抄録
The Stage II type fatigue crack propagation processes in 18-8 austenitic steel were studied upon observation of the substructure formed in the vicinity of its fatigue crack, by the X-ray microbeam diffraction technique and by the replica electron microscopic method. The main results obtained are as follows.
(1) Although 18-8 austenitic steel has very low stacking fault energy, it breeds subgrains in the vicintiy of the fatigue crack, characteristically indistinct and indefinable in observation by microbeam X-ray diffraction technique.
(2) The area of the substructural zone thus formed in the vicinity of the fatigue crack is small as compared with that formed in the metals where screw dislocation cross slips easily and is nearly limited within the grain that contains the fatigue crack. The fatigue deformation in the grain is remarkably uneven. In this way the X-ray Laue method is considered to be suitable for the detailed investigation of substructure formed in the fatigue deformation area, and various information can be obtained from each grain by its application.
(3) An α-martensite has been detected in the vicinity of fatigue crack and fatigue crack tip. Fatigue crack can be grown by the repeated cleavage-like fracture of the martensitic structure, and therefore, the fatigue fracture mechanism in 18-8 austenitic steel is considerably different from other metals.
(4) Well defined cross slip lines are observed in the fatigued 18-8 austenitic steel, but the region in which this cross slip occurs is limited to the spot near the grain boundary. This will suggest that the grain boundary acts as the strong obstruction to dislocation motin.
