Abnormal growth of TiB2 crystals during sintering of TiB2-Ni3(Al,Ti) cemented borides

Abstract

TiB2-Ni3(Al,Ti) cermets present both normal and abnormal growth of faceted titanium diboride (TiB2) grains during liquid-phase sintering. Abnormal grain growth (AGG) is preferentially found at high sintering temperatures in specimens processed from powder mixtures with a wide particle size distribution. The WC additions to the initial powder mixtures have proved efficient in reducing the number and size of these large TiB2 grains. However, the sinterability of these materials is dramatically reduced, which suggests that TiB2 AGG control is obtained by decreasing TiB2 dissolution kinetics in the liquid phase. On the other hand, an alternative method based on intensive powder milling not only reduces TiB2 AGG but also the porosity levels obtained by previous powder processing routes. TiB2 cermets produced by aggressive milling present a higher amount of alumina particles in the matrix after sintering, which, in addition, appear more homogeneously dispersed in the microstructure. The distortion produced by these particles on the facets of TiB2 growing grains suggests a possible dragging effect responsible for the AGG reduction found in these cermets. Moreover, aggressive milling removes large TiB2 particles from the powder mixtures, which could act as seeds for TiB2 uncontrolled growth. TiB2-Ni3(Al,Ti) cermets obtained by intensive milling combine hardness over 20 GPa with K IC of about 10 MPa √m, data clearly out of the range covered so far by other TiB2-based materials.