Omega Precipitation Research Articles

Mechanisms underlying enhanced strength-ductility combinations in TRIP/TWIP Ti-12Mo alloy engineered via isothermal omega precipitation

β Ti-alloys can achieve a high strain-hardening rate and tensile ductility by taking advantage of transformation induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects. While nano-precipitation of isothermal ω (ωiso) can have a substantial strengthening effect in these alloys, it usually has a detrimental effect on ductility leading to embrittlement. To overcome the above problem, this work proposes as a novel strategy based on coupling of ωiso formation and mechanical twinning/martensitic transformation to enhance the strength while preserving good ductility in case of the TRIP/TWIP Ti-12Mo alloy. An unprecedented combination of tensile properties, yield stress at 865MPa (80% higher than classic Ti-12Mo) with uniform elongation of 0.35, are recorded after 200°C aging for 60s. Higher yielding stress (990MPa) is achieved when increasing the aging duration to 150s where mechanical twinning is still active. In-situ investigations under traction/heating, and atom probe tomography are performed to clarify the ωiso formation process and the interactions between ω phase and the operating deformation mechanisms, i.e. mechanical {332}<113> twinning, β → α″ martensitic transformation and dislocation glide. The early stages of formation of ωiso precipitates, mediated via Mo partitioning at low aging temperature, and its consequent impact on the deformation mechanisms operative in the β matrix has been characterized. The transformation partition mapping method, based on statistical electron backscatter diffraction characterization developed in our previous work, is employed to individually assess the evolution of the critical resolved shear stresses of each operating deformation mechanism as a function of the ωiso nucleation.

Influence of isothermal omega precipitation aging on deformation mechanisms and mechanical properties of a β-type Ti-Nb alloy

In this study, the influence of ωiso precipitates on the active deformation mechanisms and the mechanical properties of the biomedical β-type Ti-40Nb alloy are revealed. Low temperature heat treatments (aging) at 573 K for durations up to 108.0 ks were carried out for a cold-rolled and recrystallized sample state. After an aging time of 3.6 ks the ωiso phase was determined by means of synchrotron XRD and the fraction and the crystallite size of ωiso increased progressively with increasing aging time. Due to the high intrinsic Young’s modulus of the ωiso phase, the Young’s modulus increased gradually with the aging time from 63 GPa, for the recrystallized reference condition, to values of 70 GPa (3.6 ks), 73 GPa (14.4 ks), 81 GPa (28.8 ks) and 96 GPa (108.0 ks). Depending on the aging time, also a change of the active deformation mechanisms occurred, resulting in significantly altered mechanical properties. For the single β-phase reference microstructure, stress-induced martensite (SIM) formation, {332}113̅ twinning and dislocation slip were observed under tensile loading, resulting in a low 0.2% proof stress of around 315 MPa but a high elongation at fracture of 26.2%. With increasing aging time, SIM formation and mechanical twinning are progressively hindered under tensile loading. SIM formation could not be detected for samples aged longer than 3.6 ks. The amount and thickness of deformation twins is clearly reduced with increasing aging time and for samples aged longer than 14.4 ks deformation twinning is completely suppressed. As a result of the changed deformation mechanisms and the increase of the critical stress for slip caused by ωiso, the 0.2% proof stress of the aged samples increased gradually from 410 MPa (3.6 ks) to around 910 MPa (108.0 ks). With regard to application as new bone implant material, a balanced ratio of a low Young’s modulus of E = 73 GPa and higher 0.2% proof stress of 640 MPa was achieved after an aging time of 14.4 ks.

