Mechanical Properties and Microstructures of Ni20Cr Micro-wires with Abnormal Plastic Deformation

Fabrication of Ni20Cr micro-wire

The Ni20Cr micro-wire used in the study had the chemical composition of Ni-20.037 % Cr-1.037 % Si-0.194 % Mn-0.141 % Fe-0.101 % Ti-0.087 % Al-0.023 % Cu (in wt %). Specimens were prepared by cold drawing from the as-received micro-wire with diameter of 25.6 µm through one die, as shown in Figure 1.

Figure 1:

Schematic diagram of cold-drawn from the as-received micro-wire through one die.

The traditional cold-drawing process of ultra-fine wire includes multiple-step drawing and annealing. One step usually comprises several passes, and each pass includes five to ten drawing dies. The abnormal engineering strains achieved through cold-drawing is approximately 15.7–46.6 % through one die in one pass, which is equal to the traditional engineering strain through two to five dies as shown in Table 1. Engineering strain was defined as

Here l0 and r0 are the original length and radius, respectively, and l1 and r1 are the current length and radius, respectively.

Annealing process

Annealing process was carried out in a horizontal oven in the N₂ atmosphere with a constant velocity and tension of 6.5 g. Schematic diagram and temperature field distribution of the annealing equipment are shown in Figure 2. Temperatures were measured using thermocouples placed in the oven at different distances from the left opening. From Figure 2, the temperature distribution is approximately symmetrical from the entrance to the exit of the oven. The length of constant temperature zone is about 10 mm, wherein the difference in temperature is less than 5 K relative to the set value observed in the oven. The annealing temperature is defined as the temperature of the constant temperature zone, and the annealing time is defined as the time required for the wires to pass through a distance of 550 mm at a certain velocity. The N₂ atmosphere was used to prevent Ni20Cr micro-wire oxidation. The inlet of the N₂ gas was specially designed to restrain the temperature decreasing and increase the asymmetry of annealing temperature field. The deformed samples with engineering strain of approximately 46.6 % were investigated with the effect of annealing temperature on the mechanical properties over annealing times of 2.4–22 s.

Figure 2:

Schematic diagram and temperature field distribution of annealing equipment.

Tensile test

The mechanical properties of a wire mainly include its ultimate strength, elongation and elastic modulus; these properties are usually characterized by tensile tests. The gauge length of the Ni20Cr micro-wire specimens was 45 mm. All tensile tests were performed at a constant strain rate of 4.8 × 10⁻⁴ s⁻¹ and room temperature, as shown in Table 2. Both ends of the tensile specimen were held tightly in place by a clamp using a screw to adjust the degree of tightness and grips attached to the upper and lower pull rods of the machine. At least five samples were tested under each of conditions.

Mechanical properties of Ni20Cr micro-wires

The mechanical properties of Ni20Cr micro-wires obtained under different engineering strains and annealing temperatures are shown in Figure 3. From Figure 3(a), as-drawn Ni20Cr micro-wires showed brittle fractures with limited elongation. The yield strength (YS), ultimate tensile strengths (UTS) and tensile elongation of the as-received micro-wire are about 520 MPa, 781 MPa, 17.2 %, respectively. Flow localization in the deformed sample is observed almost immediately beyond the yield stress. The UTS significantly increases from 781 to 1,147 MPa and the elongation decreases from 17.2 % to 1 % with engineering strains increasing, no evident YS observed. Before the flow localization stage, the stress-strain states are uniform and the samples retain cylindrical shapes. After flow localization, the strain hardening behaviors of the deformed micro-wires differ from that of the as-received micro-wire, likely because plastic deformation increases the strength of micro-wires and decreases their plasticity. When the engineering strains were added from 36.1 % to 46.6 %, the stress-strain curves obtained are nearly identical. It indicted that the maximal deformation strain of Ni20Cr alloy was achieved by drawing through one pass, i. e. strain hardening in the alloy reaches the maximum. The experiment results prove that strength-limiting involves dislocation emission from grain boundary sources in metals with grain sizes between approximately 10 and 500–1,000 nm [16].

