*3.2. SEM*/*EDX Analysis*

The SEM microstructure of the 12-layered specimen demonstrates a typical SPD structure (Figure 3a). The microstructural modification by HSHPT results from three opposing effects. In the first stage, low pressure and high speed of the punch act together, leading to an increase in temperature of the material to almost 800 ◦C (estimated using a temperature sensor-CT laser radiation pyrometer T2 MHCF OPTC).

**Figure 3.** SEM image of HSHPT-processed metallic composite: (**a**) 12 layers, (**b**) 24 layers.

At this point, high pressure was exerted on the discs, leading to severe deformation. Besides shearing, recovery and recrystallization of grains took place due to the samples attaining high temperature. Grain refining during HSHPT should result from dynamic recrystallization that takes place at high temperature; it could be said that rapid cooling to room temperature "freezes" the UFG structure produced at high temperature. On the surface of the sample, only curved lines could be observed, and not grain boundaries. The materials were welded without a detectable intermediate

area. The adhesion of layers was noticeable. The presence of smooth interfaces between layers was attributed to the specific condition created during HSHPT. Other methods of manufacturing the metallic composite led to the occurrence of an intermetallic layer at the interface.

To investigate the distribution of elements in the Ni rich/Ti-rich areas, a line scan by EDX was run on the nine layers of the disc (Supplementary material: Figure S1). The nine layers were emphasized by the variation of the Ti (a) and Ni (b) content, respectively. Across the layers (quasi 20 μm), alternating areas Ni-richer or Ti-richer could be identified. EDX characterization was performed in an area comprising the nine-layers of the composite. Figure S2 of the Supplementary Materials presents the opposing variations of Ni and Ti contents in the successive nine layers of the composite. The severity of plastic deformation introduced by HSHPT produced rotation and plastic flow of large volumes of material caused by upper punch rotation at high speeds [19]. The 3-D images of the surfaces (seen in Figure S2 of Supplementary Material) sugges<sup>t</sup> the arrangemen<sup>t</sup> of the distinct layers.

### *3.3. Transmission Electron Microscopy*

Figure 4 illustrates a TEM micrograph (bright field) of the four-layered Ni50.3Ti/Ni49.6Ti composite. The UFG structure with an equiaxed morphology prevailed after HSHPT. The average size of the grains was about 200–300 nm.

**Figure 4.** TEM image of the 4 layered Ni50.3Ti/Ni49.6Ti composite.

The image highlights equiaxed subgrains isolated by dislocation cells. Between them, several nanograins were also interspersed. These grains, with a size of under 50 nm, formed clear boundaries. Various dislocation cell configurations, including high-density dislocations, zones of dislocation tangles or condensed dislocation boundaries, were characteristic of the rather inhomogeneous microstructures of the sample.
