In the manufacturing industry of electrical contacts, the prohibition of the use of toxic metals (Cd, Be), and desire to avoid the alloying with precious metals (Ag) has created a need for new alloys with good electrical conductivity and high mechanical properties at elevated temperatures. As a potentially useful material for this purpose, we have analyzed the continuously cast Cu-Fe-Ni-P alloy in various heat treatment conditions. Sequence of phase transformations during heat treatment was followed by 4 point D.C. electrical resistivity measuring method, and analyzed by scanning (SEM) and transmission (TEM) electron microscopy. Measurements of Vickers hardness and electrical conductivity after various heat treatment procedures indicate on high potential of Cu-Fe-Ni-P alloys as a material for electrical contacts

B. Karpe

B. Kosec

G. Brovč

G. Dražič

G. Lojen

I. Djordjević

M. Bizjak

English

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Synthesis and characterization of hardened Cu-Fe-Ni-P alloy

Brovc, G.

Drazić, G.

Karpe, B.

Đorđević, Igor

Lojen, Gorazd

Kosec, Borut

Bizjak, M.

School of dental Medicine dIgitaL archivE

51METALURGIJA 54 (2015) 1, 51-54G. BROVČ, G. DRAŽIČ, B. KARPE, I. DJORDJEVIĆ, G. LOJEN, B. KOSEC, M. BIZJAK SYNTHESIS AND CHARACTERIZATION OF HARDENED Cu-Fe-Ni-P ALLOYReceived – Prispjelo: 2014-02-21Accepted – Prihvaćeno: 2014-07-15Original Scientific Paper – Izvorni znanstveni radISSN 0543-5846METABK 54(1) 51-54 (2014)UDC – UDK 669.3:621.3.035:620.178.1=111G. Brovč, G. Dražič, B. Karpe, B. Kosec, M. Bizjak, University of  Ljubljana, Faculty of Natural Sciences and Engineering, Ljubljana, Slovenia; I. Djordjević, University of Belgrade, School of Dental Medi-cine, Belgrade, Serbia; G. Lojen, University of Maribor, Faculty of Me-chanical Engineering, Maribor, SloveniaIn the manufacturing industry of electrical contacts, the prohibition of the use of toxic metals (Cd, Be), and desire to avoid the alloying with precious metals (Ag) has created a need for new alloys with good electrical conductivity and high mechanical properties at elevated temperatures. As a potentially useful material for this purpose, we have ana-lyzed the continuously cast Cu-Fe-Ni-P alloy in various heat treatment conditions. Sequence of phase transforma-tions during heat treatment was followed by 4 point D.C. electrical resistivity measuring method, and analyzed by scanning (SEM) and transmission (TEM) electron microscopy. Measurements of Vickers hardness and electrical con-ductivity after various heat treatment procedures indicate on high potential of Cu-Fe-Ni-P alloys as a material for electrical contacts.Key words: copper alloys, precipitation hardening, electrical contacts, electrical conductivityINTRODUCTIONThe increasing mechanical and electrical property requirements and inexpensiveness of contact materials remain the main challenge for the producers of copper and copper alloys for the automotive and electro indus-try. The electrical conductivity of pure copper (100 % I.A.C.S) is in the majority of practical cases useless, if the mechanical and metallurgical properties don’t meet required demands. Especially resistance to erosion by arcing and undesired welding of contacts, recrystalli-zation temperature, mechanical and chemical stability at elevated temperatures are sometimes more important than the electrical conductivity by itself. Selecting the proper electrical contact material is always a compro-mise between electrical and other properties required by a certain application.In the present industrial practice a number of copper alloys are used as contact materials. Their selection de-pends mostly by the type of electrical contact and the magnitude of current and voltage they have to conduct or break. Most of copper based alloys for high current/medium or low voltage make-break contacts contain precious metals (Ag alloyed or Ag plated, Cu-Te) or toxic metals (Cd, Be) [1, 2]. Desire for replacement is therefore obvious.Hardening of pure copper can be achieved by cold working, solute hardening, and precipitation or disper-sion hardening. Pure copper is, in general, unsuitable for contact tips due to its tendency to welding at high currents and medium or low voltages [3]. Because sol-ute hardening has a detrimental influence on electrical conductivity, there is no other alternative as precipita-tion or dispersion hardening. For both, not only chemi-cal composition, but also proper heat treatment has to be applied [4]. As a potentially useful and economically attractive material for electrical contacts, Cu-Fe-Ni-P alloy showed some promising properties [5,6].EXPERIMENTALThe CuFe0,3Ni0,5P0,2 alloy with 0,3 mass. % Fe, 0,5 mass. % Ni and 0,2 mass.