Study On The Microstructure And Mechanical Properties Of Fe-Cu-Si-B-(Al) Amorphous/Nanocrystalline

The family of Fe-Cu-Nb-Si-B alloys (also called Finemet alloys) is a kind of excellent soft magnetic materials and has been widely used in industrial applications. This type of the alloys is typically produced from amorphous precursors that are partially devitrified, resulting in nano-scaled ferromagnetic grains embedded in the residual amorphous matrix. However, the price of producing Finemet alloys is very high due to the high cost of the Nb element. Thus, looking for a substitution for Nb element is a hot topic around the world. In this thesis, a new series of Finemet-type alloys were designed using Al substituting Nb, and the microstructures and mechanical properties were studied. It has been shown that:(1) Fe76.5-xCu1Si13.5B9Alx (x=0,1,2,3,5,7at.%) amorphous ribbons were fabricated by single-roller melt-spinning method. The experimental results indicated that Al addition has little effect on the amorphous formation ability of the alloys. However, increasing the Al content can substantially increase Tx2, and inhibit the formation of Fe borides.(2) Three microstructures of Fe74.5Cu1S13.5B9Al2ribbons, including amorphous, amorphous/crystalline and ordered atom clusters/amorphous were fabricated by single-roller melt-spinning method. Compared to amorphous ribbons, the appearance of nano-scaled a-Fe(Si) improves the hardness and Young’s modulus of the ribbons. In addition, higher cooling rate increases the content of free volume in amorphous ribbons and enhances the nucleation and propagation of the shear bands, resulting in the decrease of the hardness and elastic modulus.(3) Isothermal annealing results in the formation of body-centered cubic a-Fe(Si) nanocrystals in the Fe74.5Cu1Si13.5B9Al2amorphous matrix. Increasing the annealing time increases the grain size of the nanocrystals. Isothermal annealing effectively increases the hardness and Young’s modulus of the Fe74.5Cu1Si13.5B9Al2alloy.(4) Fe76.5Cu1Si13.5B9alloy rod fabricated by copper mold suction casting method has different microstructures that comprise a complete amorphous structure, an amorphous/crystalline composite structure and a complete crystalline structure. Compression and nanoindentation tests showed that the hardness, strength and elastic modulus of the alloy increase with the crystalline component.This research provides a basis for fabricating Nb-free Finemet-type alloys and supports a theoretical reference for developing new Fe-based functional and structural materials.