Omega Phase Research Articles

Tensile behavior of dual-phase titanium alloys under high-intensity proton beam exposure: Radiation-induced omega phase transformation in Ti-6Al-4V

A high-intensity proton beam exposure with 181 MeV energy has been conducted at Brookhaven Linac Isotope Producer facility on various material specimens for accelerator targetry applications, including titanium alloys as a beam window material. The radiation damage level of the analyzed capsule was 0.25 dpa at beam center region with an irradiation temperature around 120 °C. Tensile tests showed increased hardness and a large decrease in ductility for the dual α+β-phase Ti-6Al-4V Grade-5 and Grade-23 extra low interstitial alloys, with the near α-phase Ti-3Al-2.5V Grade-9 alloy still exhibiting uniform elongation of a few % after irradiation. Transmission Electron Microscope analyses on Ti-6Al-4V indicated clear evidence of a high-density of defect clusters with size less than 2 nm in each α-phase grain. The β-phase grains did not contain any visible defects such as loops or black dots, while the diffraction patterns clearly indicated ω-phase precipitation in an advanced formation stage. The radiation-induced ω-phase transformation in the β-phase could lead to greater loss of ductility in Ti-6Al-4V alloys in comparison with Ti-3Al-2.5V alloy with less β-phase.

Twins or the omega phase: Which is it in high carbon steels?

Electron diffraction coupled with aberration-corrected (AC) scanning transmission electron microscopy (STEM) is used to image, identify and confirm the structure producing double diffracted spots seen in electron diffraction patterns in twinned high-carbon steels. The experimental and simulated data show that regions comprising: (i) overlaps between a twin and the matrix or, (ii) overlapping twins, create projected structures that are similar to the ω phase observed in other metastable bcc systems. Thus, AC STEM is proficient in the definitive identification of twins in bcc high carbon steels or nanoscale phases in metastable bcc systems.

Omega Transition Accompanied by Mechanical Induced Twinned Martensite in Medium Manganese Steels

The omega transition process was firstly analyzed combined with martensitic transformation in this investigation. In order to avoid the influence of auto-tempering on metastable omega phase, martensitic transformation process was inspired by deformation at room temperature. Omega phase was found only exist within twinned martensite with single variant, and the results indicate that omega phase formed by the lattice invariant twin shear during the dynamic transformation of twinned martensite. Besides, the nature poor ductility of twinned martensite was discussed.

A low-cost metastable beta Ti alloy with high elastic admissible strain and enhanced ductility for orthopaedic application

Abstract In this work, a low-cost biomedical titanium alloy Ti–5Mo–Fe–3Sn (atomic percent) was successfully developed. The microstructure, tensile properties and deformation behaviour were investigated at ambient temperature. It was found that the combined addition of Sn and Fe suppressed the formation of athermal omega phase and introduced solid solution strengthening. An excellent combination of low elastic modulus (52 GPa) and high yield strength (740 MPa) was achieved, leading to a high elastic admissible strain (1.42%). Transmission electron microscopy results revealed that with an increase in tensile strain, the {332} twin system was initiated first, and then secondary {332} twinning and ternary {112} twinning were also observed. The evolution of multi-twin system during deformation was responsible for the enhanced strain hardening rate and plasticity (elongation ∼30%).

Density functional theory study of ω phase in steel with varied alloying elements

AbstractThe presence of a long‐abandoned hexagonal omega (ω) phase in steel samples is recently gaining momentum owing to the advances in transmission electron microscopy (TEM) measurements, even though it is already reported in other transition‐metal alloys. The stabilization of this metastable phase is mainly investigated in presence of C, even though the formation of the ω phase is attributed to the combined effect of many factors, one among which is the enrichment of solute elements such as Al, Mn, Si, C, and Cr in the nanometer‐sized regimes. The present study investigates the effect of the above alloying elements in ω‐Fe using density functional theory (DFT) calculations. It is seen that the magnetic states of the atoms play a major role in the stability of ω‐Fe. Cohesive energy calculations show that the alloying elements affect the energetics and stabilization of ω‐Fe. Further, density of states calculations reveal the variation in d‐band occupancy in the presence of alloying elements, which in turn affects the cohesive energy. Phonon band structure calculations show that only ω‐Fe with substitutional C shows positive frequencies and hence possess thermodynamic stability. Finally, we confirm the existence of ω‐Fe using TEM measurements of a steel sample containing the same alloying elements. Our results can shed light on the stabilization of the ω in other transition‐metal alloys as well, in the presence of minor alloying elements.

