*3.4. DSC*

The DSC curves for samples extracted from the midradius of the discs of the Ni50.3Ti/Ni49.6Ti composite with 4, 16 and 32 layers after HSHPT are shown in Figure 5. The equipment is capable of measuring temperatures in the range of –150 ◦C to 150 ◦C, covering the martensite transformation temperature range for both alloys. The thermograms of severe plastic-deformed composites revealed two peaks upon heating and two peaks after cooling. It was possible to identify the transformation temperatures corresponding to the Ni-rich (lower temperatures) and Ti-rich (higher temperatures) SMAs. The peaks between 80 ◦C and 120 ◦C represented the martensite (B19')-to-austenite (B2) reversible transformation for the shape memory Ni49.6Ti50.4 (Ti-rich) alloy.

**Figure 5.** DSC curves of the 4 (black), 16 (blue) and 32 (red) layered Ni50.3Ti/Ni49.6Ti composites after HSHPT.

The peaks between 10 ◦C and –30 ◦C represented the austenite (B2)-to-martensite (B19') reversible transformation for Ni-rich Ni50.3Ti49.7 alloy. Making successive deformation to produce 4, 16, 32 layers of composite did not significantly change the transformation temperatures. This result was obtained despite the fact that the degrees of deformation of the multilayered bimetallic composites with 4, 16 and 32 layers ranged from 1.17, 2.41 to 4.65. Likewise, the grain size was significantly reduced, while the cumulative degree of deformation increased. The reasonably stable transformation temperatures may be explained by the fact that the HSHPT process imposed a complex strain due to the high pressure concomitant with high rotational speed of the upper punch. The friction between punches and sample heated the severely plastic-deformed disc, thereby contributing to the rearrangemen<sup>t</sup> and decrement of the lattice defects. However, the intensity of the peaks was seen to decrease slightly with the increasing number of layers.

On the other hand, the Ms temperature slightly increased in the initial state in both alloys, as compared to the Ni-Ti SMAs that were rich in titanium and nickel. These results can be attributed to the grain refinement brought about by severe plastic deformation.

The Ni50.3Ti/Ni49.6Ti composite showed reversible martensitic transformation subsequent to SPD. Postdeformation annealing was not required in contrast to deformation using other severe plastic deformation methods. The HSHPT technique combines SPD imposed on the sample at RT by HPT with PDA caused due to the heat generated because of friction occurring between the anvils and the sample.

Another important element that could be observed was the absence of the second step of the martensitic transformation, even in the Ni-Ti SMA that was rich in nickel. In the initial state, this alloy showed a two-stage phase transition of B19' ↔ R-phase ↔ B2 (Figure S3 of Supplementary Materials). But the DSC curves corresponding to severely plastic-deformed discs exhibited just one strong transformation stage, namely B19' ↔ B2. After HSHPT, the intermediate R-phase transition was suppressed, as observed in our earlier results from research on a Ni-rich Ni-Ti alloy [22].

The DSC curves at the center and edge of the disc, which had 32 layers after processing, are illustrated in Figure 6.

**Figure 6.** DSC curves for the 32 layers of Ni50.3Ti/Ni49.6Ti composite after HSHPT. Thicker lines: center sample; thinner lines: edge sample.

In the HSHPT condition, the DSC curves revealed two peaks upon cooling and two upon heating, both for the edge sample and the middle sample only in the four-layered composite; for the 16- and 32-layered samples, the DSC peaks associated with the Ni-rich layers were observed only for the center and edge samples, respectively. The exothermic and endothermic peaks of the Ti-rich alloy broadened, perhaps because of the increased density of the dislocations. The severe plastic deformation of Ni-rich alloy led to a considerable broadening of the exothermic peaks which were, however, still visible. Processing by HSHPT is complex because the imposed strain varies, both with the rotational speed of superior punch applied to the composite and with the position within the disc. Consequently, the thickness of the discs was slightly higher at the center where the cooling speed would be expected to decrease. The microstructure developed more rapidly at the edge than at the center of the disc in all plastic deformation processes by torsion at high pressure. The absence of transformation peaks associated with the Ni-rich alloy could be related to heterogeneous deformation of the HSHPT material [23].
