*3.2. Mechanical Properties at Room Temperature*

Typical RT tensile stress–strain diagrams of the SR-, ST-, and HIP-treated specimens are shown in Figure 4a–c, along with the stress–strain diagram of the wrought annealed alloy in Figure 4d. For the build orientation dependency evaluation, tensile diagrams for the horizontal and vertical specimens have been superimposed.

**Figure 4.** Tensile stress–strain diagrams (RT) of the (**a**) SR-, (**b**) ST-, and (**c**) HIP-treated LPBF alloy and (**d**) wrought annealed alloy.

An examination of these diagrams reveals that at room temperature, the SR specimens manifested the strongest build orientation dependency and the most hardened mechanical behavior as compared to their ST and HIP counterparts: after SR, the yield strength (YS) and the ultimate tensile strength (UTS) of the horizontal specimens were higher, while their elongations to failure (ε) were lower than those of the vertical specimens (Figure 4a). After ST, both the strength characteristics and their orientation dependency decreased (Figure 4b), while the elongations to failure increased, as compared to the corresponding values for the SR specimens. The YS and UTS values of the horizontal ST specimens were still slightly greater than those of the vertical ST specimens, while the elongations were lower. The HIP specimens did not manifest any orientation dependency and their elongations to failure were similar to those of the ST specimens, while their strength characteristics were slightly lower (Figure 4c).

#### *3.3. Mechanical Properties in the 25–871* ◦*C (1600* ◦*F) Temperature Range*

Typical stress–strain diagrams of the SR-, ST-, and HIP-treated LPBF specimens and the wrought annealed alloy are presented in Figure 5 for the temperature ranging from 25 to 871 ◦C (1600 ◦F).

*J. Manuf. Mater. Process.* **2019**, *3*, 75

**Figure 5.** Stress–strain diagrams of the (**a**) SR-, (**b**) ST-, and (**c**) HIP-treated specimens and (**d**) wrought annealed alloy in the 25–871 ◦C temperature range.

Globally, at elevated temperatures, the horizontal SR specimens exhibited higher YS and UTS and lower elongations as compared to their vertical counterparts (Figure 5a), which was similar to the room temperature behavior of these specimens. It was seen, however, that the higher the testing temperature, the lower the orientation dependency in terms of the YS and UTS values, but the higher this dependency in terms of their elongations to failure. As shown in Reference [16], after low temperature ST and HIP treatments, the mechanical behaviors of the corresponding specimens became build orientation-independent, and therefore, only vertical specimens were tested in the present work (Figure 5b,c).

In the 427–538 ◦C temperature range, the SR specimens manifested the highest YS and UTS values as compared to those of the ST (HIP) and wrought alloy specimens, but at the expense of lower elongations. In this specific temperature region, the LPBF specimens (both ST and HIP) and wrought specimens manifested similar mechanical behavior. However, starting at 593 ◦C and up, the ductility of the ST and HIP specimens decreased, while that of the wrought alloy increased (the same trend was observed for the SR specimens at 649 ◦C and up). Vertical ST and HIP specimens manifested lower elongations than their vertical SR counterparts, but higher elongations than their horizontal SR counterparts. Thus, in the 593–649 ◦C temperature range, the mechanical behavior of the LPBF alloy was characterized by increasing brittleness, whereas the wrought alloy showed only a slight decrease in ductility.

At 760 ◦C, both the SR and wrought specimens exhibited yield strength peaks, while the ST and HIP specimens both manifested significant work hardening. At 871 ◦C, yield strength peaks and work softening were observed for all the specimens. At this temperature, the mechanical strength parameters of all the specimens were similar, while the elongations to failure of the LPBF alloy specimens were significantly lower than those of the wrought alloy, irrespective of the post-processing conditions of this study.

Additionally, the fracture analysis of the SR and HIP specimens after their tensile testing at 538 and 760 ◦C confirmed that in this temperature range, the specimens manifested fracture mechanisms corresponding to the transition from ductile to brittle behavior [24]. As seen in Figure 6, at 538 ◦C, the fracture surfaces showed a mostly ductile fracture mode characterized by dimples (Figure 6a,c), while at 760 ◦C, intergranular brittle fracture occurred (Figure 6b,d). Moreover, at 760 ◦C, after HIP, the fracture surface reflected visible triple points between equiaxed grains.

**Figure 6.** Fracture surfaces of the (**a**,**b**) SR (horizontal) and (**c**,**d**) HIP specimens, tensile-tested at (**a**,**c**) 538 ◦C and (**b**,**d**) 760 ◦C.