% P was made from indus-trially pure oxygen free copper, iron, nickel, and master alloy (Cu +8,8 mass. % P). Synthesis started with in-duction melting (4 kHz, 60 kW - Leybold furnace) fol-lowed by continuous casting by vertical continuous casting apparatus (Technical Gauss) with the graphite mould. Melting and continuous casting of rods was car-ried out in a vacuum (0,5 mbar) protective atmosphere. The casting parameters are given in Table 1.By continuous casting we achieved better longitudi-nal homogeneity of castings, but due to the solidifica-tion mechanism in the continuous casting process, non-homogenity through the perpendicular cross-section of Table 1 Casting parametersCasting procedure P1 P2 P3Alloy /mass. % Cu-Ni 0,5-Fe 0,3-P 0,2Melt temperature /⁰C 1 180Rod diameter /mm 16Pulling cycle /mm 3 5 10Pause I /s 0,2 0,2 0,2Average casting velocity /mm/min 290 380 48052  METALURGIJA 54 (2015) 1, 51-54G. BROVČ et al.: SYNTHESIS AND CHARACTERIZATION OF HARDENED Cu-Fe-Ni-P ALLOYthe rod still remain. To eliminate this non-homogeneity, the rods were solution annealed in a protective argon atmosphere at the temperature 930 °C for one hour and quenched in water.The sequence of the microstructural changes in an-nealed and quenched samples during heating was fol-lowed by the in-situ measurements of the electrical re-sistance. For measurements of the electrical resistance during heating, the four-point D.C. method with the changeable polarity of the measuring current was ap-plied. The disturbing effect of electromagnetic fields from the surrounding was successfully eliminated by the use of special measuring cables and electromagnetic shields [7,8]. All measurements were made at a constant heating rate of 5 K/min in the tube furnace with protec-tive argon atmosphere. The temperature of the furnace was simultaneously measured by an S-type thermocou-ple [9]. Schematic diagram of the measuring system is shown in Figure 1.After determination of the entire electrical resist-ance temperature dependency up to 800 °C, individual samples, taken from the same continuous cast rod, were heated in the argon protective atmosphere to the distinc-tive temperatures of electrical resistance deviations, and slowly cooled in the furnace. Microstructure transfor-mations were analyzed by scanning (SEM) and trans-mission electron microscopy (TEM) [10]. Thin foils for TEM observation were prepared by grinding the sawed foils (200 m) to approximately 40 m, cutting out the 3 mm diameter disks and ion thinned with Gatan PIPS M,691 apparatus.RESULTS AND DISCUSSIONMicrostructure of a continuous cast rod in as cast state consists of columnar zone, with decreasing grain size and more directional orientation (parallel to the rod axis) in the center of the rod (Figures 2 and 3). Morphol-ogy of solidification is dendritic with alloying elements microsegregation in interdendritic spaces (Figure 4).By solution annealing at 930 °C for 1 hour and quenching in water, a homogeneous supersaturated sol-id solution αCu was obtained. The crystal grains didn’t grow considerably due to relatively short annealing time (Figure 5).The results of the electrical resistance measurements during heating with constant heating rate of 5 K/min are RE-Recipient, IS-Isolation, CR-Ceramic retort, WC-Water cooling, H- -Heater, SA-Sample, SS-Sample support, P1, P2, P3, P4-Spring loaded measuring contacts, EF-Electric (cable) feed-trough, TC-Thermocouple, RP-Rotary pump, TP-Turbomolecular pump, HV-High vacuum valve, AV-Argon dosing valve, PI 1-Pirani vacuum gauge, PI 2-Pirani vacuum gauge, PE-Penning vacuum gauge, MV-Membrane vacuum gaugeFigure 1  A scheme of a measuring system for in-situ electrical