Preferential growth of coherent precipitates at grain boundary

Materials Letters xxx (xxxx) xxx

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Preferential growth of coherent precipitates at grain boundary Hongyuan Wen a, Bingbing Zhao a,b,c,⇑, Xianping Dong a,b,c, Feng Sun a,b,c, Lanting Zhang a,b,c a

School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, China Materials Genome Initiative Center, Shanghai Jiao Tong University, Shanghai, China c Shanghai Key Laboratory of High Temperature Materials and Precision Forming, Shanghai Jiao Tong University, Shanghai, China b

a r t i c l e

i n f o

Article history: Received 23 October 2019 Received in revised form 7 November 2019 Accepted 9 November 2019 Available online xxxx Keywords: Grain boundaries Coherent precipitate Interfaces M23C6 NiAl

a b s t r a c t Coherent precipitates are known to be more effective than their incoherent counterparts in pinning grain boundaries and impact the properties of alloys. In the present work, we showed the location preference of coherent M23C6 and NiAl precipitates formed at the grain boundaries in an Al-containing austenitic steel. In particular, the NiAl precipitates, which have a N-W orientation relationship with the c-matrix, were found to completely reside in the grain with which they are coherent, while the M23C6 precipitates, which follow the cubic-cubic coherent relationship with the matrix, preferably grew into the interior of the grain without coherency, forming incoherent interfaces. These observations were rationalized by the mobility of precipitate/matrix interface and the competition between elastic strain energy and interfacial energy. Ó 2019 Elsevier B.V. All rights reserved.

1. Introduction Second-phase particles are known to greatly impact the microstructures and properties of alloys [1,2]. It has been more than 70 years since Zener and Smith [3] first formulated a theory to describe the pinning mechanism of particles on grain boundary (GB) motion, under the assumption of an incoherent spherical particle with isotropic interfacial energy. Various modifications to the Zener pinning framework have been made over the past years [4–8]. Specifically, Ashby et al. [4] showed that coherent precipitates are more effective in pinning boundaries than their incoherent counterparts. In this context, one would spontaneously ask how the coherent precipitates at GBs maintain their coherency with grains on each side of the boundary. Thanks to recent progress on the modeling capability, different computational techniques such as molecular dynamics (MD) [9–11] and phase field models [12–15] have been used to explore the effect and the underlying mechanisms of precipitate coherency at GBs. These studies have led to the development of possible interaction mechanisms [16,17] that are yet to be validated in experiments. By meticulously carrying out site-specific microstructural analysis, here we provide an insight into this topic, and show the location preference of two different coherent precipitates (M23C6

⇑ Corresponding author at: School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, China. E-mail address: [email protected] (B. Zhao).

and NiAl) formed at the grain boundaries at the same time in an Al-containing austenitic steel. 2. Materials and experiments Alumina-forming austenitic (AFA) stainless steel, a new promising class of steels with potential for use in high temperature applications [18–20], was prepared by vacuum induction melting. Details of the chemical composition and thermo-mechanical treatment of the alloy can be found in our previous report [19]. The sample was subjected to creep testing at 700 under a load of 150 MPa for 50 h, using an RDL-50 creep test machine. The microstructure of GB was characterized by TEM (FEI Metrios), with a Super-X electron dispersive spectroscopy (EDS) system. Sitespecific TEM foils containing particular GB were prepared by a TESCAN GAIA3 dual-beam focused ion beam (FIB) microscope using a Ga ion beam for milling. 3. Results and discussion Fig. 1(a) shows a typical GB of the austenitic steel after creep, where the GB was fully covered by continuous precipitates with size ~150 nm. These precipitates were found to grow into both grains on either side of GB, i.e. grain A and grain B. Depending on the certain diffraction condition, only some of the precipitates showed clear contrast, while others were less conspicuous. The corresponding scanning TEM (STEM)-EDS mapping image (Fig. 1 (b)) exhibits an unambiguous distribution of the boundary precip-

https://doi.org/10.1016/j.matlet.2019.126984 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.

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Fig. 1. GB precipitates and GB orientation: (a) TEM bright field (BF) image of a GB completely covered by precipitates; (b) STEM-EDS mapping of the precipitates; (c) Kikuchi pattern of grain A; (d) Kikuchi pattern of grain B.

h i Fig. 2. Preferred growth orientation of the M23C6 carbide. (a) TEM DF image of M23C6/grain A (zone axis 112 ); (b) the corresponding SAD pattern of (a); (c) TEM DF image of M23C6/grain B (zone axis [0 1 1]); (d) the corresponding SAD pattern of (c); (e) the distribution of Cr element (corresponding to M23C6) by STEM-EDS mapping; (f) schematic view of M23C6 location at GB.

