SEM photos for very coarse grained and high ductile polycrystalline pure Al show following characteristics:A few fracture sources are concentrated,the crack propagates in fan-shape near them,and the fatigue striations are cyclic cleavage facets.Each striation consists of a wide cleavage facet and a narrow cleavage stage.Those stages often blunt due to plastic deformation.The fatigue striations are clear and continual,and distribute over whole zone of stage Ⅱ crack propagation.Two kinds of fatigue striations form in different levels,are conjoined by a“twisted baked piece of pastry”band.The secondary crack along the fatigue striations was observed frequently.Some of them have already developed into secondary macrocracks and secondary fatigue striations were found clearly on secondary fracture sur- face.The fracture surface can be divided into two parts,initiation and propagation zone of the crack,however,no statical-fracture zone was observed.The mechanism to form striations was preliminarily discussed.
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