## 3 Results
### Fatigue Performance of Heat Treatment
#### 3.2.2 Fracture Surfaces of Different Specimens  
Figure 7 shows the representative fracture surface and crack initiation zone of the as-built specimen. The entire fracture surface is divided into three zones, as shown in Figure 7a. Among them, Zone I (the zone included in the red box) is the crack initiation zone. Figure 7b is an enlarged view of the crack initiation zone. Zone II is the crack propagation zone, which occupies most of the fracture surface area. Zone III is the transient interruption zone, which occurs when the load-bearing area is insufficient. Cracks occur in the polished as-built specimen at minor near-surface defects, which are LOF. The defect is located about 26 \(\upmu\)m from the specimen's surface, and its fatigue life is 38,600 cycles. During fatigue propagation, the dominance of the multi-faceted fracture morphology can be seen, indicating the lower toughness of the as-built specimen. There are also many holes in the crack propagation surface near the transient fracture zone, indicating that the fatigue fracture is brittle and associated with the \"brittleness\" of the acicular martensite of the as-built specimen. The lower fatigue life also indicates that the acicular martensite structure has a low resistance to crack initiation--that is, the acicular martensite is more sensitive to porosity. Figure 8 shows representative fracture surfaces of different heat-treated specimens. The figure indicates that each heat treatment fatigue specimen initiates cracks from the defects on or near the surface, and all propagate from a single source. The fracture surfaces are clearly divided into three zones. By comparison, it is found that the fatigue propagation surface (zone II) of 850 FC and 950 FC is relatively smooth (Figure 8a,b), 850 WAC and 950 WAC followed (Figure 8d,e), and 1050 FC and 1050 WAC (Figure 8c,f) exhibited a rougher surface. No obvious river lines can be seen on the smooth surface, meaning that it has good plasticity. On the contrary, the river pattern is obvious on the rough surface, which means brittleness, which is also consistent with the performance of the microstructure. The \(\beta\) phase in the two-phase zone improves the plasticity of the material, while with the coarsening of the \(\alpha\) phase, the dislocation stress increases and the plasticity decreases, so 850 FC shows a smoother surface. Comparing zone III, it is found that 1050 FC and 1050 WAC show a smaller area of the transient fracture area, indicating that the \(\alpha\) colony exhibits better resistance to fatigue fracture. Figure 7: Macroscopic fatigue fracture surface (**a**) and the crack initiation zone (**b**) of the as-built specimen. Figure 9 shows the crack initiation zone of the heat-treated specimens corresponding to Figure 8. The figure indicates that all of the heat-treated specimens were fatigue fractured due to LOF defects near the surface, and the fatigue cracks propagated in a fan shape. The crack initiation of 850 FC (Figure 9a) is about 15 \(\upmu\)m from the specimen's surface, with a fatigue life of 2,733,000 cycles, showing a high fatigue life. The crack initiation of 950 FC (Figure 9b) is about 3 \(\upmu\)m from the specimen's surface, with a fatigue life of 575,400 cycles. The crack origin of 1050 FC (Figure 9c) is about 8 \(\upmu\)m away from the specimen's surface, with a fatigue life of 78,700 cycles. The surface of the source zone shows obvious cleavage morphology, and a large red arrow can be observed. The larger the grain size, the larger the cleavage plane, which also verifies that the macroscopic mechanical properties are determined by the size of \(\alpha\) colony. 850 WAC (Figure 9d) nucleated from relatively large surface defects, with a fatigue life of 1,521,300 cycles. Both 950 WAC (Figure 9e) and 1050 WAC (Figure 9f) nucleated from tiny pores on the surface, with a fatigue life of 671,100 and 141,700 cycles, respectively. The cleavage characteristics of the 1050 WAC source zone are not obvious, and secondary cracks appear in the propagation zone (red arrow).