Α2 Laths Research Articles

Microstructure evolution and precipitation behavior in a Y2O3-bearing TiAl alloy during creep

The paper studies the microstructure stability of a Y2O3-bearing Ti–48Al–2Cr–2Nb alloy during creep. Tensile creep tests were performed at 800–850 °C under 100 or 275 MPa. Compared to thermal exposure, the low creep stress promotes the microstructure degeneration of lamellar colonies, mainly including α2-lath dissolution, γ-lath coarsening and βo phase precipitation. The high creep stress further accelerates the α2 → γ transformation, but goes against the precipitation of βo grains inside α2 laths. Similarly, more Cr-rich precipitates with the composition of Ti(Al, Cr)2 are observed in creep samples, which are identified as the C14-Laves phase. It has a {0001}C14//{111}γ and <101¯0>C14//<11¯0>γ orientation relationship with the γ phase. The high imposed stress also facilitates the precipitation behavior. It should be noted that there are a few C14-Laves phases precipitating along the Y2O3 interface by heterogeneous nucleation. The high strain energy around Y2O3 particles, especially under the high creep stress, contributes to the precipitation of C14-Laves phase not only at the Y2O3 interface, but also in the γ matrix.

The influence of stress levels on the creep behavior of Ti–42Al–6Nb–0.05Y2O3 alloy

The paper studied the creep behavior of Ti–42Al–6Nb–0.05Y2O3 alloy. Short-term creep tests were performed at the temperature range of 750–900 °C and the stress range of 100–300 MPa. The results reveal that the stress exponent varies from 1.85 to 5.01 when the creep stress increases from 100 to 300 MPa at 850 °C. Interface sliding and dislocation climb are the main creep mechanisms at 100 and 300 MPa, respectively. Formation of intergranular cracks is the major cause of the failure above 150 MPa. Instead, under 100 MPa, nucleation of cracks along lamellar interfaces plays a dominant role in the rupture. There is a critical temperature at which the apparent activation energy (Qc) increases suddenly. For the lamellar alloy, the increase of Qc has a close association with the dislocation movement in minority α2 laths. Below the activation energy transition temperature (AETT), the deformation of α2 phase is mainly governed by a→ type dislocations. Whereas above the AETT, the activation of 2c→+a→ type dislocations plays a dominant role. Moreover, the AETT at 300 MPa is determined to be 800–850 °C, whereas that at 100 MPa is 850–900 °C. There is an increase of AETT with the decrease of creep stress. At the low stress (100 MPa), the AETT depends on lattice resistance rather than long-range internal stress, since the flow stress may be less than Peierls-Nabarro force for slipping of 2c→+a→ type dislocations, resulting in a high AETT.

Effect of thermal exposure on microstructure evolution and mechanical properties of cast beta-solidifying TiAl-based alloy doped with Gd

Evolution of an initial nearly-lamellar (γ+α2+β0) structure of a new cast β-solidifying TiAl-based alloy Ti-44.5Al-2V-1Nb-2Cr-0.1Gd (at.%) after isothermal exposures at 800 °C for up to 10,000 h has been systematically investigated using SEM, TEM, EDS, and XRD analyses. During the exposures the alloy microstructure degraded by the α2 laths and β0 phase dissolution, resulting in coarsening of lamellar structure and rising of the γ phase volume fraction. A 2.5- and 3-fold increase in the number of nano-sized gadolinium oxide (Gd2O3) particles have been observed after 10,000-h exposure in lamellar colonies and in grain boundaries, respectively. The gadolinium oxides had monoclinic (C2/m) and cubic (Ia-3) structures and precipitated predominantly along γ/α2 and γ/β0 interfaces and inside the γ lamellae. Furthermore, the applied creep loading was found to contribute significantly to the precipitation process of Gd2O3 nano-particles, since dislocation networks being the sites for their preferred formation. Nanohardness of the γ phase increased from 4.4 ± 0.3 to 5.0 ± 0.4 GPa after the exposures, but no changes were observed for the β0 phase. As a result of structure evolution the tensile strengths reduced by 10–12 and 11–14 % measured at 20 and 750 °C testing temperatures, respectively. Meanwhile, the plastic elongation values remained essentially unchanged. We interpret this as insignificant effect of the precipitation of Gd-rich oxide nano-particles on the alloy ductility.

