Isothermal Omega Phase Research Articles

Role of fine nano-scaled isothermal omega phase on the mechanical and superelastic properties of a high Zr-containing Ti–Zr–Nb–Sn shape memory alloy

This work studied the microstructure, superelastic and mechanical performances of a high Zr-containing Ti–40Zr–8Nb–2Sn (at.%) shape memory alloy aged at 300 °C for 1 h. Nanodomains and quite fine nano-scaled ωiso phases with 2.1–2.9 nm wide and 2.9–4.4 nm long were formed in the aged alloy. The aged alloy in this study exhibited excellent superelasticity with a large total recovery strain of 7.1%, high ductility of 11.6% and high yield stress of 578 MPa, indicating that a good balance of functional and mechanical properties was achieved in the aged alloy. The stress-induced β → α" martensitic transformation occurred in the nanodomain-containing aged Ti–40Zr–8Nb–2Sn alloy.

Plastic deformation of beta-Ti-Mo alloys with isothermal omega phase

Aging of metastable beta-Ti alloys leads to the precipitation of omega phase, resulting in a significant loss in ductility (omega brittleness). This presentation reports the results of 3D observations of isothermal omega phase by an orthogonal FIB/SEM system. It was revealed that the shape of omega phase particles varies with the particle size. By serial sectioning, it was possible to obtain a 3D image for the volume of several micrometer cube containing numerous omega particles. The volume fraction of omega phase was qualitatively determined. The value was much lower than the previous investigation by X-ray diffraction by Hickman in 1969. The SEM and TEM revealed the formation of shear bands in the cold rolled samples, where the shearing of omega phase and transformation into beta phase while maintaining the depletion of Mo was found. After the HPT deformation, white etching layers are formed near the median plane of the disc, where the omega phase particles were absent and exhibit significantly lower hardness values. These findings may help to clarify the cause of brittleness in beta Ti alloys with isothermal omega phase.

Effects of Mo segregation on Charpy absorbed energy in Ti-12Mo alloys

Beta phase stabilizing elements such as Mo have strong tendency to segregate. We have introduced swirly type segregation of Mo in Ti-12Mo (mass %) alloy through groove bar rolling. After solution treatment and low temperature aging, hard omega phase was precipitated heterogeneously, which improved the room temperature tensile elongation values without sacrificing tensile strength. In this study, the effect of Mo segregation and heterogeneous distribution of omega phase on Charpy absorbed energy was investigated in Ti-12Mo alloy. Samples with two types of segregation were prepared; namely, swirly segregation in bar rolled sample and layered segregation in plate rolled sample. For comparison, we have also prepared Ti-12Mo bar samples with lesser Mo segregation, through high temperature thermomechanical treatment. Charpy impact tests were carried out at room temperature, 373 K and 473 K, respectively, using the samples after aging to introduce isothermal omega-phase. The samples with the segregation exhibited higher Charpy absorbed energy, especially at higher temperature of 473 K, while the sample with the swirly segregation showed higher Charpy absorbed energy than that with the layered segregation. The sample with lesser Mo segregation exhibited brittle intergranular fracture surface after Charpy testing. On the contrary, samples with Mo segregation exhibited ductile transgranular fracture surfaces.

Role of isothermal omega phase precipitation on the mechanical behavior of a Ti-Mo-Al-Nb alloy

The formation and the evolution of the athermal and isothermal omega phase precipitates in a commercial metastable β–titanium alloy, β-21S, under controlled heat treatments, and its consequent impact on tensile properties has been presented in this paper. The structural and compositional changes of the omega precipitates have been tracked using SEM, TEM, and atom probe tomography (APT). Upon quenching from above the β-transus, the microstructure consisted of a single phase β without any discrete athermal omega precipitates and this condition exhibited the highest tensile ductility. Following β -solutionizing, when the samples were aged below the omega solvus temperature, there was an increase in the size scale of the omega precipitates with increasing time period. The larger and more solute-depleted omega precipitates, resulted in increased tensile strength and reduced ductility. The details of the deformation behavior were further analyzed using electron backscattered diffraction (EBSD) and TEM. Two different modes of deformation were noted in the system, depending on the initial microstructure. While the single phase condition (β solutionized sample) exhibited deformation occurring mainly via (110)β slip, the presence of larger isothermal omega precipitates in the aged condition (β solutionized + 350 °C/100H) resulted in the activation of not only (110) β slip but also (112) β slip. The reasonably high tensile ductility noted in this aged condition has been attributed to the activation of both these slip systems.