resistance measurementFigure 4  Continuous cast CuFe0.3Ni0.5P0.2 rod dendritic microstructure (as cast state, eched by fericloride)Figure 2  Microstructure of a continuous cast rod in as cast condition. Perpendicular cross-section, P1 casting parametersFigure 3  Microstructure of a continuous cast rod in as cast condition. Longitudinal cross-section, P1 casting parameters53METALURGIJA 54 (2015) 1, 51-54G. BROVČ et al.: SYNTHESIS AND CHARACTERIZATION OF HARDENED Cu-Fe-Ni-P ALLOYpresented in Figure 6. At lower temperatures, up to ap-proximately 200 °C, CuFe0,3Ni0,5P0,2 alloy exhibits practically linear increase of electrical resistance with increasing temperature.The main reason for this are increased thermal atom vibrations, which scatters electrons and consequently increases electrical resistivity of the alloy. At higher temperatures, the electrical resistivity temperature depen-dency becomes nonlinear, with several major deviations at distinctive temperature intervals. All measured curves exhibit two temperature intervals, where electrical resis-tance decreases with increasing temperature. First one, between 200 and 350 °C with the maximum rate of the electrical resistivity decrease at derivation minima Tm1 at 234 °C and the second between 440 and 550 °C with the Tm2 at 496 °C. There are also two temperature inter-vals with minor nonlinear changes in electrical resist-ance with the minima at 584 and 675 °C. The minimal represent the temperature of the fastest response of electrical resistance on the microstructural changes in a certain temperature interval. This can be seen more pre-cisely in the derivative curve dRdT (see Figure 6). The exact values of the temperature intervals are heating rate dependant and shift towards higher temperatures with increasing heating rate, but the shape of the electri-cal resistance curve remains the same. From the meas-urements of electrical resistance (Table 2) we can see that electrical resistance of the samples decreases with increasing temperature during heating up to the third temperature interval (620 °C). Decrease of electrical re-sistance is steeper at lower temperatures. In samples heated above the third temperature interval (750 °C) of the electrical resistance deviation, electrical resistance of the samples starts to increase again.During heating over the first temperature interval of electrical resistance deviation (350 °C), supersaturated αCu solid solution gradually decomposed by precipitation of fine 10 to 20 nm large particles. TEM electron micros-copy revealed, that some of the particles are aggregates, composed of two different crystal phases (Figure 7).It has been reported that iron and nickel form phos-phides [8,9] such as: Fe2P, Ni2P [11], Fe3P [12], Ni5P2,Ni12P5 [13] and ternary compound phosphides with manganese FeMnP [14] and NiMnP [15].Major contribution to the electrical resistivity de-crease during heating over the second temperature in-Figure 6  Electrical resistance temperature dependency curve and its temperature derivative curve for CuFe0,3Ni0,5P0,2 alloyFigure 5  The CuFe0,3Ni0,5P0,2 alloy microstructure after annealing and quenchingFigure 7  TEM-micrograph of the twin type particle, precipitated from the αCu supersaturated solid solution after heating up to 350 °CTable 2  The electrical resistance of CuFe0,3Ni0,5P0,2 alloy samples after heating to distinctive temperaturesAnnealing temperature /°CElectrical resistance before heat treatment /mΩElectrical resistance after heat treatment /mΩΔR/%ΔR i- ΔR i-1 /%350 55,024 42,791 22,23 22,23550 44,156 27,315 38,14 15,91620 40,986 20,350 50,34 12,20750 41,932 23,369 44,27 -6,07Figure 8  The microstructure of the CuFe0,3Ni0,5P0,2 alloy after heating up to 550 °C (TEM)54  METALURGIJA 54 (2015) 1, 51-54G. BROVČ et al.: SYNTHESIS AND CHARACTERIZATION OF HARDENED Cu-Fe-Ni-P ALLOYterval of electrical resistance deviation (550 °C) has a depletion of αCu solid solution by precipitation of iron and nickel phosphides.Aggregate type precipitates were still present in the same form and approximately of the same size (Figure 8), but the density of the particle distribution increased considerably.In microstructure of the samples heated above the