Elevated temperature performance and creep behavior of Y2O3 reinforced Ti-48Al-6Nb alloy at the brittle-ductile transition temperature

This study investigated the elevated temperature properties and creep behavior of Y2O3-bearing Ti-48Al-6Nb alloy. Tensile and creep tests of Ti-48Al-6Nb and Ti-48Al-6Nb-0.05Y2O3 alloys were performed. The results show that the brittle-ductile transition temperatures (BDTTs) of both alloys are determined to be 825–850 ℃ by tensile tests at the strain rate of 5 × 10−4 s−1. And the BDTTs of both alloys are determined to be 800–825 ℃ by creep tests under a constant stress of 325 MPa, reducing about 25 ℃ due to the slower creep rates. Y2O3 improves the creep properties of the alloys by refining α2/γ lamellar spacing, decreasing segregation and preventing dislocation movement. Furthermore, creep deformation behavior of α2 phases at the BDTT was discussed. Deformation of α2 laths during creep is mainly provided by a→ type dislocations below the BDTT. While above the BDTT, 2c→+a→ type dislocations were activated due to the active thermal activation and decreasing Peierls-Nabarro stress. The presence of 2c→+a→ slip systems in α2 laths contributes to ductile deformation during creep.

Effect of Hf addition on solidification and hot isostatically pressed microstructures of the Ti-48Al-2Cr-2Nb (at%) intermetallic alloy

The microstructural evolutions during solidification and hot isostatic pressing (HIP) of Ti-48Al-2Cr-2Nb-xHf (4822-xHf, x = 0, 2, 4, 6 at%) intermetallic alloys were investigated using both experimental observations and thermodynamic calculations. The primary solidification phase of all the studied alloys was β, albeit it was predicted by Thermo-Calc to be α for the alloys containing 4% and 6%Hf. In agreement with the Thermo-Calc calculations, the solidification path of the base 4822 alloy encountered a peritectic reaction. Although alloying by 2%Hf did not change the solidification path, further increase in Hf content caused an additional eutectic reaction following the peritectic one. The eutectic microstructure was composed of two orthorhombic phases with TiAl2 and Al3Hf2 compositions and this finding was the first evidence of such phases in a quinary alloy with Cr, Nb, and Hf. It was found that Hf acted as a weak β stabilizer as it showed a low partitioning tendency in β and the β single-phase field was not formed in the high-Hf alloys. A finer dendritic microstructure was characterized in the high-Hf alloys due to the Hf solubility limit in the 4822 alloy. Interestingly, α2 laths dissolved in the Hf-containing alloys after HIP treatment and instead rode-like precipitates of Al3Hf2 emerged at newly formed γ/γ interfaces. The precipitation mechanism of Al3Hf2 particles was discussed.

High Nb–TiAl Intermetallic Blades Fabricated by Isothermal Die Forging Process at Low Temperature

In this study, the isothermal die forging process of high Nb–TiAl (Ti-44Al-8Nb-0.2W-0.2B-Y, at.%) alloy blades was simulated using the ABAQUS V6.11 software and the blades were fabricated successfully. The influence of a low forging temperature (lower than 1000 °C) and strain rate on the distributions of effective strain and stress were analyzed. The results indicate that the effective strain exhibits negative temperature sensitivity and positive strain rate sensitivity. The stress exponent (n = 3.02) and the apparent activation energy (Q = 293.381 kJ/mol) of the present alloy suggests that this as-forged high Nb–TiAl alloy exhibits good deformability at low temperatures. With the reduction in strain rate and the increase in forging temperature, the effective stress decreases. Finally, high-quality high Nb–TiAl alloy blades were fabricated using an isothermal die forging technology at a rate of 0.01 mm/s and temperature of 950 °C, chosen on the basis of the simulations results. Scanning electron microscopy (SEM) and electron back scatter diffraction (EBSD) results indicated that the center of the TiAl alloy blade possessed a duplex microstructure, consisting of remnant lamellar colonies and recrystallized γ/B2 grains. The refined α2 laths showed a typical forging flow line feature in the edge position, whereas the γ laths had broken down and recrystallized.

