*3.1. Optical Microscopy*

Figure 2 illustrates bright and dark field optical micrographs (OM) of the multilayers observed in the Ni50.3Ti/Ni49.6Ti alloy composite, whose sample was cut as a cross-section around the middle of the discs. The bright field micrograph of the three-layered Ni50.3Ti/Ni49.6Ti/Ni50.3Ti alloy composite is shown in Figure 2a.

**Figure 2.** Optical micrograph of Ni50.3Ti/Ni49.6Ti composite discs: (**a**) bright field image of three layers and (**b**) dark field image of 24 layers.

This feature is consistent with the refined microstructure resulting from severe plastic deformation by HSHPT. While some precipitate particles could be observed, the grain boundaries were not resolved, as the size of the microstructural features was beyond the resolution range of the optical microscope. The bonding between the sheets was discontinuous, narrow and could hardly be detected, since the chemical compositions were almost the same. The dark field optical micrograph of the 24-layered composite (Figure 2b) showed flowlines and waviness, as highlighted by the color contrast. This is in good agreemen<sup>t</sup> with HPT findings for bi-layers in the Al/Mg composite [18]. The thickness of the layers in this composite was about 22 μm. The interfacial layers were less obvious as the number of layers of the composite increased. The quality of the bonding was evident.
