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The TNM alloy, a βo-phase-containing TiAl alloy, has been withdrawn from use as a last-stage turbine blade in commercial jet engines as it suffered frequent impact fractures in service, raising doubts regarding the necessity of the βo-phase in practical TiAl alloys. Here, we evaluate the practical properties required for jet engine blades for various TiAl alloys and investigate the effects of the βo-phase thereupon. First, we explore the influence of the βo-phase content on the impact resistance and machinability for forged Ti–43.5Al–xCr and cast Ti–46.0Al–xCr alloys; the properties deteriorate significantly at increasing βo-phase contents. Subsequently, two practical TiAl alloys—TNM alloy and TiAl4822—were prepared with and without the βo-phase by varying the heat treatment temperature for the former and the Cr concentration for the latter. In addition to impact resistance and machinability, the creep strength is significantly reduced by the presence of the βo-phase. Overall, these findings suggest that the βo-phase is an undesirable phase in practical TiAl alloys, especially those used for jet engine blades, because, although the disordered β-phase is soft at high temperatures, it changes to significantly more brittle and harder βo-phase after cooling.

Cry toxins from Bacillus thuringiensis (Bt) have been extensively applied in agriculture to substitute the use of chemical insecticides. We have previously reported the use of a coexpression system in which late embryogenesis abundant (LEA) peptides under the control of the lac promoter increase the expression of insecticidal proteins in Bt. The use of lactose to induce the expression of LEA peptides may be a desirable alternative to isopropyl β‐D‐thiogalactopyranoside, the most frequently used inducer for recombinant protein expression. In this study we investigated the use of lactose as an inducer for optimal protein expression. We observed enhanced insecticidal Cry protein expression by applying a simple technique based on intermittent induction, and then optimized concentration and the point of induction time from the 11th h to the 15th h. Our data suggest that intermittent induction of lactose might be a new technique for the enhancement of bacterial protein expression.

In the aviation industry, materials are being developed to improve the performance of jet engines. TiAl is used in turbine blades because of its high strength at high temperatures and its light weight. However, due to the difficulty of machining TiAl, additive manufacturing has been expected in recent years. Electron-beam-melted TiAl4822 has a unique layered microstructure composed of duplex-like fine-grain layers and a band of coarser γ-grains. This unique microstructure is known to bring about the anisotropy of mechanical properties. This study investigated the effects of building direction on creep properties. The creep test was conducted in accordance with the ASTM standard. The creep life of an additive manufacturing specimen was longer than that of a cast specimen due to the higher percentage of lamellar structure in the former. A specimen with building angle θ = 0° (vertical direction) had the longest creep life, which is attributed to the deformation of layers. When the building angle is θ = 45° or 90°, slip occurs between the layers, resulting in the formation of a large void along the vertical grain boundary of the γ band, which may cause high creep elongation.

TiAl (titanium aluminide) has been applied to turbine blades in aerospace industry, because it is a promising lightweight material with high strengths at high temperatures. In recent years, the production method of TiAl by additive manufacturing has been studied. In this research, the effect of dimensions of TiAl specimens by additive manufacturing had on the tensile and creep properties was studied. It was confirmed that the near-net specimen showed higher tensile strengths and lower creep resistance and life than those of the specimen cut from the block with larger cross section and volume than near net specimen. The results showed that the difference of the mechanical properties was because of the different microstructure caused by the powder melting process including hatching to melt inside the cross section and contouring perimeter. The difference in microstructure is considered to be due to the energy density. The energy density of contours is nearly 4 to 10 times higher than that of hatching. Therefore, it is considered that rapid heating and rapid cooling occur in the contour process, resulting in a fine structure.

The application of Ti-48Al-2Cr-2Nb (TiAl4822) in such industries as aerospace and automotive has been studied and commercialized in some mass production parts. Additive manufacturing (AM) technology is one of the research areas to expand the application using super alloys. Electron beam melting (EBM), with a vacuum chamber and a heating system, is currently a major technique to build TiAl4822. The present study shows the mechanical properties of additively manufactured TiAl4822 by EBM. There are many reports describing the specific mechanical property or microstructure, but very few reports included the holistic investigation including different mechanical properties such as tensile, fatigue, and creep with observation of microstructure in TiAl4822. In addition, the previous researches reported by different groups applied relatively small specimens with different dimensions and conditions. This makes it difficult to compare the results. In this research, the ASTM specimens for tensile and fatigue tests were prepared adopting same conditions in EBM process to understand the important mechanical properties at 23℃ and 750℃. The effect of test direction on tensile and fatigue properties were evaluated because it has been known that AM gives anisotropic properties. Microstructure was analyzed by scanning electron microscope (SEM) and electron back scatter diffraction (EBSD) to understand the effect of the differences in microstructure on the mechanical properties. The results of tensile test at 750℃ showed that the elongation tested in 45° to the build direction was the largest due to the ductile γ phase is parallel to 45°. Regardless of the stress amplitude, fatigue life in 0° showed the shortest because the relatively low strength between layers could affect the tensile strength in 0°. The fracture surface and cross section of specimen after fatigue test indicated that the delamination between layers can propagate in the material. Fatigue limit in 45° was the greatest, reflecting tensile property in 45°. However, the observation of the cross section after high cycle fatigue test revealed that the delamination between the layers can also occur in 45° because shear stress along the layers is maximum in 45°. In was also found that the anisotropy in EBM specimens in fatigue property depends on the number of cycles, while all the EBM specimens with relatively small grain size showed the longer fatigue life than that of casting specimens. Understanding these anisotropic mechanical properties and microstructure is important when the AM parts are designed.

One of the lightweight heat-resistant alloys, Ti-48Al-2Cr-2Nb (TiAl4822), has already been adopted to turbine blades for jet engines, but is manufactured by casting which requires expensive post process. Aerospace industry is intensively studying the additive manufacturing (AM) process for the engine parts. In this research, tensile and creep properties of additively manufactured TiAl4822 by electron beam melting (EBM) were studied. The mechanical tests were performed at 750 ℃ which is similar to the actual condition in low pressure turbine module. The effect of hot isostatic pressing (HIP) treatment on these properties was investigated because such information is very limited, even though HIP treatment is required in many aerospace parts. Near net shape specimens were prepared for both tensile and creep tests. The effect of the HIP treatment on the microstructure in the specimens was observed by scanning electron microscope (SEM). Electron back scatter diffraction (EBSD) was used to analyze the effect of the HIP treatment on microstructure such as area fraction of each phase and grain size in the specimen. The results showed that the benefit of sintering during the HIP treatment decreased the voids with a size of up to 40 μm and the defects with a size of millimeter order due to lack of fusion in the specimens. The HIP treatment improved both tensile and creep properties at 750 ℃. It was found that the HIP treatment improved creep life by 15% and elongation by 60%, and delayed the starting point of tertiary creep. This is mainly because the HIP treatment eliminated the defects caused by lack of fusion and made the material more ductile than as-built specimen. On the other hand, the microscopic analysis by EBSD revealed that the HIP treatment increased the average diameter of the grain and widened the distribution of grain size, with the increase of the area fraction of Ti3Al(α2). The increase of grain diameter could decrease the strength according to the Hall-Petch Law. In this research, however, the elimination of the defects such as the voids and the defects due to lack of fusion improved the tensile and creep properties. The results suggested that the post heat treatment including cooling process after HIP treatment to control the microstructure is important.