Ωo Phase Research Articles

The microstructure evolution and phase transformation behavior of a β-solidifying γ-TiAl alloy during creep

The microstructural evolution and creep behavior of the Ti-43.5Al–4Nb–1Mo-0.1B alloy have been investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). The excellent creep property was obtained with a fully lamellar (FL) microstructure containing the least grain boundary βo phase (GB-βo). TEM results revealed that after creep testing the α2 →βo phase transformation was observed in the FL microstructure. The formation βo phase is associated with the accumulation of Mo element, which is confirmed by the energy-dispersive X-ray spectroscopy (EDS). Moreover, the formation of βo precipitation in α2 lamellae effectively decreased the generation of dislocations in (α2/γ) lamellae, thereby improving the creep resistance. For the near gamma (NG) microstructure of the as-forged sample, a large number of dislocations and dislocation tangles were observed in the globular γ phase (γ-glob), which are considered to be the dominant creep mechanism. Moreover, the ellipsoidal ωo phase was observed in the GB-βo phase, accompanying with dislocations and sub-boundaries formation. In sum, the excellent creep property of the β-solidifying γ-TiAl alloy is attributed to the fine FL structure with a small amount of GB-βo phase and the formation of βo precipitation in (α2/γ) lamellae.

Phase stabilities and equilibria of the Ti-Al-Nb ternary system at intermediate temperatures II. The 980 °C isothermal section and reactions

This paper is a continuation of our recent publication [S. Xu, et al. Journal of Alloys and Compounds 900 (2022) 163399–163407]. We systematically characterized the diffusion multiples using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM) and constructed the whole composition range of the 980 °C isothermal sections. The constructed range includes nine stable phases, twenty two-phase regions and ten three-phase regions at 980 °C. Six phase reactions occurring between 900 °C and 980 °C were determined. The ωo phase was formed by the βo+ γ→ωo reaction at a temperature just below 980 °C. The βo phase vanished through the βo→α2 + ωo+O reaction at a temperature just above 900 °C. These experimental results will bridge the existing research gap by providing the missing thermodynamic data. We believe that accurate predictions of phase diagrams and reactions occurring at intermediate temperatures using the obtained data are now feasible and the recomputations will resolve previous inconsistences and disputes.

A TEM study of a dislocations in the ßo phase of an intermetallic TNM-TiAl alloy

The structure and mobility of a<001> dislocations in the βo phase of an intermetallic TNM-TiAl alloy produced by Spark Plasma Sintering were investigated by transmission electron microscopy. We find that these dislocations are dissociated into two identical superpartial dislocations separated by a stacking fault. Furthermore, a<001> dislocations are elongated along their screw orientation and anchored at small pining points assumed to be nano-sized precipitates of the ωo phase. In situ room temperature straining experiments show that dislocation propagation occurs through jumps between these elongated configurations.

Inhibition of ωo phase precipitation in TNM-based TiAl alloys by Cr and Mn

In modern β-solidified γ-TiAl alloys, ωo phase usually precipitates very easily within βo phase during heat treatment at intermediate temperatures. This phase transformation leads to increased brittleness and reduced service performance of the alloy. In this paper, the evolution of the ωo phase is investigated in the TNM-based TiAl alloys by the addition of Mn or Cr elements. The results show that both Mn and Cr have a strong inhibiting effect on the precipitation of ωo phase, while the inhibiting effect with Mn addition is more obvious. Specifically, in the Ti-43.5Al–4Nb-0.6Mn-0.1B alloy exposed to 750 °C for 1000 h, the ωo phase is almost completely inhibited. The composition range in which Mn and Cr effectively suppressed the ωo phase precipitation were determined by reasonable modulation. The phase transformation temperature of ωo phase was lowered by the addition of alloying elements, making it difficult to precipitate at intermediate temperatures. The corresponding phase transformation mechanisms were discussed.