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itates. The enrichment of Cr and Al elements corresponds to two different types of precipitates. The Kikuchi patterns of grain A and grain B under the same tilt angle are given in (Fig. 1(c, d)). On the basis of the Kikuchi patterns [21], the misorientation matrix of this specific GB is then derived as 0 1 0:808 0:544 0:225 @
0:493 0:834
0:247 A, specifying a normal high angle GB.
0:322 0:088 0:943 The regions rich in Cr (Fig. 1(b) and Fig. 2(e)) correspond to the M23C6 carbide (mainly Cr23C6), based on the selected area diffraction (SAD) patterns (Fig. 2(b, d)). The STEM-EDS mapping for Cr (Fig. 2(e)) identifies 10 particles, marked as M23C6 ①–⑩ resided at the GB. M23C6 precipitates in the austenitic steel follow the cubic-cubic coherent relationship with the matrix. Specifically, Fig. 2(a) exhibits the TEM dark filed (DF) image of h i M23C6 taken from its 112 zone axis of grain A, and the diffraction spot indicated in Fig. 2(b), showing the five M23C6 particles (①–⑤) h i h i that follow 112 // 112 and ð220ÞA //ð220ÞM23 C 6 coherent oriA

M 23 C 6

entation relationship with grain A. Interestingly, these particles possessed relative flat coherent interfaces with grain A, and they all grew into grain B, forming curved incoherent interfaces there. Similar observations can be made by examining the TEM DF image of M23C6 (Fig. 2(c)) taken from its [0 1 1] zone axis of grain B, and

the diffraction spot indicated in Fig. 2(f), showing the five M23C6 particles (⑥–⑩) that were located mostly in grain A and had     ½011B //½011M23 C 6 , 111 // 111 coherent orientation relaB

M 23 C 6

tionship with grain B. Such coherency relationship and precipitate growth morphology is schematically illustrated in Fig. 2(f), that is, the M23C6 precipitates preferably grew into the interior of the grain without coherency, forming incoherent interfaces. In comparison, the regions populated with Al at the same GB (Figs. 1(b) and 3(a)) correspond to the B2-NiAl phase (BCC structure), as evidenced in the SAD pattern (Fig. 3(c)) taken from the [112] zone axis of grain A. The NiAl precipitates displayed a Nishiyama-Wasserman (N-W) coherent orientation relationship h i     with Grain A, i.e., 112 //½101NiAl and 111 // 101 . A

A

NiAl

The Al map in Fig. 3(a) identifies 11 NiAl particles. Fig. 3(b) shows the TEM DF image using the diffraction spot of NiAl in Fig. 3(c), and there are five bright particles. These NiAl particles both coherent with grain A, and they all grow into the coherent grain A, as schematic viewed in Fig. 3(d). In summary, the NiAl precipitates preferably grew into the interior grain with which they are coherent. It is worth mentioning that, these analyses were conducted on four randomly selected GBs and the results were in good agreement with each other. M23C6 and NiAl precipitates demonstrated

Fig. 3. Preferred growth orientation h i of the NiAl phase. (a) STEM-EDS mapping of Al element; (b) TEM DF image using the NiAl diffraction spot in (c); (c) the corresponding SAD pattern of grain A with 112 A//½101NiAl; (d) schematic view of NiAl location at GB.

