Study On Toughening Of Fe-based Metallic Glasses And Underlying Mechanism Through Energy State Modulation

Energy shortage,climate change and environmental pollution have posed severe challenges to human survival and development.Carbon peak and neutralization have become the strategic goals of sustainable development in our country.Fe-based amorphous soft magnetic alloy is a new kind of green and energy-saving material,which has excellent soft magnetic properties such as high saturation flux density and low coercivity,and can greatly reduce the energy consumption of power electronic equipment.However,the negligible plasticity of Fe-based metallic glasses（MGs）greatly limits their wide application.The traditional method to improve the ductility of Fe-based MGs is composition modification,but this method is time-consuming and labor-intensive,and often destructs other properties,such as the strength,soft magnetic properties amd glass forming ability.Therefore,it is of great significance to maintain the excellent soft magnetic properties and solve the“contradiction”between high strength and large plasticity in order to obtain Fe-based MGs with excellent comprehensive properties.In this dissertation,by modulating energy state through cryogenic thermal cycling（CTC）treatment,the compressive plasticity at room temperature,tensile ductility at high temperature and annealing induced embrittlement of Fe-based MGs were effectively improved.At the same time,the large GFA,high strength and excellent soft magnetic properties maintained.The structure variation and origin of large plasticity were indentified by advanced experimental methods.The research provides theoretical guidance for the development and preparation of Fe-based MGs with high performance,and also provides an ideal model for study of deformation mechanism and structure evolution.The research contents of this dissertation are shown as follows:1.Taking Fe Co BSi Nb,which is the classical system of Fe Co-based MG,as object.By adjusting the parameters during CTC treatment,[(Fe_0.5Co_0.5)_0.75B_0.2Si_0.05]₉₆Nb₄ MG withσ_y of4060 MPa and?_p of 6.1%was obtained.The sample rejuvenated and the content of free volume increased,meanwhile,the number of crystal-like ordered structures around 1-2 nm in the amorphous matrix increased,both of which can provide nucleation sites for shear transition zones（STZs）and facilitate the formation of shear bands（SBs）.In addition,the SBs formed preferentially are easily hindered by relatively hard crystal-like structures to avoid rapid shear failure,as a result,the plastic deformation capacity was improved.2.By small addition of Cu element to the[(Fe_0.5Co_0.5)_0.75B_0.2Si_0.05]₉₆Nb₄ alloy,a Fe-based MG with?_p of 7.4%andσ_y of 4350 MPa was successfully prepared,and the excellent soft magnetic properties maintained after CTC treatment.The optimal thermal cycling temperature decreased from 513 to 393 K compared with that without Cu element,which effectively reduced energy consumption.The addition of a small amount of Cu element effectively increased the nanoscale heterogeneity,leading to large difference in thermal expansion coefficient and the introduction of large local internal stress,which was conducive to the rejuvenation process.After CTC treatment,the nano-scale soft zones increased and the the initial ordered structures were destroyed,the disordering degree increased and effectively improved its plastic deformation capacity.However,with the continuous increase of Cu element,a large number of crystal-like structures were easy to precipitate in the matrix,which grow rapidly in the process of CTC treatment,resulting in a lower rejuvenation degree and stress concentration during deformation,thus the improvement of plasticity was not significant.3.The effect of CTC treatment on tensile properties of Fe-based MG ribbons was studied.It was found that theβrelaxation peak of[(Fe_0.5Co_0.5)_0.75B_0.2Si_0.05]₉₆Nb₄ amorphous ribbons became more obvious after cycling,so it exhibited better tensile plasticity at corresponding temperatures.Interestingly,theβrelaxation peaks appeared at two different locations with different thermal cycling temperatures.Through systematic characterization of structure evolution and relaxation spectrum analysis,the enhancement of tensile plasticity after CTC treatment was due to the increased number of local liquid-like regions.In addition,although the CTC treatment is aimed to rejuvenate MGs,once the cycling temperature is higher,it can also induce relaxation behavior.The energy state oscillation caused the size/type changes of liquid-like zones,which was the reason of differentβrelaxation peak positions.The phenomena thatβrelaxation peak indensity increased after CTC treatment and different thermal temperatures induced different peak positions were also observed in Fe39Ni39B12.82Si2.75Nb2.3P4.13 MG.4.The annealing brittleness of Fe-based MG ribbons was effectively improved by the CTC treatment.When annealing at the temperature and time with best soft magnetic properties,the MG ribbons experienced obvious relaxation and obtained a compact and uniform structure,there was no significant rejuvenation effect and no obvious improvement of bending plasticity.However,by slightly reducing the annealing time,the MG ribbons can retain more free volume and a non-uniform structure.Therefore,the Fe-based MG ribbons with excellent soft magnetic properties along with good bending plasticity can be obtained after CTC treatment.