对粗晶纯Al多晶体的疲劳断口进行了扫描电镜分析,在应变控制循环形变下,晶粒粗大而延性很好的金属的疲劳断口具有某些特征:断裂源少而集中,断裂源附近裂纹呈扇形扩展,疲劳条纹为循环解理刻面,每一条纹由较宽的解理刻面和窄的解理台阶两部分组成,台阶往往由于塑性形变而钝化;在裂纹扩展第Ⅱ阶段的疲劳条纹清晰、连续,布满裂纹扩展第Ⅱ阶段的整个区域,不同水平面形成的条纹间以“麻花形”的条带连接;多处观察到沿条纹形成的二次微裂纹,有些已发展成二次宏观裂纹,并有二次疲劳条纹;在断口上可观察到解理小舌台;断口上无最后拉断的静断区,对疲劳条纹的形成机制进行了初步的探讨。
SEM photos for very coarse grained and high ductile polycrystalline pure Alshow certain following characteristics: A few fracture sources are concentrated, the fan--shapedcrack propagated near them, and the fatigue striations are cycle cleavage facets. Each striationconsists of wide cleavage facet and narrow cleavage step. These steps often blunt due to plasticdeformation; The fatigue striations are clear and continual, and distribute to whole fracture sur-face in propagation zone of II stage. Two fatigue striations, formed in different level, areconjoined by a "twisted baked piece of pastry" band; The secondary crack along fatiguestriations were observed frequently. Some of them have already developed into secondarymacrocracks and secondary fatigue striations were found clearly on fracture surface of secon-dary crack; The whole fracture surface can be divided into two parts, crack initiation and crackpropagation zones, however, no statical--fracture zone was observed. The mechanism of fatiguestriation formation was preliminarily discussed.
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