Effect of microgravity on grain coarsening during liquid phase sintering in the Fe-Cu system

Samples within the Fe-Cu system with three different volume fractions of solid (50, 60 and 70 vol % Fe) and four different sintering times (2.5, 5, 17 and 66 min) were liquid phase sintered (LPS) in microgravity. Particle coarsening during LPS is generally known to increase with increasing volume fraction of solid. Contrary to expectations, there was an enhancement in particle coarsening with a decrease in the volume fraction of solid. The agglomerated microstructures observed in these samples (especially those with a lower volume fraction of solid) also exhibited a higher grain growth constant consistent with their higher 3D coordination number. The relevant analysis discussed in this paper strongly suggests that agglomeration is promoted by Brownian motion that dominates any density-driven force in the absence of gravity. The observed particle growth characteristics were in excellent agreement with the Lifshitz-Slyozov encounter modified theory, which incorporates the effect of higher solid volume fraction and particle coalescence into the LSW theory. The particle distributions appear to remain unchanged with processing time beyond 2.5 min, suggesting thereby, that agglomeration promotes an equilibrium particle configuration early on in the process and enables scaled grain growth with time.