The Effect of B2-like Structure on Hardening of Cu Bearing High Purity Steel Huiping REN a, Haiyan WANG b, Zongchang LIU c

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1 Advanced Materials Research Online: ISSN: , Vol. 51, pp doi: / Trans Tech Publications, Switzerland The Effect of B2-like Structure on Hardening of Cu Bearing High Purity Steel Huiping REN a, Haiyan WANG b, Zongchang LIU c School of Material and Metallurgy, Inner Mongolia University of Science and Technology, Baotou Inner Mongolia , China. a renhuiping@sina.com, b windflower126@163.com, c lzchang75@163.com. Keywords: copper, steel, B2-like structure, hardening Abstract. The precipitate behavior of copper in the high purity structural steel was investigated by means of transmission electron microscope (TEM), and aging hardening mechanism was investigated based on the corresponding phase transformation mechanism. The results show that lots of Cu rich clusters exist in supersaturated ferrite matrix in solid solution, which evolve to B2-like structure during aging. It is found that the hardening in the initial stage is controlled by the coherency relationship of the B2-like structure with matrix that forms the obstacle of the dislocation motion, while the decrease in hardness after the peak is attributed to the loss of coherency, which should highly likely be the dominant reason of aging hardening in Cu bearing high purity steels. Introduction Precipitations of copper in iron and steels have been studied extensively for its remarkable aging hardening effect. Recently the beneficial role of Cu received renewed attention due to the potential of combining high hardness with high formability in automotive applications [1]. Copper precipitation has been examined with a variety of experimental techniques such as field ion microscopy, (FIM) [2], high resolution transmission electron microscopy (HRTEM) [3]), (EXAFS [4]), and more recently in our previous work [5,6]. Based on these studies, it is now generally accepted that the following sequence is characteristic of precipitation in this system: α-fe (supersaturate solid solution) BCC copper 9R copper FCC ε-cu, but the aging precipitation rules have not been clarified in details for the complicated precipitation sequence, and the contribution of the precipitates to the hardness of the steels is less well characterized. It is the purpose of present work, to evaluate the precipitation process in a Fe-1.03%Cu alloy. An in-depth understanding is sought using an investigation, which includes the evolution of microstructure and the corresponding mechanical properties using a variety of experimental techniques. The emphasis of the micro structural investigation is to combine transmission electron microscopy (TEM), which gives direct information about the type, morphology and nucleation mechanisms of the precipitates. In addition, tensile tests are conducted to relate the precipitation characteristics quantitatively to the mechanical property evolution of the material. The effect of precipitation, coupled with the evolution of solid solution content, on the overall hardening behavior is carried out after they are subjected to solution and aging treatment. Experimental The high purity Fe-1.03 %Cu binary alloy was prepared by vacuum melting. The chemical composition (mass fraction, %) is C , Si -0.13, Mn-0.018, Cu-1.03, O , N , Al t , P 0.001, S The alloy was forged and rolled. The samples were prepared with the size of 10mm 8mm 8mm, then treated for solution at 850 o C for 120 minutes to reach a fully solid solution state and quenched into salt water. This solution treatment resulted in a fully equiaxed All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-08/05/16,20:39:41)

2 50 Engineering Materials III ferrite structure with a grain size of approximately 50µm. An aging treatment was then conducted in a salt-bath furnace at 550 o C for a time range of 100s to 100h followed by air-cooling. The microstructure observation of super-saturated solid solution and aging precipitation course was carried out in a TEM of JEOL JEM-2010 operated at 200KV. Disks for transmission electron microscopy were ground to the thickness of about 30µm, punched into discs of 3mm diameter, and electrolytically polished by the twin jet method. Image analysis was conducted on samples after several heat treatment conditions. Results and discussions Specimens of Fe-1.03%Cu alloy were prepared by solution-treatment at 850 o C for 2 hours and subsequently aged at 550 o C to precipitate Cu particles. Figure 1 shows changes in hardness of the specimens during the aging at 550 o C as a function of aging time. Typical age hardening and over-aging softening behavior are observed. These behaviors are closely related to the behavior of Cu precipitation and growth. The alloy shows little hardening in the first stage of aging. However, after 100 seconds the hardness has increased to a substantial amount. The hardness curve shows peak by precipitation after aged approximately 1000 seconds at 550 C, i.e. the second stage. Subsequently, the hardness of aged steels decreased when aged from to seconds at the third stage. The age hardness curve indicated that the microstructures of the alloy changed with increasing aging time, which results in the change of mechanical performance. Figure 1 Age hardness curve of Fe-1.03wt%Cu, aging at 550 o C (a) (b) (c) Figure 2 Microstructure of Fe-1.03 %Cu alloy, solid solution treatment dissolved at 850 o C for 2 hours(a) TEM image (b) Electron diffraction pattern, B= [110] (c) Sketch map of atom orderly arrange in bcc crystal lattice

