Ab Initio Molecular Dynamics Study On The Atomic Structure And Properties Of B-Sm(La)-Co Amorphous Alloys

The novel developed B-based B-Sm（La）-Co amorphous alloys have ultrahigh thermal stability and hardness.The properties of the amorphous alloys are closely related to their local atomic structure.However,due to the absence of long-range atomic order and the limitions of the experimental techniques,the local atomic structure and the“structure-property”relationships of amorphous alloys have not been well understood.Computer simulation has become an effective apporch for studying the atomic-scale structural details of materials.In this paper,the ab initio molecular dynamics（AIMD）method is used to study the B-based amorphous alloys.Firstly,the structural features and the origin of the high thermal stability and hardness of the B₅₀Sm₁₀Co₄₀ amorphous alloy were studied by comparing the atomic and electronic structures of B-based B₅₀Sm₁₀Co₄₀ and Co-based Co₆₅Sm₁₀B₂₅ amorphous alloys.Secondly,we analyzed and compared the local atomic structures of B₅₀Sm₁₀Co₄₀,B₅₀Sm₂₀Co₃₀,B₅₀La₁₀Co₄₀ amorphous alloys to study the influence of the rare earth elements Sm and La on the microstructure of the B-based amorphous alloy.The main results show that:（1）The results of the pair distribution function（PDF）and Voronoi polyhedral（VP）analysis show that the feature of short-range order（SRO）for B₅₀Sm₁₀Co₄₀ has distinct difference with Co₆₅Sm₁₀B₂₅amorphous alloys:the SRO of B₅₀Sm₁₀Co₄₀ alloy is dominated by B-centered tricapped trigonal prisms（TTPs）and bicapped square antiprisms（BSAPs）clusters,while the content of icosahedral-like clusters is very low.The dense-packed of these small and compact prism-type clusters,composed of covalent B-B and Co-B bonds,contribute to the high Vickers hardness（H_v）and Young’s modulus（E）of the B₅₀Sm₁₀Co₄₀ amorphous alloy.The SRO of the Co₆₅Sm₁₀B₂₅ alloy which has lower H_v and E is dominated by TTPs and icosahedral-like clusters composed of Co-B and Co-Co atom pairs.（2）The mean-square displacement（MSD）analysis shows that the atomic diffusivity of B₅₀Sm₁₀Co₄₀ alloy is much smaller than that of Co₆₅Sm₁₀B₂₅ alloy at each temperature,which implies the higher thermal stability of B₅₀Sm₁₀Co₄₀.There are high contents of stable clusters mainly composed of the covalent B-B and Co-B bonds in the B₅₀Sm₁₀Co₄₀ amorphous alloy,which limit the long-range diffusion and migration of atoms and result in a high thermal stability.（3）By comparing the atomic and electronic structures of B₅₀Sm₁₀Co₄₀ and B₅₀Sm₂₀Co₃₀amorphous alloys,it was found that the content of TTP clusters significantly increase with the increase of Sm,and the content of TTPs in B₅₀Sm₂₀Co₃₀ is about 1.7 times than that in B₅₀Sm₁₀Co₄₀ alloy.The content of covalent B-B and Co-B bonds in B₅₀Sm₂₀Co₃₀ is also higher,and the average atomic distance of B-B,B-Sm,Co-Sm is lower.The increase of Sm with larger atomic size provides more free volume in B₅₀Sm₂₀Co₃₀ alloy,which is benefical to the migration of B and Co atoms at high temperature and result in a lower thermal stability of B₅₀Sm₂₀Co₃₀ amorphous alloy.（4）By comparing the atomic and electronic structures of B₅₀Sm₁₀Co₄₀ and B₅₀La₁₀Co₄₀amorphous alloys,we found that the featured clusters of SRO for B₅₀La₁₀Co₄₀ are mainly composed of B-B,Co-B and Co-Co atom pairs which is different with B₅₀Sm₁₀Co₄₀ alloy,and the content of BSAP clusters is significantly higher than B₅₀La₁₀Co₄₀ alloy.The bonding character is same in two alloys,both La and Sm formed ionic bonds with B and Co atoms.The MSD analysis shows that the thermal stability of studied amorphous alloys shows:B₅₀Sm₁₀Co₄₀>B₅₀La₁₀Co₄₀>B₅₀Sm₂₀Co₃₀>Co₆₅Sm₁₀Co₂₅,which is consistent with the experimental results.