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

Abstract

cermets present both normal and abnormal growth of faceted titanium diboride (TiB 2 ) 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 TiB 2 grains. However, the sinterability of these materials is dramatically reduced, which suggests that TiB 2 AGG control is obtained by decreasing TiB 2 dissolution kinetics in the liquid phase. On the other hand, an alternative method based on intensive powder milling not only reduces TiB 2 AGG but also the porosity levels obtained by previous powder processing routes. TiB 2 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 TiB 2 growing grains suggests a possible dragging effect responsible for the AGG reduction found in these cermets. Moreover, aggressive milling removes large TiB 2 particles from the powder mixtures, which could act as seeds for TiB 2 uncontrolled growth. TiB 2 -Ni 3 (Al,Ti) cermets obtained by intensive milling combine hardness over 20 GPa with K IC of about 10 MPa , data clearly out of the range covered so far by other TiB 2 -based materials.1m

Effect of (Ti:B) atomic ratio on mechanical properties of TiB2–Fe composites “in situ” fabricated via Self-propagating High-temperature Synthesis

“…In order to eliminate the effect of impurities those cermets were fabricated using high purity elemental powders. Similar attempts to modify the matrix phase were emphasized by Azkona et al [4] for Ni-containing TiB 2 -based composites, as well as reported recently by Zhang and coworkers [15] for Al-matrix cermets.…”

“…MPa [4] can be reached for nearly monophasic TiB 2 material, while the same properties for unreinforced Fe are E= 210GPa and σ 0.2 = 135MPa, respectively [30]. The broken pieces after the compression test revealed cracks propagated in a plane oriented at 45° to the loading direction.…”

“…Due to extraordinary microhardness (34.0GPa), outstanding tribological properties [1][2][3][4], low density (4.52g/cm 3 ), and good thermal and electrical conductivity [5][6][7][8][9] (60W/m -1 K -1 ) [10], the TiB 2 is considered as a ceramic phase reinforcing advanced cermets [6,11]. The Young's modulus (E) of 529GPa [3] and transverse rupture strength of even 1500MPa [4] can be reached for nearly monolithic TiB 2 material.…”

“…The Young's modulus (E) of 529GPa [3] and transverse rupture strength of even 1500MPa [4] can be reached for nearly monolithic TiB 2 material. Therefore, TiB 2 -rich Fe matrix cermets with high volume fraction of TiB 2 are potentially good candidates for machine parts for aerospace, automotive, military, defense, and marine, characterized by high wear resistance as well as for nuclear industry [12][13][14].…”

Strategies and practices for suppressing abnormal grain growth during liquid phase sintering

“…As the physical properties of materials can change drastically with the formation of abnormal grains, the control of AGG has been an essential subject for materials development in academia as well as in industry. Many theories and models have been developed to explain AGG in both ceramic and metallic systems .…”