High-strain rate superplasticity of nanocrystalline silicon nitride

超塑性在现象学上被定义为多晶材料在高温和相对低应力下表现出极大伸长率的能力。氮化硅是一种坚硬、坚固的材料。它很脆，在环境温度下缺乏金属的延展性。超塑性的应用使陶瓷元件的制造与超塑性金属一样成为可能。氮化硅等共价陶瓷的高应变速率超塑性使超塑性成形成为一种有吸引力的高效成形技术。1) Si_3N_4-TiN-Si_2N_2O纳米复合材料实现了高应变速率超塑性。该纳米复合材料在1600℃下可达到1 × 10^<-2> s^<-1>的形变，应变速率是常规超塑性Si_3N_4的10 ~ 100倍。在1300℃（比si_3n_2o_3 - al_2o_3 - sio_2的形成温度低300℃）下，纳米复合材料也可以变形。在晶界处采用含有Y_2O_3-Al_2O_3-SiO_2的Si_3N_4玻璃，研究了i…More晶间玻璃的粘度对变形的影响。流动应力与玻璃成分的关系定性地对应了化学成分对玻璃粘度的影响。液相烧结超细碳化硅材料的晶界玻璃膜的稳定性采用空间分辨电子能量损失谱（EELS）分析。在高温变形过程中，YAG相从晶间玻璃中结晶出来。YAG相的结晶抑制了晶界滑动，导致断裂。4)采用细晶SiC材料，研究了掺杂剂对共价陶瓷材料变形的影响。少量硼的掺杂显著增强了SiC的超塑性变形。晶界扩散对晶界滑动调节过程的贡献随着硼掺杂量的减少而减小。位错滑动的表观贡献随着硼掺杂的减少而增加

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