ScienceDirect Effects of microalloying with lanthanum

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ScienceDirect Procedia Engineering 81 (2014) 203 – 208

11th International Conference on Technology of Plasticity, ICTP 2014, 19-24 October 2014, Nagoya Congress Center, Nagoya, Japan

Effects of microalloying with lanthanum on recrystallization of cold rolled pure copper Yan Chena, Shi-Hong Zhanga, *, Ming Chenga, Hongwu Songa, Jinsong Liua,b, Shuangkui Xiongc a

Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China School of Materials Science and Engineering, Shenyang Ligong University, Shenyang 110168, China c Guangdong Longfeng Precise Tube Co., Ltd, Zhuhai 519045, China

b

Abstract The pure copper ingots with different microalloying lanthanum (La) addition were cold rolled to a cumulative area reduction of 80% at room temperature and then subjected to various annealing treatments. The results show that the recrystallization of alloys was retarded at first and then promoted with increasing La concentration. It is noted that the recrystallization of alloys with a small amount of La was retarded. But, the recrystallization of alloys with a large amount of La was accelerated. In addition, the models of recrytallization kinetics of alloys were established to predict the recrystallisation degree of the deformed pure copper with different lanthanum addition during an isothermal annealing. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Nagoya University and Toyohashi University of Technology. Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University Keywords: Rolled pure copper; La; Recrystallization; Microstructure evolution; Kinetics

1. Introduction Microalloying has an importance effects on the improvement of mechanical properties of alloys, and has been widely applied to various alloy systems [1, 2]. In Cu-based alloys, microalloying additions such as Cr [3], Si [4],

* Corresponding author. Tel.: 024-83970196; fax: 024-23900631. E-mail address: [email protected]

1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University doi:10.1016/j.proeng.2014.09.151

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Yan Chen et al. / Procedia Engineering 81 (2014) 203 – 208

and P [5] to copper can get high strength and good electric conductivity. Very fine precipitates are formed in the copper matrix through high temperature quenching and subsequent aging, which leads to the improvement of the strength and electrical conductivity. Nevertheless, rare earth microalloying additions have obvious effects on the recrystallization temperature and softening temperature of Cu and Cu-based alloys. Guo et al. [6] reported the recrystallization temperature of CuFe-P-Cr with rare earth microalloying additions was improved because the addition of rare earth could effectively increase the volume fraction of strengthening phases. The increase of recrystallization temperature of Cu-0.6wt.%Fe composites with rare earth additions was mainly owing to the fact that rare earth in the composites result in increased amount of the Fe precipitates [7]. Zhang et al. [8] reported Ce-rich second-phase particles aggregate around the defects, and retard the motion of defects and inhibit the formation and growth of subgrain during the recrystallization process. It is well known that rare earth elements are easy to react with Cu and other elements in the copper, and form the RE-rich second phase particles. However, the effects of second-phase particles on recrystallization are cRPSOH['XULQJDQQHDOLQJWKHSULPDU\HIIHFWRIFORVHO\VSDFHGVPDOOSDUWLFOHVȝP LVWR pin grain boundaries (Zener pinning) [9, 10@EXWWKHGHIRUPDWLRQKHWHURJHQHLWLHVDWODUJHSDUWLFOHV!ȝP PD\EH sites at which recrystallization originates particle stimulated nucleation [11, 12]. The previous investigations mainly focused on disclosing the influence of rare earth on the recrystallization temperature of Cu and Cu-based alloys. However, it was uncertain that RE-rich particles formed in the Cu matrix have effects on the kinetics and microstructural evolution of recrystallization of Cu and Cu-based alloys. In the present article, the specific objective of the work was to determine the general effect of microalloying element La on the recrystallization of cold rolled pure copper. In particular, it was examined whether La has a positive effect on retarding recrystallization. 2. Experimental procedure The chemical compositions of the copper ingots are pure copper as 0#, Cu-0.022La as 1#, Cu-0.04La as 2#, Cu0.089La as 3#, Cu-0.18La as 4# and Cu-0.32La as 5#, respectively. Firstly, the copper ingots with different La contents were cold rolled to a thickness of 10.5 mm and then annealed for 90 min at 550°C followed by furnace cooling. The annealing sheet with a thickness of 10.5mm was then cold rolled in 8 passes to 2.1 mm final thickness, corresponding to a thickness reduction of 80%. Samples of the final cold-rolled sheet with a size of 7x3x2mm3 (in the rolling, transverse and normal directions, respectively) were isothermally annealed in a salt bath at various temperatures (250, 300 and 350°C), for different times (1, 5, 10, 30, 60, 120, 240 and 480 minutes), following the water cool. Recrystallization microstructures were observed in the longitudinal plane perpendicular to the transverse direction by optical microscopy. The microstructure and second-phase particles were also characterized by a JEM 2010 transmission electron microscopy. Microhardness of the samples was tested by Vickers hardness tester under the load of 300 g and holding 15 s. Every value shown in this paper was the average value of 10 points. The size of grains in these specimens was determined by using linear intercept method. 3. Results 3.1. Microstructure evolution The microstructure evolution of alloys with different La additions during the processes is shown Fig.1. After rolling, the microstructure of the cold-rolled sheet is characterized by elongated grains with an average thickness in WKHUDQJHIURPȝPWRȝP)LJ 1(a), (b), (c) and (d)). Moreover, the microstructure also shows the evidence of some wavy shear bands [13]. At the initial of annealing process, partial recrystallization occurs, and nucleation of recrystallization at the shear bands and the original grain boundaries can be observed (Fig. 1(e)). Recrystallized grains of between 3ȝPDQG7ȝPDUHREVHUYHG)LJ 1(f), (g) and (h)). At the middle of annealing process, an half of the deformed alloys are recrystallized after annealing at 250°C for 30 minutes (Fig. 1(i), (j), (k) and (l)). At the final of annealing process, when the annealing temperature was increased to 350°C, more than 90% of the sample


