- Email: [email protected]
Al composites via in-situ micropillar compression
Journal Pre-proof Unveiling the deformation behavior and strengthening mechanisms of Al3BC/Al composites via in-situ micropillar compression Yongfeng Zhao, Arun Sundar S. Singaravelu, Xia Ma, Qingdong Zhang, Shery L.Y. Chang, Xiangfa Liu, Nikhilesh Chawla PII:
S0925-8388(20)30205-X
DOI:
https://doi.org/10.1016/j.jallcom.2020.153842
Reference:
JALCOM 153842
To appear in:
Journal of Alloys and Compounds
Received Date: 12 November 2019 Revised Date:
6 January 2020
Accepted Date: 13 January 2020
Please cite this article as: Y. Zhao, A.S.S. Singaravelu, X. Ma, Q. Zhang, S.L.Y. Chang, X. Liu, N. Chawla, Unveiling the deformation behavior and strengthening mechanisms of Al3BC/Al composites via in-situ micropillar compression, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.153842. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Y. Zhao – writing – original draft, investigation A.S.S. Singaravelu, X. Ma, Q. Zhang, S.L.Y. Chang – investigation X. Liu, and N. Chawla – supervision, writing – reviewing and editing
Unveiling the deformation behavior and strengthening mechanisms of Al3BC/Al composites via in-situ micropillar compression Yongfeng Zhaoa,b, Arun Sundar S. Singaravelub, Xia Maa, Qingdong Zhangb, Shery L. Y. Changc, Xiangfa Liua, *, and Nikhilesh Chawlab, *
a
Key Laboratory for Liquid–Solid Structural Evolution & Processing of Materials,
Ministry of Education, Shandong University, Jinan 250061, China b
c
Center for 4D Materials Science, Arizona State University, Tempe, AZ 85287, USA
School of Molecular Sciences and Eyring Materials Center, Arizona State University,
Tempe, AZ 85287, USA
* Nikhilesh Chawla Tel.: +1 4809652402; fax: +1 4807279321; E-mail: [email protected]; 501 E. Tyler Mall, ECG 303. Arizona State University. Tempe, AZ 85287-6106, US * Xiangfa Liu Tel.: +86 531 88392006; fax: +86 531 88395414; E-mail: [email protected]; 17923 Jingshi Road, Jinan, Shandong Province, 250061, China
Submitted to: Journal of Alloys and Compounds Nov. 2019
Abstract Al3BC has been proved to be a promising candidate as the reinforcement for Al alloys. However the deformation behavior and strengthening mechanisms remain unclear hitherto. In this work, the deformation behavior and strengthening mechanisms of Al3BC/Al composites were investigated via in-situ micropillar compression. Wrinkled slip bands and slip homogenization were observed in the composites instead of large parallel slip bands in pure Al. The interaction between Al3BC
and
dislocations
was
investigated
in
this
work.
