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I.

INTRODUCTION

THE most distinguishable change of microstructure from the intermediate to the final stage in solid-state sintering is the collapse of interconnected open pores into isolated or closed form. This pore closing process is important in the analysis of later stage sintering behavior because of its large influence on the densification and grain growth kinetics. It,21 Several experiments have been conducted to demonstrate the instability of long cylindrical pores intentionally introduced in the solid matrix, t3,4'51 The classical perturbation theory proposed by Rayleigh ~6t and others 131 was used to predict pore closing conditions. However, in real systems of fine powder compacts, the understanding of the process leading to the development of closed pores is quite poor because of structural nonunifortuity and complex material transport behavior. Thus, in a previous article t7J in which we considered an external diffusion (densification) field affecting the instability of pore morphology, we suggested that the pore closing kinetics can be much different from those associated with a Rayleigh instability. 161 In the present investigation, pore closing behavior is directly observed in the W-Ni system, for which the sintering mechanism is well known to be the rapid diffusive flow of matrix atoms through additive Ni-rich grain boundary. [8,9J The objective of this study is to obtain a better understanding about the effect of the external diffusion field on the collapse of interconnected open pores. II.

EXPERIMENTAL PROCEDURES

The samples were prepared by conventional powder metallurgy techniques, t~~ To decrease the sintering rate, a relatively coarse W powder of approximately 5/xm in size was used. Nickel was added as a sintering activator, and its content was varied from 0.03 to 0.1 wt pct. For JOON-WOONG NOH, SUNG-SOO KIM, Senior Researchers, Materials Research and Development Division, and KIL-SUNG CHURN, Director, are with the Advanced Technology and Research Center, Agency for Defense Development, Taejon 300-600, Korea. Manuscript submitted July 15, 1991. METALLURGICAL TRANSACTIONS A

a uniform distribution of the activator, the salt NiC12 96H20 was slurry mixed with preweighed W powder in methyl alcohol. The evaporation of alcohol at about 80 ~ leaves a coating of salt on the W powder surface. Reducing the salt for 2 hours at 800 ~ in a hydrogen atmosphere provided the W powder coated with Ni. This W powder was pressed under 100 MPa into cylindrical compacts, 10 mm in diameter and about 6 mm in height. The compacts were sintered at 1300 ~ under flowing hydrogen. The heating and cooling rates were approximately 160 ~ The density and closed porosity of sintered specimens were determined by an immersion method described in the ASTM specification C20-83. Scanning electron micrographs were taken of fractured surfaces to analyze the pore and grain morphology. Dihedral angle measurements were carried out on polished sample surfaces. For each sample, about 100 pore surface-grain boundary junctions were meas