Intermetallic compound TiAl has been expected to be one of the most promising candidates for high-temperature structural materials to replace conventional titanium alloys and nickel-based superalloys. This is because the TiAl has an attractive properties such as low density, high melting point, high strength-to-weight ratio and excellent creep resistance relative to conventional superalloys and titanium-based alloys. But the poor ductility at ambient temperature must be overcome for the practical use of this compounds. In recent, therefore, there have been enormous research works to improve room temperature ductility through microalloying and microstructural control. As a result, in the last few years marked improvement in the mechanical properties of two-phase TiAl compounds have been achieved mainly through appropriate alloying and controlling their microstructure by thermomechanical processing. However, such treatments have tendency to reduce the desired mechanical properties such as toughness and creep resistance. Therefore, there has been a great demanding to develop new tow-phase alloys and fabrication processes in order to obtain the balanced mechanical properties.
Nowadays research interest has been focused on the two-phase Ti-rich TiAl compounds with the fully lamellar structure which are beneficial for toughness and high-temperature strength. Directional solidification (DS) processing of two-phase Ti-rich TiAl alloys with fully lamellar structure has great possibilities to avoid inverse relations between tensile properties and fracture/high-temperature capabilities, and to take advantage of the anisotropic nature of the lamellar microstructure. If the lamellar orientation can be aligned parallel to the growth direction, the combination of strength and toughness would be optimized in the resulting microstructure consisting of columnar grains with the lamellar orientation aligned parallel to the growth direction. The main difficulty in the lamellar microstructure control of the two-phase Ti-rich TiAl alloys by DS processing is that the lamellar microstructure is not formed from the liquid but from the solid state.
There are two ways to control the lamellar boundary orientation by the DS process: one is β-solidification technique without seeding procedure and the other is seeding technique. Lee et al. have reported that the lamellar orientation could be successfully controlled parallel to the growth direction by the DS with a seeding technique, and their room temperature tensile properties as well as creep properties were dramatically improved. However, it is difficult to apply the seeding technique to practical applications, such as turbocharger wheels, engine valves and NNS (near-net shape) forming of DS ingot. In the method of the β-solidification technique, the only requirement is the full transformation of TiAl alloy to bcc β phase from the liquid. Therefore, selection of the alloy composition which transforms to the fully β phase from the liquid is one of the most important factors in controlling the lamellar boundary direction by the modified solidification procedure and it could be determined from the phase diagram in the Ti-Al system. However, phase diagrams for the Ti-Al-X system around melting temperature are not clear. Basic data for the high temperature phase diagram needed to determine the alloy composition of β solidification have not been established.
Hence, in this study, in order to obtain an accurate phase diagram around the alloy composition of β solidification, the microstructure and the compositions of liquid/solid interfaces in Ti-44, 46, 48, 50, 52at.%Al alloys instantaneously quenched during DS were analyzed, and based on the results, the types and the transformation compositions of the phase at the beginning of solidification were confirmed. With direct measurement of the liquid, liquid/solid interface, solid phase temperatures during DS process, liquidus temperatures of selected compositions were inspected. At the same time, liquidus temperatures were verified by DTA. Considering all the results together, a high temperature phase diagram for Ti-44, 46, 48, 50, 52at.%Al alloy compositions was established. And also, in order to maintain the Ti/Al atomic fraction at 53/47 52/48, the β stabilizers, such as Mo, Re and W, were added as ternary elements. Partial liquidus projections in ternary Ti-Al-X (X=Mo, Re and W) alloy systems were investigated to identify the β stabilizing effects. On the basis of the partial liquidus projections completed in this study, the Ti-47Al-2W (at.%) alloys, which was thought to fully transform to the β phase at the solid/liquid interface, were directionally solidified by using a Bridgman type and a floating zone (FZ) type DS apparatus at the growth rate of 30, 90 and 270 mm/h. The microstructure of the DS ingots was analyzed by an optical microscope (OM), a scanning electron microscope (SEM) and a transmission electron microscope (TEM) and the mechanical properties were evaluated by a tensile test at room temperature and 800℃.
In order to control the lamellar orientation of TiAl alloy by β solidification technique, bcc β dendrite must grow to β direction at the beginning of DS process. However, it is very difficult to directly observe the growth of bcc β dendrite, because it grows at high temperature near a melting point. In this study, therefore, the orientation of γ lamellae in the DS specimens was determined by analyzing the Kikuchi diffraction patterns obtained from an electron backscattered diffraction (EBSD) detector in SEM and by TEM. On the basis of the γ lamellar orientation analysis, the preferred growth direction of bcc γ dendrites could be estimated. By observing the inside of the cavity, which is formed by quenching during DS by FZ type apparatus, the beginning procedure of the solidification at high temperature was also discussed.
In the summary, in order to control the lamellar orientation of TiAl alloys to be parallel to the growth direction by DS process, the selection of β solidification composition, the DS apparatus with high temperature gradient and the adequate growth rate, etc., are thought to be essential factors. Besides, when the TiAl alloy with β solidification composition is directionally solidified and the temperature gradient of the DS apparatus is given by a certain value, it is thought that, at the low growth rate, β solidification will be no more valid because of seeding effects which is due to the planar interface growth, and that β dendrite will grow to β direction at too high growth rate, so the microstructure control will be impossible. Therefore, to control the lamellar orientation of TiAl alloys by β solidification technique, the DS must be conducted at the proper growth rate at which β dendrite can grow stably to β direction.