Microstructural
characterization reveals that the high density dislocations and Al3BC-induced ultrafine sub-grains determine its excellent strengthening effects on Al matrix. Keywords: Al3BC;
Al
composites;
micropillar compression;
strengthening
mechanism
1. Introduction Particulate reinforced metal matrix composites (PRMMCs) have been widely applied in many structural and functional parts due to their combination of high specific strength ， high stiffness and isotropic properties, etc. [1-3] Aluminum boron-carbide phases (such as Al3BC3, Al8B4C7, AlB24C4, etc.) in the Al–B–C system have been reported to be promising candidates as reinforcements of Al alloys [4-6]. The Al-rich phase，Al3BC, was discovered in B4C/Al composites by Halverson et al. in 1985 [7]. It has a hexagonal crystal structure with lattice parameters of a= 3.491(2) Å, c= 11.541(4) Å [8]. The crystal structure can be described as close packed Al atoms (layer sequence of ABACBC) with isolated boron atoms placed in all
octahedral voids between layers A and C while isolated carbon atoms occupy trigonal voids in the layer B [9]. One point need to be mentioned is that Al3BC has no B-C covalent bonds, which is different with some other ternary M/B/C compounds, resulting in its combination of high strength like ceramic particles but with more metallic characters [9]. Solozhenko et al. [10] reported its bulk modulus of 116 GPa and its thermal stability up to 1700 K under a pressure of 1.6–4.8 GPa. Wang et. al. [11] theoretically calculated its bulk modulus (152 GPa), shear modulus (140 GPa) and Young’s modulus (E along the a direction is 326 GPa, while E along the c direction is 294 GPa) through a first-principle calculation. With low density, excellent thermal stability and remarkable modulus, Al3BC is expected to be a competitive candidate as the reinforcement for Al composites. Therefore, a novel reinforcement strategy using Al3BC as reinforcement of Al alloys was presented in our previous work [12-14], and an excellent strengthening effect of Al3BC on Al matrix have been verified. However, the deformation behavior of Al3BC during deformation and strengthening mechanisms are still not clearly investigated yet. With the popularity of the focused ion beam (FIB), micropillar compression has been widely used to quantify uniaxial mechanical properties at small-length scales [15-16]. This technique is not only a good way to obtain micro-mechanical properties but also a way to understand the deformation mechanisms of the materials. In this work, the Al3BC/Al composites with Al3BC mass fraction of 25% (Al/Al3BC/25p) were in-situ fabricated. The deformation behavior and strengthening mechanisms of
the composites were investigated and elucidated through an in-situ micropillar compression. 2. Experimental methods Al/Al3BC/25p composites were fabricated in situ through a liquid-solid reaction method at 750oC. Hot extrusion was conducted to make the material fully dense. More details on the synthesis methods have been documented in our previous work [12]. Commercial pure Al was also extruded in the same parameters as a reference sample in this work. No further heat treatment was done after the extrusion. To investigate the micromechanical properties of the composites, micropillars with size of 5×15 µm (Fig. 1c) were milled using a dual-beam focused ion beam (FIB) in a Zeiss Auriga FIB-SEM. For comparison, the micropillar compression tests on commercial pure Al were also conducted. Ex-situ micropillar compression tests were carried out on a nanoindenter XP system (MTS, Agilent Technologies, AZ) but using a flat diamond punch with a square cross-section. A CSM technique was used during the compression tests with nominal strain rate of 10-3 s-1. At least three pillars were tested to calculate the average properties. The in-situ compression tests were conducted using the Femto tools-NMT03 nanomechanical testing system in the Zeiss Auriga FIB-SEM. The pillar size was choose to be 4×8 µm according to the force limit and the size of the tungsten punch with a diameter of 7 µm. The compression process was videoed all the time, however we dwell to take high resolution images with an displacement interval of 0.2 µm. The post-compressed specimens for TEM characterizations were prepared through lift out
method using FIB. The aberration-corrected Titan E-TEM (Thermofisher), operated at 300kV, was used to acquire the data. 3. Results and Discussion The microstructure of the Al/Al3BC/25p composites was first characterized by SEM, as shown in Fig. 1a-b. The nano-scale platelet Al3BC particles uniformly distribute in the Al matrix, so the particles distribute both inside the α-Al grains and in the grain boundary area. Fig. 1d-f shows a representative stress-strain curve and microstructures of one post-compressed pillar of pure Al. Some large strain burst phenomena are observed on the stress-strain curve during plastic deformation (Fig. 1d), which can be attributed to the occurrence of slip bands during deformation. The large parallel slip bands observed on the post-compressed pillar (Fig. 1e-f) also verify this point. The number of slip bands on the pillar is the same with the number of strain bursts on the stress-strain curve. Without any reinforcements inside pure Al, the slip band propagates rapidly without any obstacle once the applied shear stress reach the critical resolved shear stress, thereby resulting in the large strain bursts on the stress-strain curve. A similar phenomenon has been also reported in some other Al alloys [16-17]. With the incorporation of Al3BC particles, the deformation behavior of the composite pillar is totally changed. Fig. 1g displays one representative stress-strain curve of the Al/Al3BC/25p composites. The compression strength of the composites is significantly improved to 773±11 MPa, about 14 times of that of pure Al (56±4 MPa). Excellent strengthening effects of Al3BC on Al matrix is verified. The strain burst
steps on the stress-strain curve are smaller and the strain at which the strain burst initiates is larger than that of pure Al. This can be attributed to the impeding effects of Al3BC on the initiation and propagation of slip bands. Al3BC particles are hard to be deformed for their strong elastic and strength, which will act as obstacles to the dislocation motion and delay the initiation of slip events. As a result, the initial strain of slip bands increases after the incorporation of Al3BC particles. Another point worth to note is that the strain hardening rate of the composites improve obviously in comparison with the pure Al, which can be attributed to the high dislocation density caused by Al3BC.
