**2. Experimental Procedure**

For this study, multilayered shape memory Ni50.3Ti/Ni49.6Ti composites were produced using HSHPT severe deformation at ambient temperature. The materials used in this investigation were cut from commercial shape memory Ni49.6Ti50.4 (at.%) sheets and superelastic Ni50.3Ti49.7 (at.%) rods. The martensitic transformation temperature Ms for the SMA that is rich in titanium is 51 ◦C, while it is –16.5 ◦C for Ni-rich alloy.

The details of the HSHPT procedure and the ad hoc machine used in the present work are given in our earlier papers [19,21]. To enable microstructural refinement concurrent with bonding of layers, the SPD process variables were chosen utilizing an EATON SVX024A1-4A1B1 frequency converter via PLC XC 200. The speed of rotation of the upper punch was maintained at 900 rpm. Initially, a pressure of 20 bars was applied using the bottom punch. The pressure levels monitored making use of the Hottinger Spider 8 equipment were between 0.01 GPa and 0.68 GPa, depending on the number of layers. The maximum torque reached was 42 Nm. The processing time lasted between 11 and 28 s. The maximum pressure was applied for less than 5 s.

HSHPT was first applied on each Ni-rich sample (about 9.5 mm × 7.4 mm and 2.35 mm in thickness) and Ti-rich sample (9.5 mm in diameter and ~2.35 mm in thickness) with austenitic and martensitic structures, respectively, at room temperature. The second step involved was fabricating the composites. To obtain two- and three-layer composites, discs with di fferent chemical compositions were made to overlap alternatively in di fferent successions. In the third step, these modules were cut in half and assembled as sandwich stacks. Four-layered composites were obtained by halving the two-layered composite and overlapping the parts in the HSHPT machine. The same procedure was used to obtain 8, 16 and 32 layers. Three-layered composites were obtained by overlapping the obtained Ti-rich, Ni-rich and T-rich disks. Five-layered composites were obtained by overlapping half of the obtained three- and two-layered composites. Six-layered composites were obtained by overlapping the obtained three-layered composite (Ti-rich, Ni-rich and T-rich half disks) with another three-layered composite (Ni-rich, Ti-rich and Ni-rich half disks). Nine-layered composites were obtained by overlapping the half of obtained four- and five-layered composites. Twelve-layered composites were obtained by overlapping the obtained 6-layered composite, and 24-layered composites were obtained by overlapping the obtained 12-layered composite (Figure 1). The cumulative degrees

of deformation of multilayered bimetallic composites, calculated using the formula ε = *h*0 *h*1 , (where h0 is initial thickness of the sample and h1 is the final thickness of the sample), ranged from 0.95 to 4.65. The SPD discs produced were with d ≤ 40 mm and t = 1.5–0.15 mm.

Microstructural examinations highlighted the ability to manufacture multilayered composites and revealed the reliable bond of layers, as well the reduction in grain diameter accomplished using the HSHPT technique. Investigation of the multilayered Ni50,3Ti/Ni49,6Ti microstructure was done using an OLYMPUS BX51 (manufactured by Olympus microscopes, Tokyo, Japan) optical microscope, with the QCapture (QuickPHOTO MICRO 2.3, Prague, Czech Republic) software package, under bright and dark field modes. The microstructure was studied using a Zeiss (ZEISS EVO MA15, manufactured by Carl Zeiss Microscopy GmbH, Jena, Germany SEM/EDX (Scanning Electron Microscope coupled with Energy Dispersive X-ray analyzer) to study the grain structure and the quality of the joints.

**Figure 1.** Processing route of Ni50.3Ti/Ni49.6Ti composite discs cut in half and assembled as sandwich stacks with: (**a**) number of layers multiple of 2 and (**b**) number of layers multiple of 3.

An in-depth microstructural analysis was also carried out using a TEM (Transmission Electron Microscope, Model Tecnai 20G2, FEI, Hillsboro, OR, USA), operating at a voltage of 200 kV. The martensitic transformation temperatures were measured using a differential scanning calorimeter. The DSC tests were run using a DSC 204 F1 Phoenix model from Netzsch (Selb, Germany). The tests were performed between −150 ◦C and 150 ◦C using a cooling and heating rate of 10 ◦C/min, under a protective gaseous nitrogen atmosphere. Specimens (15–20 mg) were obtained from the HSHPT discs for 4, 16, 32 layered composites:


Prior to DSC, an etching solution of HF:HNO3: H20 (1:5:10 in volume) was used to remove the oxidation of the surface layer and the regions affected by the cutting process.

#### **3. Results and Discussion**
