Since the directly bonded interface between TiAl alloy and SCM440 includes lots of cracks and generated intermetallic compounds(IMCs) such as TiC, FeTi, and Fe2Ti, the interfacial strength can be significantly reduced. Therefore, in this study, Cu is selected as an insert metal to improve the lower tensile strength of the joint between TiAl alloy and SCM440 during friction welding. As a result, newly formed IMCs, such as Cu2TiAl, CuTiAl, and TiCu2, are found at the interface between TiAl alloy and Cu layer and the thickness of IMCs layers is found to vary with friction time. In addition, to determine the relationship between the thickness of the IMCs and the strength of the welded interfaces, a tensile test was performed using sub-size specimens obtained from the center to the peripheral region of the friction-welded interface. The results are discussed in terms of changes in the IMCs and the underlying deformation mechanism. Finally, it is found that the friction welding process needs to be idealized because IMCs generated between TiAl alloy and Cu act to not only increase the bonding strength but also form an easy path of fracture propagation.

Si3N4 is a ceramic material attracting attention in many fields because of its excellent abrasion resistance. In addition, Ti and TiAl alloys are metals used in a variety of high temperature environments, and have attracted much attention because of their high strength and high melting points. Therefore, study of the interface reaction between Si3N4 / Ti and Si3N4 / TiAl can be a useful practice to identify phase selection and diffusion control. In this study, Si3N4 / Ti5Si3 + TiN / TiN / Ti diffusing pairs were formed in the Si3N4 / Ti interfacial reaction and Si3N4 / TiN(Al) / Ti3Al / TiAl diffusion pathway was identified in the Si3N4 / TiAl interfacial reaction. The diffusion layers of the interface reactions were identified and, to investigate the kinetics of the diffusion layer, the integrated diffusion coefficients were estimated.

Unidirectionally solidified TiAl alloys were prepared by optically-heated floating zone method at growth rates of 10 to 70 mm/h in flowing argon. The microstructures and tensile properties of these crystal bars were found to depend strongly on the growth rate and alloy composition. TiAl alloys with composition of 47 and 50 at.%Al grown under the condition of 10 mm/h showed Ti3Al(α2)/TiAl(γ) layer structures similar to single crystals. As the growth rate increased, the alloys with 47 and 50 at.%Al compositions showed columnar-grain structures. However, the alloys fabricated under the condition of 10 mm/ h had a layered structure, but the alloy with increased growth rate consisted of γ single phase grains. The alloy with a 53 at.%Al composition showed a γ single phase regardless of the growth rate. Room-temperature tensile tests of these alloys revealed that the columnar-grained material consisting of the layered structure showed a tensile ductility of larger than 4 % and relatively high strength. The high strength is caused by stress concentration at the grain boundaries; this enhances the secondary slip or deformation twinning across the layered structure in the vicinity of the grain boundaries, resulting in the appreciable ductility.

Morphology and formation processes of lamellar grain boundaries in titanium rich binary TiAl intermetallics were studied. TiAl alloys containing aluminum content of 44 to 48 at.% were induction-heated to 1723 K followed by helium-gasquenching at various temperatures. For the Ti-44%Al, few lamellae were observed in samples quenched from higher than 1473 K. Although small peaks of beta phase were detected using X-ray diffraction, only the ordered hexagonal phase (α2) with clear APB contrast was observed in TEM observation. For the Ti-48 at.%Al alloy, almost no lamellar structure, and straight grain boundaries were observed in samples quenched from higher than 1623 K. The formation of lamellae along grain boundaries was observed in the sample quenched from 1573 K. The fully lamellar microstructures with serrated boundaries were observed in samples quenched from lower than 1473 K. It was found that the formation of γ platelets took place at higher temperatures in Ti-48 at.%Al than in Ti-44 at.%Al. Although the size of the serration is different, serrated lamellar grain boundaries could be obtained for all alloy compositions employed. The serration appeared to be due to the grain boundary migration induced by precipitation and growth of γ. Differences in transformation characteristics with aluminum content are discussed.

Fundamental studies of microstructural changes and high temperature deformation of titanium aluminide (TiAl) were conducted from the view point of the effect of Al content in order to develop the manufacturing process of TiAl. Microstructures in an as cast state consisted mainly of lamellar structure irrespective of Al content. By homogenization at 1473 K, the microstructures of Ti-49Al and Ti-51Al were transformed into an equiaxial structure which was composed of γ-TiAl, while the lamellar structure that was observed in Ti-46Al and Ti-47Al was much more stable. We found that the reduction of Al content suppressed the formation of equiaxial grains and resulted in a microstructure of only a lamellar structure. On Ti-49Al and Ti-51Al, dynamic recrystallization occurred during high temperature deformation, and the microstructure was transformed into a fine equiaxial one, while the microstructures of Ti-46Al and Ti-47Al contained few recrystallized grains and consisted mainly of a deformed lamellar structure. We observed that on the low-Al alloys the lamellar structure under hard mode deformation conditions deformed as kink observed B2-NiAl. High temperature deformation characteristics of TiAl were strongly affected by Al content. An increase of Al content resulted in a decrease of peak stress and activation energy for plastic deformation and an increase of the recrystallization ratio in TiAl.

Nanostuctured TiAl powder was synthesized by high energy ball milling. A dense nanostuctured TiAl was consolidated using pulsed current activated sintering method within 2 minutes from mechanically synthesized powders of TiAl and horizontally milled powders of Ti+Al. The grain size and hardness of TiAl sintered from horizontally milled Ti+Al powders and high energy ball milled TiAl powder were 35 nm, 20 nm and 450 kg/, 630 kg/, respectively.

