In an Fe-5Cr-1Mo-2Cu-0.5P-3C (wt%) alloy prepared by liquid phase sintering at 1120℃ the intergranular liquid films migrate during cooling and isothermal heat-treatment at temperatures where the solid and liquid phases coexist. The liquid film migration (LFM) occurs extensively even during rapid cooling, apparently because of a large driving force. The migrating liquid films solidify to form network carbides. In the regions swept by the migrating liquid films the concentrations of the substitutional solute atoms are slightly different from those in the original grains. When cooled rapidly, martensites are abundant in these regions. It is possible that the driving force for LFM in this alloy stems in part from the C concentration change associated with the concentration changes of the slowly diffusing substitutional solute atoms.
When the mixture of a Fe-base alloy (Fe-5Cr-1Mo-2Cu-0.5P) powder and graphite is heated to liquid phase sintering temperature, liquid phase is produced along the grain boundaries as the C diffuses into the alloy powder particles. The thin films of liquid phase formed along the grain boundaries migrate leaving behind a new solid solution depleted with Cr, Mo, and P and enriched with Si, Cu, and C compared to the original alloy composition. As the amount of the liquid phase increases, the solid/liquid interfaces become unstable and show corrugated morphology. The liquid film migration and interface instability enhance the composition homogenization and dominate microstructural development process at the initial stage of liquid phase sintering.
When the mixture of a commercial prealloyed Fe-powder (Fe-5Cr-1Mo-2Cu-0.5P) and graphite is relatively rapidly heated (＞200℃/min) to and liquid phase sintered at 1160℃, liquid precipitates with various shapes form within solid grains during the initial stage of sintering. The shapes of the liquid precipitate changes with the increment of their size from sphere (with radius ＜ 0.3㎛), a transient polyhedron with more than 7 faces (1-2㎛), cuboid (3-5㎛), and finally to sphere (＞5㎛). The shapes of liquid precipitates closely resemble the growth shapes predicted on the basis of solid-liquid interfacial energy and the coherency strain energy with anisotropic elastic constants in the diffusion zone around the precipitates.
When Fe-X-C (X=Mo,P) alloys sintered in two (γ+1) phase region were heat treated at temperatures lower than those for sintering, chemically induced interface migration (CIIM) was induced, producing new solid solution behind the migrated boundaries. The migration characteristics varied substantially with the composition of specimens and heat treatment temperature. The migration of liquid film and grain boundary and the chemically induced recrystallization (CIR) were observed in the specimens of Fe-12Mo-3.5C (wt%), while, in the specimens of Fe-4Mo-2.8C, Fe-7Mo-2.5C, and Fe-P-C alloys, only the migration of liquid film was observed. The specimen of Fe-12Mo-3.5C showed discontinuous precipitation (DP) when heat treated below 950℃. During the migration of grain boundary and recrystallizing grain boundary liquid droplets were precipitated and moved along with the migrating boundary. These droplets were pronouncedly developed in the specimens heat treated at below 1000℃. As the migration proceeds, the liquid droplets merge into continuous liquid film. These liquid films solidified and remained as network like carbides along the boundaries after cooling. The migration distance increased with heat treatment time. The migration rate decreased with time especially at temperatures lower than 1000℃. The driving force for the migration is believed to arise from the coherency strain produced in the frontal diffusion layer ahead of the migrating boundary. The variation of the solubilities of the solutes (Mo, P, C) in γ solid grain with temperature change led to the production of the coherency strain. The Fe-Mo-C system provides a unique case of interface migration, in which both the substitutional specie Mo and the interstitialcy C having a diffusivity greater than that of Mo in γ by $10^4$-$10^5$ orders of magnitude are involved. The effect of the interstialcy C under such a condition is discussed.
