High-strain rate superplasticity of nanocrystalline silicon nitride

超塑性是指多晶材料在升高的温度和相对较低的应力下表现出非常大的伸长率的能力。氮化硅是一种坚硬、坚固和坚硬的材料。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构件的制造成为可能，就像超塑性金属一样。氮化硅等共价键陶瓷的高应变速率超塑性使超塑性成形成为一种有效的成形技术。1）Si_3N_4-TiN-Si_2N_2 O纳米复合材料具有高应变速率超塑性。该复合材料在1600℃下的变形速率可达1 × 10 ~（<-2>-3）s <-1>~（-1），比传统超塑性Si_3N_4的应变速率高10 ~ 100倍。该复合材料在1300℃也能发生形变，比Si_3N_4的形成温度低300 ℃。 ...更多信息 用含Si_3N_4的Y_2O_3-Al_2O_3-SiO_2系晶界玻璃研究了晶界玻璃对形变的影响。流变应力与玻璃成分的关系定性地对应于化学成分对玻璃粘度的影响。但由于Si_3aN_4的溶解改变了晶界玻璃相的组成，使得流动应力与玻璃的粘度不成正比。3）采用空间分辨电子能量损失谱（EELS）分析了液相烧结超细碳化硅材料中晶界玻璃膜的稳定性。YAG相在高温变形过程中从晶间玻璃中结晶。结晶后的YAG相阻碍了晶界滑移，导致晶界开裂。4）以细晶SiC为材料，研究了掺杂对共价键陶瓷材料变形的影响。少量的硼掺杂显著增强了SiC的超塑性变形。随着硼掺杂量的减少，晶界扩散对晶界滑动调节过程的贡献逐渐减小。位错滑移的表观贡献随着硼掺杂的增加而增加

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