AlM 2 B 2 (M  =  Cr, Mn, Fe, Co, Ni): a group of nanolaminated materials

Combining theory with experiments, we study the phase stability, elastic properties, electronic structure and hardness of layered ternary borides AlCr B , AlMn B , AlFe B , AlCo B , and AlNi B . We find that the first three borides of this series are stable phases, while AlCo B and AlNi B are metastable. We show that the elasticity increases in the boride series, and predict that AlCr B , AlMn B , and AlFe B are more brittle, while AlCo B and AlNi B are more ductile. We propose that the elasticity of AlFe B can be improved by alloying it with cobalt or nickel, or a combination of them. We present evidence that these ternary borides represent nanolaminated systems. Based on SEM measurements, we demonstrate that they exhibit the delamination phenomena, which leads to a reduced hardness compared to transition metal mono- and diborides. We discuss the background of delamination by analyzing chemical bonding and theoretical work of separation in these borides.