Effect of groove rolling on the microstructure and properties of Cu–Nb microcomposite wires

1.   Introduction

Cu–Nb microcomposites have attracted interest for their excellent mechanical strength, high conductivity, and high temperature resistance [1–3]. The Cu–Nb microcomposite has a good combination of mechanical strength and conductivity properties, and is often the best candidate for a high magnetic field magnet. Therefore, it is of great significance to develop and research Cu–Nb microcomposites to further their performance [4]. The microstructural stability of Cu–Nb microcomposites wires fabricated by a bundling and drawing process has been successfully applied to the Wuhan National High Magnetic Field Center, China, and a pulsed high magnetic field of 90.6 T has been achieved. However, there is a need for further improvements in the performance of Cu–Nb microcomposite wire to aid the rapid development of the pulsed high magnetic field.

In theoretical predictions to surpass a pulse magnetic field of 100 T, the tensile strength of the conductor material needs to be greater than 1 GPa; however, it also must have good conductivity [5]. Cu–Nb microcomposite wire is mainly fabricated by an accumulative drawing and bundling (ADB) process. This preparation technology is approaching the theoretical limit of Cu–Nb microcomposite wire and the tensile strength of the Cu–Nb microcomposite wire has stayed at 900–1000 MPa; however, the conductivity of the Cu–Nb microcomposites decreased rather than increased as the tensile strength increased. Methods to refine the size of the Nb filaments, increase the Cu/Nb contact area, and increase the number of the Cu/Nb interfaces can improve the mechanical properties of the Cu–Nb microcomposite [6]. Therefore, new structural materials have been successfully prepared by optimizing the structural design such as Cu–Nb–Cu and Cu–Nb–Ag [7–8]. Although the properties of these materials have been improved, the material structure and preparation process are more complicated; therefore, they are only suitable for experimental research but not suitable for engineering applications. Shimoyama et al. [9] reported that the (0001) basal texture of AZ31 sheets were weakened by a periodical straining rolling process, and the microstructure and texture gradient were produced by controlling strain distribution. It has been recently reported that Cu/Ta nanolamellar with a layer thickness of 50 nm show a {100}_Ta[110]‖{110}_Cu[111] rolling texture, and the tensile strength of the Cu/Ta nanolamellar was up to 950 MPa after rolling [10]. Therefore, the rolling process may be beneficial for obtaining good texture and high properties. However, there are few reports on the groove rolling of a Cu–Nb microcomposite wire. In the present paper, a groove rolling method was developed to improve the plasticity of Cu–Nb microcomposite wire, refine the filament size, and improve the mechanical and electrical properties. This study mainly focused on the effects of groove rolling on the microstructure and properties of Cu–Nb microcomposites. The microstructure of the Cu–Nb groove-rolled and non-groove-rolled microcomposite wires was compared, and the microstructure evolution and micro-mechanisms affecting the properties were discussed.

2.   Experimental

As previously reported [1], Cu–Nb microcomposite wires with diameter of 7.74 mm were prepared by the ADB process. An intermediate annealing treatment for the groove-rolled sample was annealed at 700°C for 4 h in a vacuum furnace to eliminate work hardening due to the severe plastic deformation. Sample was subject to one pass of groove rolling after annealing, resulting in deformation of up to 60%. The groove-rolled sample and non-groove-rolled sample were drawn to corresponding diameters (ϕ2.83 mm, ϕ2.39 mm, and ϕ2.02 mm), and samples were finally characterized and analyzed.

The diffraction patterns of the samples were investigated using X-ray diffraction (XRD, Bruker D8 Advance) with Cu K_α radiations. The morphology of the Cu–Nb composites was characterized by scanning electron microscopy (SEM, JSM-6700). The stress–strain curves of the Cu–Nb composites were measured by a tensile testing machine (Instron Model 5982) at room temperature. The conductivities of the Cu–Nb composites were measured by a four-probe method.

3.   Results and discussion

3.1   XRD analysis

Figs. 1(a) and 1(b) show the XRD patterns of groove-rolled and non-groove-rolled (conventional drawn) samples with cross-section different diameters (ϕ2.83 and ϕ2.02 mm), respectively. The main diffraction peaks of the groove-rolled sample were stronger and sharper than those of the non-groove-rolled sample (Fig. 1(a)), indicating the main orientation of the material changed significantly after groove rolling. When the strain was increased, the diffraction peaks of the groove-rolled sample gradually weakened, but were still stronger than those of the non-groove-rolled sample (Fig. 1(b)). Groove rolling is different from conventional plastic deformation due to the symmetry of the side-wall. There is a large deformation in reduction, which leads to a stretch in the longitudinal direction of the material; therefore, the longitudinal deformation of the groove-rolled sample is greater than that of the non-groove-rolled sample. Knezevic and Bhattacharyya [11] reported that a more uniform <110> fiber texture was reduced in the cross-section of the rod using square-to-round shape rolling. During plastic deformation, a large number of dislocations will occur at the large-angle grain boundaries that significantly increase the deformation resistance [12]. Due to the strong driving force of the rolling deformation, the original orientation relationship of the Cu–Nb material was destroyed, which promoted the migration and coupling of the large-angle boundary and dislocations and a new orientation of the material was formed [13]. Therefore, large internal stresses were produced inside the material due to the huge deformation during the groove rolling, which caused the diffraction peaks to become wider, which were a measure of the residual stresses.

