Preparation And Plastic Deformation Behavior Of Fe-Based Soft Magnetic Bulk Metallic Glasses With High Performance

Metallic glasses exhibit much better properties compared with the traditional crystalline counterparts,due to their unique atomic packing of disordering in long-range but ordering in short-range.Among them,Fe-based bulk metallic glasses（BMGs）have attracted more attentions for their high strength,high hardness,high elastic limit,excellent soft magnetic properties,high-efficient degrading ability and relatively low material cost.However,owing to the poor glass forming ability（GFA）and limited microscopic plasticity,their application as structural and functional materials is restricted.Usually,in order to gain high GFA,a great deal of metalloid and metallic glass forming elements need to be added.Whereas the addition of large amount of glass forming elements especial for large-sized metallic glass forming elements will decrease the soft magnetic properties.In addition,the commonly used strategy of introducing a second phase is not suitable for development of ductile soft magnetic metallic glasses,because precipitation of the second phases usually decreases the GFA and strength,and also deteriorates the soft-magnetic properties of ferromagnetic BMG because the magnetic domain will be pinned by these second phases.The high GFA and excellent soft magnetic properties,the high strength and large plasticity seem to be two“contradictory”which cannot be mediated.Nevertheless,many application fields require soft magnetic materials with cobination of these merits,which make the development of metallic glasses with combined high GFA,good soft magnetic properties,high strength and large plasticity of great significance.In addition,the relationship between the macroscopic properties and the microstructure of the BMGs was systematically studied based on the new Fe-based BMGs,and the toughening and brittleness transformation mechanism of the Fe-based BMGs was also revealed,which further provides guidelines to develop new Fe-based BMGs with excellent comprehensive properties.In this dissertation,based on the FeP-and FeB-based BMG system,two FeNiMoPCBSiand FeNiBSiNbP BMG system with combination of high GFA,excellent soft magnetic and mechanical properties were developed through composition adjustment and fluxing treatment technical.The origin of large plasticity was investigated by X-ray photoelectron spectroscopy,synchrotron radiation X-ray and corrected spherical aberration transmission electron microscopy.This work gives a new ideal for development of high-performance magnetic metallic glasses and provides an ideal model material to study the basic sciences of magnetic bulk amorphous alloys during deformation.The research contents of this dissertation are shown as follows:1.Based on the FeP-based metallic glasses,through micro-alloying of Siand the partial substitution of Feby Ni,a ductile Fe₅₆Ni₂₀Mo₄P₁₁C₄B₄Si₁ BMG with critical diameter of 2 mm,saturation magnetic flux density（B_s）of 0.93 T and coercivity（H_c）of 1.9 A/m,yield strength of2730 MPa and plastic strain of 7%was developed.The charge transfer and exchange effect between the components of alloys lead to the variation of the soft magnetic properties of metallic glasses.In addition,adding Nielement decreases the binding force between each atomics,which leads the structure more instability under applied loading,meanwhile,the addition of Sielement results in a large number of potential shear transition zone sites caused by the loose structure,leading to the formation of multiple shear bands,thereby resulting in a complex deformation process follow the self-organized critical dynamics.2.Based on the FeB-based metallic glass,through optimizing the ratio of Feand Ni,the proportion of B and P,A novel Fe₃₉Ni₃₉B_12.82Si_2.75Nb_2.3P_4.13 BMG with high notch toughness of72.3 MPa m^1/2,large plastic strain of 9.8%,high yield strength of 2930 MPa,and a large critical diameter of 2.5 mm was successfully developed.This BMG also exhibits excellent soft magnetic properties,i.e.,higher B_s of 0.86 T,extremely low H_c of 0.65 A/m,and high effective permeability of 23250 with high frequency stability.Its toughness value is the highest among Fe-based BMG family.It was found that the high toughness are attributed to atomic-scale structural heterogeneity that resulted from large free volume,high content of metal-metal bonds and high structural disorder degree,which can promote the multiple shear bands and hinder the subsequently propagation.3.The strength and plasticity of Fe₃₉Ni₃₉B_12.82Si_2.75Nb_2.3P_4.13 BMG are improved simultaneously through fluxing treatment.The compression strength increases from 3074 to4220 MPa,and the plastic strain is enlarged from 10.7%to more than 50%.The combination of large plasticity and high strength of this FeNiBSiNbP BMG is a breakthrough among Fe-based BMG family,which originates from the optimization of atomic-scale structure.More icosahedral-like clusters（ILCs）and crystal-like clusters（CLCs）are found in this FeNi-based BMG with fluxing treatment,and the ILCs are usually surrounded by CLCs.Furthermore,phase separation and a sandwich-like heterogeneous structure of shear band are also observed during deformation,indicating the multiscale deformation mechanism and a stable shear-band evolution,which is induced by the different responses of clusters with various structural characteristics participating in the deformation process.The unique“ILC surrounded by CLCs”structure and phase separation lead to a stable plastic deformation process with strong interactions of multiple shear bands,thereby the improved plasticity and strength.4.The serrated flow dynamics of Fe-based BMGs transform from chaotic state to self-organized critical state with increasing plasticity,leading to the changing of plastic deformation mechanism from single shear-band dominated to multiple shear-bands dominated.The origin of this transformation is attributed to the increasing atomic-scale heterogeneity caused by the increasing free volume and the characteristics,amount and distribution of short-to-medium range order,which facilitates a higher frequency of interaction and multiplication of shear bands,thereby results in a brittle to ductile transition in Fe-based BMGs.