third distinctive temperature interval, up to 620 °C, the twin type particles were no longer detected. The process of continuous particle growth and coalescence took place during further heating up to 750 °C (Figure 9).The connection between small electrical resistance increase (Table 2) in samples heated above Tm4 could be explained by the EDS spot micro-analysis [16], which re-vealed transformation of phosphide particles with high-er Ni/P ratio to a lower Ni/P ratio, which is in a good agreement with the authors [17, 18], who found Fe2P and Ni2P as stable phases after long periods of aging at 450 °C. It’s well known that any kind of crystal lattice distortion, increases electrical resistivity of the metal. Solute atoms are particularly effective scattering cen-ters for electrons and have the greatest effect on the electrical resistivity increase. The greatest influence on the electrical resistivity increase has P, followed by Si, Fe, Mn, Al, Ni, Sn, Zn.Based on the electrical resistance measurements and metallographic researches, we set the temperature of precipitation annealing at 495 °C. Table 3 shows meas-ured values of specimen electrical conductivities and hardnesses of the CuFe0,3Ni0,5P0,2 alloy after anneal-ing at 495 °C for various holding periods.Maximum electrical conductivity (73,71 % I.A.C.S. at 20 °C) as well as hardness (150 HV) was achieved after 3 hours of annealing at 495 °C. At longer periods of annealing, electrical conductivity starts to decrease.CONCLUSIONSFrom the microstructure of samples heated to dis-tinctive temperatures of electrical resistance deviations can be seen, that decomposition of supersaturated Cu solu-tion already started at Tm1 by precipitation of fine 10 to 20 nm large metastable particles.During heating over the second distinctive tempera-ture Tm2 complete precipitation of iron and nickel stable and metastable phosphides with higher Fe/P or Ni/P ra-tio had occurred. At even higher temperatures, redisso-lution of nickel back into Cu solution takes place, by transformation of metastable to stable Ni2P phosphides with lower Ni/P ratio.REFERENCES[1]  P. G. Slade: Electrical Contacts, Principles and Applica-tions, Marcel Dekker, New York, 1999.[2]  M. Braunovic, N.K.Myshkin, V. Konchits: Electrical Con-tacts, Taylor & Francis, London, 2006.[3]  D. Klobčar, J. Tušek, M. Bizjak, V. Lešer: Metallurgy, 53 (2014) 4, 509 – 512.[4]  J.R. Davis, Copper and Copper Alloys, ASM International, Materials park, Ohio, 2001.[5]  Y. Yanamoto, G. Sasaki., K. Yamakawa, M. Ota: Hitachi Cable Review No. 19, Tokio, 2000.[6]  Y. Yamamoto, H. Takano, M. Ohta., C. Tong., A. Kunishi-ge: Hitachi Cable Review No. 26, Tokio, 2007.[7]  M. Bizjak, B. Kosec, IRT 3000, 6 (2011) 36, 32-33.[8]  P. Pečin, M. Bizjak, M. Pribošek, M. Godec, S. Ribarič, J. Rozman, Materials and Technology, 47 (2013) 6, 837-843.[9]  B. Zorc, L. Kosec: Revista de Metallurgia, 36 (2000) 2, 100 – 107.[10]  B. Zorc, L. Kosec: Metall, 58 (2004) 5, 290 – 294.[11]  D. Klobčar, L. Kosec, B. Kosec, J. Tušek: Engineering Fai-lure Analysis, 20 (2012) 1, 43-53.[12]  H. Cao, J.Y. Min, S.D. Wu, A.P. Xian, J.K.Shang: Material Science and Engineering , A 431 (2006), 86-91.[13]  ASM Handbook Volume 3: Alloy Phase Diagrams, Mate-rials Park, Ohio, 1992.[14]  J.H.Choi: Material Science and Engineering, A 550 (2012), 183-190.[15]  J.H. Choi, D.N. Lee: Materials Science and Engineering A, 458 (2007), 295-302.[16]  G. Kosec, A. Nagode, I. Budak, A. Antic, B. Kosec: Engi-neering Failure Analysis, 18 (2011) 1, 450-454.[17]  J.S. Byun J.S, J.H Choi, D.N. Lee: Scripta Materialia, 45 (2000), 637-643.[18]  P.D. Lu, J. Wang, W.J. Zeng, Y. Liu, L. Lu, B.D. Sun: Ma-terials Science and Engineering A, 421 (2006), 254-259.Note:  The responsible translator for English language is Urška Letonja Grgeta, MOAR, Podgora, Slovenia.Figure 9  Precipitates after heating over Tm4 of electrical deviations: SEM (below) and TEM (above)Table 3  Electrical conductivity and hardness after annealing at 495 °C for various holding periodsAnnealing time  /MSm-1Δ=a-q /MSm-1 I.A.C.S. /%Hardness HVQuenched 19,14 / 33,00 851 h 38,27 19,13 65,98 1422 h 41,58 22,54 71,69 1453 h 42,75 23,61 73,71 1504 h 42,18 22,78 72,72 149

