UDK 621.762.5:669.715 Izvirni znanstveni članek ISSN 1580-2949 MTAEC9, 38(5)245(2004) INFLUENCE OF ALUMINA CONTENT ON THE SINTERABILITY OF THE Cu-Al2O3 PSEUDO ALLOY (COMPOSITE) VPLIV VSEBNOSTI GLINICE NA SPOSOBNOST SINTRANJA V SISTEMU Cu-Al2O3 Zoran Anđić1, Miloš Tasić1, Marija Korać2, Branka Jordović3, Aleksa Maričić3 1Scientific Research Center, Nikole Pasica 26, 31000 Užice, Serbia and Montenegro 2Faculty of Metallurgy and Technology, Karnegijeva 4, 11000 Belgrade, Serbia and Montenegro 3Faculty of Engineering, Trg Svetog Save, 32000 Čačak, Serbia and Montenegro nicueŽverat.net Prejem rokopisa – received: 2004-06-10; sprejem za objavo - accepted for publication: 2004-10-07 Mechanism and kinetics of the thermally activated processes of the occurring by annealing of cold pressed samples of Cu-Al2O3 powder. With electrical resistance measurements in non-isothermal and isothermal conditions and with microstructural analysis, the influence of Al2O3 and temperature on the sintering process of Cu(1-x)-Al2O3(x) powders was determined. By measuring the isothermal change of specific electrical resistance at temperatures below the recovering temperature (400-680 K), the kinetics parameters of the process are determined. The sintering was performed in hydrogen at the temperatures of (1073, 1173, 1273) K for (15, 30, 60, 120) min. The results show that with increasing Al2O3 content the sintering time increases at all the examined temperatures. Key words: dispersion strengthening, Cu-Al2O3 composite, specific electrical resistance, microstructure Članek opisuje mehanizem in kinetiko termično aktiviranih procesov med žarjenjem hladno stisnjenih prahov kompozitne zlitine Cu-Al2O3. Z meritvami električne upornosti v izotermnih in neizotermnih razmerah in z analizo mikrostrukture je bil raziskan vpliv temperature na proces sintranja kompozitov Cu(1-x)-Al2O3(x). Z meritvami specifičnega električnega upora v izotermnih razmerah pod temperaturo poprave (400-680 K), so bili določeni kinetični parametri procesa. Sintranje se je izvršilo v vodiku pri temperaturah (1073, 1173, 1273) K v časih (15, 30, 60, 120) min. Rezultati meritev kažejo, da čas sintranja raste pri vseh temperaturah z rastjo vsebnosti Al2O3. Ključne besede: disperzijska utrditev, Cu-Al2O3, specifična električna upornost, mikrostrukturna analiza 1 INTRODUCTION Due to its low mechanical strength, a highly conductive copper matrix needs to be dispersion strengthened and new composite materials, with superior characteristics are obtained. The most important application area of these materials, electrical engineering, sets more and more complex demands for the material synthesis. The parameters, which effect the optimization of the characteristics of dispersion strengthened copper, are the dispersoide content and the sintering temperature 1,2,3. All dispersed systems can be obtained with powder compaction using different methods 4,5. By cold pressing internal stresses are generated and than relaxed with annealing at increased temperature. In several papers 6,7,8, the mechanism of thermally activated relaxation was determined for the pressed and cold deformed powder mixtures. It has been shown that grain size reduction of the dispersoide leads to an increasing decrease of the rate of relaxation of internal stresses. In this paper the influence of Al2O3 content on the kinetics and the mechanism of the thermally activated processes of relaxations of internal stresses in cold pressed samples Cu-Al2O3 composite during sintering was investigated. 