Coarsening of Cobalt Precipitates in Copper- Cobalt Alloys
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COARSENING OF COBALT PRECIPITATES IN COPPER- COBALT ALLOYS
T.
Eguchi*, Y. Tomokiyo**, K. Oki* and Y. Seno* *Department of Materials Science and Technology, Graduate School of Engineering Sciences and **Research Laboratory of HVEM Kyushu University, *Kasuga-shi 816 and **Fukuoka-shi 812, Japan
ABSTRACT The Ostwald ripening of precipitates in Cu-Co alloys was investigated by electron microscopic observation. The variation of the mean particle radius T, and the distribution of relative particle sizes, f(r/ir , were obtained as functions of the aging time at 873K . The shape of f(rlrwas rather sharp and symmetrical around r =T in the early stage. It became broad with the aging and was similar, in the late stage, to the theoretical distribution of Ardell. It appeared that the shape of f(r1T) depends on the volume fraction but the growth rate of F scarcely depends on it. The rate of change of r was proportional to the cube root of the aging time from the early stage of aging.
INTRODUCTION Precipitate particles in an alloy grow at the expence of smaller ones after the precipitation from the matrix almost completes. This competitive growth, or Ostwald ripening was treated theoretically by Lishitz and Slyozov [1] and Wagner [2]. We will abbreviate this toLSW theory here. Ardell [3] and other investigators [4,5] have modified LSW theory in order to take the effect of the volume fraction of precipitates into account. The experimental studies of the Ostwald ripening have been made mainly on Ni-based alloys [6 - 11]. However, the experimental investigation to clarify the effect of the volume fraction are very scarce. Furthermore, little attention has been paid to the condition of the steady state, whereas the result of LSW theory is valid only when the steady state is attained. In the present paper the coarsening process of precipitate particles in Cu-Co alloy was investigated by means of transmission electron microscopy. The Cu-Co alloy is of interest because many factors such as the composition of precipitates and the lattice misfit between the precipitates and the matrix are different from those of Ni-based alloys. Our attention is focussed on the Co concentration dependence, namely the effect of the volume fraction on the coarsening process, and the aging time dependence of the distribution of particle size. In the following sections the observed distributions of particle size and the growth rate for the mean particle radius are shown. The coarsening process is simulated using the rate equation of LSW theory and compared with experimental results.
EXPERIMENTAL PROCEDURE The Cu-Co alloys with 1.3, 3.07 and 4.30at%Co were prepared from pure Cu and Co in an induction furnace in Ar atmosphere. From now on, we call these alloys Cu-l%Co, Cu-3%Co and Cu-4%Co alloys, respectively. Electron microscopic observation was carried out at 200kV with JEM-200 or JEM-20OBelectron microscope of HVEM LAB, Kyushu University. Precipitates in Cu-l%Co and Cu-3%Co alloys were observed by bright field micrographs taken under the twobeam kinemat
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