Titanium and Ti-based materials are advanced materials that have found applications across various fields of science and engineering. Over the past three decades, significant efforts have been made to employ severe plastic deformation (SPD) via the high-pressure torsion (HPT) method on Ti-based materials. These efforts aim to achieve ultrafine grains with a high density of lattice defects, control phase transformations, and synthesize new compounds. Pure titanium, and Ti-containing alloys, intermetallics, ceramics and composites have been processed by HPT to enhance mechanical and functional properties. These properties include high hardness, high strength combined with plasticity, biocompatibility, superconductivity, dielectric performance, photocatalysis, electrocatalysis and photovoltaic capabilities. Additionally, the synthesis of new high-entropy alloys and ceramics via HPT has emerged as a promising direction, contributing to various disciplines such as photocatalysis, biomedical devices and hydrogen storage. This article provides a brief overview of the SPD concept and the HPT method, followed by a summary of recent advances in the HPT processing of Ti-based materials.

The onset of plasticity in engineering titanium alloys with bimodal microstructures depends on the activation of slip in the equiaxed α grains. We use protocols based on slip trace offset analysis to measure the ratios of critical resolved shear strengths (CRSS) in the equiaxed α grains in bimodal microstructures of Ti 6242 and Ti 6246 alloys. The resulting CRSS ratios for prismatic 〈a〉/basal 〈a〉 and pyramidal 〈a〉/basal 〈a〉 for Ti 6242 vary depending on whether the probability of slip is estimated at high Schmid factors for these slip systems or average Schmid factors over the range of orientations of equiaxed α used in the dataset. We compare these ratios to the existing data on these and other titanium alloys. We then examine slip in transformed β in these bimodal structures and conclude that deformation in transformed β is influenced strongly by slip transfer from equiaxed α arising from orientation relationships between equiaxed α and β rather than global Schmid factors associated with the load axis.

This study investigates the microstructural evolution, mechanical properties, and biological response of the Ti-34Nb-2Ta-3Zr-0.5O (mass%) alloy processed by High-Pressure Torsion (HPT) at ambient temperature, focusing on its suitability for biomedical applications. The HPT process significantly increased the fraction of high-angle grain boundaries, particularly in the range of 50–60°. Grain refinement was observed after both 2 and 4 turns of HPT; ultra-fine grain fraction increased with an increase in the number of turns of HPT. No phase transformations occurred during deformation. The bulk texture indicates the formation of a fibrous texture post-HPT processing. The hardness of the samples increased up to 348 HV after 4 turns compared to the alloy not subjected to HPT (280 HV). The HPT samples demonstrated higher E_corr and lower I_corr values, indicating improved corrosion resistance in phosphate-buffered saline. In vitro cell studies revealed that HPT did significantly affect the viability and growth of osteoblasts on the TNTZ alloy. Taken together, these results establish HPT as a promising processing route for enhancing the biomedical performance of TNTZ toward developing high-performance bone implants.

In this current study, polycrystal modelling has been utilized to understand the anisotropic dwell fatigue behavior of Cp titanium along rolling and transverse directions. A 2D microstructure of polycrystalline Cp titanium obtained through Electron Back Scattered Diffraction (EBSD) microstructure which is used to study the heterogeneous stress distribution, slip activity, and evolution of texture during tensile and dwell fatigue deformation through experiments and crystal plasticity fast Fourier transform (CPFFT) simulations. A significant anisotropy in the strain hardening is observed when deformed along the rolling direction when compared to the transverse direction which was attributed to the difference in texture which eventually leads to high dwell fatigue life along RD when compared to TD. The grains which developed a higher amount of stress post-deformation are labelled as stress hotspots which interestingly were observed to be higher during tensile deformation when deformed along RD when compared to TD and during dwell fatigue deformation the number fraction of stress hotspots are high along TD when compared to RD. The tensile deformation stress hotspots along RD have initial orientations // ⟨1010⟩ and ⟨2110⟩ direction whereas in case of TD grains the stress hotspots have initial orientations which are closer to ⟨0001⟩ and ⟨2110⟩. The stress hotspots grains tend to reorient towards the ⟨1010⟩ direction along RD and towards ⟨0001⟩ and ⟨1010⟩ along TD during tensile deformation. The present study shows that crystallographic texture is a great tool to understand the anisotropy of dwell fatigue life of Cp titanium.

