A challenge to the sintering of nanocrystalline metals: Organic burnout and oxide reduction compete with powder consolidation

In nanocrystalline materials atomic transport is enhanced by diffusion along grain boundaries and triple junctions, aggressively accelerating sintering kinetics to the point that the onset of consolidation can precede organic debinding reactions at low temperature. What is more, nanocrystalline metals typically have (or at one time had) more surface area, and thus a greater degree of native oxide that can react with organics upon sintering. In this paper, we explore the sintering behavior of high-energy ball-milled nanocrystalline Ni-10Fe powder, quantitatively evaluating the interference of densification with the process of organic removal. Early sintering leads to trapping of evolved gas and substantial swelling of a nominally sintering compact. Volatilizing process additives remain trapped until very high temperatures, causing microstructural degradation through creep foaming. Finally, in an alloy system with more than one native oxide phase, there are shown to be multiple oxide reduction events that can renew gas evolution to very high temperatures. After quantitatively evaluating these mechanisms in the Ni-10Fe powder specifically, we also generalize their consideration to a range of nanocrystalline metals, to provide guidance for development of powder processing routes for those materials.