Figure 3:

Stress-strain curves of Ni20Cr micro-wires with the different engineering strains and annealing temperatures (the annealing time of 7.3 s).

The annealed Ni20Cr micro-wires showed extended plastic deformation. From Figure 3(b), the YS (ε ~0.2 %) decreases from 1,246 to 890 MPa and the UTS decreases from 1,257 to 782 MPa when the annealing temperature is reduced from 700 °C to 890 °C. Elongation increases from 1.1 % to 14.3 % in the same range of annealing temperatures. When the annealing temperature is less than 700 °C, stress-strain curves are similar with the UTS and elongation slightly decreasing. After annealing at 700 °C, the deformation behavior of the Ni20Cr micro-wires begins to exhibit plastic deformation characteristics, similar to other specimens annealed at higher temperatures. Micro-wires are fractured when the annealing temperature exceeded 890 °C.

The annealing process includes two main parameters: the annealing temperature and the annealing time. Specially, their cooperation is worth noting. Figure 4 shows variations in the UTS and elongation of deformed specimens with engineering strains of 46.6 % as a function of annealing time. In Figure 4(a), when the annealing temperature is below 850 °C, the UTS of the specimens increases as the annealing time increases from 2.4 to 4.4 s and then decreases as the annealing time increases from 4.4 to 22 s. Above 850 °C, the UTS of the specimens decreases with increasing annealing time from 2.4 to 22 s. At an annealing temperature of 890 °C, the annealing process fails when the annealing time exceeds 7.3 s. In Figure 4(b), it reveals that the elongation of the specimens increases as annealing time increases from 2.4 to 22 s.

Figure 4:

Ultimate strength and elongation of the deformed specimen with engineering strain of 46.6 % as a function of annealing times.

UTS and elongation are approximately linearly increasing as the annealing time increases from 2.4 to 22 s at 870 °C. However, this tendency clearly differs when the annealing temperature is above or below 870 °C under the conditions of our present experiment. This result indicates that the annealing temperature clearly affects the mechanical property tendency of deformed Ni20Cr micro-wire, on the contrary, the annealing time only affects a variable quantity of mechanical properties along a special tendency. For the second drawing, the elongation of deformed Ni20Cr micro-wire must be recovered closely to the level of raw micro-wire, similar as the UTS. Based on the results shown in Figures 3 and 4, it demonstrated that the optimal Ni20Cr annealing parameters were the temperature of 890 °C and the time of 7.3 s in the case of 6.5 g of tension. (The tension effect on mechanical properties will be discussed in detail at other articles.)

After annealing at 870 °C and 7.3 s, specimens with a diameter of 21.14 µm are suitable for second drawing. Figure 5 shows UTS and elongation of deformed specimens under different engineering strains. The strain hardening efficiency is different between the first pass and second pass. However, the UTS of the raw micro-wires are approximately equal to that of annealed micro-wires with engineering strains of 46.6 %. The elongation of drawn specimens in the second pass is lower than that in the first pass. As the number of drawing pass increases, elongation decreases evidently. The D-value first increases and then decreases to a stable value, likely because of the damage caused by the drawing process and inadequate annealing. The decreasing of UTS cannot be clearly accounted for the deformation mechanism, strains and deformation microstructures of the micro-wires. Relative to the UTS, elongation is more sensitive during plastic deformation.

Figure 5:

Ultimate strength and elongation of the deformed specimen with different engineering strains.

Microstructures of Ni20Cr micro-wires

The microstructures of as-deformed Ni20Cr micro-wires were characterized using TEM and XRD. The XRD patterns (diffraction intensity normalization processing) of drawn Ni20Cr micro-wires with different engineering strains are shown in Figure 6. Here, scanning was performed at 2θ from 30° to 130°. There are XRD peaks of the (111), (200), (220), (311) and (400) planes, no (222) plane. Peak intensities of (111), (311) and (400) planes decrease, and peaks broadening. It indicates that grains reduce with [111], [311] and [400] orientation. There are two possible reasons to lead to these phenomena.

Figure 6:

XRD patterns (diffraction intensity normalization processing) of drawn specimens with different engineering strains.