Titanium alloys exhibit superplastic behavior—mainly in the two-phase (α+β) alloys—with the presence of a fair amount of β phase in the alloy. SP-700 (Ti-4.5Al-3V-2Mo-2Fe) is one of the β rich (α+β) titanium alloys which show superplasticity at temperature approx. 100°C lower than the popular Ti-6Al-4V alloy without much increase in the flow stresses. However, it is still a relatively lesser explored alloy and only limited studies are available in this regard. Thus, a comprehensive study was needed to understand its superplastic behavior in terms of microstructural evolutions such as volume fractions of α and β phases; role of texture and so on. In the current study, superplastic behavior of SP-700 alloy in the 90 pct. (α+β) rolled condition was evaluated in two-phase conditions at 700°C and 775°C temperatures and 10⁻³ s⁻¹ strain rate on the sub-size flat specimens. The tested specimens were characterized from grip, central gauge, and near fracture regions by EBSD technique to understand the roles played by the α and β-phases during the superplastic deformations. While grain boundary sliding (GBS) was found to be the dominant deformation mechanisms during superplastic deformation at higher temperature; reasonable contribution from the slip-based deformation was also observed along with GBS at lower temperatures.

In the present work, the microstructure and texture evolution in commercially pure titanium subjected to blast assisted deformation has been investigated by means of electron backscattered diffraction and transmission electron microscopy. The evolved texture in the deformed material is primarily attributed to the dominant ⟨1010⟩ 64.4° contraction twinning. Other deformation twins were also observed in a low fraction. A uniform dislocation background with other microstructural features observed suggested the formation of a superimposed microstructure in the material. {1121} extension twins (ET2) with a thick uniform pile-up of dislocation along its boundary have formed in the material. Parts of ET2 have detwinned leaving gaps along the twin length, revealing the unstable nature of ET2 twins. The in-depth microstructural analysis reveals the ET2 twin formation mechanism, where the instability developed in the material due to progressive lattice rotation via accumulative slip is relieved by ET2 twin formation by atomic shuffling. The gaps (local detwinning) observed in the present case is attributed to the transient oscillatory response of the material under impulsive loading, which has been previously reported in the literature.

Additive manufacturing of workhorse Ti6Al4V aerospace alloy components by direct metal laser sintering (DMLS) offers cost-efficiency in producing complex designs. However, the rapid and complex heating and cooling cycles during manufacturing can lead to undesirable microstructures and mechanical properties that are inferior to the wrought product. The present investigation aims to study the microstructure-fracture toughness paradigm for the heat treated DMLS Ti6Al4V sample with conventional thermos-mechanically processed microstructures. To this end, DMLS Ti6Al4V samples were subjected to isothermal heat treatment at 800°C for 1 h to obtain a basketweave α+β dual phase microstructure, while hot rolled samples with equiaxed α+β microstructure were subjected to heat treatment at 1000°C for 1 h to obtain a lamellar α+β microstructure. Fracture toughness tests were performed in three-point bend geometry on fatigue pre-cracked specimens for the three distinct microstructures. The fracture toughness of the heat treated DMLS parts is comparable to the thermo-mechanically treated lamellar α+β microstructure and superior to the equiaxed microstructure. Full-field strain measurement was performed during fracture toughness testing using digital image correlation and detailed microstructural characterisation was performed using electron backscatter diffraction and synchrotron diffraction. It was revealed that the deformation within the lamellar α+β phase delays the crack nucleation, while further crack propagation through thicker α laths and prior β grain boundaries contribute to pronounced crack tortuosity or crack path deflection resulting in a more corrugated fracture surface and enhanced fracture toughness.

Commercially pure titanium rolled sheets with different grain sizes of 20, 50, and 80 µm were applied to tensile tests to investigate the effects of grain size on the relationship between mechanical properties and activities of slip systems. While the ductility was independent of grain size, the activity of first order pyramidal slips and second order pyramidal slips decreased with decreasing grain size. In addition, grain boundary sliding was found to contribute to ductility when the grain size was small. Prismatic slips were activated in all of the specimens when yielded. Activity of pyramidal slips increased with increasing strain and decreased with decreasing grain size. Tensile tests were interrupted and slip lines were observed after the unloading in this study. Yield stress increment was observed when reloaded in interrupted tensile tests, but not in immediate reloading tensile tests. We found room temperature strain aging in pure titanium sheets. Yield stress increment increased with increasing strain.

