Formation and reversion of stress induced martensite in Ti-10V-2Fe-3Al

Multiple Deformation Mechanisms in Adiabatic Shear Bands of a Titanium Alloy during High Strain Rate Deformation

The occurrence of adiabatic shear bands, as an instability phenomenon, is viewed as a precursor to failure caused by instability at high strain rates. Metastable β titanium alloys are extensively utilized due to their excellent mechanical properties, which are often subjected to high strain rate loads in service conditions. Understanding and studying their adiabatic shear instability behavior is thus crucial for preventing catastrophic failure and enhancing material performance. In this study via detailed microstructural analyses in the adiabatic shear region of a Ti-10V-2Fe-3Al alloy subjected to high strain rates, it was observed that α″ martensitic transformation and nano-twinning plus β-to-α phase transformation with α″ martensite as an intermediate phase occurred, in addition to substantial fine grains. The grain refinement mechanisms were mainly related to dynamic recovery dominated by dislocation migration alongside severe plastic deformation.

Effect of Si Content on the Thermal Expansion of Ti15Mo(0-2 Si) Biomaterial Alloys during Different Heating Rates

Thermal expansion measurements were used to characterize phase transformations in metastable β-Ti alloys (Ti15MoxSi) without and with various Si additions (where x = 0, 0.5, 1.0, 1.5, and 2 in wt.%) during linear heating at two heating rates of 5 and 10 °C/min up to 850 °C. For this study, five alloys were developed and examined in terms of their presence phases, microstructures, and starting and final transformation temperatures. According to the results, all of the as-cast samples primarily include an equiaxed β-Ti phase. The influence of phase transformation on the material dimensions was discussed and compared with the variations in Si contents. The transformation was investigated using a dilatometric technique for the developed alloys during continuous heating and cooling. The dilatometric curve of heating revealed two distinct reflection points as the heating temperature increased. The starting transformation temperature (T_s) to obtain the ω-phase was reported at 359 °C without Si addition; whereas the final transformation temperature (T_f) of the dissolution of α-phase was obtained at 572 °C at a heating rate of 10 °C/min. At 2 wt.% Si, the first derivative curves reported T_s and T_f transforming temperatures of 314-565 °C (at a 5 °C/min heating rate) and 270-540 °C (at a 10 °C/min heating rate), respectively. The T_s and T_f transforming temperatures were significantly decreased with Si additions, which decreased the β-transus temperature. Moreover, the thermal expansion coefficient curves of the investigated alloys without and with 2 wt.% Si were studied. The transformation heating curves have an S-shaped pattern, according to the results.

Shape memory effect and aging behavior of Bi-added Ti–Cr alloys for biomedical applications

For the further development of biocompatible metastable β (bcc) Ti alloys, the purpose of this study is to evaluate the potential of bismuth (Bi) addition in terms of shape memory properties and phase stability. It was found that the shape memory effect appeared in Ti-5Cr-1.6Bi (mol%) alloy. However, permanent (unrecoverable) deformation due to dislocations or twinning was also introduced simultaneously from the early stage of deformation. Regarding the formation of isothermal ω phase and the resulting hardness change by aging, it was found that the hardness change was large and that the isothermal ω phase formed in Ti-5Cr-1.6Bi alloy, while age hardening was small and no isothermal ω phase formed in Ti-5Cr-6.1Bi alloy. These results indicate the suppression of not only athermal ω but also isothermal ω phase by Bi addition. However, considering the fact that the alloy becomes brittle when Bi addition is over 3 mol%, it can be concluded that 1-3 mol% Bi addition is worth for the improvement of shape memory effect, suppression of ω phase, X-ray imaging, magnetic resonance imaging, and biocompatibility in metastable β Ti alloys.