The first possible reason is [200] and [220] orientation textures form in face-centered cubic (fcc) structure Ni20Cr under plastic deformation. Grains with [111], [311] and [400] orientation rotate along the drawing direction. So the diffraction peak intensities of (200) and (220) planes increase, with other peak intensities decreasing. In addition, internal strains rise with engineering strains increasing leading to peaks broadening.

The second possible reason is abnormal plastic deformation had induced phase transformations in Ni20Cr alloys like decomposition, formation of solid solution and solid-state amorphization. Straumal reported phase transitions in metallic alloys driven by the high-pressure torsion [12–14]. Their results reveal that severe plastic deformation is equivalent to the heat treatment at a certain elevated temperature (effective temperature). The effective temperature is about 30 °C to 1,450 °C in different alloy systems and components. It leads to phase transformations, e. g. amorphization, nanocrystallization, dissolution and decomposition, and so on. Sheng had investigated on the amorphization of Zr–Al solid solution by ball milling [15]. Their results reveal that observable two-phase (solid solution/amorphous phase) coexists caused by external forcing at certain Al concentration and milling temperature. The observations are proposed that the effective temperature dependence of the external forcing effects brought in by the nonequilibrium milling process.

On the condition of traditional engineering strain, the maximum the temperature rise is about 85 °C brought in the drawing process [16]. In addition, Wright demonstrated the impact of drawing temperature on the microstructure of copper wire was secondary recrystallization [17]. In our condition of abnormal engineering strain, the maximum the temperature rise is about 145 °C brought in drawing. It cannot result in amorphization of Ni20Cr alloys. But, according to Straumal’s and sheng’s reports, effective temperature is sufficiently high brought by abnormal plastic deformation. The effective temperature induces phase transformations of solid-state amorphization in Ni20Cr alloys. Solid-state amorphization also result in peak intensities decreasing and peaks broadening, with special appearing higher angles planes. According to the NiCr binary equilibrium phase diagram, the effective temperature is about 1,255 °C for the Ni20Cr brought by abnormal plastic deformation, taking off temperature rise contributed in drawing process. From Figures 3 and 6, we think the quantity of amorphous phase is few relative to crystal phase. So it hardly affects the mechanical properties of Ni20Cr micro-wires.

Figure 7 shows the differential scanning calorimetry (DSC) trace of Ni20Cr micro-wires with engineering strain of 46.6 %. For individual Ni20Cr solid solution and amorphous phases, heating to 1,050 °C results in only one narrow exothermic peak corresponding to the transformation into equilibrium compounds. This curve before the main peaks may arise from the irreversible recovery of the deformed crystals through strain relief, defects annihilation, grain growth or structural relaxation in amorphous phase [18, 19]. Figure 7(b) indicates that the decomposition (γ′⇌Ni+Cr) of Ni20Cr solid solutions do not happen below 900 °C, because there is no obvious peak (dW/dT). Another reason is that the concentration of Cr does not reach the critical value of γ′ phase at 500 °C.

Figure 7:

DSC trace of Ni20Cr micro-wires with engineering strain of 46.6 %, at a constant heating rate of 10 °C/min.

Two thinning preparation techniques were used in preparing TEM specimens of the drawn direction and wire cross section [20]. Figure 8 shows selective area electron diffraction (SAED) patterns and deformation twin of the Ni20Cr alloy with strain of 36.1 % in the wire cross section. The SAED pattern shows the (110), (111), (200), (220), (311) and (222) lattice planes of Ni. The presence of twinning reactions and an inhomogeneous distribution of twins are observed in the wire cross section. The relationship of deformation twinning among grains is marked in Figure 8(b) by several white lines. There are certain steps between twin boundaries, as marked by GB1, GB2 and GB3. The twinning angle is approximately 146°, which is slightly different from the twinning angle of approximately 141° observed in the drawn direction (D. D.) cross section [20]. Figure 9 shows SAED, the lattice plane and dislocation configuration of Ni20Cr micro-wire with strain of 46.6 % in the wire cross section. The SAED pattern indicates that the samples are nanocrystalline (nc) with (111), (220), (311) and (420) crystal planes as shown in Figure 9(a). There were (420) and distorted (311) crystal planes without (200) crystal planes. The microstructures were observed, including (111) and (200) lattice planes in regions A and B as shown in Figure 9(b). The structure of region B was processed by fast Fourier transforming (FFT), masking and inverse FFT as shown in Figure 9(c). From it, we can observe that edge dislocations, which are marked white, exist at the grain interior.