Surface Layers of Zr-18%Nb Alloy Modified by Ultrasonic Impact Treatment: Microstructure, Hardness and Corrosion

Near-surface layers in Zr-18%Nb alloy were modified using ultrasonic impact treatment (UIT). The effects of the UIT processing time on a microstructural formation, omega/alpha precipitations, microhardness and corrosion are analyzed. XRD analysis, TEM and SEM observations and EDX characterization allow establishing the links between the microstructure, microhardness and corrosion behavior of the surface layers formed. At the strain extent up to e ≈ 0.3, structural formation occurs under influence of deformation induced heating, which facilitates omega precipitation in beta phase and mechanically induced oxygen transport and oxide formation. XRD analysis reveals moderate compressive residual stresses (− 160 MPa) and pronounced {110} texture after the UIT process. Generation of dislocations and hindering of their movement by nanoscale omega precipitates manifest themselves as the broadening of diffraction peaks occurred mainly owing to the lattice microstrains, and they provide marked strain hardening. The enhanced anticorrosion properties of Zr-18%Nb alloy in saline solution were concluded to be a result of the formation of a protective oxide film, {110} texture and compressive stresses.

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.

Coupling Atom Probe Tomography with Aberration-Corrected Scanning Transmission Electron Microscopy and First-Principles Computations to Investigate Omega Precipitation in Titanium Alloys

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Structurally induced enhancement in corrosion resistance of Zr–2.5%Nb alloy in saline solution by applying ultrasonic impact peening

Near surface layers in Zr–2.5%Nb alloy are modified using severe plastic deformation process induced by ultrasonic impact peening (UIP). The effect of the UIP processing time on the microstructural formation, microhardness and corrosion is analyzed. XRD analysis and TEM observations allow establishing the links between microstructure in the surface layers formed at different extents of the effective strain e¯ and their microhardness and corrosion resistance. Structural formation occurs under influence of deformation induced heating, which facilitates dynamic recrystallization and omega precipitation in beta grains. Subdivision of alpha grains is observed to be a dislocation-mediated process mainly. Deformation twinning may be effective in the early stages of the UIP process till the strain extents of approx. e¯≈0.2. Generation and rearrangement of dislocations result in formation of elongated cell structure at e¯≈0.43. Further straining to e¯≈0.8 leads to formation of ultrafine-grained (UFG) structure through the subdivision of elongated subgrains into equiaxed ones and subsequent increase in their misorientation. Average grain size in the formed UFG structure was about 150–200nm. XRD analysis reveals high compressive residual stresses and strong basal texture after the UIP process. It is suggested that the sliding character of the impact loads occurred at the UIP process plays an important role in the crystallites' reorientation in superficial layer. Formed UFG surface layer with strong basal texture provides an essential increase in surface microhardness from 2 to 3.15GPa and marked enhancement of the corrosion resistance of Zr–2.5%Nb alloy in saline solution due to the broadening of passivity region on the anodic potentiodynamic polarization curve. Decreased surface roughness and large compressive residual stresses also promote higher corrosion resistance.

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.

Investigations of Omega Precipitation in Titanium Molybdenum Alloys by Coupling 3D Atom Probe Tomography and High Resolution (S)TEM

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Isothermal Aging Behavior of Beta Titanium&amp;ndash;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]

Ageing of C-Containing and C-Free Ti-15Cr and Ti-15-3

Samples of Ti-15Cr and Ti-15V-3Sn-3Al-3Cr (wt%) containing controlled additions of carbon up to 0.2wt% and different oxygen contents have been quenched and aged at temperatures between 400 and 600°C. Optical, scanning and analytical transmission electron microscopy have been used to characterise the microstructures of the quenched and aged samples. Hardness testing has been used to follow the kinetics and extent of age hardening, which are accelerated in Ccontaining samples. The addition of carbon results in the formation of Ti(CxOy) precipitates which pin grain boundaries in forged samples so that the grain size in the quenched C-containing samples is about a factor of ten less than that in the C-free samples. In the C-free samples coarse grain boundary alpha tends to form, but in the C-containing samples alpha precipitation is more uniform throughout the beta grains. The extent of omega precipitation is very different in the two alloys; the Ti-15Cr alloy forms athermal omega in the as-quenched samples and large omega precipitates are formed on ageing at 400°C. No evidence for omega has been obtained in the Ti-15-3. The hardening responses and microstructural observations are interpreted in terms of the different grain boundary oxygen contents in the C-containing and C-free samples and the different roles of omega and of carbon in the two alloys.