Heat Treatments of Metastable β Titanium Alloy Ti-24Nb-4Zr-8Sn Processed by Laser Powder Bed Fusion

Titanium alloys, especially β alloys, are favorable as implant materials due to their promising combination of low Young's modulus, high strength, corrosion resistance, and biocompatibility. In particular, the low Young's moduli reduce the risk of stress shielding and implant loosening. The processing of Ti-24Nb-4Zr-8Sn through laser powder bed fusion is presented. The specimens were heat-treated, and the microstructure was investigated using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The mechanical properties were determined by hardness and tensile tests. The microstructures reveal a mainly β microstructure with α″ formation for high cooling rates and α precipitates after moderate cooling rates or aging. The as-built and α″ phase containing conditions exhibit a hardness around 225 HV5, yield strengths (YS) from 340 to 490 MPa, ultimate tensile strengths (UTS) around 706 MPa, fracture elongations around 20%, and Young's moduli about 50 GPa. The α precipitates containing conditions reveal a hardness around 297 HV5, YS around 812 MPa, UTS from 871 to 931 MPa, fracture elongations around 12%, and Young's moduli about 75 GPa. Ti-24Nb-4Zr-8Sn exhibits, depending on the heat treatment, promising properties regarding the material behavior and the opportunity to tailor the mechanical performance as a low modulus, high strength implant material.

Transformations of the Microstructure and Phase Compositions of Titanium Alloys during Ultrasonic Impact Treatment Part II: Ti-6Al-4V Titanium Alloy

Experimental and theoretical studies enabled the reveal of patterns of the microstructure formation in the surface layer of Ti-6Al-4V titanium alloy subjected to ultrasonic impact treatment. The mixed amorphous and nanocrystalline structure of the 200 nm thick uppermost surface layer of titanium dioxide TiO2 was demonstrated using transmission electron microscopy. The 5 µm thick intermediate layer containing nanocrystalline α grains, and the 50–60 µm thick lower layer containing fragmented α-Ti grains with retained β phase were also observed. The refinement of the β-Ti phase during ultrasonic impact treatment was accompanied by the formation of the orthorhombic (α″) martensitic phase. Molecular dynamics simulation of strains of a vanadium-doped titanium crystallite subjected to ultrasonic impact treatment revealed the formation of striped dislocation substructures as well as the development of reversible β→α phase transformations. Ab initio calculations of the atomic structure of V-doped Ti crystallites containing α, β or α″ phases of titanium were carried out. On the basis of the results of the experimental observations, a molecular dynamics simulation and ab initio calculations a mechanism was proposed, which associated the development of the strain-induced β→α″ phase transformations in Ti-6Al-4V alloy with the presence of oxygen. The role of the electronic subsystem in the development of the strain-induced phase transformations was discussed.

A Study of Low Young's Modulus Ti-15Ta-15Nb Alloy Using TEM Analysis

The microstructural characteristics and Young’s modulus of the as-cast Ti–15Ta–15Nb alloy are reported in this study. On the basis of the examined XRD and TEM results, the microstructure of the current alloy is essentially a mixture (α + β+ α′ + α″ + ω + H) phase. The new H phase has not previously been identified as a known phase in the Ti–Ta–Nb alloy system. On the basis of examination of the Kikuchi maps, the new H phase belongs to a tetragonal structural class with lattice parameters of a = b = 0.328 nm and c = 0.343 nm, denoting an optimal presentation of the atomic arrangement. The relationships of orientation between these phases would be {0001}_α//{110}_β//{1¯21¯0}_ω//{101¯}_H and (011¯0)_α//(11¯2)_β//(1¯010)_ω//(121)_H. Moreover, the Young’s modulus of the as-cast Ti–15Ta–15Nb alloy is approximately E = 80.2 ± 10.66 GPa. It is implied that the Young’s modulus can be decreased by the mixing of phases, especially with the presence of the H phase.

Predicting the available work from deformation-induced α′′ martensite formation in metastable β Ti alloys

Deformation-induced α′′ martensite formation is essential to the mechanical properties of a variety of metastable β Ti alloys by extending elasticity or contributing to work-hardening during plastic deformation. Nevertheless, to date, a comprehensive analysis of the effect of β texture and applied stress state on the martensitic transformation to α′′ is still lacking. The present study therefore provides a detailed analysis of the work which is made available from the shape strain of the martensitic transformation under a variety of in-plane stress states and as a function of β crystal orientation. The available work was found to strongly depend on the applied stress state and the parent grain orientation. The shape strain of the martensitic transformation was obtained from applying the phenomenological theory of martensite crystallography. In cases where this theory was not applicable, an approximation of the shape strain by the Bain strain was found to provide a good approximation of the available work. Analysis of three different metastable β Ti alloys showed no strong effect of the alloy composition on the available work. Martensite formation from typical cold- and warm-rolling β texture components under different stress states is discussed. Cases are highlighted to show how the cold- and warm-rolling β textures can be tailored to hinder martensite formation upon subsequent industrial forming operations.