Development and Research of Low-Cost Titanium Alloys, Especially Case of Japan

Titanium (Ti) exhibits many attractive properties that enable practical applications. It is also considered to be a ubiquitous element, since it has the ninth highest Clarke number among all the elements. However, the principal beta-stabilizing elements for Ti, molybdenum and vanadium, can be very expensive, and so many Ti alloys are also costly. For this reason, less expensive alloying elements would be preferable. Iron (Fe) and manganese (Mn) are beta stabilizers for Ti alloys that are readily available, since they have the fourth and eleventh highest Clarke numbers, respectively. Furthermore, since Fe has a large diffusion coefficient in the beta phase of Ti, precipitation of the omega phase occurs more quickly when Fe is added. The behaviors of Ti-Mn and Mn-Fe alloys during heat treatment have been investigated and it has been found that, in some alloys, the isothermal omega phase is precipitated. Because this phase can lead to brittleness of the alloy, it is very important to suppress its precipitation. Since it is well known that aluminum (Al) suppresses isothermal omega precipitation, the present work investigated the effects of Al content on the phase constitution and heat treatment behavior of Ti-8.5 mass%Mn-1 mass%Fe-0, 1.5, 3.0 and 4.5 mass%Al alloys using electrical resistivity, Vickers hardness, and X-ray diffraction measurements. In the case of each of these alloys, whether solution-treated or water-quenched, only the beta phase was identified. The resistivities at room and liquid nitrogen temperatures were found to increase monotonically with Al content, while the Vickers hardness decreased up to 3 mass% Al and then remained constant. The addition of Al was found to suppress omega precipitation.

Effect of Al Addition on Phase Constitution and Heat Treatment Behavior in Ti-8.5mass%Mn-1mass%Fe-Al Alloys

Titanium is considered to be a ubiquitous element since it has the 9th-highest Clarke number of all elements. Iron and manganese can also be used as beta stabilizers for Ti alloys, and can be considered to be ubiquitous because of their 4th- and 11th-highest Clarke numbers, respectively. However, investigations into the behavior of Ti-Mn-Fe alloys during heat treatment have shown that in some alloys, the isothermal omega phase is precipitated. Because this phase can lead to brittleness, it is very important to prevent it from forming. It is well known that aluminum can suppress the precipitation of the isothermal omega phase. Thus, in the present study, we investigated the effect of Al content on the phase constitution and heat-treatment behavior of Ti-8.5mass%Mn-1mass%Fe-0 to 4.5mass%Al alloys using electrical resistivity, Vickers hardness, and X-ray diffraction measurements. In all solution-treated and quenched alloys, only the beta phase was identified, thus confirming the suppression of omega-phase precipitation. The resistivity was found to increase monotonically with Al content, while the Vickers hardness decreased up to 3 mass% Al and then remained constant.

Influence of Fe Content of Ti-Mn-Fe Alloys on Phase Constitution and Heat Treatment Behavior

If Mn could be partly substituted by Fe, Ti-Mn-Fe alloys would be less costly than Ti-Mn alloys. Furthermore, the use of iron as a beta-stabilizing element is more suitable than the use of manganese from a situation of an element strategy. In this study, 4.26 was admitted as the average ratio of valence electrons to atoms, e/a. The compositions of Mn and Fe were chosen under 4.26 as e/a. We investigated the influence Fe in selected Ti-Mn-Fe alloys by performing electrical resistivity, Vickers hardness, and X-ray diffraction measurements. In solution-treated and water-quenched 10Mn alloy, the beta and athermal omega phases were identified, while only the beta phase was identified in 8.7Mn-1Fe, 6.1Mn-3Fe, and 3.5Mn-5Fe alloys. In all alloys, equiaxial beta grains were observed by optical microscope. The resistivities at room and liquid-nitrogen temperatures and the Vickers hardness were relatively invariant across all Ti-Mn-Fe alloys, except for the Vickers hardness of the 5Fe alloy. During aging at 773 K, an isothermal omega phase precipitated in only the 3.5Mn-5Fe alloy, whereas only the alpha phase precipitated in the others.

