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Hydroxyapatite (HA) coating has been carried out on a titanium alloy Ti–29Nb–13Ta-4.6Zr (TNTZ) to improve the biocompatibility and bioactivity of the alloy in order to attain a high degree of osseointegration and low inflammatory reactions in bone tissue after implantation in orthopedic implant applications. The coating process was carried out using the electrophoretic deposition (EPD) method. TNTZ implants with and without HA coating were installed in the tibia of the Rattus norvegicus Wistar test rats; the test animals were then terminated after two weeks. The degree of osseointegration was measured using a removal torque tester, the Tumor Necrosis Factor-Alpha (TNF-α) level was measured by using the Enzyme-Linked Immunosorbent Assay (ELISA) method, and a histopathological analysis was carried out on the implanted tissue. TNTZ implants with HA coating exhibit significantly higher osseointegration compared to those without HA coating, whereas TNTZ implants without HA coating can trigger significantly higher TNF-α levels. Greater osteogenesis occurs in a bone tissue fitted with TNTZ implants with HA layers. The HA coating on the surface of the TNTZ implant can reduce excessive inflammation, enhance the osteogenesis process, and then improve osseointegration in the bone tissues of test animals. It is also expected to be suitable for orthopedic implant applications.

In this study, the TiO2-matrix coatings including hydroxyapatite (HA) and tricalcium phosphate (TCP) were successfully deposited on the β-type Ti–29Nb–13Ta–4.6Zr alloys by plasma electrolytic oxidation (PEO) process to enhance the in-vitro corrosion resistance of the alloys. An electrolyte containing calcium hydroxide (CaOH) and sodium phosphate dodecahydrate (Na3PO4·12H2O) were used to prepare micro-arc oxidized samples. The effect of duty cycle, deposition frequency, coating thickness, and surface morphologies have been discussed in the view of corrosion phenomena. The coated morphologies were characterized with scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The corrosion behaviors of the coatings were investigated using potentiodynamic polarization scanning (PDS) technique in Ringer's solutions at body temperature. In addition, the immersion tests on the coated and uncoated samples were performed to observe the apatite phase formation on the surface. X-ray diffraction technique (XRD) has been used in order to determination Ca- and P-based phases on the surfaces. It was found that the corrosion rates of the all coated samples were approximately 4–14 times lower than the uncoated TNTZ sample. The highest resistant coating to corrosion was obtained in the oxidized TNTZ samples deposited at 500 Hz with 30% duty cycle by PEO.

Friction stir welding (FSW) was applied to a butt joint between the 2-mm-thick plates of an (α+β)-type titanium alloy with high hot workability, Ti-4.5Al-2.5Cr-1.2Fe-0.1C alloy (Ti531C), to obtain defect-free welds. Refined equiaxed grains (equiaxed structure) were obtained and anisotropic mechanical properties of the parent material disappeared by grain subdivision of the α phase during FSW. However, grain diameter and crystal texture of the equiaxed structure remained unchanged during annealing. The tensile strengths of the welded joints with and without annealing were comparable to that of the Ti531C parent plate. However, the elongations of the welded joints were lower than that of the Ti531C parent plate because the stir zones are harder than the base metal, which results in deformation preferentially in the base metal region of the specimens. Tensile testing caused a failure in the base metal region in all welded joints. Annealing improved the high-cycle fatigue strength of the welded joints greatly. However, the high-cycle fatigue strength of the welded joints subjected to annealing was still slightly lower than that of the Ti531C parent plate. Fatigue failure point was in the stir zone or at the boundary between the stir zone and the base metal, which are different from the tensile test. These results imply that failure factors are different between tensile and fatigue tests. The small portion of the isothermal ω phase in the β phase in the stir zone and discontinuous microstructure at the boundary between the stir zone and the base metal were potential stress concentration sites, inducing high-cycle fatigue failure.

The growth process of α phase in β-type bio-Titanium alloy TNTZ during aging at 723 K is investigated. The results show that acicular α phase has a Burgers orientation relationship with β matrix of {1−10}β//(0001)α, 〈111〉β//〈11−20〉α. The kinetics equation of acicular α phase length meets l = 333(t-3.95)^0.0764, while the kinetics equation of diameter meets d = 7.50(t-2.83)^0.353. The growth process of α phase can be divided into three stages. During stage I, α phase grows rapidly along both the long direction and the diameter. At beginning of stage I, fine acicular α phase grows with the long direction parallel to each other. Subsequently, α phase grows with another long direction beside the former one in a ‘V-shape’ or in a ‘near perpendicular shape’. During stage II, α phase grows slowly along the diameter, while, the growth along long direction will be gradually restricted with the other α around it. The phase distributes more homogeneously, and present 5 classic possible orientations between α variants. During stage III, α phase grows slowly along the diameter, and stops growing along long direction. The growth of α phase is affected by distortion energy, interface energy and diffusion anisotropy, and it is mainly controlled by Nb diffusion through the ledge riser of the interface between α and β phases.

In this study, Ti3.6Nb1.0Ta0.2Zr0.2 based coatings were deposited on anodized and non-anodized aluminum surfaces by PVD. For this purpose, a target for the magnetron sputtering PVD system was prepared from a β-type Ti–29Nb–13Ta–4.6Zr bulk alloy via hot-forgings. Surface morphologies and elemental composition analysis of the TNTZ-based PVD coatings were conducted using scanning electron microscopy attached with an energy dispersive spectroscopy incorporated (SEM-EDS). A micro-scratch and hardness tester was used to evaluate the adhesion and mechanical properties of the deposited coatings. The contact angle and electrochemical corrosion measurements were applied to assign the wettability levels and potential usability of the coated surfaces under in-vitro conditions. Also, the corrosion resistance of the coatings was compared with commercial Ti and Ti6Al4V alloys. The results showed that the coated surfaces were exhibited a hydrophilic behavior and adequate in-vitro resistance in simulated body fluids (SBF). Besides, the TNTZ coatings on anodized surfaces were presented different cracking mechanisms and higher adhesion resistance than that of their non-anodized surfaces. The reason for the behavior has been discussed in the present study.