The mechanical behaviours of the Ti-10V-2Fe-3Al alloy under the high-temperature and dynamic loading conditions

In this work, we develop a high-temperature dynamic testing technology for characterizing the properties of Ti-1023 alloy. The modified Hopkinson torsion bar system includes a high-temperature furnace subsystem and a water cooling subsystem. High-temperature dynamic tests of the alloy are performed. The dependence of the yield stress on the temperature and the shear strain rate is studied. The scanning electron microscopy observations show that the microstructural evolution makes up of two steps: the first one is caused by thermal loading and the second one by the mechanical loading. The high-temperature dynamic constitutive model is established based on the experimental results.

Giant thermal expansion and α-precipitation pathways in Ti-alloys

Ti-alloys represent the principal structural materials in both aerospace development and metallic biomaterials. Key to optimizing their mechanical and functional behaviour is in-depth know-how of their phases and the complex interplay of diffusive vs. displacive phase transformations to permit the tailoring of intricate microstructures across a wide spectrum of configurations. Here, we report on structural changes and phase transformations of Ti–Nb alloys during heating by in situ synchrotron diffraction. These materials exhibit anisotropic thermal expansion yielding some of the largest linear expansion coefficients (+ 163.9×10⁻⁶ to −95.1×10⁻⁶ °C⁻¹) ever reported. Moreover, we describe two pathways leading to the precipitation of the α-phase mediated by diffusion-based orthorhombic structures, α″_lean and α″_iso. Via coupling the lattice parameters to composition both phases evolve into α through rejection of Nb. These findings have the potential to promote new microstructural design approaches for Ti–Nb alloys and β-stabilized Ti-alloys in general.

Complex phase transformations in β-stabilised titanium alloys can dramatically change their α and β microstructures, providing tailorability for aerospace or biomaterial applications. Here the authors show that Ti-Nb alloys exhibit giant thermal expansions and identify two new pathways that lead to α phase formation.

Influence of Hot Rolling and Heat Treatment on the Microstructural Evolution of β20C Titanium Alloy

The microstructural evolution and underlying mechanism of a new high strength, high toughness near β titanium alloy, β20C, during hot deformation, and heat treatment were studied qualitatively and quantitatively. It was found that dynamic recovery occurs mainly in β phase, while α phase undergoes both a dynamic recovery and continuous incomplete dynamic recrystallization with a fraction of high-angle grain boundaries (≥15°) of 21.1% under hot-rolling. Subsequently, α phase undergoes static recrystallization with an increasing fraction of high-angle grain boundaries (21.1%→60.7%) under annealing, while the grains are equiaxed with refined grain sizes of 1.63 µm observed from the rolling direction (RD) and 1.66 µm observed from the transverse direction (TD). Moreover, the average aspect ratio of the lamellar α phase was 2.44 observed from the RD and 3.12 observed from the TD after hot rolling, but decreased to 2.20 observed from the RD, and 2.53 observed from the TD after annealing. Furthermore, the strict Burgers' relationship between α and β phases changed after hot-rolling and remains the distortion, even after the static recrystallization process of α phase during annealing.

Effect of thermomechanical processing on the mechanical biofunctionality of a low modulus Ti-40Nb alloy

Different hardening strategies were evaluated regarding their potential to improve the mechanical biofunctionality of the cast and solution-treated low modulus β-Ti alloy Ti 40Nb. The strategies are based on thermomechanical treatments comprised of different hot- and cold-rolling steps, as well as annealing treatments aiming at the successive exploitation of different hardening mechanisms (grain boundary hardening, work hardening and precipitation hardening). Quasi-static tensile testing revealed that grain refinement by one order of magnitude has only a small impact on improving the mechanical biofunctionality of Ti-40Nb. However, work hardening effectively improves the tensile strength by 30% to a value of 650MPa, while retaining Young׳s modulus at 60GPa. The α-phase precipitation hardening was verified to have an increasing effect on both, strength and Young׳s modulus. Thereby, the change of Young׳s modulus dominates the change of the strength, even at low α-phase fractions. The pseudo-elastic behavior of Ti 40Nb is discussed under consideration of the microstructural changes due to the thermomechanical treatment. The texture changes evolving upon cold-rolling markedly influence the recrystallization behavior. However, the present results do not show a significant effect of the texture on the mechanical properties of Ti-40Nb.