2 EXPERIMENTAL The mixture commercial electrolytic copper powder and Al2O3 powder in the range of composition 85-95 % Cu and 15-5 % Al2O3 was homogenizated in a mixer of the "double-cone" type for 30 min. The mixture was than compacted with a compressive force of 100 MPa from both sides to specimens of size (8 × 32 × 2) mm. The sintering of samples was performed in hydrogen at the temperatures of (1073, 1173, 1273) K for (15, 30, 60, 120) min. The electrical resistance was measured during the sintering using of a two-channel recorder ISKRA TZ-2000 with a sensitivity in the range 10–6 V. For microstructural investigation, the sintered samples were, after grinding, electro-polished and electro-etched for 20 s. As electrolyte, the solution of nitric and methyl alcohol of 1 : 2 was used. The microstructural analysis of the sintered samples was performed with an automatic quantitative image analyser. 3 RESULTS AND DISCUSSION a) Recovering kinetics The kinetics recovering parameters were determined by measuring the time related electric resistance by MATERIALI IN TEHNOLOGIJE 38 (2004) 5 245 Z. AN\I] ET AL.: INFLUENCE OF ALUMINA CONTENT ON THE SINTERABILITY OF THE Cu-Al2O3 1.4 Cu+15% Al2°3 a) 468 K 1.2 " \ Č\ b)--------478K c).......513K 1.0 : \\ x °-6 : \\ \ 0.4 l \ \ a) \ ! \ 0.0 \ -0.2 : \ \ -0.4 V \ c) \ -0.6 Č b) -200 200 400 600 800 r/s 1000 1200 1400 1600 Figure 1: Isothermal dependence specific electrical resistance lnp(r) on sintering time for the specimens Cu + 15 % Al2O3; a) T = 468 K; b) T=478K;c) T=513 K Slika 1: Izotermna odvisnost lnp(-c) za kompozite Cu + 15 % Al2O3; a) T = 468 K; b) T = 478 K, c) T = 513 K isothermal annealing at (468, 478, 513) K for the sample with 15 % Al2O3. The obtained isothermal dependence of specific electrical resistance versus time is shown in Figure 1 for all temperatures. By differentiation of the curves a, b and c two linear dependences (Figure 2) were obtained, which show that the recovering process occurs in two stages. From the slope of A ln p'/Ar and A ln p''/Ax the rate k ' and k'' was determined for both stages of the recovering process. In both stages a linear dependence the form of ln k on T1 (Figure 3) is found and from the slope of the linear dependence the activation energy was determined applying the relation: Alnk a č M(1/T) For the samples with 5 % and 10 % of Al2O3 the same dependence was obtained as with the 15 % of Al2O3 specimen. After differentiation of the curves and the related calculations the results Table 1 were obtained. It is, therefore, confirmed that the relaxation of internal 1.5-1.0- 0.5- 0.0 -•-a1)]npW,468K AbJtapWČK ¦Cj)]npW,513K a2)hip"(r),4éSK Č)lnp"(r),4HK Cj)lnp"(r),5BK ¦ A \ \ • *%) \ A V -Ab,) Abl) •. Č"•Č \.aj) ¦cl) 0 100 200 300 400 500 600 700 ?/s Figure 2: Two new dependences specific electric resistance versus sintering time ln ?'(?) and ln ?''(?) obtained with differentiation of curves in Figure 1. Composites: Cu + 15 % Al2O3; a1, a2) T = 468 K; b1, b2) T = 478 K; c1, c2) T=513 K Slika 2: Odvisnostln ?'(?) in ln ?''(?) za kompozite Cu + 15 % Al2O3 ; a1, a2) T = 468 K; b1, b2) T = 478 K; c1, c2) T = 513 K 4- ČȦČČČČ ¦ Ink' A Ink" 3- Č-----ČČ ¦ 2- ČČČ 1- ¦ČČČČČ 0- I----ČČ -1- Č_ČČ A -2- A 1.95 2 .0 5 1000/T/K–1 Figure 3: Dependence of rates of change of electric resistance as ln k' = f (1000/T) in ln k'' = f (1000/T) for the curves in figure 2. Alloy Cu + 15 % Al2O3 Slika 3: Odvisnostln k' = f (1000/T) in ln k'' = f (1000/T) za izoterme pri kompozitu Cu + 15 % Al2O3 0.130 0.125 0.120 0.115 0.110 0.105 0.100 0.095 0.090 0.035 0.030 Cu + 5%AI203 ¦ —¦— 1073 K -•— 1173 K -a— 1273 K •-____ A------ 60 r/min b *-• Cu + 10%AI2CČ 0.24 0.23 Čmč-___Č —¦— 1073 K -•— 1173 K —*— 1273 K 02? čččččm---------- 0.21 0.20 019 60 80 r/min 1.0 0.35 05 0.34 0.33 00 0.32 -0.5 0.31 -1.0 0.30 0.29 -1.5 c ¦. Cu+15%AI203 —¦— 1073 K -•— 1173 K a 1273 K * AČ • ?