2019 Volume 60 Issue 7 Pages 1111-1115
Microstructure and mechanical properties induced either by heavy cold rolling or multi-directional forging of Cu–Al alloy were investigated and compared. While both microstructures were developed mainly by a mechanism of mechanical twinning, the features were completely different. The former exhibited a typical heterogeneous nano-structure where “eye” shaped twin domains were surrounded by deformation bands and they were further embedded in low-angle lamellae. The latter also showed complicated feature where twin domains and ultrafine grains composed of equi-axed nano-grains were randomly distributed. Even while the latter possessed finer grain size, the former showed more superior mechanical properties, yield strength of 863 MPa and ultimate tensile strength of 1168 MPa than the latter, yield strength of 720 MPa and UTS of 870 MPa.
This Paper was Originally Published in Japanese in J. Japan Inst. Copper 57 (2018) 59–64.
Different microstructures evolved in a Cu–7 mass%Al alloy by (a) multidirectional forging to ΣΔε = 6.0 and (b) 90% cold rolling. Areas having largely different lattice lines, which suggest evolved nano-grains, were indicated by circles in (a). Lamellae were subdivided by nano-twins in (b).
Many researches on ultrafine-grained (UFGed) structures developed by means of severe plastic deformation (SPD) of metals and alloys have been carried out aiming at improvement of various properties. SPD enables formations of UFGed structures with average grain size smaller than 1 µm, which is difficult to achieve by conventional thermo-mechanical processes, and various particular properties are being improved or discovered.1–5) The majority of the SPD researches are on strengthening and actually notably higher strengths compared with those achieved by means conventional thermo-mechanical processes are frequently reported. These researches have also suggested possibility of fabrication of high strength “pure” metals, i.e., strengthening without any element additions for solid solution and/or precipitation hardening. The fabrication of high strength pure metals is accompanied by some advantages such as easier recycling, lower cost, etc. However, SPD processes are quite complicated compared with the conventional processes and essentially carried out under condition of a fixed sample shape. Therefore, application of SPD processes to industrial mass production seemed quite difficult. Thus, development of much easier process to fabricate UFGed structures and suitable for mass production is being demanded.
Miura et al. have attained UFGed structures of Cu–Zn alloy and SUS316L austenitic stainless steel with average grain sizes finer than 10 nm by multi-directional forging (MDFing) employing a mechanism of mechanical twinning,4,5) where MDFing is one of methods for SPD. The evolved UFGed structures by mechanical twinning possessed superior balance of mechanical properties. For example, UFGed SUS316L stainless steel exhibited extraordinary high ultimate tensile strength (UTS) of 2.2 GPa and elongation of 10%.5) Grain fragmentation and evolution UFGed structure by mechanical twinning are result of impediment of dislocation glide and stress relaxation due to mechanical twinning in the alloys with low stacking-fault energy (SFE).4–6)
Miura et al., furthermore, heavily cold rolled Cu–Be alloys with low SFE and successfully developed UFGed structure by dense introduction of mechanical twins. Still more, they have demonstrated superior mechanical properties comparable with those achieved by SPD, i.e., 1.8 GPa UTS and 4% elongation after ageing.7) The microstructure developed in a heavily cold-rolled Cu–Be alloy looked quite different from the conventional one observed in cold-rolled metals and alloys, that is, “eye-shaped twin domains” were surrounded by shear bands and they were further embedded in ultrafine low-angle lamellae. They named this characteristic microstructure as “heterogeneous nano-structure” because all the microstructural components were of heterogeneously deformation induced. Evolution of the heterogeneous nano-structure was confirmed also in heavily cold-rolled Cu–Ti–Co alloy with 2 mass% content of Ti.8) This Cu–Ti alloy exhibited superior properties of 15% electric conductivity as well as excellent mechanical properties of 1.2 GPa UTS and 15% elongation owing to the reduction of Ti content. In short, well-balanced various superior properties could be derived by means of a simple process of heavy cold rolling of Cu alloys with low SFE and by evolution of heterogeneous nano-structures. Heavy cold rolling is a quite simple and conventional process actually employed for the industrial mass productions.
However, the researches on the heterogeneous nano-structure and the properties have just started. The related reports are, therefore, still quite limited. This is the motivation of the present study. In the present study, evolved microstructure developed by heavy cold rolling of a solid-solution hardened Cu–Al alloy and the mechanical properties are precisely investigated. The Cu–Al alloy is also MDFed, and the microstructure and mechanical properties are examined for comparison.
