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Abstract A series of bending tests was conducted on specimens of different TiAl alloys to investigate the nucleation of microcracks under tensile stress. The distribution of microcracks and the deformation traces around them were characterized to study the role of plastic deformation intersecting with microstructural features in the microcrack nucleation. Results show that the microcrack nucleation plays an important role in the fracture of multi-phase high Nb-containing TiAl alloys. The microcracks tend to nucleate at colony boundaries in near fully lamellar and fully lamellar alloys, while they prefer B2/γ phase boundaries in (B2+γ) alloy. The heterogeneous deformation in different lamellar colonies was caused by plastic anisotropy of the lamellar structure. It leads to the nucleation of microcracks at colony boundaries. Meanwhile, due to the stress concentration at lamellar interfaces in the transversal deformation mode, a few microcracks nucleate inside lamellar colonies which have an orientation angle Φ near to 90°. The existence of brittle B2 phase leads to strain incompatibility and facilitates microcrack nucleation at B2/γ boundaries. Highlights • A systematic study of microcrack nucleation in multi-phase TiAl alloys. • The microcrack nucleation is correlated with heterogeneous deformation. • The effect of plastic anisotropy of lamellar structure and B2 phase is investigated. [ABSTRACT FROM AUTHOR]

A series of fracture toughness tests was conducted on specimens of multi-phase high Nb-containing TiAl alloys with three different microstructures. The fracture path profiles and microscopic toughening mechanisms were analyzed to investigate the effect of constituent phases and microstructures on the fracture toughness. The results show that the fracture toughness of the multi-phase TiAl alloys mainly depends on the lamellar structure and the phase arrangement at colony boundaries. A combination of microscopic toughening mechanisms including crack deflection, shear ligaments, plasticity and microcracking contributes to the toughness of high Nb-containing TiAl alloys. Twinning and dislocation glide in γ phase as well as prismatic slip in α 2 phase promotes the plasticity toughening. B2 phase is normally detrimental to the crack resistance due to its brittleness. However, the B2 phase located at colony boundaries results in microcracking away from the main crack, which is regarded as a toughening way. [ABSTRACT FROM AUTHOR]

To understand the fracture behavior and cavity evolution of high Nb-containing TiAl alloys, tensile tests were conducted at low strain rates and various temperatures around brittle to ductile transition temperature (BDTT). Then, the fracture morphology and cavitation behavior were investigated. The results show that cavitation occurs above BDTT as the fracture mode changes from cleavage fracture to ductile fracture. And the cavitation of this alloy is dominated by diffusion-controlled cavity nucleation, which is promoted at higher temperature. The growth and coalescence of cavities are correlated with heterogeneous deformation and facilitated at higher strain rate. The cavity growth and coalescence are accelerated at boundaries between hard and soft orientated lamellar colonies due to the strain incompatibility. Additionally, β/B2 phase may facilitate the cavity nucleation due to its high diffusivity. These findings may provide important implications for optimizing the hot-working processing of high Nb-containing TiAl alloys. [ABSTRACT FROM AUTHOR]

A series of continuous cooling tests were performed on TiAl alloys using a Gleeble3500 machine to investigate the effect of thermal stresses on the microstructure. The results show that macroscopic thermal stresses promote correlated nucleation of γ lamellae. The trend of the dominance of one twin-group γ variants in local regions is weakened, and the γ/γ interfaces tend to be true twin and pseudotwin boundaries rather than 120° rotational faults under macroscopic thermal stresses. Meanwhile, thermal-induced deformation generated under the effect of both microscopic and macroscopic thermal stresses results in numerous low angle grain boundaries (LAGBs) and dislocations. The LAGBs and dislocations distribute heterogeneously among lamellar colonies and phases. No mechanical twins are observed due to the low strain and low strain rate characteristics of the thermal-induced deformation. These findings could shed light on understanding and preventing the cracking of TiAl components during cooling process. [ABSTRACT FROM AUTHOR]

In this paper, ingot breakdown process of a high containing alloy with a chemical composition of -42.63-8.11-0.21-0.09 (at.%) has been investigated under conventional forging conditions. It was found that the present alloy possesses superior hot-workability that can be successfully forged by conventional upsetting route due to the appearance of large amount of β/B2 phase, though shear band was observed in the forged-pancake. Further studies revealed that hot-working performed in (α + β) phase region which can effectively impede the β → α transformation and thus significantly increase the volume fraction of β/B2 phase. In contrast, the amount of β/B2 phase was notably reduced by heat treatment at the same conditions. This stress-induced effect is considered to be responsible to the superior hot-workability of the present alloy and the mechanism has been discussed and reasonably clarified. It was also suggested that the stress-induced effect has practical significance that it allows the implementation of conventional multi-step forging process which can develop fine and uniform microstructures suitable for secondary processing. [ABSTRACT FROM AUTHOR]