The crystal structures and morphologies of precipitates in L10-ordered TiAl intermetallics containing nitrogen were investigated by transmission electron microscopy (TEM). Under aging at an approximate temperature of 1073 K after quenching from 1423 K, TiAl hardens appreciably due to the nitride precipitation. TEM observations revealed that needle-like precipitates, which lie only in one direction parallel to the [001] axis of the L10-TiAl matrix, appear in the matrix preferentially at the dislocations. Selected area electron diffraction (SAED) pattern analyses showed that the needle-shaped precipitate is perovskite-type Ti3AlN (P-phase). The orientation relationship between the P-phase and the L10-TiAl matrix was found to be (001)p//(001)TiAl and [010]p//[010]TiAl. By aging at higher temperatures or for longer periods at 1073 K, plate-like precipitates of Ti2AlN (H-phase) with a hexagonal structure formed on the 111 planes of the L10-TiAl matrix. The orientation relationship between the Ti2AlN and the L10-TiAl matrix is (0001)H//(111)TiAl and H//TiAl.

The research involves the development of a powder metallurgical route for producing good quality TiAl targets for making physical vapour deposition (PVD) coatings. Mixtures of elemental titanium and aluminium powders were mechanically milled using a novel discus milling technique under various conditions. Hot isotropic pressing (HIP) was then employed for consolidation of the mechanically alloyed powders. A cathodic arc vapour deposition process was applied to produce a TiAlN coating. Microstructural examination was conducted on the target material and PVD coatings, using X-ray diffractometry (XRD), X-ray photoelectron spectrometry (XPS) and scanning electron microscopy (SEM). It has been found that combining mechanical alloying and HIP enable us to produce fairly good quality of TiAl based target. The PVD coatings obtained from the TiAl target showed very high microhardness values.

The present work studies the influence of high-energy milling (HEM) and sintering cycle of Ti and Al powders on the obtainment of TiAl. This study shows that HEM modifies the diffusion processes during the sintering stage. The samples were obtained by cold uniaxial and isostatic pressing, pre-sintered at different temperatures, and heated up to the sintering temperature. This study also shows the effect of powder additions processed by HEM on the sintering behavior of elemental Ti and Al powders.

본 논문에서는 Ti-48.5at%Al-0.96at%Mo 조성을 갖는 γ-TiAl 합금은 항온 단조 및 후속 조직제어 열처리 시 발달하는 집합조직의 변화에 관해 연구하였다. 특히, 동적 재결정 후 조직제어 열처리 온도와 시간의 증가에 따라 발생하는 lamellar volume fraction 과 집합조직 변화에 주목하여 관찰하였다. 동적 재결정 후 집합조직은 ND⊥(302) 선분과, TD⊥(100) 및 이에 비해 상대적으로 다소 약한(111) 성분들이 발달하였으며, 열처리 온도와 시간이 증가함에 따라 lameller volume fraction 은 증가했고 동일한 성분의 집합조직도 점차 강하게 발달함을 알 수 있었다. 하지만 상온인장시험 결과는 lameller volume fraction이 증가할수록 낮은 상온 연신을 보였는데, 이는 준 층상조직을 갖는 γ-TiAl합금의 상온인장특성이 집합조직의 영향보다는 lameller volume fractio과 같은 미세조직 특성에 더욱 강하게 영향을 받기 때문인 것으로 판단된다.

Ti 과 AI의 고순도 원소 박판을 이용하여 열간프레스장치에서 고온자전합성법으로 TiAI계 금속간화합물을 제조하였다. 원소 박판에서 TiAl3 금속간화합물을 제조하는 데 승온속도, 압력, 온도 등의 변수가 고온자전합성에 영향을 미치는 중요한 인자다. 특히 승온속도는 반응합성온도를 결정하는 인자로서 본 실험에서 DTA 분석을 이용하여 공정변수를 결정하였다. DTA 분석결과에 따르면, Ti와 AI의 계면에서 반응합성은 AI의 용융점 이하와 이상의 온도에서 두 번 발생함을 알 수 있다. 또한 승온속도가 증가할수록 두 반응합성온도는 증가하였다. 10층의 Ti 박판과 9층의 AI 박판을 20˚C/min의 승온속도로 고온자전합성시킨 후, 810˚C와 240MPa의 압력에서 4시간 동안 열처리한 결과 700μm 두께의 TiAI계 금속간화합물 판재를 제조하였으며, XRD 회절과 SEM으로 확인하였다.

Engine valve-shaped TiAl-Mn intermetallics containing 43.5 to 47.5at%Al (Mn/Al=0.036) are successively fabricated by reactive sintering the elemental powder mixtures near-net shaped by extrusion and die forging. A duplex structure consisted of lamellar grains and equiaxed grains is developed for all compositions, and the areal fraction of the lamellar grains(or equiaxed grains) decreases (or increases) with increasing Al content. As Al content increased, the elongation increases with accompanying decrease in yield strength and ultimate tensile strength at both room temperature and 80. This indicates that the suitable composition is Ti-45at%Al-1.6at%Mn in considering the balance of ambient and elevated tensile properties. The reactive-sintered Ti-45Al-1.6Mn alloy shows superior oxidation resistance not only to the plasma arc melted one but also to the heat resistance steel STR35(representative exhaust valve head material for automotive engine). The reactive-sintered Ti-45Al-1.6Mn alloy coated with an oxidizing scale exhibits a better wear resistance than induction hardened martensitic steel STR11(representative exhaust valve tip material for automotive engine).