J. Cent. South Univ. (2020) 27: 1158-1167

DOI: https://doi.org/10.1007/s11771-020-4356-4

Mechanical property and corrosion behavior of aged Cu-20Ni-20Mn alloy with ultra-high strength

TANG Shuai-kang(唐帅康)¹, LI Zhou(李周)^{1, 2}, GONG Shen(龚深)¹, XIAO Zhu(肖柱)^{1, 3}

1. School of Materials Science and Engineering, Central South University, Changsha 410083, China;

2. State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, China;

3. Key Laboratory of Non-ferrous Metal Materials Science and Engineering, Ministry of Education,Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: The ultra-high strength Cu-20Ni-20Mn alloy was prepared by vacuum melting and its mechanical property and corrosion behavior were investigated. After thermomechanical treatment, the alloy exhibited an ultra-high tensile strength of 1204 MPa and the applicable elongation of up to 6.2%. With the increasing exposure time in 3.5% NaCl solution, the corrosion current of the alloy decreased, while the polarization resistance and the charge-transfer resistance of the corrosion surface increased. The corrosion products formed on the surface of the alloy exposed for 1 d, and further corrosion made the corrosion product layer much dense, increasing the corrosion resistance and protecting the alloy from further corrosion.

Key words: CuNiMn alloy; ultra-high strength; elongation; corrosion

Cite this article as: TANG Shuai-kang, LI Zhou, GONG Shen, XIAO Zhu. Mechanical property and corrosion behavior of aged Cu-20Ni-20Mn alloy with ultra-high strength [J]. Journal of Central South University, 2020, 27(4): 1158-1167. DOI: https://doi.org/10.1007/s11771-020-4356-4.

1 Introduction

Copper alloys have been widely applied in industry due to their high electrical and thermal conductivity, excellent mechanical properties and good corrosion resistance in seawater environment [1-4]. Especially in marine engineering, copper alloys have been used as deep-sea pipelines and valves, bearings and propellers, as the copper ions have a stronger antimicrobial effect to inhibit the adhesion and growth of the marine organisms [5-7]. Phosphorus-added deoxidized copper (TP2) pipes were generally used as seawater piping for ships, however, their low strength limited the further application for the high load structure components in marine engineering. In recent years, BFe10-1-1 and BFe30-1-1 copper alloys were developed to meet the demand of advanced marine engineering [8, 9]. However, none of the above copper alloys can achieve a strength above 1000 MPa. Cu-Be copper alloy was one of the most widely used ultra-high strength copper alloys. The tensile strength of QBe2 and QBe1.9-0.1 reached 1200 MPa, however, their elongation was less than 3.0%. The low ductility of these alloys not only made them difficult to process and decreased the product yield, but also reduced the safety and reliability of the component due to the brittle fracture in service.

Recently, CuNiMn alloys have attracted many attentions due to their ultra-high strength and relative high ductility. Many studies on CuNiMn alloy mainly focused on their structure, phase transformation and mechanical properties. PAN [10] investigated the mechanical properties of Cu-20Ni-20Mn alloy and found that the alloy had a strength of higher than 1200 MPa, with an elongation of up to 5%. XIE et al [11] and LUO et al [12] reported that the main precipitation- strengthening phase of CuNiMn alloys was the NiMn precipitate with face-centered tetragonal (FCT) structure, and its orientation relationship with the Cu matrix was (020)_Cu//(001)_NiMn and [001]_Cu//[010]_NiMn. Although some literatures [13-15] reported that the Cu-Ni binary alloy had a good corrosion resistance in salt environment, there was little work reported in the literatures on the corrosion behavior of CuNiMn alloys with ultra-high strength.

In this paper, a small amount of Ce element was added into Cu-20Ni-20Mn alloy to refine the microstructure of the alloy. The mechanical property and corrosion behavior of Cu-Ni-Mn-Ce alloy were investigated, and its corrosion mechanism in 3.5% NaCl solution was analyzed.