Influence of Si addition on the microstructure and mechanical properties of Ti-35Nb alloy for applications in orthopedic implants

In the development of new materials for orthopedic implants, special attention has been given to Ti alloys that show biocompatible alloy elements and that are capable of reducing the elastic modulus. Accordingly, Ti-Nb-Si alloys show great potential for application. Thus, this is a study on the microstructures and properties of Ti-35Nb-xSi alloys (x=0, 0.15, 0.35 and 0.55) (wt%) which were thermally treated and cooled under the following conditions: furnace cooling (FC), air cooling (AC), and water quenching (WQ). The results showed that Si addition is effective to reduce the density of omega precipitates making beta more stable, and to produce grain refinement. Silicides, referred as (Ti,Nb)3Si, were formed for alloys containing 0.55% Si, and its formation presumably occurred during the heating at 1000°C. In all cooling conditions, the hardness values increased with the increasing of Si content, as a result from the strong Si solid solution strengthening effect, while the elastic modulus underwent a continuous reduction due to the reduction of omega precipitates in beta matrix. Lower elastic moduli were observed in water-quenched alloys, which concentration of 0.15% Si was more effective in their reduction, with value around 65 GPa. Regarding Ti-35Nb-xSi alloys (x=0, 0.15 and 0.35), the "double yield point" phenomenon, which is typical of alloys with shape memory effect, was observed. The increase in Si concentration also produced an increase from 382 MPa to 540 MPa in the alloys' mechanical strength. Ti-35Nb-0.55Si alloy, however, showed brittle mechanical behavior which was related to the presence of silicides at the grain boundary.

Biocompatibility of Ti-alloys for long-term implantation

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.

Effect of Zr Addition on Mechanical Properties of Ti-Nb-Zr Alloys for Biomedical Applications

Artificial bone implant is concerned to improve their substantial features such as biocompatibility and mechanical properties. Ti-Nb alloys were considered to be one of the competitive materials because of their good biocompatibility and pseudoelasticity. In a present work, the effect of Zr addition as a third element on mechanical properties and pseudoelasticity of Ti-Nb alloys with Nb-content of 22-23at% were investigated by using cycling tests. The alloy ingots were fabricated by an arc melting method. The ingots were homogenization treated at 1273 K for 3.6ks followed by cold-rolled to a reduction ratio of 90% in thickness. All specimens were heat-treated at 873 K and some of them were aging treated at temperature ranging from 573 to 673 K after heat-treatment. Pseudoelasticity and mechanical behavior were evaluated by cycling test at room temperature. The results suggested that psuedoelasticity was confirmed in specimens without aging treatment irrespective of alloy compositions. Maximum recovery strain recovery increases with increasing Zr content. From all information acquired, it can be concluded that Ti-22Nb-(3-4)Zr(at.%) and Ti-23Nb-(2-3)Zr(at.%) alloys are the most optimum for artificial bone.

Fatigue properties of a metastable beta-type titanium alloy with reversible phase transformation