The microstructure of one post-compressed pillar of
Al/Al3BC/25p composite is shown in Fig. 1h-i. Unlike the large parallel slip bands in the pure Al, some wrinkles which actually can be regarded as tortuous slip bands homogenously distributed on the whole pillar surface, indicating a more uniform deformation.
Fig. 1 Microstructure and micropillar compression tests: (a-b) microstructure of Al/Al3BC/25p composites; (c) morphology of one representative pillar of Al/Al3BC/25p composites before compression; (d-f) compression stress-strain curve and post-compressed pillar of pure Al; (g-i) compression stress-strain curve and post-compressed pillar of Al/Al3BC/25p composites In-situ micropillar compression tests on the Al/Al3BC/25p composite was conducted to further investigate how the wrinkled slip bands form and how the Al3BC particles contribute to the slip homogenization. Fig. 2a shows the force-displacement curve during the in-situ compression and the period force decrease is caused by the dwell to take an image. Fig. 2b-f shows the morphologies of the pillar with a variety of displacement. During the elastic deformation stage, nothing special happens. After 1.3 µm displacement, which is above the yield point, some slip behavior initiate on the surface of the pillar as shown in Fig. 2c. With a further increase of the displacement, more and more wrinkled slip bands occur (Fig. 2d-f, Supplementary
video). Comparatively speaking, the pillar of the composites was deformed more uniformly than the pure Al, indicating a slip homogenization effect caused by Al3BC. Fig. 2g clearly shows the wrinkled slip bands on the pillar surface. This wrinkled slip behavior is actually caused by the interaction between slip bands (dislocations) and Al3BC particles. When the slip bands (dislocations) propagate to Al3BC particles, they cannot shear the Al3BC particles and leave behind steps, that is, they have to bypass the particle and cause tortuous slip bands, finally resulting in the formation of the wrinkles described above. The interaction between Al3BC and dislocations during compression has been verified in the following TEM analyses (Fig. 3). The slip homogenization caused by Al3BC can be attributed into the following reasons. At the yield point, the sample usually tends to slip along the weakest planes, but the strong Al3BC nanoparticles can effectively block a further slip along those weak planes, thereby preventing a localized deformation along weak atomic planes and enabling the activation of slips along other atomic planes. In addition, as the inter-particle spacing gets smaller, there are less regions for the dislocations to move to. Thus, it is more likely to get large homogenous networks of dislocations and the slips become more homogenized [18-19]. Moreover, the α-Al grain structure will also contribute to the slip homogenization. Unlike the pillar of the pure Al matrix alloy which is single crystal (inset of Fig. 1d), the pillars of the composites are polycrystalline (inset of Fig. 1g). With the incorporation of Al3BC, the nucleation of α-Al grains is facilitated and the growth of them is retards by impeding the grain boundary movement during dynamic recrystallization in the extrusion process [20-21],
and
Fig. 2 In situ compression test on the Al/Al3BC/25p: (a) Force-displacement curve; (b-f) morphologies of the pillar with a variety of displacement; (g) high magnification images of the pillar after compression；(h-i) FIB cross-section of the post-compressed pillar; (j) a schematic showing the slip behavior of Al/Al3BC/25p composites.
hence the α-Al grain of the composites is significantly refined. For the polycrystalline Al, the slip behavior is separated into different grains and results in a more uniform deformation. Fig. 2h-i demonstrates the FIB cross-section of the pillar and no voids, interface debonding or cracked particles were observed, which also indicates a homogeneous deformation happening in this material. Fig. 2j shows the schematic which illustrates the slip behavior in Al/Al3BC/25p composites. Both the impeding
effects of Al3BC particles and the polycrystalline structure contribute to its slip homogenization.
