Effect of Al Element on the Microstructure and Properties of Cu-Ni-Fe-Mn Alloys

materials

Article Effect of Al Element on the Microstructure and Properties of Cu-Ni-Fe-Mn Alloys

Ran Yang 1, Jiuba Wen 1,2,*, Yanjun Zhou 1,2, Kexing Song 1,2,* and Zhengcheng Song 1

1 School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471023, China; [email protected] (R.Y.); [email protected] (Y.Z.); [email protected] (Z.S.) 2 Collaborative Innovation Center of Nonferrous Metals, Henan University of Science and Technology, Luoyang 471023, China * Correspondence: [email protected] (J.W.); [email protected] (K.S.)

Received: 28 August 2018; Accepted: 14 September 2018; Published: 19 September 2018

Abstract: The effects of aluminum on the mechanical properties and corrosion behavior in artificial seawater of Cu-Ni-Fe-Mn alloys were investigated. Cu-7Ni-xAl-1Fe-1Mn samples, consisting of 0, 1, 3, 5, and 7 wt % aluminum along with the same contents of other alloying elements (Ni, Fe, and Mn), were prepared. The microstructure of Cu-7Ni-xAl-1Fe-1Mn alloy was analyzed by Transmission Electron Microscopy (TEM), and its corrosion property was tested by an electrochemical system. The results show that the mechanical and corrosion properties of Cu-7Ni-xAl-1Fe-1Mn alloy have an obvious change with the aluminum content. The tensile strength has a peak value of 395 MPa by adding 3 wt % aluminum in the alloy. Moreover, the corrosion rate in artificial seawater of Cu-7Ni-3Al-1Fe-1Mn alloy is 0.0215 mm/a which exhibits a better corrosion resistance than the commercially used UNS C70600. It is confirmed that the second-phase transformation of Cu-7Ni-xAl-1Fe-1Mn alloy follows the sequence of α solid solution → Ni3Al → Ni3Al + NiAl → Ni3Al + NiAl3. The electrochemical impedance spectroscopy (EIS) shows that the adding element aluminum in the Cupronickel can improve the corrosion resistance of Cu-7Ni-xAl-1Fe-1Mn alloy.

Keywords: Cu-Ni-Al alloy; microstructure; mechanical property; corrosion resistance

1. Introduction Due to their high resistance to biofouling, heat transfer ability, and favorable resistance to the ravages of the marine environment, copper-based alloys are being offered as solutions to a range of industries requiring reliability in seawater to meet today’s exacting engineering challenges. It is well known that the addition of nickel to copper matrix can usually improve the comprehensive properties of the alloy, including corrosion and mechanical properties [1–3]. For instance, H. Nady et al. [4] investigated the stability of Cu-Al-Ni alloy in neutral chloride solutions with different nickel contents. It is conﬁrmed that the corrosion rate of the alloy decreases as the content of nickel increases. However, its cost is quite high due to its high nickel content. In the 1930s, it was found that adding appropriate amounts of iron and manganese to copper–nickel alloys could improve corrosion–erosion resistance [5–7]. The most commonly used for seawater pipelines is the UNS C70600. The desired properties, including corrosion and mechanical properties, can be achieved by alloying elements aluminum [8–11]. One of the copper-based alloys Cu-14.5Ni-3Al-1.3Fe-0.3Mn has found increasing use in recent years. In previous studies [12–14], the good corrosion resistance of Cu-Ni-Al alloy in chloride solution was found to be attributed to the formation of a duplex oxide ﬁlm. However, the effect of aluminum on the combination property of special Cupronickel alloys Cu-7Ni-xAl-1Fe-1Mn has not been in-depth studied. Thus, a study of aluminum effect of Cu-7Ni-xAl-1Fe-1Mn on the mechanical properties and corrosion resistance is necessary.

Materials 2018, 11, 1777; doi:10.3390/ma11091777 www.mdpi.com/journal/materials Materials 2018, 11, x FOR PEER REVIEW 2 of 11 Materials 2018, 11, 1777 2 of 11 xAl-1Fe-1Mn has not been in-depth studied. Thus, a study of aluminum effect of Cu-7Ni-xAl-1Fe- 1Mn on the mechanical properties and corrosion resistance is necessary. For copper–nickel alloys, its ability of both mechanical strength and corrosion resistance in For copper–nickel alloys, its ability of both mechanical strength and corrosion resistance in seawater is crucial for engineering application. Therefore, we tried to reduce nickel content by adding seawater is crucial for engineering application. Therefore, we tried to reduce nickel content by adding aluminumaluminum and and expected expected better better results results with with the the CupronickelCupronickel alloy of of Cu-7Ni- Cu-7Ni-xxAl-1Fe-1MnAl-1Fe-1Mn alloy alloy (wt.%, (wt.%, usedused throughout throughout the the whole whole paper). paper). WeWe preparedprepared Cu-7Ni-Cu-7Ni-xAl-1Fe-1Mn with with different different aluminum aluminum contentscontents and and carried carried out out tensile tensile tests tests and and immersionimmersion test.test. The The experimental experimental results results will will provide provide guidanceguidance for for the the research research and and industrial industrial application application ofof Cu-7Ni-Cu-7Ni-xAl-1Fe-1Mn alloy. alloy.

