Grain growth inhibition by connected porosity in sintered niobium

Pore–boundary interaction plays an important role in densification during solid-state sintering. This paper reports the evolution of porosity and grain size in niobium sintered at 2073 and 2273 K for different sintering times. The densification curves show a decrease in porosity up to 8 and 4 vol.% after 10,800 s for the samples sintered at 2073 and 2273 K, respectively. Grain growth is observed to take place together with this decrease in porosity. A new model for grain growth inhibition during sintering is proposed for connected porosity. This model considers that the moving grain boundaries and the outer surface of cylindrical pores remain in contact during grain growth and that energy dissipation takes place owing to the fact that the grain boundary is moving relative to the porosity. Our mechanism is akin to a friction between the grain boundary and the connected porosity at their contact region. In contrast to the traditional particle–grain boundary bypassing mechanisms, the present model is not a purely geometrical relationship but is material dependent. The model gives agrees well with experimental results obtained in this paper for sintered niobium as well as for other sintered materials reported in the literature. Our model is a novel approach to treating grain growth inhibition by pores during the sintering stage in which the porosity becomes interconnected.