This Paper was Originally Published in Japanese in J. JILM 74 (2024) 421–426.

This study investigates the mechanical properties of fully-lamellar Ti-6Al-4V, focusing on thermally activated processes behind yielding. Temperature dependences of yield stress, effective stress, activation volume, and activation enthalpy were examined within the temperature range of 77 K to 650 K. The change in the activation volume indicates a change in the deformation mechanism as a thermally activated process. The activation enthalpy in the fully-lamellar Ti-6Al-4V was closer to that of the basal slip in single-crystalline commercially pure Ti, indicating the dominance of basal slip in the fully-lamellar Ti-6Al-4V. Additionally, this study compares the difference in activated slip systems of this alloy with bimodal Ti-6Al-4V and Ti-0.45O, and suggesting changes in deformation mechanisms.

Ni-TiB₂ composites can also maintain high hardness at high temperatures, which has a wide range of application prospects, but due to the low self-diffusion coefficient of TiB₂, dense sintering is more difficult. Adding Ni as the binder phase and using the SPS sintering process can significantly reduce the sintering temperature of the composites and obtain the composites denser and improve the toughness. In this research, we investigated the sintering process of Ni-TiB₂ composites at 5%–15% Ni content, sintering temperatures between 1273 K–1473 K, holding times between 300 s–1200 s, in vacuum environments with 50 MPa of pressure. Using Box-Benhnken (BBD) design and Response Surface Methodology (RSM) were applied to optimize the process parameter variables including sintering temperature, nickel content and holding time, and hardness as a response variable. Based on the statistical model, the predicted optimal sintering process is Ni-10%, sintering temperature is 1433 K and holding time is 930 s. At the optimum process, the hardness and relative density of the composites were 2036 HV and 96.8%. The experimental predictions yielded an optimal average hardness of 2065 HV, an optimal density of 4.56 g/cm³, and an optimal relative density of 96.5%. The experimental results are highly consistent with the model prediction.

This study presents measurements of electrical conductivity and Vickers microhardness of ω phase in pure Ti. Samples containing 100% ω phase is produced by a high-pressure synthesis under an elevated temperature. The results are compared with those of 100% α phase in an as-received state and of the samples processed by high-pressure torsion (HPT) where severe plastic strain is imposed under a high pressure. For the electrical conductivity measurement, a contactless method using a superconducting quantum interference device magnetometer is employed, which allows the measurement over a wide range of temperature down to the liquid helium temperature. Vickers microhardness measurement is conducted for the ω phase under different applied loads to minimize the effect of reverse transformation from the ω phase to the α phase during the measurement. Microstructures are observed by electron back scatter diffraction analysis, showing that the grain size is of ∼12 µm containing less dislocations, and this structure is in contrast with the HPT-processed sample having high densities of dislocations and grain boundaries. This difference in the microstructure results in appreciably lower electrical conductivity in a temperature range below ∼100 K for the HPT-processed sample. No anomaly of a superconductive signal is detected in the ω phase down to the temperature of 1.8 K, suggesting that a superconductive state does not exist at ambient pressure in the corresponding temperature range.

This study investigates the effects of severe plastic deformation (SPD) techniques, particularly high-pressure torsion (HPT) and high-pressure sliding (HPS), on the microstructural evolution and mechanical properties of commercially pure (Grade 2) Ti. The experiments were conducted under pressures of 2, 5, and 6 GPa. For the crystallographic analyses, X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used. Nanostructured Ti was obtained after processing by HPT and HPS, and the phase transformation from alpha (α) to omega (ω) phase was confirmed under pressures of 5 and 6 GPa. Vickers microhardness and tensile tests confirmed that HPT-processed samples exhibited increased strength under higher pressures, while the HPS process produced more homogenous material properties, along with a promising strength-to-ductility ratio. These findings indicate that the HPS process may offer better control over microstructure and mechanical performance, making it a promising technique to enhance the mechanical properties of pure Ti for biomedical applications.