2. Experimental2. Experimental

2.1.2.1. Materials Materials TheThe nominal nominal compositions compositions of theof designedthe designed Cu-Ni-Al Cu-N alloysi-Al alloys are listed are listed in Table in1 .Table The raw1. The materials raw werematerials electrolytic were copper,electrolytic nickel copper, block, nickel electrolytic block, electrolytic aluminum, aluminum, Cu-10 mass% Cu-10 iron mass% master iron alloy, master and Cu-22alloy, mass% and manganeseCu-22 mass% master manganese alloy. The master raw materialsalloy. The were raw smeltedmaterials in were medium smelted frequency in medium vacuum inductionfrequency furnace vacuum by feedinginduction sequence furnace by of copper,feeding nickel,sequence Cu-Fe of copper, master nickel, alloy, Cu-Fe Cu-Mn master master alloy, alloy, Cu- and electrolyticMn master aluminum. alloy, and The electrolytic raw materials aluminum. were The melted raw inmaterials proper were ratios melted at 1170–1200 in proper◦C ratios and deoxidized at 1170– by1200 magnesium °C and deoxidized alloy. by magnesium alloy.

TableTable 1. 1.The The nominal nominal chemical chemical compositions compositions ofof the designed copper copper alloys alloys (wt (wt %). %).

Number NumberCu Cu Ni Ni Fe Fe Mn Mn Al Al 1 Bal1 Bal7.0 7.0 1.01.0 1.01.0 0 0 2 Bal2 Bal7.0 7.0 1.01.0 1.01.0 1.0 1.0 3 Bal3 Bal7.0 7.0 1.01.0 1.01.0 3.0 3.0 4 Bal4 Bal7.0 7.0 1.01.0 1.01.0 5.0 5.0 5 Bal5 Bal7.0 7.0 1.01.0 1.01.0 7.0 7.0

2.2.2.2. Experimental Experimental Method Method MechanicalMechanical properties properties of of Cu-7Ni- Cu-7Ni-xxAl-1Fe-1MnAl-1Fe-1Mn alloy were were measured measured in in accordance accordance with with the the uniaxialuniaxial tensile tensile test. test. TensileTensile samplessamples were were prepared prepared according according to the tothe GB/T GB/T 228.1-2010 228.1-2010 method method of test of testat at room room temperature. temperature. Figure Figure 1 shows1 shows the tensile the tensile specimens specimens for room for temperatur room temperaturee tests. Tensile tests. testing Tensile testingwas performed was performed using a using SHIMADZU a SHIMADZU AG-I250KN AG-I250KN precision precisionuniversal testing universal machine testing with machine a constant with a constantstrain rate strain of 1 rate mm/min. of 1 mm/min. For transmission For transmission electron microscopy electron microscopy (TEM), thin (TEM), foil specimens thin foil specimens with a withdiameter a diameter 3 mm 3 mmwere were prepared prepared by means by means of both of both a double-jet a double-jet electropolisher electropolisher and and an anion ion thinner. thinner. MicrostructuresMicrostructures of of alloys alloys were were observed observed by by JEOL JEOLJEM-2100. JEM-2100.

FigureFigure 1. 1.Drawing Drawingof of tensiletensile specimensspecimens (unit: mm). mm).

TheThe corrosion corrosion resistance resistance of of Cu-7Ni-Cu-7Ni-xxAl-1Fe-1MnAl-1Fe-1Mn alloy was was measured measured by by static static immersion immersion corrosioncorrosion test test based based on on JB/T JB/T 7901-1999, 7901-1999, andand the corro corrosionsion rate rate was was calculated calculated by by weight weight loss loss method. method. TheThe sample sample size size was wasΦ30 Φ mm30 mm× 2.5 × 2.5 mm, mm, and and each each group group had had three three parallel parallel specimens. specimens. The The surface surface area of the sample was measured before immersion corrosion, and the quality was accurately determined by an analytical balance.

The corrosion medium was artiﬁcial seawater; the artiﬁcial seawater composition is shown in Table2[ 15]. The test was carried out at room temperature for 10 days, and the corrosion medium was Materials 2018, 11, x FOR PEER REVIEW 3 of 11

area of the sample was measured before immersion corrosion, and the quality was accurately determined by an analytical balance.

MaterialsThe2018 corrosion, 11, 1777 medium was artificial seawater; the artificial seawater composition is shown3 of 11in Table 2 [15]. The test was carried out at room temperature for 10 days, and the corrosion medium was replaced every seventh day. After completion of the test, with the volume ratio of 1:2 replacedhydrochloric every acid seventh solution, day. on After the corrosion completion after of cleaning, the test, the with corrosion the volume products ratio were of 1:2 removed hydrochloric after acidcleaning solution, with onalcohol, the corrosion and then after the cleaning, sample was the corrosionput in a drying products oven were drying removed to calculate after cleaning the quality with alcohol,loss of samples and then before the sample and after was putcorrosion. in a drying The ovencorrosion drying rate to calculateR was calculated the quality by loss the ofchange samples of beforesample and quality after corrosion.before and The after corrosion corrosion. rate RThenwas calculatedthe corrosion by therate change was calculated of sample qualityaccording before to andEquation after corrosion.(1): Then the corrosion rate was calculated according to Equation (1):

