Mechanical Properties and Microstructural Evolution of CuZn Alloys via Pre-Torsional Deformation

Mechanical properties and microstructural evolution of Cu-10wt.%Zn and Cu-30wt.%Zn alloys after pre-torsional deformation have been investigated in this study. The results indicated that pretorsional deformation can significantly enhance the strength of both Cu-10wt.%Zn and Cu-30wt.%Zn alloys without changing their shape and size, but with the sacrifice of the ductility. The pre-torsion deformation can introduce gradient hardness and microstructures along the radial direction, with the microstructures along the radial direction in Cu-30wt.%Zn alloy being "dislocations→stacking faults→twins (low density)". The stacking fault energy affects significantly the microstructure along the radial direction.


Introduction
Plastic deformation is an effective technique for producing bulk metals with tailoring properties 1 .Recently, severe plastic deformation (SPD) techniques have attracted much attention due to their ability to fabricate bulk ultrafine grained (UFG) and/or nanocrystalline materials with an average grain size less than 1 µm 2 .Several SPD techniques, including equal channel angular pressing (ECAP), high pressure torsion (HPT), multidirectional forging (MDF) and accumulative roll bonding (ARB), are available for applying high strains on bulk materials 3 .Amongst all SPD techniques, HPT is the most convenient and effective technique where a thin disc or ring is placed between two massive anvils under a high pressure and intense shear strain is introduced by rotating the two anvils with respect to each other [4][5][6] .Materials treated by HPT usually have high strength but relatively low ductility at ambient temperature [7][8][9] .In addition, the dimensions of the samples prepared by SPD are too small, which limit their wide applications in modern industry.
Recently, improving the mechanical properties of structural metals via pre-tension 10 , pre-compression 11 , pre-rolling 12 and surface mechanical attrition treatment (SMAT) 13 have been extensively reported.It is commonly accepted that pre-deformation greatly affects the deformation behaviors of metals.Compared to the other deformation modes, pre-torsion with only pure shear strain doesn't induce any change in the sample's shape and size.Thus, simple torsional deformation has been widely reported to tailoring the properties of metals recently [14][15][16][17][18][19] .Pre-torsional treatment of a TWIP steel resulted in the formation of gradient nanotwin density that successfully overcame the strength-ductility trade-off dilemma 16 .While, pre-torsional treatment of copper and a magnesium alloy did not introduce gradient nanotwin structures but produced gradient grain size and dislocation substructures that improved the tensile strength but with the ductility trade-off 17,18 .
It is clear from the above-mentioned studies that pretorsional treatment significantly affects the strength and ductility, which are highly dependent on the processing materials.Stacking fault energy (SFE) is an important factor affecting the deformation mechanisms of materials.Reducing SFE significantly increases the propensity of deformation twinning, which increases the strain hardening rate, dislocation storage capacity, and ductility [20][21][22] .In this paper, Cu-10wt.%Zn and Cu-30wt.%Znalloys with different SFEs (~ 35 mJ/m 2 and ~ 13 mJ/m 2 , respectively 23 ) were pre-torsional deformed, and the effects of pre-torsional deformation on the mechanical properties and microstructural evolution of the alloys were investigated.

Experimental
Raw materials of Cu-10wt.%Zn and Cu-30wt.%Znalloys were bought from market.These two alloys are single phase solid solutions without any precipitate phase 24 .Dog-boneshaped specimens with a gauge size of Φ6 mm × 20 mm were fabricated by wire electrical discharge machine for the free-end torsional deformation.The torsional treatment was carried out with a constant rate of 1 rpm for 0°, 45°, 90°, 180° and 360°, respectively, using MTS 858 Mini Bionix II testing machine.The specimens in this paper were labeled as "composition-torsional angle".The composition and torsional angle of different specimens and the corresponding specimen names were listed in Table 1.The tensile experiments were conducted using an Instron 3369 testing system with a tensile rate of ~10 -5 s -1 .The hardness was measured along the radial direction using an FEI-VM50 PC Vickers hardness testing machine, with the indentation load and dwelling time being a State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, P.R.China 10 g and 15 s, respectively.The hardness at each position along the radial direction was averaged from 10 independent measurements.The pre-torsional treated specimens were cut using a low speed diamond saw for microstructural characterization.The microstructures of the specimens were examined from the diametrical cross-section by a JEOL 2100F transmission electron microscope (TEM) with an accelerating voltage of 200 kV.The TEM investigations were taken at regions of ~ 0 mm, ~ 1.5 mm and ~ 3 mm from the center.The TEM samples were electropolished using an electrolyte of nitric acid (33%) and methyl alcohol (67%) at a temperature of 243 K. Dual beam focused ion beam/scanning electron microscope (FIB/SEM, FEI 600i) method was used to prepared the TEM specimens of some special positions.