Experimental evidence of elastic interaction between ω nanoparticles embedded in a metastable β titanium alloy

Elastic interactions by means of correlation effects between precipitate particles often play an important role in determining the microstructure of systems in which a precipitation reaction takes place. Thus, the role of strain fields and the ensuring mechanochemical processes have historically been discussed in connection with the classic problem of precipitation and growth of misfitting metallic clusters in an anisotropic matrix [1]. However, although the case of coherent γ ′ precipitates has been intensively investigated in Ni based alloys [2–4], very little information is available in the literature concerning such mechanisms in isothermal ω phase growth kinetics in β titanium alloys. Such knowledge is of a great interest since metastable β Ti alloys can be primarily strengthened through precipitation of coherent secondary phases (ω and α), leading to extremely refined high strength microstructures [5]. The aim of this paper is to present preliminary results concerning the morphological aspect of the ω nanoparticles embedded in a BCC β matrix under the influence of coherent strain field interactions. Resistivity measurements and transmission electron microscopy (TEM) dark field imaging have been carried out and a short range elastic interaction between ω particles is proposed. The composition (by weight/%) of the titanium alloy provided by Timet was 87.2% Ti, 6.8% Mo, 4.5% Fe, 1.5% Al. In order to obtain the metastable β-Ti structure at room temperature, this alloy was heat treated (1.8 ks annealing at 1123 K) in a high vacuum induction furnace and then water quenched. The electrical resistance (R) measurements were made by a computer driven four point technique. R/Ri (Ri is the initial resistance) was plotted as a function of time and temperature. In this way, microstructural evolution and transformation kinetics can be investigated [6–8]. TEM observations were carried out on a Jeol 2000FX operating at 200 kV. From previous studies [8], ω phase has been shown to precipitate between 423 and 623 K in the Ti-6.8Mo4.5Fe-1.5Al alloy. The typical ω phase morphology is displayed by the TEM dark-field image in Fig. 1. On this micrograph, ω ellipsoidal nanoparticles are clearly observed to precipitate randomly through the β-Ti cubic matrix after a thermal treatment of 3.6 ks at 473 K. It is now reasonably well established that the ω phase can nucleate through the well-known (111)β collapsing mechanism first proposed by De Fontaine [9]. The classical orientation relationship between the two phases β and ω have been confirmed in the present TEM investigation and are consistent with the following epitaxial relationships: (111)β //(0001)ω and [110]β //[1120]ω. As a consequence, the lattice misfit between the metastable ω phase and the β matrix is expected to be strong and this lattice mismatch is accommodated by elastic displacements generating strain fields within both phases. Isothermal growth kinetics of ω phase were also investigated by resistivity measurements. Fig. 2 presents the change in resistivity of quenched Ti-6.8Mo-4.5Fe1.5Al as a function of time at three different isothermal heat treatments within the omega precipitation range (473, 508 and 523 K). The detailed growth mechanisms of ω phase have not been clarified yet [5], however, Ho et al. [7] assumed that resistivity changes could be correlated with an

Effects of oxygen and heat treatment on the mechanical properties of alpha and beta titanium alloys