7 7 8.768.76×10× 10×(M× (−MM−)M1) R =R = 1 (1)(1) STDSTD where R is the corrosion rate, mm/a; M is the quality before metal corrosion, g; M1 is quality of where R is the corrosion rate, mm/a; M is the quality before metal corrosion, g; M1 is quality of the the sample after removing the corrosion product, g; S is the surface area of the sample, m2; T is the sample after removing the corrosion product, g; S is the surface area of the sample, m2; T is the corrosion time, h; and D is the density, Kg/m3. corrosion time, h; and D is the density, Kg/m3. Electrochemical tests were conducted in artificial seawater and exposed to laboratory air. The Table 2. Artiﬁcial seawater composition. working electrodes were plastic pipes by epoxy resin with a surface area of 1 cm2 to contact the solution.Chemical A three-electrode Compound system Concentration, with graphite g/L electrode Chemical Compoundas auxiliary electrode Concentration, and saturated g/L

calomel as referenceNaCl electrode was used 24.53 in the electrochemical MgCl experiment.2 The AC impedance5.20 were measured inNaHCO the frequency3 range from0.201 0.1 to 105 Hz. The amplitude CaCl2 of the superimposed1.16 ac-signal was 10 mV. EachKBr experiment was conducted 0.101 at least three times KCl to ensure the stability 0.695 of the test. H3BO3 0.027 Na2SO4 4.09 SrCl2 0.025 NaF 0.003 Table 2. Artificial seawater composition.

ElectrochemicalChemical Compound tests were Concentration, conducted ing/L artiﬁcial Chemical seawater Compound and exposed Concentration, to laboratory g/L air. The workingNaCl electrodes were plastic24.53 pipes by epoxy resinMgCl with2 a surface area of 15.20 cm 2 to contact the solution.NaHCO A three-electrode3 system0.201 with graphite electrode CaCl as2 auxiliary electrode1.16 and saturated calomel as referenceKBr electrode was used0.101 in the electrochemicalKCl experiment. The AC0.695 impedance were measured inH the3BO frequency3 range from0.027 0.1 to 105 Hz. The amplitudeNa2SO4 of the superimposed4.09 ac-signal was 10 mV. EachSrCl experiment2 was conducted0.025 at least three times NaF to ensure the stability 0.003 of the test.

3. Results and Discussion

3.1. Microstructure of As-Cast AlloyAlloy Figure2 2a,ba,b showsshows the TEM and and corresponding corresponding selected selected area area diffraction diffraction patterns patterns (SADP) (SADP) of ofas- as-castcast Cu-7Ni-1Fe-1Mn Cu-7Ni-1Fe-1Mn alloy, alloy, respectively. respectively. Comparing Comparing with with the thestandard standard electron electron diffraction diffraction (SED), (SED), the themicrostructure microstructure of the of alloy the alloy was wasa α copper a α copper matrix matrix with withthe incident the incident axis of axis the of [100] the [100]band, band, and there and therewas no was visible no visible second second phase phase structure. structure.

Figure 2. Microstructures of as-castas-cast Cu-7Ni-1Fe-1Mn alloy:alloy: (a) TEM image; and (b) the corresponding SADP of ((aa).).

Figure 3a,c shows two TEM images and a corresponding SADP of as-cast Cu-7Ni-1Al-1Fe-1Mn alloy, respectively. Compared with cast Cu-7Ni-1Fe-1Mn alloy, a dispersed, small spherical second Materialsphase with2018, 11a ,diameter 1777 of about 2–10 nm began to appear on the copper substrate (Figure 43a,b). of 11 According to the analysis of diffraction patterns (Figure 3c), the precipitates is one of the nickel– analysisaluminum of diffractionintermetallic patterns compounds, (Figure3 namelyc), the precipitates Ni3Al, with is onea body of the centered nickel–aluminum cubic structure. intermetallic Phase compounds,relationship namelyof copper Ni 3matrixAl, with and a body precipitate centered is cubic(1 11) structure. Cu//(111)Ni Phase3Al, relationship (022) Cu//(011)Ni of copper3Al, matrix[011] andCu//[0 precipitate11] Ni3Al. is (111) Cu//(111)Ni3Al, (022) Cu//(011)Ni3Al, [011] Cu//[011] Ni3Al.

Figure 3.3. MicrostructuresMicrostructures of of cast cast Cu-7Ni-lAl-1Fe-1Mn Cu-7Ni-lAl-1Fe-1Mn alloy: alloy: (a)bright-ﬁeld (a) bright-field image; image; (b) dark-ﬁeld (b) dark-field image; andimage; (c) and the corresponding(c) the corresponding SADP of SADP (a). of (a).

