Investigation of the reaction zone between TiAl and Mo
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I.
INTRODUCTION
MATERIALS based on the intermetallic compound TiA1 are of practical importance due to their low density, high melting temperature, relatively good mechanical properties, and environmental resistance at elevated temperatures. However, their applications as lightweight high-temperature materials are largely limited by their poor workability and ductility at low or intermediate temperatures. In order to overcome the poor workability of TiA1, a reactive powder processing has been developed. [~-4]Using this processing, near-net-shaped parts can easily be made from an elemental Ti and A1 powder mixture. But, an improvement of the room-temperature ductility has not been achieved with the reactive powder processed TiA1 materials.[5,6] It is known, however, that the problem of poor ductility may be solved by incorporating metallic ductile phases in the brittle TiA1 matrix, as reported by some authors, t7,8] In addition, this measure can also lead to a significant increase in fracture toughness,tg,l~ Keeping possible high-temperature applications of TiAIbased materials in mind, refractory metals such as Nb, Mo, W, etc. should be chosen as ductile reinforcements for TiAI. Attempts using Nb have already achieved quite promising results: large improvements in both monotonic fracture toughness and cyclic fatigue threshold compared to monolithic TiA1 are observed in Nb-particle-reinforced TiA1.t12,13]In contrary, few reports about Mo and W as ductile reinforcements for TiAI have been found in the open literature. The primary purpose of the present study is to verify whether Mo is suitable as a ductile reinforcement of TiA1. Since the nature of the interface between the matrix and reinforcements normally plays an important role for properties of composite materials, the main focus of this article is the interface microstructure between TiAI and Mo.
F.-Y. HSU and H.-J. KLAAR are with GFE, Aachen University of Technology, D-52056 Aachen, Germany. G.-X. WANG is with the Department of Materials Science and Engineering, Zhejiang University, 310027 Hangzhou, People's Republic of China. F. PIRWITZ is with the Institute for Materials Research, GKSS Research Center, 21502 Geesthacht, Germany. Manuscript submitted December 1, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
II.
EXPERIMENTAL
There were two kinds of specimens prepared in this study: the so-called sandwich specimen (Figure 1) and the coaxis specimen (Figure 2). Details of the specimen preparations are discussed in Sections A and B.
A. Preparation of the Sandwich Specimen Figure 1 shows the manufacturing process used for preparation of the sandwich specimen. Elemental titanium and aluminum powders of sizes smaller than 100/zm, with purities of 99.8 and 99.9 pct, respectively, were mixed in air to the desired composition Ti-48 at. pct A1. The powder mixture was precompacted at room temperature using a 1000 kN load to a green compact of 50-mm diameter. This compact was further extruded to a 10 • 20-mm rectangular bar, which was then rolled to a 1.0-mm-thick
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