Studies on Substructures around a Fatigue Crack in <I>fcc</I> Metals and Alloys

To establish the relationship between the substructure formation and fatigue crack propagation, substructures around a fatigue crack in aluminium, copper, α-brass and aluminium-bronze have been examined by means of an X-ray microbeam technique and transmission electron microscopy.The results are: (1) So-called plastic zones have been observed in the vicinity of fatigue cracks in all the metals and alloys examined. (2) In α-brass and aluminium-bronze, subgrains are formed only in the plastic zones. (3) In copper and aluminium, subgrains in the plastic zones are smaller than those in the matrix. (4) The higher the stacking-fault energy, the more pronounced becomes the substructure formation. (5) Fatigue cracks in aluminium have been found to propagate through a specimen of the well-developed substructure more easily than through a specimen of the less developed substructure. Direct evidence that fatigue cracks propagate along sub-boundaries has been obtained. (6) Strain-concentrated regions as well as the plastic zones have been observed around fatigue cracks by an electron microscope.In view of the above experimental results, the substructure formation is concluded to play an important role in the propagation of fatigue cracks.

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