Page 78 - MSAM-3-2

Materials Science in Additive Manufacturing Mechanical properties of NiTi TPMS

toward quasi-cleavage fracture mechanisms in RGCS-A4, treatment at 350°C, the density of dimple and tear ridge in
RGCS-A6, RGCS-A8, and RGCS-A10. the NiTi alloy increased, accompanied by the appearance
Figure 12 depicts the fracture morphology of SGCS of micro-cracks, indicative of ductile fracture. Subsequent
after the compression test. Similar to RGCS-A0, the heat treatment at 450°C resulted in the emergence
fracture morphology of SGCS-A0 exhibited characteristics of substantial microcracks, with the cleavage surface
typical of river patterns and cleavage steps, without evident becoming predominant. In addition, grain refinement was
macroscopic plastic deformation. This observation suggests observed after solution treatment, leading to a decrease in
that brittle fracture is the primary fracture mechanism. the average particle size from 52.43 µm to 15.45 µm.
After aging heat treatment, numerous dimpled fractures 3.3. Effect of aging heat treatment on the
were observed in SGCS-A2 and SGCS-A4, indicating a superelasticity
shift toward ductile fracture mechanisms in these samples.
With aging time exceeding 6 h, the dimples in SGCS Superelasticity is an important characteristic of NiTi SMA,
became shallower and fewer in number. Notably, the tear denoting its capacity to undergo significant deformation on
ridge observed in SGCS-A6 was significantly higher than the application of stress to the austenite phase, exceeding
in SGCS-A8 and SGCS-A10. These results suggest that its elastic limit strain, and subsequently automatically
54
quasi-cleavage fracture was the fracture mechanism in reverting to its original shape during unloading.
SGCS-A6, SGCS-A8, and SGCS-A10. Figures 13 and 14 depict the stress-strain curves of RGCS
and SGCS subjected to 2%, 4%, 6%, and 8% compressive
Yan observed a significant presence of ductile dimples strain at 100°C, respectively. RGCS-A0, RGCS-A2,
39
on the fracture surface of samples treated with a solid RGCS-A6, and RGCS-A8 exhibited the capability to
solution, indicating the dominance of ductile fracture. withstand a compressive strain of 8%, accompanied by a
After compression, the fracture pattern resembled a river- reduction in unrecoverable strain from 3.12% to 1.27%
like formation, indicating rapid crack propagation speed. with increasing aging time. Conversely, SGCS exhibited
This phenomenon was attributed to the absence of a plastic commendable superelasticity up to 6% compressive strain,
zone in the NiTi sample to prevent crack propagation with a maximum unrecoverable strain not exceeding
during compression. Bhardwaj noted that after aging heat 1.35%. However, only the SGCS-A6 and SGCS-A8 samples
53
A B C

D E F

Figure 16. Shape memory behavior of rod-shaped gyroid cellular structure (RGCS) under different strains with different aging times. (A) 0 h; (B) 2 h;
(C) 4 h; (D) 6 h; (E) 8 h; and (F) 10 h.

Volume 3 Issue 2 (2024) 15 doi: 10.36922/msam.3137