**2. Material and Experimental Techniques**

Commercially-pure Al 1050 rods (Fe content of 0.352 ± 0.015 wt%) were inserted into 99.9% pure copper tubes with an outer diameter of 10 mm and wall thicknesses of 0.4 and 0.7 mm, in such a manner that compression was exercised on the aluminium rod surface, Figure 1. The copper tube was heated to 150 ◦C to extend its diameter sufficiently to insert the cold aluminium rod. After the tube cooled down, it shrank and created the compressive stresses at the surface of the rod. The initial average grain sizes of the aluminium and copper were around 58 ± 25 μm and 96 ± 37 μm, respectively.

**Figure 1.** Aluminium rods cladded by copper with different thicknesses before deformation. (The left image is a backscattered SEM image and the right one is an optical microscope image.).

Samples were subjected to one and two passes of equal channel angular pressing (ECAP) using Route A (no rotation between passes) and Route BC (90◦ rotation between passes). This was followed by annealing according to two different schedules—namely, 200 ◦C for 5 min and 120 ◦C for 2 h.

The alloy's microstructure in all conditions was characterised by high resolution scanning electron microscope (HRSEM) Zeiss Ultra Plus, Jena, Germany). The interface zone, which was formed by intermixing and diffusion, was characterised by energy-dispersive X-ray spectroscopy (EDX) (Oxford Instruments, UK) in HRSEM.

A Vickers hardness test was performed using a Buehler MMT-7 micro-hardness tester (Lake Bluff, IL, USA). The samples were polished with 500 grit sandpaper and 10 hardness measurements were performed for each sample using a load of 200 g. The standard deviations from the mean value were calculated as between 2.8 and 3.6.

The electrical resistivity of all these samples was measured at room temperature using the four-point constant-current (DC) method. The resistivity was calculated as:

$$
\rho = \frac{V}{I} \cdot \frac{S}{l'} \tag{1}
$$

where *V* is the voltage change between two points on the side of the tubular samples at a distance of *l'* = 5 mm from each other, measured by a Keithley 2700 multi-meter. In addition, *I* = 10 A was the constant current applied by a TDK Lambda current source through the top and bottom sides of the samples over an area S of the tube cross-section, which was in contact with copper plates under ~250 Pa pressure.