Synthesis and characterization of hardened cu-fe-ni-p alloy

Hrčak - Portal of scientific journals of Croatia

51METALURGIJA 54 (2015) 1, 51-54G. BROVČ, G. DRAŽIČ, B. KARPE, I. DJORDJEVIĆ, G. LOJEN, B. KOSEC, M. BIZJAK SYNTHESIS AND CHARACTERIZATION OF HARDENED Cu-Fe-Ni-P ALLOYReceived – Prispjelo: 2014-02-21Accepted – Prihvaćeno: 2014-07-15Original Scientific Paper – Izvorni znanstveni radISSN 0543-5846METABK 54(1) 51-54 (2014)UDC – UDK 669.3:621.3.035:620.178.1=111G. Brovč, G. Dražič, B. Karpe, B. Kosec, M. Bizjak, University of  Ljubljana, Faculty of Natural Sciences and Engineering, Ljubljana, Slovenia; I. Djordjević, University of Belgrade, School of Dental Medi-cine, Belgrade, Serbia; G. Lojen, University of Maribor, Faculty of Me-chanical Engineering, Maribor, SloveniaIn the manufacturing industry of electrical contacts, the prohibition of the use of toxic metals (Cd, Be), and desire to avoid the alloying with precious metals (Ag) has created a need for new alloys with good electrical conductivity and high mechanical properties at elevated temperatures. As a potentially useful material for this purpose, we have ana-lyzed the continuously cast Cu-Fe-Ni-P alloy in various heat treatment conditions. Sequence of phase transforma-tions during heat treatment was followed by 4 point D.C. electrical resistivity measuring method, and analyzed by scanning (SEM) and transmission (TEM) electron microscopy. Measurements of Vickers hardness and electrical con-ductivity after various heat treatment procedures indicate on high potential of Cu-Fe-Ni-P alloys as a material for electrical contacts.Key words: copper alloys, precipitation hardening, electrical contacts, electrical conductivityINTRODUCTIONThe increasing mechanical and electrical property requirements and inexpensiveness of contact materials remain the main challenge for the producers of copper and copper alloys for the automotive and electro indus-try. The electrical conductivity of pure copper (100 % I.A.C.S) is in the majority of practical cases useless, if the mechanical and metallurgical properties don’t meet required demands. Especially resistance to erosion by arcing and undesired welding of contacts, recrystalli-zation temperature, mechanical and chemical stability at elevated temperatures are sometimes more important than the electrical conductivity by itself. Selecting the proper electrical contact material is always a compro-mise between electrical and other properties required by a certain application.In the present industrial practice a number of copper alloys are used as contact materials. Their selection de-pends mostly by the type of electrical contact and the magnitude of current and voltage they have to conduct or break. Most of copper based alloys for high current/medium or low voltage make-break contacts contain precious metals (Ag alloyed or Ag plated, Cu-Te) or toxic metals (Cd, Be) [1, 2]. Desire for replacement is therefore obvious.Hardening of pure copper can be achieved by cold working, solute hardening, and precipitation or disper-sion hardening. Pure copper is, in general, unsuitable for contact tips due to its tendency to welding at high currents and medium or low voltages [3]. Because sol-ute hardening has a detrimental influence on electrical conductivity, there is no other alternative as precipita-tion or dispersion hardening. For both, not only chemi-cal composition, but also proper heat treatment has to be applied [4]. As a potentially useful and economically attractive material for electrical contacts, Cu-Fe-Ni-P alloy showed some promising properties [5,6].EXPERIMENTALThe CuFe0,3Ni0,5P0,2 alloy with 0,3 mass. % Fe, 0,5 mass. % Ni and 0,2 mass.