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different morphologies and covered the GBs completely, which could strongly influence the mechanical and other properties [22]. M23C6 particles are ellipsoids with small wetting angles at GBs, indicating similar GB wetting tendency to other carbides in the steel [23]. The complete wetting of GB by M23C6 precipitate was interrupted by NiAl particles, which had a strong interfacial anisotropy and grew straightly into the matrix. Generally speaking, the precipitate grows preferentially toward the grain where it does not maintain a specific phase relationship [24–26] and the preferential growth direction can be explicated by the mobility of precipitate/matrix interface or the degree of supersaturation [26,27], but that explains only one type of GB precipitate. Recently, some simulation studies [14,15] addressing the coherency maintenance at the precipitate/GB interfaces in terms of the competition between elastic strain energy and interfacial energy. Coherency strains produce an elastic strain energy which is proportional to the volume of the precipitate and scales with the square of the lattice misfit. In the case of M23C6 precipitates in this study (Fig. 2), the measured M23C6/c lattice misfit from SAD pattern is relatively large (~1.6%). while the majority part grew into the incoherent grain. In the case of the NiAl precipitates, however, the precipitates were found to grow into the grain with which they are coherent. Such an apparent discrepancy can be partly attributed to the strong lattice misfit anisotropy with the matrix grain. The lattice mismatch between the (1 1 0)NiAl and the (1 1 1)matrix is close to 0, which lowers the energy cost associated with the coherent interfaces. However, the lattice misfit in the perpendicular direction between the (0 2 0)NiAl and the (2 2 0)matrix is ~15%. The B2-NiAl/matrix interfaces with an N-W orientation relationship actually contain a certain number of misfit dislocations that relax the strain energy, and thus technically belong to complex semicoherent (and even incoherent) interfaces. Hence, there is different location preference of coherent M23C6 and NiAl precipitates formed at the grain boundaries. 4. Conclusion This study provides a neat example demonstrating the variation in the precipitate/matrix coherency at the grain boundaries. In particular, we showed the location preference of coherent M23C6 and NiAl precipitates formed at the grain boundaries, where the NiAl precipitates completely resided in the grain with coherency, while the M23C6 precipitates preferably grew into the interior of the grain without coherency, forming incoherent interfaces. Our study highlights the importance of both mobility and detailed energy landscape for the structural configuration of the precipitate/matrix interfaces at GBs.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Financial supports from the National Key Research and Development Program of China (Grant No. 2016YFB0701405), the National Natural Science Foundation of China (Grant No. 51501114) and the Science and Technology Committee of Shanghai Municipality (Grant No. 16DZ2260602) are gratefully acknowledged. References [1] K. Huang, K. Marthinsen, Q. Zhao, R.E. Logé, Prog. Mater Sci. 92 (2018) 284– 359. [2] K. Zhang, H. Wen, B. Zhao, X. Dong, L. Zhang, Mater. Charact. 155 (2019) 109792. [3] C.S. Smith, Trans AIME. 175 (1948) 15. [4] M. Ashby, J. Harper, J. Lewis, Trans. Met. Soc. AIME. 245 (1969) 413–420. [5] E. Nes, N. Ryum, O. Hunderi, Acta Metall. 33 (1985) 11–22. [6] P.A. Manohar, M. Ferry, T. Chandra, ISIJ Int. 38 (1998) 913–924. [7] A. Harun, E.A. Holm, M.P. Clode, M.A. Miodownik, Acta Mater. 54 (2006) 3261– 3273. [8] V.Y. Novikov, Mater. Lett. 178 (2016) 276–279. [9] J. Zhou, W. Li, B. Zhao, F. Ren, Acta Mater. 148 (2018) 1–8. [10] H.R. Peng, W. Liu, H.Y. Hou, F. Liu, Materialia. 5 (2019) 100225. [11] R.L. Morrison, S.J. Fensin, J.L.W. Carter, Materialia. 7 (2019) 100383. [12] N. Wang, Y. Wen, L.-Q. Chen, Philos. Mag. Lett. 94 (2014) 794–802. [13] N. Wang, Y. Ji, Y. Wang, Y. Wen, L.-Q. Chen, Acta Mater. 135 (2017) 226–232. [14] K. Chang, J. Kwon, C.-K. Rhee, Comput. Mater. Sci. 142 (2018) 297–302. [15] T. Chakrabarti, S. Manna, Comput. Mater. Sci. 154 (2018) 84–90. [16] J. Zhou, S. Zhang, X. Wang, B. Zhao, X. Dong, L. Zhang, Scr. Mater. 116 (2016) 100–103. [17] R.K. Koju, K.A. Darling, K.N. Solanki, Y. Mishin, Acta Mater. 148 (2018) 311– 319. [18] B. Zhao, J. Fan, Y. Liu, L. Zhao, X. Dong, F. Sun, et al., Scr. Mater. 109 (2015) 64– 67. [19] B. Zhao, J. Fan, Z. Chen, X. Dong, F. Sun, L. Zhang, Mater. Charact. 125 (2017) 37–45. [20] B. Zhao, K. Chang, J. Fan, Z. Chen, X. Dong, L. Zhang, Mater. Lett. 176 (2016) 83– 86. [21] C.T. Young, J.H. Steele, J.L. Lytton, Metall. Trans. 4 (1973) 2081–2089. [22] B.B. Straumal, A.A. Mazilkin, S.G. Protasova, S.V. Dobatkin, A.O. Rodin, B. Baretzky, et al., Mater. Sci. Eng., A 503 (2009) 185–189. [23] B.B. Straumal, Y.O. Kucheev, L.I. Efron, A.L. Petelin, J.D. Majumdar, I. Manna, J Mater. Eng. Perform. 21 (2012) 667–670. [24] H.U. Hong, B.S. Rho, S.W. Nam, Mater. Sci. Eng., A 318 (2001) 285–292. [25] C.P. Luo, G.C. Weatherly, Acta Metall. 37 (1989) 791–801. [26] C.S. Smith, Trans. Am. Soc. Metals. 45 (1953) 533–575. [27] D. Lee, H.-U. Jeong, K.-H. Lee, J.B. Jeon, N. Park, Mater. Lett. 250 (2019) 127– 130.

Please cite this article as: H. Wen, B. Zhao, X. Dong et al., Preferential growth of coherent precipitates at grain boundary, Materials Letters, https://doi.org/ 10.1016/j.matlet.2019.126984