recrystallized (Fig. 1(m), (n), (o) and (p)). It is noted that the average recrystallized grain sizes of alloys are ȝP/D ȝP/D ZW ȝP/D ZW DQGȝP/D ZW UHVSHFWLYHO\

Fig. 1. Optical micrographs of cold rolled pure copper with different La contents after different processes: (a) 0#, (b)2#, (c)3# and 5# after rolling; (e) 0# (250°C x5min), (f) 2# (250°C x60min), (g) 3# (250°C x30min), (h) 5# (250°C x10min), (i) 0# (250°C x30min), (j) 2# (300°C x1h), (k) 3# (300°C x2h), (l) 5# (300°C x30min), (m) 0# (300°C x1h), (n) 2# (350°C x4h), (o) 3# (350°C x4h) and (p) 5# (350°C x1h) after annealing.

3.2. Size and volume fraction of second-phase particles Fig.2 shows the backscattered electrons morphology of cold rolled pure copper with different La contents. The white spherical phases are La-rich particles (as shown by the arrows), and the gray phases are copper matrix. In order to accurately determine the size and volume fraction of second-phase particles in the matrix, several backscattered electrons LPDJHV ZLWK D ȝP VFDOH RI DOOR\s were analyzed. The average diameter and volume fraction of measured particles were shown in Table 1. It can be found that the size and volume fraction of particles are increased substantially with the increasing of La content. In addition, the volume fraction to radius of the particle size (F V /r) is calculated. The local volume fraction to radius (F V /r) is this ratio that controls the Zener pinning pressure that the dispersoids exert on grain and subgrain boundaries [14]. Table 1. Volume fraction (F V ), mean radius (r) and pinning effectiveness (F V /r) of particles for alloys. Content of La (wt.%) 0.022 0.04 0.089 0.18 0.32

F V (%) 0.136 0.215 0.337 0.556 0.983

r(ȝm) 0.84 0.87 1.7 1.77 1.84

F V /r(ȝm-1) 0.162 0.247 0.198 0.314 0.534

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Fig. 2. Backscattered electron images of cold rolled pure copper with different La contents: (a) 1# (b) 2# (c) 3# (d) 4# and (e) 5#

3.3. Recrystallization kinetics The change in microhardness as a function of annealing time and temperature, for obvious alloy with different La addition, is shown in Fig. 3(a), (b) and (c), respectively. It is obvious that the microhardness falls gradually with the increase of time at each annealing temperature. Fig. 3(e) shows the microhardness as a function of annealing temperatures for 5 min. It is obvious that the ability of softening resistance of alloys is improved gradually with the increase of La contents.

Fig. 3. Microhardness evolution of alloys with different La contents: during (a) 250°C, (b) 300°C and (c) 350°C with the increase of time; (d) for 5 min with the increase of temperature.

