Spinodal decomposition of a Co-Ti-Nb alloy
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I.
INTRODUCTION
PHASEtransformations in cobalt-rich Co-Ti
alloys have been investigated to some extent. ~L-s~The decomposition of alloys of cobalt with 3, 5, and 9 pct Ti has been investigated by several techniques upon aging between 773 and 1173 K. Precipitation was shown to be preceded by compositional fluctuations along (100) directions, which could be detected by the appearance of satellites in single crystal oscillation pattern^[2] ~ and side bands in powder patterns. ~3]The precipitate phase was T-Co3Ti with the L12-Cu3Au structure. The T-phase had a cuboidal morphology. Thompson t9'~~ established that the early stage of precipitation of Co-Ti alloys (5 to 10 at. pct Ti) occurred through the spinodal decomposition mode. In the later stages of aging, a discontinuous mode of reaction was shown to set in. In the C0-5 and 9 pct Ti alloys, both heterogeneous and homogeneous modes of decomposition were observed at 973 K.t12] Discontinuous precipitation was observed for Co-3 pct Ti at 973 K and for Co-9 pct Ti at 973 and 1073 K. Piearcey et al. H3I have investigated age-hardening in cobalt-rich Co-Nb alloys with 3.8 to 11.4 pct Nb at 1123 K and have identified the precipitate as CosNb 2 with a C36 Laves phase structure. Kokorin and Chuistovt14'~5]examined a 4 pct Nb alloy and observed diffuse satellite peaks about the principal peak in the as-quenched alloy as well as alloys aged at 1023 K for short times. They reported the existence of a new intermediate phase Co3Nb with the ordered L12 (Cu3Au) structure. In a l a t e r r e p o r t [16A7]they interpreted their X-ray diffraction and electron microscopy observations in terms of composition fluctuations in the early stages of aging. They also concluded that two types of precipitation occurred in this alloy. The intermediate metastable precipitating phase C03Nb occurred through a continuous mode of transformation followed by the formation of the equilibrium Co2Nb phase through cellular precipitation. However, no explicit mention of spinodal decomposition in the early stage of aging was made. There appears to have been no 9rior detailed investigation on the effect of a third element J. SINGH, formerly Visiting Assistant Professor, Department of Mechanical and Industrial Engineering, University of Illinois at UrbanaChampaign, 1206 West Green Street, Urbana, IL 61801, is with EMTL, Aircraft Engine General Electric Company, Mail Drop H-99, Cincinnati, OH 45215. C. M. WAYMAN, Professor with the Department of Materials Science, and J. MAZUMDER, Associate Professor with the Department of Mechanical and Industrial Engineering, are with The University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, IL 61801. S. LELE is Professor with the Department of Materials Science and Engineering, Institute of Technology, Banaras Hindu University, Varanasi, India. S. RANGANATHAN is Professor with the Department of Materials Science and Engineering, Indian Institute of Science, Bangalore, India. Manuscript submitted November 25, 1986. METALLURGICALTRANSACTIONS A
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