In Fig. 2, both the cross-sectional and longitudinal SEM images of drawn Cu-Nb microcomposite wires show Cu matrix in dark regions and Nb phases in light regions. Nb phase is highly curling on cross section (Fig. 2a), and highly aligned and elongated along the drawing direction (DD) (Fig. 2b). They are (25 ± 5) nm thick and (1036 ± 20) nm wide, giving an aspect ratio of around 41.4. The aspect ratio is defined by w/t, where w is the width of the ribbon and t is the thickness. Thus, the Nb phase exhibits a thin ribbon shape on the longitudinal section and can be treated as finite thin films, since the ratio is much higher than 10 in the work by Sandim et al. [1].

Fig. 2

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	Fig. 2 SEM images showing the microstructures of as-drawn wires: a cross section; b longitudinal section. The arrow shows the drawing direction or the wire axis

Annealing at different temperatures provides different microstructures (Fig. 3). At 400 ° C or below (Fig. 3a), both the edge and surface of each Nb ribbon/film are well defined, as shown in the inset in Fig. 3a. It preserves a severe curling shape on cross section and around 25 nm in thickness, indicating that no obvious changes occurred during the heat treatment. The broke up of Nb films shown in Fig. 3a is mainly attributed to the removal of Cu matrix and ultrasonic cleaning during specimen preparation. As shown in Fig. 3b, a number of small holes/pits form within Nb films after annealing at 500 ° C. This phenomenon is different from those of the reports [1, 32, 33, 34], most of whom found the boundary splitting and groove formation along the boundaries after annealing [1, 35]. Sharma et al. [10] presented several major models for the coarsening of Nb including direct cylinderization, edge spheroidization, termination migration, discontinuous coarsening, and boundary splitting. None of these models depict the microstructure changes deriving from the wide surface of Nb thin ribbon/films. The coarsening of Nb in the present work is believed to derive from the regions with certain low-angle misorientations within Nb films, where elastic energy carried by the stress was built during the wire drawing process. Diffusion of Nb atoms from these strained regions to less-strain regions took place during annealing [36, 37], resulting in the formation of the grooves and pits as observed in this study. Besides, the curvature of Nb film is believed to provide a driving force for particle migration and coalescence within Nb films. At higher temperatures (Fig. 3c-f), recrystallization, spheroidization, and coarsening occurred, even Nb precipitation might take place [38, 39], affecting the magnetoresistance and hardness of the wires. Details for the microstructural changes will be addressed in another paper.

Fig. 3

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	Fig. 3 Longitudinal microstructures of Cu-Nb microcomposite wires after annealing at different temperatures for 1 h: a 400 ° C, the inset showing the cross-sectional image, b 500 ° C, c 600 ° C, d 700 ° C, e 800 ° C, f 900 ° C; the arrows showing drawing direction (DD) or the wire axis

Table 1 presents the sizes of Cu and Nb phases and the mean free paths (l_o) of electrons in two phases at 25 and -196 ° C. We can see that after annealing at 400 ° C, the sizes are almost unchanged which are about 25 nm for Nb and 55 nm for Cu, indicating no recrystallization occurred during the annealing, while annealing at 900 ° C both appeared obvious recrystallization followed by coarsening of Nb and Cu phase. This is coincident with the results in Fig. 3.

Table 1

Table 1

Table 1 Microstructural sizes of Nb and Cu phases (Nb phase in thickness or diameter, Cu phase in width, and electron mean free path lo) Phase lo (nm) [11, 40] Phase size (nm) 25 ° C -196 ° C 25 ° C 400 ° C 900 ° C Nb 3.1 17.6 25 25 104 Cu 43-45 138-145 54 55 133

Table 1 Microstructural sizes of Nb and Cu phases (Nb phase in thickness or diameter, Cu phase in width, and electron mean free path lo)

A large number of small Nb particles can also be observed in Fig. 3e and f, which provide large density of the phase boundaries. These small Nb particles in annealed wires are expected to be precipitates either due to large interface curvature and internal stress during wire fabrication [20] or Ostwald ripening where small particles form to decrease elastic energy [41]. More discussion will be addressed in the future.