Decomposition and phase transformation mechanisms of α2 lamellae in β-solidified γ-TiAl alloys

The decomposition of (α2/γ) lamellae during service is an important microstructure degradation phenomenon that is worth studying. In this study, the decomposition mechanism of (α2/γ) lamellae was systematically investigated by transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) techniques. The results showed facilitated α2 →βo phase transformation by [112¯0]α2/[01¯1]γ semi-coherent α2 /γ interface due to its larger misfit than that of [112¯0]α2/[1¯10]γ interface. Ellipsoidal ωo particles were formed at βo /α2 interface and nucleated at the interface dislocations. The energy-dispersive X-ray spectroscopy (EDS) results demonstrated feasible depletion of Mo and enrichment of Ti at α2 /βo interface to form ωo phase. In addition, lath-shaped ωo precipitates were observed between two precipitated large βo grains formed by direct α2 →ωo phase transformation. The formation mechanism of the lath ωo phase was attributed to both element segregation and strain accommodation caused by βo precipitate in α2 lamellae. The α2 /βo interface served as preferential nucleation sites for ωo phase due to the atomic structure of the interface, which crystallographically was closely associated with the structure of ωo phase. Moreover, βo precipitates at low stress introduced stacking faults in α2 lamellae, while delaying the parallel decomposition of α2 lamellae at high stress. In sum, these findings provided profound insights into the decomposition mechanism of (α2/γ) lamellae, relevant as theoretical basis for controlling the microstructures and improving the performance of β-solidified γ-TiAl alloys.

Plasticity and brittleness of the ordered βo phase in a TNM-TiAl alloy

In order to identify the reasons for the brittleness of a TNM-TiAl alloy (Ti51.05Al43.9Nb4Mo0.95B0.1 in at.%), the plastic deformation behaviour of the ordered βo phase is investigated. The corresponding powder was densified by Spark Plasma Sintering to produce a near lamellar microstructure made of γ/α2 lamellar colonies surrounded by γ and βo grains. Then, the room temperature tensile behaviour is studied by carrying out tensile tests and additional in-situ straining experiments in a transmission electron microscope (TEM). The TNM alloy exhibits a limited ductility, with the fracture surface of test specimens showing cleavage facets along lamellar interfaces. In addition, the βo grains contain nano-precipitates of the ωo phase and deform plastically by <111> superdislocations dissociated into two superpartial dislocations separated by an antiphase boundary. These dislocations glide in {011} planes and are observed to be localized into pile-ups, which is related to the ωo strengthening precipitation within the βo grains. This localised hardening behaviour leads to stress concentration in grain boundaries which is assumed to be responsible for delamination along lamellar interfaces in neighbouring lamellar colonies.

Study on the evolution of phase relations in the Ti–Al–Nb/Cr systems at 0–50 at.% Al region

β-Solidifying γ-TiAl based alloys have gained scientific attention and showed potential applications as structure materials. When adding the β-stabilizing elements (e.g., Nb and Cr), ordered B2 and ωo phases are often formed during the production process of the TiAl based alloys. The information about the evolution of these phases as the temperature changes is necessary and important for design and preparation of desired TiAl based alloys with excellent properties. In this work, phase relationships and evolvement of Ti–Al–Nb/Cr systems have been investigated in the service temperature ranging from 950 to 700 °C. In the Ti–Al–Nb ternary system, B2 phase will directly transform into ωo phase at the temperature range of 913.6–921.1 °C, which results in an increase of the microhardness (i.e. 460 HV to 530 HV). At 700 °C, ωo phase has the composition of 15.8–20.4 at.% Nb, 29.8–33.3 at.% Al, and 46.3–54.4 at.% Ti. Equilibrium composition of ωo is defined as Ti3Al2Nb. The ternary compound O_Ti2AlNb has been found to be formed via β(Ti)+Ti3Al + Nb2Al→O at 981.5 ± 6.5 °C. As for the Ti–Al–Cr ternary system, when temperature decreases from 950 to 900 °C, the center section of the β(Ti) phase region shrinks and separates, resulting in the formation of independent B2 phase region. The island-like B2 region can remain stable at 700 °C. According to the evolution of phase relationship, one solid transition reaction Ti3Al + C14→B2+C15 at 900–800 °C has been deduced to occur in the Ti–Al–Cr ternary system. Present work provides an important guide for fabricating Ti–Al–Nb/Cr alloys with different ratio and distribution of plastic/brittle phases.