Good ductility, low magnetic susceptibility, and tunable Young’s modulus are highly desirable properties for materials usage as spinal fixation rods. In this study, the effects of niobium content on the microstructure, magnetic susceptibility, and mechanical properties of Zr– x Nb (13 ≤ x≤23 wt%) alloys were investigated. For the Zr–15Nb and Zr–17Nb alloys, a remarkable increase in Young’s modulus was achieved due to the occurrence of deformation-induced ω phase transformation. This was the result of the competition of two factors associated with the Nb content: an increase of the stability of β phase and a decrease of the amount of athermal ω phase with increasing Nb content. When the Nb content was 15% or 17%, the amount of deformation-induced ω phase was maximum. Moreover, the magnetic susceptibility decreased with the deformation-induced β → ω phase transformation, and the Zr–17Nb alloy with apparent kink bands exhibited a smaller amount of springback than the Zr–15Nb alloy with {3 3 2} 〈1 1 3〉 mechanical twins. Furthermore, the ions released from the Zr– x Nb alloys in accelerated immersion tests were at a very low level. The combination of low initial Young’s modulus, and its remarkable variation induced by deformation, low magnetic susceptibility, good ductility, and smaller springback make the Zr–17Nb alloy a potential candidate for spinal fixation rods. Statement of Significance For the rods of spinal fixation devices, it is important but difficult to lower the springback for bending formativeness while keeping the low initial Young′s modulus for biocompatibility and lower the magnetic susceptibility for postoperative examination simultaneously. In this study, Zr–17Nb alloy was successfully developed via deformation-induced ω phase transformation during loading, simultaneously meeting the abovementioned properties for spinal fixation rods.

Deformation-induced ω-phase transformation during tensile deformation was investigated in the developed spinal-support alloy, Ti-9Cr-0.2O. Both preferential single-variant ω transformation along its [0001] direction and growth and/or assembling of uniformly distributed ω particles undergo in the alloy during tensile deformation. It is confirmed that this deformation-induced ω-phase transformation can be triggered by elastic strain or stress without plastic deformation. Furthermore, a re-orientation process works for this transformation; that is, the ω1 variant may re-orientate into the ω2 variant via {001} ⟨110〉 twinning-type mechanism probably related to the external loading condition and the orientations of ω and β phases.

The surface treatment technology known as ‘cavitation peening’ was employed in this study in order to enhance the durability of spinal implant fixture applications, which are subject to fretting fatigue. Cavitation peening can be realized by a technique in which a high-speed water jet is injected into water through a nozzle. It utilizes a phenomenon by which surface impacts due to collapsing cavitation bubbles induce work-hardening by introducing residual compressive stress near the surface. A fretting fatigue test was conducted on a spinal implant rod made of Ti-6Al-4V ELI in accordance with the ASTM F1717 standard, which is the established method for testing spinal implants after they are treated by cavitation peening. The residual stress was evaluated by using X-ray diffraction analysis. The hardness over the cross-sectional area was also measured using an indentation test. The obtained results show that cavitation peening drastically improves the fretting fatigue properties of spinal implant fixtures by as much as 2.2 times compared to untreated ones. This can be attributed to a significant increase in the hardness from 5.0 to 9.6 GPa and a high compressive residual stress of over 600 MPa induced by cavitation peening.

A non-toxic and non-allergic β-type Ti–29Nb–13Ta-4.6Zr (hereafter abbreviated as TNTZ) target material was prepared by the vacuum arc-melting process and then hot-forged, successfully. The TNTZ target has been deposited on the pure magnesium (Mg) and AZ31 Mg alloys by the physical vapor deposition (PVD) method. The characterization of the coatings was carried out by using SEM, EDS, AFM, and XRD techniques. The coating adhesion and its hardness were determined by scratch and Vickers tests, respectively. The corrosion resistance of the coatings was analyzed in simulated body fluid (SBF) under in-vitro conditions. Also, the coating wettability was compared by contact angle measurements. The results showed that the coatings had a very dense columnar and compact microstructure. The grain sizes were calculated between 20 and 40 nm, and the porosity of the coating was about 8.5% (±1.5). The hydrophilicity of the Ti-based PVD coating was better than the uncoated Mg. The failure mechanism of the coating during the scratch test was formed as conformal cracks. The in-vitro corrosion tests indicated that the Ecorr values of the TNTZ coated pure Mg and AZ31 alloy were nobler about 400 mV than the uncoated Mg-based samples. At the intensive stage of the corrosion, remarkable corrosion products, grown pits, and deep cracks were also observed on TNTZ-based coating layers due to the heavy corrosion attacks in SBF.

The effect of oxygen on stability of isothermal omega precipitates in Ti-29Nb-13Ta-4.6Zr was examined using X-ray powder diffraction, transmission electron microscopy, and atom probe tomography. Two alloys with 0.1 and 0.4 mass% oxygen were subjected to single step, and two-step annealing heat-treatments to respectively promote omega and alpha formation. After second step annealing, large volume fraction of omega precipitates was retained in 0.4 mass% O alloy while mainly alpha phase was observed in TNTZ-0.1O. The enhanced stability of omega in the higher oxygen containing TNTZ alloys questions the conventionally accepted understanding that oxygen destabilizes the omega phase in titanium alloys.

Bioactive oxide layers were fabricated on Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) by three different alkali solution treatments: hydrothermal (H), electrochemical (E), and hydrothermal-electrochemical (HE). The adhesive strength of the oxide layer to the TNTZ substrate was measured to determine whether this process achieves sufficient adhesive strength for implant materials. Samples subjected to the HE process, in which a current of 15mA/cm(2) was applied at 90°C for 1h (HE90-1h), exhibited a comparatively higher adhesive strength of approximately 18MPa while still maintaining a sufficiently high bioactivity. Based on these results, an oxide layer fabricated on TNTZ by HE90-1h is considered appropriate for practical biomaterial application, though thicker oxide layers with many cracks can lead to a reduced adhesive strength.

Nanostructured hydroxyapatite (HA)–graphene nanosheet (GN) composites have been fabricated by spark plasma sintering consolidation. Nanostructual evolution of the bioceramic-based composites during further high temperature heat treatment is characterized and enhanced mechanical strength is assessed. GN keeps intact after the treatment and its presence at HA grain boundaries effectively inhibits HA grain growth by impeding interconnection of individual HA grains. Microstructural characterization discloses strong coherent interfaces between GN and the (300) plane of HA crystals. This particular matching state in the composites agrees well with the competitive theoretical pull-out energy for single graphene sheet being departed from HA matrix. The toughening regimes that operate in HA–GN composites at high temperatures give clear insight into potential applications of GN for ceramic matrix composites.

Titanium and its alloys are suitable for biomedical applications owing to their good mechanical properties and biocompatibility. Beta-type Ti–Mn alloys (8–17 mass% Mn) were fabricated by metal injection molding (MIM) as a potential low cost material for use in biomedical applications. The microstructures and mechanical properties of the alloys were evaluated. For up to 13 mass% Mn, the tensile strength (1162–938 MPa) and hardness (308–294 HV) of the MIM fabricated alloys are comparable to those of Ti–Mn alloys fabricated by cold crucible levitation melting. Ti–9Mn exhibits the best balance of ultimate tensile strength (1046 MPa) and elongation (4.7%) among the tested alloys, and has a Young’s modulus of 89 GPa. The observed low elongation of the alloys is attributed to the combined effects of high oxygen content, with the presence of interconnected pores and titanium carbides, the formation of which is due to carbon pickup during the debinding process. The elongation and tensile strength of the alloys decrease with increasing Mn content. The Ti–Mn alloys show good compressive properties, with Ti–17Mn showing a compressive 0.2% proof stress of 1034 MPa, and a compressive strain of 50%.