2 Experimental

2.1 Alloy preparation

Cu-20Ni-20Mn-0.15Ce (wt%) alloy was prepared by vacuum melting and mold casting. After removing the surface defects, the ingots were homogenized at 920 °C for 8 h and subsequently hot-rolled at 850 °C from a thickness of 25 to 7.5 mm. The hot-rolled plate was then annealed at 750 °C for 2 h and cold-rolled to 1.5 mm. Finally, the cold-rolled strips were solid solution treated at 700 °C for 2 h, followed by aging at 400 °C for different time.

2.2 Mechanical and electrochemical tests

Hardness test was performed on a HV-5 Vickers hardness tester with a load of 5 kg and holding time of 15 s. Each sample was measured five times and the average value was taken. The room temperature tensile tests of the alloy were carried out on the WDW-200KN microcomputer control electronic universal testing machine according to GB/T 228.1—2010.

The peak-aged samples (aged at 400 °C for 24 h) used for electrochemical test were exposed in 3.5% NaCl solution at (35±0.5) °C for 1, 3, 7, 10 and 15 d. The surface of each sample was sealed by polymethymethacrylic, leaving an uncovered area of 10 mm×10 mm in the experiment. The exposed surface was ground with the 1200# sandpaper, subsequently polished with the diamond polishing paste. The polished samples were cleaned with anhydrous ethanol and deionized water, air dried and then immersed in 3.5% NaCl solution. The containers containing the samples were placed in a thermostat water bath cauldron.

Electrochemical tests were carried out on the Multi AUTOLAB M204 electrochemical workstation. Pt electrode was used as the auxiliary electrode, and saturated calomel electrode (SCE) was used as the reference electrode. First, the samples were subjected to an open circuit potential test (OCP). The OCP test time was 500 s, and the average values of the last 50 s were used as the test data during processing. The polarization curve and the electrochemical impedance spectroscopy (EIS) were measured. The testing range for the polarization curve was between -600 mV and 400 mV, with the scanning speed of 1 mV/s. The measured data were analyzed by CHI660C software. The frequency range of the EIS test was between 100 kHz and 10 mHz, and 71 points were measured. The amplitude of the sinusoidal AC disturbance voltage was 10 mV. The measured data were analyzed by Zview 2 software.

2.3 Microstructure observation

Optical metallographic (OM) observation was carried out on the Leica EC3 microscope. The microstructures of the aged samples and corrosive samples were observed using a Sirion200 field emission scanning electron microscope (SEM). The microstructures of the aged samples were observed by a Tecnai G2 F20 transmission electron microscope (TEM).

3 Results and discussion

3.1 Microstructure and mechanical properties of alloy

The composition of Cu-20Ni-20Mn alloy was shown in Table 1. It can be seen that there are few impurity elements in the alloy. Figure 1 shows the OM images of the samples after different treatments. Coarse typical dendritic structure grains formed in the casted ingot (Figure 1(a)), due to the diffusion difficulty of the high content alloying elements and the slow cooling rate in vacuum induction furnace [10]. After the homogenization treatment at 920 °C for 8 h, the dendritic structure disappeared and the large grains formed (Figure 1(b)). The large grains were broken after hot-rolling and cold-rolling treatment, and elongated along the rolling direction (Figure 1(c)). Recrystallization occurred in the aged sample, leading to fine grain in the alloy (Figure 1(d)). The average grain size of the aged sample was 15-30 μm.

Table 1 Compositions of designed alloy (wt%)

Figure 2(a) shows the variation of hardness of the alloy aged at 400 °C. The hardness increased with the aging time, and reached its peak value of HV 413 after 24 h aging. Figure 2(b) shows the SEM image of the peak aged sample. Fine equal crystal grain was found, however, little coarse second phase appeared in the alloy. Figures 2(c) and (d) show the TEM image and the selected area electron diffraction (SAED) pattern of the peak aged sample. Some larger discontinuous precipitates still formed along the direction perpendicular to the grain boundary (Figure 2(c)). However, a large number of the nanoscale NiMn particles precipitated in the alloy and the precipitates distributed uniformly inside the grains (Figure 2(d)).

