Page 351 - IJB-10-1

International Journal of Bioprinting Corrosion behavior of SLM-prepared 316L steel

corresponded to the locations at which the microstructure
observations were performed (see section 2.2.3). Each
E 111  υ 111  sample was then mechanically ground and polished using
111
111
σ ij =  ε 111 + 111 ( ε 11 + ε 22 + ε 111 ) ij (III)
33
−
( + υ 111 )  12 υ  diamond solutions with the coarseness down to 3 µm,
1
cleaned, and consequently rinsed in deionized water.
where E is 247.8 GPa, υ is 0.24 , d 111 is the inter-
68
111
0
111
planar distance of {111} planes for original material with 2.2.3. Structure observations
The structure observations were performed on the
no residual stress, and are indices denoting stress tensor cross-sectional cuts from the swaged pieces. Before the
ij
components (if principal stresses are considered, the non- investigations, the feature of the rotary swaging process
zero stress components are for hoop orientation, for was considered (i.e., the fact that the shear strain is
11
22
radial orientation, and for axial orientation).
33 imposed from the periphery of the processed workpiece
Last but not least, spurious strains, which occurred in toward its axis; see ref. ). Therefore, microstructure
73
the vicinity of the workpiece periphery due to incomplete analyses were performed in the mid-radius distance for
overlapping of the workpiece and gauge volume, were each swaged workpiece. In other words, the observations
eliminated. These strains can cause aberration peak shifts for samples 20, 17, and 15 were performed in the distance
and lead to misleading interpretation of the acquired data of 5 mm, 4.25 mm, and 3.75 mm from the periphery of
(the principles of the used method can be found in ref. ). the workpiece, respectively. The examined locations
69
The residual stress data was eventually depicted graphically corresponded to the locations in which the corrosion
as contour plots with five major and eight minor levels testing was performed. As for the methods, scanning
using the Origin Pro 8.1 software (OriginLab Corporation, and transmission electron microscopy (SEM and TEM)
Northampton, USA). were used; for SEM, we applied the electron backscatter
diffraction (EBSD) method. The used devices included a
2.2.2. Electrochemical corrosion Tescan Lyra 3 XMU FEG/SEMxFIB microscope (Tescan
Biocompatibility of the AISI 316L stainless steel and Orsay Holding a.s., Brno, Czech Republic) equipped with a
(possible) effects of its usage within a human body on the Symmetry EBSD detector (Oxford Instruments, Abingdon,
surrounding tissues have been investigated by numerous United Kingdom), and a JEM-2100 TEM microscope
researchers 70-72 . We performed the electrochemical (JEOL, Tokyo, Japan).
corrosion testing in a simulated body fluid (SBF), which is
a solution with ion concentrations closely similar to those The SEM-EBSD analyses were carried out on the
in the human blood plasma. However, the mentioned transversely cut, cross-sectional samples prepared by
correspondence is only an approximate. Thus, Oyane et manual grinding, manual polishing, and finally electrolytic
al. 50-52 introduced i-SBF, whose ion concentrations match polishing. In order to reliably assess the substructural
those of the blood plasma in dissociated amounts. Besides, features, such as low angle grain boundaries (LAGB)
i-SBF is highly suitable to assess the in vitro activity of and local (Kernel) misorientations, the scan step was
bio-applicable metallic materials. The preparation and selected to be 0.4 µm. The TEM substructure analyses
chemical composition of the i-SBF used in this study were were performed on manually ground foils, which were
also inspired by other published works 50-52 . The pH of the then electrochemically etched. All the acquired data was
used i-SBF was 7.40 ± 0.05 at 37°C. After preparation, the then evaluated using the AZtecCrystal software (Oxford
fluid was deaerated for 15 min with a flow of nitrogen gas at Instruments, Abingdon, United Kingdom). The considered
5 ml/min. As for the experimental setup, we used standard limits for the grains and grain boundaries were 5° for
three-electrode electrochemical cell supplemented with an LAGB and 15° for high angle grain boundaries (HAGB);
SP-150 potentiostat (BioLogic Company, Inc., Seyssinet- the (ideal) texture orientations were evaluated with 15°
Pariset, France) and a standard calomel reference maximum deviation.
electrode. Open circuit potential (OCP) measurement was
performed for at least 60 min before the potentiodynamic 3. Results
scan was initiated. The potentiodynamic scan with 1 mV/s 3.1. Residual stress
scan rate at 25°C started from -0.6 V (below OCP) and The results of analyses of residual stress distribution within
SCE
ended at 1.2 V (above OCP), or at the breaking current the sample 20 in the hoop, radial, and axial orientations
SCE
of 100 µA. are graphically depicted in Figure 2A–C), respectively. As
The samples for the analyses were prepared from cross- can be seen, the residual stress distributions in the hoop
sectional cuts from the swaged workpieces acquired after and radial directions (Figure 2A and B), which were both
each of the three last swaging passes; the exposed locations oriented perpendicular to the axis of the workpiece (see

Volume 10 Issue 1 (2024) 343 https://doi.org/10.36922/ijb.1416