ZrAlCoNb bulk metallic glasses with different Nb contents were fabricated by copper mold casting. Corrosion behavior, mechanical properties and cell cytotoxity were investigated. The investigated Zr-based bulk metallic glasses exhibit high corrosion resistance due to the enrichment of Zr and Al in the oxide layer. The yield strength of 1975 MPa and a plastic strain of 3.5% for Zr56Al16Co23Nb5 BMG are obtained. The cell viability is improved with increasing of Nb content.

In order to develop new low-cost and high-strength β-type titanium alloys, a Ti–13Mn was fabricated by metal injection molding. For improving its tensile strength, Ti–13Mn was subjected to cold-rolling at reduction ratios of 60% and 90%, respectively. The solutionized Ti–13Mn has pores and titanium carbide (Ti carbide) precipitates and consists of a β phase and an athermal ω phase. The porosity of the alloy decreases from 6.1% to 0.01% after cold-rolling at a reduction ratio of 90%. Moreover, during cold-rolling, the Ti carbides are fragmented and a deformation-induced ω phase is formed. The ultimate tensile strength, 0.2% proof stress, Vickers hardness, and Young's modulus of Ti–13Mn increase from 888 MPa to 1852 MPa, from 827 MPa to 1823 MPa, from 279 Hv to 461 Hv, and from 96 GPa to 108 GPa, respectively, after cold-rolling at a reduction ratio of 90%. On the other hand, the elongations of both the solutionized and cold rolled Ti–13Mn are less than 2%. Although the elongation of Ti–13Mn is less than 2%, the tensile strength of the cold rolled Ti–13Mn is extremely high compared with that of existing titanium alloys. This large-improvement in the tensile strength of the cold rolled Ti–13Mn is attributed to the increase in the dislocation density, decrease in grain size, decrease in porosity, and formation of a deformation-induced ω phase.

The alloy Ti–9Cr–0.2O has been developed as a potential material for implant rods used in spinal fixation applications, since it exhibits good mechanical properties and a remarkably “changeable Young's modulus”, which is achieved by suppressing the athermal ω-phase formed upon quenching and enhancing the deformation-induced ω-phase transformation. In this study, athermal and deformation-induced ω-phase transformations in Ti–9Cr–0.2O were investigated systematically by transmission electron microscopy. This was done in order to understand the nature of these ω-phase transformations, as well as the specific functionality—the “changeable Young's modulus”—resulting from them. In solution-treated alloy samples, in addition to ideal ω-structures, structures considered as initial ω-structures associated with incommensurate ω-phase were observed. This might be attributed to the composition heterogeneity, heterogeneity of oxygen distribution, and/or the inhomogeneous distribution of defects such as vacancies and locally strained areas. Following cold rolling, some of the selected area electron diffraction patterns of the alloy showed that the reflections of one ω-variant had increased significantly in intensity while those of the other ω-variant had decreased sharply. This vanishing of one type of variant ω-structures is attributable to two possible mechanisms: (i) a reversal mechanism, under which the particular partial dislocations transform the corresponding ω-variants back into β-phase or (ii) a re-orientation mechanism, according to which the ω-variants unfavorable with regard to the loading direction re-orient and turn into the preferred ω-variants.

Along with a high strength, ductility, and work hardening rate, a variable Young’s modulus is crucial for materials used as implant rods in spinal fixation surgery. The potential in this context of Ti–(9, 8, 7)Cr–0.2O (mass%) alloys is reported herein. The microstructural and mechanical properties of the alloys were systematically examined as a function of their chromium content, and the ion release of the optimized alloy was investigated to assess its suitability as an implant material. In terms of the deformation-induced ω-phase transformation required for a variable Young’s modulus, the balance between β-phase stability and athermal ω-phase content is most favorable in the Ti–9Cr–0.2O alloy. In addition, this composition affords a high tensile strength (>1000 MPa), elongation at break (∼20%), and work hardening rate to solution-treated (ST) samples. These excellent properties are attributed to the combined effects of deformation-induced ω-phase transformation, deformation twinning, and dislocation gliding. Furthermore, the ST Ti–9Cr–0.2O alloy proves resistant to metal ion release in simulated body fluid. This combination of a good biocompatibility, variable Young’s modulus and a high strength, ductility, and work hardening rate is ideal for spinal fixation applications. Statement of significance Extensive efforts have been devoted over the past decades to developing β-type titanium alloys with low Young’s moduli for biomedical applications. In spinal fixation surgery however, along with excellent mechanical properties, the spinal-support materials should possess high Young’s modulus for showing small springback during surgery to facilitate manipulation but low Young’s modulus close to bone once implanted to avoid stress shielding. None of currently used metallic biomaterials can satisfy these abovementioned requirements. In the present study, we have developed a novel alloy, Ti–9Cr–0.2O. Remarkably variable Young’s modulus and excellent mechanical properties can be achieved in this alloy via phase transformations and complex deformation mechanisms, which makes the Ti–9Cr–0.2O preferred material for spinal fixation surgery.

Ti–xMg (x = 17, 33, and 55 mass%) alloy films, which cannot be prepared by conventional melting processes owing to the absence of a solid-solution phase in the phase diagram, were prepared by direct current magnetron sputtering in order to investigate their biocompatibility. Ti and Mg films were also prepared by the same process for comparison. The crystal structures were examined by X-ray diffraction (XRD) analysis and the surfaces were analyzed by X-ray photoelectron spectroscopy. The Ti, Ti–xMg alloy, and Mg films were immersed in a 0.9% NaCl solution at 310 K for 7 d to evaluate the dissolution amounts of Ti and Mg. In addition, to evaluate the formation ability of calcium phosphate in vitro, the Ti, Ti–xMg alloy, and Mg films were immersed in Hanks' solution at 310 K for 30 d. Ti and Mg form solid-solution alloys because the peaks attributed to pure Ti and Mg do not appear in the XRD patterns of any of the Ti–xMg alloy films. The surfaces of the Ti–17Mg alloy and Ti–33Mg alloy films contain Ti oxides and MgO, whereas MgO is the main component of the surface oxide of the Ti–55Mg alloy and Mg films. The dissolution amounts of Ti from all films are below or near the detection limit of inductively coupled plasma-optical emission spectroscopy. On the other hand, the Ti–17Mg alloy, Ti–33Mg alloy, Ti–55Mg alloy, and Mg films exhibit Mg dissolution amounts of approximately 2.5, 1.4, 21, and 41 μg/cm2, respectively. The diffraction peaks attributed to calcium phosphate are present in the XRD patterns of the Ti–33Mg alloy, Ti–55Mg alloy, and Mg films after the immersion in Hanks' solution. Spherical calcium phosphate particles precipitate on the surface of the Ti–33Mg film. However, many cracks are observed in the Ti–55Mg film, and delamination of the film occurs after the immersion in Hanks' solution. The Mg film is dissolved in Hanks' solution and calcium phosphate particles precipitate on the glass substrate. Consequently, it is revealed that the Ti–33Mg alloy film evaluated in this study is suitable for biomedical applications.

