Study On The Correlation Between Mechanical Properties And Structure Of Fe Based Amorphous Alloys Through Ab Initio Molecular Dynamics Simulations

In this thesis,the Ab initio molecular dynamics?AIMD?simulation method based on the density functional theory is used to study the relationship between mechanical properties and structure of Fe_80-xNi_xP₁₄B₆?x=0,20,30,40,50 at.%?and Fe_80-xMo_xP₁₄B₆?x=3,6,9 at.%?amorphous alloys from the perspective of short-range order?SRO?and medium-range order?MRO?.The main results are summarized as follows:?1?To understand the structural origin of the significant improvement on the plasticity of Fe-based bulk metallic glasses?BMGs?through the replacement of Fe by Ni,the SRO and MRO structures of Fe_80-xNi_xP₁₄B₆?x=0,20,30,40,50 at.%?amorphous alloys are investigated by AIMD simulation in the first work.It is found that the substitution of Ni for Fe causes the significant change in the local environment of B atoms.As the Ni replaces the Fe,the number of B-B bonding decreases,and the fraction of<0 3 6 0>polyhedron decreases and that of<0 2 8 0>polyhedron increases among the major B-centered Voronoi polyhedrons?Vps?.Additionally,the average coordination number?CN?of the present Fe-Ni-P-B amorphous alloys decreases with increasing the Ni content.The decreases of the number of B-B bonding and the average CN are conducive to plastic deformation,which may be responsible for the improvement on the plasticity of the present Fe-Ni-P-B amorphous alloys through the Ni addition.The bond pair analysis indicates that,with the increase of Ni content,the fraction of 15xx bond pair with a five-fold symmetry and the number of pentagons decrease,and that of 1431,142x and 1311bond pairs of non-fivefold symmetry increase in the present Fe-Mo-P-B amorphous alloys,indicating that the reduction of fivefold symmetrical SRO is benefit to the intrinsic plasticity.The MRO investigation indicates that,as increasing the substitution content of Ni for Fe,the vertex-shared connection increases and the edge-shared connection decreases,which result in the increase of cavity and boundary between the SRO atomic clusters,and thus will be conducive to the improvement of the plasticity of the present Fe-Ni-P-B amorphous alloys.?2?In the second work,we perform an AIMD simulation to investigate the structural origin of the significant improvement on the plasticity of Fe_80-xMo_xP₁₄B₆?x=3,6,9 at.%?amorphous alloys through the replacement of Fe by Mo.From the distribution of pair distribution functions?PDFs?and VPs,it can be seen that,for the series of Fe_80-xMo_x P₁₄B₆?x=3,6,9 at.%?amorphous alloy,the number of B-B bonds,the fraction of icosahedron-like<0 1 10 2>VP and perfect icosahedral<0 0 12 0>VP are the most dominant bond and VPs,and meanwhile the strength is the largest when x=6,indicating that the number of icosahedrons-like VPs is closely related to the strength of the present amorphous alloys.With the increase of Mo content,the proportion of pentagons is reduced,and meanwhile the experimentally measured plasticity of the Fe-Mo-P-B BMGs is increased,indicating that the reduction of five symmetries in SRO can improve the plasticity of the Fe-Ni-P-B amorphous alloys.The density of state?DOS?analysis results show that the number of hybrid bond between 3d-electrons of Fe/Mo atoms and p-electrons of P/B atoms is the least when the content of Mo is 6 at.%,which may account for the experimental results that the Fe₇₄Mo₆P₁₄B₆ amorphous alloy has the largest plasticity.With the increase of Mo content,the negative spin maximum value of Fe 3d band first increases and then decreases,indicating that the interaction between Fe atoms first increases and then decreases,which is consistent with the variation of the experimentally measured compressive strength of the Fe-Mo-P-B amorphous alloys with the Mo content.The investigation on MRO indicates that,as the Mo replaces the Fe,the vertex-shared connection increases first and then decreases,the edge-shared connection decreases first and then increases,and face-shared connection changes little,i.e.,cavity and boundary between the SRO atomic clusters increase first and then decreases,thus resulting in the plasticity of the Fe-Mo-P-B amorphous alloys increases first and then decreases,which is agreement with the experimental results.