9Nb–6Zr–3 V–O, which contain large amounts of heavy Group-5 elements such as Nb and Ta. In the present study, to improve the specific strength of a multifunctional beta Ti alloy, three alloys (Ti–20Nb–5Zr–1Fe–O, Ti–12Zr–10Mo–4Nb–O, and Ti–24Zr–9Cr–3Mo–O) were designed by satisfying the above three requirements while adding Fe, Mo, and Cr, which are not only lightweight but also have strong hardening effects. Microstructural and mechanical property analyses revealed that Ti–20Nb–5Zr–1Fe–O has a 25% higher specific strength than gum metal while maintaining an ultralow elastic modulus.

A facile approach for the single-step preparation of branched polythiophenes using a copper(II) catalyst via a dual initiation polymerization process is reported. The synthetic route of dual initiation polymerization involves both oxidative polymerization and metal-catalyzed radical polymerization. In this process, two kinds of polymerization can be performed concurrently as a one-pot reaction using a Cu2+/Cu+ switchable catalytic system. Differential scanning calorimetry and photoluminescence studies confirmed the formation of the π-conjugated polymer structure. Our results provide new insights on the various grafting of copolymers with polythiophene copolymers and functional branched chains using a single-step process. This process could lead the way for enhanced processability towards novel functionalized π-conjugated polymers

Microstructure, mechanical properties as well as phase transformation behavior of newly developed Ni 49 Ti 36 Hf 15-x Ta (x=0, 3, 6, 9, 12) high temperature shape memory alloys has been investigated. Ni 49 Ti 36 Hf 15 alloy consists of (Ti,Hf) 2 Ni phase which is brittle and incoherent with the martensite matrix. However, addition of Ta in place of Hf suppresses the brittle (Ti,Hf) 2 Ni phase formation with white Ta rich phase. More than 70% reduction in area has been observed in Ta added Ni 49 Ti 36 Hf 15 high temperature shape memory alloys. The austenitic transformation temperature is well above 200 °C (473 K) for lower Ta concentrations with up to 6 at. %; however; at higher Ta concentrations (i.e. above 9 at. %) decreases significantly. Overall, the addition of Ta greatly influences the hot deformation behavior and high temperature shape memory properties of Ni 49 Ti 36 Hf 15 alloy. The present paper discusses the detailed investigation on Ni 49 Ti 36 Hf 15-x Ta (x=0, 3, 6, 9, 12) high temperature shape memory alloys.

A systematic parametric study was conducted using thermal finite element method simulations in order to calibrate the laser penetration depth and absorptivity as a function of the laser power and scan speed for single tracks of Alloy 718 processed by laser powder bed fusion. A methodology was developed to calibrate both laser penetration depth and absorptivity using an algorithm proposed. Calibrated laser penetration depths and absorptivities captured experimentally observed variations of the melt pool depth and width with the laser power and scan speed, and showed strong correlations with a modified energy density, which is the laser power normalized by a square root of the scan speed (W/(m/s)1/2). The result indicated that the laser penetration depth and absorptivity heavily influence the determination of the melt pool depth and width, respectively. Variations of calibrated laser penetration depths and absorptivities with the laser power and scan speed reasonably depicted physical phenomena related with how incident laser beam interacts with the melt pool under different input energy densities, and were in quantitative agreement with those calculated from an analytical model and observed from the experiment.

