Ordered Structures And Their Influence On Mechanical And Magnetic Properties In Iron-Cobalt Soft Magnetic Alloys

Fe-Co soft magnetic alloy is a kind of soft magnetic alloy widely used in practice.Its outstanding performance advantages are very high saturation magnetization and high Curie temperature.However,with the expansion of the application field of soft magnetic alloys,new requirements have been put forward for the performance of alloys.For example,a typical basic electromagnetic structural component is the addition of a soft magnetic core in a current carrying coil to generate a strong magnetic field controlled by current.Iron core alloys not only need to have good magnetic properties to meet the functional requirements of electromagnetic components,but also must have good strength,plasticity,and impact toughness requirements at the same time.At present,the Fe-Co soft magnetic alloy,which has been widely used,has good magnetic properties,but its mechanical properties,especially impact toughness,still need improvement.According to the binary Fe-Co phase diagram,Fe-Co alloys undergo a disorderorder transition with the decreasing temperature in wide-range compositions.The existing engineering Fe-Co soft magnetic alloys are all in such composition range,which seriously affects the impact toughness and plastic-deformation ability.In this paper,the mechanical properties of Fe-Co soft magnetic alloys are improved to meet the needs of some special applications.Aiming at the LRO and SRO in low-cobalt ironcobalt alloys,molecular dynamics(MD)simulations.first-principles calculations,and experiments were performed to study the ordered structures in iron-cobalt alloy’s and their effects on mechanical and magnetic properties at different scales.Also,the ordering transitions in Fe-Co alloys and its relationship with the deformability and impact toughness of the alloys were studied.The research contents and main conclusions of this paper are as follows:(1)The EAM FeCo potential is improved to make it suitable for ordered B2-FeCo alloy,especially the antiphase boundary energies.The antipliase boundary energies are in good agreement with the experimental data.The grain boundary strength decreases with the increasing order degree.Under tensile loading,due to the additional dislocation resistance generated by APB in the completely ordered FeCo alloy,the poor plasticdeformation compatibility at the grain boundaries cannot maintain continuous plastic deformation.Therefore,microcracks occur at grain boundaries and lead to intergranular fracture.With the decreasing order degree at grain-boundary nanolayer,migration barrier of grain boundary gradually decreases and grain boundary movement is activated.The movement of grain boundaries can improve the plastic-deformation compatibility and release the concentrated stress at grain boundary,preventing the crack nucleation and intergranular fracture.(2)First-principles calculations were performed to study the effects of alloying elements on the plastic deformation ability of FeCo alloys.The solution elements V,Nb,Ta,Mo,and W can significantly reduce the maximum ordering degree of FeCo alloys.Furthermore,Nb and Ta can effectively delay the ordering kinetics.V,Nb,Ta,Mo,and W atoms at the antiphase boundary can redistribute the charge at antiphase boundary,stabilizing the antiphase boundary and reducing the antiphase boundary energies in FeCo alloys.(3)The existence of L60-Fe3Co in the slowly cooled Fe-24Co alloy was proved by XRD.TEM observation shows that the width of superdislocations in slowly cooled Fe24Co and 1J22 alloys are29 nm and 8.8 nm,consistent with the theoretical values.The ordering of Fe3Co reduces the elongation of slowly cooled Fe-24Co.Fe-22Co,Fe20Co,and Fe-18Co,but still exhibits good tensile plasticity.Ordering of Fe3Co significantly increases the work hardening rate under tensile and compressive loading.The ordering of Fe3Co leads to impact brittleness,which can only be improved by quenching to increase the cooling rate.Within the composition range where Fe is the main component.Fe Co alloy exhibits various continuous long-range ordered structures between L60-Fe3Co and B2-FeCo.These ordered structures form the largest proportion of Fe-Co first nearest neighbor atomic pairing.Therefore.the long-range order in the low temperature region within a wide range of components including FeCo and Fe3Co in the phase diagram is the through type,rather than the coexistence of L60-Fe3Co and B2-FeCo in phase equilibrium.(4)SRO-induced notch brittleness and the brittleness mechanisms were studied by experiments and molecular dynamics simulations.The SRO structures were observed by STEM in Fe-22Co alloy quenched at 500℃,containing B2 and L60-Fe3Co structures.Average SRO size and area ratio are 1.2 nm and 40%,respectively.SRO significantly increase the tensile strength of Fe-22Co and Fe-23Co alloys,but can reduce the elongation after fracture.SRO make dislocation slip mode change from cross slip to plane slip,and significantly increased the work hardening rates.Under tensile loading,SRO caused notch brittleness,resulting in the brittle fracture with a mixed mode of intergranular and cleavage.Under impact loading,SRO causes impact notch brittleness with cleavage brittle fracture.Molecular dynamics simulation results show that SRO cause greater stress concentration coefficient at the notch,and reduce the grain boundary strength,making an easier nucleation of the intergranular microcracks.SRO significantly increase the antiphase boundary energy and γisf/γusf in Fe-Co alloys.In the process of crack propagation,SRO can suppress the dislocation and twin plasticity at the crack tip,resulting in rapid crack propagation and notch brittleness.(5)The effects of LRO,SRO,and antisite defect on the magnetic properties were studied by first-principles calculations and experiments.The ordering of B2 and SRO in FeCo alloys leads to volume expansion,while the ordering of Fe3Co and SRO in Fe25Co alloys leads to volume shrinkage.Correspondingly,the ordering of B2 and SRO in FeCo alloy leads to an increase in atomic magnetic moment,while the ordering of Fe3Co and SRO in Fe-25Co alloy leads to a decrease in atomic magnetic moment.The anti-site defect in B2 FeCo reduces the atomic magnetic moment,while increases the magnetic moment of L60-Fe3Co.By detecting the magnetic properties of the alloy,it was found that the long-range and short-range ordering in Fe-23 wt%Co significantly reduced the saturation magnetic polarization intensity Js.Inhibiting long-range and short-range ordered structures in low cobalt alloys can simultaneously improve their impact toughness and saturation magnetic polarization strength...