Figure4 4a–ca–c exhibitsexhibits the the TEMTEM imagesimages andand correspondingcorresponding selected selected area area diffraction diffraction patterns patterns (SADP) (SADP) of as-cast Cu-7Ni-3Al-1Fe-1Mn alloy. alloy. As As can can be be seen seen in in Figure Figure 4a,b,4a,b, many many second second phases phases having having a adiameter diameter of ofabout about 5–15 5–15 nm nm were were distributed distributed ov overer the copper substrate.substrate. Compared Compared with the precipitated structure of the cast Cu-7Ni-1Al-1Fe-1Mn alloy, thethe morphologymorphology ofof the second phase did not change, and and the the size size increased increased from from 2–10 2–10 to to 5– 5–1515 nm. nm. As As shown shown in Figure in Figure 4c,4 thec, the structure structure of the of theprecipitate precipitate did didnot notchange, change, the orientation the orientation relation relationshipship between between precipitate precipitate and andthe surrounding the surrounding Cu- Cu-matrix is found as follows: (111) Cu//(111)Ni Al, (022) Cu//(011)Ni Al, [011] Cu//[011] Ni Al. matrix is found as follows: (111) Cu//(111)Ni3Al, (022)3 Cu//(011)Ni3Al, [0311] Cu//[011] Ni3Al. 3 Figure 55a–da–d showsshows TEMTEM imagesimages andand SADPSADP ofof as-castas-cast Cu-7Ni-5Al-1Fe-1MnCu-7Ni-5Al-1Fe-1Mn alloy, respectively.respectively. As shown shown in in Figure Figure 5a,b,5a,b, many many sphere-shaped sphere-shaped precip precipitatesitates distribute distribute on the on copper thecopper substrate substrate retained. retained. Moreover, phase relationship between copper substrate and Ni Al is found as follows (see Moreover, phase relationship between copper substrate and Ni3Al is found3 as follows (see Figure 5c): Figure5c): (002) Cu//(100)Ni Al, (020) Cu//(010)Ni Al, (022) Cu//(110)Ni Al, [100] Cu//[001] Ni Al. (002) Cu//(100)Ni3Al, (020) Cu//(010)Ni3 3Al, (022) Cu//(110)Ni3 3Al, [100] Cu//[001]3 Ni3Al. Particularly,3 Particularly,a few irregular a fewlumps irregular in the grain lumps boundary in the grain are also boundary observed are in also Figure observed 5d. Analysis in Figure based5d. Analysison Figure based5e indicates on Figure the 5precipitatese indicates theare precipitatesone of the nick areel–aluminum one of the nickel–aluminum intermetallic compounds, intermetallic namely compounds, NiAl, namelywith a body NiAl, centered with a bodycubic centered structure. cubic Phase structure. relationship Phase between relationship copper between substrate copper and NiAl substrate is found and NiAl is found as follows: (131) Cu//(100)NiAl, (311) Cu//(110)NiAl, [114] Cu//[001] NiAl. as follows: (1 3 1) Cu//(100)NiAl, (311) Cu//(110)NiAl, [11 4 ] Cu//[001] NiAl. Figure6a,b shows a TEM image and a corresponding SADP of as-cast Cu-7Ni-7Al-1Fe-1Mn alloy, respectively. Compared with cast Cu-7Ni-3Al-1Fe-1Mn alloy, there were signiﬁcant changes in the size and morphology of precipitated phase. As observed in Figure6a, the nano-scale precipitates still exist and the elongated precipitates appear at the same time. A precipitate having a width of several hundred nm is randomly distributed on the Cu-matrix. As seen from Figure6b, the precipitates are one of the nickel–aluminum intermetallic compound,s namely NiAl3, with a orthorhombic structure. The crystal orientation relationship between the matrix and precipitate of Ni3Al and NiAl3 is: (022) Cu//(110) Ni3Al, (111) Cu//(031) NiAl3, [011] Cu//[110] Ni3Al//[500] NiAl3. Materials 2018, 11, 1777 5 of 11 Materials 2018, 11, x FOR PEER REVIEW 5 of 11 Materials 2018, 11, x FOR PEER REVIEW 5 of 11

FigureFigure 4. 4.Microstructures Microstructures of of cast cast Cu-7Ni-3Al-1Fe-1MnCu-7Ni-3Al-1Fe-1Mn alloy: alloy: ((a a) )) bright-fieldbright-field bright-ﬁeld image; image; (b ( )b dark-field) dark-ﬁelddark-field image;image; and and (c )( c the) the corresponding corresponding SADPSADP SADP of ofof (((aaa).).).

FigureFigure 5. 5.Microstructures Microstructuresof ofcast cast Cu-7Ni-5Al-1Fe-1MnCu-7Ni-5Al-1Fe-1Mn alloy: alloy: (a (a) )bright-field bright-field image; image; (b ()b dark-field) dark-field image;image; (c )(c the) the corresponding corresponding SADP SADP ofof ((aa);); ((dd)) bright-fieldbright-field image image in in th thee grain grain boundary; boundary; and and (e) ( ethe) the Figure 5. Microstructures of cast Cu-7Ni-5Al-1Fe-1Mn alloy: (a) bright-field image; (b) dark-field correspondingcorresponding SADP SADP of of (d ().d). image; (c) the corresponding SADP of (a); (d) bright-field image in the grain boundary; and (e) the

corresponding SADP of (d).

