Myung-Hoon Oh

Composition and growth rate effects in directionally solidified TiAl alloys

Abstract Effects of alloy compositions and growth rates on the microstructure and the deformation behavior of fully lamellar TiAl alloys were systematically investigated by directional solidification (DS) techniques. Some β stabilizing elements, such as Mo, Nb and Cr, have been added in order to increase the β phase field in the binary system. It was found that the lamellar orientation was aligned nearly 0–45° to the growth direction in Ti–46Al–2Nb and Ti–46Al–2Cr(at.%) DS ingots grown at the growth rate of 90 mm h −1 . Therefore, it could be concluded that there is a good possibility to control the lamellar orientation by adding some β stabilizing elements, such as Mo, Nb and Cr, in the TiAl system. In the Ti–47.5Al–2.5Mo DS alloy, the lamellar boundary was nearly perpendicular to the growth direction at growth rates of 180 and 360 mm h −1 , however, it was nearly parallel to the growth direction at the growth rate of 90 mm h −1 . These results indicate that the lamellar boundary orientation of DS ingot has been affected by the growth rate as well as the alloy composition.

Microstructure control of TiAl alloys containing β stabilizers by directional solidification

Abstract Phase transformation at elevated temperatures in Ti-Al-X(X=Mo, Re, W) systems has been investigated by analyzing the dendrite morphology of arc-melted buttons. It was found that the addition of β stabilizers, such as Mo, Re and W, shifted the β phase field at liquid/solid temperatures to the high Al composition side. So, the β phase formed as a primary crystal even in higher Al compositions in Ti-Al-X ternary systems compared with Ti-Al binary. The addition of W was found to be the most effective β (bcc) stabilizer among Mo, Re, W, and the Ti-47Al-2W composition was selected for β solidification. In the directional solidification of Ti-47Al-2W alloy at the growth rate of 90 mm/h using the Bridgman type DS apparatus, it was found that the lamellar orientation in the columnar grains was parallel or inclined with the angle of 45° to the growth direction. This means that the β phase forms at the solid/liquid interface in the Ti-47Al-2W alloy. The tensile elongation of DS alloys was clearly improved compared with that of the polycrystalline alloy with a fully lamellar microstructure, while maintaining the same yield stress level.

High temperature phase equilibria near Ti–50 at% Al composition in Ti–Al system studied by directional solidification

Abstract High temperature phase diagram near the stoichiometric composition of TiAl has been established by the directional solidification and quenching technique. The quenched dendrite morphologies showed that the first solidified phase was the β phase in Ti–44, 46, 48 at% Al alloys and the α phase in Ti–50, 52 at% Al alloys. From the EDS analysis of the quenched dendrite tips and measurement of the temperature gradient directly recorded during directional solidification in Ti–(44–52 at%)Al alloys, the solid-liquid phase equilibria could be determined. The phase transformation temperatures were also confirmed by DTA. The phase equilibria established in this study agreed with the phase diagram that Okamoto proposed, while the β+γ two phase region, which Murray suggested, was not found near the TiAl composition. The lamellar orientation in TiAl alloys has been reported to be controlled in the growth direction in the presence of the primary β phase from a liquid. A composition in which the liquid phase was fully transformed to the β phase was selected, Ti–44 at% Al alloy, and directional solidification was performed at the growth rate of 45 mm/h. It was found that the lamellar orientation was aligned at nearly 0° and 45° to the growth direction. It is thought that the success in controlling the lamellar orientation is due to β solidification of the Ti–44 at% Al alloy at the beginning of liquid/solid transformation.

Effects of Al–21Ti–23Cr coatings on oxidation and mechanical properties of TiAl alloy

Abstract Ti–48Al specimens were coated with Al–21Ti–23Cr film of approximately 10 μm thickness at 200 W, 0.8 Pa and 300 °C using RF magnetron sputtering. The oxidation behavior of the coated specimens was investigated through isothermal and cyclic oxidation tests, and the effect of coating on the mechanical properties of TiAl alloy was, in particular, assessed by means of focusing on the tensile properties, which also provided the key to investigate the adhesion between TiAl substrate and Al–21Ti–23Cr coating and between Al–21Ti–23Cr coating and Al 2 O 3 scale. The isothermal and cyclic oxidation curves showed that the Al–21Ti–23Cr coating was very effective in improving the oxidation resistance of Ti–48Al at 1000 °C. This excellent oxidation resistance is attributable to the formation of a protective Al 2 O 3 scale on the surface of the Al–21Ti–23Cr film. Although extensive cracking in a transverse direction was observed on the surface of the film, the delamination of the coating from the substrate and the spallation of the Al 2 O 3 scale were not found. From the tensile test, it was confirmed that the Al–21Ti–23Cr coating enabled Ti–48Al to maintain its tensile properties regardless of exposure to oxidizing atmosphere. In addition, the result of the microhardness test indicated that the Al–21Ti–23Cr coating was very effective in suppressing the embrittlement of the Ti–48Al when exposed at 1000 °C. Thus, it could be suggested that the Al–21Ti–23Cr coating provides the key, not only to improve oxidation resistance, but also to maintain the mechanical properties of TiAl alloy.