Rolling parameters, microstructure control, and mechanical properties of powder metallurgy Ti–44Al–3Nb-(Mo, V, Y) alloy: The impact of rolling temperatures

Flow softening behavior, microstructure evolution and hot working parameters of powder metallurgy Ti–44Al–3Nb-(Mo, V, Y) alloy were systematically studied by isothermal hot compression. Based on the analysis of processing maps and the corresponding microstructure, power dissipation efficiency peak occurred at two domains, which were associated with dynamic recrystallization (DRX) and super-plasticity. Pile-up of high density dislocation and a large number of twins were the primary deformation mechanisms at low temperature and high strain rate. As the temperature increases, fine DRX grains promote the movement and rotation of grain boundary, the softening of β phase and the transformation of β→α/α2+γ above 1200 °C improve the deformation capacity of the alloy significantly. Crack-free TiAl sheets were successfully obtained by direct hot rolling process at 1080 °C and 1220 °C. The corresponding microstructure and mechanical properties were analyzed. The sheets rolled at 1080 °C was characterized by near γ (NG)+β0 microstructure with a streamline along the rolling direction, while duplex (DP)+β0 microstructure appeared to be the main microstructure at rolling temperature of 1220 °C. The as-rolled sheet exhibits excellent mechanical properties at 800 °C. The sheet rolled at 1080 °C exhibits 320 MPa yield strength and 468 MPa ultimate tensile strength with 142% elongation. The yield strength, ultimate tensile strength, and elongation of the as-rolled sheet are 446 MPa, 586 MPa, and 3.0%, respectively, when rolled at 1220 °C. The excellent ductility of the sample rolled at 1080 °C is attributed to formation of fine DRX grains due to consumption of twins and substructure under tensile stress, which promote grain boundary slip. Lamellar colonies with fine interlamellar spacing and α2 laths formed at 1220 °C can effectively prevent dislocation motion, resulting in a significant increase in the strength of the alloy.

Microstructural stability, phase transformation and mechanical properties of a fully-lamellar microstructure of a Mo-modified high-Nb γ-TiAl alloy

Microstructural instability modes, phase transformation and mechanical properties evolution of a fine-grained fully-lamellar Ti–44Al–6Nb–1Mo−0.3(B, Y) (at.%) alloy were investigated systematically. During isothermal exposure at application temperatures, dramatic discontinuous precipitation of γ and βo (ωo) grains proceeded along boundaries of prior lamellar colonies, i.e. via a transformation of L(α2/γ) → γ + βo (ωo). Meanwhile, the internal lamellar structures degraded by α2-lath dissolution and simultaneous γ-lath coarsening. Furthermore, thermal loading was found to contribute to the above decomposition processes. However, almost no βo and βo (ωo) grains formed in the interiors of lamellar structures. The precipitation of equilibrium phases γ, β and ωo abided by normal crystallographic orientation relations with their respective parent phases. The present microstructural decomposition was considered to be essentially induced by thermodynamic instability of α2 laths and high interfacial energy of nanoscaled lamellar structures. Finally, it was demonstrated that this microstructural degradation caused both high-temperature strength and room-temperature ductility to deteriorate.

Microstructure Evolution of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y Alloy during Long-Term Thermal Exposure.

The hot-rolled alloy Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y was exposed to 700 °C air for up to 10,000 h. The changes in microstructure were observed using scanning and transmission electron microscopies. It was found that the α2 laths, α2 + γ lamellae, and B2(ω) structure of the alloy showed thermodynamic instability. There were three types of phase transformation in the alloy during long-term thermal exposure. The first was α2 → γ, which occurs in the interior and boundary of the α2 + γ lamellae. The second was α2 + γ → B2(ω), which occurs on the α2 + γ boundary. In addition, B2(ω) also precipitates on the γ/γ interfaces. The third was B2(ω) → γ, which describes the precipitation of micron-scale γ phases in the B2(ω) area after thermal exposure of 5000 h. The volume fraction and size of the B2(ω) area and equiaxed γ grains continued to increase throughout the exposure process. Large-sized γ grains and a B2 area of tens of microns appeared in the microstructure after long-term thermal exposure. The volume fractions of the B2 area and the equiaxed γ grains after thermal exposure of 10,000 h reached 16.8% and 63.2%, respectively.