Designing a piezoelectric energy harvester (PEH) with high power density and high fatigue resistance is essential for the successful replacement of the currently using batteries in structural health monitoring (SHM) systems. Among the various designs, the PEH comprising of a cantilever structure as a passive layer and piezoelectric single crystal-based fiber composites (SFC) as an active layer showed excellent performance due to its high electromechanical properties and dynamic flexibilities that are suitable for low frequency vibrations. In the present study, an effort was made to investigate the reliable performance of hard and soft SFC based PEHs. The base acceleration of both PEHs is held at 7 m/s2 and the frequency of excitation is tuned to their resonant frequency (fr) and then the output power (Prms) is monitored for 107 fatigue cycles. The effect of fatigue cycles on the output voltage, vibration displacement, dielectric, and ferroelectric properties of PEHs was analyzed. It was noticed that fatigue-induced performance degradation is more prominent in soft SFC-based PEH (SS-PEH) than in hard SFC-based PEH (HS-PEH). The HS-PEH showed a slight degradation in the output power due to a shift in fr, however, no degradation in the maximum power was noticed, in fact, dielectric and ferroelectric properties were improved even after 107 vibration cycles. In this context, the present study provides a pathway to consider the fatigue life of piezoelectric material for the designing of PEH to be used at resonant conditions for long-term operation.

Direct energy deposited (DED) Ti-5Ni (wt.%) alloys promote significant grain refinement with fine equiaxed grains (~30–50 μm); they have discontinuous grain boundary α, fine eutectoid α laths (~1 μm width), and α + Ti2Ni phases. Post-heat treatment below beta transus for 24 h stimulates the formation of near equiaxed α, and the temperature significantly influenced the microstructural constituents, which included pre-eutectoid α + retained β + Ti2Ni (760°C), pre-eutectoid α + α’ + Ti2Ni (800°C), and fully α’ + Ti2Ni (850°C) phases. The tensile properties of the as-fabricated and the heat-treated alloy remarkably improved compared with the other conventionally processed eutectoid Ti-Ni binary alloys. Thus, the proposed approach opens up new opportunities and possibilities for fabricating other eutectoid-forming alloy systems with improved manufacturing efficiency, which can overcome limitations in the other manufacturing processes.

Rapid electric current heat treatment has been successfully applied to a cold-rolled sheet of commercially pure titanium (CP Ti). The electric current heat treatment was conducted at various temperatures (400, 500, 600 and 700 °C) by altering the current density (A/mm2). The detailed microstructure and texture evolution was studied using electron backscatter and X-ray diffraction analysis. For comparison, conventional heat treatment at 400, 500 and 600 °C were also applied to the cold-rolled sheets. The electrically heat-treated sample showed a much smaller and uniform grain size with a relatively weak texture than the conventionally heat-treated one. As a result, the electrically heat-treated samples exhibited better tensile properties than conventionally heat-treated samples. Furthermore, the electric current treatment produced minimum sheet distortion and good oxidation resistance compared with the conventional heat treatment.

In this study, Ti-4.5Al-2.5Fe-0.25Si and Ti-4.6Al-2.0Fe-0.25Si alloys demonstrating martensite reorientation-induced plasticity (MRIP) were investigated via ex-situ electron backscattering diffraction and field-emission transmission electron microscopy. The alloys were designed such that the annealed microstructure consisted of primary α, retained β, and athermal martensite with sheared nano-domains. The alloys exhibited deformation-induced reoriented martensite during deformation. Three reorientation sites of martensite, retained β, nano-domain, and sheared nano-domain, were observed inside the athermal martensite. These transformations resulted in grain refinement and martensite reorientation. This MRIP alloys exhibited an appreciable combination of strength (yield strength: 882 MPa and ultimate tensile strength: 1474 MPa) and ductility (23.7%).

For this study, several TiAl–Nb–Si-based alloys were designed for a ductility improvement, whereby the high-temperature strength and oxidation resistance were not sacrificed. The environmental properties under the cyclic oxidation behaviors of the TiAl alloys were evaluated at 900 °C for up to 360 cycles. The compositions of the as-cast alloys determined their microstructures, and the cyclic oxidation behavior of the selected alloy was relatively comparable to that of a commercial TiAl alloy that is currently used in automotive engines.

