Β-type Ti Alloys Research Articles

Texture Evolution in a Ti-Ta-Nb Alloy Processed by Severe Plastic Deformation

Titanium alloys are extensively used in a variety of applications because of their good mechanical properties, high biocompatibility, and corrosion resistance. Recently, β-type Ti alloys containing Ta and Nb have received much attention because they feature not only high specific strength but also biocorrosion resistance, no allergic problems, and biocompatibility. A Ti-25Ta-25Nb β-type titanium alloy was subjected to severe plastic deformation (SPD) processing by accumulative roll bonding and investigated with the aim to observe the texture developed during SPD processing. Texture data expressed by pole figures, inverse pole figures, and orientation distribution functions for the (110), (200), and (211) β-Ti peaks were obtained by XRD investigations. The results showed that it is possible to obtain high-intensity share texture modes ({001}〈110〉) and well-developed α and γ-fibers; the most important fiber is the α-fiber ({001} $$ \left\langle {1\bar{1}0} \right\rangle $$ to {114} $$ \left\langle {1\bar{1}0} \right\rangle $$ to {112} $$ \left\langle {1\bar{1}0} \right\rangle $$ ). High-intensity texture along certain crystallographic directions represents a way to obtain materials with high anisotropic properties.

Long-Term Corrosion Behavior of Biocompatible β-Type Ti Alloy in Simulated Body Fluid

The corrosion behavior of the Ti-33.5Nb-5.7Ta alloy (Ti-Nb-Ta) as a biocompatible β-type Ti alloy during long-term immersion in simulated body fluid was investigated. Like pure Ti, pitting corrosion did not occur on Ti-Nb-Ta during anodic polarization. Thus, alloying of Ti with Nb and Ta did not change the chloride-ion sensitivity. Metal ion release of Ti, Nb, and Ta was detected after 7-d immersion in the solution; however, the amounts of ions were much smaller than those from Type316L stainless steel. X-ray photoelectron spectroscopy revealed that the fractions of Nb and Ta in the passive layer increased during the immersion while that of Ti decreased. The corrosion rate of Ti-Nb-Ta determined by electrochemical impedance spectroscopy kept decreasing over a period of 15 d while the thickness of the passive layer did not change after 1 d. Thus, the reconstruction of the passive layer of the alloy was proven to be important for metal ion release during long-term implantation in a living body. Thus, Ti-Nb-Ta has sufficient corrosion resistance as a biocompatible β-type Ti alloy.

Single-crystal growth of Ti–Nb–Ta–Zr–O alloys and measurement of elastic properties

Single crystals of β-type Ti alloy system Ti–Nb–Ta–Zr–O have been grown successfully in an Ar gas flow by a floating zone method. The growth orientations were determined approximately by using seed crystals with the desired orientations. The various growth conditions were realized by choosing the gas purity, the gas flow rate, and the growth rate as variables. Composition analysis of the grown crystals was done to check any variation from the values of the raw material along with the bulk homogeneity, followed by measurements of the lattice parameter and the hardness, which provides the following results: (1) the composition of oxygen varies with respect to the flow rate, or is increased as the purity is degraded, (2) the lattice parameter is increased with increasing composition of oxygen, (3) which is also the case with the hardness. Measurements of Young's moduli were performed to investigate the elastic properties. The results indicate that the crystals exhibit the anisotropy which was expected previously. The elastic constants were estimated from the moduli, giving the ideal stress 1.7–1.9GPa which is on a level with the real strength. Additionally, the tensile stress–strain curve for the crystallographic direction 〈110〉 exhibited nonlinear elasticity and hysteresis.

Effect of Nb Content on Deformation Textures and Mechanical Properties of Ti-18Zr-Nb Biomedical Alloys

Recently, β-type Ti alloys composed of non-toxic elements such as Nb, Ta, Zr, Mo and Sn have been studied for biomedical applications. The present author’s research group has also developed these alloys including Ti-Zr-Nb as new biomedical superelastic materials. They reveal strong textures which are formed during thermo-mechanical processing and cause the anisotropy in mechanical properties. In this study, the effect of Nb content on the deformation texture of Ti-18Zr-Nb alloys was investigated. The anisotropy in mechanical properties of Ti-18Zr-Nb alloys was also investigated. Ti-18Zr-(15∼18)Nb (at%) alloy ingots were fabricated by an Ar arc melting method and then homogenized at 1273 K for 7.2 ks. The ingots were cold rolled with a reduction up to 99% in thickness. For the as-rolled alloys, a weak γ-fiber texture was observed in Ti-18Zr-(15, 16)Nb alloys, whereas a well developed {001}⟨110⟩ texture was confirmed in the Ti-18Zr-18Nb alloy. The former alloys revealed weak anisotropy in Young’s modulus due to the weak texture, while the latter alloy exhibited strong anisotropy due to the strong texture.

Stress-induced α″ martensitic (110) twinning in β-Ti alloys

A fully transformed α″ martensite with stress-induced nanoscale {110}, {021}-type compound twin or a 90° rotation twin has been experimentally explored and unambiguously characterized in traditional β-type Ti alloys, which usually undergo a partial martensitic transformation [a β grain partially transformed into α″ with internal (111) twin] by quenching. This newly observed twinning, which matches with the predication based on the deformation twinning theory of Bilby and Crocker [Proc. R. Soc. 288, 241 (1965)], can help to explain the deformation mechanism and aid future development of advanced materials.

