AlM₂B₂ (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 AlCr₂B₂, AlMn₂B₂, AlFe₂B₂, AlCo₂B₂, and AlNi₂B₂. We find that the first three borides of this series are stable phases, while AlCo₂B₂ and AlNi₂B₂ are metastable. We show that the elasticity increases in the boride series, and predict that AlCr₂B₂, AlMn₂B₂, and AlFe₂B₂ are more brittle, while AlCo₂B₂ and AlNi₂B₂ are more ductile. We propose that the elasticity of AlFe₂B₂ 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.