A supplied Cu–7 mass%Al alloy plate with thickness of 15 mm and with an initial grain size of 94 µm was employed, which was prepared in advance by cold rolling followed by annealing. The SFE of the Cu–7 mass%Al alloy is ultra low and its value is reported to be 1.7 mJ/m2.9) It was cold rolled up to 90% reduction in thickness. Rolling conditions were as follows; oil lubricant, 10% reduction per pass and rolling speed of 2 m/min. The evolved microstructure was examined by transmission electron microscopy (TEM) at an accelerating voltage of 200 kV. Specimens for TEM observations were prepared using an ion thinning apparatus (ion slicer/Jeol EM-09100IS) and the observation was carried out on the planes normal to the transverse direction (T.D.) of the cold-rolled sheet. Mechanical properties were examined by tensile tests on an Instron-type universal mechanical testing machine. Samples for the tensile tests were prepared by discharged machining to have gauge dimensions of 6.0 × 3.0 × 1.0 mm3. Tensile tests were carried out along the rolling direction (R.D.) at an initial strain rate of 3.0 × 10−3 s−1 and at room temperature.
On the other hand, the supplied Cu–Al alloy plate was cut into a rectangular shape with dimensions of 15.0 × 18.3 × 22.5 mm3 and MDFed on an Amsler-type mechanical testing machine at an initial strain rate of 3.0 × 10−3 s−1 at room temperature employing pass strains of Δε = 0.4 up to a cumulative strain of ΣΔε = 6.0. Because the cumulative strain is a simple summation of forging pass strains ignoring changes in the forging direction during MDFing, the above value can be simply converted to an equivalent strain of 6.0. The equivalent strain after 90% reduction is 2.3. The equivalent strain induced by MDFing to ΣΔε = 6.0 is, therefore, 2.6 times larger than that of 90% rolling. The specimens for TEM observation were prepared in the same way as shown above. TEM observation was carried out from the direction normal to the final forging axis of MDFing. The conditions of tensile test were also the same with those of the cold-rolled samples. The tensile axis was normal to the final forging axis. After the above experiments, differences in the microstructures and mechanical properties between the cold-rolled and the MDFed samples were compared in detail.
TEM micrographs of the cold-rolled sample are displayed in Fig. 1. It looks mainly composed of ultrafine lamellae and shear bands (Fig. 1(a)), in which the lamellar spacing was approximately 100 nm. An “eye-shaped twin domain” could be also observed at the left side in Fig. 1(a). However, it seems yet half way of the formation of typical one because not so sharply and clearly appeared. The eye-shaped twin domain is composed of a large number of mechanical twins, in which the {111} twinning plane is parallel to the rolling plane.5) It can be recognized from the selected-area-diffraction pattern with long arcs in Fig. 1(a) that increase in grain-boundary misorientation distribution and, therefore, grain-orientation randomization appears proceeding.
TEM micrographs of the 90% cold-rolled Cu–Al alloy observed from transverse direction; typical areas of (a) lamellar and (b) “eye shaped” twin domain. Selected-area-diffraction pattern taken in the area indicated by a circle is also exhibited in (a). R.D. indicates rolling direction.
A typical microstructure of eye-shaped twin domain is exhibited in Fig. 1(b). It can be confirmed that the twin domain is composed of ultrafine lamellar twins with spacing of about 40 nm and it is surrounded by shear bands with an average thickness of roughly about 150 nm to form so called “eye-shaped twin domain”. The total volume fraction of the eye-shaped twin domains was estimated to be about 20% at maximum.
On the other hand, ultrafine twins with a spacing of about 10 nm were partially evolved in the low-angle lamellae to subdivide them (Fig. 2). Although the twinning planes appear at a glance parallel from each other in the matrix lamellae which aligned to form ⟨001⟩{110} texture, they have rather large misorientation distribution up to 10 degrees.10) This misorientation distribution among the ultrafine twins in the low-angle lamellae would be understood from continuous formation, crystal rotation of matrices and shear banding during cold rolling. The formation of different variant ultrafine twins (at low-left part in Fig. 2), which twinning plane is parallel to the rolling plane, indicates evolution of extremely complicated UFGed structure.
Nano-twins developed in lamellae in the 90% cold-rolled Cu–Al alloy. R.D. is the rolling direction.
