Grain boundary sliding and component shape distortion during liquid-phase sintering
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I. INTRODUCTION
THE production of components by liquid-phase sintering with desirable properties and precise dimensions requires microstructure control through the sintering cycle. Previous studies[1–6] have established linkages between the component’s structural rigidity and microstructure features. Key microstructural parameters are the volume fraction of solid, dihedral angle, grain coordination number, solid contiguity, and grain size. Upadhyaya[4] observed that compacts slumped after liquid-phase sintering also exhibited microstructures with a grain connectivity (two-dimensional) less than 3. Alternatively, investigations on shape distortion during liquid-phase sintering show a solid contiguity over 0.38 is needed to create a solid skeleton, a necessary condition to avoid distortion.[5,6] Both contiguity and connectivity provide information on the solid skeleton microstructure in liquid-phase sintering. Our prior research on shape distortion during liquid-phase sintering used small cylindrical samples with the option of microgravity.[1–6] As part of our research, a 10-mm-high, cylindrical compact with a composition of 78 wt pct W, 15.4 wt pct Ni, and 6.6 wt pct Fe was sintered on the space shuttle Columbia (flight STS-65 in July 1994) at 1500 ⬚C for 120 minutes with a peak acceleration of 3⭈10⫺5 m/s2.[6] After sintering, the sample was spherical, illustrating that without gravity there was insufficient solid skeleton to retain shape. However, ground-base sintering of a similar compact under the same conditions produced a less distorted “elephant foot” geometry. Since surface tension is a weaker force when compared to gravity, it is evident that gravity plays a role in inducing grain contacts that favorably retard distortion. When liquid-phase sintering is performed on a large compact or one with cantilevered sections, gravitational forces can induce distortion even in high contiguity systems. High stress regions undergo shape distortion, even if the solid contiguity JIANXIN LIU, Senior Materials Engineer, is with the Prometal Division, Extrude Hone Corporation, Irwin, PA 15642. RANDALL M. GERMAN, Brush Chair Professor in Materials, and Director, The Center for Innovative Sintered Products, is with the P/M Lab, 147 Research West, The Pennsylvania State University, University Park, PA 16802-6809. Manuscript submitted June 7, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
is greater than 0.38, because the local stress induces grain sliding at the grain contacts. The strength of a liquid-phase sintering system varies directly with a parameter measuring the percolated solid grain structure, known as the backbone fraction. According to the percolation theory, the backbone fraction depends on the microscopic details of the system. At the percolation threshold, the backbone fraction is almost zero. To relate percolation concepts to liquid-phase sintering, note the solid phase is nearly engulfed in the liquid, while contacts between the solid grains provide rigidity if the solid skeleton percolates through the stru
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