The Evolution Of Ordering Formation And Its Effect On Mechanical Properties In Fe-6.5Wt%Si Alloy

Fe-6.5wt%Si alloy (high silicon electrical steel) is an excellent soft magnetic material with high permeability, high resistivity, low iron loss and near-zero magnetostriction, which can be used to improve the efficiency of electric devices, save energy, and reduce noise. However, its room temperature embrittlement and poor workability seriously hinder its practical application in industry. Recently, cold rolled Fe-6.5wt%Si alloy sheet with0.05-0.1mm in thickness has been successfully fabricated by the advanced technique of stepwise ductilization in our group. Terefore, the purpose of this thesis is to understand its embrittlement mechanism, analyze the influencing factor of ductility, and investigate the stepwise ductilization mechanism in the fabrication of high silicon steel sheets.In this thesis, the order-disorder phase transition and its influence on mechanical properties were systematically investigated, clarifying the brittleness essence of the Fe-6.5wt%Si alloy. The effects of cooling rate and deformation on ordered phases were studied, which will provide methods and ideas for the performance improvement of the alloy. A quantitative ordering analysis method was proposed to investigate the ordering evolution in the whole fabrication of high silicon steel, providing the theoretic foundation for stepwise ductilization technique.(1) The order-disorder transition temperatures of the Fe-6.5wt%Si alloy were determined. The transition temperature for A2-B2and B2-D03are760℃and640℃respectively. The B2structure is formed from A2by unlike-atom (Fe and Si) pairing of the first nearest neighbors, and its growth follows the ideal kinetic equation for grain growth. Additional ordering between the second neighboring atoms causes the phase change from B2to D03When developing to a sufficient extent, the D03domain inside will decompose into another D03’domain along<100> direction with different Si atom occupation, forming the regularly arranged strip contrast.(2) The ordering processing in the D03phase region deteriorates the mechanical properties of the Fe-6.5wt%Si alloy. The disordered Fe-6.5wt%Si alloys are heat treated within B2and D03phase region for different time respectively to attain alloys with different ordering content. For the alloys annealed in the B2phase region, their flow stress has no obvious regularity, and their micro-hardness remains generally stable, and all of them exhibit good ductility; while for the alloy annealed in D03phase region, the flow stress and micro-hardness tend to rise with time within1h, afterwards they remain stable, and moreover, the ductility is gradually reduced with time and the fracture mode belongs to brittle cleavage fracture.(3) High cooling rates during heat treatment or fabrication process, e.g. quenching, air cooling method and rapid solidification, can suppress the formation of DO3structure and improve the ductility of Fe-6.5wt%Si alloy. While the slow cooling rates, e.g. furnace cooling and common vacuum induction melting, are helpful for the formation and growth of the DO3structure. DO3ordering can decrease the mobility of dislocations. Dislocations tend to gather and react to form networks within the grain or around the boundaries, which further impedes the slip of dislocations and easily cause stress concentration, so the ductility is reduced and the alloy is easily failed with brittle intragranular fracture and intergranular fracture.(4) The deformation process can destroy the B2or DO3ordering, during which the domain size, ordering content, and ordering degree are gradually reduced, meaning that the deformation induces disordering in Fe-6.5wt%Si alloy. Deformation induced disordering is the result of the movement of superlattice dislocations, and it starts from the mechanical APBs on the slip plane. The movement of numerous superlattice dislocations will expand the disordered region and decrease the degree of order gradually. When the Fe-6.5wt%Si alloy is deformed within the disordered region and followed by air cooling, the existent high density of dislocations will restrict the formation of ordering and reduce the ordering content greatly.(5) Fe-6.5wt%Si alloy exhibits remarkable work softening behavior when being deformed within intermediate ordered phase region, and the softening mechanism is deformation induced disordering and dynamic recovery. During the deformation, the domain size, ordering content and ordering degree are decreased gradually. At the later stage, dynamic recovery occurs. Plenty of dislocation cells and sub-grains are formed, which greatly reduce the dislocation density and effectively remove part of work hardening. The two aspects soften the alloy together.(6) A new quantitative ordering analysis method using integrated electron diffraction intensity is attempted, and an algorithm is developed. The method is verified to be reasonable and feasible by experiment, and is applied to analyze the ordering evolution during the whole fabrication process of Fe-6.5wt%Si alloy sheets. It is found that the ordering content is gradually reduced during the cast- forging-hot rolling-warm rolling-cold rolling process, which provides theoretical support for the successful fabrication of ultra-thin sheet by stepwise ductilization technique.