A new class of ultra-hard materials based on AlMgB14

In this study, aluminum magnesium boride combined with 5 to 30 mol.% additives (AlMgB14:X where X = Si, P, C, AlN, TiB2, or BN), were prepared by mechanical alloying and consolidated by vacuum hot pressing. Matkovich and Economy first reported the orthorhombic AlMgB14 intermetallic compound (oI64, space group Imam, a - 0.5848 nm, b = 0.8112 nm, c = 1.0312 nm), and the structure determination was later refined by Higashi and Ito. The unit cell is based on four B12 icosahedral units centered at (0, 0, 0), (0, 0.5, 0.5), (0.5, 0, 0), and (0.5, 0.5, 0.5) within the unit cell. The remaining eight B atoms lie outside the icosahedra, bonding to the icosahedral B atoms and to the Al and Mg atoms. The Al atoms occupy a four-fold position at (0.250, 0.750, 0.250), and the Mg atoms occupy a four-fold position at (0.250, 0.359, 0). The icosahedra are arranged in distorted, close-packed layers. The unique electronic, optical, and mechanical properties of this material are due to a complex icosahedra (intericosahedral bonding). The highest hardness was observed in the AlMgB14 + 30%TiB2 material, which possesses a multi-phase microstructure. Here again, an increase in hardness accompanying the introduction of additional phases is somewhat surprising and difficult to explain. The possible compounds that may form at the AlMgB14-TiB2 interface during hot pressing are numerous