This study investigates the effect of heterogeneous precipitation induced by the segregation of substitutional and interstitial elements in Ti–29Nb–13Ta–4.6Zr alloy (TNTZ) on its mechanical properties. For this, samples both with and without segregation of substitutional elements were prepared, with only the latter being subjected to long-term homogenization. It was found that micro-scale segregation of substitutional elements such as Nb, Ta, and Zr does not significantly affect mechanical properties such as fatigue strength, not even if heterogeneous precipitation occurs as a result of this segregation. On the other hand, segregation of interstitial elements was achieved by controlling the aging time. The segregation of interstitial elements creates precipitate-free zones (PFZs), grain boundary (GB) plates, and Widmanstatten α phases with migrating O atoms that all significantly affect the mechanical properties. Specifically, the PFZs have the potential to improve fatigue life, while the Widmanstatten α phase increases the tensile strength and reduces the fatigue ratio, the GB-plates reduce elongation, These results indicate that the formation of a Widmanstatten α phase by the migration of interstitial elements has a varying influence on the tensile and fatigue properties.

A β-type Ti–29Nb–13Ta–4.6Zr (TNTZ) alloy containing various amounts of TiB reinforcements has been developed to achieve higher fatigue strength for biomedical applications. Fatigue tests were performed at room temperature, and the effects of TiB on fatigue crack initiation and propagation were investigated. The results indicate that the fatigue limit of the TNTZ alloy was substantially improved by the TiB reinforcements. However, the fatigue strength first increased and subsequently decreased as the B concentration increased. TNTZ alloy with 0.10% B has the largest fatigue limit, which is 67% greater than that of the TNTZ alloy. The effects of TiB on the improvement of the fatigue properties are due to the following two factors. First, sliding of the dislocations can be blocked by TiB particles, which results in resistance to fatigue crack initiation. In addition, the crack deflection and crack bridging by the TiB particles could delay the crack propagation. Conversely, debonding of some larger TiB particles may occur, especially when the B concentration is higher than 0.20%. Therefore, the crack initiates from the debonding of the TiB particles and easily propagates along the voids from the interfacial decohesion, which could exert a deleterious influence on the fatigue strength.

The predominant factor determining the wear properties of a new titanium alloy, Ti–29Nb–13Ta–4.6Zr (TNTZ) and a conventional titanium alloy, Ti–6Al–4V extra-low interstitial (Ti64) was investigated for TNTZ and Ti64 combinations in metal-to-metal contacting bio-implant applications. The worn surfaces, wear debris, and subsurface damages were analyzed using a scanning electron microscopy combined with energy-dispersive spectroscopy and electron-back scattered diffraction analysis. The volume loss of TNTZ is found to be larger than that of Ti64, regardless of the mating material. The wear track of TNTZ exhibits the galled regions and severe plastic deformation with large flake-like debris, indicative of delamination wear, which strongly suggests the occurrence of adhesive wear. Whereas, the wear track of Ti64 have a large number of regular grooves and microcuttings with cutting chip-like wear debris and microfragmentation of fine oxide debris, indicative of abrasive wear combined with oxidative wear. This difference in the wear type is caused by severe and mild subsurface deformations of TNTZ and Ti64, respectively. The lower resistance to plastic shearing for TNTZ compared to that of Ti64 induces delamination, resulting in a higher wear rate.

The new β-type Ti–29Nb–13Ta–4.6Zr (TNTZ) alloy containing trace amounts of CeO2 additions has been designed as a biomedical implant with improved fatigue properties achieved by keeping Young׳s modulus to a low value. The results show that the microstructure is refined by the addition of CeO2; the β grain size becomes a little larger when Ce content increases from 0.05% to 0.10%. This occurs because dispersed CeO2 particles can act as nucleation sites for β grains; thus, the effect of rare earth oxides on microstructure refinement mainly depends on the size and dispersion of the rare earth oxides. Young׳s moduli of TNTZ with CeO2 additions are maintained as low as those of TNTZ without CeO2, while the fatigue limit is highly improved. The 0.10% Ce alloy exhibits the best fatigue strength among the experimental alloys; its fatigue strength is increased by 66.7% compared to that of pure TNTZ. The mechanism by which rare earth oxides affect fatigue performance is dominated by dispersion strengthening. The stiff rare earth oxides can hinder the movement of dislocations, resulting in resistance to the formation of fatigue cracks. Rare earth oxides also change the crack propagation direction and the crack propagation route, effectively decreasing the crack propagation rate.

In order to elucidate the microstructural refinement mechanism and the effect of secondary phase on the microstructural evolution of β -type titanium alloy, severe plastic deformation was conducted on samples of a precipitation-hardened Ti–29Nb–13Ta–4.6Zr (TNTZ). Specifically, TNTZ that was precipitation-hardened through an aging treatment (TNTZ AT ) was subjected to high-pressure torsion (HPT) processing (TNTZ AHPT ). The microstructure of TNTZ AHPT , which has been evaluated as a function of the torsional rotation number, N , exhibits ultrafine elongated β grains. The needle-like α precipitates in TNTZ AT , which exhibit a diameter of approximately 12 nm, are homogeneously distributed within the β grains. The dislocation density and subgrain diameter, estimated by X-ray line profile analysis, saturate at approximately 4.2 × 10 16 m −2 and 12.2 nm, respectively, at N ⩾ 10. The β grains contain nanostructured subgrains having non-uniform morphologies surrounded by blurred and wavy boundaries. A saturated hardness distribution at approximately 450 HV indicates that microstructural homogeneity has been achieved at N ⩾ 10. The α precipitates enhance the β grain refinement and microstructural homogeneity is achieved in TNTZ AHPT at N ⩾ 10, whereas this occurs at later stages ( N > 20) in TNTZ which is solution-treated and therefore does not contain any α precipitates.

