## 3 Results and Discussion
### Effect of DB and LTGN on Fatigue Behavior of 304 CNASS
#### 3.3.2. Fractography  
Figure 13 shows the fracture surface of the fatigue specimen from Group I (stress amplitude of 590 MPa; number of cycles to failure: 250,300). Several centers of macrocrack formation are observed, but only the one outlined in red is a major macrocrack (Figure 13a). This crack did not necessarily form first. It is possible that before this, another macrocrack formed and developed, but split into several sections upon encountering a local obstacle. As a result, the local stress, which is the cause of macrocrack growth, sharply decreases below its critical value, whereupon the development of another macrocrack begins. The mechanism of formation and development of the main macrocrack is shown in Figure 13b. Region A shows the nucleation of microcracks that merge and form the macrocrack. These microcracks are localized in the microzone of opposing slip systems, causing displacement of dislocations in two intersecting planes. This microzone is situated at the boundary between the hard, but brittle, AL and the bulk material. Thus, the microzone is a natural stress concentrator and the formation site of the macrocrack that develops and causes a transcrystalline crack in the AL. The main macrocrack formed develops under the action of critical local stress between planes, with a different direction of movement of the cyclic wave (zone 1 and zone 2 in Figure 13b--general view). Zone 1 has weak slip lines, and zone 2 has an intercrystalline crack formed under the condition of hindered deformation. On both sides of the main macrocrack, linear cracks (regions B and C) are observed, as well as a region with extrusion, which is characteristic of plasticity due to high local stresses. On this basis, it can be assumed that the development of the macrocrack has a rather plastic character. Figure 12. S-N curves of specimens subjected to LTGN after different prior treatments. Figure 14 shows the fracture surface of the fatigue specimen from Group II (stress amplitude of 605 MPa; number of cycles to failure: 106,200). In Figure 14a, three zones are outlined. The micro-area of nucleation of the microcracks and macrocrack formation is located at the border between the AL and the bulk material in zone 1. The direction of the cyclic wave differs by a small angle from the radial direction. At the boundary between zones 1 and 2, an extrusion region is formed, almost perpendicular to the cyclic wavefront. The extrusion region exerts a strong resistance to the propagation of the cyclic wave and  
Figure 13: Fracture surface of the fatigue specimen (stress amplitude of 590 MPa; number of cycles to failure: 250,300), treated by turning + LTGN: (**a**) main macrocrack; (**b**) mechanism of formation and growth of main macrocrack. greatly increases the local alternating stresses. The resistance to the cyclic wave increases, and a static boundary zone forms that separates zone 2 from zone 3. As a consequence of the hindered development of the cyclic wave (leading to a sharp increase in local stresses), grain cracks (intercrystalline destruction) are formed in zones 2 and 3. The shape of the grain boundary cracks gives an indication of the tough-plastic behavior of the material. One of the prerequisites for the formation of the three zones is the linear arrangement of dislocations on separate polygonization planes. The arrangement of dislocations along grain boundaries is a prerequisite for intercrystalline destruction. Figure 14: Fracture surface of the fatigue specimen (stress amplitude of 605 MPa; number of cycles to failure: 106,200), treated by smoothing DB + LTGN: (**a**) general view; (**b**) mechanism of formation and growth of main macrocrack. The mechanism of macrocrack formation and development (Figure 14b) is similar to that of the specimen in Figure 13, but the crack-developing mechanism is more pronounced. The material below the AL shows tough-plastic behavior, and the destruction has a pronounced plastic character.