Study On Recrystallization And Mechanical Behaviors Of Fe-Si Alloy

Iron-silicon alloys have been widely applied on electric power and telecommunications industries due to their excellent magnetic properties. With the increase in silicon content, the magnetic properties can be improved significantly.However,high silicon content can also make the alloys more brittle,making it hard to form. This severely hinders the application of high silicon alloys. Previous researches show that the forming properties of alloys can be improved significantly through controlling the microstructures, specifically directional microstructure and special grain boundary characterization. In this thesis,recrystallizations of Fe-6.5wt% Si and Fe-6.0wt% Si are investigated during both iso-thermal and directional annealing. The main focuses are on the effects of annealing parameters on the microstructure evolution, including the texture and grain boundary character distribution. The mechanical behaviors of alloys after iso-thermal and directional annealing are also evaluated.The research results indicated that under isothermal annealing, the microstructure evolution includes the typical recovery, recrystallization and growth stages. Above 1000?,abnormal grain growth can be observed. The grain size and grain growth rate increase with the increase in temperature and annealing time.During recrystallization, the fraction of small angle grain boundaries decreases.The numbers of defects, such as dislocations are also decreased during recrystallization, leading to a decrease in hardness of alloys and a slight increase in elongation. Grain coarsening after annealing at high temperature lowers the mechanical properties. Isothermal annealing at 700 ? leads to the formation of cubic texture and a decrease in the number of CSL ?13 grain boundary,the tensile strength of Iron-silicon alloys in this tempreture is 770MPa.However, increasing the annealing temperature to 850? leads to a decrease in cubic texture while an increase in the number of CSL ?13 grain boundaries. These indicate that the formation of grain boundary is dependent on the annealing temperature and recrystallization processes.The Recrystallization activation energy of Iron-silicon alloys with 70%deformation is 181KJ/mol.Columnar grain structures with grain aspect ratio of ?10 are successfully obtained through directional annealing. With a annealing temperature of 1000? and withdrawing velocity of 5-8?m/s, the largest aspect ratio of grains can be obtained.Below 950 ? and above 1100?, it is hard to obtain the structures composed of columnar grains. In the specimen with 70% thickness reduction,directional annealing leads to a smaller amount of small angle grains and larger amount of CSL ?13 grain boundaries, as compared to that in the isothermal-annealed specimen. While in the forged specimen, directional annealing produces a grain boundary character distribution with a high frequency of both low angle and low ? coincidence site lattice boundaries. A thickness reduction during cold rolling of ?30% can be obtained in the resulting material.