Effect of liquid content on distortion and rearrangement densification of liquid-phase-sintered W-Cu
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I. INTRODUCTION
THE lack of solubility between tungsten and copper enables composites of these materials to be processed with useful combinations of properties for electrical contacts and thermal management materials.[1–14] Composites with 10 to 20 wt pct (19.3 to 35.1 vol pct) Cu are most commonly used for thermal management, but the limited solubility of W in liquid Cu makes liquid phase sintering of these compositions to full density difficult. Submicron W powders and intimate mixing techniques, such as mechanical alloying,[13,15–21] co-reduction of oxides,[22–27] or other thermochemical processes[28] are required to achieve pore-free components via liquid phase sintering. Such conditions promote solid-state sintering of the W skeleton in the presence of liquid Cu, which has been shown to sufficiently account for the experimentally observed densification behavior.[29] At copper contents above 20 wt pct (35.1 vol pct), the amount of liquid phase is theoretically sufficient to achieve full density through particle rearrangement alone;[30] however, even in these cases, lower densities have been observed for W-Cu.[22,31] Another unique aspect of the W-Cu system is a lack of distortion with liquid contents as high as 80 vol pct.[32,33] Densification and distortion are two separate processes with distortion delayed until densification is practically complete. Rigidity during densification is attributed to the capillary forces arising from the presence of pores.[34,35] Even at high liquid volume fractions, residual porosity in liquid-phase-sintered W-Cu may provide sufficient capillary forces for structural rigidity. However, the inability of these capillary forces to fully eliminate porosity at liquid volume fractions of 0.35 to 0.80 has not been fully explained. JOHN L. JOHNSON, Senior Research Associate, JUSTIN J. BREZOVSKY, Undergraduate Student, and RANDALL M. GERMAN, Brush Chair Professor in Materials and Director, are with the Center for Innovative Sintered Products, The Pennsylvania State University, University Park, PA 16802-6809. Contact e-mail: [email protected] Manuscript submitted November 24, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
In general, densification during liquid phase sintering occurs through a combination of particle rearrangement due to capillary forces, grain shape accommodation by solution reprecipitation, and sintering of the skeletal structure by solid-state diffusion.[36,37,38] For W-Cu, the low solubility of W in Cu makes the contribution of solution reprecipitation to densification negligible, so densification can be attributed to either rearrangement or solid-state sintering. However, densification by rearrangement may also be restricted by bonds between the W particles that form during heating. In conventional tungsten heavy alloy systems, these bonds dissolve when the liquid phase forms,[39] but in W-Cu, they may stay intact due to their limited solubility. A rigid W skeleton can form even at compositions with 40 wt pct Cu as confirmed by dissolution of the copper matrix with
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