The size of the precipitates at the grain boundaries was larger than that inside the grains. This could be attributed to the higher energy at the grain boundaries of the alloy, where the second phase tended to nucleate and grow preferentially. The fine grain and tiny precipitates contributed to the strength of the alloy.

Figures 2(e) and (f) show the tensile curve and fracture morphology of the alloy, respectively. The tensile strength of the alloy was up to 1204 MPa, the yield strength was 971 MPa, and the elongation reached 6.2%. The fracture photographs of the alloy showed obvious dimples, suggesting a ductile fracture mode. The size of the dimples was related to the geometric size and the distribution of the second phase. The depth of the dimples was dependent on the nearby metal matrix when the dimples were connected, and it increased with the ratio of deformation. During the aging process, a large number of second phase precipitated and the distance between the particles was very small. These precipitates were difficult to deform during tensile test, leading to shallow and tiny dimples.

Figure 1 Optical metallographic images of alloys after different treatments:

Figure 2 Variation of hardness of alloy aged at 400 °C (a), SEM image of peak aged sample (b), TEM image (c) and SAED pattern (d) of peak aged sample, stress-strain curve of peak aged sample (e), fracture morphology of peak aged sample (f)

The main strengthening mechanisms of the studied alloy are mainly attributed to solid solution strengthening, precipitation strengthening, and grain refinement strengthening. According to Figure 2 and Ref. [11], a large number of NiMn precipitates formed in Cu-20Ni-20Mn alloy during aging, which could hinder the movement of dislocations and increase the strength of the alloy. Ni and Mn elements dissolved in the matrix could also improve the strength and hardness of the alloy. Furthermore, the fine grain could contribute to the increase of both the strength and the plasticity of the alloy. The influence of the grain size on the strength of the alloy can be estimated by Hall-Petch equation:

                            (1)

where τ_s and τ₀ are the alloy yield stress and the resistance of dislocation motion within grains, respectively; K is a constant associated with the material, and d is the average grain diameter.

The strength of the alloy increased with the decrease of grain size. The grain boundaries could hinder the motion of sub-structure, leading to a remarkable increase in strength of the alloy [16]. Besides, the large-area grain boundaries within a unit volume of the small grain played a cooperative deformation role during deformation, improving the plasticity of the alloy. Therefore, the alloy maintained a high strength and plasticity. The addition of Ce in the alloy could also enhance the flow stress and the strength of the copper alloy [17, 18].

Table 2 shows the mechanical properties of the studied alloy and some typical ultra-high strength copper alloys reported in literatures. Cu-20Ni- 20Mn had a tensile strength of more than 1200 MPa and an elongation of more than 6%. The strength- ductility product was up to 7465 MPa%, which was almost twice that of other ultra-high strength copper alloys. In addition, the alloy had a hardness of HV 413, which was much higher than that of the aged BFe30-1-1 alloy (HV 225). The ultra-high hardness could improve the wear resistance performance of the alloy used as seawater pipelines during service.

3.2 Corrosion behavior of alloy

3.2.1 Open circuit potential test

The open circuit potential (OCP) characterizes the magnitude of the thermodynamic trend of the material in the solution. The more negative the potential value is, the higher the tendency of the material is likely to corrode in solution [25].Figure 3 shows the variation of the open-circuit potential with exposure time for studied alloy in 3.5% NaCl solution. At the early stage of corrosion, the open-circuit potential moved positively rapidly due to the rapid formation of a corrosion film on the surface, then shifted to the negative values gradually, and reached the steady state at about 5 d. This process was related to the reaction process occurring on the surface of the copper alloy [26-28].

3.2.2 Polarization curve test

Figure 4 shows the polarization curves of samples exposed in 3.5% NaCl solution for different days. The anode curves of samples displayed a typical passivation phenomenon, indicating that a protective corrosion product film formed on the surface of samples. The formation of positive film could prevent the surface of alloy from further corrosion [29]. The polarization resistance (R_p) can be calculated by the Stern-Geary equation [27]:

                   (2)

where b_a and b_c are the anode and cathode Tafel slopes, respectively, and i_corr is the self-corrosion current density.