The effect of the magnet field strength on the resistivity of the Cu-Nb composites is shown in Fig. 4. It can be seen that at RT (25 ° C), the effect of the magnetic field on the resistance of the specimens is fairly small. The MR values are only a few percent (less than 5%), even at 19.1 T, as shown in Fig. 4a. These slight variations can be negligible.

Fig. 4

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	Fig. 4 MR of wires plotted as annealing temperatures: a tested at 25 ° C; b tested at -196 ° C. The annealing temperature for the as-drawn specimen is assumed to be 25 ° C

Annealing states at 25 and 400 ° C, both the Cu sizes are comparable with l_o. This is the case where the resistivity is expected to increase and the scattering by the interfaces, and it is considered as the major source for the increased resistivity [6, 8, 11]. The MR at -196 ° C (Fig. 4b) shows no significant change up to a magnetic field of 5 T, where the Lorentz force is not sufficient for notable change in resistance, particularly for the wires annealed at temperatures below 500 ° C. This is attributed to the delay of recovery and recrystallization in Cu matrix by well-defined and high density of Cu-Nb interfaces [42, 43]. At higher fields, the Lorentz force is sufficiently high to force the electrons to change their direction from straight lines to helical paths, and thus reduces the path length they can travel before they are scattered by the grain boundaries or interfaces [44]. This leads to increase in MR. The MR of the wires annealed at 900 ° C can reach 22% at a field of 19.1 T (Fig. 4b).

At temperatures below 800 ° C (Fig. 4b), the MR increases with the annealing temperature. This may be due to the small Nb particles observed in these annealed wires. However, the MR decreases notably in the wire annealed at 900 ° C in 10 and 15 T (Fig. 4b). This decline is owed to the decrease in surface area and dislocation scattering by spheroidization and coarsening at 900 ° C annealing [6, 45].

The evolution of the microhardness before and after annealing is shown in Fig. 5. Data from some reports are also presented in Fig. 5 for comparison. It can be seen that the hardness values in present work show minor decrease at temperatures below 500 ° C, which is possibly attributed to the recovery and recrystallization of the submicro-Cu matrix (S-Cu) [42, 46] (Fig. 2a). After annealing at temperatures above 500 ° C, more drops in hardness can be observed, while the hardness obtained by Sandim et al. (blue line in Fig. 5) shows almost linear drop, which is possibly due to the larger scale of their microstructure, and the hardness of Cu matrix dropped at around 200 ° C as expected (red line in Fig. 5) [8]. Our previous study found that the lattice relaxation, recovery, and crystal rotation in Cu matrix could be suppressed by high density of interfaces, which interacted with dislocations and precipitate strengthening component of the wires [4, 47]. The microstructure did not change until the wires were annealed at 500 ° C. At temperatures over 600 ° C, the hardness of Nb drops dramatically as a result of recovery and recrystallization of Nb (green line in Fig. 5) [48]. Therefore, the minor drop at temperatures below 500 ° C is mainly due to the recovery and recrystallization of S-Cu, while the sharp drop in hardness at temperatures above 500 ° C is primarily attributed to the perforation, necking, and spheroidization of Nb ribbon/films. Furthermore, annealing, on one hand, decreases the hardness by reducing the lattice defects formed during drawing; on the other hand, it may introduce small Nb particles to reduce elastic energy, as described above, inducing lattice distortion and, consequently, leading to solid solution hardening [20]. Thus, two mechanisms, solid solution hardening and annealing softening, should be considered for mechanical property analysis during annealing process. In our wires, the sharp drop in Fig. 5 shows that the softening outdoes the solution hardening, and the change of Nb from films/ribbon to bamboo/rod in shape in Fig. 3 induces severe decrease in hardness, making softening be the dominate mechanism during high-temperature annealing.

Fig. 5

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	Fig. 5 Hardness response after annealing