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Polysynthetic twinned (PST) TiAl single crystal possesses great potentials for high-temperature applications due to its excellent combination of strength, ductility and creep resistance. However, a critical property for high-temperature application of such material involving high-temperature fatigue properties remains unknown. Here, the high-temperature high-cycle fatigue performance of PST TiAl single crystal has been studied. The result shows that PST TiAl single crystal can withstand more than 107 cyclic loadings at 975 °C under a stress amplitude of 270 MPa, which is significantly higher than traditional TiAl alloys. Experimental observations and atomistic simulations indicate that the improvement of fatigue resistance is attributed to the plastic strain delocalization in uniform lamellar structure, and the plastic deformation is well-distributed and sufficient in each lamella. Even in the α2 lamella with difficult slippage, a large number of stacking fault structures can be observed. The 〈c + a〉 dislocations in α2 tend to dissociate into a Frank partial with b = 1/6 2 ¯ 0 3 ¯ >, forming a ribbon of I1 fault which ensures the continuity of deformation.

Contrary to previous researches of cracking behaviors in lamellar materials, a novel phenomenon has been found in polysynthetic twinned (PST) TiAl single crystals that the specimens with crack divider (type II) own much higher toughness than those with crack arrester (type I). By combining microscopic characterization and theoretical calculation, we reveal that this contrary is caused by the coherency of interlamellar boundaries. For PST TiAl single crystal with coherent interlamellar boundaries, delamination in type I specimen is suppressed by the coherent boundaries and the crack tends to penetrate lamellae along the deformation twinning planes. While sessile dislocations produced in type II specimen hinder the further motion of other dislocations and induce the passivation at crack tip. As a result, the specimen with type II owns higher fracture toughness than that with type I. A toughness ratio, RT=Ttype I / Ttype II, is proposed to evaluate the cracking behavior with different levels of boundary coherency in lamellar materials. In previous studies, RT > 1 because their boundaries are not coherent, which promotes the delamination and crack tip passivation. The present work sheds light on the relationship of interlamellar boundaries and crack propagation, which provides a new perspective for the development of lamellar materials.

NiCrAlY coatings without and with a Cr or Mo diffusion barrier were fabricated on Ti-42Al-5Mn alloys by vacuum plasma spray. The microstructure evolution and inter-diffusion behavior were studied via isothermal oxidation tests conducted at 950 °C. The results revealed that substantial inter-diffusion happened in the coating without a diffusion barrier, accelerating the degradation of NiCrAlY coating. While the addition of Cr or Mo diffusion barrier deferred the inter-diffusion phenomenon, and Mo performed much better than Cr due to its high stability. The thickness of the inter-diffusion zone with the Mo diffusion barrier was just about one-fifth of that for NiCrAlY coating after 200 h oxidation. The results could provide comprehensive understanding of the microstructural evolution of NiCrAlY coating, which was essential for controlling the inter-diffusion behavior and improving the durability of NiCrAlY coating.

The effects of withdrawal rate on the microstructure and segregation behavior of a nickel-base single crystal superalloy were investigated. The results indicate that by increasing the withdrawal rate, the primary dendrite arm spacing (λ1), the secondary dendrite arm spacing (λ2) and the size of γ' precipitates decrease gradually. Moreover, the shape of γ' precipitates is more regular at higher withdraw rates. Also, the size of γ/γ' eutectic becomes small while the total volume fraction increases. Considering the composition segregation is determined by diffusion time of the constituent elements, when withdrawal rate is elevated, diffusion time of solutes decreases, and the effects of segregation are enhanced.

The porous SiC ceramic foam/Zr-based metallic glass interpenetrating phase composites were prepared by water quenching after pressure infiltration. Effects of pore density on microstructure and mechanical properties of interpenetrating phase composites are reported. Microstructural investigations show that the metallic glass is completely infiltrated into the open pores of the SiC ceramic foams and a typical interconnected amorphous phase is formed. A ~10 µm thick diffusion layer in which the reactant ZrC is generated can be found between the ceramic phase and the glassy phase. The average size of the metallic glass phase decreases with increasing the pore density. The results of the quasi-static compression tests indicate that the compressive strength of the composites increases with the pore density of the SiC foams due to the formation of more high-quality interfaces. The brittle fracture of the composites is mainly attributed to the premature destruction of the SiC foams. The results of friction tests imply that compared with monolithic Zr-based BMG samples, the reinforcement of SiC retards the adhesive wear and plastic deformation of Zr-based BMG, and reduces the friction coefficient of the composites.

The interface structure of {2 2 ‾ 01} α2-Ti3Al deformation twins formed during tensile deformation of polysynthetically twinned (PST) TiAl single crystals at 900 °C has been studied systematically by aberration-corrected scanning transmission electron microscopy. Numerous superlattice intrinsic stacking faults (SISFs) are formed on the (0001) basal plane of α2 twins, thereby resulting in the formation of steps on the twin boundaries. The height of steps equals to the spacing of the (2 2 ‾ 01) twinning plane. The basal-plane SISFs formed in α2 twins can't be attributed to the glide of Shockley superpartial dislocations. Lattice correspondence analyses based on classical twinning theory reveal that complex atomic shuffle and interchange shuffling are responsible for the formation of twinning and basal-plane SISFs in α2 twins. In addition, basal-plane antiphase boundaries (APB) are formed at the vicinity of α2 twin tips to relieve the stress concentration imposed by misfit dislocations.

The isothermal oxidation behavior of a new Re-free nickel-based single-crystal superalloy in air at 950 °C for 200 h was studied by scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results indicate that oxidation kinetics obeys parabolic law approximately, and the mass gain increases rapidly during initial oxidation stage and then gradually slows down. The oxidation scales are composed of three layers: the outer layer mainly consists of NiO with a small amount of CoO; the intermediate layer is mainly composed of Cr2O3 with a small amount of spinel compounds such as CrTaO4, NiCr2O4, CoCrAl2O4, CoAl2O4, and NiAl2O4; and the inner layer is composed of A12O3. Inner Al2O3 layer suppresses the diffusion of elements between oxygen and alloy elements, slows down the alloy oxidation speed, and also suppresses the growth of the oxide scale and reduces the oxidation rate, which is agreeable with the oxidation kinetics.

TiAl alloys are lightweight, show decent corrosion resistance and have good mechanical properties at elevated temperatures, making them appealing for high-temperature applications. However, polysynthetic twinned TiAl single crystals fabricated by crystal-seeding methods face substantial challenges, and their service temperatures cannot be raised further. Here we report that Ti-45Al-8Nb single crystals with controlled lamellar orientations can be fabricated by directional solidification without the use of complex seeding methods. Samples with 0° lamellar orientation exhibit an average room temperature tensile ductility of 6.9% and a yield strength of 708 MPa, with a failure strength of 978 MPa due to the formation of extensive nanotwins during plastic deformation. At 900 °C yield strength remains high at 637 MPa, with 8.1% ductility and superior creep resistance. Thus, this TiAl single-crystal alloy could provide expanded opportunities for higher-temperature applications, such as in aeronautics and aerospace.