A newly developed Ti–46Al–6Nb-0.5W-0.5Cr-0.3Si-0.1C alloy was oxidized isothermally and cyclically in air, and its high-temperature oxidation behavior was investigated. When the alloy was isothermally oxidized at 700 °C for 2000 h, the weight gain was only 0.15 mg/cm 2 . The parabolic rate constant, k p (mg 2 /cm 4 ·h), measured from isothermal oxidation tests was 0.002 at 900 °C and 0.009 at 1000 °C. Such excellent isothermal oxidation resistance resulted from the formation of the dense, continuous Al 2 O 3 layer between the outer TiO 2 layer and the inner (TiO 2 -rich, Al 2 O 3 -deficient) layer. The alloy also displayed good cyclic oxidation resistance at 900 °C. Some noticeable scale spallation began to occur after 68 h at 1000 °C during the cyclic oxidation test.

A hybrid study of in-situ transmission electron microscopy experiments and finite element analyses was carried out in order to understand the effect of precipitates on crack initiation and propagation in TiNi–(Hf) alloys. Samples of Ti-rich (Ti–46Ni–15Hf) and Ni-rich (Ti–50.3Ni–15Hf) alloys were prepared. The alloys showed differences in room temperature ductility and the ability of rolling at 900 °C. From the in-situ straining transmission electron microscopy experiments, it was revealed that the crack growth behavior is different in the two TiNi–(Hf) alloys. Cracks were easily initiated and propagated along the interface between the matrix and precipitates of Ti–46Ni–15Hf alloy which showed no ductility. However, in the Ti–50.3N–15Hf alloy with some ductility at room temperature, the precipitates were not brittle, and the cracks did not interact with the precipitations instead propagating in a ductile manner with a lot of dislocation activities. The fracture toughness of an interface crack between the matrix and the precipitates was determined by matching numerical crack opening profiles in finite element analyses with the experimental counterpart.

Owing to the high production cost of a high alloying system, the commercialization of β-Ti alloys with high elastic admissible strain (= yield strength/elastic modulus) for biomaterials is challenging. To reduce the alloying elements and maintain a high elastic admissible strain, a new alloy design of the metastable α+β-Ti alloy (Ti–4Al–2Mo–2Fe–2Cr) is proposed in this study. The microstructure of the alloy consists of an α phase with a high yield strength and a metastable β phase with a low elastic modulus. The existence of the hard α phase increases the critical stress for stress-induced martensitic transformation due to stress and strain partitioning. In addition, heat treatment in the α+β regime is an effective method for grain refinement, resulting in high strength without increasing the elastic modulus. This alloy exhibited an excellent combination of strength and ductility with a high elastic admissible strain. Moreover, electron backscattering diffraction and field-emission transmission electron microscopy were used to understand the mechanical behavior of the α+β Ti alloy.

In this study, the low-cycle fatigue behavior of a cryogenic-rolled commercially pure titanium alloy was investigated, and compared with those of undeformed and room-temperature rolled ones. The amounts of deformation twins increased to >69.8% after cryogenic temperature rolling, and to 27.5% after room temperature rolling. Strain-controlled low-cycle fatigue tests were performed at total strain amplitudes 0.4 % ≤ Δ e t 2 ≤ 1.2%. The Coffin-Manson and hysteresis energy-based models confirmed that low-cycle fatigue resistance was remarkably improved with increasing the volume fraction of deformation twins by pre-deformation. As the proportion of deformation twins in the microstructure increased, the hysteresis loop area decreased, the striation spacing decreased, and the severity of crack-deflection behavior increased. At low Δ e t 2 = 0.4%, dislocation recovery was suppressed in the pre-deformed microstructure, so cyclic behavior was stable. However, at Δet/2 ≥ 0.8%, the recovery became the predominant mechanism, so well-defined cell structures formed and cyclic softening occurred. The smaller dislocation cells formed in RTR and CTR were considered to cause more severe crack arrest than in AR.