Figure 8:

SAED patterns and deformation twin of Ni20Cr micro-wires with engineering strain of 36.1 % in wire cross section by transmission electron microscope (TEM).

Figure 9:

SAED patterns and deformation microstructures of Ni20Cr micro-wires with engineering strain of 46.6 % in wire cross section by TEM.

The XRD patterns of Ni20Cr micro-wires annealed at different temperatures (annealing time of 7.3 s) are shown in Figure 10(a). Here, scanning was performed at 2θ from 30° to 110°. The peaks in the XRD patterns demonstrate the polycrystalline nature of the Ni20Cr micro-wires. Changes in the intensity of the XRD peaks of the (200), (220), (111), (311) and (222) planes as a function of annealing temperature are shown in Figure 10(b). Obvious variations in the peak intensity of each crystallographic plane are noted after annealing, especially at annealing temperatures above 850 °C. During annealing below 850 °C, the peak intensities of all of planes except for the (222) plane evidently increase with increasing annealing temperatures. At annealing temperatures above 850 °C, the relative intensities of the (111), (311) and (222) planes increase rapidly from approximately 40 %, 10 % and 0 to 100 %, 20 %, and 10 %, respectively. By contrast, the relative intensities of the (200) and (220) planes decrease rapidly from approximately 100 % and 70 % to 80 % and 50 %, respectively.

Figure 10:

XRD patterns and diffracted intensity height of the deformed specimen with engineering strain of 46.6 % as a function of annealing temperatures.

Changes in relative intensity demonstrate that the volume of grains with <111>, <311>, and <222> orientations increases, which indicates the effects of annealing on the micro-structural evolution of the wires. Most of the grains show orientations parallel to the <100> and <110> planes after drawing. These structures are related to shear deformation and the original microstructures and differ from the typical deformation structure of fcc metals exhibiting a majority of <111> fibers and a minority of <100> fibers. TEM analysis reveals that the deformation microstructures include amorphous GB, crystal GB, edge dislocations, twins, and lattice distortion [20]. During annealing from 650 °C to 850 °C, intense height increases contribute to the recovery of a few distorted lattices and crystallized amorphous GB. During annealing at 890 °C, grain growth occurs in the <111>, <311>, and <222> regions by consuming <200> and <220> regions.

The full width at half-maximum (FWHM) of the (111), (200), (220) and (311) planes are shown in Figure 11(a) as a function of annealing temperature. According to XRD theory, diffraction ray broadening is caused by micro-internal stress and grain refinement. As the annealing temperatures increase, the FWHM of all planes decreases. When annealing temperatures are below 700 °C, the decreasing of FWHM may be mainly attributed to elimination of micro-internal stress. When annealing temperatures are above 700 °C, decrease in FWHM was caused by grain growth.

Figure 11:

FWHM and grain sizes of the deformed specimen as a function of annealing temperatures.

We can observe different grain sizes from different crystal planes, as shown in Figure 11(b). As the lattice plane spacing increases, the calculated grain size increases. The average grain size of all planes is close to the grain sizes calculated from (200) and (220) regions. Changes in the average grain size of the Ni20Cr micro-wires as a function of annealing temperature are shown in Figure 11(c). During annealing temperatures below 700 °C, the main grain size increases because of the decrease in micro-internal stress. At 750 °C, the deformed microstructures recrystallize and the average grain size decreases. Coarsening occurs in all regions of the micro-wire as the annealing temperature increases of 750–890 °C. During annealing from 850 °C to 890 °C, grain growth in the <111>, <311> and <222> regions consumed the <200> and <220> regions. The growth mechanisms observed in later stages are different from the previous stages found.