Mechanical properties
Fig. 1 shows the torque versus twist curves.The torque increases with increasing the torsional angle, and the torque of the Cu30Zn alloy is higher than that of the Cu10Zn alloy at the same torsional angle, indicating that Cu30Zn might have a high strength/hardness due to more effective solid solution strengthening.
Fig. 2 shows the Vicker's hardness of the specimens before and after pre-torsional treatment.The hardness along the radial direction of the as-received sample is homogeneous, and after pre-torsional treatment the hardness is clearly seen to increase along the radial direction.The hardness also increases with increasing the pre-torsional degree.It should be noted that the hardness of Cu30Zn alloy is higher than that of Cu10Zn alloy after pre-torsional deformation for the same degree at the same position, which agrees well with the torque during the torsional treatment (shown in Fig. 1).Similar to high pressure torsion (HPT) treatment 25 , the pre-torsional treatment can also introduce gradient strain along the radial direction.
Fig. 3a shows the engineering strain-engineering stress curves of the Cu10Zn alloy before and after pre-torsional treatment.With increasing the torsional degree, the yield strength and ultimate tensile strength (UTS) of the specimens increase substantially.The yield strength increases from ~220 MPa of Cu10Zn-0 sample to ~450 MPa of Cu10Zn-360 sample, and the UTS increases from ~450 MPa of Cu10Zn-0 sample to ~570 MPa of Cu10Zn-360 sample.For the ductility, the uniform elongation of samples was also changed with the torsional treatment, which is also highly dependent on the torsional degree.Generally, the ductility decreases with increasing the torsional degree, and the reduction of the ductility reaches ~64.5% after torsional treatment for 360°.Fig. 3b shows the engineering strain-engineering stress curves of the Cu30Zn alloy before and after pre-torsional treatment.The evolutions of yield strength, UTS and ductility of Cu30Zn alloy after pre-torsional treatment are similar to those of the Cu10Zn alloy.The maximum yield strength and UTS of Cu30Zn-360 sample were ~550 MPa and ~600 MPa, respectively.The reduction of the ductility is ~71.4% after torsional treatment for 360°.Fig. 3c and 3d is the corresponding strain hardening rate curves of the Cu10Zn  and planar slip can all be observed.For the Cu30Zn-180 specimen, several dislocations can be observed at the center of the specimen (Fig. 4d).In the region where r/R ≈ 0.5 (see Fig. 4e), a large number of the stacking faults (also see the insert image) and dislocations were observed.In the region where r/R ≈ 1 (see Fig. 4f), some deformation nanotwins can be observed.

