Influence of Fe Addition on the Microstructure and Mechanical Properties of Cu Alloys

The influence of Fe addition on the microstructure and mechanical properties of Cu alloys was studied. Combined with phase identification via EDS, X-ray diffraction shows that only Cu matrix and α-Fe phase exist in Cu-5Fe, Cu-10Fe and Cu14Fe alloys. With the addition of Fe, the grain size refines from 1011 μm in pure Cu to around 46 μm in Cu-Fe alloys. Phase identification of the alloys via EBSD shows that the α-Fe phase with an average grain size of about 4 μm is located in grain boundaries and in the grain interior. With increasing Fe content, the yield stress largely increased while maintaining the high elongation to failure. The results show that grain refinement; second phase strengthening and texture strengthening effect play a descending-order role in the overall strengthening effect.


INTRODUCTION
industry. Y Li [6] studied the mechanical properties of Cu-10Fe alloy added with Ag and found that when the amount of Ag was 3 wt%, the conductivity of Cu-10Fe alloy increased by 1-4 % IACS after aging, and the tensile strength increased by  MPa. In addition, it is found that the addition of only 2 wt% Si on the Cu-10Fe alloy can lead to the microstructural evolution such as the microstructural refinement and the formation of secondary separated phases with the yield strength enhanced from 180 MPa to 285 MPa [4]. During the solidification of Cu-Fe alloy, the liquid phase separation happens [7][8][9][10]. The liquid phase separation is a kind of uneven distribution of Fe phase, which would result in severe macrosegregation, and will deteriorate the overall performance of Cu-Fe alloys.
The higher the Fe volume fraction, the more sever the macrosegregation. Then, the composition, microstructure and properties of the Cu-Fe alloy are not uniform. Therefore, Cu-Fe alloy with a relatively uniform composition and uniform structure (especially alloys with an Fe content of more than 5 wt%) is hard to be prepared by the traditional casting method [11]. This is the case in China that Cu-Fe rolled strips can be successfully produced when Fe content is less than 5%. How different amounts of Fe addition (more than 5% Fe) influence the microstructure and properties of Cu-Fe alloys remains unclear. This work places great emphasis on the influence of Fe addition on the microstructure and mechanical properties in Cu alloys. The results find that the higher fraction of Fe addition, the harder the alloys. The relevant mechanism is discussed.
Pure Cu is purchased from Jiangxi Copper Group Co., LTD, and pure Fe from Nanchang Iron & Steel Co. LTD. The samples were cast into a plate-shape graphite mold of 45 mm thickness. Table 1 shows the compositions of different cast alloys.
Tension tests along the long direction of the plate (denoted as RD) at room temperature were performed on a CMT5205 machine using a strain rate of 2 ×10 -3 s -1 . The specimens for tension tests were dog-bone shaped with a gauge length of 50 mm and across-section of 8×0.2 mm. At least three samples were tested for each condition.

Microstructure and texture measurements
Phase identification of the alloys was performed using X-ray diffraction (XRD, Shimadzu XRD-7000). Electron backscatter diffraction (EBSD) mapping was conducted on a scanning electron microscope (SEM, EVO18) equipped with a HKL-EBSD system using a step size of 0.5 μm combined with energy dispersive X-ray spectrometry (EDS, X-Max20). The resulting inverse pole figure map, crystallographic orientation analysis, and pole figures were processed using commercial TSL OIM or Channel 5 software. Cu-14Fe alloy. It is found that the α-Fe phase with an irregular shape formed both in grain boundaries and grain interior.   table 2 show that for all alloys in the cast condition, the texture is random.

Mechanical behavior
The true stress-strain curves under tension are displayed in Fig. 7 with the relevant mechanical properties listed in

DISCUSSION
The mechanical properties of alloys are partially governed by grain size. A reduction in grain size serves to enhance yield stress as formulated by the well known Hall-Petch relationship [12,13].
where σ y is the yield stress, σ o is the friction stress when dislocations glide on the slip plane, d is the average grain size, and k is the stress concentration factor. The relationship provides guidance for a straightforward way to produce stronger and harder materials. Grain boundaries impede dislocation slip and smaller grains limit dislocation pile-up sizes which affects how easily dislocations can traverse grain boundaries and propagate from grain to its neighboring grain. A higher applied stress is then necessary to propagate dislocations between neighboring grains and this effectively increases the yield stress and hardness [14]. In the present work, for pure Cu in the cast state, the grain size is as large as 1011 μm. With the addition of 5%Fe, the grain is largely refined to 46 μm. Grain refinement strengthening should be one of the main strengthening factors. On the opposite, with more Fe addition, the average grain size hardly changes and the yield stress increase is relatively small. Therefore, grain refinement should be an important strengthening factor.
Second phase strengthening should be another important strengthening factor. For Cu-5Fe alloy, a large number of fine α-Fe phase is induced. Those α-Fe phases largely located in grain boundary and in the grain interior as shown in Fig.   8. The fine and dispersed α-Fe phases would largely hinder dislocations slip and then strengthen the alloy. The case is almost the same as Cu-10Fe and Cu-14Fe. The volume fraction of α-Fe phase is much higher in Cu-10Fe and Cu-14Fe than that in Cu-5Fe, the yield stress is therefore higher as shown in Texture could also be a strengthening factor, which is referred as the texture strengthening effect. Schmid factors (SFs) as a function of the relative spatial position in EBSD maps and relevant distribution for slip under tension along the RD for the four alloys are depicted in Fig. 9, with the relevant average SF listed in value of average SF is similar for Cu-5Fe, Cu-10Fe and Cu-14Fe.
The lower the SF, the higher the texture strengthening effect.
The SF in α-Fe phase is also calculated as shown in Table 4. The SF in α-Fe phase is slightly higher than that in Cu matrix, but the value of average SF in α-Fe phase is similar for Cu-5Fe, Cu-10Fe and Cu-14Fe. The hardness of α-Fe phase is much higher than that of Cu matrix [15], the Cu matrix would undergo plastic deformation ahead of the α-Fe phase during the yielding stage. It is therefore the SF in Cu matrix that turns out to be a more important factor for the texture strengthening effect.    All in all, grain refinement and second phase should be two important parameters in strengthening effect. However, the grain refinement should be the more efficient factor, since the volume fraction of α-Fe phase is highest in Cu-14Fe, but the yield stress between Cu-5Fe, Cu-10Fe and Cu-14Fe is similar.
The texture strengthening effect should also play a minor role in the strengthening effect.

DATA AVAILABILITY
All relevant data supporting the findings of this study are contained in the paper and its supplementary information files.
All other relevant data are available from the corresponding authors on request.