Particle Coarsening of Dispersoids in Rapidly Solidified Ti-5Sn-3Y
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PARTICLE COARSENING OF DISPERSOIDS IN RAPIDLY SOLIDIFIED
Ti-5Sn-3Y*
Y.Z. Lu and B.C. Giessen, Materials Science Division, Barnett Institute, Northeastern University, Boston, MA 02115, S.H. Whang, Department of Metallurgy and Materials Science, Polytechnic University, Brooklyn, NY 11201
ABSTRACT Rapidly solidified Ti alloys containing rare earth metal dispersoids, such as La or Er, have excellent resistance to coarsening at elevated 0 temperature (700-900 C). In particular, the coarsening of the dispersoid particles in RSP Ti-5Sn-3Y was studied; these particles were found to be Y5 Sn 3 . The measured growth kinetics were found to be compatible with the modified LSW model; it was concluded from the observed parameters that in this alloy the diffusion rate of Y is the rate determining factor, providing a measurement of the diffusion coefficient of Y in a Ti matrix and yielding its activation energy Q = 272 kJ/mole.
INTRODUCT ION To date, the design of Ti alloys has been based on the utilization of solution strengthening and basic microstructural manipulation. This design approach has led, e.g., to the development of Ti-6A1-4V [1]. In recent years, new approaches going significantly beyond the conventional design practice have been taken to raise the high temperature 0 capability of Ti alloys further, to 700-1000 C. In one of these approaches, new coarsening-resistant dispersions are introduced into the Ti matrix using rapid solidification processing [2-5]. It was found by trial and error that rare earth metal dispersoids, such as La or Er, have excellent coarsening resistance in Ti alloys at elevated temperatures 0 (700-900 C) [6-7]; by contrast, silicide particles were observed to coarsen rapidly [8]. A preliminary study of the Ostwald ripening behavior of La dispersoids had been reported [6]; however, there is no similar study for other rare earth dispersoids or rare earth compounds in Ti and the particle coarsening mechanism for rare earth dispersoids in Ti is not yet understood. In this paper, the coarsening of yttrium-containing dispersoid particles in Ti-5Sn-3Y alloys is reported. (Here and in the following, percentages are in weight percent).
EXPERIMENTAL PROCEDURE For the present experiments, alloy buttons were prepared from Ti(99.99%) and Y(99.9%) by repeated melting in an arc furnace under argon gas atmosphere. The oxygen level of the Ti used is < 150 ppm. Small alloy pieces from the buttons were splat quenched into thin foils by the hammer and anvil technique. Disk shaped specimens of 20 gm thickness were obtained from the central section of the foil. For heat treatment, samples were wrapped in Ta foil and sealed in quartz tubing with additional Ti foil as getter material under a vacuum of 10-5 torr. The tubes were annealed isothermally for the desired temperatures and times. In order to identify the chemical composition and crystal structures of the dispersoids in the heat treated alloys, carbon extraction replicas prepared from the annealed foils were studied by STEM (HB5) and electron
Mat. Res. Soc
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