Materials used for implant rods in spinal fixation systems must show changeable Young’s moduli, high strength and large elongation-to-failure. A d-electron design method was used to determine the chemical composition of Ti–10Cr, which showed all these properties and significant work-hardening characteristics owing to multiple plastic deformation mechanisms, such as deformation-induced ω-phase transformation, {3 3 2} mechanical twinning and dislocation gliding. Therefore, Ti–10Cr exhibits great potential for use in spinal fixation applications.

Ag–Pd–Au–Cu alloys have been used widely for dental prosthetic applications. Significant enhancement of the mechanical properties of the Ag–20Pd–12Au–14.5Cu alloy as a result of the precipitation of the β ′ phase through high-temperature solution treatment (ST), which is different from conventional aging treatment in these alloys, has been reported. The relationship between the unique hardening behavior and precipitation of the β ′ phase in Ag–20Pd–12Au– x Cu alloys ( x =6.5, 13, 14.5, 17, and 20 mass%) subjected to the high-temperature ST at 1123 K for 3.6 ks was investigated in this study. Unique hardening behavior after the high-temperature ST also occurs in Ag–20Pd–12Au– x Cu alloys ( x =13, 17, and 20) with precipitation of the β ′ phase. However, hardening is not observed and the β ′ phase does not precipitate in the Ag–20Pd–12Au–6.5Cu alloy after the same ST. The tensile strength and 0.2% proof stress also increase in Ag–20Pd–12Au– x Cu alloys ( x =13, 14.5, 17, and 20) after the high-temperature ST. In addition, these values after the high-temperature ST increase with increasing Cu content in Ag–20Pd–12Au– x Cu alloys ( x =14.5, 17, and 20). The formation process of the β ′ phase can be explained in terms of diffusion of Ag and Cu atoms and precipitation of the β ′ phase. Clarification of the relationship between hardening and precipitation of the β ′ phase via high-temperature ST is expected to help the development of more effective heat treatments for hardening in Ag–20Pd–12Au– x Cu alloys.

The springback behavior of Ti–12Cr and Ti–29Nb–13Ta–4.6Zr (TNTZ) during deformation by bending was investigated; and the microstructures of the non-deformed and deformed parts of both alloys were systematically examined to clarify the relationship between microstructure and springback behavior. For the deformed Ti–12Cr alloy, deformation-induced ω-phase transformation occurs in both the areas of compression and tension within the deformed part, which increases the Young׳s modulus. With the deformed TNTZ alloy, deformation-induced ω-phase transformation is observed in the area of compression within the deformed part; while a deformation-induced α″ martensite transformation occurs in the area under tension, which is likely to be associated with the pseudoelasticity of TNTZ. Among these two alloys, Ti–12Cr exhibits a smaller springback and a much greater bending strength when compared with TNTZ; making Ti–12Cr the more advantageous for spinal fixation applications.

Some newly developed β-type titanium alloys for biomedical applications exhibit distinctive heterogeneous structures. The formation mechanisms for these structures have not been completely revealed; however, understanding these mechanisms could lead to improving their properties. In this study, the heterogeneous structures of a Ti-29Nb-13Ta-4.6Zr alloy (TNTZ), which is a candidate for next-generation metallic biomaterials, were analyzed. Furthermore, the effects of such heterogeneous structures on the mechanical strength of this alloy, including fatigue strength, were revealed by comparing its strength to that of homogenous TNTZ. The heterogeneous structures were characterized micro-, submicro- and nano-scale wave-like structures. The formation mechanisms of these wave-like structures are found to be different from each other even though their morphologies are similar. It is revealed that the micro-, submicro- and nano-scale wave-like structures are caused by elemental segregation, crystal distortion related to kink band and phase separation into β and β', respectively. However, these structures have no significant effect on both tensile properties and fatigue strength comparison with homogeneous structure in this study.

For spinal-fixation applications, implants should have a high Young's modulus to reduce springback during operations, though a low Young's modulus is required to prevent stress shielding for patients after surgeries. In the present study, Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) with a low Young's modulus was modified by adding Cr to obtain a higher deformation-induced Young's modulus in order to satisfy these contradictory requirements. Two newly designed alloys, TNTZ-8Ti-2Cr and TNTZ-16Ti-4Cr, possess more stable β phases than TNTZ. These alloys consist of single β phases and exhibit relatively low Young's moduli of65GPa after solution treatment. However, after cold rolling, they exhibit higher Young's moduli owing to a deformation-induced ω-phase transformation. These modified TNTZ alloys show significantly less springback than the original TNTZ alloy based on tensile and bending loading-unloading tests. Thus, the Cr-added TNTZ alloys are beneficial for spinal-fixation applications.

The design of new low-cost Ti-alloys with high biocompatibility for implant applications, using ubiquitous alloying elements in order to establish the strategic method for suppressing utilization of rare metals, is a challenge. To meet the demands of longer human life and implantation in younger patients, the development of novel metallic alloys for biomedical applications is aiming at providing structural materials with excellent chemical, mechanical and biological biocompatibility. It is, therefore, likely that the next generation of structural materials for replacing hard human tissue would be of those Ti-alloys that do not contain any of the cytotoxic elements, elements suspected of causing neurological disorders or elements that have allergic effect. Among the other mechanical properties, the low Young's modulus alloys have been given a special attention recently, in order to avoid the occurrence of stress shielding after implantation. Therefore, many Ti-alloys were developed consisting of biocompatible elements such as Ti, Zr, Nb, Mo, and Ta, and showed excellent mechanical properties including low Young's modulus. However, a recent attention was directed towards the development of low cost-alloys that have a minimum amount of the high melting point and high cost rare-earth elements such as Ta, Nb, Mo, and W. This comes with substituting these metals with the common low cost, low melting point and biocompatible metals such as Fe, Mn, Sn, and Si, while keeping excellent mechanical properties without deterioration. Therefore, the investigation of mechanical and biological biocompatibility of those low-cost Ti-alloys is highly recommended now lead towards commercial alloys with excellent biocompatibility for long-term implantation.

The ω transformation and its correlation with elastic properties were investigated in cold-worked Ti–36Nb–2Ta–3Zr–xO mass% alloys with low body-centered cubic (β) phase stability, known as gum metal. Analysis of the temperature dependence of the ω (hexagonal) phase formation using transmission electron microscopy and of the elastic properties of solution-treated and cold-worked alloys using resonant ultrasound spectroscopy revealed that in the solution-treated 0.36% and 0.51% O alloys, the high concentration of oxygen suppressed ω-phase formation from room temperature to a fairly low temperature of ∼13 K. However, the ω phase was formed by cold working at room temperature in the 0.30% and 0.47% O alloys. Importantly, the fraction of the ω phase clearly increased upon cooling, which indicates that the formation of the ω phase is thermodynamically favorable near and below room temperature in the cold-worked 0.30% and 0.47% O alloys. This formation of the ω phase and the low stability of the β phase related to the low electron/atom (e/a) ratio were the dominant factors determining the elastic properties near and below room temperature in the cold-worked Ti–Nb–Ta–Zr–O alloys.