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.

Relationship between phase composition and wear resistance of titanium alloys

The wide use of titanium alloys is often due to their unique properties. Availability of a wide variety of mechanical, physical and chemical properties is associated with a large number of polymorphic modifications of titanium alloys. During the last decade, the metastable omega phase piques the largest interest among all titanium modifications. This study attempts to explain the different wear behavior of some titanium alloys with different Nb content during dry friction impact.

Effect of phase transformations during aging on tensile strength and ductility of metastable beta titanium alloy Ti–35Nb–7Zr–5Ta-0.35O for orthopedic applications

Present work describes the aging response of biocompatible metastable beta titanium alloy Ti–35Nb–7Zr–5Ta (TNZT alloy for short) solution treated (ST) in (α + β) phase field. The primary focus of this study is to gain insight into the correlation among phase transformations, microstructure and mechanical properties of TNZT alloy pertinent for orthopedic applications. Complete recrystallization is observed during heat treatment at 750° C-8 h and these ST samples were subsequently aged from 300 °C to 550 °C for times up to 64 h. Very low elastic moduli in the range of 41–46 GPa are observed which is explained in terms of calculated values of alloying parameters bond order, d orbital energy value and electron per atom ratio. Lower aging temperatures have improved ductility (greater than 13% elongation), whereas higher temperature aged samples are failed in brittle manner (elongation < 5%). Among the aging treatments, 350 °C-16 h aging has resulted in an optimum strength and ductility combination of 966 MPa and 13.7%, respectively. Transmission electron microscopy analysis has confirmed the vital role of omega phase precipitation during 350 °C-16 h aging contributing to enhanced tensile and yield strength. Moreover, this investigation also indicates that the reduced ductility observed in case of 500 °C-8 h aging can be attributed to the coarsened lenticular grain boundary α precipitation and intragranular coarser alpha laths. Hence from the mechanical aspects of the current investigation, 350 °C-16 h aging can be considered to be a suitable heat treatment parameter for orthopedic applications.

Microstructural evolution in a high-silicon medium carbon steel following quenching and isothermal holding above and below the Ms temperature

In this study, the microstructural features evolved in a high-Si, medium-carbon steel (Fe-0.53C-1.67Si-0.72Mn-0.12Cr) subjected to quenching and isothermal holding at temperatures above and below the martensite start temperature (Ms) for one hour have been examined. Both laser scanning confocal and transmission electron microscopy were employed for detailed microstructural characterization, supported by dilatometry, X-ray diffraction, and hardness measurements. In the case of isothermal treatment above Ms at 300 °C, besides bainite transformation marked by typical S-shaped dilatation curve, high-carbon martensite is formed during the final cooling to room temperature. In the case of isothermal treatment below Ms at 250 °C, the initial martensite formation and subsequent carbon partitioning to austenite is followed by the formation of bainite containing carbides and some high-carbon martensite that forms during the final cooling to room temperature. Also, selected area diffraction patterns (SAED) for both of Q&B and Q&P heat treated samples showed there are extra weak diffraction spots, presumably due to the presence of omega phase (ω) as an intermediate phase during fcc (face- centered cubic) austenite to bcc (body- centered cubic) martensite transformation and is considered as a common substructure in bcc metals and alloys with a coherent interface with the matrix: aω = √2 × abcc and cω = √3/2 × abcc that appeared in twinning martensite or martensite regions with dislocations as a substructure.