Fig. 1.  XRD patterns of the rolled and non-rolled (conventional drawn) Cu–Nb samples with different cross-section diameters: (a) ϕ2.83 mm; (b) ϕ2.02 mm. Relationships between the crystal orientation and full width at half maxima (FWHM) of samples with different diameters: (a) ϕ2.83 mm; (b) ϕ2.02 mm. (e) Nb (110) peaks of samples in (a).

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The change in the full width at half maxima (FWHM) of the diffraction peaks with different diameters was measured using Jade software, as shown in Figs. 1(c) and 1(d). The FWHM of the rolled sample was greater than that of the non-rolled sample due to the large internal stresses of the material. According to the Debye-Scherrer formula, the average grain size of the groove-rolled sample was approximately 28 nm; however, the average grain size of the non-groove-rolled sample was approximately 40 nm, which showed that the groove rolling can promote further refinement of the grain size. As shown in Fig. 1(e), the Nb (110) peak exhibited a slight shift toward higher angles after groove rolling. This higher angle was likely a reflection of the lattice distortion caused by macro-residual stress as the diffraction inside the material was due to compressive stress [10]; therefore, the diffraction peak was shifted.

3.2   Microstructure analysis

3.2.1   Macromorphology analysis

The metallographic photographs of the groove-rolled and non-groove-rolled Cu–Nb samples are shown in Fig. 2. As shown in Fig. 2(a), the filaments of the groove-rolled sample were more regular without distortion and maintained a regular hexagon after the large deformation, which indicated that the filaments of groove-rolled Cu–Nb sample had a more uniform shape. As shown in Fig. 2(b), slight differences in the deformation between the filaments at the leading edge and in the middle were observed; furthermore, the average size of the filaments was approximately 50 μm. Therefore, for the groove-rolled sample, the refinement of the filaments was more obvious. However, for the non-groove-rolled sample, the differences in the deformation between the filaments at the leading edge and in the middle were more evident; the filaments exhibited an irregular shape and were stretched (Fig. 2(d)). Thus, the filament morphology with the regular hexagon was destroyed due to the asynchronous deformation of the material.

Fig. 2.  Metallography of the rolled and non-rolled Cu–Nb samples with a diameter of ϕ2.83 mm: (a, b) different magnifications of rolled sample; (c, d) different magnifications of non-rolled sample.

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It is well known that the rolled material exhibits a heterogeneous microstructure with a high density of dislocations [14]. After groove rolling, a large number of dislocations are generated inside the material due to the large deformation and the deformation of the different grains is well coordinated by massive dislocations. Therefore, the filament deformation of the groove-rolled sample is more uniform. It is noteworthy that due to the mismatch of the crystal structure types (face-centered cubic (fcc) Cu, body-centered cubic (bcc) Nb), a large number of interfacial misfit dislocations and internal stresses are generated inside the material after large plastic deformation, and the stress states between the Cu matrix and Nb filaments are different [15]. Due to the large deformation of groove rolling, simultaneous deformation between the matrix and filaments can be coordinated by increasing the density of dislocations, causing the filaments to deform uniformly.

3.2.2   Micromorphology analysis

Fig. 3 presents the cross-SEM and fracture morphologies of groove-rolled and non-groove-rolled Cu–Nb samples. It is well known that microstructure uniformity in a composite is a key factor for determining the material quality. For the groove-rolled sample, the filament micromorphology became more regular (Fig. 3(a)); furthermore, after multiple compounding, the filaments still maintained a regular hexagon (Fig. 3(b)), indicating that the deformation of the filaments with different layers were more even. The diameter of the filament was tens of nanometers, and the refinement effect of the filaments became more obvious due to the large deformation of the groove rolling. However, the regular hexagonal array of filaments in the non-groove-rolled sample was destroyed, as shown in Fig. 3(d). The filaments gradually stretched and distorted after the severe plastic deformation, and it was further confirmed that the deformation of filaments with different layers was uneven for the non-groove-rolled sample. The large deformation led to more uniform deformation of the filaments with different layers by coordinating the deformation of grains with different crystal orientations for Cu–Nb groove-rolled sample. The Cu–Nb sample exhibited a subgrain structure due to entanglement of the dislocations after rolling [15].