Micro-arc oxidation (MAO) was performed on a β-type Ti–29Nb–13Ta–4.6Zr alloy (TNTZ) in this study to improve its bioactivity in a body fluid and its hard-tissue compatibility. The surface oxide layer formed on TNTZ by MAO treatment in a mixture of calcium glycerophosphate and magnesium acetate was characterized using various surface analyses. The oxide layer was mainly composed of two types of TiO 2 (rutile and anatase), and it also contained Ca, P, and Mg, which were incorporated from the electrolyte during the treatment. The calcium phosphate formation on the surface of the specimens after immersion in Hanks’ solution was evaluated to determine the bioactivity of TNTZ with and without MAO treatment. As a result, thick calcium phosphate layers formed on the TNTZ specimen that underwent MAO treatment, whereas only a small amount of precipitate was observed on TNTZ without treatment. Thus, the MAO treatment is a promising method to improve the bioactivity and hard-tissue compatibility of TNTZ.

For implant rods as spinal fixation system, low Young's modulus, easy bending to an intended shape (low spring-back), and retaining initial bended shape during a long implantation (low aging-back) are essential properties. The rods were fabricated using a biomedical β-type titanium alloy, Ti–29Nb–13Ta–4.6Zr alloy (TNTZ), and their above-mentioned properties were evaluated in comparison with conventional biomaterials, commercially pure titanium, Ti–6Al–4V ELI, and SUS316L stainless steel. In the results of an animal experiment using implant rods made by TNTZ subjected to solution treatment with low Young's modulus, no negative effects are seen in the radiographs, and no inflammatory cells are observed in histological observation. In spite of low Young's modulus, the spring-back of TNTZ subjected to solution treatment is comparable to that of Ti–6Al–4V ELI. Further, its aging-back is quite small among the other materials.

The complex microstructure of a high hot-workable α + β-type titanium alloy, Ti–4.5Al–2.5Cr–1.2Fe–0.1C with a continuously varying α phase in terms of its size, distribution, morphology, and crystal orientation from the welded zone to the matrix, including a trace amount of welding defect, was investigated by several microstructural and crystallographical analysis techniques such as optical microscopy, scanning electron microscopy, and X-ray diffraction to elucidate the crucial factors determining its mechanical properties such as tensile properties and fatigue etc. The alloy was processed with laser welding to prepare parts for use in next-generation aircraft. The tensile properties of welded samples exhibit a strength–ductility balance similar to that of non-welded sample. All the failures in these samples occur at their matrices because the hardness values of welded zone on the cross section perpendicular to loading direction of the welded samples are higher than that on the same plane of non-welded sample, which is related to crystal texture of α phase. However, the fatigue strengths of welded samples are lower than that of non-welded sample. Such the decrease in fatigue strength of welded samples is caused by the presence of pores formed during welding.

Metallic implant rods used in spinal fixtures should have a Young’s modulus that is sufficiently low to prevent stress shielding for the patient and sufficiently high to suppress springback for the surgeon. Therefore, we propose a new concept: novel biomedical titanium alloys with a changeable Young’s modulus via deformation-induced ω phase transformation. In this study, the Cr content in the range of 10–14 mass% was optimized to produce deformation-induced ω phase transformation, resulting in a large increase in the Young’s modulus of binary Ti–Cr alloys. The springback and cytotoxicity of the optimized alloys were also examined. Ti–(10–12)Cr alloys exhibit an increase in Young’s modulus owing to deformation-induced ω phase transformation. In this case, such deformation-induced ω phase transformation occurs along with {3 3 2} β mechanical twinning, resulting in the maintenance of acceptable ductility with relatively high strength. Among the examined alloys, the lowest Young’s modulus and largest increase in Young’s modulus are obtained from the Ti–12Cr alloy. This alloy exhibits smaller springback than and comparable cytocompatibility to the biomedical Ti alloy Ti–29Nb–13Ta–4.6Zr.

The mechanical strength of a beta titanium alloy such as Ti–Nb–Ta–Zr alloy (TNTZ) can be improved significantly by thermo-mechanical treatment. In this study, TNTZ was subjected to solution treatment, cold caliber rolling, and cold swaging before aging treatment to form a rod for spinal fixation. The { 110 } β are aligned parallel to the cross-section with two strong peaks approximately 180 ∘ apart, facing one another, in the TNTZ rods subjected to cold caliber rolling and six strong peaks at approximately 60 ∘ intervals, facing one another, in the TNTZ rods subjected to cold swaging. Therefore, the TNTZ rods subjected to cold swaging have a more uniform structure than those subjected to cold caliber rolling. The orientation relationship between the α and β phases is different. A [ 110 ] β // [ 121 ] α , ( 112 ) β // ( 210 ) α orientation relationship is observed in the TNTZ rods subjected to aging treatment at 723 K after solution treatment and cold caliber rolling. On the other hand, a [ 110 ] β // [ 001 ] α , ( 112 ) β // ( 200 ) α orientation relationship is observed in TNTZ rod subjected to aging treatment at 723 K after cold swaging. A high 0.2% proof stress of about 1200 MPa, high elongation of 18%, and high fatigue strength of 950 MPa indicate that aging treatment at 723 K after cold swaging is the optimal thermo-mechanical process for a TNTZ rod.

The objective of this research was to investigate the effect of constitutional phases on the unique hardening behavior of as-solutionized dental Ag–Pd–Au–Cu alloy fabricated by cold rolling. The commercial dental Ag–Pd–Au–Cu alloy fabricated by cold rolling consists of Cu-rich α1, Ag-rich α2, and β phases. On the other hand, the Ag–Pd–Au–Cu alloy fabricated by the liquid rapid solidification (LRS) method consists of single α phase. They were subjected to various heat treatments, respectively. The microstructures were observed by scanning electron microscope, transmission electron microscope and X-ray diffraction. The hardness was evaluated by a Vickers micro-hardness tester. In the Ag–Pd–Au–Cu alloy fabricated by cold rolling, the fine L10-type-ordered β′ phase is precipitated and the coarse β phase is remained after solution treatment at 1123 K. The hardness increases drastically. On the other hand, in the Ag–Pd–Au–Cu alloy fabricated by LRS method, the single α phase was decomposed into the α1 phase and the α2 phase after solution treatment at 1023 K and its hardness change was small. However, after aging treatment at 673 K, the fine β phase is precipitated in the α phase and the hardness increases greatly even in the Ag–Pd–Au–Cu alloy fabricated by LRS method. It is considered that the precipitation of the fine L10-type-ordered β′ phase may contribute strongly to the unique hardening in the as-solutionized dental Ag–Pd–Au–Cu alloy fabricated by cold rolling.

