一种镍基双晶材料疲劳裂纹扩展效应

对晶界平行裂纹和晶界垂直裂纹的双晶体进行三点弯曲疲劳实验，研究了双晶材料的疲劳裂纹扩展规律，测定了双晶的疲劳扩展速率，揭示了晶界对晶粒疲劳裂纹扩展的屏蔽效应：当裂纹距晶界某一特定长度时，裂纹扩展速率最快；而裂纹顶端交于晶界时，裂纹扩展速率最侵．进一步的晶体滑移有限元数值分析揭示了这种屏蔽效应的机理：晶界附近不协调的塑性变形，导致了裂纹尖端应力场的重新分布．

SHIELDING EFFECT AT NI-BASE BICRYSTALS GRAIN BOUNDARY

Fatigue behavior is studied experimentally for a Ni-base bicrystal. Two three-point- bend(TPB) specimens are tested. One TPB specimen is named Bicrystal I, whose pre-crack is along the grain boundary and the applied load parallels to pre-crack direction, the other TPB specimen is named Bicrystal 2, whose pre-crack is perpendicular to the grain boundary and the applied load parallels also to the pre-crack. The crack growth rate of the bicrystals is measured. The grain boundary-induced shielding effect on fracture is found in the Bicrystal 2. The fatigue behavior of Bicrystal 2 specimens is dependent on the distance between the crack tip and the grain boundary. The crack growth rate is highest when the crack tip is at the critical distance to the grain boundary, while the rate was the lowest when the crack tip reachs the grain boundary. After the crack grows across the grain boundary, the crack growth rate increases. With the help of SEM analyses and numerical calculations, the mechanisms for the grain boundary-induced shielding are discussed. The grain boundary-induced redistribution of stresses near the crack tip, which is caused by incompatible plastic strain at grain boundary, induces the grain boundary-induced shielding.