/min Figure 4: Dependence specific electric resistance versus sintering time for different temperature; a) Cu + 5 % Al2O3, b) Cu + 10 % Al2O3 and c) Cu + 15 %Al2O3 Slika 4: Izotermna odvisnost specifična električna upornost – čas za različne temperature sintranja; a) Cu + 5 % Al2O3, b) Cu + 10 % Al2O3 in c) Cu + 15 % Al2O3 2O 21C 215 0 30 90 20 0 2J 40 100 120 5 0.27 -20 -20 0 246 MATERIALI IN TEHNOLOGIJE 38 (2004) 5 Z. AN\I] ET AL.: INFLUENCE OF ALUMINA CONTENT ON THE SINTERABILITY OF THE Cu-Al2O3 ... stress in cold pressed specimens in temperature range from 450 K to 650 K occurs in two stages, which are related to the change in the arrangement and concentration of lattice defects in the copper matrix. The first stage consists of the relaxation of internal stresses, introduced into the material by the pressing of powders and of the removal the point defects, while, the second stage of the structural relaxation is related to line defects. In the first stage the rate of relaxation is considerably faster due to the fact that point defects more efficiently diffract the conductive electrons than the line defects. With the increase of the Al2O3 content, the rate of change of electrical resistance is decreased and activation energy related to the relaxation processes is increased. b) Isothermal sintering Figure 4 shows the isothermal dependence of the specific electric resistance versus time for different sintering temperatures and the composites: a) Cu + 5 % Al2O3, b) Cu + 10 % Al2O3 and c) Cu + 15 % Al2O3. As 0.080- 1050 1100 1150 1200 1250 1300 TIK 0.19- 1050 1100 1150 1200 1250 1300 1050 1100 1150 1200 1250 1300 TIK Figure 5: Dependence specific electric resistance versus temperature for determined tempering times; a) Cu + 5 % Al2O3, b) Cu + 10 % Al2O3 and c) Cu + 15 %Al2O3 Slika 5: Odvisnost specifične električne upornosti od temperature za različne čase sintranja; a) Cu + 5 % Al2O3, b) Cu + 10 % Al2O3 in c) Cu + 15 % Al2O3 measure of the structural stability of the system at a determined temperature the time when the electrical resistance achieves a constant value is taken, i. e., Ap/Ar = 0. For the system with 5 % of dispersoide during sintering at 1073 K the sintering process is not completed after 120 min, at 1173 K the process is finished after 30 min, and at 1273 K after 15 min of annealing. The sintering of the composite with 10 % Al2O3 during annealing at1073 K is notcompleted after 120 min, at1173 K, itis finished after 60 min, and at 1273 K after 30 min. For the composite with 15 % of dispersoide, during annealing at the stated temperatures, the sintering was completed even after 120 min. The results show that with the increasing Al2O3 content, the time to the completion of sintering increases. This finding is in agreement with the activation energy for the sintering of composites with (5, 10, 15) % Al2O3 (Table I). Finally, the sinterability of the Cu-Al2O3 composites decreases with the increasing Al2O3 content. The diagrams in Figure 5 show the dependence of the specific electrical resistance of the sintering temperature after different sintering times. For a given Al2O3 content, temperature and for a given time, the specific electrical resistance decrease with increasing sintering temperature. The results also show that with the increasing dispersoide content and for a selected temperature-time regime, the specific electrical resistance increases. Microstructural analysis (Table 2) shows that, with increasing sintering temperature for a given time, the volume share of porosity decreases and that with increasing the Al2O3 content, the porosity increases. The porosity, as measure of the structural integrity of the system, has, thus, a significant influence on the specific electrical resistance after sintering and the porosity. It is also clear that at selected temperatures at which Ap/Ar i 0 after finished