Preparation,Magnetic And Mechanical Properties Of Cobalt Based Bulk Metallic Glasses

Compared to other bulk metallic glasses?BMGs?,Co-based BMGs have excellent magnetic properties,i.e.,high saturation magnetization?B_s?,high effective permeability??_e?and low coercivity?H_c?.On the other hand,these alloy systems also exhibit superior mechanical properties,i.e.,high fracture strength??_f?and high hardness.Due to the excellent magnetic and mechanical properties,Co-based BMGs have attracted attentions for engineering applications as structural and functional materials.However,the alloy systems do not appear the macroscopic plastic deformation at room temperature.Usually,the unstable propagation of shear bands in alloys leads to catastrophic fracture,which prevents their wide applications as new structural materials.Therefore,it is urgent to improve the plasticity of Co-based BMGs at room temperature while maintaining good soft magnetic properties and glass forming ability of Co-based BMGs at the same time.This is also the focus of this study.In this paper,we choose the Co-Fe-B-Si-Nb alloy system as the research object.The effects of Ni,Cu and both of these two elements on the glass forming ability,microstructure,magnetic and mechanical properties of the bulk metallic glasses system were investigated.The internal mechanism of adding elements is analyzed in detail.The main results are as follows:?1?The effects of Ni substitution for Fe or Co in(Co_1-x-yFe_xNi_y)₆₈B_21.9Si_5.1Nb₅ on the thermal,magnetic properties and mechanical properties were investigated.It is found that saturation magnetization of the alloys decreased with the addition of Ni element,but the plasticity is obviously increased from 0.8%to 3.8%.The compressive fracture strength of the alloys is above 4000 MPa.Combining with rods surface SEM image analysis,the increase of plastic deformation is caused by multiple shear bands and abundant vein patterns.The Poisson's ratio of alloy system is increased and the G/K value is decreased with the addition of Ni element.It promotes the increase of shear transition zone and makes the alloy have better fracture resistance,which is more conducive to plastic deformation.In addition,the low T_g corresponds to the decrease of shear modulus,which means that the alloy system is prone to shear deformation.While the deformation-induced microstructural change could take place easily in the metallic glass with narrow?T in the process of mechanical deformation.Therefore,the addition of Ni increases the plastic deformation ability of the alloy system.?2?TheeffectsofCucontentsontheglass-formingabilityof[(Co_0.7Fe_0.3)_0.68B_0.219Si_0.051Nb_0.05]_100-xCu_x alloys were investigated.Bulk glassy rods with a critical diameter of 3.5-5.5 mm were prepared by copper mold casting.With the Cu addition,the Co-based BMGs exhibit superior mechanical properties,such as superhigh?_f of 4310-4400MPa and large?_p successfully elevated to 2.5%,and excellent magnetic properties,such as low H_c of 0.78-1.85 A/m,high B_s of 0.69-0.73 T.It is found that the mechanical and magnetic properties are closely correlated with their inner atomic cluster structure.The existence of?-?Fe,Co?is effective in improving not only B_s,but also the plasticity of the alloy by producing abundant of vein patterns on the fracture surface and high density multiple shear bands on the flank of the rod specimen.The high strength together with the promising plasticity is encouraging for further development of Co-based BMGs with potential applications.?3?The effects of the simultaneous addition of Ni and Cu on the microstructure and properties of(Co_0.7Fe_0.2Ni_0.1)_68-xB_21.9Si_5.1Nb₅Cu_x BMGs were investigated.The alloy system has high saturation magnetization of 0.48-0.60 T and low coercivity of 0.41-2.11 A/m.In addition,the alloy system also has superior mechanical properties:ultrahigh strength of 4275-4770 MPa,large plasticity of 0.8-5.5%.The obvious work hardening phenomenon has been observed.Especially,(Co_0.7Fe_0.2Ni_0.1)_67.7B_21.9Si_5.1Nb₅Cu_0.3 BMGs exhibit high saturation magnetization,low coercivity,ultrahigh strength and large plasticity.The microstructure analysis shows that the proper match of size ratio and volume fraction between?-?Fe,Co?phases and glassy matrices are very crucial to control the mechanical properties.Besides,the simultaneous addition of Ni and Cu with high Poisson's ratio and low G/K reduces the activation barrier for viscous flow,hinders the formation of cracks,promotes the formation of multiple shear bands,and improves the shear deformation ability of the alloy.Moreover,the low T_g means the low shear activation energy,while the narrow?T is easily to induce the internal microstructure change during the deformation.Therefore,these factors work together to improve the plastic deformation ability of the alloy system.