Papers by Keyword: TiAl Intermetallic Compound

Abstract: Although metal matrix composites (MMC) for the high temperature structural material have been investigated extensively for many years, applications of MMC have been still limited. Among many combinations between the ceramic fibers and the matrix materials, combination of SiC fiber and TiAl based intermetallic compounds has been expected to be one of the best combination, since both SiC fiber and TiAl have demonstrated the capabilities of the low density heat resistant materials. SiC fiber reinforced TiAl composites have been successfully fabricated using hot press method. Optimum temperature and pressure have been determined. SiC/TiAl composite having relatively low fiber volume fraction shows nearly an ideal elastic property applying the law of mixture. Effects of interface layers on the mechanical properties of composites have been studied in detail. Micro-indentation on a single fiber was carried out to examine the pull out strength of SiC fiber quantitatively. Estimated shear stress on the interface was 145-195MPa, those values are quite reasonable since the tensile strength of TiAl matrix was 420MPa and the maximum shear stress would be the half of tensile strength according to Schmid law. Three-point bending tests have been carried out to evaluate the mechanical properties of composites. Fiber volume fraction 8.9% specimen shows ideal bending stiffness compare with the calculated values based on the low of mixture. Reaction layers and the interface between SiC fiber and TiAl have been analyzed by SEM-EDS and XRD. At least two or more reaction layers have been identified. These reaction layers can be explained based on the Si-Ti-C ternary equilibrium phase diagram at 1373K. Optimum conditions of interface structure will be discussed

Abstract: Air plasma sprayed thermal barrier coatings (APS-TBCs) deposited on the TiAl intermetallic compounds was heat exposed in air at different temperatures and times to evaluate the microstructural change and delamination behavior. The thermal barrier coating (TBC) layer, bond coat (BC) layer and substrate were composed of 4 mol% Y₂O₃ stabilized ZrO₂, CoNiCrAlY alloy (Co-32Ni-21Cr-8Al-0.5Y (mol%)) and TiAl intermetallic compound (Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1 Si (mol%)), respectively. Due to the heat exposure, diffusion of the elements occurred between the BC layer and the substrate, and diffusion layers were formed on both the BC layer and the substrate. A thermally grown oxide (TGO) layer was formed between the TBC layer and the BC layer. The thickness of the TGO layer and the diffusion layer increased with increasing exposure temperature and time. In the TBCs heat exposed at 1273 K for 200 h, a composite oxide of Al₂O₃ and TiO₂ was formed in the BC layer. Regarding the TBCs which were as-deposited and heat exposed at 1073, 1173 K up to 200 h and at 1273 K for 10 h, delamination occurred in the TBC layer near the BC layer. In the TBCs exposed at 1273 K for 50 h or more, delamination occurred at the vicinity of the interface between diffusion layer on the substrate side and the unreacted side of the substrate too. In case that the TBCs were heat exposed at 1073 and 1173 K, the shear strength decreases after reaching the maximum value of the shear strength at 10 h heat exposure. When the TBCs were exposed to heat at 1273 K, the shear strength indicated a constant value after the shear strength increased up to 50 h. This change may be due to the change in crack path after exposure for 50 h at 1273 K.

Abstract: Effects of holding temperature and time at (β+γ) two phase region on the microstructure of fully lamellar Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si (mol%) intermetallic compounds are studies. Fully lamellar microstructure is observed after homogenization heat treatment for 3.6 ks at 1643 K (α single phase state). Fine β phased grains precipitate at fully lamellar structure after heat treatment of homogenized material at 1373 K. Holding the homogenized material for 72 ks at 1373 K decompose partially the lamellar structure. Heat treatment of homogenized material at 1273 K also precipitates the fine β phased grains in fully lamellar structure. In this temperature range, decomposition of lamellar structure is not observed up to 72 ks heat treatment. The toughness of homogenized material is ~ 15 MPa√m. Heat treatment of homogenized material at 1373 K and 1273 K for 3.6 ks indicates maximum fracture toughness in each temperature range. This may due to the precipitation of fine β phased grains. The fracture toughness decreases with the increase in heat treatment time up to 18 ks and/or 36 ks. Then, the value of fracture toughness became constant. Specimens heat treated at 1373 K for 36 ks and 72 ks indicate lower toughness than homogenized material. However, when the specimens are heat treated at 1273 K for 36 ks and 72 ks, the toughness is higher than that of homogenized material. This change is due to the decomposition of the lamellar structure.

Abstract: The effects of massive transformation and subsequent heat treatments on the microstructure of Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si (mol%) intermetallic compounds are studied. Massive transformation occurs at the center region of the specimen by cooling from α single phase state. At the surface side of the specimen, α phase has remained. Fine convoluted microstructure with α₂, γ phases and lamellar structure has formed by heating at (α+γ) two phase state after massive transformation. Colony size or grain size is about 25 μm. Fine fully lamellar structure is obtained after heat treatment of convoluted microstructure at α phase for 60 s. Fracture toughness seems to be increasing with the increase in lamellar colony size. However, some massively transformed specimens show lower toughness due to the formation of microdamage present in samples before the test.

Abstract: TiAl based intermetallic compound claddings were produced on TA15 alloy surface by using laser depositing technology to melt Ti-46Al-2Cr metal powders. An interface layer between TA15 substrate and the TiAl claddings was formed. The influence of laser power, scanning speed and number of cladding layers on the interface layer and the hardness of TiAl claddings were investigated. Higher laser power and lower scanning speed made the interface layer thicker. Increasing the laser power and especially the scanning speed could improve the hardness of the TiAl claddings. When the second TiAl layer was deposited, there was no interface layer formed between the two TiAl layers, but the hardness of the first layer decreased and the second TiAl layer was softer than the first layer due to the rough microstructure.