Phase Equilibria in the Ti-Rich Part of the Ti-Al-Nb System—Part I: Low-Temperature Phase Equilibria Between 700 and 900 °C

Precise knowledge of the phase equilibria in the Ti-Al-Nb system between 700 and 900 °C is of crucial importance for the urgently needed improvement of TiAl-based turbine materials already in industrial use to achieve further energy savings. As a result of the occurrence of the two ternary intermetallic phases ωo (“Ti4NbAl3”) and O (“Ti2NbAl”), which form in the solid state just in the range of the application-relevant temperatures, the phase relations are very complex and not well studied. In the present investigation, isothermal sections of the Ti-rich part of the Ti-Al-Nb system at 700, 800, and 900 °C were determined by a systematic study of 15 ternary alloys, one solid-solid diffusion couple, and three liquid-solid diffusion couples. Using scanning electron microscopy, electron probe microanalysis (EPMA), x-ray diffraction (XRD), high-energy XRD (HEXRD), differential thermal analysis (DTA), and transmission electron microscopy (TEM) investigations, type and composition of phases as well as phase transitions were determined. With these results, the phase equilibria were established. A focus of the investigations is on the homogeneity ranges of the two ternary phases ωo and O, which both are stable up to temperatures above 900 °C. Based on the compositions measured for the ωo phase and its crystal structure type, a new formula (Ti,Nb)2Al is suggested. The results also indicate that the phase field of the ωo phase is split into two parts at 900 °C because of the growing phase field of the ordered (βTi,Nb)o phase.

Thermomechanical instability and deformation behavior of βo(ω) phase region in a Ti-43Al-8Nb-0.2W-0.2B alloy under high-temperature rotary-bending fatigue

Phase transformation and deformation behavior of the βo(ω) phase region under high-temperature rotary-bending (HTRB) fatigue condition are studied in the current work. Rotary-bending fatigue tests of a Ti-43Al-8Nb-0.2W-0.2B (at.%) alloy were performed at 800 °C under 325, 340, 355 and 370 MPa in atmosphere condition. The quasi-fatigue limit (σ-1) was calculated as 355 MPa [4]. Microstructures were examined using scanning and transmission electron microscopy (SEM &TEM) and thermodynamic calculation was adopt to interpret the phase transition mechanisms. Under HTRB fatigue, the phase transition of ωo → βo occurred and the newly formed βo phase is completely coherent with the pre-existing βo phase. Thermodynamic calculation revealed that a tensile stress could promote the phase transition of βo → ωo. The local recrystallization was found at lamellar colony boundary and grain interior, where is consisted of γ and ω grains. The deformation mechanisms of βo phase might be the dislocation glide on the 〈111〉{123} system. The dislocations have a strong tendency for slip localization into slip bands. Disordered ω phase was observed in the residual ωo phase and it may be a transition state in the phase transformation of ωo → βo.

Phase stabilities and equilibria of the Ti-Al-Nb ternary system at intermediate temperatures I. The 900 °C isothermal section

Determination of intermediate temperature phase diagrams is a challenging task due to sluggish transformation kinetics, metastable phases and low diffusion coefficients encountered under these conditions. The development of Ti-Al-Nb alloys has been hampered by scarcity of intermediate temperature phase diagrams. Through guidance from diffusion simulation results, we designed the composition of the diffusion components in a diffusion multiple capable of efficiently and accurately testing the Ti-Al-Nb ternary phase diagrams at intermediate temperatures, which is the service temperature of Ti-Al-Nb alloys. We systematically characterized the diffusion multiples using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), electron backscatter diffraction (EBSD), transmission kikuchi diffraction (TKD) and transmission electron microscope (TEM), and constructed the whole composition range of the 900 °C isothermal section. The constructed range includes ten stable phases, nineteen two-phase regions and ten three-phase regions. Two ternary phases (O-Ti2AlNb and ωo-Ti4Al3Nb phases) were determined and the ωo phase was defined on the experimental phase diagram for the first time. The previous thermodynamic database at 900 °C is unreliable, based on the present work. The present experimental results will bridge the existing research gap by providing the missing thermodynamic data. The present diffusion multiple is expected to construct more isothermal sections of Ti-Al-Nb ternary system at intermediate temperatures in the future. The constructed phase diagrams are also invaluable tools to researchers and industry for development of Ti-Al-Nb alloys.

