Simulation Study On Deformation Mechanism Of Nanocrystal-Metallic Glasses Composites

Metallic glasses(MGs)show brittle failure caused by local shear deformation under loading condition,which limits its application.To solve this problem,designing metallic glass composites is one of the effective ways to enhance the strengthening and toughening property of metallic glasses.Molecular dynamics simulations are conducted to investigate the mechanical properties and deformation mechanism of CuZr nanocrystal-metallic glass composites(NCMGCs)under the influence of crystallographic direction,spatial distribution and volume fraction of B2 CuZr nanocrystals.For improving the plasticity of metallic glass composites,NCMGCs with different crystal defects were designed,and the effects of defects on the deformation behavior of amorphous composites were discussed.The above research reveals the deformation mechanism of the amorphous matrix and B2 CuZr nanocrystals,which is of great significance for optimizing the performance of metallic glass composites and expanding its application.The conclusions of this research can be summarized as follows:(1)The effects of B2 CuZr nanocrystals on the formation and expansion of shear transition zones(STZs)and shear bands(SBs)in amorphous matrix.The simulation results show that the amorphous matrix and the amorphous-crystalline interfaces(ACIs)serve as nucleation points of the STZs,which induces immature shear bands(ISBs).As the stress increases,STZs penetrate and eventually expand to form mature SBs.The strength of the metallic glass composites is higher than that of the pure MGs due to the existence of the nanocrystals.The outer layer atoms of nanocrystals gradually become amorphized during the deformation process,and the amorphized atoms participate in the formation of SBs.(2)The effects of crystallographic direction,distribution and size of B2 CuZr nanocrystals on the mechanical properties of NCMGCs were studied.We find that the crystallographic direction of the nanocrystals have great influence on elastic modulus of composites.Moreover,the spatial distribution of nanocrystals plays an important role in SBs propagation direction.In particular,by increasing the volume fraction of nanocrystals,the nanocrystals begin to participate in plastic deformation.The co-deformation of the nanocrystals and the amorphous is achieved for the[110]&AA&d=7.4 nm and the[111]&AA&d=7.4 nm samples,resulting in two stages of yielding on the stress-strain curves.The first stage of yielding corresponds to the occurrence of stacking faults(SFs)in the nanocrystals.This process releases part of the stress in nanocrystals,resulting in the recovery of lattice distortion.The second stage of yielding corresponds to the shear deformation dominated by the STZs in the amorphous matrix.STZs develop into ISBs and eventually expand to a mature SB.This indicates that the appearance of SFs in the nanocrystals retards the formation of a mature SB with the increase of volume fraction of the nanocrystals.(3)The mechanical properties and deformation mechanism of NCMGCs with voids,dislocations,SFs and grain boundaries(GBs)were studied.Due to the introduction of defects,the nucleation core of the SF increases,and it is easier to give priority to plastic deformation in the form of SFs during the deformation.The SFs and ACIs work together to stimulate the STZs in the amorphous phase,so that the amorphous phase and the nanocrystals jointly participate in the plastic deformation.1)The presence of voids stimulates a large number of SFs in the nanocrystals.SFs and ISBs distributed on the amorphous matrix in a network-like distribution,making the sample showed uniform plastic deformation.2)The distribution of dislocations affects the orientation and density of SFs.Moreover,the introduction of dislocations makes the nanocrystal preferentially deformed,but it also reduces the yield strength of the composite.SFs crossed each other and excite STZs at ACIs,both of which carry strain together to avoid obvious local shear deformation in the amorphous.3)For composites containing SFs in different orientations,the existence of the SFs reduce the yield strength of the composite,but the softening degree of the nanocrystals is not as obvious as that of nanocrystals with dislocations.For the nanopolycrystal-metallic glass composites,nanocrystal is divided into smaller grains due to the size effect,and the strain is uniformly distributed on the ACIs and amorphous matrix.Therefore,for designing NCMGCs,continuous optimization of structural design schemes can provide the necessary theoretical basis for the deformation behavior and deformation mechanism of MGs during compression deformation,which provides the necessary theoretical basis for experimental research.