*3.2. Interface Formation during Room Temperature Deformation*

Interface formation is typically governed by three mechanisms, namely, intermixing, inter-diffusion and phase formation [14,15]. Room temperature deformation leads to intermixing when the surface asperities are crushed during contact under high hydrostatic pressure and severe shear deformation [14], mixing the constituents in a swirl flow (Figure 3). As a result of SPD; however, grain refinement by rearrangement of accumulated dislocations takes place and many new defects are introduced (for example, solid solutions of constituent atoms).

**Figure 3.** The inter-mixing zone resulting from intensive shear of constituents under high hydrostatic pressure (black lines show the boundary of the intermixing zone where random islands of copper within the aluminium can be seen).

Subsequent static annealing results in accelerated inter-diffusion, which adds to the formation of the interface, Figure 4. SEM-EDX analysis of selected points in the vicinity of interface points 1–5 shows the diffusion of copper atoms into the aluminium matrix to a depth of about 750 nm while aluminium atoms diffused into the copper matrix to a depth of around 250 nm, Figure 4a. The concentration of aluminium at a distance of ~200 nm from the interface within the copper matrix is around 12 at%, Figure 4b, which is below the concentration required to form any intermetallic compound [7]. These results are similar to previously reported observations of interface formation in aluminium/copper bimetallic tubes made by high-pressure tube shearing [11], and the formation of intermetallic compounds detrimental to conductivity was not observed.

(**a**)

**Figure 4.** *Cont.*

(**b**)

**Figure 4.** SEM-EDX analysis of selected points in the vicinity of the interface at two locations: (**a**) and (**b**).

#### *3.3. Hardness*

It can be seen, Figure 5, that the aluminium hardness raises gradually as the number of passes increases. Due to the small number of ECAP passes; however, this increase is insignificant, from ~36 HV at the initial annealed conditions to 40–42 HV after one ECAP pass and to 45–48 HV after two ECAP passes. In contrast, the copper cladding hardness rises significantly from its initial annealed value of 58 HV to 85–105 HV after deformation. The Cu-Al interface hardness displays a gradual increase compared to the aluminium hardness due to non-homogeneous intermixing of constituents.

**Figure 5.** Hardness after deformation versus distance from the centre measured at six points in the aluminium part (points 1–4), at the interface (point 5) and in the copper cladding (point 6). (The two similarly coloured points at the interface and at the copper cladding represent measurements for thick and thin sheaths).

It should be noted that annealing results in a decrease in the aluminium hardness and an increase in the interface and copper sheath hardness (Figure 6). The effect of increased strength in ultrafine-grained (UFG) materials due to annealing has been observed and discussed quite recently in several publications [16,17]. The reason for this phenomenon was believed to be the segregated impurities at the grain boundaries. Nevertheless, the issue has still not been elucidated as high purity metals in a nano-crystalline state demonstrated a similar effect. Further, several UFG materials show hardening by annealing while others do not. For example, copper annealed after high-pressure torsion at temperatures in the range of 0.25 to 0.3 Tm (depending on the copper purity) demonstrated similar hardening behaviour, which was explained by agglomeration and annihilation of deformation-induced vacancies [18].

**Figure 6.** Comparison of hardness after deformation (two passes Route BC) and annealing measured at six points located in the aluminium part (points 1–4); at the interface (point 5); and in the copper part (point 6) (measured for sample with thick sheath).

In our case, the temperature of annealing was chosen within the range defined for copper in [17]. Subsequently, hardening of the copper and the interface zone was observed. The chosen temperature; however, was high enough to change the crystallite size and dislocation density in aluminium, causing softening of the aluminium core.