We investigated the effects of surface texturing dimple-shaped patterns via picosecond pulsed laser processing on the wear behaviors of titanium alloy discs in contact with zirconia (ZrO₂) ball by comparing three different alloy types. As an α-type titanium alloy, commercially pure titanium (CP-Ti) was used in the present study, and the results were compared with those obtained using an α+β- and β-type titanium alloy, Ti-6Al-4V ELI (Ti64) and Ti-29Nb-13Ta-4.6Zr (TNTZ), in the previous study. The wear behaviors of these alloys were dominated by different wear modes depending on the alloy type. Abrasive and adhesive wear modes were dominant in Ti64 and TNTZ, respectively. Conversely, adhesion of wear debris was observed in CP-Ti but the amount of adhered wear debris was smaller than that in TNTZ. In Ti64, the wear debris acted as abrasive particles. By contrast, the wear debris easily adhered to the TNTZ disc surface and formed a hard wear-protective layer. The wear debris adhered to the CP-Ti disc surface too but the amount was smaller, and the protective effect was weaker than that observed for TNTZ. These differences in wear debris characteristics lead to different impacts of surface texturing on the wear behavior of titanium alloys. As the dimples fabricated on the surface can trap wear debris, they effectively reduce wear in Ti64 but are detrimental to TNTZ. Both these effects on wear appeared for the dimples on CP-Ti because its wear debris was adhesive just after its generation, due to its low proof stress, similar to that of TNTZ. However, the wear debris was more prone to oxidation and hardened more easily, resulting in its transition to abrasive particles, similar to those of Ti64.

This study examines the effect of shock loading on allotropic transformation of pure Ti. The samples are initially processed by high-pressure torsion under 2 and 6 GPa to impart intense shear strain and they are subjected to shock loading at an impact speed of 702 m/s. X-ray diffraction analysis as well as hardness measurement is carried out to check the formation of ω phase. It is shown that intense shear strain before shock loading does not promote the ω phase formation but concurrent shear straining under high pressure is effective for the ω phase formation.

The production of ultra-thick Ti-6Al-4V (Ti-64) alloy plates is critical for aerospace applications, however, meeting the demanding mechanical and microstructural standards presents significant challenges. This study focuses on optimizing the manufacturing process of 75 mm ultra-thick Ti-64 plates to meet the stringent requirements of the AMS4905E specification. Initial attempts revealed that the presence of the primary hcp-α phase, even after β-annealing, severely limited ductility and work-hardening capacity. The mechanical performance, particularly elongation, failed to meet the standard despite achieving adequate yield strength (YS) and ultimate tensile strength (UTS). To overcome these limitations, an optimized process was developed, incorporating a homogenization step prior to hot forging and a globularization step after hot rolling. This approach was aimed at achieving a more uniform microstructure and enhancing the α-to-β phase transformation during heat treatment. Microstructural characterization using optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) confirmed the elimination of the primary hcp-α phase, with the formation of a refined and homogenous Widmanstatten structure. These changes resulted in a significant improvement in mechanical properties, particularly elongation, which increased from 4–7.5% in the preliminary trials to 10–11.5%, fully meeting the AMS4905E requirements. In addition to mechanical properties, the size of the prior β grains was carefully controlled to remain within the specified limits of the standard, ensuring full compliance with both microstructural and performance criteria. This optimized process not only enhances the mechanical performance of 75 mm ultra-thick plates but also establishes a reliable foundation for the future production of 100 mm thick Ti-64 plates, which are currently under development. The advancements demonstrated in this study contribute significantly to the field of high-performance titanium alloys for aerospace applications.