Phase transformation and deformation behavior of a TiAl–Nb composite under quasi-static and dynamic loadings

To improve the high-temperature deformability of TiAl intermetallics, a TiAl–Nb intermetallic matrix composite was prepared by powder metallurgy to try to introduce the deformable β phase. The microstructure and deformation behavior of the TiAl–Nb intermetallic matrix composite under quasi-static and dynamic conditions were systematically investigated. It is found that the microstructure of the composite is composed of a near-gamma TiAl matrix and Nb-rich regions. In the Nb-rich regions a coexistence of βo, ωo and γ phases was identified where ωo and γ phases were supposed to directly precipitate in the βo matrix. The ultimate tensile strength of the composite exhibits different temperature-dependent behaviors under quasi-static and dynamic loading conditions. It increases with temperatures up to 650 °C and then declines with temperature under quasi-static loading, while it does not decrease from 650 to 850 °C under the dynamic loading. In the deformed microstructures, the Nb-rich regions at all conditions are still composed of βo, ωo and γ phases, indicating that the solvus temperature of the ωo phase could be above 850 °C. Additionally, the TiAl matrix appears to not significantly contribute to the plastic deformation of the composite, where the propensity of dislocations and mechanical twins do not significantly change when varying loading conditions. However, the γ particles formed in Nb-rich regions contain substantial mechanical twins and dislocations under both quasi-static and dynamic loading conditions. Thus, the γ particles directly transformed from the βo phase exhibit better deformability than the γ phase in the matrix. This could be probably explained by a decreased stacking fault energy for the newly formed γ phase due to a decrease in Al fraction and a significant increase in Nb fraction. Meanwhile, twin intersections occurred in the γ particles, and the induced stress concentration in the intersection regions was relieved by lattice distortion and detwinning processes. This twin intersection behavior could be beneficial for improving the composite strength without sacrificing much ductility.

Experimental Phase Equilibria and Isopleth Section of 8Nb-TiAl Alloys

The 8Nb isopleth section of a Ti-Al-Nb system is experimentally determined based on thermal analysis and thermodynamic calculation methods to obtain the phase transformation and equilibrium relations required for material design and fabrication. The phase transus and relations for the 8Nb-TiAl system show some deviations from the calculated thermodynamic results. The ordered βo phase transforms from the disordered β/α phases at 1200–1400 °C over a large Al concentration range, and this transformation is considered to be an intermediate type between the first- and second-order phase transitions. Moreover, the βo phases are retained at the ambient temperature in the 8Nb-TiAl microstructures. The ωo phase transforms from the highly ordered βo phase, rather than from α2 or βo with a low degree of atom ordering B2 (LOB2) structure, with Al concentration of 32–43 at% at approximately 850 °C. From the experimental detection, the transition of the ωo phase from the βo phase is considered to be a further ordering process.

Thermal Stability and Lattice Strain Evolution of High‐Nb‐Containing TiAl Alloy under Low‐Cycle‐Fatigue Loading

The micromechanical behavior and the effect of temperature on the micromechanical mechanism of high‐Nb‐containing TiAl alloy during low‐cycle fatigue still remain uncertain. Herein, in situ and ex situ synchrotron‐based high‐energy X‐ray (HEXRD) experiment results reveal that the γ and ωo phases suffer compressive lattice strains but the lattice strain in the α2 phase evolves from tensile to compressive during low‐cycle fatigue at 900 °C. In addition, the three phases suffer compressive lattice strains during cooling to room temperature, which could result in larger compressive lattice strains in γ and ωo phases and the change of the lattice strain state in the α2 phase. The peak‐broadening results show γ recrystallization is dominant in the interrupted low‐cycle‐fatigue samples, whereas inhomogeneous deformation occurs in the failed low‐cycle‐fatigue samples. The performed synchrotron diffraction experiments offer a deeper insight into the phase transformations and micromechanism of TiAl alloy during low‐cycle fatigue.

