表面缺陷对单晶高温合金高周疲劳性能的影响

在定向凝固炉中采用螺旋选晶法制备了一种单晶高温合金试棒，标准热处理后加工成旋转弯曲高周疲劳试样，试样中间位置用电火花加工成不同尺寸的孔洞以模拟叶片的表面缺陷，在980 ℃、应力分别为400 MPa和500 MPa条件下，研究表面孔洞对合金高周疲劳性能的影响，用扫描电镜分析了疲劳试样的断口形貌.结果表明，与标准试样相比，带有孔洞合金的高周疲劳寿命都有不同程度的降低，随着表面孔洞尺寸增大，合金的疲劳寿命逐渐减小.在合金试样的高周疲劳断口上可见疲劳源区、裂纹扩展区和瞬断区.相对于标准试样，带有孔洞试样疲劳源除了试样表面，还有表面孔洞，所有试样都为多源疲劳断裂.与高温下拉伸持久的断裂机制不同，高温下旋转弯曲高周疲劳为类解理断裂. 

Effect of surface defects on the high cycle fatigue properties of a single crystal superalloy

A nickel-based single crystal superalloy was casted using spiral crystal selection in the directional solidification furnace. The standard cylindrical specimens for rotary bending high cycle fatigue test were machined after standard heat treatment. The void with different size was made by an electro-spark machine in the middle of the fatigue sample to simulate blade surface defects. The effect of surface void on high fatigue property of the alloy at 980 ℃ with stress 400 MPa and 500 MPa was investigated respectively. SEM was used to examine the fracture surface of fatigue specimens. The results show that the fatigue lives of the sample with surface void decreased to some extent compared with standard sample and it decreased with the rising size of surface void. SEM observations show that the crack initiation site, the fatigue crack propagation area and the rapid fatigue fracture area can be observed. Compared with standard sample, the fatigue cracks of the sample with surface void initiated at void including surface. All the samples were multi-source fatigue fracture. The rotary bending high cycle fatigue at high temperature all exhibited qusi-cleavage fracture which is different from tensile and stress rupture at same temperature.