Generally, the ductile fracture characteristics that the material exhibits are obvious: the grain boundaries are weakened, and a large number of dimples appear in the fracture morphology. The fracture morphology of the composite shows a large number of dimples and tear ridges, and its fracture characteristic is typical of dimple polymerized ductile fracture [16]. The formation mechanism of the dimples is essentially microporous polymerization, the dimple-like pattern plastically deformed in the micro area is micro-cavities that nucleate, grow, and aggregate, finally connecting leaving a trace on the fracture surface [17]. As shown in the fracture morphology in Fig. 3(c), due to more uniform deformation of filaments, the fracture of the sample was relatively smooth and flat, showing large local necking and a large number of dimples, confirming ductile fracture. Therefore, the elongation of the Cu–Nb composite improved significantly. However, the fracture of the non-groove-rolled Cu–Nb sample was relatively rough, the local necking was small and the aggregated grains were significant, showing characteristics of brittle fracture (Fig. 3(f)).

Fig. 3.  Microstructures and fracture morphologies of the rolled and non-rolled Cu–Nb samples with a diameter of ϕ2.83 mm: (a, b) different magnifications of rolled sample; (c) SEM image of the fracture surface of rolled sample; (d, e) different magnifications of non-rolled sample; (f) SEM image of the fracture surface of non-rolled sample.

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3.3   Performance analysis

Fig. 4 shows the stress–strain curves of groove-rolled and non-groove-rolled Cu–Nb samples. The tensile strength of the groove-rolled Cu–Nb sample significantly increased by nearly 4.6%, and the elongation of the Cu–Nb groove-rolled sample increased by nearly 23.8%, compared to the sample without groove rolling. Notably, the tensile strength of the groove-rolled sample with a diameter of 2.02 mm was greater than 1 GPa. Accordingly, its conductivity reached about 68% of the International Annealed Copper Standard (IACS, Table 1). As a result, a Cu–Nb microcomposite wire with high tensile strength and high conductivity was obtained. The filament size was a few tens of nanometers. Due to the mechanism for fault-zone strengthening, the strength of the material will increase proportionally [18]. Therefore, material properties, such as higher strength and larger elongation, are improved after groove rolling.

Sheets of Al alloys with an ultra-fine grained structure and enhanced strength are produced by constrained groove pressing, with changes in the dislocation structure as the predominant mechanism [19]. The dislocation structure in the material is changed after groove rolling, and the dislocation density inside the material is also increased due to large plastic deformation. The plastic deformation resistance gradually increases as the true strain of the material increases. In addition, it is apparent that groove rolling can efficiently enhance the plastic deformation ability (Fig. 4(b)), the elongation of the rolled sample significantly increases, therefore, the effect of filaments refinement are more obvious, which is consistent with the previous description of the microstructure. The smaller and more uniform the grain size, the greater the plasticity of the material [20]. The conductivity of samples with different diameters is presented in Table 1. Generally, the tensile strength of the material increased and the conductivity decreased with the diameter decrease; however, the conductivity of the material slowly decreased with the tensile strength increased after groove rolling. The deformation temperature of the material may have increased due to large deformation during groove rolling, which led to recovery of the material. Although the shape of the crystal grains and strength of the material were basically unchanged during the recovery process due to the short-distance diffusion of metal atoms, which led to a reduction of the lattice distortion and internal stress during the recovery process. Therefore, the scattering of electrons was reduced [21].

Fig. 4.  (a) Diameter–strength curves of the rolled and non-rolled Cu–Nb samples; (b) stress–strain curves of the samples with a diameter of ϕ2.02 mm.

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Table  1.  Tensile strength and conductivity parameters of the rolled and non-rolled samples with different diameters

Sample	Diameter / mm	Tensile strength / MPa	Conductivity relative to IACS / %
Rolled sample	2.83	871	70.4
	2.39	959	70.1
	2.02	1038	67.7
Non-rolled sample	2.83	861	68.8
	2.39	946	70.3
	2.02	991	68.4

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4.   Conclusions

In this paper, a Cu–Nb composite was successfully prepared by a groove rolling process. Through characterization of the diffraction peak, morphology, and fracture morphology of the Cu–Nb groove-rolled and non-groove-rolled samples, the effect of groove rolling on the microstructure and properties of the Cu–Nb composite was determined:

(1) The main diffraction peaks of the groove-rolled samples were stronger and sharper than those of non-groove-rolled samples, indicating that large internal stresses were produced inside the material due to the large deformation during groove rolling, which widened the diffraction peaks. The XRD results showed that the main orientation of the material changed significantly after groove rolling.

(2) The filaments of the groove-rolled sample were more regular without distortion, which still maintained a regular hexagon after the large deformation, and the refinement of the filaments were more obvious, indicating that the groove rolling technology can have simultaneous deformation between the matrix and filaments by increasing the density of dislocations, causing the filaments to deform uniformly.

(3) The Cu–Nb composite with a combination of high tensile strength and high conductivity were obtained. Groove rolling can efficiently enhance the plastic deformation ability. The increased tensile strength was mainly attributed to the dislocation strengthening and the elongation was attributed to the increased dislocation density. The conductivity of the material slowly decreased after groove rolling and the deformation temperature increased potentially due to the large deformation during groove rolling, which led to recovery of the material. The lattice distortion and internal stress were reduced during the recovery process; therefore, the scattering of electrons was reduced.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (No. 2016YFA0401701) and the National Natural Science Foundation of China (No. 51601151).