Improvement in fatigue strength in spite of maintaining low Young's modulus was achieved in Ti–29Nb–13Ta–4.6Zr (TNTZ) by hard-particles dispersion. A certain amount of Y2O3 additions was added into TNTZ. TNTZ with 0.05–1.00mass%Y consists of a β-phase with a small amount of Y2O3. Young's moduli of TNTZ with 0.05–1.00mass%Y are maintained low, and are almost similar to that of TNTZ without Y2O3. The tensile strength of TNTZ with 0.05–1.00mass%Y is slightly improved and the elongation does not deteriorate by Y2O3 additions. However, the 0.2% proof stress decreases with the increase in Y concentration. Although tensile properties are not changed drastically, the fatigue strength is significantly improved by Y2O3 additions. The dispersion of Y2O3 particle increases the resistance to fatigue initiation. However, Y2O3 with too large diameter at the surface of the specimen works harmfully as the fatigue initiation site. The Y2O3 diameter and volume fraction increase with the increase in Y concentration. As a result, the fatigue limit of the alloys with 0.05–1.00mass%Y firstly increases and then decreases with the increase in Y concentration. TNTZ with 0.1mass% Y exhibits the best combination of higher fatigue strength and low Young's modulus.

The relationship between the microstrucutural changes of L1 0 -type ordered β′ phase and hardening behavior in as-solutionized dental Ag–Pd–Au–Cu alloys was investigated by changing the cooling rate and the solution treatment temperature. Additionally, the formation process of the β′ phase in as-solutionized Ag–Pd–Au–Cu alloy was attempted to clarify. The microstructural changes were observed by X-ray diffraction (XRD) method and transmission electron microscopy (TEM). The hardness was evaluated using a Vickers microhardness tester. The β′ phase is precipitated regardless of the cooling rate, after solution treatment at 1123 K. TEM dark field images show that the size of the β′ phase decreases and the number of β′ phase increases with an increase in the cooling rate. The Vickers hardness value increases with an increase in cooling rate. TEM dark field images show that the microstructure of β′ phase is similar when the solution treatment temperature increases from 1123 K to 1173 K. However, the Vickers hardness increases with an increase of solution treatment temperature. It is of great significance to reveal that the β′ phase precipitated in as-solutionized Ag–Pd–Au–Cu alloy is formed during cooling after high-temperature solution treatment and that the growth of the β′ phase is diffusion controlled.

β-Type titanium alloys with composition of Ti–24 at% Nb–(0, 2, 4) at% Zr were prepared and subjected to aging treatment for different durations. The addition of Zr suppressed the formation and growth of isothermal ω phase during aging treatment, and therefore the investigated Ti–Nb–Zr alloys with different Zr content performed various deformation mechanisms, mechanical properties and super-elastic effects with the changes of aging time. The isothermal ω phase exerted inhibitory effects on twinning and stress-induced martensitic transformation. Desirable properties were obtained in Ti–Nb–Zr alloy by adding suitable Zr content and processing short time aging treatment. Ti–24 at% Nb–2 at% Zr alloy subjected to aging treatment at 573 K for 7.2 ks exhibited an ultimate tensile strength of 710 MPa, an elongation of 13%, a Young's modulus of 64 GPa, and a super-elastic recovery strain of 4.3%. Ti–24 at% Nb–4 at% Zr alloy subjected to aging treatment at 573 K for 1.8 ks, 3.6 ks, and 7.2 ks performed a low Young's modulus of 62 GPa and ultimate tensile strength around 600 MPa.

Composite materials that consisted of a biomedical β-type titanium alloy, Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) with low Young's modulus and segment polyurethane (SPU) have been fabricated for application in biomedical devices. The effects of different kinds of terminal functional groups and the thickness of the silane layers (SIL) on the adhesive strength between TNTZ and SPU were investigated by means of shear bonding tests. The following silane coupling agents were employed in this study: 3-methacryloxypropyltrimethoxysilane (γ-MPTS), aminopropyltriethoxysilane (APS), and 3-mercaptopropyltrimethoxysilane (γ-MPS). Furthermore, the shear bonding strength of the TNTZ/SIL/SPU interface was also characterized after immersion in water for 30 d. Silane coupling treatment produces a ten-fold increase in the shear bonding strength, independent of the type of terminal functional groups and the thickness of the silane layers. Scanning electron microscopic evaluation of the fracture surfaces of the TNTZ/SIL/SPU composites after the shear bonding tests coupled with energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy revealed that the TNTZ/SIL/SPU composites are partially fractured at the interfaces of the TNTZ/SIL while the rest of the fracture occurs at the interfaces of the SIL/SPU in single sample. The shear bonding strength decreases after immersion in water for 30 d when APS and γ-MPS are used as the silane coupling agents, because stable chemical bonding is not achieved between the silane layer and SPU, whereas the bonding of the γ-MPTS composite is not affected by exposure to water.

Nowadays, there is a significant research focus on the development of bio-implant materials that have not only a low Young’s modulus but also other unique characteristics such as a changeable Young’s modulus and the ability to prevent calcium phosphate formation. Taking advantage of deformation-induced phases is an effective way to obtain the changeable Young’s modulus. This study investigated the relationship between the various deformation-induced products and the mechanical properties–including Young’s modulus, microstructure, and tensile properties–of Ti–30Zr–(5,6,7)mass%Mo alloys subjected to solution treatment (ST) and cold-rolling (CR). After ST, each alloy is composed of a β phase and a small amount of athermally formed ω phase, and exhibits a low Young’s modulus. During CR, deformation-induced phase transformation occurs in all the alloys. The change in Young’s modulus due to CR is highly dependent on the types of deformation-induced products. The decrease in Young’s modulus due to CR is related to the deformation-induced α ′ phase transformation accompanying with the disappearance of athermal ω phase, and the increase in Young’s modulus is attributed to the deformation-induced ω phase, which mainly exists in { 332 } β mechanical twins.

