Study on the impact of Cr content on the microstructure and properties of Cu-Cr alloys

Copper has excellent electrical conductivity, but the disadvantages of low strength and hardness limit its application range. In this paper, Cu-XCr (X=5, 7, and 9, where 5, 7, and 9 are all mass percentages) alloys were treated as the object of study; the impacts of Cr contents on the microstructure, Vickers hardness, and conductivity of Cu-XCr alloys were investigated using an XHC-SV2 upright metallographic microscope, a digital display Vickers hardness tester, and a Sigma2008C digital metal conductivity meter; the optimal Cr content was determined by the comprehensive performance parameter Z. The research results showed that the second phase of Cu-XCr alloys contained dendritic, nearly spherical, and other shapes. The size was relatively small, and the distribution was relatively uniform; the Vickers hardness improved and the electrical conductivity declined with climbing Cr content; when X=7, the comprehensive performance parameter Z of Cu-XCr alloys reached a maximum of 6.7×105 HV2·%IACS.


Introduction
Copper is used universally in electronics, electric power, machinery, metallurgy, military industry, and transportation fields because of its good thermal conductivity, electrical conductivity, corrosion resistance, and ductility.However, its disadvantages, such as low strength and hardness, make it almost impossible to be used as a structural component, thereby reducing its application range.Therefore, there is an urgent need to increase the hardness and strength and to obtain a good combination of strength and electrical conductivity for copper [1,2].
In recent decades, binary Cu-X alloys Cu-Cr, Cu-Fe, Cu-Ag, and Cu-Nb, etc., have become one of the key conductive materials due to their high strength and conductivity [2,3].However, Ag and Nb are precious metal materials, which are difficult to be widely used in practical production.At the same time, at high temperatures the solubility of Fe is high and at low temperatures the precipitation from Cu is slow, therefore the electrical conductivities of Cu-Fe alloys are relatively low [4,5].Cr has relatively low cost, high strength, and low solubility in the Cu matrix.Accordingly, the Cu-Cr alloy is a binary alloy with great application prospects [5,6].

Experimental details
Cu-XCr (X=5, 7, 9) alloys were melted using a medium-frequency melting furnace.The specific steps were as follows: First, electrolytic copper and industrial pure chromium (99.94 wt%) were matched according to the above mass percentage ratio and put into a magnesia crucible in the melting furnace for melting, and then the alloy in the molten state was poured into a graphite casting mold with a size of 36×380 mm.The alloy composition was analyzed using a novel 300-flame atomic absorption spectrometer.The result indicated that the composition of the as-cast Cu-XCr alloys was consistent with the mass percentage of the experimental ratio.
Several 15×15×8mm samples were cut from different as-cast Cu-XCr alloys by machining.The samples were embedded using an XQ-2B inlay machine and were ground and polished using 400#, 1000#, and 1500# sandpapers and a P-1 metallographic sample polishing machine respectively.The sample surface was etched using a test solution prepared with 5gFeCl 3 +25mlHCl+100mlH 2 O.The microstructure was observed by an XHC-SV2 upright metallographic microscope.The Vickers hardness was determined by a digital display Vickers hardness tester.The conductivity of the samples was determined by a Sigma2008C digital metal conductivity meter, each was tested five times, and took the average value as the experimental result.To avoid errors caused by work hardening, three locations far away were selected for hardness measurement for each sample and took the average value as the experimental result.

Microstructure
Figure 1 shows the microstructure of Cu-XCr (X=5, 7, 9) alloys.The microstructure of Cu-XCr alloys was composed of Cu matrix and primary Cr dendrites.The second phase contained dendritic, nearly spherical, and other shapes.The second phase of Cu-5Cr alloy had a large size and poor distribution uniformity, as shown in Figure 1 (a).The second phase size of Cu-7Cr was relatively small and the distribution was relatively uniform, as shown in Figure 1 (b).As observed in Figure 1 (c), the second phase size of Cu-9Cr was relatively small, but there were problems such as partial aggregation and network distribution.

Conductivity
Figure 3 presents the as-cast Cu-XCr (X=5, 7, 9) alloys' conductivity.As observed in Figure 3, the Cu-XCr alloys' conductivity decreased with increasing Cr content.This is because the interfacial scattering resistivity and impurity scattering resistivity are the main components of the resistivity of Cu-XCr alloys.The interface area and impurity content in the alloy increased with climbing Cr content, which resulted in a decrease in the Cu-XCr alloys' conductivity.At X=5, the conductivity of Cu-XCr alloy was the highest, and its value was 55.33% IACS.

Comprehensive performance
In practical production applications, people not only need Cu-Cr alloys to have good strength but also need them to have excellent electrical conductivity, to achieve a fine match between electrical conductivity and strength for Cu-Cr alloys.Related investigations have studied the comprehensive performance of the electrified railway contact wire, and obtained the comprehensive performance parameter Z of the material.The specific expression was as follows: (1) In Equation (1), σ b was the Vickers hardness of the material; φ was the electrical conductivity of the material.
Figure 4 presents the comprehensive performance parameters Z of as-cast Cu-XCr (X=5, 7, 9) alloys.It could be found from Figure 4 that the comprehensive performance parameter Z of Cu-XCr alloys first improved and then declined with climbing Cr content.At X=7, the comprehensive performance parameter Z of Cu-XCr alloy was the largest, and its value was 6.710 5 HV 2 %IACS.This shows that the Cu-7Cr alloy had the best comprehensive performance.

Conclusions
(1) The microstructure of Cu-XCr (X=5, 7, 9) alloys was composed of Cu matrix and primary Cr dendrites.The second phase contained dendritic, nearly spherical, and other morphologies.At X=7, the second phase size of Cu-XCr alloy was relatively small and the distribution was relatively uniform.
(2) The Cu-XCr alloys' Vickers hardness improved with climbing Cr content.At X=9, the Cu-XCr alloy's Vickers hardness was up to 125.69HV.
(4) The comprehensive performance parameter Z of Cu-XCr alloys first improved and then declined with climbing Cr content.At X=7, the comprehensive performance parameter Z of Cu-XCr alloy was up to 6.710 5 HV 2 %IACS.
(5) The research is beneficial for expanding the application fields of Cu-Cr alloys.The next step should be to explore new microalloying and preparation processes to further increase the Cu-Cr alloys' comprehensive performance.