Study On Plastic Deformation And Microstructure Of FeCoCrMoYCB Bulk Amorphous Alloy

In this thesis, Fe-based(Fe41) metallic glass with a diameter of 4 mm prepared using copper mold suction method has been investigated. The samples were annealed with under static pressure at different temperatures, and X-ray diffraction(XRD) and differential thermal analysis(DSC) were applied to study the effect of temperature and static pressure on the crystallization behavior of the BMGs in therm-mechanical process. High temperature compression tests of this Fe-based amorphous alloy were carried out using Gleeble 3500 to analyze the effects of temperature and strain rate on the plastic deformation. The fracture morphologies were observed by scanning electron microscope(SEM) to discuss the plastic deformation mechanism in the supercooled liquid region. Therefore, the effect of plastic strain on the structure and thermal stability of the BMG under the same compression condition was studied,respectively.The results of thermo-mechanical experiment reveal that the crystallization behavior of Fe41 is retarded by the pressure when annealing temperature is higher than glass transition temperature. Pressure inhibits the precipitation of Fe3 B phase when the annealing temperature is 720 ℃, the average grain size of the none pressure annealing sample is 35 nm, the average grain size of the pressure annealing sample is28 nm; Fe23B6 phase is also inhibited by pressure at 820 ℃, and the average grain size of the none pressure and pressure annealing samples are 45 nm and 57 nm,respectively. By calculating the percentage of crystallization, it’s found that the crystallization percentage of the pressure annealed specimen is lower than that of the non pressure annealed specimen at different temperatures.The results of high temperature compression test show that the plastic deformation of Fe41 bulk amorphous alloy has a great sensitivity to temperature and strain rate,and the alloy exhibits a good ductility of 75 % with no crystallization when the temperature is 620 ℃ and the strain rate is 1×10-2 s-1. The shear zone of the "fringe" and "river" like characteristics are randomly distributed on the high temperature compressive fracture surface of the bulk amorphous alloy, and plastic flow characteristics are found on the fracture surface of the BMG in the range of 1×10-3 s-1~ 1×10-2 s-1 at 620 ± 5 ℃. The high temperature deformation of bulk amorphous mayoccur under the joint action of free volume and shear zone.No crystallization phase is found in the specimens after deformation of 20 %, 40 %and 75 % at the condition of 620 ℃ and 1×10-2 s-1, and with the increase of deformation, the crystallization temperature(Tx) and glass transition temperature(Tg)enlarged and the width of the supercooled liquid region increases, especially, when the deformation is 75 %, the width of the supercooled liquid region raises from original 40 ℃ to 59 ℃. In the case of this bulk amorphous alloy, the plastic deformation in supercooled liquid region makes the thermal stability more excellent.The microhardness of the bulk amorphous alloy is also affected by the plastic deformation in supercooled liquid region. It performs that the hardness in large strain position is higher than that in small strain position.