The origin of the low Young’s modulus of cold worked Ti–36Nb–2Ta–3Zr–xO mass% polycrystals with a body-centered cubic (β-phase) structure, referred to as gum metal, was investigated with a focus on the roles of oxygen concentration, the electron–atom (e/a) ratio, and the cold working process. Analysis of the temperature dependence of the microstructures and elastic properties of single crystals at x = 0.09, 0.36, 0.51% O using transmission electron microscopy and an electromagnetic acoustic resonance method, respectively, revealed that the shear moduli c′ and c44 of the 0.36 and 0.51% O alloys softened upon cooling near room temperature (RT) and exhibited low values at RT. This was because suppression of the α″ martensitic transformation by oxygen addition led to retention of the low stability single β-phase state at RT. The Hill approximation indicated that the low c′ and c44 values caused by softening gave rise to the low Young’s modulus, which is common to some Ti–Nb-based alloys with an e/a ratio of ∼4.24. Analysis of the microstructures and elastic properties of solution-treated and cold worked x = 0.06, 0.30, 0.47% O alloy polycrystals at RT revealed that the Young’s modulus increased upon 90% cold working due to formation of the α″ martensite phase (0.09% O) and ω phase (0.09, 0.30, and 0.47% O) with a high elastic modulus in the β-phase matrix. However, increasing the oxygen concentration suppresses the increase in Young’s modulus because oxygen addition decreases the amount of α″ and ω phases formed while retaining the low stability β phase. Therefore, cold working combined with oxygen addition produces a low Young’s modulus compatible with high strength.

Porous titanium (pTi) can possess a low Young's modulus equal to that of human bone, depending on its porosity. However, the mechanical strength of pTi deteriorates greatly with increasing porosity. On the other hand, certain medical polymers exhibit biofunctionalities, which are not possessed intrinsically by metallic materials. Therefore, a biodegradable medical polymer, poly-L-lactic acid (PLLA), was used to fill in the pTi pores using a modified in-situ polymerization technique. The mechanical and biodegradable properties of pTi filled with PLLA (pTi/PLLA) as fabricated by this technique and the effects of the PLLA filling were evaluated in this study. The pTi pores are almost completely filled with PLLA by the developed process (i.e., technique). The tensile strength and tensile Young's modulus of pTi barely changes with the PLLA filling. However, the PLLA filling improves the compressive 0.2% proof stress of pTi having any porosity and increases the compressive Young's modulus of pTi having relatively high porosity. This difference between the tensile and compressive properties of pTi/PLLA is considered to be caused by the differing resistances of PLLA in the pores to tensile and compressive deformations. The PLLA filled into the pTi pores degrades during immersion in Hanks' solution at 310 K. The weight loss due to PLLA degradation increases with increasing immersion time. However, the rate of weight loss of pTi/PLLA during immersion decreases with increasing immersion time. Hydroxyapatite formation is observed on the surface of pTi/PLLA after immersion for ≥8 weeks. The decrease in the weight-loss rate may be caused by weight gain due to hydroxyapatite formation and/or the decrease in contact area with Hanks' solution caused by its formation on the surface of pTi/PLLA.

In order to develop a novel alloy with a changeable Young’s modulus for spinal fixation applications, we investigated the microstructures, Young’s moduli, and tensile properties of metastable Ti–30Zr–(Cr, Mo) alloys subjected to solution treatment (ST) and cold rolling (CR). All the alloys comprise a β phase and small athermal ω phase, and they exhibit low Young’s moduli after ST. During CR, deformation-induced phase transformation occurs in all the alloys. The change in Young’s modulus after CR is highly dependent on the type of deformation-induced phase. The increase in Young’s modulus after CR is attributed to the deformation-induced ω phase on {3 3 2} mechanical twinning. Ti–30Zr–3Cr–3Mo (3Cr3Mo), which exhibits excellent tensile properties and a changeable Young’s modulus, shows a smaller springback than Ti–29Nb–13Ta–4.6Zr, a β-type titanium alloy expected to be useful in spinal fixation applications. Thus, 3Cr3Mo is a potential candidate for spinal fixation applications.

TiB2 was added to β-type Ti–29Nb–13Ta–4.6Zr (TNTZ) to enhance its mechanical properties and lower its Young's modulus, and the microstructures and mechanical properties of TNTZ with 0.05–0.50 mass%B were investigated. The B concentration of TNTZ was controlled by varying the amount of added TiB2. TNTZ with 0.05–0.50 mass%B consists of a β-phase with a small amount of TiB. As the B concentration increases, these TiB precipitates become increasingly efficient at preventing texture movement. The microstructure of TNTZ is refined by TiB, and the TNTZ grain size decreases as the B concentration increases. Moreover, the Young's moduli of TNTZ with 0.05–0.50 mass%B remain below 70 GPa, and are almost similar to that of TNTZ without TiB. The tensile strength of TNTZ with 0.05–0.50 mass%B is slightly improved, and the optimum tensile strength is enhanced by approximately 8%. TNTZ with 0.10 mass%B exhibits a good balance between tensile strength and the elongation. The results of this study are expected to contribute to the creation of a TNTZ-based material with improved mechanical properties and maintained Young's modulus that can be used as dental and orthopedic biomaterials.

Plastic deformation behavior and its relation to tensile properties were investigated in an attractive β-type titanium alloy (Ti–29Nb–13Ta–4.6Zr) with the oxygen content of 0.1–0.7 mass% subjected to hot rolling and solution treatment after hot rolling. Hereafter, Ti–29Nb–13Ta–4.6Zr is abbreviated to TNTZ. With the increase of oxygen content, the tensile strength and 0.2% proof stress of all the samples increase, however, their elongation indicates special change, which is contradictory to that reported conventionally. The elongation firstly decreases and then increases with the increase in the oxygen content. Therefore, TNTZ with high strength and high ductility due to the addition of high oxygen content (0.7 mass%) is obtained. Remarkable yielding phenomenon and strain hardening are observed in TNTZ, which can be explained by the interaction between oxygen atoms and a lot of screw and edge dislocations leading to the easy activation of the multiple slip systems. The deformation behavior changes with the addition of oxygen in TNTZ. The plastic deformation mode changes from the deformation-induced martensite transformation to slip mechanism. It is realized that there is a specific compositional area of oxygen in which the TNTZ exhibits strain hardening and high strength, and appropriate Young's modulus value.

The composition and temperature dependence of the elastic properties and phase stability of quaternary Ti–Nb–Ta–Zr β-phase alloys with a body-centered cubic structure, developed for biomedical applications, were investigated using their single crystals, in order to clarify the origin of the low Young’s modulus in polycrystals. Transmission electron microscopy observations clarified that α ″ martensitic transformation occurred in a temperature range that depended on the β-phase stability below room temperature. Electromagnetic acoustic resonance measurements clarified that the shear moduli c ′ and c 44 of single crystals softened upon cooling from room temperature and became rather low near the martensitic transformation start temperature, i.e. the lower limit of β-phase stability. An analysis by the Hill approximation indicates that low c ′ and c 44 caused the low Young’s modulus, and thus it is probable that the softening in c ′ and c 44 is the origin of the low Young’s modulus.