Βo Phase Research Articles

Effects of trace alloying elements on the phase transformation behaviors of ordered ω phases in high Nb-TiAl alloys

Ordered ω phases are equilibrium phases at intermediate temperatures in TiAl alloys. However, a systematical research on the effects of alloying elements on the ordered ω phase transformations is still lacking. The effects of alloying elements on the ordered ω phase formation should be considered in future design of TiAl alloys. In this study, the effects of Cr, Mn and Ni on the ordered ω phase formation are investigated. The results show that the precipitation of ordered ω phase is promoted by addition of Ni, i.e., the sizes of ordered ω phase were in micron-level. However, the addition of Mn greatly suppresses the growth of ordered ω phases. During thermal exposure at 850°C, a Ti-Al-Ni ternary phase formed within the original βo phase areas in the Ni-containing alloy. Meanwhile, no ordered ω phase existed in an annealed Mn-containing alloy. In both as-cast and annealed conditions, the effects of Cr seemed to be similar to those reported in Mo and W-containing alloys. Differential scanning calorimetry (DSC) experiments suggested that the precipitation temperatures of ordered ω phase in these three alloys were affected by minor alloying elements, which fitted well with the observed phenomena in the electron microscopy study.

Phase transformation mechanisms in a quenched Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy after subsequent annealing at 800 °C

During the processing of high Nb-containing TiAl (Nb-TiAl) alloys, the cooling rate at certain parts of the components can be very high, especially at thin parts. Moreover, annealing treatment must be applied to TiAl alloys to improve their mechanical properties. In this study, the phase transformation mechanisms in a quenched Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy during subsequent annealing were characterized using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The results show that βo, α2, and massively transformed γ phases co-exist in the as-quenched microstructure. Some fine γ laths nucleated in the primary α2 phase in the quenched samples. After annealing at 800 °C for 1 h, numerous extremely fine γ laths precipitated in the bulk α2 phase and could only be recognized using TEM imaging. The ωo particles at sizes of 0.5–1 μm precipitated in the retained βo phase and nearly consumed all of the βo areas. More interestingly, some coarsened γ grains in true-twin relationship were observed at the boundaries of the lamellar colony and βo(ω) regions. The orientation relationship between βo(ω) and coarsened γ was confirmed to be the following: [110]β//[112¯ 0]ω//[111]γ, (111¯)β//(0001)ω//(11¯ 0)γ. After annealing at 800 °C for 100 h, the βo phase region transformed into small ωo particles and equiaxed γ grains and still followed the above-mentioned orientation relationship. The α2 phase only existed as thin laths in the lamellar structures in a small volume fraction. These results indicate that the ωo phase is stable at 800 °C. Possible mechanisms of these phase transformations are discussed.

Silicon distribution and silicide precipitation during annealing in an advanced multi-phase γ-TiAl based alloy

Intermetallic TiAl alloys solidifying via the disordered β phase, such as the TNM alloy, are promising γ-TiAl based materials as balanced mechanical properties are attained by conventional manufacturing routes and subsequent heat-treatments. However, a further enhancement of creep resistance is required to exceed present service temperature limitations. In this regard, alloying with light elements, such as C and Si, is of particular interest. Underlying strengthening mechanisms are well established in case of C, but matter of debate in case of Si.In this study the microstructural evolution and preferential distribution of the alloying elements in the constituent phases of a derivative of the TNM alloy containing Si is investigated after a two-step heat-treatment by atom probe tomography, scanning and transmission electron microscopy as well as X-ray diffraction. During γ lamellae formation upon annealing, Si is repelled from the γ phase and accumulates in the α2 phase. In both phases Si preferentially resides on Al lattice sites as evidenced by ab initio calculations. Neither a Si excess nor precipitate formation at lamellar α2/γ interfaces is observed. Precipitates are, however, perceived to form in the regions between the colonies, more precisely in the βo phase. These particles are identified as ζ-silicides, which are enriched in Nb and Al. Accompanying ab initio calculations, used to study the phase preference, provide evidence that in βo phase containing alloys, silicides most likely precipitate in this phase, whereas silicide precipitation within the other phases, γ, α2, or ωo, is improbable as these phases are energetically stabilized by the incorporation of Si atoms.

Preparation Methods for Examining the ωo-Phase Formation in a β-Solidifying TiAl Alloy via Atom Probe Tomography

Abstract Examinations with the atom probe are an excellent way to analyze all phases of a material on an atomic level, thus enabling a better understanding of their formation. This study aims to provide an appropriate method for the preparation of atom probe specimens made of multiphase intermetallic titanium aluminide alloys. With the help of a focused ion beam system and transmission kikuchi diffraction, tips of defined phases and phase interfaces were prepared, as is shown by the example of the ωo-Ti4Al3Nb phase, which is precipitated from the βo phase. The chemical composition of the two phases was determined by subsequent atom probe measurements which allowed to draw conclusions about the phase formation.