Mechanical properties of low modulus [formula omitted] titanium alloys designed from the electronic approach

Titanium alloys dedicated to biomedical applications may display both clinical and mechanical biocompatibility. Based on nontoxic elements such as Ti, Zr, Nb, Ta, they should combine high mechanical resistance with a low elastic modulus close to the bone elasticity (E=20 GPa) to significantly improve bone remodelling and osseointegration processes. These elastic properties can be reached using both lowering of the intrinsic modulus by specific chemical alloying and superelasticity effects associated with a stress-induced phase transformation from the BCC metastable beta phase to the orthorhombic α ″ martensite. It is shown that the stability of the beta phase can be triggered using a chemical formulation strategy based on the electronic design method initially developed by Morinaga. This method is based on the calculation of two electronic parameters respectively called the bond order ( B o ) and the d orbital level ( M d ) for each alloy. By this method, two titanium alloys with various tantalum contents, Ti–29Nb–11Ta–5Zr and Ti–29Nb–6Ta–5Zr (wt%) were prepared. In this paper, the effect of the tantalum content on the elastic modulus/yield strength balance has been investigated and discussed regarding the deformation modes. The martensitic transformation β → α ″ has been observed on Ti–29Nb–6Ta–5Zr in contrast to Ti–29Nb–11Ta–5Zr highlighting the chemical influence of the Ta element on the initial beta phase stability. A formulation strategy is discussed regarding the as-mentioned electronic parameters. Respective influence of cold rolling and flash thermal treatments (in the isothermal omega phase precipitation domain) on the tensile properties has been investigated.

Effect of Oxygen Addition on Isothermal Omega Phase Stability in Ti-29Nb-13Ta-4.6Zr

A peculiar effect of oxygen on ω-phase stability, i.e., the enhancement of the isothermal ω-phase formation during aging due to oxygen addition was observed in the Ti-29Nb-13Ta-4.6Zr; this effect is contradictory to that reported conventionally. The effect was analyzed from the viewpoint of distribution of alloying elements. Oxygen and/or zirconium may dissolve in the ω phase during aging, resulting in the stabilization of the ω phase in this alloy.

Phase Constitution and Heat Treatment Behavior of Ti-7mass% Mn-Al Alloys

Titanium exhibits many attractive properties. It is considered to be ubiquitous since it has the 9th-highest Clarke number of all the elements. However, the principal beta-stabilizing elements for titanium can be very expensive, making many titanium alloys expensive. Manganese is a beta stabilizer for titanium alloys and it is also considered to be ubiquitous since it has the 11th-highest Clarke number of all the elements. The behavior of Ti-Mn alloys during heat treatment has been investigated and it was found that in some alloys the isothermal omega phase is precipitated. Because this phase can lead to brittleness, it is very important to suppress its precipitation. Since it is well-known that aluminum suppresses isothermal omega precipitation, we investigated the effect of adding aluminum using Ti-7mass% Mn-0, 1.5, 3.0 and 4.5mass% Al alloys by performing electrical resistivity, Vickers hardness, and X-ray diffraction measurements. In solution-treated and water-quenched 0 and 1.5 alloys, only beta phase was identified, while hcp martensite and bate phase were identified in 3.0 and 4.5Al alloys. The resistivities at room and liquid-nitrogen temperatures were found to increase monotonically with increasing Al content. Isothermal  precipitation was suppressed by aluminum addition, while alpha precipitation was accelerated by Al addition.

Isothermal Aging Behavior of Beta Titanium–Manganese Alloys

Although titanium is considered to be a ubiquitous element as it has the 10th-highest Clarke number of all the elements, it is actually classified as a rare metal because the current refinement process for the metal is more environmentally damaging than the processes used to refine iron and aluminum. Furthermore, the principal alloying elements of titanium alloys are very expensive, owing to their low crustal abundances; this is especially true of the beta-stabilizing elements. Manganese is also considered to be a ubiquitous element as it has the 12th-highest Clarke number of all the elements. Therefore, manganese is promising as an alloying element for titanium, especially as a beta-stabilizing element. In order to develop beta titanium alloys as ubiquitous metallic materials, it is very important to investigate the properties of Ti-Mn alloys. In this study, the phase constitutions and isothermal aging behaviors of Ti-6.0 to 14.8 mass%Mn alloys were investigated by electrical resistivity and Vickers hardness measurements, X-ray diffraction (XRD), and optical microscopy. In 6.0 mass%Mn alloys quenched from 1173 K, both hexagonal close-packed martensite and the beta phase were identified by XRD, whereas only the beta phase was detected in 8.7 and 14.8 mass%Mn alloys. The resistivity at liquid nitrogen temperature was greater than that at room temperature between 6.0 and 14.8 mass%Mn. The Vickers hardness decreased with an increase in the Mn content up to 11.3 mass%Mn and then increased slightly. On aging at 673 K, the isothermal omega phase was precipitated in 6.0 to 11.3 mass%Mn alloys, while it was precipitated by aging at 773 K in 6.0 and 8.7 mass%Mn alloys. The Vickers hardness increased drastically on isothermal omega precipitation, whereas it increased slightly in the case of direct alpha precipitation with no isothermal omega precipitation. [doi:10.2320/matertrans.MA200902]