% P was made from indus-trially pure oxygen free copper, iron, nickel, and master alloy (Cu +8,8 mass. % P). Synthesis started with in-duction melting (4 kHz, 60 kW - Leybold furnace) fol-lowed by continuous casting by vertical continuous casting apparatus (Technical Gauss) with the graphite mould. Melting and continuous casting of rods was car-ried out in a vacuum (0,5 mbar) protective atmosphere. The casting parameters are given in Table 1.By continuous casting we achieved better longitudi-nal homogeneity of castings, but due to the solidifica-tion mechanism in the continuous casting process, non-homogenity through the perpendicular cross-section of Table 1 Casting parametersCasting procedure P1 P2 P3Alloy /mass. % Cu-Ni 0,5-Fe 0,3-P 0,2Melt temperature /⁰C 1 180Rod diameter /mm 16Pulling cycle /mm 3 5 10Pause I /s 0,2 0,2 0,2Average casting velocity /mm/min 290 380 48052  METALURGIJA 54 (2015) 1, 51-54G. BROVČ et al.: SYNTHESIS AND CHARACTERIZATION OF HARDENED Cu-Fe-Ni-P ALLOYthe rod still remain. To eliminate this non-homogeneity, the rods were solution annealed in a protective argon atmosphere at the temperature 930 °C for one hour and quenched in water.The sequence of the microstructural changes in an-nealed and quenched samples during heating was fol-lowed by the in-situ measurements of the electrical re-sistance. For measurements of the electrical resistance during heating, the four-point D.C. method with the changeable polarity of the measuring current was ap-plied. The disturbing effect of electromagnetic fields from the surrounding was successfully eliminated by the use of special measuring cables and electromagnetic shields [7,8]. All measurements were made at a constant heating rate of 5 K/min in the tube furnace with protec-tive argon atmosphere. The temperature of the furnace was simultaneously measured by an S-type thermocou-ple [9]. Schematic diagram of the measuring system is shown in Figure 1.After determination of the entire electrical resist-ance temperature dependency up to 800 °C, individual samples, taken from the same continuous cast rod, were heated in the argon protective atmosphere to the distinc-tive temperatures of electrical resistance deviations, and slowly cooled in the furnace. Microstructure transfor-mations were analyzed by scanning (SEM) and trans-mission electron microscopy (TEM) [10]. Thin foils for TEM observation were prepared by grinding the sawed foils (200 m) to approximately 40 m, cutting out the 3 mm diameter disks and ion thinned with Gatan PIPS M,691 apparatus.RESULTS AND DISCUSSIONMicrostructure of a continuous cast rod in as cast state consists of columnar zone, with decreasing grain size and more directional orientation (parallel to the rod axis) in the center of the rod (Figures 2 and 3). Morphol-ogy of solidification is dendritic with alloying elements microsegregation in interdendritic spaces (Figure 4).By solution annealing at 930 °C for 1 hour and quenching in water, a homogeneous supersaturated sol-id solution αCu was obtained. The crystal grains didn’t grow considerably due to relatively short annealing time (Figure 5).The results of the electrical resistance measurements during heating with constant heating rate of 5 K/min are RE-Recipient, IS-Isolation, CR-Ceramic retort, WC-Water cooling, H- -Heater, SA-Sample, SS-Sample support, P1, P2, P3, P4-Spring loaded measuring contacts, EF-Electric (cable) feed-trough, TC-Thermocouple, RP-Rotary pump, TP-Turbomolecular pump, HV-High vacuum valve, AV-Argon dosing valve, PI 1-Pirani vacuum gauge, PI 2-Pirani vacuum gauge, PE-Penning vacuum gauge, MV-Membrane vacuum gaugeFigure 1  A scheme of a measuring system for in-situ electrical resistance measurementFigure 4  Continuous cast CuFe0.3Ni0.5P0.2 rod dendritic microstructure (as cast state, eched by fericloride)Figure 2  Microstructure of a continuous cast rod in as cast condition. Perpendicular cross-section, P1 casting parametersFigure 3  Microstructure of a continuous cast rod in as cast condition. Longitudinal cross-section, P1 casting parameters53METALURGIJA 54 (2015) 1, 51-54G. BROVČ et al.: SYNTHESIS AND CHARACTERIZATION OF HARDENED Cu-Fe-Ni-P ALLOYpresented in Figure 6. At lower temperatures, up to ap-proximately 200 °C, CuFe0,3Ni0,5P0,2 alloy exhibits practically linear