Materials 2018, 11, x FOR PEER REVIEW 6 of 11 Materials 2018, 11, x FOR PEER REVIEW 6 of 11 Figure 6a,b shows a TEM image and a corresponding SADP of as-cast Cu-7Ni-7Al-1Fe-1Mn alloy, Figure 6a,b shows a TEM image and a corresponding SADP of as-cast Cu-7Ni-7Al-1Fe-1Mn alloy, respectively. Compared with cast Cu-7Ni-3Al-1Fe-1Mn alloy, there were significant changes in the respectively. Compared with cast Cu-7Ni-3Al-1Fe-1Mn alloy, there were significant changes in the size and morphology of precipitated phase. As observed in Figure 6a, the nano-scale precipitates still size and morphology of precipitated phase. As observed in Figure 6a, the nano-scale precipitates still exist and the elongated precipitates appear at the same time. A precipitate having a width of several exist and the elongated precipitates appear at the same time. A precipitate having a width of several hundred nm is randomly distributed on the Cu-matrix. As seen from Figure 6b, the precipitates are hundred nm is randomly distributed on the Cu-matrix. As seen from Figure 6b, the precipitates are one of the nickel–aluminum intermetallic compound,s namely NiAl3, with a orthorhombic structure. one of the nickel–aluminum intermetallic compound,s namely NiAl3, with a orthorhombic structure. The crystal orientation relationship between the matrix and precipitate of Ni3Al and NiAl3 is: (022) The crystal orientation relationship between the matrix and precipitate of Ni3Al and NiAl3 is: (022) Materials 2018, 11, 1777 6 of 11 Cu//(110) Ni3Al, (111) Cu//(031) NiAl3, [011] Cu//[110] Ni3Al//[ 5 00] NiAl3. Cu//(110) Ni3Al, (111) Cu//(031) NiAl3, [011] Cu//[110] Ni3Al//[ 5 00] NiAl3.

Figure 6. Microstructures of cast Cu-7Ni-7Al-1Fe-1Mn alloy: (a) bright-field image; and (b) the Figure 6.6. MicrostructuresMicrostructures of cast Cu-7Ni-7Al-1Fe-1Mn alloy: ( (aa)) bright-field bright-field image; image; and and ( b) the corresponding SADP of (a). corresponding SADP of (a).). As shown in Figures 2–6, the precipitation behavior of Cu-7Ni-xAl alloy with the change of As shown in Figures2 2–6,–6, thethe precipitationprecipitation behaviorbehavior ofof Cu-7Ni-Cu-7Ni-xAl alloy with the change of aluminum content has a series of phase transformation. The results show that, with the increase of aluminum content has a series of phase transformation.transformation. The results show that, with the increase of aluminum content from 0% to 7%, the second-phase transformation of Cu-7Ni-xAl alloy follows the aluminum content from 0% to 7%, the second-phase transformation of Cu-7Ni-xAl alloy follows the sequence of: α solid solution → Ni3Al → Ni3Al + NiAl → Ni3Al + NiAl3. sequence of: α solid solution → Ni3AlAl →→ NiNi33AlAl + + NiAl NiAl →→ NiNi3Al3Al + +NiAl NiAl3. 3. 3.2. Effect of Aluminum Content on Mechanical Properties 3.2. Effect of Aluminum ContentContent on Mechanical Properties Figure 7 shows the tensile strength with different aluminum content. For comparison, aluminum Figure7 7 shows shows thethe tensiletensile strengthstrength withwith differentdifferent aluminumaluminum content.content. ForFor comparison,comparison, aluminumaluminum has been found to be able to improve the mechanical strength of alloy. With the increase of aluminum has been found to be able to improve improve the mechanical mechanical strength of alloy. With the increase of aluminum content, the tensile strength of Cu-Ni-Al alloy firstly increases and then decreases, but all of them content, thethe tensiletensile strength strength of of Cu-Ni-Al Cu-Ni-Al alloy alloy firstly firstly increases increases and and then then decreases, decreases, but allbut of all them of them were were higher than that of the copper alloy without aluminum content. The copper alloy with 3% higherwere higher than that than of that the copperof the alloycopper without alloy aluminumwithout aluminum content. Thecontent. copper The alloy copper with alloy 3% aluminum with 3% aluminum has the highest tensile strength, 395 MPa. It was notable that, with the addition of hasaluminum the highest has tensilethe highest strength, tensile 395 MPa.strength, It was 395 notable MPa. that,It was with notable the addition that, with of aluminum the addition content, of aluminum content, the elongation of Cu-7Ni-xAl-1Fe-1Mn alloys firstly increases and then decreases, thealuminum elongation content, of Cu-7Ni- the elongationxAl-1Fe-1Mn of Cu-7Ni- alloysx firstlyAl-1Fe-1Mn increases alloys and firstly then decreases,increases and but then still moredecreases, than but still more than 40% higher than without aluminum content. The addition of aluminum element 40%but still higher more than than without 40% higher aluminum than content. without The alumin additionum content. of aluminum The addition element of did aluminum not cause element a severe did not cause a severe impairment of tensile ductility. impairmentdid not cause of a tensile severe ductility.impairment of tensile ductility.

420 100 420 100 400 Tensile strength 400 Tensile strength Elongation 90 380 90 380 Elongation

360 / % Elongation

360 80 / % Elongation 340 80 340 320 70 320 70 300 300 60 280 60 280 260 260 50 240 50 Tensile strength /MPa 240 Tensile strength /MPa 220 40 220 40 200 200 01234567 01234567

Al addition/% Al addition/% FigureFigure 7. 7. EffectsEffects of of Al Al content content on on tensile tensile strength strength and and elongtion elongtion of of Cu-7Ni- Cu-7Ni-xxAl-1Fe-1MnAl-1Fe-1Mn alloy. alloy. Figure 7. Effects of Al content on tensile strength and elongtion of Cu-7Ni-xAl-1Fe-1Mn alloy. As has been reported in Ref. [16,17], comparing copper with aluminum, the atomic radius and elastic modulus are quite different. After adding aluminum content, the elastic deformation degree of copper alloy matrix is very high. Aluminum is a very effective solid solution strengthener in copper alloy. The solid solubility of aluminum in Cu-Ni alloy is lower, and nickel–aluminum intermetallic compounds compound (Ni3Al, NiAl or NiAl3) are produced in Cu-Ni-A1 alloy (see Figures3–6), which has obvious precipitation hardening effect. From this, it is speculated that, in this study, solid solution hardening and precipitation hardening of aluminum are the reasons for enhancing the tensile strength. In particular, precipitation hardening is the main strengthening mechanism of Cu-Ni-Al Materials 2018, 11, x FOR PEER REVIEW 7 of 11