Growth Kinetic Model for Isothermal Omega Phase Particles in Low-Cost Beta Titanium Ti–6.8Mo–4.5Fe–1.5Al Alloy

The growth kinetics of the isothermal omega phase in Ti–6.8Mo–4.5Fe–1.5Al can be monitored by the continuous measurement of the electrical resistivity as shown in our previous publication in this Journal. The rate law for the growth of these isothermal omega particles radius riso is obtained assuming the resistivity to be proportional to the surface of the interface between the beta matrix and the quasi spherical isothermal omega particles. The rate law r3iso∝t is observed at the beginning of the growth process, that is before the interaction of the outward diffusion fields from the particles. A mathematical model based on a diffusional transfer of the beta stabilizers from the ωiso particles through the interface between the ωiso particles into the beta matrix is developed. The growth rate slows down when the outward diffusional fields from the particles interact. Ostwald ripening is not observed.

Design of new titanium alloys for orthopaedic applications.

Parallel to the biofunctionalisation of existing materials, innovation in biomaterials engineering has led to the specific design of titanium alloys for medical applications. Studies of the biological behaviour of metallic elements have shown that the composition and structure of the material should be carefully tailored to minimise adverse body reactions and to enhance implant longevity, respectively. Consequently, interest has focused on a new family of titanium alloys: Ti-6Mo-3Fe-5Ta, Ti-4Mo-2Fe-5Ta and Ti-6Mo-3Fe-5Zr-5Hf alloys. The non-toxicity of the specially designed titanium alloys compared with osteoblastic cells has been ascertained using MTT and RN tests. In addition, phase transformations upon thermal processing have been investigated, with comparison with a well-defined beta titanium alloy. Optimum thermal processing windows (above 550 degrees C) have been designed to generate a stable and nanostructured alpha phase from the isothermal omega phase that precipitates in a low temperature range (150-350 degrees C). The generation of such nanostructured microstructures should provide a promising opportunity to investigate tissue-biomaterial interactions at the scale of biomolecules such as proteins.

Ti-Fe-Cr合金の相構成と引張特性に及ぼすβ単相領域温度からの冷却速度の影響

The influence of cooling rate on phase constitution and tensile properties of Ti-2.9Fe-4.9Cr and Ti-4.2Fe-6.9Cr alloys was investigated by electrical resistivity and Vickers hardness measurements, X-ray diffraction, optical microstructure observation and tensile testing.In only Ti-2.9Fe-4.9Cr alloy quenched at an average cooling rate of 25 Ks−1, reflections from isothermal omega phase were identified, whereas only reflections from beta phase were identified by XRD with no reflections of isothermal omega phase present in Ti-2.9Fe-4.9Cr alloy cooled at other cooling rates and Ti-4.2Fe-6.9Cr alloy quenched at all cooling conditions. HV for Ti-2.9Fe-4.9Cr alloy cooled at 25 Ks−1, was about 400 and HV of Ti-4.2Fe-6.9Cr alloy cooled at all cooling conditions, even 25 Ks−1, was around 320. Therefore, quench sensitivity of Ti-4.2Fe-6.9Cr alloy is lower than that of Ti-2.9Fe-4.9Cr alloy.Tensile strength and reduction in area of Ti-2.9Fe-4.9Cr alloy cooled at three different cooling rates except for 25 Ks−1 showed about 1200 MPa and 45%, respectively. About 1050 MPa in σB and 60% in φ were obtained in all quenched Ti-4.2Fe-6.9Cr alloys, respectively. The strength/ductility balances of Ti-4.2Fe-6.9Cr alloy quenched at four different cooling rates are comparable with those of developed beta titanium alloys in solution treated and quenched state.

Decomposition processes prior to detection of the omega phase in aged TiCr alloys

Transmission electron microscopy has been employed to study the microstructural processes that occur prior to the detection of the isothermal omega phase in beta stabilized TiCr binary alloys. The alloy compositions were selected to provide a suitable incubation period prior to the initiation of the omega phase during isothermal aging. It has been observed that the metastable beta phase undergoes a phase separation reaction, ß → ß 1 + ß 2 prior to the detection of the omega phase in the TiCr alloys. The ß 2 precipitate regions produced by this reaction do not act as preferred sites for the subsequent nucleation of the omega phase.