Thermodynamics of isolated pores filling with liquid in sintered composite materials
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I.
INTRODUCTION
ISOLATED pores in composite materials are centers of crack initiation and account for low performance of sintered articles. [''2j Therefore, the solution of the problem on formation and overgrowth of isolated pores was studied in a number of experimental and theoretical works. At present, the mechanism and the kinetics of an isolated pore filling with liquid, the removing of vapor from the pore, and the overgrowth of the pore have been studied thoroughly.[3-9] Motive forces of these processes were studied in. II~ The authors have shown that the liquid in the pore is acted upon not only by capillary forces but also by forces which tend to return the liquid from the pore into the composition body. The latter was ambiguously considered as a suction pressure, I~~ a negative pressure, t~jT sphering forces, tl2] etc. The action of these forces was experimentally observed in WC-Ni, Co-Cu, W-Fe, and Fe-Cu compositions, tj~,]2J3] Our investigations have shown that there are compositions (e.g., WC-Cu, Cr3C2-Cu, Ni-Pb, and Cu-Pb) which are free from suction pressure, negative pressure, and sphering forces. Because of this, some scientific conceptions formulated in References 10 through 12 cannot be applied to these compositions. We consider this deficiency to be removable if the thermodynamics of an isolated pore filling with liquid is studied. Experimental and theoretical datat3-~s] provide a good basis for an efficient formulation of the thermodynamic problem of the study. Thermodynamic studies provide an added knowledge of the problem and allow development of a concept of pore filling with liquid which is applicable for all compositions. Thermodynamics considers only the changes in the energy of the system transition from one state into another and does not give any information on the kinetics and the mechanism of the process under study. Because of this, the kinetics and the mechanism of an isolated pore filling with liquid are not considered in the work. A.F. LISOVSKY, Head of a Laboratory, is with the V. Bakul Institute for Superhard Materials of the Ukrainian Academy of Sciences, Kiev, 254153, Ukraine. Manuscript submitted July 26, 1991. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
THEORY
Consider the system which consists of vapor, liquid, and solid phases. A solid phase is presented by highmelting particles which are equal in volume but different in geometrics. They make contact with each other and with liquid to form a skeleton. A liquid phase is presented by a continuous medium which permeats the skeleton and provides proper wetting of particles. This structure adequately simulates compositions which consist of high-melting particles and a binding metal. In the system under study, this structure fills region I, and region II is filled with vapor (Figure 1). The regions are enclosed in a rigid envelope. The composition I in its initial state A has an arbitrary shaped isolated pore which is surrounded with particles and filled with vapor (Figure 1(a)). Two stages of the process of the pore fill
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