increase of electrical resistance with increasing temperature.The main reason for this are increased thermal atom vibrations, which scatters electrons and consequently increases electrical resistivity of the alloy. At higher temperatures, the electrical resistivity temperature depen-dency becomes nonlinear, with several major deviations at distinctive temperature intervals. All measured curves exhibit two temperature intervals, where electrical resis-tance decreases with increasing temperature. First one, between 200 and 350 °C with the maximum rate of the electrical resistivity decrease at derivation minima Tm1 at 234 °C and the second between 440 and 550 °C with the Tm2 at 496 °C. There are also two temperature inter-vals with minor nonlinear changes in electrical resist-ance with the minima at 584 and 675 °C. The minimal represent the temperature of the fastest response of electrical resistance on the microstructural changes in a certain temperature interval. This can be seen more pre-cisely in the derivative curve dRdT (see Figure 6). The exact values of the temperature intervals are heating rate dependant and shift towards higher temperatures with increasing heating rate, but the shape of the electri-cal resistance curve remains the same. From the meas-urements of electrical resistance (Table 2) we can see that electrical resistance of the samples decreases with increasing temperature during heating up to the third temperature interval (620 °C). Decrease of electrical re-sistance is steeper at lower temperatures. In samples heated above the third temperature interval (750 °C) of the electrical resistance deviation, electrical resistance of the samples starts to increase again.During heating over the first temperature interval of electrical resistance deviation (350 °C), supersaturated αCu solid solution gradually decomposed by precipitation of fine 10 to 20 nm large particles. TEM electron micros-copy revealed, that some of the particles are aggregates, composed of two different crystal phases (Figure 7).It has been reported that iron and nickel form phos-phides [8,9] such as: Fe2P, Ni2P [11], Fe3P [12], Ni5P2,Ni12P5 [13] and ternary compound phosphides with manganese FeMnP [14] and NiMnP [15].Major contribution to the electrical resistivity de-crease during heating over the second temperature in-Figure 6  Electrical resistance temperature dependency curve and its temperature derivative curve for CuFe0,3Ni0,5P0,2 alloyFigure 5  The CuFe0,3Ni0,5P0,2 alloy microstructure after annealing and quenchingFigure 7  TEM-micrograph of the twin type particle, precipitated from the αCu supersaturated solid solution after heating up to 350 °CTable 2  The electrical resistance of CuFe0,3Ni0,5P0,2 alloy samples after heating to distinctive temperaturesAnnealing temperature /°CElectrical resistance before heat treatment /mΩElectrical resistance after heat treatment /mΩΔR/%ΔR i- ΔR i-1 /%350 55,024 42,791 22,23 22,23550 44,156 27,315 38,14 15,91620 40,986 20,350 50,34 12,20750 41,932 23,369 44,27 -6,07Figure 8  The microstructure of the CuFe0,3Ni0,5P0,2 alloy after heating up to 550 °C (TEM)54  METALURGIJA 54 (2015) 1, 51-54G. BROVČ et al.: SYNTHESIS AND CHARACTERIZATION OF HARDENED Cu-Fe-Ni-P ALLOYterval of electrical resistance deviation (550 °C) has a depletion of αCu solid solution by precipitation of iron and nickel phosphides.Aggregate type precipitates were still present in the same form and approximately of the same size (Figure 8), but the density of the particle distribution increased considerably.In microstructure of the samples heated above the third distinctive temperature interval, up to 620 °C, the twin type particles were no longer detected. The process of continuous particle growth and coalescence took place during further heating up to 750 °C (Figure 9).The connection between small electrical resistance increase (Table 2) in samples heated above Tm4 could be explained by the EDS spot micro-analysis [16], which re-vealed