As has been reported in Ref. [16,17], comparing copper with aluminum, the atomic radius and elastic modulus are quite different. After adding aluminum content, the elastic deformation degree of copper alloy matrix is very high. Aluminum is a very effective solid solution strengthener in copper alloy. The solid solubility of aluminum in Cu-Ni alloy is lower, and nickel–aluminum intermetallic compounds compound (Ni3Al, NiAl or NiAl3) are produced in Cu-Ni-A1 alloy (see Figures 3–6), which has obvious precipitation hardening effect. From this, it is speculated that, in this study, solid Materialssolution2018 ,hardening11, 1777 and precipitation hardening of aluminum are the reasons for enhancing7 of 11the tensile strength. In particular, precipitation hardening is the main strengthening mechanism of Cu- alloys.Ni-Al When alloys. the When aluminum the aluminum content is 3%, content the maximum is 3%, the tensile maximum strength tensile is the resultstreng ofth precipitation is the result of of precipitation of a large amount of nano-Ni3Al. The experimental results have confirmed that the a large amount of nano-Ni3Al. The experimental results have conﬁrmed that the addition of aluminum signiﬁcantlyaddition of affectsaluminum the mechanical significantly properties affects the of mechanical the Cu-Ni-Al properties alloy. of the Cu-Ni-Al alloy. TheThe plastic plastic fracture fracture ofof metalmetal materials is is the the process process of of forming forming and and growing growing microvoids. microvoids. The Thefracture fracture is the isthe formation formation of microvoids of microvoids in the in pl theace place where where the plastic the plastic deformation deformation is serious. is serious. Under Underthe action the action of tension, of tension, a large a large amount amount of of plasti plasticc deformation deformation makes brittlebrittle inclusionsinclusions break break or or disconnectdisconnect inclusions inclusions with with the the interface interface of of the the matrix matrix to to form form holes holes [18 [18].]. Once Once the the holes holes are are formed, formed, theythey begin begin to to grow grow up up and and gather, gather, and and the the result result of of the the aggregation aggregation is is the the formation formation of of cracks, cracks, which which eventually lead to fracture. Smaller size and densely distributed Ni3Al particles promote dimple eventually lead to fracture. Smaller size and densely distributed Ni3Al particles promote dimple nucleationnucleation and and form form smaller smaller and moreand dimplemore dimple patterns. pa Thetterns. size isThe larger size and is thelarger irregular and distributionthe irregular distribution NiAl3 particles promote crack growth and form larger dimple patterns. The change of NiAl3 particles promote crack growth and form larger dimple patterns. The change of the mechanical propertiesthe mechanical corresponds properties to the corresponds microstructure to the of themicrostructure alloy. of the alloy.

3.3.3.3. Effect Effect of of Aluminum Aluminum Content Content on on Corrosion Corrosion Resistance Resistance InIn Figure Figure8, the8, the corrosion corrosion rate rate of of Cu-7Ni- Cu-7Ni-xAl-1Fe-1MnxAl-1Fe-1Mn alloy, alloy, after after immersing immersing simulated simulated seawater seawater forfor 10 10 days days is is plotted. plotted. The The corrosion corrosion rate rate of of copper coppe alloyr alloy after after addition addition of of aluminum aluminum is is lower lower than than thatthat of of the the copper copper alloy alloy without without aluminum. aluminum. It It can can be be determined determined that that the the addition addition of of aluminum aluminum has has a positivea positive effect effect on on the the corrosion corrosion resistance resistance of of the the alloy. alloy. With With increasing increasing aluminum aluminum content, content, corrosion corrosion raterate ﬁrstly firstly decreases decreases and and then then has a has slight a increase.slight incr Whenease. theWhen aluminum the aluminum content is content 3 wt %, is the 3 corrosionwt %, the ratecorrosion is the lowest, rate is whichthe lowest, is 0.0215 which mm/a. is 0.0215 mm/a.

4.5

4.0 corrosion rate

3.5 mm/a -2

3.0

2.5

2.0 Corrosion rate / 10

1.5 01234567 Al addition/%

Figure 8. Effect of Al content on corrosion rate of Cu-7Ni-xAl-1Fe-1Mn alloy after immersing in Figure 8. Effect of Al content on corrosion rate of Cu-7Ni-xAl-1Fe-1Mn alloy after immersing in artiﬁcial seawater for 10 days. artificial seawater for 10 days. Such behavior has been explained in the literature on copper and copper alloys [19–21]. When Such behavior has been explained in the literature on copper and copper alloys [19–21]. When immersed in chloride-containing solutions, the main anodic reaction of copper and its alloys is the immersed in chloride-containing solutions, the main anodic reaction of copper and its alloys is the dissolution process of copper according to dissolution process of copper according to − − − Cu + 2Cl → CuCl2 + e (2) the cathode reaction is mainly the process of oxygen reduction