In this study, the effects of slow cooling treatment on the microstructure of Ti-xZr (x = 15–45 at%) binary alloys were investigated. The alloys fabricated by vacuum arc melting were annealed at 1000°C (β phase field) for 2 h and subsequently cooled to room temperature (α phase field) at 50°C/h. The crystal structure was investigated using X-ray diffraction, confirmed that the constituent phase of the slow-cooled Ti-xZr alloys was only α phase. However, optical microscope and scanning electron microscope observations for the alloys with x = 33–37 revealed that the layered microstructure was formed at various locations within α grains. In addition, the layer thickness decreased with increasing x. Compositional analyses for the layered microstructure were performed by energy dispersive X-ray spectroscopy, showing that Zr-rich and Zr-lean layers were arranged alternately. Therefore, it was found that a compositional modulation was occured in the layered microstructure. To investigate the crystallographic orientation of the layered microstructure, electron backscatter diffraction measurement was conducted. This result revealed that the c-axis direction of the hexagonal close-packed structure was oriented parallel to the layers. Moreover, the kernel average misorientation value in Zr-rich layer was higher than that in the Zr-lean layer. In fact, transmission electron microscope observation indicated that the unit cell orientation in the Zr-rich layer was slightly misaligned from that in the Zr-lean layer. The Vickers hardness of the layered microstructure was slightly lower than that of the random microstructure. From these results, it is expected that the slow cooling treatment for the Ti-Zr alloys is an effective approach to introduce periodicity into the microstructure.

In TiC-Ti₃SiC₂ composites, the introduction of the TiC hard phase could improve the hardness of the Ti₃SiC₂ phase and prepare high hardness, high toughness and self-lubricating materials. In this study, elementary Ti, Si and graphite powders in the molar ratio of 3:1+x:2 were used as starting powders, a small amount of Al powder was added as a sintering aid, and spark sintering technology was used to sinter in the temperature range of 1373 to 1673 K at 50 MPa in vacuum. The reacted phases were identified by X-ray diffraction, and microstructure characteristics were observed. The formation mechanism of TiC-Ti₃SiC₂ composites was investigated based on the sintering behavior of the elemental powders during spark sintering. Pure TiC-Ti₃SiC₂ composites could be prepared from Ti, Si, graphite powders and Al at a molar ratio of 3:1.1:2:0.3 at 1573 K with a holding time of 0.9 ks. In conclusion, dual phase consist of TiC and Ti₃SiC₂ composite could be synthesized through in-situ reaction by adjusting process parameters such as sintering temperature, holding time and the content of Si and Al in element blending method.

Titanium has the highest strength-to-weight ratio of any metal. Titanium and its alloys are strong and lightweight, and are therefore used in many industrial applications. The ω-phase is known to form metastably during quenching and aging processes of titanium alloys, and the presence of this phase causes increases in strength and decreases in ductility. However, the mechanical properties of the ω-phase are poorly measured because this phase precipitates as nanograins in titanium alloys. Here we report the fabrication of bulk polycrystalline ω-phase samples in pure titanium under high pressure and temperature conditions of 12 GPa and 400°C. They are single-phase and randomly oriented polycrystalline materials with an average grain size of 21 ± 8 µm. The 0.2% offset yield strength was determined from compressive stress-strain curves to be 913 ± 3 MPa. The ω-phase is more than twice as strong as the α-phase in pure titanium and could be used as a non-toxic structural material for biomedical applications.

Treatment of severe bone fractures is possible using Ti alloy-based artificial bone. However, infections caused by bacterial proliferation at the replacement material remain a critical issue. This study aimed to add antibacterial properties to Ti by adding Cu, which was recognized for its antibacterial effects. Ti-(5, 8, 10 at%)Cu alloys were fabricated, and heat treatments were performed at temperatures ranging from 300 to 1000°C in the Ar and Ar-2% O₂ gas atmospheres. The effect of heating temperature and atmospheres on the distribution of Cu at the sample’s surface was investigated. Cu distribution was evaluated using XPS depth profiles. The Cu concentration at the surface of the as-polished sample and the heat-treated sample at temperatures above 700°C were lower than the alloy composition. The enrichment of Cu at the surface of the oxide layer formed on the sample’s surface is achievable through the heat treatments in an Ar or Ar-2% O₂ atmosphere at 300 to 500°C. The appropriate heat treatments avoid the formation of the Cu-depleted layer on the surface of Ti-Cu alloys, expecting to maintain the antimicrobial properties of the alloy.