Table 3 summarizes the electrochemical parameters obtained from the polarization curves of the alloy, where E_corr is the self-corrosion potential of the alloy. According to the calculation results, the anode Tafel slope b_a and the cathode Tafel slope b_c were independent. This result was similar with the corrosion parameters of Cu-30Ni alloy in sulfur-containing seawater reported in Ref. [30]. With the increase of exposure time in 3.5% NaCl solution, the corrosion current of samples decreased and the polarization resistance gradually increased in the whole testing period. The equivalent resistance of the sample increased with the exposure time, suggesting that the corrosion product film protected the surface of the alloy.

Table 2 Mechanical properties of some ultra-high strength copper alloys

Figure 3 Variation of OCP with time for Cu-20Ni-20Mn alloy in 3.5% NaCl solution

Figure 4 Polarization curves of samples after exposed in 3.5% NaCl solution for different time

3.2.3 Impedance measurement test

Figure 5 shows the impedance spectrum of samples after exposed in 3.5% NaCl solution for different time. Figure 5(a) is the Nyquist diagram of the alloy samples. The pattern was roughly semi-circular in the high-frequency and intermediate-frequency zone. The typical Nyquist semi-circle diameter of the alloy gradually increased with the immersion time, indicating that the charge transfer resistance on the surface of the alloy gradually increased with the immersion time [31]. Figure 5(b) is the Bode diagram of the alloy samples. In the low frequency region, lg|Z| also increased with the immersion time gradually. For example, at 10^-2 Hz, the lg|Z| increased form 3.37 at 1 d to 3.78 at 15 d, indicating that the protective oxide film formed and became thick on the surface of samples in the testing period with the immersion time [32]. In the Bode diagram of samples exposed for more than 3 d, there was an inverse relationship between lg|Z| and lgf with a slope close to -1 in the low-frequency and intermediate-frequency zones, due to the capacitance of the electric double layer (EDL) between the metal surface and the solution or the corrosion product film and the metal surface or both [31]. This indicated that a complete oxide film formed on the sample surface. This capacitive behavior disappeared in the lower frequency range between 10^-2 Hz and 10^-1 Hz, suggesting that the corrosion mechanism was mainly diffusion- controlled [33].

Figure 5(c) is the Bode-phase angle diagram of the samples. During the corrosion process from 1 to 15 d, only one peak appeared in the frequency range between 10^-2 Hz and 10³ Hz, suggesting that single oxidation layer formed on the surface. In the low frequency region, the angle of the peak was related to the EDL, and its value gradually increased with the increase of the immersion time, indicating that the corrosion rate decreased with the immersion time. The impedance spectrum of the samples showed that a protective corrosion product film formed on the surface of the alloy in 3.5% NaCl solution, and the corrosion product layer became thicker and denser with the exposure time.

Table 3 Electrochemical parameters of polarization curves of samples after exposed in 3.5% NaCl solution for different time

Figure 5 Impedance spectra of samples after being exposed in 3.5% NaCl solution for different time:

3.2.4 SEM corrosion morphology analysis

Figure 6 shows the SEM images of the surface and cross section of samples exposed a in 3.5% NaCl solution for different time. The corrosion products formed on the surface of sample exposed for 1 d. Further increasing the exposure time, the corrosion products layer became dense, which was consistent with the results of electrochemical tests. The distribution of the corrosion product film was relatively uniform, without pits on the grain boundaries and inside the grains. The SEM image of the cross section of the sample after exposed in 3.5% NaCl solution for 15 d showed that the interface between the matrix and the corrosion product layer was quite straight, suggesting that the alloy exhibited a general corrosion characteristic. This could be related to the uniformity of the microstructure of the alloy [27]. The EDX analysis results in Figure 6(d) showed that the corrosion product layer mainly contained Cu, Ni, Mn and O, suggesting that the corrosion product layer consisted of the oxides of Cu, Ni and Mn.