− O2 + 2H2O + 4e → 4OH (3) Materials 2018, 11, x FOR PEER REVIEW 8 of 11

+→−−− + Cu 2Cl CuCl2 e (2) the cathode reaction is mainly the process of oxygen reduction Materials 2018, 11, 1777 ++→− 8 of 11 O22 2H O 4e 4OH (3)

AccordingAccording to to H. H. Nady et al. [4], [4], the copper ch chlorideloride on on the alloy surface will will undergo undergo further further hydrolysishydrolysis reaction, according to: −−++→ ++ 2CuCl22− H O Cu 2 O 4Cl− 2H + (4) 2CuCl2 + H2O → Cu2O + 4Cl + 2H (4) leading to the formation of cuprous oxide. leadingIn the to the quinary, formation aluminum of cuprous containing oxide. alloys can form the presence of Al-oxide, which can significantlyIn the quinary,reduce the aluminum chloride-induced containing corro alloyssion compared can form thewith presence Cu metal, of according Al-oxide, to: which can significantly reduce the chloride-induced corrosion compared with Cu metal, according to: −−− Al+→ 4Cl AlCl + 3e (5) − 4 − − Al + 4Cl → AlCl4 + 3e (5) −+−+→++ AlCl42 2H− O Al 23 O 3H+ 4Cl − (6) AlCl4 + 2H2O → Al2O3 + 3H + 4Cl (6) FigureFigure 99 isis thethe impedanceimpedance response of the differentdifferent alloysalloys in the artificialartificial seawater after 10 days. The impedance spectrum shown shown in in Figure 99 exhibitsexhibits aa similarsimilar characteristic,characteristic, whichwhich isis aa singlesingle semicircularsemicircular type. type. The The overall overall trend trend of of the the diamet diameterer change change of of the the Nyquist Nyquist diagram diagram is is the gradual increaseincrease of of the the impedance impedance Z, Z, which which means means an an increase increase in in the the transfer transfer resistance resistance of of the the oxide oxide layer layer on on thethe sample sample surface. surface. The The resistance resistance of of oxide oxide film film fi firstrst increased increased and and then then decreased decreased as as the the Al Al content increasesincreases which which is in good agreement with the corrosion rate data.

70 Cu-7Ni-xAl-1Fe-1Mn 60

50 2

/kΩ cm x=0 i 20

Z x=1 10 x=3 x=5 0 x=7

0 102030405060 Z /kΩ cm2 r FigureFigure 9.9. NyquistNyquist impedance impedance plots plots of theof differentthe different Cu-7Ni- Cu-7Ni-xAl-1Fe-1MnxAl-1Fe-1Mn alloy afteralloy immersing after immersing artiﬁcial artificialseawater seawater for 10 days. for 10 days.

TheThe excellent corrosion corrosion resistance resistance of of copper copper alloy alloy is is attributes to to the the protective film film formed on thethe surface. surface. The The addition addition of of aluminum aluminum can can form form a a compact compact oxide oxide film film [22] [22] (mainly (mainly Cu, Cu, Ni Ni and and Al Al oxideoxide film), film), which which improves improves the the quality quality of of passive passive fi filmlm and and thus thus shields shields the the alloy alloy from from corrosive corrosive ions. ions. However,However, there is a certain potential difference difference be betweentween the the precipitated precipitated phase phase and and the the substrate substrate [9], [9], which is not conducive to the improvementimprovement ofof corrosioncorrosion resistance.resistance. Continuing Continuing to to enhance the aluminumaluminum content in in the the alloy, alloy, the content content of of micr micron-scaleon-scale metal metal compound compound distributes distributes on the the copper copper matrix,matrix, accelerating accelerating the alloy alloy surface surface passivation passivation film film damage, damage, which which is is unfavorable unfavorable to to the the reduction reduction of the corrosion rate of the alloy. These results are in agreement with the electrochemical results in Figure9.

3.4. Mechanical Properties and Corrosion Resistance of Different Cupronickel The engineering stress–strain curves of the Cu-7Ni-3Al-1Fe-1Mn alloy and commercial alloy UNS C70600 are shown in Figure 10. To compare these alloys with each other directly, all data were Materials 2018, 11, x FOR PEER REVIEW 9 of 11 of the corrosion rate of the alloy. These results are in agreement with the electrochemical results in Figure 9.

3.4. Mechanical Properties and Corrosion Resistance of Different Cupronickel The engineering stress–strain curves of the Cu-7Ni-3Al-1Fe-1Mn alloy and commercial alloy Materials 2018, 11, 1777 9 of 11 UNS C70600 are shown in Figure 10. To compare these alloys with each other directly, all data were obtained using the same method of the GB/T 2059-2008. Comparing the several Cupronickel alloys showsobtained that the using tensile the samestrength method becomes of the higher GB/T 2059-2008.with increasing Comparing nickel the content, several Cupronickelwith the designations alloys of alloyshows B30-1-1 that the eutectic tensile strengthcomposition becomes Cu-30Ni-1Fe-1Mn higher with increasing showing nickel the content, best tensile with the strength. designations As shown of alloy B30-1-1 eutectic composition Cu-30Ni-1Fe-1Mn showing the best tensile strength. As shown in in Figure 10, the tensile strength of Cu-7Ni-3Al-1Fe-1Mn is almost 61% higher than that of the Figure 10, the tensile strength of Cu-7Ni-3Al-1Fe-1Mn is almost 61% higher than that of the commercial commercial alloy UNS C70600 (Cu-10Ni-1Fe-1Mn). The addition of 3% aluminum has shown not alloy UNS C70600 (Cu-10Ni-1Fe-1Mn). The addition of 3% aluminum has shown not only an improved only tensilean improved strength tensile but also strength a much but ﬁner also corrosion a much resistance, finer corrosion with the resistance, corrosion rate with decreasing the corrosion from rate decreasing0.0359 mm/a from to0.0359 0.0215 mm/a mm/a. to Therefore, 0.0215 mm/a. excellent Therefore, corrosion resistance excellent and corrosion good mechanical resistance properties and good mechanicalare expected properties for Cu-7Ni-3Al-1Fe-1Mn. are expected for Cu-7Ni-3Al-1Fe-1Mn.