Discussion
It is well known that SFE is an important factor affecting the deformation mechanism of materials.Decreasing SFE can reduce the possibility of cross slip and thus lead to the transition of dislocation slip from wavy slip to planar slip 26 .On the other hand, reducing SFE significantly increases the propensity of deformation twinning, making it a feasible deformation mechanism during deformation 27 .SFE can also significant affect the thickness of the twins, and previous investigation indicated that when the twin boundary spacing is reduced to the nanometer scale, the twin boundary strengthening becomes dominant 28 and the nanoscale twin boundaries can provide as much strengthening as conventional high-angle grain boundaries via blocking dislocation motion 29 .Previous study indicated that SFE can also affects the formation of incoherent twin boundaries, which lead to the various of split length of 9R phase 30 .In this paper, Cu-10wt.%Zn and Cu-30wt.%Znalloys were treated by the same pre-torsional treatment.After pre-torsional treatment, the microstructures of different positions in the same specimen and the same position in different specimens are various.For the Cu10Zn-180 specimen, in the region where r/R ≈ 0.5 (Fig. 4b) the dislocations slip is wavy slip and dislocation cells were formed.In the region where r/R ≈ 1 (Fig. 4c) the dislocation planar slip can be observed.For the Cu30Zn-180 specimen, in the region where r/R ≈ 0.5 (Fig. 4e) the dislocation planar slip and SFs can be observed.On the other hand, several deformation twins can be observed in the region where r/R ≈ 1 (Fig. 4f).During the torsional processing, the shear strain along the radial direction is gradient 15 , leading to the various microstructure along the radial direction.
For the same material, different deformation processing method can lead to the different twinning capabilities, which will affect the mechanical properties 23 .For Cu-32wt.%Zn, high-pressure torsion (HPT) treatment induced more deformation twins than that of equal-channel angular press (ECAP) treatment, which leads to both higher strength and ductility after HPT treatment 23 .Nanoscale twins promote dislocation-twin-boundaries interactions and create more room for dislocation storage, leading to more pronounced strain hardening.The presence of the nanoscale twins in the grains changes the dislocation activities considerably, since nanoscale twin boundaries not only act as effective barriers for dislocation slip, but also provide space for dislocation and Cu30Zn alloys.In the first part of the strain hardening rate curves, the strain hardening rates are very high and drop rapidly due to the formation of a large number of the dislocations tangles all over the interior of the grains.In the second part of the strain hardening rate curves, the strain hardening rates of all the specimens decrease continuously, but decreasing speeds are various and depend significantly on the torsional degree.

Microstructural evolution
Fig. 4 shows the TEM microstructures of the Cu10Zn-180 sample (Fig. 4a-4c) and Cu30Zn-180 sample (Fig. 4d-4f).Here r/R indicates the position from which the EBSD images were obtained (R is the radius of the cross section of the specimen, r is the radius of the recorded position).It can be seen from Fig. 4a that several dislocations can be observed in the region where r/R ≈ 0 (central region in the specimen) in the Cu10Zn alloy.Because the specimen center where r/R = 0 was not pre-strained, the structure observed at the specimen center was actually the same as that before torsional treatment.In the region where r/R ≈ 0.5 (see Fig. 4b), profuse dislocations can be observed, the dislocations tangled and formed the dislocation cells.In the region where r/R ≈ 1 (see Fig. 4c), dislocation cells, dislocation pile-ups  accumulation and storage 31,32 .In this paper, after pre-torsional treatment only several deformation twins were observed in the surface region of Cu-30wt.%Znalloy (Fig. 4f).Due to the limit number of the deformation twins, no obvious strain hardening rate improvement was observed in the Cu-30wt.%Znalloy after pre-torsional treatment (Fig. 3d).Thus, the mechanical properties evolutions of both Cu10Zn and Cu30Zn alloys after pre-torsional treatment include improved strength but sacrificed ductility, and the evolution tendency for both alloys is the same (Fig. 3a and 3b).

Conclusion
In the study, Cu-10wt.%Zn and Cu-30wt.%Znalloys with different SFEs were treated by pre-torsional deformation.The results showed that after pre-torsional deformation a gradient hardness was formed along the radial direction and the pre-torsional deformation can significantly improve the strength but decrease the ductility of both the Cu-10wt.%Zn and Cu-30wt.%Znalloys.After torsional treatment, different microstructures were formed along the radial direction in Cu-10wt.%Zn and Cu-30wt.%Znalloys, due to the relative lower SFE of the Cu-30wt.%Znalloy compared to that of the Cu-10wt.%Znalloy.The microstructures along the radial direction in Cu-30wt.%Znalloy were "dislocations→stacking faults→twins (low density)".

Figure 1 .
Figure 1.The torque versus torsion angle curves of the Cu10Zn and Cu30Zn alloys.

Figure 2 .
Figure 2. The evolution of the Vicker's hardness of the (a) Cu10Zn and (b) Cu30Zn alloys before and after pre-torsional treatment.

Figure 3 .
Figure 3. Engineering strain-engineering stress curves of the (a) Cu10Zn and (b) Cu30Zn alloys before and after pre-torsional treatment, and the corresponding strain hardening rate curves of (c) Cu10Zn and (d) Cu30Zn alloys.

Table 1 .
The composition, torsional angle and corresponding specimen names of different specimens.