Ab Initio Molecular Dynamics Simulation On The Structure And Properties Of Ni-metalloid Amorphous Alloys

In this thesis,Ni₈₀（P,B,C）₂₀ and Ni-containing Fe₈₀P₁₃C₇ amorphous alloys are chosen to study by ab initio molecular dynamics simulation.The main work includes the following two aspects.（1）Ni₈₀P₁₄B₆,Ni₈₀B₁₄C₆ and Ni₈₀P13C7 amorphous alloys are chosen to study by ab initio molecular dynamics simulation,so that the effects of the metalloid elements M（M: P,B and C）on the atomic structure and glass forming ability（GFA）of Ni-metalloid amorphous alloys can be investigated.The PDF analysis indicates that the bonding of Ni-P and Ni-C is stronger than that of Ni-B,and further,P-P,P-B,P-C and C-C atomic pairs attract each other in Ni₈₀P₁₄B₆ and Ni₈₀P13C7 alloys,which is benefit to the GFA.On the contrast,the B-B and B-C atomic pairs tend to repel each other in Ni₈₀B₁₄C₆ alloy,which goes against the GFA.The analysis of the P-centered Voronoi polyhedrals（VPs）shows that P atoms have two placeholder ways,which one is to locate in the center of the antiprism and the other is to locate at the substitute position of Ni atoms.Furthermore,compared with Ni₈₀P13C7 alloy,P atoms in liquid Ni₈₀P₁₄B₆ alloy have more icosahedral-like P-centered clusters,which leads to a more complex structure of melts and thus is benefit to the GFA.The PDFs and VPs analyses suggest that the GFA of the three Ni-metalloid alloys is dropped in the order of Ni₈₀P₁₄B₆,Ni₈₀P13C7 and Ni₈₀B₁₄C₆.（2）Fe₈₀P₁₃C₇ and Fe50Ni30P13C7 are chosen to study by ab initio molecular dynamics simulation,so that we can investigate the reason why the plasticity of Fe₈₀P₁₃C₇ amorphous alloy can be significantly improved by the addition of Ni.The results indicates that the primary SRO in Fe₈₀P₁₃C₇ amorphous alloy is icosahedrons clusters while the proportion of Archimedean antiprism clusters are significantly increased in Fe50Ni30P13C7 amorphous alloy,meaning that the degree of fivefold symmetry decreases.Compared with other types of atomic coordination,the SRO with the fivefold symmetry results in a denser packing and higher configurational transition barrier in amorphous,and thus the more reluctant to undergo shear transformation events.Therefore,the simulation results show that the addition of Niwill lead to the plasticity improvement of Fe₈₀P₁₃C₇ amorphous alloy from the perspective of the amount of five symmetries in the short-range order.Further,the figure of charge density shows that,compared with Fe50Ni30P13C7 amorphous alloy,the charge density between the metal and metalloid atoms in Fe₈₀P₁₃C₇ amorphous alloy is higher,indicating more prone to form covalent bonds.Compared with the metallic bonding,the covalent bonding with the larger bonding strength leads to the larger shear modulus,the less Poisson’s ratio,and thus more resisitance for shear flow,which is opposite to the plasticity of materials.Additionally,the covalent bonds are highly localized.When the amorphous alloy is stressed,the electrons remain localized until the stress is sufficient enough to expel them from a bond,which leads to the materials fracture catastrophically.Therefore,from the perspective of the atomic bonding,the simulation results also indicate that the addition of Ni will result in the improvement of the plasticity of Fe₈₀P₁₃C₇ amorphous alloy.