Epiphot 300

In order to reveal the microstructure, a standard metallographic technique was used, consisting of its grinding with SiC paper sheet, polishing to a mirror finish and etching with Kroll reagent (HF + HNO₃ and deionized water, 1:3:96) [9 ]. The microstructure was observed with a Nikon EPIPHOT 300 optical microscope coupled to a Nikon FDX-35 camera (Nikon Instruments Europe B.V., Amsterdam, The Netherlands).

The samples were prepared for microstructural observation following a standard metallographic technique: smoothing with 400-, 600-, 1200- and 1500-grit SiC paper sheet, polishing with 0.3 μm alumina to a mirror finish and chemical etch with Kroll’s reagent (HF + HNO₃ and distilled water in 1:3:96 proportions) [34 ]. The microstructure was characterized using a Nikon EPIPHOT 300 optical microscope coupled to a Nikon FDX-35 camera (Nikon Instruments Europe B.V., Amsterdam, The Netherlands).

Prior to performing the corrosion tests, the discs were subjected to metallographic preparation in order to detect their initial microstructure, which allowed comparison with the microstructure after the performed corrosion tests. Samples from the discs were placed in a hot investment mass, so that it was possible to grind the surface with abrasive paper of 180-4000 # on the grinding and polishing machines, Buhler Automet 250 and Eco Met 250 (Buhler, Swiss). Polishing with felt and polishing with an Al₂O₃ suspension of 1 μm followed on the same devices. After polishing, the discs were cleanedusingultrasound. This was followed by mild chemical etching with a solution of 3 mL HF, 6 mL HNO₃, and 100 mL H₂; etching time was 105 s [23 (link)].
The Nikon Epiphot 300 (Japan) optical microscope was used for observations of the initial output microstructure. Figure 1b shows the representative optical microstructures of the NiTi discs. The microstructure is typically a lamellar eutectic structure with phase composition: NiTi and Ni₃Ti, which is consistent with previous research [26 (link)]. The bright phase is NiTi, containing around Ni = 50 at.% and Ti = 50 at.%, while the second darkminority phase is a eutectic phase composed of NiTi_eut. and Ni₃Ti_eut. (Ni = 75 at.% and Ti = 25 at.%). The grains were almost the same size (about 30 μm), and the grain boundaries were clearly noticeable.