Interdiffusion Between Mo and TiAl
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INTERDIFFUSION BETWEEN Mo AND TiAI
Min-Xian Zhang, Ker-Chang Hsieh* and Y. Austin Chang Department of Materials Materials Science and Engineering, University of Wisconsin, Madison, WI 53706 *On leave from Institute of Materials Science and Engineering, Sun Yat-Sen University, Kaohsiung, Taiwan, ROC ABSTRACT Diffusion couple experiments were carried out for Mo/y-TiAl at 900, 1000 and 1 100*C for periods of time ranging from 121 to 553 hrs. Using the Boltzmann-Matano analysis, the two diagonal interdiffusion coefficients for the two phases formed in these couples were obtained assuming the cross diagonal terms to be negligible. These two phases are 8-(Mo,Ti) 3 Al and Jý-(Mo,Al)Ti. The three intrinsic diffusion coefficients were also obtained using the Darken-type relationships between the interdiffusion and intrinsic diffusion coefficients. The calculated interdiffusion coefficients from the three intrinsic diffusion coefficients are in reasonable accord with those obtained directly from the Boltzmann-Matano analysis. INTRODUCTION Composite materials to be used as high-temperature structural components are becoming an important class of engineered materials. One such system which has been proposed consists of an intermetallic compound as the matrix and an oxide fiber as the reinforcement. An example is a composite of y-TiAl and sapphire, which are in thermodynamic equilibrium with the exception of oxygen solubility in y. A recent study on the fracture resistance of metal/ceramic/intermetallic interfaces by Evans, Bartlett, Davis, Flinn, Turner and ReimanisI reported tLat y-TiAI/A12 0 3 interfaces are resistant to debonding2 . According to Evans et al. 1, brittle debonding of metal/ceramic interfaces has been observed only for Mo/Al 2 0 3 3 and W/AJ2 0 3 1. On the basis of these observations, if one were to fabricate a y-TiAl/A12 0 3 composite, the sapphire fibers need to be coated with Mo in order to enhance interfacial debonding during mechanical deformation. 3ut on the other hand, there is a need to know the chemical compatibility between the Mo coating and the Y-TiAI matrix. The primary objective of the present study is to determine the extent of interactions between Mo and y-TiAI at 900, 1000 and 1100*C. Experimental Method Diffusion couples of Mo/y-TiAl were made by sputter deposition of -300 Pim Mo on well-characterized y-TiAl. The y-TiAI substrate was under constant rotation during the deposition process with the substrate temperature kept at about 870*C. The total sputter time was approximately 55 hrs. The diffusion-couple samples sealed in evacuated quartz capsules were annealed at 900, 1000 and 1100 0 C. The samples were sealed under a vacuum of -10-3 Torr. However, they were evacuated and then back-filled with high purity argon several times before sealing. In order to prevent interaction between the samples and the quartz capsules, the surfaces of Mat. Res. Soc. Symp. Proc. Vol. 273. 01992 Materials Research Society
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the samples as well as the inside surfaces of the capsules were coated with Y20 3.
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