Creep-induced ωo phase precipitation and cavity formation in a cast 45.5Ti-45Al-9Nb-0.5B alloy

The precipitation behavior of ωo phase and creep properties of a Ti-45Al-9Nb-0.5B alloy at intermediate temperatures are investigated in this study. The prior βo phase located at lamellar colony boundaries in the as-cast microstructure is entirely replaced by the ωo phase after creep tests. The imposed stress evidently contributes to the nucleation and growth of the ωo phase. The precipitation of ωo phase inside α2 lamellae can be initiated in front of deformation twins located in adjacent γ lamellae, which are impeded at the lamellar interface. Higher applied stress is expected to further enhance the α2→γ phase transformation and to generate more deformation twins in the γ lamellae. A stress exponent of n = 4.2 is determined for the present creep tests. The microstructural degradation is predominantly induced by dynamic recrystallization (DRX). The decomposition of α2 lamellae via α2→ωo and α2→γ transformation reduces the creep resistance. The distribution and size of ωo precipitates have a pronounced impact on the cavity evolution at the tertiary creep stage. The ωo phase formation under the synergistic effects of the thermal exposure and applied stress accelerates creep failure.

Creep-Induced Phase Instability and Microstructure Evolution of a Nearly Lamellar Ti–45Al–8.5Nb–(W, B, Y) Alloy

Microstructure degradation and stress-induced transformation of a high Nb-containing TiAl alloy with nearly lamellar microstructure during creep were investigated. Tensile creep experiments were performed at 800, 850 and 900 °C under 150 MPa in air. Microstructures before and after creep tests were examined using scanning and transmission electron microscopy (SEM and TEM). Dislocations within the lamellar structure and βo(ω) region and twin intersection in massive γ grains were investigated. Dislocation sliding played a critical role in the deformation of ωo phase, which preferentially occurred on the (0002)ωo plane. Possible deformation mechanisms were revealed. A stress-induced γ → α2 phase transformation took place during the creep test at 850 and 900 °C. α2 lamella could directly decompose into the ωo phase at 850 °C. The instability of high-temperature microstructure can weaken the creep resistance and promote the plastic deformation of the lamellar matrix, thus could be detrimental to the creep properties. The correlations between creep properties and microstructure instability were discussed.

Microstructure and phase transformations of ωo-Ti4Al3Nb based alloys after quenching and subsequent aging at intermediate temperatures

The existence of ωo (ordered ω) phase in TiAl alloys in different compositions at intermediate temperatures has been widely reported. In this study, the phase transformation behaviors of ωo-based alloys at 750–900 °C were studied by electron microscopy. It is found that fine-grained microstructures mainly composed of γ+ωo and a tiny amount of α2 phase precipitated in the alloys quenched from high temperature during aging. Twins form in α2 phase after aging, which is analogous to observations reported in deformed TiAl alloys. The γ phase precipitates originate from the stacking faults of α2 laths and grow up by coarsening at 750 °C. Semi-coherent interfaces between γ and ωo phases are formed and a high density of stacking faults can be observed inside the α2 laths. At higher temperatures, the direct nucleation of γ phase from the ωo/βo matrix becomes active. Various orientation relationships are found between ωo, γ and α2 phases, leading to a number of grain clusters having smooth interfaces in between. In addition, the effects of Ta, V and Zr on the microstructure are investigated. The effect of these elements on the stability of the ωo phase is not as obvious as reported for some other elements.