Due to recent concern about allergic and toxic effects of Ni ions released from TiNi alloy into human body, much attention has been focused on the development of new Ni-free, metastable beta-type biomedical titanium alloys with a reversible phase transformation between the beta phase and the alpha'' martensite. This study investigates the effect of the stress-induced alpha'' martensite on the mechanical and fatigue properties of Ti-24Nb-4Zr-7.6Sn (wt.%) alloy. The results show that the as-forged alloy has a low dynamic Young's modulus of 55GPa and a recoverable tensile strain of approximately 3%. Compared with Ti-6Al-4V ELI, the studied alloy has quite a high low-cycle fatigue strength because of the effective suppression of microplastic deformation by the reversible martensitic transformation. Due to the low critical stress required to induce the martensitic transformation, it has low fatigue endurance comparable to that of Ti-6Al-4V ELI. Cold rolling produces a beta+alpha'' two-phase microstructure that is characterized by regions of nano-size beta grains interspersed with coarse grains containing alpha'' martensite plates. Cold rolling increases fatigue endurance by approximately 50% while decreasing the Young's modulus to 49GPa along the rolling direction but increasing it to 68GPa along the transverse direction. Due to the effective suppression of the brittle isothermal omega phase, balanced properties of high strength, low Young's modulus and good ductility can be achieved through ageing treatment at intermediate temperature.

TiNi-Base and Ti-Base Shape Memory Alloys

The basic characteristics of TiNi-based and Ni-free Ti-based shape memory alloys are reviewed. They include the crystal structures of the parent and martensite phases in both the alloys, the recoverable strain associated with the martensitic transformation, the transformation temperatures, the temperature and orientation dependence of deformation behavior, etc. The sputter-deposited Ti-Ni thin films are also reviewed briefly because of their possibility of expanding into micromechanical system applications as the most powerful microactuator.

Elastic deformation behaviour of Ti-24Nb-4Zr-7.9Sn for biomedical applications

In this paper, the elastic deformation behaviour of a recently developed beta-type titanium alloy Ti-24Nb-4Zr-7.9Sn (wt.%) that consists of non-toxic elements and is intended for biomedical applications is described. Tensile tests show that this alloy in the as hot-rolled state exhibits peculiar non-linear elastic behaviour with maximum recoverable strain up to 3.3% and incipient Young's modulus of 42GPa. Solution treatment at high temperature has trivial effect on super-elasticity but decreases strength and slightly increases the incipient Young's modulus. Ageing treatment in the (alpha+beta) two-phase field increases both strength and Young's modulus and results in a combination of high strength and relatively low elastic modulus. In spite of the formation of the alpha phase, short time ageing has no effect on super-elasticity, whereas the non-linear elastic behaviour transforms gradually to normal linear elasticity with the increase of ageing time. We suggest sluggish, partially reversible processes of stress-induced phase transformation and/or incipient kink bands as the origin of the above peculiar elastic behaviour.

Recent Development of New Alloys for Biomedical Use

Metallic materials are widely used in medicine not only for orthopedic implants but also for cardiovascular devices and other purposes. New alloys for biomedical use are developed all over the world continuously to decrease corrosion, toxicity and fracture during implantation and increase interfacial and dynamical tissue compatibility. Most of efforts are made to develop titanium alloys, especially in β-type alloys whose Young’s modulus is as low as cortical bone. Nickel-free alloy is also necessary to prevent nickel allergy: nickel-free austenitic stainless steels and shape memory alloys are developed. To increase iocompatibility, the controls of surface morphology and surface treatment or modification are necessary.

Structure and properties of cast binary Ti-Mo alloys

Structure and properties of a series of binary Ti-Mo alloys with molybdenum contents ranging from 6 to 20 wt% have been investigated. Experimental results indicated that crystal structure and morphology of the cast alloys were sensitive to their molybdenum contents. The hexagonal alpha' phase c.p. Ti exhibited a feather-like morphology. When Mo content was 6 wt%, a fine, acicular martensitic structure of orthorhombic alpha" phase was observed. When Mo content was 7.5 wt%, the entire alloy was dominated by the martensitic alpha" structure. When Mo content was increased to 10 wt% or higher, the retained beta phase became the only dominant phase. Among all Ti-Mo alloys, the alpha" phase Ti-7.5Mo alloy had the lowest hardness. The bending strength of Ti-7.5Mo was similar to that of Ti-15Mo and Ti-13Nb-13Zr, and higher than c.p. Ti by nearly 60%. The bending modulus of the alpha"-dominated Ti-7.5Mo alloy was lower than that of Ti-15Mo by 22%, of Ti-6A1-4V by 47%, of Ti-13Nb-13Zr by 17%, and of c.p. Ti by 40%.