Experimental study of the dissolution and reprecipitation behaviors of ωo phase in high Nb containing TiAl alloy

The dissolution and reprecipitation behaviors of ωo phase during isothermal annealing and ensuing re-aging at 700–800°C are investigated on an as-cast high Nb containing TiAl alloy with nominal composition of Ti–45Al–8.5Nb–0.2W–0.2B–0.02Y (at.%). The microstructural characterization revealed that numerous micron-sized ωo particles exist within the ordered βo phase in the as-cast alloy. These ωo particles have high thermal stability when annealing at 800°C and 850°C for 100h. Increasing the temperature to 900°C, the ωo particles began to decompose into βo phase with a low transformation rate. Further increasing temperature causes accelerated dissolution kinetics of ωo phase. Ensuing re-aging at 700°C and subsequent quenching lead to the reprecipitation of nano-sized ωo particles accompanied by extremely fine γ plates. When the aging temperature increases to 800°C, the micron-sized ωo particles precipitate from βo phase accompanied by coarsened γ plates. Possible mechanism of the reprecipitation behavior of ωo phase is discussed.

Ordered ω phase transformations in Ti-45Al-8.5Nb-0.2B alloy

The ordered ω phase in as-cast Ti-45Al-8.5Nb-0.2B alloy and its phase transformation during heat treatment are investigated. Ordered ω variants are observed to uniformly precipitate within the βo area in as-cast Ti-45Al-8.5Nb-0.2B alloy. After annealing at 850 °C for 500 h, the βo areas are replaced by large B82-ωo grains. Small γ precipitates are observed at the grain boundaries of the ωo phase and are thought to be transformed from the βo phase. Moreover, the ωo precipitates directly transformed from the parent α2 laths are found within the lamellar colonies. The orientation relationship between the ωo phase and the lamellar structure is < 110]γ//[0001]ωo//[112¯ 0]α2; (111)γ//(112¯ 0)ωo//(0001)α2. The interfaces between the ωo and γ are semi-coherent. The ωo phase is an equilibrium phase at 850 °C in high Nb-containing TiAl alloys. When annealed at 1250 °C, the ordered ω is eliminated in a short time, and the βo phase is substituted by the coarsened α2 laths in the lamellar colonies after 12 h annealing.

Carbon distribution in multi-phase γ-TiAl based alloys and its influence on mechanical properties and phase formation

Advanced intermetallic γ-TiAl based alloys are attractive light-weight materials for high-temperature application. In order to extend their service temperature limits, alloying with low-density elements, such as C, is of particular interest and has been shown to effectively increase high-temperature strength as well as creep resistance.In the present study the local chemical composition of the constituent phases of the so-called TNM alloy and a C-containing derivative thereof is characterized by atom probe tomography. In both alloys Mo is found to preferentially locate in the βo phase, in contrast to Nb, which is dispersed in similar levels in all phases. In the C-containing alloy, C is enriched in the α2 phase, dissolved in the γ phase, but depleted in the βo phase. Furthermore, the investigation of interfaces through site-specific sample preparation reveals segregation of C at phase interfaces and their close vicinity. Finally, a correlation of the mechanical properties with the C distribution is established by nanoindentation technique. Both the γ and the α2 phase significantly harden through the addition of C, which is in good agreement with the C concentration present within these phases as observed by atom probe tomography. However, the βo phase softens through the addition of C, which is not a direct consequence of the C distribution, but follows from the absence of finely dispersed ωo particles in the βo phase of the C-containing alloy.

ωo phase precipitation in annealed high Nb containing TiAl alloys

The ordered ω phases in high Nb containing TiAl (Nb-TiAl) alloys have been garnering increasing attention in the recent years. However, the investigations on the Nb dependence on the ωo precipitation are scarce. In this study, the effect of Nb content on the ωo precipitation in high Nb (6–10 at%) containing TiAl alloys after long-time annealing at 850°C has been studied. The results show that small ordered ω particles in the retained βo phase cannot be discerned under scanning electron microscope (SEM) but can be observed using transmission electron microscopy (TEM). Although the Nb segregation can be eliminated after the homogenization heat treatment, the ωo phase precipitated in all the alloys studied after annealing at 850°C. TEM examination reveals that the orientation relationship between the ωo and α2 phases can be derived as: [0001]ωo//[112̄0]α2; (112̄0)ωo//(0001)α2, which indicates that the ωo phase is directly transformed from the parent α2 phase. Small γ particles are also observed within the ωo areas. The α2→ωo+γ decomposition process is expected during annealing. It is concluded that ωo phase is an equilibrium phase in high Nb-TiAl alloys at 850°C.