Experimental equilibria of several β/βo–Ti–Al–Nb alloys containing ωo phase

The phase equilibria of several (6, 8, 10) at% Nb–TiAl alloys are experimentally determined by the morphologic, crystallographic and thermal analysis methods. The thermodynamically determined phase equilibria reveal that ωo phase transforms from α2 phase, having no β/βo phase retained at ambient temperature. While, the β/βo phase are experimentally observed in the samples annealed at 1400 °C followed by a slow cooling rate (∼10 °C min−1). Furthermore, the transformation of βo ↔ ωo, as well as the disorder-order transition of β ↔ βo, is unambiguously detected, within the certain transformation temperatures.

Coupling effects of deformation and thermal exposure on the precipitation behaviors of βo(ω) phases in a high Nb-containing TiAl alloy

The precipitation behaviors of βo(ω) phases are investigated in order to understand the effects of deformation on the microstructural stability during thermal exposure. Deformation can promote the precipitation of the βo(ω) phases not only at the lamellar boundaries but within the lamellar colonies. The higher exposed temperature and lager engineering strain are in favor of the precipitation of the βo(ω) phases. Four different ωo variants are identified at the βo regions of the deformed samples, indicating the residual stress induced by deformation cannot change the preferential formation of ωo variants during exposure. Diffusion during the parallel decomposition promotes the formation of ωo phase by making Nb enriched in the remaining α2 laths. During the growth of ωo grains, W is excluded and enriched in local regions of the α2 laths, leading to the formation of βo phase. The residual stress of the deformed samples is measured by the hole-drilling method. After deformation, the absolute value of residual stress comes to a peak value and then gradually decreases during exposure. The Vickers hardness shows a similar varying trend with the residual stress. Deformation can significantly decrease the microstructural stability and promote the precipitation of the βo(ω) phases.

Microstructure stability and micro-mechanical behavior of as-cast gamma-TiAl alloy during high-temperature low cycle fatigue

This study systematically investigated the low cycle fatigue deformation of a high Nb-containing TiAl alloy with a nominal chemical composition of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y at 850 °C by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and synchrotron-based high-energy X-ray diffraction (HE-XRD) techniques. Cyclic stress-strain (CSS) behavior, lattice strain, and peak broadening of {100}α2, {201}γ, and {202}ωo planes, phase transformations, and crack propagation behavior were obtained for samples with three total strain amplitudes: Δεt/2 = ±0.25%, Δεt/2 = ±0.28%, and Δεt/2 = ±0.30%. At early deformation stages, α2lamellae transformed into ωo phase with a distinct orientation relationship, and a certain orientation relationship (OR) between them was observed after the following cyclic deformation. Furthermore, γ particles precipitated within the single ωo area. In addition, according to the peak intensity and peak broadening results, the ωo → B2 phase transformation occurred, leading to the appearance of single B2-phase areas. The lattice strains in the ωo phase were always in tension during the cyclic deformation and large differences of the lattice strains were found in the γ phase and α2 phase, not only the values but also the directions, which resulted in crack nucleated at and propagated along the α2/γ lamellar interface. This study provides a better understanding of the low cycle fatigue deformation of TiAl alloys.

Nucleation behavior of ωo phase in TiAl alloys at different elevated temperatures

In this study, the location, morphology and composition of the ωo phase, which transformed from B2 phase in Ti–45Al–8.5Nb-(W, B, Y) (at.%) alloy, were investigated by short-time heat treatments in an intermediate temperature range. The results showed that the characteristics of B2 → ωo phase transformation differ with the reduction in temperature. Below the ωo solves, the B2 → ωo phase transformation will take place within B2 area. There are existing two stages in the phase transformations during cooling from 850 to 700 °C. When the temperature is high (850 °C), granular ωo populated along the dislocations existing in B2 phase and B2 grain boundaries, due to higher energy and the enrichment of Nb element at the B2/γ boundaries. It is worth noting that only two ωo variants nucleate at 850 °C. With the decrease in temperature, the nucleation sites of ωo phase would be migrated from the B2 boundaries to the inner area. At the temperature of 700 °C, four ωo variants with different orientations nucleate homogenously within B2 grain. The ordered ωo formation process of Ti–45Al–8.5Nb-(W, B, Y) alloy during cooling was postulated in this study.