Peculiar elastic behavior of Ti–Nb–Ta–Zr single crystals

The cause of a low Young’s modulus was investigated in quaternary β-type Ti–Nb–Ta–Zr alloys, as the modulus is decreased to prevent bone absorption and degradation of bone quality when these alloys are implanted into human bones. This investigation was carried out using the alloys′ single crystals. Acoustic measurements and analysis by the Hill approximation revealed that a low Young’s modulus in a polycrystalline form is caused by the low shear modulus c′, related to the low β-phase stability, low c 44, and relatively low bulk modulus B compared with those of binary Ti-based alloys. Furthermore, it was found that the single crystals had strong orientation dependence on Young’s modulus, where that in the 〈1 0 0〉-direction E 100 is the lowest of all crystallographic orientations. For quaternary Ti–29Nb–13Ta–4.6Zr alloy (mass%), E 100 is only ∼35 GPa, which is similar to Young’s modulus of human cortical bones as a result of the low B and c′. These results indicate that decreases in c′, c 44 and B are essential for decreasing Young’s modulus of novel β-type Ti alloys which are expected to be developed in the near future.

生体用 Ti-18Zr-Nb 合金の加工集合組織と機械的特性に及ぼす Nb 濃度の影響

Recently, β-type Ti alloys composed of non-toxic elements such as Nb, Ta, Zr, Mo and Sn have been studied for biomedical applications. The present author's research group has also developed these alloys including Ti-Zr-Nb as new biomedical superelastic materials. They reveal strong textures which are formed during thermo-mechanical processing and cause the anisotropy in mechanical properties. In this study, the effect of Nb content on deformation textures of Ti-18Zr-Nb alloys was investigated. The anisotropy in mechanical properties of Ti-18Zr-Nb alloys was also investigated. Ti-18Zr-(15~18) Nb(at%) alloy ingots were fabricated by an Ar arc melting method and then homogenized at 1273 K for 7.2 ks. The ingots were cold rolled with a reduction up to 99% in thickness. For the as-rolled alloys, a very weak γ-fiber texture was observed in Ti-18Zr-(15, 16)Nb alloys, whereas a well developed {001}<110> texture was confirmed in the Ti-18Zr-18Nb alloy. The former alloys revealed weak anisotropy in Young's modulus due to the weak texture, while the latter alloy exhibited strong anisotropy due to the strong texture.

Snoek‐Type High‐Damping Alloys Realized in β‐Ti Alloys with High Oxygen Solid Solution

High strength and a huge Snoek damping peak—properties required for high-damping alloys, which find applications in mechanical structures in attempts to eliminate noise and vibration—are reported to have been realized in β-type Ti alloys with an oxygen solid solution above 1.0 at %. The properties of some Ti–25 Nb–x O (at %) alloys (see figure) are superior to those of most previously developed high-damping alloys.

General approach to phase stability and elastic properties of β-type Ti-alloys using electronic parameters

A general correlation between the phase stability and the elastic properties in β(body-centered cubic)-type Ti-alloys is investigated with the aid of two electronic parameters. The extent of the single β-phase field is clearly represented using these parameters. It is also shown that the addition of Al, O, and Sn is effective in modifying greatly the stability of the β-phase, because they work as the β-stabilizing elements in these β-type alloys, resulting in significant changes in the elastic properties of alloys.

Electron microscopy study on a new phase in β-titanium alloys aged at a high temperature

In a Ti–20mass%Mo alloys aged by two steps aging at 623 and 823 K, a new phase has been observed. In the first step aging, ω-phase crystals appear and at the second step aging, these crystals disappear and are replaced with the newly discovered phase crystals. Observations in the dark field image and analysis of composition of the new phase crystals have shown that the formation is closely related to the ω-phase structure. The new phase crystals have been also observed in four kinds of β-type Ti alloys and a (β+α) type Ti alloy aged at high elevated temperatures. Based on high-resolution electron microscope observations, the atomic structure of the new phase is assumed.

Ｔｉ基ろうによるＴｉ−２１Ｖ−４Ａｌβ型チタン合金の液相拡散接合

Diffusion brazeability of β-type Ti alloy (Ti-21V-4Al) using newly developed Ti-20Zr-2OCu-2ONi amorphous filler has been investigatedd by electron microscopy, EDX microanalyses, and tensile tests of the joints. The brazeability was compared with solid state diffusion bondability of the same material, and the effect of aging heat treatments of the joint generally carried out for the alloy was also examined.Results of metallurgical examination of the joints indicated that the liquid layer at the joint interface became to disappear during holding at the joining temperature, and that the joint structure and composition finally became similar to those of the base metal.As for mechanical properties of the joint, even the joint made at 850C for 15 min under the joining pressure of 0.6 MPa, which is fairly lower than that required in solid state diffusion bonding of the same material, was fractured in the base metal with tensile strength of 680 MPa. Furthermore, the joint strength could be increased up to 1400MPa by aging heat treatment after joining.These results demonstrate that the use of developed Ti-base filler metal improves the diffusion weldability, and make it possible to produce the joint equivalent to the base metal even under a fairly low joining pressure.