Relationships between character and mechanical properties of symmetric and asymmetric grain boundaries

本文用SiO_2粒子观察法测定了Cu中取向差为θ=38.9 °和50.5°的<$9和<$11 [110]非对称倾斜晶界在φ=0 ~ 90°范围内的晶界能。本方法没有边界开槽法和三结法所不可避免的缺点。在φ=10、64.8和90°处存在三个深尖点，而在φ= 35.3°处出现一个浅尖点。而对于φ 9边界，边界能随倾角的增大而单调减小，在φ=65°时达到最小值。当倾斜角大于65°时，晶界能略有增加.在1173 K下对Cu双晶进行48 h的Bi掺杂，然后在1173 K下均匀化48 h.这些双晶体的拉伸试验在77 K和室温下在Instron-type机器上沿垂直于晶界的方向沿着进行。在室温下，φ=0 ~ 32.8°的双晶以沿晶方式断裂，而φ=54 ~ 90°的双晶以穿晶方式断裂。相反，在77 K时，在φ= 54 ~ 64°范围内观察到穿晶断裂。这种断裂方式对倾角的依赖性可以用边界能的倾角依赖性来解释。

A silica particle observation method was utilized to determine experimentally the boundary energiesof Σ9 and Σ11 [110] asymmetric tilt boundaries with misorientation angles of θ=38.9 and 50.5°, respectively, in Cu for various inclination angles between φ=0 and 90°. The present method has nodrawbacks which are inevitable in boundary grooving and tri-junction methods. In the boundary energy versus inclination angle diagram for the Σ11 boundaries, three deep cusps exist at φ=10, 64.8 and 90° whereas a shallow cusp appears at φ= 35.3°. On the other hand, in the case of the Σ9 boundaries, the boundary energy rather monotonically decreases with increasing inclination angle, and then reaches to the minimum value at φ=65°. As the inclination angle becomes greater than 65°, the boundary energy increases slightly.Copper bicrystals with the Σ9 boundaries were doped with bismuth at 1173 K for 48 h and then homogenized at 1173 K for 48 h. Tensile tests of these bicrystals were carried out on an Instron-type machine at 77 K and room temperature along the direction normal to the grain boundaries. At room temperature, the bicrystals with φ=0 to 32.8° fracture in an intergranular manner, whereas those with φ=54 to 90° fracture in a transgranular manner. On the contrary, the transgranular fracture was observed at φ= 54 to 64° at 77 K.Such dependence of the fracture manner on the inclination angle can be accounted for by the inclination angle dependence of the boundary energy.