It is well known that the analytical descriptions of the kinetics of recrystallization can be characterized in terms of the Johnson-Mehl-Avrami -Kolmogorov (JMAK) equation [15]: X c (t ) 1 exp( Kt n ) ,

(1)

where X c (t) is the recrystallized fraction at time t, t is the annealing time, K is the Avrami rate constant containing the nucleation and the growth parameters, and n is the Avrami exponent the value. In this research, the recrystallized fraction X c (t) is given by the following relation[16]: X c (t )

HVi HV (t ) , HVi HV f

(2)

where HV i is the microhardness of the initial deformed state, HV f is the microhardness after complete recrystallization, and HV(t) is the microhardness of the recrystallized with annealing time (t) at a given annealing temperature T. Furthermore, the parameters in Eq (1) are usually determined by taking the double logarithm .The value of k and n could be obtained by fitting the experimental data, which are listed in Table 2.

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Yan Chen et al. / Procedia Engineering 81 (2014) 203 – 208 Table 2. Values of Avrami exponent (n) and parameter (K) for recrystallization kinetics of the cold rolled pure with different La addition during annealing at different temperatures. 250°C 0# 1# 2# 3# 4# 5#

n 1.222 1.072 1.043 0.917 0.974 1.280

K 7.46×10-5 4.93×10-5 3.41×10-5 1.45×10-4 1.09×10-4 1.33×10-6

300°C 0# 1# 2# 3# 4# 5#

n 0.905 0.882 0.861 0.757 0.870 0.924

K 7.69×10-3 1.66×10-3 1.58×10-3 4.58×10-3 2.26×10-3 2.44×10-4

350°C 0# 1# 2# 3# 4# 5#

n 0.916 0.895 0.869 0.788 0.855 0.956

K 4.68×10-2 1.59×10-2 1.23×10-2 2.33×10-2 2.00×10-2 4.56×10-2

4. Discussion It can be observed from Fig.2 that in alloy containing La less than 0.1wt% several small particles are present in the copper matrix, while there are a lots of large particles in copper matrix in the case of La content more than 0.1wt%. However, recrystallization can be observed to nucleate primarily on the large eutectic constituent particles that form during casting (Fig.4). It is recognized that the higher the content of La is, the more nucleation sites is for the more perfect recrystallization. Therefore, the more perfection of crystals indicates the increase of the Avrami exponent (n). However, the primary recrystallization mechanism in the high-La copper alloys results from particle stimulated nucleation of eutectic constituent particles.

Fig. 4. TEM images of alloys during different annealing processes: (a) 5# alloy at 350°C for 1 hour and (b) 5# alloy at 300°C for 30 minutes

Generally speaking, the pinning effectiveness of a particle distribution in preventing grain boundary migration or not is expected to depend on the volume fraction to radius (F V /r) ratio according to the Zener pinning equation. Table 1 summarizes the F V /r measurements for second-phase particles. It is found that when the content of La is less than 0.1 wt%, the value of F V /r is below 0.3 ȝP-1, and the process of recrystallizaton is retarded. While the content of La is more than 0.1, the value of F V /r is over 0.3 ȝP-1, and the process of recrystallization is accelerated. It is clear that there is a critical particle size and volume fraction for the transition accelerated to retarded recrystallization. Previous studies have shown that analysis of many experimental investigations focused mainly on either the condition of large particles and low volume fraction or the case of small particles and high volume fraction [17, 18]. Nevertheless, their conclusions are that retardation of recrystallization occurs when the parameter F V /r is greater than the critical value. If F V /r is less than the critical value, it is found that recrystallization is accelerated [14]. In the present work, an important finding which is contrary to the previous conclusions is that when F V /r is less than the critical value, recrystallization is accelerated. While F V /r is more than the critical value, acceleration of recrystallization occurs. It is obvious that the F V U UDWLR RI ȝP-1 is the critical value for the transition from accelerated to retarded recrystallization. In our previous work, it is found that the particles become ribbons during deformation, but on annealing, they break-up and spheroidise. Therefore, no geometrically necessary dislocations are produced and there is no increase in the driving pressure for recrystallization. In addition, the effective interparticle spacing decreases during deformation, thus increasing the effectiveness of the particles in pinning boundaries. It is noted that alloys containing deformable particles should exhibit retarded recrystallization more readily than one containing non-GHIRUPDEOH SDUWLFOHV +HQFH VHYHUDO VPDOO SDUWLFOHV ȝP ZKLFK DUH

