OBRABOTKAMETALLOV Vol. 27 No. 3 2025 technology a b Fig. 3. Photographs of the obtained sheet from ingot 2-1 (TiNiHf alloy) with a thickness of 2.2 mm (a) and ingot 3-1 after fracture (b) Further deformation by hot bar rolling (HBR) and rotary forging (RF) was only done on ingot 2-2 of the TiNiHf alloy with 5 at.% Hf. Initially, two passes were rolled in one caliber from a single heating cycle, taking advantage of deformation heating. However, when switching to the second caliber, cracks appeared at the back end of the ingot. This suggests that the alloy has a narrow temperature range for plastic deformation. To prevent the ingot from cooling down, it was reheated after each pass. Changing the rolling pattern and eliminating the second pass without preheating resulted in successful rolling without fracture or new cracks. This produced a rectangular bar measuring 6.9 × 8.5 × 236.4 mm. After bar rolling, a 150 mm long sample was cut from the bar for rotary forging. Rotary forging was carried out at a deformation temperature of 850 °C, with a relative strain per pass below 10%, and the ingot was heated between each pass for 10-15 minutes. This produced a rod with a diameter of 119 mm. It’s worth noting that, unlike the approach in [23], the section of ingot 2 used here was rotary forged after prior deformation rather than in the as-cast state. This highlights the potential for combining hot bar rolling and rotary forging to produce TiNiHf SMA rods. Fig. 4 shows a photo of the bars after rolling and rotary forging. a b Fig. 4. Photograph of a TiNiHf alloy rod after caliber rolling (a) and rotary forging (b) It should be noted that rotary forging had difficulties with rod alignment and chipping at the ends, likely due to cooling. Because of this, further rotary forging to smaller diameters (which would cool even faster) wasn’t attempted. Next, a 2.1 × 27.5 × 56 mm sample was cut from the hot-rolled sheet from ingot 2-1 for cold rolling (CR). After cutting, the sample was cleaned to remove the surface oxide layer by grinding and chemical etching in a mixture of nitric and hydrofluoric acids. The sheet thickness before rolling was 2.1 mm. Cold rolling was carried out with a relative strain per pass below 10%. Prior work found the critical relative strain for cold rolling TiNiHf alloy to be 20% [22]. Therefore, cold rolling was carried out with intermediate annealing at a temperature of 850 °C for 10 minutes upon reaching a relative degree of deformation close to 20%. After cold rolling to 1.04 mm, a sample was cut from the sheet for further cold rolling to fracture to redetermine the critical strain. Fig. 5 shows the sheet before and after cold rolling, thus concluding our description of the cold rolling process.
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