Stellite12钴基合金热循环冲击前后拉伸断裂机理研究

Investigation of Fracture Mechanism Before and After Thermal Cycling Stellite12 Cobalt-based Alloy with Tensile Tests

对不同缺口的Stellite12钴基合金试样(700℃/20℃进行不同次数的热循环冲击和未冲击)进行原位拉伸,并结合试验数据的分析以及断口形貌的扫描电镜观察,分析了Stellite12钴基合金热循环冲击前后的拉伸断裂过程和断裂机理。结果发现:热循环冲击后不同半径试样的断裂过程略有不同,热循环冲击后的小圆弧缺口试样在缺口根部产生表面微裂纹,试样边缘及微裂纹两侧产生氧化微孔;原位拉伸时,该试样热冲击过程产生的裂纹先向试样厚度方向扩展,待厚度方向贯通,然后裂纹尖端的基体发生变形、黑相(白相)穿晶开裂、少量沿氧化微孔裂开,试样瞬间发生断裂;而经历热循环冲击后的大圆弧试样表面并未产生明显的裂纹,拉伸加载过程经历大圆弧根部基体变形、黑白相内开裂、边缘氧化微孔张开,试样突然断裂;对于未冲击试样,在加载过程中,试样的断裂过程经历基体变形、黑白相内部开裂,能量聚集到一定程度试样突然断裂。对于未热冲击的三种不同试样其断裂过程基本类似,仅仅是由于小圆弧半径的试样应力集中程度更大,从而使得其断裂应力低于平板以及大圆弧试样。

By means of in-situ tensile tests of Stellite12 cobalt-based alloy(after the thermal cycling shock at 700℃/20℃for different times and unshocked)with different notch size,in combination with the analysis of experiment data and SEM observation of fracture,the tensile fracture process and fracture mechanism were investigated before and after thermal cycling shock of Stellite12 cobalt-based alloy.The result of in-situ tensile experiment reveals that after the thermal cycling,the fracture process of specimen with different size notch has a little difference.There were micro surface cracks on the root of small circular notch specimen and many oxydic micropores on the specimen edge and side of the cracks after thermal cycling.When the sample experienced in-situ tensile test,cracks extended in the direction of the thickness of the specimen firstly on the process of thermal shock,to be connected in thickness direction,and then the substrate phase of the crack tip deformation occurred,black phase(white phase)transgranular cracking,oxydic micropores expanding,specimen instantaneously fracturing in the end.But the great circle specimen after thermal cycling shock generated no obvious crack on the surface.During in-situ tensile test,the substrate phase of the crack tip deformation occurred,with black phase(white phase)transgranular cracking,oxydic micropores expanding,specimen instantaneously fracturing.While,the original unshocked specimen experienced deformation,transgranular cracking and fracture when energy accumulating.Three kinds of original specimens are similar to the process of fracture because the stress concentration of the small arc radius specimen is bigger,so as to make its fracture stress lower than the tablet and the great circle sample.