High-strain rate superplasticity of nanocrystalline silicon nitride

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Superplasticity is phenomenologically defined as the ability of a polycrystalline material to exhibit extraordinarily large elongation at elelvated temperatures and at relatively low stresses. Silicon nitride is a hard, strong and stiff material. It is brittle, and lacks the ductility of metals at ambient temperatures. The application of superplasticity makes it possible to fabricate ceramic components just like superplastic metals. High-strain-rate superplasticity of covalent ceramics, such as silicon nitride, renders the superplastic forming an attractive technology for shaping components efficiently.1)High-strain-rate superplasticity was achieved in Si_3N_4-TiN-Si_2N_2O nanocomposite. The nanocomposite could be deformed at 1 X 10^<-2> s^<-1> at 1600℃ which was 10 to 100 times higher than strain rate of conventional superplastic Si_3N_4. The nanocomposite could be deformed also at 1300℃, which was 300 degree lower than the forming temperature of Si_3N_4.2)The effect of viscosity of i … More ntergranular glass on deformation was studied by using Si_3N_4 containing Y_2O_3-Al_2O_3-SiO_2 glass at grain boundaries. The relation between flow stress and glass composition corresponded to the effect of chemical composition on viscosity of the glass qualitatively. However, the flow stress was not proportion to the viscosity of the glass, because the composition of intergranular glass phase had changed by dissolving Si_3aN_4.3)The stability of grain boundary glass film was used by using spatially resolved electron energy-loss spectroscopy(EELS) analysis in liquid-phase-sintered ultrafine silicon carbide material. The YAG phase was crystallized from the intergranular glass during deformation at elevated temperatures. The crystallized YAG phase prohibited grain boundary sliding, and brought about fracture.4)The effect of dopant on deformation of covalent ceramic materials was studied by using fine-grained SiC. The small amount of boron doping enhanced superplastic deformation of SiC significantly. The contribution of grain-boundary diffusion to the accommodation process of grain boundary sliding decreased as the amount of boron doping decreased. The apparent contribution of the dislocation glide increased with boron doping Less