Carbon content driven high temperature γ-α2 interface modifications and stability in Ti–46Al–4Nb intermetallic alloy

The effect of carbon addition on the microstructure modifications and hardness response of a two-phase lamellar Ti–46Al–4Nb intermetallic alloy was studied. The alloy was heat treated as to have an initial tow-phase refined lamellar microstructure consisting of α2 laths and γ phase characterized by nanometric wide twinning. The effect of carbon in solid solution in the α2 phase and the onset of the H–Ti2AlC phase precipitation within the γ phase was addressed. A threshold carbon limit of C ≅ 3000 wt ppm was identified as lower limit solute concentration for the precipitation initiation of the H phase particles. The major alloy hardening factors were identified as the refined interlamellar spacing, the fine twinning and the amount of carbon in solid solution. The role of H phase formation on the alloy hardening with carbon content was addressed and it was essentially such to induce a hardening saturation effect.

Effect of alloying additions on work hardening, dynamic recrystallization, and mechanical properties of Ti–44Al–5Nb–1Mo alloys during direct hot-pack rolling

There is significant gap in the understanding of the effect of alloying elements, particularly V and B, on the dynamic recrystallization (DRX) behavior, and the corresponding microstructure evolution in Ti–44Al–5Nb–1Mo-(V, B) alloys during uniaxial hot compression. In this regard, we have studied here the relationship of work hardening rate (θ) and critical strain of dynamic recrystallization (εc) on strain, deformation parameters. The θ values showed typical dynamic recrystallization characteristics, and were sensitive to temperature and strain rate. Deformation mechanism involving twinning-dislocation interactions and phase transformation contributed to characteristics of θ. DRX occurred in Ti–44Al–5Nb–1Mo–2V-0.2B alloys even at higher rolling speed and at ambient temperatures over a wide range, which implied a wider processing window during the direct hot-pack rolling process. Crack-free sheets were directly obtained from the as-HIPed alloys at 1250 oC with rolling speeds of 0.3 m/s, and 10%–20% reduction per pass. Their microstructure, high-temperature tensile properties and fracture mechanisms are discussed. As rolling progressed, Ti–44Al–5Nb–1Mo alloy was characterized by residual lamellae with locally dissolved α2 laths, broadening γ laths, and fine sub-grains. Dislocation-induced DRX and phase transformation of α2/γ lamellae were the primary deformation modes. In contrast, on the addition of V and B, deformation twinning was activated. Multi-phase microstructure was characterized by equiaxed grains of ~ 5–20 μm. The as-rolled sheets of both the alloys exhibited superior performance at 800 oC. Ti–44Al–5Nb–1Mo alloy showed a higher ultimate tensile stress of ~689 MPa, while superior ductility of greater than 75% was obtained for Ti–44Al–5Nb–1Mo–2V-0.2B alloy. The microstructure with different degree of dynamic recrystallization contributed to various aspects of high-temperature tensile properties.

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.

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.

High-resolution electron microscopy characterization of modulated structure in high Nb-containing lamellar γ-TiAl alloy

The microstructure in a lamellar γ-based Ti-45Al-8.5Nb alloy has been investigated by transmission electron microscopy (TEM). The results show that there exists a nano-scale modulated structure with tweed contrast within α2 laths and the orthorhombic phase as a constituent of the modulated structure has the O1 structure with a random occupancy of Ti and Nb atoms on the Wyckoff sites 8 g and 4c2. The orthorhombic phase has a thin plate-like morphology with orientation relationships of {001}O//{0001}α2 and <110>O//<112¯0>α2. The habit plane for orthorhombic phase variants lies at {350}O//{134¯0}α2, and the O1/α2 interface is not atomic flat. The results further exhibit an enrichment of Nb in the orthorhombic phase in comparison with the α2 parent phase. Based on the TEM evidences, the formation of the orthorhombic phase arises from a phase separation reaction in the α2 parent phase, which is dominated by a diffusion mechanism.

Morphology and stability of orthorhombic and hexagonal phases in a lamellar γ-Ti-42Al-8.5Nb alloy-A transmission electron microscopy study

Intermetallic titanium aluminides are often alloyed with niobium to improve oxidation resistance and mechanical properties. Depending on the overall alloy composition and on the solubility of the present phases for niobium, new phases may be introduced in the alloys' microstructure upon niobium addition. Here we describe the microstructure of the alloy Ti-42Al-8.5Nb produced by powder metallurgy after annealing at 550 °C and 650 °C. We identify the orthorhombic O-phase within α2 laths of lamellar (α2 + γ) colonies by single crystal electron diffraction and high-resolution imaging. Domains of O-phase variants and α2 phase form columnar crystallites in α2 (0001) planes having low indexed α2 {11-20} and {1-100} interface planes. The nm-sized domains are rotated with respect to each other resulting in elastic strains across the domain boundaries. No elemental segregation of Nb was detected in α2/O-phase lamellae.

