6.7 Microstructural Changes

When a green compact is sintered, the original particle boundaries can no longer be observed. Instead, the structure becomes similar to that of the metal in wrought and annealed conditions, except that it contains pores. Pores are of two types open or closed. With progression of sintering, pores continue to shrink. At about 5% total porosity, the formation of closed pores commences. Many of the microstructures seen in P/M parts are caused by porosity and by the blends of elemental powders that constitute many alloys. These blends do not always result in homogeneous, well diffused structures. Such heterogeneity is not necessarily detrimental and in certain nickel steels and diffusion alloy steels, may be advantageous.

The heating or annealing of cold worked metals leads to three distinct micro structural changes: recovery, recrystallization and grain growth. Grain growth is the most important one for sintering. There is normally a large driving force for grain growth. Grain growth in a sinter mass can be somewhat different than the conventional metal, since the former can be treated as a two phase material, the second phase being pore. Porosity in green compacts and in the developing sinter mass represents a very effective hindrance to grain growth. The addition of other component in the powder blend may drastically hinder bonding between adjacent particles and the formation of grain boundaries. Grain boundary grooves, may also tend to inhibit grain growth, since the movement of grain boundary away from its groove leads to an increase...