Microstructural development and phase transformation in the liquid-phase and the solid-state sintering

Abstract

Microstructural development and phase transformation have been studied in liquid-phase and solid-state sintering. Since all physical properties of the materials are related to their microstructure, understanding and thereby controlling the microstructure is essential for the improvement of the properties of the materials. In part one, the development of the liquid pocket within Fe grains in the liquid-phase sintered Fe-Cu alloy was studied. Since the grain boundary breaking away from the dihedral junction is energetically impossible during liquid-phase sintering, the formation of the liquid pocket was attributed to the solid-state grain boundary migration during heating, and was experimentally verified. The specimens prepared to have favorable conditions for extensive grain boundary migration in the solid-state (such as high compacting pressure and slow heating rate) contained a much larger number of the liquid pockets than those normally prepared. Two mechanisms were suggested for the formation of the liquid pocket. One is the solid-state trapping of Cu by the Fe grain during heating. The other is the development of the C-shaped Fe grain in the solid-state, which eventually leads to the formation of liquid pockets during liquid-phase sintering by solution and reprecipitation. This is because the interior part of the C-shaped Fe grain has a negative curvature. In part two, the massive transformation of the Fe grain in the liquid-phase sintered Fe-Cu alloy was studied. The non-oriented growth of the massive ferrite was observed regardless of slow cooling (even at 0.1℃/sec). The typical mode of massive growth was also observed in a specimen held for 48 hours at 810℃. In this case, the driving force for the massive transformation did not exist for the initial composition of the Fe particle. The continuous precipitation of Cu-rich epsilon phase was regarded to be responsible for such an isothermal massive reaction. During isothermal holding at 830℃, however, the ...