This study investigates the spark plasma sintering behavior of TiB₂-10vol.%Ni powder composites, focusing on the effects of TiB₂ particle size variation on microstructure and toughness. The experimental results confirm that utilizing TiB₂ powders with mixed particle sizes enhances sintering performance and improves fracture toughness. Two types of TiB₂ powders were used: fine TiB₂ powder (STiB₂, ∼3 µm) and a mixed powder containing larger TiB₂ particles (LTiB₂, average size ∼15 µm). These powders were mixture with Ni powder and using spark plasma sintering preparation of composite material. The sintering behaviors corresponded to the obtained characteristic microstructures, as follows: (1) For 100STiB₂-10Ni, the relative density increased from the lowest temperature compared with others, which corresponds to random distribution of STiB2 and Ni. (2) For 100LTiB₂-10Ni, heterogeneous area consisting of fractured particles by initial loading corresponded to high level in initial relative density. (3) For 50STiB₂+50LTiB₂-10Ni, there were both similar areas to in 100STiB₂-10Ni and 100LTiB₂-10Ni, which correspond to density increase at initial stage and sintering promotion after medium temperature. The highest final density level was obtained by deformation of continuous Ni layer in heterogeneous region, such as100LTiB₂-10Ni. The sintering curve of 50STiB₂+50LTiB₂-10Ni could be explained by the behavior of both 100STiB₂-10Ni and 100LTiB₂-10Ni. Its relative density showed the highest value, and its sintering rate was close to that of pure Ni, which showed a high value. Furthermore, it was found that 50STiB₂+50LTiB₂-10Ni exhibited high fracture toughness values.

This study investigates the impact of nozzle cavity structure on the flow and thermal field of aluminum melt to enhance casting nozzle design. FLUENT was used to perform numerical simulations on an ultra-wide 8021 aluminum alloy Hunter-type nozzle. The findings reveal that the tertiary flow distribution method exhibits a more dispersed flow, larger vortices, and greater temperature changes, particularly from the center to the edge of the nozzle, compared with the secondary flow distribution method. Consequently, optimizing the two-stage flow distribution significantly reduces velocity and temperature fluctuations in the aluminum melt at the nozzle cavity exit. This optimization enhances both the thermal field and uniformity of the melt flow, confirming the feasibility of the improved nozzle structure design.

In order to achieve sustainable development goals (SDGs) in the casting process and become a decarbonized society by 2050, decarbonization in the manufacturing process, such as the utilization of converted biomass resources, should be promoted. Recently, using recycled steel scraps that contain small amount of carbon tends to increase as a cheap iron source. In this case, the total amount of carburizer has to increase. Therefore, it is necessary to examine the characteristics of biomass, which is a sustainable carbon source and carbon-neutral, as a substitution for coal coke and to examine the casting method that is friendly to the environment.

Biocoke is a solid product to utilize biomass effectively. Biocoke can be produced from herbaceous biomass. The possibility to promote the domestic production of the carburizer can be expected by using bamboo as a raw material. The carburizing effect of biocoke as a carburizer in the melting process using a high frequency induction melting furnace has been confirmed. However, the carburizing process and mechanism of carburizer made from biomass have not been studied.

In this study, Bio-coke with different degrees of carbonization was carried out in the casting process to clarify the carburizing process. The graphitization degree was used for the evaluation of the carbon on the contact surface between bio-coke and molten metal using the graphitization degree. The consideration of the carburizing process was carried out from the carburizing effect and graphitization degree. The lower the degree of crystallinity of carbon in bio-coke, the faster the carbonization was carried out. It was found that the carburizing was performed when the R-value, which indicates the degree of graphitization, reached around 0.8.

This Paper was Originally Published in Japanese in J. JFS 95 (2023) 9–15.

The stress-induced martensitic transformation and superelasticity at room temperature were investigated by tensile tests on a series of single crystal Cu-17Al-xMn (x = 11.4, 12, 13, and 14 at%) shape memory alloys with a 〈110〉 orientation, aiming to increase the critical stress for martensitic transformation without plastic deformation. As the Mn content increased, the martensitic transformation temperatures decreased, and the critical stress for martensitic transformation increased from 180 MPa (11.4Mn) to 590 MPa (14Mn), exhibiting excellent superelasticity. For the 14Mn alloy, which showed a high critical stress, superelasticity was successfully obtained at temperatures ranging from 163 K to 293 K, and the temperature dependence of critical stress was revealed. Based on these data, the critical stress in Cu-17Al-Mn alloys with the 〈110〉 orientation was approximately formulated as functions of Mn content and testing temperature.