Phase transformation and decomposition mechanisms of the βo(ω) phase in cast high Nb containing TiAl alloy

High Nb containing TiAl (Nb–TiAl) alloys are thought to be used at higher temperatures than the conventional TiAl alloys, typically up to 900°C. However, the βo and ω-related phases are usually induced by Nb-segregation in cast alloys. It is important to reveal the transformation mechanisms of the βo and ω-related phases at elevated temperatures for better design of the alloy and optimizing the heat treatment process. In this study, the interconversion mechanism between the ordered ω and βo phases and the decomposition process of the βo phase in the as-cast Ti–45Al–8.5Nb–0.2W–0.2B–0.02Y (at.%) alloy are studied. The ωo particles grow up by richening Nb and rejecting W to the surrounding βo matrix. The βo and ωo phase are separated after annealing at 900°C, whereas the ωo phase dissolves into the βo matrix after annealing at 950°C. The βo and ωo phases undergo βo→γ and ωo (B82 structure)→D88-ω (D88 structure)→γ transformation respectively during annealing, and the βo→γ transformation can be achieved in several ways. The ωo particles with a high Nb content act as pinning points at the βo/γ boundaries during the βo→γ transformation. The corresponding mechanisms of the transformations mentioned above are discussed.

Microstructure and Texture Evolution in an Intermetallic β‐Stabilized TiAl Alloy During Forging and Subsequent Isothermal Annealing

Microstructure and texture formation were investigated in an intermetallic Ti–43.4Al–4.2Nb–1.1Mo–0.1B (in at.%) alloy after near conventional forging in the (α + β) phase field region and subsequent isothermal annealing treatments at 1150 °C for different holding times. During forging and the following cooling process a fine grained microstructure is formed consisting of lamellar α2/γ colonies and elongated grains of the βo phase situated at the colony boundaries. The isothermal annealing treatment leads to a significant change of the forged microstructure. After hot‐forging, all phases show typical fiber textures as it is expected for uniaxial deformation. In the as‐forged state the α2(α) phase exhibits a 〈11‐20〉 fiber, the βo(β) phase has a strong and sharp fiber and the γ phase shows a and a weaker fiber. After additional annealing at 1150 °C for 8 h, the textures are almost unchanged in contrast to the microstructure.

Evolution of the ωo phase in a β-stabilized multi-phase TiAl alloy and its effect on hardness

The intermetallic β-stabilized Ti–43.5Al–4Nb–1Mo–0.1B alloy (in at.%), termed TNM alloy, is designed to be used at elevated temperatures, typically up to 750°C. To understand the evolution of the microstructures during heat treatments and subsequent creep tests, an understanding of the phase transformations and decomposition reactions that occur is necessary. The present study deals with the development and growth mechanism of the ωo phase, which forms in the βo phase during static annealing treatments and creep tests performed at 750, 780 and 800°C using an applied stress of 150MPa. In situ high-energy X-ray diffraction experiments were conducted to investigate the decomposition behaviour of the ωo phase during heating as well as to determine its dissolution temperature. High-resolution transmission electron microscopy was used to study the coarsening of ωo grains during creep. The chemical composition of βo and ωo was determined by means of energy dispersive X-ray microanalysis. In particular, the impact of the Mo content on the growth of the ωo grains within the βo matrix was investigated. Additionally, nanohardness measurements in γ, α2, βo and (βo+ωo) grains were performed by cube corner indentation. The results show that βo is the hardest phase in the TiAl–Nb–Mo alloy system when finely dispersed ωo precipitates are present.

The Transformation Mechanism of β Phase to ω-Related Phases in Nb-Rich γ-TiAl Alloys Studied by &lt;i&gt;In Situ&lt;/i&gt; High-Energy X-Ray Diffraction

In recent years intermetallic γ-TiAl based alloys with additional amounts of the ternary bcc β Ti(Al,Nb) phase attracted increasing attention due to their improved workability at elevated temperatures. Depending on alloy composition and heat treatment the ductile high-temperature β phase can transform to several ordered phases at lower temperatures. However, currently available phase diagrams of these multiphase alloys are quite uncertain and the precipitation kinetics of some metastable phases is far from understood. In the present study various transformation pathways of the third phase were observed in situ by means of high-energy X-ray diffraction using synchrotron radiation. A Ti-45Al-10Nb (at.%) specimen was subjected to a temperature ramp of repeated heating cycles (700 °C - 1100 °C) with subsequent quenching at different rates. Depending on the quenching rate reversible transformations of the B2-ordered βo phase to different ω-related phases were observed. The results indicate that the complete transformation from βo to hexagonal B82-ordered ωo consists of two steps which are both diffusion controlled but proceed with different velocities.

