The Structural Evolution and Densification Mechanisms of Nanophase Separation Sintering

Nanophase separation sintering (NPSS) facilitates low temperature, pressureless sintering through the formation of solid phase necks driven by phase separation. Systems that have been shown to exhibit this phenomenon are W–Cr, Cr–Ni and to a lesser degree Ti–Mg. Initial information on the average rate-limiting sintering kinetics in these systems was obtained using traditional master sintering curve analysis, but it is very clear that multiple processes occur during NPSS, and these should each have their own characteristic kinetics. Here we analyze these three systems in greater kinetic detail using densification rates in a Kissinger-style analysis derived explicitly for densification data. For the W–Cr and Cr–Ni systems two critical temperatures were identified: one at low temperatures for the formation of the secondary phase necks, and a second one at high temperatures corresponding to the onset of rapid densification. The activation energies of these processes are different, and reflective of bulk solute diffusion and interdiffusion, respectively. Combined with microstructural observations, these data show that the onset of rapid densification at high temperatures is facilitated by the presence of the second-phase necks, and occurs at the point where the system can fully interdiffuse, rehomogenizing those necks. These observations help explain why the Ti–Mg system does not densify well, because it does not exhibit redissolution at high temperatures. These results help clarify the conditions needed to achieve NPSS and may support design of new alloys for NPSS behavior.