# Superior Mechanical Behavior and Fretting Wear Resistance of 3D-Printed Inconel 625 Superalloy
## 3 Results and Discussion
### Tensile Tests  
Figure 4a shows one of the five performed tensile stress-strain curves of Inconel 625 alloy. The inset gives comparison results of the tensile test for Inconel 625 samples in the present work and literatures [25; 26; 27; 28; 29; 30; 31]. Apparently, the tensile strength of different processing samples are not identical. Compared with the available data of sintering-processed and welding-processed samples, the ultimate TS of AM-processed Inconel 625 is significantly improved. The sintering and welding samples have a lower ultimate TS (420-830 MPa) compared to that of AM-processed specimens (810-1070 MPa). In this work, Inconel 625 sample prepared by AM have a relatively high ultimate TS of 952 \(\pm\) 18 MPa. Previous work shows that in general, components made by AM have higher yield and tensile strengths than those of wrought materials of the alloy [32]. The yield strength in equiaxed metals normally relates to the grain size followed by the Hall-Petch low, which can be described as  
\[\sigma_{y}=\sigma_{o}+\frac{\kappa}{\sqrt{d}} \tag{1}\]  
where \(\sigma_{y}\) is the yield strength, \(d\) is the grain size and \(\sigma_{0}\) and \(k\) are material constants. To link the yield strength to the grain size of Inconel 625 alloys, yield strength results extracted both from this work and literatures are plotted versus \(d^{-0.5}\), as shown in Figure 4b. A linear fit of the yield strength versus relevant grain size gives values of \(\sigma_{0}\) = 105 MPa and \(k\) = 1380 MPa \(\sqrt{\upmu\upmu}\) to describe the Hall-Petch relationship between the yield strength and the grain size of Inconel 625. Nevertheless, there are some discrepancies in the predicted yield strength versus \(d^{-0.5}\), in which the datum in the present  
Figure 3: XRD results of the as-received initial Inconel 625 powders and specimen after AM process. work (793 MPa) does not lie on the line. The value of the yield strength is relative higher than the predicted result, which is likely due to the formation of precipitates in the AM process (Figure 3). Further analysis regarding the precipitate strengthening of the AM-Inconel 625 specimen needs to be carried out after the tensile tests. The corresponding fractography of AM-Inconel 625 tested at room temperature is shown in Figure 5. Dimples and particle facets of precipitates are clearly found in the fracture surface, while no obviously microcrack can be observed. This suggests that the failure of AM-Inconel 625 shows a ductile character. A high magnification of the fracture surface (Figure 5b) shows spherical precipitates with a size around 600 nm. In the case of precipitate strengthening (\(\Delta\sigma_{p}\)), the dislocation shearing or the Orowan dislocation bypass is believed to be the main strengthening mechanism. For the shearing mechanism, the improvement of the yield strength can be ascribed to contributions of coherency strengthening (\(\Delta\sigma_{cs}\)), modulus mismatch strengthening (\(\Delta\sigma_{ms}\)) and order strengthening (\(\Delta\sigma_{os}\)) [33]. The shearing mechanism is more significant for coherent precipitates with small radii and the transition to Orowan bypass mechanism is active when the radii of coherent precipitates reach a critical value or when the precipitate is separate [34], as can be seen in this work (Figure 5b). For the Orowan dislocation bypass mechanism, the increase in the yield strength can be expressed as  
\[\Delta\sigma_{\sigma}=M\frac{0.4 Gb}{\pi\lambda}\frac{\ln\left(\frac{2\pi}{b} \right)}{\sqrt{1-v}} \tag{2}\]  
Figure 4: (**a**) Tensile stress-strain curve of Inconel 625 alloy and (**b**) yield strengths plotted versus \(d^{-0.5}\) for Inconel 625 samples in the present work and literatures, the inset shows the comparison results of tensile strength for Inconel 625 samples in the present work and literatures. Figure 5: (**a**) Fractography of AM-Inconel 625 after tensile test at room temperature; (**b**) High magnification of the fracture surface in (**a**).