Directional solidification of TiAl-Si alloys using a polycrystalline seed

Abstract Directional solidification (DS) process of Ti–43Al–3Si alloys has been studied. Successful DS ingots having not only fully lamellar microstructure parallel to longitudinal axis but also rotated columnar grains with respect to longitudinal axis were obtained using a polycrystalline seed material. Successful seeding and growing require plane-front solidification condition during the entire DS process. Fracture toughness of the DS alloys were superior to the PST alloys, with a value of K Q =21.7–31.7 MPa (m) 1/2 for the crack arrest/divide mode and K Q =7.4–19.0 MPa (m) 1/2 for the short transverse mode. The orientation dependence of fracture toughness for the crack arrest/divide mode was improved in the DS alloy compared to the PST alloy.

A composition window in the TiAl-Mo-Si system suitable for lamellar structure control through seeding and directional solidification

Abstract The thermal stability of the lamellar microstructure of cast Ti–46Al–1.5Mo–(0–3)Si (at.%) alloys was investigated to find suitable seed compositions for growing ingots of these alloys with the aligned lamellar microstructure through seeding and directional solidification. In order for the seeding to be made successfully, the original orientation of the lamellar microstructure must be restored upon heating to and cooling from the melting temperature. In this study, the lamellar stability was determined by examining whether or not the lamellar structure in the as-cast alloys is preserved after quickly heating to just below melting temperature, holding, and then cooling to room temperature. This requirement was found to be fulfilled for the Si contents larger than 1.0 at.%. Ti–46Al–1.5Mo–1Si and Ti–46Al–1.5Mo–1.5Si (at.%) alloys are typical examples of alloys which can be used as seed materials. Directional solidification experiments of Ti–46Al–1.5Mo–1Si alloy were performed using Ti–46Al–1.5Mo–1.5Si alloy as a seed material and ingots with the aligned lamellar microstructure were obtained.

Microalloying Effects in TiAl+Mo Alloys

Abstract Ti–48.5Al–1.5Mo (at.%) and Ti–48.5Al–1.5Mo–X (X=0.3, 1.0 and 2.0C or N) alloys were prepared by vacuum arc melting and homogenized at 1390°C for the lamellar microstructure and then aged at 900°C. It was found that C (or N) additions decreased the grain size of the lamellar structure. In particular, 1.0 and 2.0%C (or N) additions refined the grain size to 1/10 of that of the mother alloy. Tensile elongation at room temperature decreased for the alloys with 0.3%C (or N) but recovered with 1.0% and 2.0%C (or N) additions compared to the value of the mother alloy. The yield stress at room temperature for these alloys containing 1.0% and 2.0%C (or N) increased up to twice that of the mother alloy, and this high stress was maintained up to 800°C. The aged alloys showed much higher tensile elongation at room temperature than did the alloys with only heat-treatment. Moreover, the alloy with 1.0%N added exhibited better creep resistance than did the mother alloy, in spite of its much smaller grain size.

Effect of nitrogen on the mean lamellar thickness of fully lamellar TiAl alloys

The effect of nitrogen addition in the 0–1.0 at.% range on the mean lamellar thickness of a fully lamellar TiAl alloy (Ti–48.5Al–1.5Mo) has been studied. Mean lamellar thickness first decreases with 0.3 at.% N addition but then increases with further increase in N level in the alloy. The increase in lamellar spacing for N levels between 0.3 and 1.0 at.% is attributed to the formation of Ti2AlN nitrides and the consequent change in Al level in the matrix. 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Lamellar boundary alignment of DS-processed TiAl−W alloys by a solidification procedure

In this study, a β solidification procedure was used to align the lamellae in a Ti-47Al-2W (at.%) alloy parallel to the growth direction. The Bridgman technique and the floating zone process were used for directional solidification. The mechanical properties of the directionally solidified alloy were evaluated in tension at room temperature and at 800°C. At a growth rate of 30 mm/h (with the floating zone approach), the lamellae were well aligned parallel to the growth direction. The aligned lamellae yielded excellent room temperature tensile ductility. The tensile yield strength at 800°C was similar to that at room temperature. The orientation of the γ lamellar laths in the directionally solidified ingots, which were manufactured by means of a floating zone process, was identified with the aid of electron backscattered diffraction analysis. On the basis of this analysis, the preferred growth direction of the bcc-β dendrites that formed at high temperatures close to the melting point was inferred to be [001]β at a growth rate of 30 mm/h and [111]β at a growth rate of 90 mm/h.

Fabrication of GDL microporous layer using PVDF for PEMFCs

The Gas Diffusion Layer(GDL) of fuel cell, are required to provide both delivery of reactant gases to the catalyst layer and removal of water in either vapor or liquid form in typical PEMFCs. In this study, the fabrication of GDL containing Micro Porous Layer(MPL) made of the slurry of PVDF mixed with carbon black is investigated in detail. Physical properties of GDL containing MPL, such as electrical resistance, gas permeability and microstructure were examined, and the performance of the cell using developed GDL with MPL was evaluated. The results show that MPL with PVDF binder demonstrated uniformly distributed microstructure without large cracks and pores, which resulted in better electrical conductivity. The fuel cell performance test demonstrates that the developed GDL with MPL has a great potential due to enhanced mass transport property due to its porous structure and small pore size.