700℃循环热冲击下钴基合金的裂纹扩展及应力计算

Crack propagation and stress calculation under the thermal cycling shock at 700 ℃ of cobalt base alloy

对不同缺口类型的钴基合金在700℃下进行循环热冲击试验,采用扫描电镜、光学显微镜和长度测量软件等,对合金在不同循环热冲击次数下产生主裂纹的情况进行统计分析,并计算缺口位置首次热冲击所致开裂或裂纹萌生的热应力。结果表明,不同缺口类型的试样在首次或者较低循环次数的热冲击下均产生一定长度的主裂纹;随循环热冲击次数增多,裂纹不断扩展,但扩展速率减小;热冲击过程中,在WC和Cr_7C_3这两相的内部以及它们与基体的界面处易产生微裂纹,微裂纹聚集长大形成主裂纹并沿着易产生微裂纹的区域(薄弱区域)扩展长大,发生弯曲和偏转,最终沿最薄弱区域断裂。缺口位置应力集中程度不同,致使所产生的裂纹长度以及裂纹产生所需要的热循环次数不同。通过有限元计算结果发现:缺口的应力集中程度对开裂产生重要影响,离缺口越远的位置产生的正应力越小。

Thermal cycle shocking test at700℃were performed for cobalt base alloy tensile specimens with different notches,main cracks at different thermal cycle shocking number were investigated by scanning electron microscopy(SEM),optical microscope(OA),and length measuring software.The thermal stress which makes crack initiation under the first thermal shock was calculated by the ABAQUS finite element software.The results show that,the main cracks with a certain length are produced at different types of notch samples under the first or less thermal shocking cycles.The more number of thermal cycle shocking,the longer the crack propagation,but the crack propagation rate become slower.There are distinct differences in physical properties between WC,Cr7C3phases and matrix,and it is easy to produce micro cracks in internal and substrate interface of two phases.Micro cracks and main cracks connect,propgate and deflect along the weakest zone,finally the specimen fractures along the weakest zone at higher thermal cycle.The difference of stress concentration at notch root leads to the difference of the crack length and the number of thermal cycle.The result of finite element calculation indicates that the degree of stress focusing concentration at notch root has an important effect on cracking,and the farer from the position of the notch root,the smaller the normal stress.