transformation of phosphide particles with high-er Ni/P ratio to a lower Ni/P ratio, which is in a good agreement with the authors [17, 18], who found Fe2P and Ni2P as stable phases after long periods of aging at 450 °C. It’s well known that any kind of crystal lattice distortion, increases electrical resistivity of the metal. Solute atoms are particularly effective scattering cen-ters for electrons and have the greatest effect on the electrical resistivity increase. The greatest influence on the electrical resistivity increase has P, followed by Si, Fe, Mn, Al, Ni, Sn, Zn.Based on the electrical resistance measurements and metallographic researches, we set the temperature of precipitation annealing at 495 °C. Table 3 shows meas-ured values of specimen electrical conductivities and hardnesses of the CuFe0,3Ni0,5P0,2 alloy after anneal-ing at 495 °C for various holding periods.Maximum electrical conductivity (73,71 % I.A.C.S. at 20 °C) as well as hardness (150 HV) was achieved after 3 hours of annealing at 495 °C. At longer periods of annealing, electrical conductivity starts to decrease.CONCLUSIONSFrom the microstructure of samples heated to dis-tinctive temperatures of electrical resistance deviations can be seen, that decomposition of supersaturated Cu solu-tion already started at Tm1 by precipitation of fine 10 to 20 nm large metastable particles.During heating over the second distinctive tempera-ture Tm2 complete precipitation of iron and nickel stable and metastable phosphides with higher Fe/P or Ni/P ra-tio had occurred. At even higher temperatures, redisso-lution of nickel back into Cu solution takes place, by transformation of metastable to stable Ni2P phosphides with lower Ni/P ratio.REFERENCES[1]  P. G. Slade: Electrical Contacts, Principles and Applica-tions, Marcel Dekker, New York, 1999.[2]  M. Braunovic, N.K.Myshkin, V. Konchits: Electrical Con-tacts, Taylor & Francis, London, 2006.[3]  D. Klobčar, J. Tušek, M. Bizjak, V. Lešer: Metallurgy, 53 (2014) 4, 509 – 512.[4]  J.R. Davis, Copper and Copper Alloys, ASM International, Materials park, Ohio, 2001.[5]  Y. Yanamoto, G. Sasaki., K. Yamakawa, M. Ota: Hitachi Cable Review No. 19, Tokio, 2000.[6]  Y. Yamamoto, H. Takano, M. Ohta., C. Tong., A. Kunishi-ge: Hitachi Cable Review No. 26, Tokio, 2007.[7]  M. Bizjak, B. Kosec, IRT 3000, 6 (2011) 36, 32-33.[8]  P. Pečin, M. Bizjak, M. Pribošek, M. Godec, S. Ribarič, J. Rozman, Materials and Technology, 47 (2013) 6, 837-843.[9]  B. Zorc, L. Kosec: Revista de Metallurgia, 36 (2000) 2, 100 – 107.[10]  B. Zorc, L. Kosec: Metall, 58 (2004) 5, 290 – 294.[11]  D. Klobčar, L. Kosec, B. Kosec, J. Tušek: Engineering Fai-lure Analysis, 20 (2012) 1, 43-53.[12]  H. Cao, J.Y. Min, S.D. Wu, A.P. Xian, J.K.Shang: Material Science and Engineering , A 431 (2006), 86-91.[13]  ASM Handbook Volume 3: Alloy Phase Diagrams, Mate-rials Park, Ohio, 1992.[14]  J.H.Choi: Material Science and Engineering, A 550 (2012), 183-190.[15]  J.H. Choi, D.N. Lee: Materials Science and Engineering A, 458 (2007), 295-302.[16]  G. Kosec, A. Nagode, I. Budak, A. Antic, B. Kosec: Engi-neering Failure Analysis, 18 (2011) 1, 450-454.[17]  J.S. Byun J.S, J.H Choi, D.N. Lee: Scripta Materialia, 45 (2000), 637-643.[18]  P.D. Lu, J. Wang, W.J. Zeng, Y. Liu, L. Lu, B.D. Sun: Ma-terials Science and Engineering A, 421 (2006), 254-259.Note:  The responsible translator for English language is Urška Letonja Grgeta, MOAR, Podgora, Slovenia.Figure 9  Precipitates after heating over Tm4 of electrical deviations: SEM (below) and TEM (above)Table 3  Electrical conductivity and hardness after annealing at 495 °C for various holding periodsAnnealing time  /MSm-1 Δ=a-q /MSm-1 I.A.C.S. /%Hardness HVQuenched 19,14 / 33,00 851 h 38,27 19,13 65,98 1422 h 41,58 22,54 71,69 1453 h 42,75 23,61 73,71 1504 h 42,18 22,78 72,72 149

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Synthesis and characterization of hardened Cu-Fe-Ni-P alloy

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