Study On The Atomic Structure And Toughening Mechanism Of Co-B-based Amorphous Alloys

In this thesis,Co_65-xB₃₅Ta_x（at.%,x=0,5,8,11）and Co_92-xTa₈B_x（at.%,x=30,32.5,35,37.5）amorphous alloys are taken as research objects.The local structure is studied in detail by ab initio molecular dynamics simulations.The relationship of alloy composition with local structure and toughness is emphatically analyzed to reveal the intrinsic toughening mechanism.The simulation results show that the 1551,1541,and 1431 bond pairs are the main bond-pair structures in Co-B-based amorphous alloys.The coordination numbers（CNs）of Co-centered polyhedra clusters are mainly 13 and 14.The CNs of B-centered clusters are mainly 9 and 10.The CNs of Ta-centered clusters are mainly14,15,and 16.Among these clusters,the deformed prism structure（B-centered<0 36 0>cluster）and the deformed icosahedron（Co-centered<0 1 10 2>cluster）dominate.The nearest-neighbor correlation index analysis indicates that B-centered<0 3 6 0>and<0 2 8 0>clusters show non-association in the space.The spatial arrangement of Co-centered<0 1 10 2>,<0 3 6 4>and<0 2 8 4>clusters changes with the alloy composition and their surrounding environment.The addition of an appropriate amount of Ta to the Co₆₅B₃₅ alloy(i.e.,Co₆₀B₃₅Ta₅)results in a decrease in the covalent Co-B bond,and an increase in the ionic B-Ta bond and Co-Ta bond.This indicates that the replacement of Co by Ta weakens the bonding force between metal and metalloid,which would reduce the stability and shear resistance of cluster,and thus improves the toughness of alloy.In addition,most types of clusters in the Co₆₀B₃₅Ta₅ alloy repel each other,and thus they are evenly distributed in the space.These clusters are connected through the coordination atoms to form loosely medium-range order structures,which have lower configurational transition barriers.So,they are more susceptible to shear deformation and have better plasticity.As we known,when the Poisson’s ratio of amorphous alloy is higher than the critical value of 0.32-0.33,it will be more easily to exhibit good toughness.The addition of Ta can significantly increase the Poisson’s ratio to above0.32 in the present alloys,which is in support of the toughness improvement.Also,this is consistent with the previous experimental result.On the other hand,with an increase in Ta content,the covalent B-B bond increases.When the covalent bond is formed between B and B,the unoccupied 2p orbital accepts the electrons transferred by Ta,and the bonding force of the B-B bond increases.Thus,B-centered clusters are more stable.The B-centered<0 3 6 0>and<0 2 8 0>clusters exhibit non-association in the space.On the contrary,the other clusters show a good correlation.The centered atoms of clusters are connected to form more stable medium-range order structures,which casuses an increase in the strength and a decrease in the toughness.In the case of Co_92-xTa₈B_x（at.%,x=30,32.5,35,37.5）amorphous alloys,with increasing B content,the covalent Co-B,B-B bond and the ionic B-Ta bond increase,while the metallic Co-Co bond decreases.This results in an increase in the mechanical strength.In the Co₆₂Ta₈B₃₀ alloy,five clusters with higher content（B-centered<0 3 6 0>,<0 2 8 0>and Co-centered<0 1 10 2>,<0 3 6 4>,<0 2 8 4>clusters）all show their own non-association.This shows that in this alloy,the same type of clusters repel each other,and the clusters are evenly distributed in the space.The clusters are connected by coordination atoms in the space to form loosely medium-range order structures,which reduces the stability of the structure,thereby increasing the toughness of the alloy.Obviously,the addition of appropriate amount of B element（30 at.%）can increase the Poisson’s ratio to above 0.33.This result suggests that the Co₆₂Ta₈B₃₀ alloy has the best toughness,which is in agreement with the experimental result.The above analysis shows that the mechanical properties of Co-B-based amorphous alloys are related to the atomic bonding and the spatial distribution of the clusters.The proper adjustment of the alloy composition can change the atomic structure and thus improve the toughness of the alloy to some extent.The study of the relationship among the alloy composition,atomic structure and mechanical properties by computational simulation has scientific guidance and reference significance for the composition design and development of new amorphous alloys.