*3.5. Fractography Observation*

Fracture property is one of the critical considerations in material design and plays a significant role in the engineering field. The longitudinal and transversal fracture morphologies are analyzed via SEM. The macroscopic fracture morphology of the tensile specimens is shown in Figure 16. It can be seen that several inter-layer lines are observed from the longitudinal specimen after fracture, as shown in Figure 16a. This phenomenon results from the inferior mechanical properties of inter-layer areas, which are treated as the weak link. It has been proven that strain tends to be concentrated in the weaker areas in the non-uniform regions and cause fracture in these regions [38]. This is consistent with the strain evolution and local strain analysis that has been previously mentioned, interpreting the anisotropic mechanical properties. The surface of the macroscopic fracture of the longitudinal

specimen is shown in Figure 16b where a bumpy and undulating surface morphology can be observed. The surface of the macroscopic fracture of the transversal specimen is illustrated in Figure 16c where the surface is smooth and located at a certain angle with the axial tensile stress.

**Figure 16.** Macroscopic fracture appearance of tensile specimens. (**a**) Longitudinal and transversal specimens after fracture; (**b**) macroscopic fracture of longitudinal specimen; (**c**) macroscopic fracture of transversal specimen.

In order to give a further illustration of this phenomenon, a comparison between the inter-layer area and deposited area on the Taylor factor is conducted by electron backscattered diffraction (EBSD). It is generally accepted that the Taylor factor is an important parameter, which determines the stress required to activate a slip system. This means that the Taylor factor plays an important role in tensile behavior, followed by the relationship:

$$
\sigma = M \tau\_{\xi} \tag{1}
$$

where σ is the applied stress; *M* is Taylor factor; and τc is the critical resolved shear stress on each of the activated slip systems [39].

The comparison results of the Taylor factor distribution are shown in Figure 17. Compared with the distribution of the Taylor factor in the deposited area shown in Figure 17b, a less uniform distribution of the Taylor factor in the inter-layer area can be observed in Figure 17a. This confirms that nonuniform strength distribution takes place in the inter-layer area. The statistics for the Taylor factor distribution are depicted in Figure 18. The standard deviation of the Taylor factor in the inter-layer area is 0.907, larger than that of the deposited area (0.865).

**Figure 17.** Comparison between the inter-layer area and deposited area on the Taylor factor: (**a**) Inter-layer area; (**b**) deposited area.

**Figure 18.** Statistics between the inter-layer area and deposited area with the Taylor factor: (**a**) Inter-layer area; (**b**) deposited area.

A higher standard deviation of the Taylor factor means less uniform strength distribution in the inter-layer area. During the tensile test, to maintain the consistence of deformation, the grains located on both sides of the boundary are forced to deform consistently. Thus, nonuniform deformation and local stress concentration take place around the boundary, resulting in zigzag surface features near the fracture of the longitudinal specimen, as shown in Figure 16b, where it is constant with the strain evolution and local strain analysis that has been previously mentioned, interpreting the anisotropic mechanical properties. The result mainly corresponds to the directional growth of the microstructure, which has been confirmed by previous investigations [40].

The fracture morphologies of the transversal and longitudinal specimens are shown in Figure 19. It can be seen from Figure 19a,b that the transversal and the longitudinal fracture surfaces both present as grey and fibrous, revealing ductile fracture. Additionally, a large number of equiaxed dimples with a relative uniform distribution are apparent on the fracture surface of the transversal specimen, shown in Figure 19c. In contrast, the mixed-rupture characteristics of quasi-cleavage and small dimples emerge on the fracture surface of the longitudinal specimen, as depicted in Figure 19d. Through the comparison of the appearance of dimple dimensions and depths of the transversal and longitudinal fracture, one can find the transversal are greater, which indicates that the transversal specimen has better ductility. Although the feature of quasi-cleavage is observed from the local part of the fracture on the longitudinal specimen, it can still show high ductility. Therefore, it is concluded from the result of the fracture analysis that the longitudinal mechanical property is inferior to that of the transversal one, which is in accordance with the above analysis.

**Figure 19.** Fracture morphology of the tensile specimens: (**<sup>a</sup>**,**b**) Macroscopic fracture morphology of the transversal and longitudinal specimens; (**<sup>c</sup>**,**d**) typical fracture features of the transversal and longitudinal specimens.

During the process of fracture analysis, a large number of uniformly distributed particles are found in the dimples, as shown in Figure 20a. The particles may be instrumental in the formation of dimples [41]. In this study, most particles are smaller than 1 μm, implying that during the process of fracture, larger stress is required for the formation of microvoids, and that the growth rate of microvoids is lower [42]. Energy dispersive X-ray spectroscopy (EDX) is applied to analyze the composition of the particles, and the results are shown in Figure 20b. It can be seen that Fe, Ti, Al, O, Mn, and Si are the main elements of the particles, which can be identified as non-metallic inclusions such as Al2O3 or Ti2O3. The existence of the particles can be considered as nucleation points for dimples.

**Figure 20.** The distribution of particles and their EDX analysis: (**a**) distribution of second phase particles; (**b**) the EDX analysis.