450

400 A: Cu-7Ni-3Al-1Fe-1Mn

350 B: C70600 A

300

250

200 B 150

100

Engineering Stress (MPa) 50

0 1020304050 Engineering Strain (%)

Figure 10. Engineering stress–strain curves at room temperature of the commercial alloy UNS C70600 Figureand 10. Cu-7Ni-3Al-1Fe-1Mn Engineering stress–strain alloy. curves at room temperature of the commercial alloy UNS C70600 and Cu-7Ni-3Al-1Fe-1Mn alloy. 4. Conclusions 4. ConclusionsIn this study, the effect of aluminum addition on mechanical properties and corrosion resistance in artificial seawater of Cu-7Ni-xAl-1Fe-1Mn alloy was investigated. Based on results obtained, In this study, the effect of aluminum addition on mechanical properties and corrosion resistance the following conclusions can be drawn: in artificial seawater of Cu-7Ni-xAl-1Fe-1Mn alloy was investigated. Based on results obtained, the following(1) The conclusions precipitation can behavior be drawn: of Cu-7Ni- xAl-1Fe-1Mn alloy with the change of aluminum content has a series of phase transformations. It is confirmed that the second-phase transformation of (1) The precipitation behavior of Cu-7Ni-xAl-1Fe-1Mn alloy with the change of aluminum content Cu-7Ni-xAl-1Fe-1Mn alloy follows the sequence of: α solid solution → Ni3Al → Ni3Al + NiAl → has aNi series3Al + NiAlof phase3. transformations. It is confirmed that the second-phase transformation of (2)Cu-7Ni-Thex tensileAl-1Fe-1Mn strength alloy of Cu-7Ni- followsx Al-1Fe-1Mnthe sequence alloy of: α first solid increased solution and → thenNi3Al decreased → Ni3Al but + NiAl all → Ni3Alof + them NiAl were3. higher than that of the copper alloy without aluminum content. The Cu-7Ni- (2) The tensilexAl-1Fe-1Mn strength alloy of with Cu-7Ni- 3wt %x aluminumAl-1Fe-1Mn has thealloy highest first tensileincreased strength and with then 395 decreased MPa. Moreover, but all of themthe were addition higher of than aluminum that of element the copper did not alloy cause without a severe aluminum impairment content. of tensile The ductility. Cu-7Ni-xAl-1Fe- (3)1MnDue alloy to with the formation 3wt % aluminum of the stable has Al the2O highest3 in the barriertensile film,strength the aluminum with 395 MPa. can improve Moreover, the the additionstability of aluminum of Cu-7Ni-x elementAl-1Fe-1Mn did alloy. not cause The corrosion a severe resistance impairment of the of Cu-7Ni- tensilexAl-1Fe-1Mn ductility. alloy in artificial seawater first decreased and then increased with the increase of aluminum content, (3) Due to the formation of the stable Al2O3 in the barrier film, the aluminum can improve the but all of them were lower than that of the alloy without aluminum content. stability of Cu-7Ni-xAl-1Fe-1Mn alloy. The corrosion resistance of the Cu-7Ni-xAl-1Fe-1Mn (4) Among the Cu-7Ni-xAl-1Fe-1Mn alloys with aluminum content from 0% to 7 wt %, the alloy in artificial seawater first decreased and then increased with the increase of aluminum alloy Cu-7Ni-3Al-1Fe-1Mn has the best mechanical properties and corrosion resistance in content, but all of them were lower than that of the alloy without aluminum content. artificial seawater. Compared with commercial alloy UNS C70600, the tensile strength of (4) AmongCu-7Ni-3Al-1Fe-1Mn the Cu-7Ni-xAl-1Fe-1Mn alloy increased alloys from with 245 MPaaluminum to 395 MPa,content and from the corrosion 0% to 7 rate wt decreased %, the alloy Cu-7Ni-3Al-1Fe-1Mnfrom 0.0359 mm/a tohas 0.0215 the best mm/a. mechanical properties and corrosion resistance in artificial seawater. Compared with commercial alloy UNS C70600, the tensile strength of Cu-7Ni-3Al-

Materials 2018, 11, 1777 10 of 11

Author Contributions: R.Y. performed the data analysis and the conception of the study; J.W. and K.S. helped the methodology and supervision; Y.Z. helped perform the analysis with constructive discussions; Z.S. drafted the manuscript. Funding: This work was supported by the National Key R&D Program of China under Grant No. 2016YFB0301400. Acknowledgments: This work was supported by the National Key R&D Program of China under Grant No. 2016YFB0301400. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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