3.2.5 Corrosion mechanism of alloy

According to EDX results and Refs. [26, 34, 35], it may be speculated that the main corrosion products in the corrosion film of Cu-20Ni-20Mn alloy could be CuO, Cu₂O, MnO, Mn₂O₃, MnO₂, NiO and Ni(OH)₂. The electrode potentials for reversible anodic dissolution of the alloying elements, Mn and Ni in the alloy, are -1.029 V and -0.257 V (vs standard hydrogen electrode), respectively. Therefore, Mn and Ni may be dissolved in the early stage of corrosion [30]:

                   (3)

            (4)

   (5)

The cathode reaction:

                     (6)

Some of Ni²⁺ may bond with OH^- [13]:

                  (7)

MnO can further react with O₂ dissolved in the solution [35]:

                      (8)

                     (9)

The corrosion of Cu-based alloy in the solution with Cl^- ions often involves the oxidation of Cu [26]:

                       (10)

Figure 6 SEM images of samples exposed in 3.5% NaCl solution for different time: Surface of sample exposed for 1 d (a), 7 d (b), 15 d (c); Cross section of sample exposed for 15 d (d)

Followed by:

                       (11)

Subsequently, CuCl^2- may decompose in the OH^- environment through the following reaction [27]:

          (12)

Copper oxide products formed by the following reactions:

             (13)

                (14)

During the corrosion process, the OCP of the initial stage of the corrosion shifted positively, which could be attributed to the formation of the corrosion film, and the negative shift in the later period should result from the transition of corrosion product from Cu₂O to CuO. The corrosion products, especially CuO and NiO, would form a dense corrosion film on the surface of the alloy, decreasing the corrosion rate and protecting the alloy from further corrosion. According to the Ref [36, 37], Ce could inhibit the corrosion of copper alloy in NaCl solution in some extent, with the inhibition efficiency of about 50%, as the corrosion product layer of the Ce-contained alloy could be bonded much tightly with the matrix.

4 Conclusions

1) The aged Cu-20Ni-20Mn-0.15Ce alloy with grain size of 15-30 μm had an ultra-high tensile strength of the alloy of 1204 MPa and elongation of 6.2%.

2) The open circuit potential of the Cu-20Ni-20Mn alloy shifted positively at the early stage of corrosion, then moved to negative direction. During the subsequent corrosion process, the open circuit potential gradually shifted negatively and gradually stabilized after 5 days of immersion corrosion. In the whole testing period, the corrosion current of the alloy generally decreased, and the polarization resistance and the charge transfer resistance increased gradually.

3) The corrosion products formed on the surface of the alloy after exposed for 1 d, and further corrosion made the corrosion product layer much dense, increasing the corrosion resistance and protecting the alloy from further corrosion.

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(Edited by HE Yun-bin)

中文导读

拥有超高强度的时效态Cu-20Ni-20Mn合金的力学性能和腐蚀行为

摘要：通过真空熔炼制备了拥有超高强度的Cu-20Ni-20Mn合金，并研究了其力学性能和腐蚀行为。结果表明：经过一系列变形和热处理工艺后，该合金表现出1204 MPa的超高拉伸强度，且伸长率高达6.2%。随着合金在3.5%NaCl溶液中暴露腐蚀时间的延长，合金样品的腐蚀电流降低，而腐蚀表面的极化电阻和电荷转移电阻增加。腐蚀产物在暴露1 d时已在合金表面上形成，并进一步腐蚀，使得腐蚀产物层变得致密。腐蚀产物层增强了合金的耐腐蚀性能，保护合金免受进一步的腐蚀。

关键词：CuNiMn合金；超高强度；伸长率；腐蚀

Foundation item: Project(2017YFB0306105) supported by the National Key R&D Program of China; Projects(51601227, 51701241) supported by the National Natural Science Foundation of China

Received date: 2020-03-03; Accepted date: 2020-03-27

Corresponding author: XIAO Zhu, PhD, Associate Professor; Tel: +86-13974910804; E-mail: xiaozhumse@163.com; ORCID: 0000- 0001-7042-8860