Two alloys, Ti-6Al-2V and Ti-2Al-16V, simulating the alpha and beta phases of Ti-6A1-4V, respectively, were prepared with oxygen concentrations from 0.07 to 0.65 wt pct (0.20 to 1.83 at. pct). Their microstructure, deformation behavior, and strength were investigated with X-ray diffraction, microscopy, and mechanical tests to determine the effects of oxygen concentration and heat treatment. In both alloys the hardness increases in identical fashion with the square root of oxygen concentration. The alloys' strengths also depend on heat treatment, but in different ways. Whereas the alpha alloy is non-age-hardenable, the beta alloy's strength can be doubled by aging. The hardening effect of oxygen is generally unaffected by heat treatment, except for the alloys with the highest oxygen concentrations. During aging of the alpha a small amount of Ti3Al can form, and slight age-hardening occurs. The ductility of the alpha alloy is little affected by aging. On the other hand, oxygen causes a change from good ductility at low oxygen concentration (0.07 wt pct) to total brittleness at 0.65 wt pct oxygen, independent of heat treatment. In the beta alloy there are complex phase transformations depending on heat treatment. Its deformation behavior varies from very ductile in solutiontreated and quenched (STQ) condition to totally brittle in aged conditions. The aging embrittlement appears to be caused by alpha and some omega precipitation. Decoration of the beta grain boundaries with precipitates accounts for the intergranular brittle fracture. Oxygen, on the other hand, is not an embrittler, although it reduces the ductility of the beta alloy.

準安定&amp;beta;型チタン合金の時効特性

Aging characteristics of the beta metastable titanium alloys Ti-15Mo-5Zr and Ti-15Mo-5Zr-3Al were investigated with detailed aging treatments by means of the tensile test, hardness measurement, microstructure observation and the X-ray diffraction method.In the as-solution treated condition, the strength decreased with increasing solution temperature and the relationships between the yield strength (YS kg/mm2) and the beta grain diameter (d mm) were given as follows:(This article is not displayable. Please see full text pdf.) The optimum solution temperature for the aged material was beta transus for Ti-15Mo-5Zr and 50°C below the beta transus for Ti-15Mo-5Zr-3Al, as the finer beta grains gave the good tensile properties after heat treatment.In the Ti-15Mo-5Zr alloy solution treated at the beta transus and aged at 400°C, the maximum age hardening and the minimum value in ductility were observed for each aging time. It was found from the X-ray diffraction results that these phenomena were related to the omega phase precipitation. In this alloy, the alpha phase precipitation led to the decrease in tensile strength and the increase in ductility. In the Ti-15Mo-5Zr-3Al alloy the omega phase precipitation during aging was suppressed by aluminum addition and the age hardening was related to the amount of the alpha phase strengthened by aluminum. The largest age hardening was observed after heat treatment at 425 to 450°C. Therefore, the mechanical properties were controlled by the omega and the alpha precipitation in the Ti-15Mo-5Zr alloy and mainly by the alpha precipitation in the Ti-15Mo-5Zr-3Al alloy. The tensile properties obtained by the optimum heat treatment conditions are as follows:(This article is not displayable. Please see full text pdf.) The notched tensile test, the impact test and the fatigue test showed that the Ti-15Mo-5Zr-3Al alloy hardened by the alpha precipitation was superior to the Ti-15Mo-5Zr alloy hardened by the omega precipitation as a commercial alloy, with good combinations of strength, ductility, toughness and fatigue property.

Relaxation processes in metastable beta titanium alloys

Extensive observations have been made on an internal friction peak in a series of binary titanium alloys containing Mo and V. The compositions of the alloys ranged from 8 to 16 at.% Mo and from 15 to 50 at.% V, respectively. The peak was observed depending on alloy compositions in temperature ranges of 120°K to 140°K at 10 4 Hz and 153°K to 210°K at 10 7 Hz, respectively. In both alloy systems, the peak strength passed through a maximum with increasing alloy content at or near the upper compositional bound for athermal omega formation. The peak persisted well beyond the composition associated with the peak strength maximum, although its strength was greatly diminished. The peak structure was not disturbed by 15 per cent room temperature plastic deformation. The peak strength was severely reduced by annealing in the temperature range of thermal omega precipitation. Activation parameters of Q = 0.2 eV and τ 0 = 2 × 10 −14 sec were observed for both Ti-10 Mo and Ti-20 V. The results of this work are compared to the properties of a similar internal friction peak reported earlier in Ti and Zr base alloys, and discussed in conjunction with a reciprocal lattice streaking phenomenon also known to occur in these metastable beta titanium alloys. It is suggested that the atomistic mechanism responsible for the internal friction peak is the same “atom shuffle” process proposed by de Fontaine, Paton and Williams to explain reciprocal lattice streaking. Limited observations on a lower temperature relaxation ( T ≤ 25° K at 10 4 Hz) in the Ti-V alloy system are also reported.

