Coarsening of cobalt grains dispersed in liquid copper matrix
- PDF / 829,232 Bytes
- 5 Pages / 594 x 774 pts Page_size
- 15 Downloads / 275 Views
INTRODUCTION
THE theory of Ostwald ripening by Lifshitz, Slyozov, ~and Wagner 2 assumes that the distance between the particles dispersed in the matrix is infinite. The effect of varying interparticle distance has been theoretically examined by Ardell, 3 Brailsford, and Wynblatt4 still assuming a system of relatively well dispersed particles. On the other hand, in the alloys prepared by liquid phase sintering--such as, for example, W-Ni-Fe heavy metal--volume fraction of the liquid matrix is typically less than 20 pct, and thus the solid grains are in contact with each other. Therefore, their structure deviates considerably from the theoretical models. When the alloy composition is altered to increase the liquid fraction and thus the interparticle distance, it was observed that the solid grains tended to sink or float because of the density difference from the liquid matrix) In the Co-Cu alloy, however, it is possible to disperse the spherical Co-rich grains uniformly in the Cu-rich liquid matrix, because densities of the two phases are nearly equal. The interparticle distance can be also varied by changing the Co-Cu ratio while maintaining the well dispersed grain structure. Therefore, Co-Cu alloy is an ideal system for testing the validity of various theoretical models in describing the coarsening of solid grains in liquid matrix. In this investigation, the experimental results of grain coarsening in Co-Cu alloy at the liquid phase sintering temperatures are compared with the theoretical predictions on the basis of the form of the rate law, the CHAN HYOUNG KANG, formerly a student in the Department of Materials Science at the Korea Advanced Institute of Science, is presently at the Korea Institute of Science and Technology. DUK N. YOON is a Professor in the Department of Materials Science at the Korea Advanced Institute of Science, P.O. Box 150 Chongyangni, Seoul, Korea. Manuscript submitted May 12, 1980. METALLURGICAL TRANSACTIONS A
grain size distribution, the rate constant, and its variation with composition and temperature. EXPERIMENTAL PROCEDURE The experimental procedure was similar to that described earlier)The alloy specimens were prepared from commercial grade Co and Cu powders of 1.25 and 4.8/~m, respectively, by Fisher Subsieve Sizer. The specimen compositions were 30 pct Co-70 pct Cu, 40 pct Co-60 pct Cu, and 50 pct Co-50 pct Cu by weight. The mixed powder was compacted under 15 MPa pressure to cylindrical specimens of 10 mm in diam and 5 to 8 mm in height; the green density was 40 to 45 pct. The compacts were annealed in a tube furnace under flowing hydrogen atmosphere. They were pushed into the furnace at about 15 ~ to 1100 ~ and then rapidly--in about 30 s--to the annealing temperature. After annealing, the specimens were pulled out rapidly to 1100 ~ The polished specimen surfaces were etched in H202 solution. Photomicrographs were taken at five or six different regions on the horizontal and vertical cross sections. After magnification, the average grain diameter was 10 to 15 mm and linear interc
Data Loading...
