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 AlCr2 B2 , AlMn2 B2 , AlFe2 B2 , AlCo2 B2 , and AlNi2 B2 . We find that the first three borides of this series are stable phases, while AlCo2 B2 and AlNi2 B2 are metastable. We show that the elasticity increases in the boride series, and predict that AlCr2 B2 , AlMn2 B2 , and AlFe2 B2 are more brittle, while AlCo2 B2 and AlNi2 B2 are more ductile. We propose that the elasticity of AlFe2 B2 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.