sintering, the microstructure has not achieved a stable state. F.i. the the microstructure of the Cu-Al2O3 alloy with 15 % of dispersoide, sintered at 1073 K and 1173 K for 120 min indicates to a correlation of the change of specific electrical resistance Table 1: Kinetic parameters for the recovering process of the pressed composite Cu-Al2O3 with (5, 10, 15) % of dispersoide Tabela 1: Kinetični parametri za proces poprava stiskanih kompo-zitov Cu-Al2O3 za (5, 10, 15) % disperzoida T/K k '/ (10–3 s–1) k''/ (10–3 s–1) E'a/ (kJ/mol) E'' a (kJ/mol) Cu + 5% AI2O3 473 17.54 0.2 24.267 34.9026 513 28.4 0.4 Cu + 10% AI2O3 473 15.84 0.14 53.765 59.148 488 23.36 0.27 498 27.62 0.36 Cu + 15% AI2O3 468 4.05 0.1 96.285 65.900 478 13.52 0.40 513 43.52 0.60 MATERIALI IN TEHNOLOGIJE 38 (2004) 5 247 Z. AN\I] ET AL.: INFLUENCE OF ALUMINA CONTENT ON THE SINTERABILITY OF THE Cu-Al2O3 Table 2: Stereological data on porosity after sintering Tabela 2: Stereološki podatki o poroznosti sintranih kompozitov Sintering Parameters Pore size d/µm Relative measuring error , % Volume share of porosity pv/% min. max. mean Cu + 5 % Al2O3 1073 K, 120 min 0.19 9.08 1.14271 3.23202 17.2 1173 K, 120 min 0.17 10.07 1.31927 3.24081 13.9 1273 K, 120 min 0.12 4.8 0.86511 2.48973 10.115 Cu + 15 % Al2O3 1073 K, 120 min 0.2 20.3 2.23306 3.76813 29.9 1173 K, 120 min 0.18 19.97 2.17131 3.65405 28.89 1273 K, 120 min 0.16 14.03 1.93522 3.62367 26.136 Figure 6: Microstructure of the composite Cu + 15 % Al2O3 alloy sintered at 1173 K for 120 min Slika 6: Mikrostruktura kompozita Cu + 15 % Al2O3, sintranega 120 min pri 1173 K Figure 7: Microstructure of the Cu + 15 % Al2O3 composite sintered at1073 K for 120 min Slika 7: Mikrostruktura kompozita Cu + 15 % Al2O3, sintranega 120 min pri 1073 K and the grain growth. Namely, for the systems with 15 % Al2O3 sintered at 1173 K for 120 min, an unhomogeneus microstructure is achieved, as consequence of anormal grain growth (Figure 6), and the specific electrical resistance is lower if compared to the same alloy sintered at1073 K for 120 min, when a more homogeneus microstructure with grains of a polyglobal shape (Figure 7) is obtained. It seems that grain growth decreases the specific electrical resistance because of the decrease of the surface of grain boundaries after sintering. 4 CONCLUSION On the basis of the results it is concluded that: – The process of stress relaxation of the cold pressed samples in temperature interval from 450 K to 650 K occurs in two stages. The first stage starts with the relaxation of internal stresses introduced with the pressing of the powder and the removal of point defects. The second stage of relaxation seems to be related to line defects. With increase of the Al2O3 content the rate of electrical resistance change decreases while, the activation energy of the corresponding processes increases; – With the increase of the sintering temperature the time to the microstructural stabilisation is shortened; – The analysis of the dependence of the specific electrical resistance on sintering time indicates that for the material with a lower Al2O3 content a shorter sintering time is needed; The analysis of the microstructure of the sintered samples shows that the porosity has a significant influence on the specific electrical resistance, as a measure of the microstructural stability of the system. Also, the analysis of the microstructure indicates a correlation between the specific electrical resistance change and the grain growth. More precisely, grain growth, due to a decrease of the overall surface of the boundaries, is related to the decrease of the specific electrical resistance after sintering. 5 REFERENCES 1 P. K. Jena, E. 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