Page 73 - MSAM-3-2

Materials Science in Additive Manufacturing Mechanical properties of NiTi TPMS

Figure 9. (Continued).

while maintaining strength, with RGCS-A6 and RGCS-A8 the maximum compressive strength of RGCS-A6 only
demonstrating the highest yield strength. exhibited a modest increase of 22.23%, reaching 29.38 MPa
In contrast, the aging heat treatment shortened the compared to RGCS-A0. These results indicate a significant
yield plateau of SGCS, leading to a rapid transition enhancement in the mechanical properties of rod-shaped
through the yield stage and prompt initiation of fracture gyroid structures due to aging heat treatment.
on reaching its strength limit. The failure strain of RGCS Five evenly distributed points were extracted from the
increased from 10% to 15%, while that of SGCS decreased DIC strain cloud diagram, from top to bottom, to analyze
from 10% to approximately 5%. These results indicate that the compression process of the gyroid, as depicted in
aging heat treatment improved the plasticity and strength Figures 7 and 8. Except for RGCS-A0, the strain at point
of RGCS but increased the brittleness of SGCS. Due to P0 from RGCS-A2 to RGCS-A10 consistently exceeded
age hardening, the strength of SGCS increased while that of the other four points after 45 s. Notably, the strain
its plasticity and toughness declined, causing it to enter at the P0 point of samples A4, A6, and A8 surpassed 2%,
the compression fracture stage more rapidly, consistent as shown in Figure 7. The upper unit of RGCS experienced
with age hardening principles. Figure 3A illustrates the the most significant compression deformation. Conversely,
precipitation of the NiTi phase in the matrix after aging the strain at the five extraction points of SGCS exhibited
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heat treatment, impeding dislocation movement within a synchronous upward trend within 20 s in Figure 8,
the alloy and thereby enhancing both yield strength and indicating that the overall structure underwent rapid,
compressive strength. Furthermore, the LPBF process uniform elastic deformation during the compression
introduced sample defects such as micropores, with SGCS process. Twenty seconds later, the rate of strain increased
exhibiting greater sensitivity to defects compared to RGCS, more rapidly at points P2, P3, and P4 in the middle
leading to faster progression to the compression fracture compared to the other points, except for SGCS-A8.
stage. This study demonstrates enhanced plasticity for Figures 9 and 10 display the strain contours of the two
RGSC after aging heat treatment while maintaining nearly structures analyzed using DIC. The compressive shear
unchanged strength. failure of RGCS was attributed to the significant stress near
the diagonal of the structure. Zhang discovered that the
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The compressive elastic modulus is depicted in Figure 6C. initial fracture of the uniform gyroid rod structure occurred
The elastic modulus of RGCS-A0 was 564.89 MPa, while on the diagonal rod at the corner with higher stress and
that of SGCS-A0 was 1011.74 MPa. Compared with the strain levels. The first fractures occurred at the bottom
untreated samples, the elastic modulus of RGCS decreased, of the cell and then extended to the upper layer. Most
whereas that of SGCS increased after aging heat treatment. fractures were observed on the diagonal rod, while very few
At 2 h of aging, the minimum elastic modulus recorded for occurred on the horizontal rod. Chen discovered that the
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RGCS-A2 was 361.52 MPa, whereas the maximum elastic homogeneous gyroid structure developed a 45° shear band
modulus observed for SGCS-A2 was 1262.82 MPa.
along the cube diagonal and fractured due to the localized
Figure 6D illustrates the compressive strength. Both shear stress aligned with the loading direction at a 45° angle.
RGCS and SGCS demonstrated higher compressive The text explained that the uniform gyroid structure was
strength after aging heat treatment compared to the prone to failure when diagonal yield deformation occurred.
untreated samples. The maximum compressive strength of However, the angle of the fracture zone of RGCS was slightly
SGCS-A4 reached 69.48 MPa, marking a 61.39% increase larger than 45°, and the position showed an offset near the
over the compressive strength of SGCS-A0. Conversely, diagonal. The RGCS-A2 and RGCS-A6 displayed a 45°

Volume 3 Issue 2 (2024) 10 doi: 10.36922/msam.3137