3 Advanced Materials Research Vol Figure 2(a) shows that there are lots of particles about 4 to 20nm dispersed in ferrite crystals observed in TEM. According to the Fe-Cu phase diagram, Fe-1.03 %Cu alloy is in the α-fe single phase region at 850 o C. So these particles could only be Cu atoms clusters. Many investigations indicate that there are always lots of Cu-rich clusters in ferrite crystals of Fe-Cu alloy [7]. The supersaturated Cu clusters in the α-fe matrix tends to evolve as Cu particles by recrystallization annealing after aged at 550 o C [8]. The diffractive stripe in clusters indicate that Cu atoms distribute regularly and tend to congregate on the (001) α planes of BCC ferrite since the elastic modulus in <001> α direction is minimal. Electron diffraction analyses prove the correctness of the idea (Figure 2(b)). As demonstrated in Figure 2(c), Cu atoms congregate on (002) α and (001) α. Hence it should also cluster on (110) α planes, which have maximal crystal plane space ( nm). Figure 3 gives the high resolution electron microscope (HRTEM) image of the stripes in Cu clusters as well as the regenerative image of Fourier transform when samples solid solution treated at 850 o C for 2 hours. Lattice image of (110) α plane can be seen clearly since (110) α have the maximal crystal plane space in Figure 3(a). When Cu atom solid dissolved into α-fe matrix, crystal lattice aberrance appears since the radius of Cu atom is bigger than that of Fe atom. Therefore, larger aberrance stress would arise at the [110] α direction, which results in crystal lattice distortion. Consequently, there was 2 o -4 o orientation deviation between the (111) fcc and (110) α crystal plane, which result in some dislocation at the boundary of Cu rich stripes and ferrite matrix ((Figure 3(b)). (a) (b) 5nm Figure 3 Nanometer microstructure of Cu clusters, solid solution treated at 850 o C for 2 hours(a) HRTEM image (b) Regenerative image of Fourier transformation Figure 4 shows changes in the mean Cu particle size and spacing as a function of aging time. Cu particles are finely dispersed within the ferrite matrix with the average size of about 24 nm at the aging peak hardness (Figure 4(a)), these particles gradually ripen with aging time and this leads to the enlargement of mean particle spacing in all specimens (Figure 4 (b) and 4 (c)). Samar Das et al. [9] have reported that stacking faults may exist in the large sized ε-cu precipitate. During the growth of these precipitates, coherency of the precipitates with matrix is gradually lost and they ultimately become incoherent that may results in stacking faults perpendicular to the most closely packed direction of <111> Cu.

4 52 Engineering Materials III (a) (c) Figure 4 Microstructure evolution of precipitate during aged at 550 o C for (a) 1000 Sec. (b) 20 hours (c) 300 hours (a) (b) (c) (d) Figure 5 TEM microstructure of sample at aging peak, aged at 550 o C for 1000 Sec. (a) HRTEM image (b) (c) (d) Electron diffraction pattern, B= [100], [110] and [111] It is known that Cu precipitates as bcc clusters which are coherent with the matrix in the early stages of aging [10,11]. The hardness is significantly influenced by the coherent strain around the Cu particle. HRTEM of sample aged at 550 o C for 1000s are presented in Figure 5(a).The crystal lattice indicate the exact (110) α plane. Lattice aberration would appear when Cu atoms dissolved into α-fe, since Cu and Fe atoms have difference in crystal character and atom size. This results in the arrangement of Cu atoms on {001} α planes, forming (110) α crystal planes. Consequently, great lattice aberration would occur in the [110] direction with the aging time increasing. When the aberration reach a certain degree, the self-organized function of system will adjust strain direction to reduce strain energy. The Cu clusters near the dislocation will forms the obstacle of the dislocation movement, which exhibit evident hardening effect on Cu bearing high purity steel. Some investigations [12] analyzed the precipitation hardening based on Frank-Read theory. They demonstrated that the precipitation belongs to metastable Cu-Fe phase, which have B2-like structure. It keeps coherency relationship with matrix, thus leads to high ductility combined with high yield hardness. Precipitations of aging hardness peak could only be at the intermediate stage. Cu atoms concentrate gradually and evolve as equilibrium phase with increasing aging time. Recently, Isheim and Seidman [13] have also shown that the precipitate is a mixture of Cu and Fe. In addition, they re-examined the source of precipitation hardening from bcc Cu precipitates [14]. Our TEM observations are in reasonably good agreement with those reports. It is obvious the diffraction pattern represents the reciprocal lattice of bcc ferrite matrix in Figure5 (b), (c) and (d). In addition, the small spot (dot, circle) at the center indicates precipitates inside the ferrite matrix, the larger black spots plus the dimmed, small spot signals B2 like structure. Nevertheless, compare to the diffraction pattern of solution state (Figure 2(b), these diffraction spots are more evident and in focus. The differences of these figures lead to an assumption that the Cu atoms move several atom space and reach the exact (001) position by way of heat activation transition during aging, which make the ordering array of Cu atoms. The aging hardness curve change can be perfectly explained under this assumption, too.