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present in the alloy containing lower La content can exert an enough pinning effect on boundaries and inhibit the migration of grain boundaries, which leads to retardation of recrystallization. 4. Conclusions (1) The primary recrystallization mechanism in the high-La copper alloys results from particle stimulated nucleation of new grains. Eutectic constituent particles that form during casting are the primary nuclei. (2) The addition of mircoalloying La to pure copper can improve the ability of softening resistance of alloy. This make the softening temperature increase from 250°C to 300°C. (3) It is found that F V U UDWLR RI ȝP-1 is the critical value for the transition from accelerated to retarded recrystallization. The acceleration of recrystallization occurs when the parameter F V /r is greater than the critical value. If F V /r is less than the critical value, it is found that recrystallization is retarded. Acknowledgements The authors would like to express their appreciation to the financial support by Science Foundation of The Chinese Academy of Sciences. The authors would like to thank Guangdong Longfeng Precise Tube Co., Ltd for supplying experimental materials in the present study. References [1] A. Ardehali Barani, F. Li, P. Romano, D. Ponge, D. Raabe. Design of high-strength steels by microalloying and thermomechanical treatment. Materials Science and Engineering: A, 2007, 463(1-2): 138-146 [2] V. Prokoshkina, L. Kaputkina, G. Khadeev. Effect of nitrogen microalloying on structure and properties of quenched martensitic steels. Journal of Alloys and Compounds, 2013, 577(S1): 559-562 [3] H. Fernee, J. Nairn, A. Atrens. Precipitation hardening of Cu-Fe-Cr alloys Part I Mechanical and electrical properties. Journal of materials science, 2001, 36: 2711-2719 [4] V. C. Srivastava, A. Schneider, V. Uhlenwinkel, S. N. Ojha, K. Bauckhage.Age-hardening characteristics of Cu–2.4Ni–0.6Si alloy produced by the spray forming process. Journal of Materials Processing Technology, 2004, 147: 174-180 [5] J. H. Choi, D. N. Lee. Aging characteristics and precipitate analysis of Cu–Ni–Mn–P alloy. Materials Science and Engineering A, 2007, 458: 295-302 [6] F. A. Guo, C. J. Xiang, C. X. Yang, X. M. Cao, S. G. Mu and Y. Q. Tang. Study of rare earth elements on the physical and mechanical properties of a Cu–Fe–P–Cr alloy. Materials Science and Engineering: B, 2008, 147(1): 1-6 [7] Z. W. Wu, Y. Chen, L. Meng. Effects of rare earth elements on annealing characteristics of Cu–6 wt.% Fe composites. Journal of Alloys and Compounds, 2009, 477(1-2): 198-204 [8] Z. F. Zhang, G. Y. Lin, S. H. Zhang, J. Zhou. Effects of Ce on microstructure and mechanical properties of pure copper. Materials Science and Engineering A, 2007, 457(1-2): 313-318 [9] 15L. Vanherpe, N. Moelans, B. Blanpain and S. Vandewalle. Pinning effect of spheroid second-phase particles on grain growth studied by three-dimensional phase-field simulations. Computational Materials Science, 2010, 49(2): 340-350 [10] J. D. Robson, D. T. Henry and B. Davis. Particle effects on recrystallization in magnesium–manganese alloys: Particle pinning. Materials Science and Engineering: A, 2011, 528(12): 4239-4247 [11] L. P. Troeger, E. A. Starke Jr. Particle-stimulated nucleation of recrystallization for grain-size control and superplasticity in an Al–Mg–Si– Cu alloy. Materials Science and Engineering: A, 2000, 293(1-2): 19-29 [12] J. D. Robson, D. T. Henry and B. Davis. Particle effects on recrystallization in magnesium–manganese alloys: Particle-stimulated nucleation. Acta Materialia, 2009, 57(9): 2739-2747 [13] W. Y. Yeung and B. J. Duggan. Shear band angles in rolled F.C.C. materials. Acta Metallurgica, 1987, 35(2): 541-548 [14] F. J. Humphreys and M. Hatherly. Recrystallization and Related Annealing Phenomena. Tarrytown: Elsevier Science, Inc., 1996. [15] M. Fanfoni and M. Tomellini. The Johnson-Mehl-Avrami-Kolmogorov model: A brief review. Nuovo cimento, 1998, 20(7-8): 1171-1182 [16] Y. Lü, D.A. Molodov, G. Gottstein. Recrystallization kinetics and microstructure evolution during annealing of a cold-rolled Fe-Mn-C alloy. Acta Materialia, 2011, 59(8): 3229-3243 [17] R. D. Doherty and J. W. Martin. Recrystallization in two-phase aluminium-copper alloys. Transactions of ASM, 1964, 57:874-884 [18] F. J. Humphreys. The nucleation of recrystallization at second phase particles in deformed aluminium. Acta Metallurgica, 1977, 25(11): 1323–1344