Abstract: The hot deformation behavior of a high Nb containing TiAl alloy with a nominal composition of Ti–42Al–8Nb–(W, B, Y) was investigated at temperatures ranging from 1000 °C to 1150 °C and strain rates from 10−3 s−1 to 0.5 s−1 on a Gleeble thermo-simulation machine. The work hardening regime and flow softening behavior of the alloy were analyzed in detail. The results revealed that the onset of dynamic recrystallization (DRX) was quite easy for the present alloy, whereas the dynamic recovery (DRV) was impeded during the hot deformation. The DRX kinetics was studied by Avrami-type equation. The low Avrami exponents of the proposed equation indicate a lower recrystallization rate compared to ordinary metals and alloys. Based on the classical hyperbolic-sine law and the kinematics of DRX, the constitutive equations of the work hardening-recovery period (i.e. flow stress before the peak) and flow softening process (i.e. flow curve after the peak stress) were established for the present alloy, respectively. Comparisons between the experimental and calculated results revealed that except the severely cracked specimens, the stress–strain curves predicted by the established model are in good agreement with experimental results. [Copyright &y& Elsevier]

Fine-grained Ti-(42.5–43.5)Al-8Nb-0.2W-0.2B-(0–0.1)Y (in at.%) alloys with two different phase constituents (i.e., (α 2 +γ) alloy and (β/B2+γ) alloy) have been prepared by means of thermo-mechanical processing. Tensile tests were performed at 1000 °C under an initial strain-rate of 10 −4 s −1 to evaluate the superplastic properties, and it was found that both alloys exhibited impressive superplasticity so that the elongations reached ∼400%. Texture measurements preliminarily revealed that intragranular deformation and the subsequent dynamic recrystallization was the predominant superplastic deformation mechanism in (β/B2+γ) alloy, while grain boundary sliding was responsible for the large elongation of (α 2 +γ) alloy. Cavities caused by superplastic deformation have been quantitatively evaluated by using SEM and X-ray tomography to reveal the effect of β/B2 phase on cavitation behavior. It was found that in (α 2 +γ) alloy the cavities preferentially nucleated in the triple-junctions of interphase boundaries, whereas most of the small cavities in (β/B2+γ) alloy were located along the β/γ interfaces. Quantitative analysis revealed that the alloys had comparable cavity number density after deformation, but the (β/B2+γ) alloy was characterized by larger cavity sizes, and thus by higher cavitation volume fraction and coalescence parameter in comparison with that of (α 2 +γ) alloy. Moreover, the large cavities in (β/B2+γ) alloy preferentially elongated along the direction parallel to the tensile axis, whereas the large cavities were more equiaxed in (α 2 +γ) alloy. The different cavity nucleation–growth–coalescence behavior of the two alloys was clarified based on the divergence in superplastic deformation mechanisms. Besides, theoretical evaluations were performed and revealed that the cavity growth rate of γ-TiAl was much higher than ordinary Al alloys under similar deformation conditions. This was believed to be the main reason for the relatively lower superplastic elongations of TiAl-based intermetallics. [ABSTRACT FROM AUTHOR]

γ-TiAl-based alloys have attracted intense attentions in the past two decades and are regarded as promising candidate materials for high-temperature applications due to their low density, high specific strength, superior creep and oxidation resistance, etc. It is well known that there are four typical microstructures in TiAl-based alloys, namely, full-lamellar (FL), near-lamellar (NL), duplex (DP) and near-gamma (NG). The FL/NL-TiAl alloys possess superior high temperature mechanical performance such as creep resistance, but their deformability is relatively lower. In contrast, the DP/NG-TiAl alloys have much better formability while the creep resistance is reduced. Hence the DP/NG microstructure is preferable for hot-working and thought to the prerequisite for secondary processing, and the present paper is primarily focused on the high temperature deformation behavior of DP/NG-TiAl alloys. Till date, considerable research efforts have been directed towards the deformation kinetics or mechanical properties of various DP/NG-TiAl alloys, and a mass of experimental data has been accumulated. In order to explore their general features of high temperature deformation kinetics, in this paper the tensile/compression mechanical data of various DP/NG-TiAl alloys from the literature published in the past twenty years has been summarized and reviewed. Three deformation mechanisms (dislocation creep, grain boundary sliding and diffusion creep) have been identified, and for each the effects of major factors (i.e., alloy composition, microstructure and grain size) on deformation kinetics have been analyzed and discussed in detail. The results revealed that the high temperature deformation kinetics of DP/NG-TiAl alloys is less sensitive to composition except some dispersion-hardened alloys such as carbon-containing TiAl alloys. Instead, the γ grain size became the major limiting factor. Based on this observation, a series of unified rate-equations has been successfully developed. [ABSTRACT FROM AUTHOR]