Ti-15Mo系&amp;beta;合金の諸性質に及ぼすAlの影響

Though Ti-15Mo-5Zr shows, as given previously, such excellent characteristics as corrosion resistance, cold workability and weldability, and is placed under several practical applications, the stability and the resistance to oxidation of the alloy is not satisfactory.With the intention of improving these imperfections of Ti-15Mo-5Zr, the effect of Al addition on the various properties, such as thermal and mechanical stability of beta phase, forgeability, cold workability, aging property, oxidation resistance, and corrosion resistance, were studied. Three Ti-15Mo-5Zr base alloys and three commercial or semicommercial alloys were used in this investigation. Though the embrittlement of Ti-15Mo-5Zr after slow cooling was much reduced by the addition of 3%Al, because Al suppresses and ratards the omega precipitation, Ti-15Mo-5Zr-3Al-2V is superior to Ti-15Mo-5Zr-3Al so far as corrosion resistance is concerned.The latter alloy showed a marked hardening during the aging at rather high temperature, because Al strengthens the precipitates. This alloy is expected to be an excellent age hardenable alloy for construction application. There was no fundamental difference of oxidation resistance between Al-bearing and Al-free Ti-15Mo-5Zr alloys. The forgeability and cold workability of the alloy was not so much affected by 3%Al addition.

A phase separation reaction in a binary titanium-chromium alloy

Experimental evidence is presented to show that a phase separation reaction occurs in the beta phase of Ti-Cr binary alloys. The reaction has been detected prior to the precipitation of both the omega and the alpha phases, and it becomes a prominent mode of decomposition when omega precipitation is inhibited either by alloying additions or through appropriate heat-treatments. It is suggested that the β 2 particles in Ti-Cr alloys are leaner in solute than the matrix. The precipitation of the alpha phase during prolonged aging has been observed to occur on the β 2 particles formed during prior phase separation.

Ti-15Mo系&amp;beta;合金の安定性に及ぼすZrの影響

The effect of Zr addition (0∼40 wt%) on the stability of a metastable beta titanium alloy, Ti-15 wt%Mo, was investigated. The measurement of the mechanical properties in various annealing conditions revealed that the stability of Ti-15Mo-Zr alloy changed abruptly at about 5%Zr.When the alloys containing less than 5%Zr were slow-cooled from the beta region, hardening and embrittlement due to the decomposition of beta was remarkable, and when quenched, a low yield/tensile strength ratio was shown because of martensitic strain transformation.On the other hand, the mechanical properties of the alloys containing more than 5%Zr did not depend on the rate of cooling from the beta region, and the alloys were stable mechanically and thermally. The TTT diagram obtained by an automatic transformation apparatus and the changes in the mechanical properties during aging showed that Zr in a meta-stable beta alloy, Ti-15Mo, suppresses and retards the omega precipitation. The beta stabilizing power of Zr which was calculated from the slope of β⁄α+β solvus in the Ti-Zr equiliblrium diagram was weak, but the above-mentioned beta stabilizing effect of Zr in the Ti-15Mo alloy was as strong as that of Mo.

Comment on the preparation of thin foils of beta titanium

The preparation of clean thin foils of beta titanium suitable for transmission electron microscopy is discussed. Special attention is given to material containing omega precipitation.

Microstructure Effects in Ti-22 at.% Nb Conductors

Composite conductors of copper and titanium-niobium, either cable or solid, have been fabricated with low-weight-percentage Nb alloys in the range 22–28 at.% niobium. The fabrication of such alloys and conductors involves heat treatment in the temperature range 300°–500°C. It has been found that omega-phase precipitation occurs in all samples. Details as to the size and distribution of omega precipitates will be given as well as the correlation between omega precipitation and critical current densities in the superconductor.