Molecular Dynamics Simulations On The Crystallization Behavior In The Heat Affect Zone Of Laser Additive Manufacturing Bulk Metallic Glasses

At present,the main method for preparing bulk metallic glasses is copper mold casting method.However,due to the limitation of heat transfer,it is difficult for the melt in the center part to reach the critical cooling rate when the bulk size is too large,and it greatly restricts the application and development of metallic glass in the industrial field.Additive manufacturing technology using laser as the power source has been considered as a potential CNAdidate to prepare BMGs without limitation of critical size,due to its point to point forming advantage.But the amorphous crystallization behavior is liklely to occur in the heat affected zone during the preparation of metallic glasses with laser additive manufacturing.With the aim to suppress the crystallization behavior in the heat affect zone,molecular dynamics simulations were carried out to study the crystallization mechanism in the heat affected zone during the preparation of bulk metallic glass with laser additive manufacturing and to study the influence of thermal history,structural inhomogeneity and compositional inhomogeneity on the crystallization behavior in the heat affect zone.The main results obtained are as follows:(1)The microstructure evolution behavior of Ni₇₅Al₂₅ amorphous system during thermal cycling process at different temperature change rates was studied by molecular dynamics simulation,the structural evolution path of amorphous system during crystallization process was clarified,and the decline of nucleation at high heating rate was verified.For the Ni₇₅Al₂₅ amorphous system,the amorphous structure is preferentially transformed to BCC structure under the thermal cycling conditions.If the subsequent relaxation time is long enough,the BCC structure transforms into mixed structure of FCC and HCP;if the subsequent relaxation time is insufficient,the BCC structure CNAnot be converted into the mixed structure of FCC and HCP,but the defective BCC structure in the system transforms into a standard BCC structure.This indicates that the structure in heat-affected zone after the amorphous crystallization is closely related to the rate of temperature change.(2)During the thermal cycle process,When the cooling rate is about 0.36 R_c,the crystallization begins in the first cooling process,the decline of nucleation occurs in the second heating process,and the large-scale crystallization behavior is observed in the second cooling process;if the cooling rate increases to 0.4R_c,the crystallization behavior begins in the second cooling process,due to the decline of nucleation,the small-sized crystal nucleus disappear in the third heating process,and large-scale crystallization behavior occurs during the third cooling process.When the cooling rate reaches 0.72R_c,no crystal lattice structure is found in the system at the end of the thermal cycle because of the occurrence of the decline of nucleation.It means that the occurrence of crystallization behavior in the heat-affected zone CNA be suppressed for laser additive manufacturing.(3)The amorphous structure is composed by the plastic zone with icosahedral grid and the brittle zone with low order polyhedral.In the crystallization process,since the Gibbs free energy barrier between the icosahedral lattice structure and the BCC structure is lower,the plastic zone crystallizes prior to the brittle zone under the same heating conditions:the standard icosahedron first transforms into the defect icosahedron,the defect icosahedron transforms into a defective BCC structure,and then the defect BCC structure transforms into the standard BCC structure.(4)Due to the imperfect alignment of the atoms at the edge of nucleus,the CNA structure analysis method often recognizes the outermost atom of the crystallization region as an amorphous atom when identifying the crystallized region.The CNA structure analysis method is improved by characterization of the lattice orientation difference for the atoms.Most of the boundary atoms of the crystallized region CNA be effectively identified as the crystal atoms with the improved CNA structure analysis method,thus enhances the accuracy of the crystallized region extraction.(5)The crystallization phenomenon of Ni₆₀Al₂₀Zr₂₀ in the supercooled liquid phase was studied.By analyzing the composition and distribution characteristics of the crystallized regions with different size features,it was found that when the size of the crystallized region is small,the proportion of Al atoms in the crystallized region is signifi CNAtly higher than its average level in the system;when the size of the crystallized region is sufficiently large,the composition of the crystallized region is consistent with the composition of the system,indicating that the region in which the Al atoms are enriched in the system is more susceptible to crystallization;If there is no structural inhomogeneity,the region in which the heat-affected zone Al is concentrated is more likely to undergo crystallization.