The use of selective laser melting (SLM) has become more common with regard to the fabrication of high-strength Ti-6Al-4V components. However, the as-SLMed Ti-6Al-4V alloy parts typically exhibit low ductility because of the formation of acicular and brittle α′ martensites. It is essential to use post-heat treatment to change their microstructures to achieve superior mechanical properties. In this study, a particular multi-step heat treatment (MSHT) was applied to the Ti-6Al-4V alloy samples fabricated via SLM. Conventional post-heat treatments were conducted for comparison. The microstructures and tensile properties of the as-SLMed and heat-treated samples were investigated. The as-SLMed Ti-6Al-4V sample is dominated by plate and acicular α′ martensites, which exhibit high strength (1280 MPa) and low ductility (9.0%). The conventional solution plus aging or stress-relieving treatment cannot result in a good combination of strength and ductility. A single step annealing process at 700 °C for 2 h changes the martensitic structure to a fine lamellar α+β structure, which provides a strong and ductile Ti-6Al-4V with an ultimate tensile strength of 1108 MPa and a total elongation of 17.6%. In contrast, the MSHT process facilitates the globularization of α and, thereby generates a nearly equiaxed structure, resulting in superior ductility (21.8% of total elongation), while maintaining a moderate ultimate tensile strength of 953 MPa.

In this study, we examined the microstructure and strength–ductility balance of pure Ti processed by cryogenic-temperature rolling (CTR) at various rolling reductions (RRs). The strength–ductility balance of the processed materials was evaluated by multiplying the yield strength by the fracture elongation, which were measured by tensile testing. Generally, the strength–ductility balance deteriorates as metallic materials are strengthened, but remarkably, CTR enhanced the strength–ductility balance while significantly strengthening pure Ti by adjusting the amount of RR. CTR formed numerous thin twins in the grains of the material with suppressed dislocation generation due to the increased twinning activity and relatively decreased slip activity at a low temperature, which cannot be achieved in typical processes performed at or above room temperature. This unique microstructure enhanced the strength–ductility balance of the processed material by intensifying grain refinement and improving resistance to necking instability. However, the strength–ductility balance deteriorated when a large RR was imposed due to the significant changes in the microstructure as the deformation proceeded. Therefore, it is important to adjust the amount of RR for CTR to impart its exceptional ability. We discuss the effects of microstructural factors on the strength–ductility balance in terms of possible strengthening mechanisms and strain-hardening capacity. The present findings could guide the design of optimized CTR for the production of pure Ti plates or sheets with excellent tensile properties and extend their use in industrial applications.

An artificial neural network (ANN) model was designed to correlate the complex relations among composition, temperature, and mechanical properties of 18Cr-12Ni-Mo austenitic stainless steels. The developed model was used to estimate the composition-property and temperature-property correlations with 97% and 91% accuracy, for train and unseen test datasets. The ANN predictions are more accurate with experimental results as compared with the calculated properties of the existing model. The effective response of the alloying elements on the mechanical properties at ambient as well as elevated temperatures was quantitatively estimated with the help of the index of relative importance (IRI). The calculated results of the ANN model beneficial for both researchers as well as designers to guide actual experiments. Hence, this proposed technique will be helpful in developing the components of austenitic stainless steel with desired properties.

Electron beam melted Ti–48Al–2Cr–2Nb alloy has undesirable creep resistance due to a near-gamma structure that is not proper for creep. We aimed to control the microstructure of EBM-built Ti–48Al–2Cr–2Nb using two-step heat treatment to improve the high-temperature (750 °C) creep resistance while also maintaining room-temperature ductility. As a result of two-step heat treatment, heat-treated EBM-built Ti–48Al-2r-2Nb alloy has a nearly lamellar microstructure with a thin lamellar structure and fine equiaxed gamma grains. High-temperature creep resistance was significantly improved after two-step heat treatment by inducing a thin lamellar structure. Room temperature ductility was also maintained even while containing a lamellar structure.