Phase transformations in Ti–34Al–13Nb alloy

The microstructure and orientation relationship (OR) of the α2 and ωo phases in the Ti–34Al–13Nb (at. %) alloy were investigated. The α2 segments nucleated at the ωo boundaries after aging at 900 °C for 2 h. The ORs between the two phases were studied by selected area diffraction patterns using a transmission electron microscope and interpreted by the superimposed stereographic projections. Two ORs were defined as follows: \( \left[ {1\bar{2}10} \right]\upalpha_{2} //\left[ {000\bar{1}} \right]\upomega_{o} \); \( (0002)\upalpha_{2} //\left[ {\bar{1}2\bar{1}0} \right]\upomega_{o} \) and \( \left[ {\bar{1}010} \right]\upalpha_{2} //\left[ {2\bar{4}2\bar{3}} \right]\upomega_{o} \); \( \left( {0002} \right)\upalpha_{2} //\left( {01\overline{12} } \right)\upomega_{o} \). Two α2 laths with parallel \( \left\langle {\bar{1}102} \right\rangle \) directions were observed in this alloy, which were explained by the different α2 variants nucleated at the ωo boundaries according to ORI and ORII.

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.

Ordered α2 to ωo phase transformations in high Nb-containing TiAl alloys

The transformation of the D019-α2 phase to the B82-ωo phase is a new type of phase transformation in high Nb-containing TiAl alloys; experimental results concerning this phase transformation are still lacking. Moreover, studies on the orientation relationships (ORs) between these two phases are scarce compared with those reported between the disordered ω and α phases, which can be more complicated due to their ordered structures. In this study, the direct α2 to ωo phase transformation is observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The ωo phase is transformed from the α2 laths in the lamellar structure after annealing over long periods at 850°C. The various ORs observed between these two phases can be merged into two typical ORs: [112¯0]α2//[0001]ωo,(0002)α2//(112¯0)ωo and [11¯00]α2//[224¯3¯]ωo,(0002)α2//(011¯2)ωo. The other ORs observed are subsets of these two ORs. The edge-to-edge matching model is applied to predict the possible ORs between the ordered α2 and ωo phases based on the calculated close-packed planes of the two phases. The simulation results agree well with the experimental results.

B19 phase in Ti–45Al–8.5Nb–0.2W–0.2B–0.02Y alloy

The B19 phase is widely observed in the α2 laths in Ti–45Al–8.5Nb–0.2W–0.2B–0.02Y alloy, forming a modulated structure together with the α2 phase. The distortion between the α2 and B19 phases exists only along the [11¯00]α2 direction and is very difficult to detect. Thin γ laths precipitate regularly within the α2+B19 structure to accommodate the lattice misfit, whereas the sizes of the γ laths are limited to a small scale. The B19 phase, however, is unstable and was eliminated after annealing at 900°C for 30min, followed by air-cooling. The pure α2 phase observed in the air-cooled sample is not in the equilibrium state. The B19 phase can be a stable phase in some temperatures below 900°C but be repressed under rapid cooling.

Phase transformation and microstructure evolution of differently processed Ti–45Al–9Nb–Y alloy

Microstructure formation in as-cast, wrought and as-rolled Ti–45Al–9Nb–Y alloys has been investigated, and near lamellar, near duplex and near gamma structures were observed, respectively. The investigation included the evolution of the β/B2-phase in these three differently processed Ti–45Al–9Nb–Y alloys. Omega-phase occurs on the expense of the β/B2-phase in the hot deformed Ti–45Al–9Nb–Y alloy. Moreover, γ laths precipitate in β/B2-phase when the alloy was hot rolled. After canned forging, a tweed like contrast which is perpendicular to the orientation of lamellae occurs in the coarsened α2 laths in residual lamellae. In addition, a rod-like microstructure which is composed of the α2 and γ-phase has been observed in as-rolled alloy. The grain size of this material is a little larger than that of the as-forged alloy, but rather smaller than of the as-cast alloy.