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.

Observation of the microstructure of the metastable Ti15Mo alloy after ageing

Omega phase particles can be observed by conventional SEM. Metastable Ti15Mo alloy was annealed at 500 °C for 16 hours. During annealing, ω phase particles grew and got chemically stabilized by expelling Mo atoms. As the result, these ellipsoidal particles, approx. 100 nm long and 50 nm wide can be observed using back-scattered electrons signal in conventional SEM due to chemical contrast. TEM study proved that these particles indeed belong to ω phase. Co-existence of β, ω and α phases was observed. Thin α lamellae were observed along with distorted ω phase particles by TEM.

Formation of omega phase induced by laser shock peening in Ti-17 alloy

Synchrotron-based high-energy X-ray diffraction and transmission electron microscopy were used to study the in-depth compressive residual strains and microstructure evolution in Ti-17 alloy subjected to two-sided laser shock peening (LSP). A noticeable difference was observed in the lattice strains and extents of peak broadening on the two sides of the sample. The compressive residual strain was smaller and the peak broadening was more pronounced on the bottom surface in comparison to the top surface. This is due to the β → ω phase transformation induced by LSP on the bottom surface. It was determined that the ω phase with fine particle morphology and incommensurate structure is located mainly in the 15 μm-thick layer under the bottom surface. The formation of the LSP-induced ω phase on the bottom surface can be attributed to the adiabatic increase in temperature due to the higher shock peak pressure on the bottom surface, and can result in the relaxation of the accumulated residual compressive strain in the original phases.

Role of omega phase evolution in plastic deformation of twinning-induced plasticity β Ti–12V–2Fe–1Al alloy

The evolution of the ω phase and its role in plastic deformation in a twinning-induced plasticity β Ti–12 V–2Fe–1Al alloy aged at 373–573 K have been studied in this work. The results show that isothermal ω forms at about 398 K and above, and the growth of isothermal ω is accompanied by a rejection of solute atoms from ω into β. The composition partitioning increases internal elastic strain energy and hydrostatic pressure resulting from the coherency of isothermal ω phase in β. Moreover, the isothermal ω phase becomes hard and even undeformable due to composition partitioning based on first-principles calculations. It is suggested that the internal elastic strain stabilizes the β phase, which suppresses {332} deformation twinning and leads to dislocation glide and stress-induced ω phase transformation in the alloy aged at low temperature. And the embrittlement of the alloy aged at high temperature is thought to arise from undeformability of isothermal ω phase.

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.

Thermomechanical treatments influence on the phase composition, microstructure, and selected mechanical properties of Ti–20Zr–Mo alloys system for biomedical applications

The study and development of new titanium alloys aim to place a metallic biomedical device with better mechanical compatibility with the bone, that is, with a smaller Young's modulus, on the market, to improve its application in the orthopedic and dental fields. This study analyzed the influence of thermomechanical treatments on the structure, hardness, and elastic modulus of the Ti–20Zr–Mo ternary alloy system, where the molybdenum content varied between 0 and 10 wt%. The treatments performed in this study involved homogenization, hot-rolling, and annealing. The structural and microstructural analyses were performed by X-ray diffraction, optical and scanning electron microscopy. An analysis of the mechanical properties of the alloy was obtained by measuring Vickers microhardness and the dynamic elastic modulus. The structural and microstructural results indicated that the Ti–20Zr alloy has only the α′ phase, the Ti–20Zr–10Mo alloy had a predominance of β phase, and the other alloys were α+β types. The hardness increased due to solid-state hardening with the increase of the molybdenum content and due to hot-rolling processing, which increased the internal stresses of the material. The elastic modulus decreased with higher levels of molybdenum and the hot-rolling processing, due to β phase stabilization. The other treatments increased the elastic modulus and may even induce the formation of the omega phase in some alloys of the system.