A schematic illustration of heterogeneous nano-structure revealed from the TEM observations of heavily cold-rolled Cu–Al alloy is displayed in Fig. 3. Twin domain is surrounded by shear bands and it is embedded into low-angle lamellae. The latter matrix lamellae are partially subdivided by ultrafine mechanical twins and by shear bands. In short, UFGed structure dominantly developed by deformation-induced microstructures could be successfully fabricated by simple heavy cold rolling of an ultra-low SFE Cu–Al alloy to an equivalent strain of 2.3.
A schematic illustration of a typical hetero-nano structure composed of “eye shaped” twin domain, shear bands and low angle lamellae. The lamellae were further subdivided by mechanical nano-twins.
It is widely known that sharp texture evolution of {112}-{011} on the cold-rolling plane of copper alloys spoils plastic deformability and bendability. In the eye-shaped twin domains mentioned above, however, {111} twinning planes are aligned on the rolling plane. Twins developed in lamellae can also contribute to distraction of the texture. In this way, sharp evolution of ⟨001⟩{110} texture is effectively suppressed.10,11) It is expected from the above observations of heterogeneous nano-structure, therefore, bendability and ductility would be improved.12) This will be discussed in detail in the section of 3.3.
It is known that grain-boundary related phenomena (strength, precipitation, segregation, corrosion, recrystallization and etc.) strongly depend on grain-boundary character.13–17) And the grain-boundary character can be described not by misorientation angle or Σ value but by grain-boundary energy, i.e., with decreasing grain-boundary energy, grain-boundary precipitation,14) grain-boundary segregation,15) grain-boundary corrosion,16) recrystallization at grain-boundary17) become more difficult and grain-boundary strength becomes higher.14,18) Because the heterogeneous nano-structure is mainly composed of twins, sub-boundaries and low-angle grain-boundaries, in short, all they are of typical low energy boundaries, fracture strength must be largely enhanced. It is assumed, therefore, so-called “grain-boundary engineering” proposed by Watanabe13) have been actually realized by a simple heavy cold rolling of ultra-low SFE copper alloy.
3.2 Observations of evolved microstructure by MDFingTypical microstructure developed by MDFing to a cumulative strain of ΣΔε = 6.0 is shown in Fig. 4. At lower magnification (Fig. 4(a)), evolution of equi-axed UFGs with an average grain size of 100 nm at a glance could be observed. The UFGs were classified roughly into to following two UFGed structures, twin domains called as “packet grains” and grains with smooth and white contrast. In spite of the above description, the packet grains were further composed of mechanical twins with boundary spacing of 20∼30 nm, which was the same structure observed in a MDFed Cu–Zn alloy.19) It is reported that the packet grains are formed by introduction of dense mechanical twins.19) On the other hand, the grains with smooth and white contrast appeared to increase with increasing cumulative strain of MDFing. Because no clear evidence of evolution of dislocation substructures in the grain interior could be confirmed, a possibility of recrystallization at room temperature due to thermal instability after SPD was considered.20) It was revealed by high resolution TEM observation and lattice-line analysis (Fig. 4(b)), however, that the areas with white contrast were composed of equi-axed extra UFGs with a diameter of 5 nm having large misorientation angles among the neighboring grains. In spite of this, they were not of twin orientations. Hence, the increment of the area with white contrast could be understood by the increase in the extra UFGs evolved by fragmentation of the other coarser UFGs and packet grains during MDFing.
Evolved microstructure after multi-directional forging of Cu–7%Al alloy to ΣΔε = 6.0. (b) is the imposed image of the area with white contrast in (a). Areas having largely different lattice lines, which suggest newly evolved nano-grains, were indicated by dotted white circles in (b).
When the evolved microstructures induced by heavy cold rolling and MDFing are compared, it was found that both were UFGed ones dominantly fragmented by introduction of dense mechanical twins, and therefore, similar twin domains were observed. The most decisive difference between the microstructures is that the feature of the former main body was of elongated along rolling direction whereas that of the latter was of equi-axed. Even while the comparison of the actual grain size should be quite difficult because of the complex structures, it is concluded from twin-boundary spacing and equi-axed grain size that the grain size after MDFing is finer than that after cold rolling.