Effect of strain rate on the mechanical response of a Ti3Al-Nb-Mo alloy

In this study, the effect of strain rate and temperature on the substructure evolution and mechanical response of a Ti3Al-Nb-Mo alloy (Ti-24.5Al-10.5Nb-l.5Mo) was investigated. The yield and flow stresses were found to decrease gradually with increasing temperature at the strain rate of 3000 s1 and no flow stress/temperature anomaly was observed. The variation of hardening behavior of Ti-24.5Al-10.5Nb-1.5Mo alloy with temperature and strain rate was found to be negligible, implying that the dynamic recovery mechanism for this alloy is rather athermal. Dynamic recovery of Ti-24.5Al-10.5Nb-l.5Mo alloy is thought to be quite difficult and slow because of its complicated dislocation core structure. Ti-24.5Al-10.5Nb-l.5Mo alloy exhibited predominantly planar dislocation debris after deformation. Following the room temperature deformation at low strain rate the majority of the dislocations were a-dislocations lying on the basal and 1storder pyramidal slip planes and the 2nd-order pyramidala/2+c slip on {1211}. Increasing the rate of deformation at room temperature to 6000 s1 resulted in the increaseda-slip on prism planes and the lessor amount of basal slip. At elevated temperatures, the substructure after high rate deformation was found to be similar to that following high rate deformation at room temperature except for an increased amount of basal slip and somewhat higher incidence of the 2nd-order pyramidal slip. The evidence of an increased amount of shearing of the βo phase was observed at elevated temperature, suggesting the decreased strength of the βo phase.

Deformation structure in a Ti-24Al-11Nb alloy

A Ti-24Al-11Nb alloy has been heat-treated so as to obtain a microstructure of coarse α2 particles (D019 structure based on Ti3Al) in a matrix of the ordered βo phase (B2 structure based on Ti2AlNb). Dislocation structures generated by tensile strains of ∼2 pct at room temperature have been analyzed by transmission electron microscopy The βo phase is shown to deform inhomogeneously on {110}, {112}, and {123} planes by α/〈211〉 slip. The slipband structure is complex, consisting of segments of heavily pinned edge dislocations with periodic cross slip of screw components on to secondary slip planes. Incompatibility stresses at α2/βo interfaces can generate fine α[100] slip as well. The α2 phase deforms independently by α dislocation slip. Slipbands in the βo phase can shear the α2 phase by activatingc +a/2 slip on $$\left\{ {2\overline 2 01} \right\}$$ and $$\left\{ {22\overline 4 2} \right\}$$ slip planes, as well asa slip on higher order pyramidal planes, where the parallelism of the specific slip system is permitted by the Burgers relationship between the two phases.

Structure, tensile deformation, and fracture of a Ti3Al-Nb alloy

The development of Ti3Al-Nb alloys is an excellent example of the recent resurgence of interest in the use of intermetallics for high-temperature applications. We examine, in this contribution, the structure of a typical alloy Ti-24A1-11Nb and show it to consist primarily of the ordered α2 phase (based on Ti3Al, DO19) and βo, (based on Ti2NbAl, B2) phases, with small amounts of a third phase, which is distorted slightly to an orthorhombic symmetry from the D019 (hexagonal) structure. Tensile properties have been examined on samples heat-treated to vary the size, shape, and volume fraction of α2 phase and the deformation and fracture behavior of the ordered, two-phase mixture established. The tensile ductility is seen to maximize at intermediate volume fractions of the α2 and βo phases (∼30 pct) at values of 6 to 10 pct elongation to fracture, depending on the grain size of the βo phase. A rationale incorporating the failure modes of the two phases—cleavage of α2 and slipband decohesion of βo—has been evolved to explain the trends in ductility with heat treatment.

Stress-Assisted Discontinuous Precipitation during Creep of Ti3Al-Nb Alloys

ABSTRACTStress-assisted discontinuous precipitation was observed during creep of Ti- 25Al-12.5Nb at.% and associated with microstructures in which large primary creep strains were observed [1]. It was found that a large shift between the equilibrium βo (B2) phase composition at the heat treatment temperature and disordered β(bcc) phase at the creep temperature provided a driving force for discontinuous precipitation of disordered beta phase. Applied stress accelerated the growth of discontinuous β phase at grain boundaries perpendicular to the principal stress axis, but did not produce a significant shift in composition. The difference between beta and ordered beta phase boundaries in the Ti-Al-Nb system at 650°C and 1040°C suggests that discontinuous precipitation or related dissolution should occur in all Ti3Al-Nb alloys.