Metastable phase-assisted nucleation in a near-alpha titanium matrix composite with network architecture

ABSTRACTThe secondary alpha (αs) phase is an important component of a novel TiBw/TA15 composite with network architecture. In order to investigate the precipitation mechanism of the αs phase, the microstructure after multi-directional forging was characterised by EPMA, EBSD, and TEM. The results showed that several differently sized αs phases separated out from the (Mo and V)-enriched phase, which was composed of a nano-sized metastable omega phase along with some retained beta phase. It could be also found that the metastable omega phase would transform to the alpha phase when adequate activation energy was provided. The precipitation mechanism of the αs phase in this composite has been discussed in detail and it can be inferred as an omega-assisted heterogeneous nucleation process.This paper is part of a thematic issue on Titanium.

Mechanical (Compressive) Form of Driving Force Triggered the Phase Transformation from Beta to Omega &amp; Alpha Phases by Means of Dislocation Slips in Metastable Beta Phase Field Ti-5553 Alloy

Most of structural alloys' applications are under static, dynamic and cyclic forms of loading, for which Ti-5553 alloy in beta phase field is being investigated to confirm the mechanism of deformation and phase transformation upon quasi-static and dynamic compression. To achieve the fully beta phase field, Ti-5553 alloy was heated at 900oC (almost 50oC above beta tarsus temp.) for one hour of soaking time followed by air quenching. Thereafter, Dynamic compression (DC) by Split Hopkinson Pressure Bar (SHPB) and Quasi-static compression (QSC) were performed at strain rate of 103/s and 10-3/s, respectively. Recovered specimens were thoroughly examined by using different tools such as OM, SEM, HRTEM, EBSD to get reliable data for justification of logical conclusions. It's found that the prominent mode of deformation was dislocation slips along with twining to some extent in both of QSC and DC but sliding and spalling of grain boundary was more noticeable in former. Transformation of phases occurred in saturated dislocation slips grains in which from beta (BCC) to omega transformation has been completed by mechanical form of driving force in a way when set of two adjacent (110)ᵦ planes covered ±1/6 of total separation distance between two next to each other (111)ᵦ planes, equal but opposite shears, in (111)ᵦ direction and it could be justified by the 3% shrinkage of two closely existed (110)ᵦ planes after transformation to omega. Furthermore, it is noticed that the omega phase and alpha transformation occurred in the grains which were saturated with dislocation slips.

Mechanical properties and microstructure of Ti-Mn alloys produced via powder metallurgy for biomedical applications

Titanium and especially its alloys are highly employed materials in biomedical applications because of their balanced mechanical properties and biocompatibility. Ti-Mn alloys (1, 5, and 10 wt%. Mn) were produced by powder metallurgy as a potential alternative material for biomedical applications. Two sets of samples were produced, one set as-sintered and the other was beta (β) forged. For the as-sintered samples with a content of up to 10 wt% Mn, the tensile strength ranged from 606 to 1070 MPa. On the other hand, for the β forged alloys the tensile strength ranged from 728 to 1224 MPa and the maximum value was for Ti-5Mn. Forged Ti-5Mn exhibits a good balance of mechanical properties such as ultimate tensile strength (1224 MPa), elongation (4.6%) and Vickers hardness (415 HV). The purely elastic properties of the Ti-10Mn alloy is attributed to the effects of the omega (ω) phase, the formation of which is due to the high amount of beta stabiliser added to Ti.

Omega phase formation and deformation mechanism in heat treated Ti-5553 alloy under high strain rate compression

This research has been carried out to focus on the formation of omega phase and deformation mechanism in Ti-5553 heat treated (900 °C for 1-h solid solution without aging) at high strain rate compression. Split Hopkinson Pressure Bar (SHPB) was used to apply load at room temperature and the effect was characterized with the help of analyzing tools. Results ended up with the conclusions that under high strain rate loading, omega phase formation arose when two adjacent beta planes (1 1 0)ᵦ in 〈1 1 1〉ᵦ direction covered 1/6th distance of total separating distance between planes and deformation through mechanical twinning occurred when dislocations could not match superimposed strain rate.