5 Advanced Materials Research Vol The above results indicate that the ε-cu equilibrium phase is obviously the results of long time aging in the third stage, which have lose coherent with matrix. Further growth of the ε-cu precipitates leads to the decrease of precipitates density and hardening effects [8], while the metastable phase with B2-like structure is highly likely the dominant hardening phase, which more recently proved by the impact properties investigation of Cu bearing low alloy steels [15]. In addition, the amount of the precipitates will increase since the composition of the precipitates was very different from pure copper if they have B2-like structure. As mentioned above, the hardening due to Cu precipitate in Fe-Cu alloys is dependent on particle size as well as structure. Therefore, in order to apply the Fe-Cu alloys for high hardness steels, it is important to add sufficient Cu and control not only size and spacing but also the structure of particles. Summary Electron diffraction analysis of solution treated and aging state in Fe-1.03 %Cu alloy indicates that the Cu clusters at the aging peak hardness should have B2-like structure with higher Cu content, which appear in the early stage aging and evolve with aging time. The increasing stress field and evolvement of Cu clusters size should be the dominant reason of aging hardening in Fe-1.03wt%Cu alloy. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No ) and Research Foundation of Inner Mongolia University of Science & Technology. REFERENCES [1] A.Deschamps, M.Militzer and W.J.Poole: ISIJ Int., 41(2001), p.196 [2] P.J. Othen, M.L. Jenkins, G.D.W. Smith and W.J.Pythian: Phil Mag Letters, 64(1991), p.83 [3] N. Maruyama, M. Sugiyama, T. Hara and H.Tamehiro: Materials Transactions, JIM, 40 (1999), p.268 [4] S. Pizzini, K.J. Roberts, W.J. Phythian, C.A. English and G.N. Greaves: Phil. Mag. Letters, 61 (1990), p. 223 [5] Ren H P, Mao W M: Precipitation behavior of B2-like particles in Fe-Cu binary alloy. Journal of University of Science and Technology Beijing, 9 (2002), p.185 [6] H.Y.Wang, H.Lu, Z.C.Liu and H.P Ren: Trans of Material and Heat Treatment, 26(2005), p.66 [7] Y.N.Yu: Metallography Principle [M], Metallurgy Industry Publishing Company, Beijing, (2000), p.76 [8] R.Monzen, K.Takada and C.Watanabe: Coarsening of Spherical Cu Particles in an α-fe Matrix [J], ISIJ Int., 44 (2004), p.444 [9] Samar Das, A.Ghosh, S.Chatterjee and P.Ramachandra Rao: Scandinavian Jounal of Metallurgy, 31(2002), p.272 [10] K.Nakashima, Y. Futamura, T.Tsuchiyama and S.Takaki: ISIJ Int., 42 (2002), p.1541 [11] A.Deschamps, M.Militzer and W.J.Poole: ISIJ Int., 43 (2003), p.1826

6 54 Engineering Materials III [12] W.M.Mao, H.P.Ren and Y.N.Yu: Transactions of materials and heat treatment, 25(2004), p.1 [13] D.Isheim, D.N.Seidman: Surf Interface Anal, 36(2004), p.569 [14] M.E.Fine, D.Isheim: Scripta Materials, 53(2005), p.115 [15] A.N.Bhagat, S.K.Pabi, S.Ranganathanv and O.N.Mohanty: ISIJ Int., 44 (2004), p.115

7 Engineering Materials III / The Effect of B2-Like Structure on Hardening of Cu Bearing High Purity Steel / DOI References [4] S. Pizzini, K.J. Roberts, W.J. Phythian, C.A. English and G.N. Greaves: Phil. Mag. Letters, 61 (1990), p /