3.3 Mechanical propertiesThe samples cold-rolled and MDFed were tensile tested and the stress-strain curves are shown in Fig. 5. The elongation was attained from the displacement of the crosshead, therefore, the strain should not be always exact. The MDFed samples exhibited an excellent balance of mechanical properties; yield stress of 720 MPa, UTS of 870 MPa and elongation of 7.5% (Table 1). Beside it, the cold-rolled one showed more superior mechanical properties of 863 MPa yield stress, 1168 MPa UTS and 9.0% elongation. It is already evident from above comparison, the mechanical properties in the cold-rolled samples should be more excellent than those in the MDFed one. Yet, the uniform elongation was 4.0% and 2.9% respectively. Hence the local contraction after the peak stress in the cold-rolled samples, which induces more rapid decrease in the flow stress, looks slightly more noticeable than that in the MDFed one.
True stress vs. true strain curves of Cu–7 mass%Al alloys 90% cold-rolled and multi-directionally forged to a cumulative strain of ΣΔε = 6.0. The strain was calculated from the displacement of the crosshead.
Uoji and Aoyagi have precisely investigated the effects of eye-shaped twin domain on strengthening by means of multi-scale crystal plastic simulation using crystal information, and yielded following results; i) twin domain largely contributes to strengthening, ii) strength increases with volume fraction of twin domains and iii) strength becomes higher along T.D. than R.D.21) Actually, 1.4 times higher tensile strength along T.D. than R.D. is reported in the SUS316LN stainless steel with heterogeneous nano-structure.11,12) Nevertheless, tensile test was carried out only parallel to R.D. in the present study. Therefore, further larger difference in the mechanical properties should be arisen if tensile test along T.D. was carried out.
As already mentioned in the section 3.1, heterogeneous nano-structure developed in Cu–Al alloy is mainly composed of twins, sub-low angle boundaries. These are of typical low-energy grain boundaries and known to possess high fracture toughness compared with general grain boundaries.14,18) This is assumed because of more difficult crack initiation and propagation at such low-energy grain boundaries. Still more, twin boundaries also contribute to strengthening as well as the other general grain boundaries.22) In contrast, grain boundaries developed by SPDs are mainly of high angle ones, in other wards, high energy ones. Therefore, the fracture toughness should be comparably lower. In summary, the tensile strength of the samples with heterogeneous nano-structure should become higher compared with the SPDed ones. Finally, it is concluded that grain-boundary control13) to derive dense high-strength grain boundaries has been realized by a simple cold rolling applicable for industrial mass production.
The relationship between texture induced by rolling and ductility is not so clearly defined as like the effect on bendability. Nevertheless, randomization of crystal orientation contributes notably to ductility of metals and alloys with large crystal anisotropy.23) Miura et al. explained the rather good elongation of stainless steel with heterogeneous nano-structure by i) weaken texture, ii) twinning-induced plasticity (TWIP), iii) shear-banding induced plasticity.11,12) The comparable uniform elongation of the heavily cold-rolled samples with those of the MDFed one should be possibly due to suppression of sharp ⟨001⟩{110} texture11,12) and TWIP effect.11) In fact, rather good elongation by TWIP is confirmed in SUS316LN stainless steel with heterogeneous nano-structure.10) From the above results and discussion, it is concluded that UFGed structure can be easily achieved by simple heavy cold-rolling and comparable or more superior mechanical properties than those obtained by SPDs can be attained when grain fragmentation takes place dominantly by dense mechanical twinning. Nevertheless, the experimental results about heterogeneous nano-structure and the properties induced by heavy cold rolling are still not sufficient. Therefore, further researches are required in the future.
Cu–7 mass%Al alloy was heavily cold rolled to have heterogeneous nano-structure composed of ultrafine lamellae, shear bands and eye-shaped twin domains. Nevertheless, the evolved ultrafine-grained (UFGed) structure appeared different from those developed by multi-directional forging, i.e., UFGed but equi-axed. Both heavy cold-rolled and multi-directional forged (MDFed) samples exhibited excellent mechanical properties. The mechanical properties of the cold-rolled samples, however, were more superior to those of MDFed ones. Finally, it is demonstrated that the same or more superior mechanical properties can be achieved by simple heavy cold rolling of ultra low-stacking-fault-energy copper alloy than those attained by MDFing to higher equivalent strain.
This research was financially supported by Japan Science and Technology Agency (JST) under Industry-Academia Collaborative R&D Program “Heterogeneous Structure Control: Towards Innovative Development of Metallic Structural Materials” and by Japan Copper and Brass Association. The authors deeply appreciate these supports.
