材料期刊网

通过扫描电镜系统分析了伸长率＞5％和零伸长率多晶Be室温拉伸断口形貌.发现多晶Be无论伸长率高低,拉伸断口均平整、无颈缩;断口宏观形貌分纤维区和放射区,微观形貌既有裂纹沿一定结晶学表面扩展形成的解理断裂特征,又有一定塑性变形产生的撕裂棱,属准解理断口.但是,伸长率＞5％的多晶Be断口纤维区和放射区界限不清晰,放射花样细小且走向多变,断口没有明显的主裂纹源,断裂是由多个裂纹源汇合所致.而零伸长率多晶Be断口纤维区和放射区界限清晰,放射花样粗大且走向单一,并且纤维区占整个断口比例极小,放射花样则几乎遍布断口通区,断口上可见明显的主裂纹源,主裂纹源中心往往存在某种组织缺陷,断裂主要是单一裂纹扩展所致.这表明多晶Be的伸长率主要来自于裂纹形核阶段,微观组织缺陷造成裂纹过早地达到临界裂纹扩展尺寸,是导致多晶Be材室温伸长率降低的主要原因.

The tensile fractography of polycrystalline beryllium with zero and ＞5％ elongations at room temperature was examined by scanning electron microscopy.The results show that the tensile fracture of polycrystalline beryllium is of flat surface without necking.Regardless of elongation of polycrystalline beryllium,its macroscopic fractography has fibrous and radical zones,and its microscopic fractography has cleavage fracture patterns produced by cracks propagating along some specific crystallographic surfaces,together with tear ridges produced by some plastic deformation.Therefore,tensile fracture of polycrystalline beryllium is of quasi-cleavage one.For the polycrystalline beryllium with ＞5％ elongation,the boundaries between fibrous and radical zones in the fracture area are not clear,and radial patterns are tiny and of multiple directions.The main crack source is not obvious in the fractograph,and the fracture is caused by the confluence of multiple cracks.In contrast,for the polycrystalline beryllium with zero elongation,the boundaries between fibrous and radical zones in the fracture area are clear,and the radial patterns are coarse with one direction.The radical patterns are mainly across almost all regions of the fracture area,together with very limited fibrous zones.The main crack source is obvious,where some kinds of microstructure defects exist,and the fracture is mainly controlled by a single crack propagation.It can thus be concluded that the elongation of polycrystalline beryllium mainly arises from the microcrack nucleation stage.Microstructure defects lead microcracks to prematurely reach the critical size of crack propagation,which is responsible for the poor ductility of polycrystalline beryllium at room temperature.