Fig. 3 TEM analysis on the post-compressed pillar of Al/Al3BC/25p composites: (a) low magnification bright field image; (b) high magnification images of the area in (a), marked by the blue rectangle; (c) schematic showing the dislocation behavior with Al3BC particle in (b); (d) high magnification images of the area in (a), marked by the red rectangle; (e) SAED of the circled area in (d), which shows the crystallographic orientation of the α-Al; (f) Schematic showing the interaction between dislocations and Al3BC particle in (d).
To further verify how the Al3BC interact with the slip behavior of dislocations, TEM analyses on the post-compressed pillar of the composites were carried out. Fig. 3 displays the TEM results of Al/Al3BC/25p composites after 20% compression. Fig. 3a shows the low magnification bright field image of the post-compressed pillar. Nano Al3BC particles distribute inside Al grains and on the grain boundaries can be both observed. Large amount of dislocations in the matrix interact with Al3BC particles were observed. Numerous investigations have reported that the dislocation
density increase a lot with the incorporation of reinforcements [22-24]. The dislocation density in Al3BC/Al composites was also obviously increased, which has been verified in our previous work [13]. The dislocations with Burgers vector parallel to the slip direction will be impeded by other dislocations whose Burgers vector don’t parallel to the slip direction and form the dislocation tangles, thereby enhancing the strength of the composites. This can also explain the higher strain hardening rate in the Al3BC/Al composites (Fig. 1g). Some investigations [25-26] have revealed that higher strain hardening will also contribute to the uniform deformation of the material. Fig. 3b shows the high magnification image of the blue rectangle area in Fig. 3a. Selected area electric diffraction (SAED) was conducted and the [110] direction was determined to be the arrow direction. For FCC Al, the slip plane is {111} and slip direction is <110> [27]. The dislocations slip along the [110] direction and are then obstructed by the Al3BC particles. For further movement of dislocations, they will get curved, detour the particle and finally leave loops around the particles (Fig. 3c). Fig. 3d displays the high magnification image of another rectangle area in Fig. 3a, in which the dislocation lines are perpendicular to the paper surface and pile up when slip to the Al3BC particle. Here the α-Al was tilted to [111] zone axis as detected by SAED (Fig. 3e). The dislocations slip along the [101] direction and pile up around Al3BC. The pileup of dislocation will result in a stress concentration and more applied stress is need for further movement of the dislocations. For further slip, dislocation climb which requires more energy happens to get around the Al3BC particle (Fig. 3f).
One point need to be noted is that the diffraction spots in Fig. 3e is arced and some spots even separate into several small spots, which indicates the formation of sub-grain.
Fig. 4 Formation of sub-grains during compression in Al3BC/Al composites: (a) Bright field images of one Al grain (circled by dashed line) in Al/Al3BC/25p composite after compression; (b-c) SAED patterns obtained from the circled area in (a); (d) Schematic shown the formation of sub-grains in Al/Al3BC/25p.
As mentioned above, some clues which indicate the formation of sub-grains have been observed in the SAED patterns shown in Fig. 3e. More analyses have been done to further verify the formation of sub-grains caused by the incorporation of Al3BC during compression progress. Fig. 4a-c shows the bright field image and related
SAED patterns of the post-compressed Al/Al3BC/25p pillar, respectively. Through the EBSD result as shown in Fig. 1g, the grain size of α-Al in extruded Al/Al3BC/25p composites is refined to about 550 nm. Fig. 4a displays the bright field image of one α-Al grain of Al/Al3BC/25p composite after compression. Large amount of dislocations were observed and they tend to align with each other to form some substructures. SAED was conducted as shown in Fig. 3b to certify the orientation of the α-Al grain. It’ clearly shows that the diffraction spots break down into several smaller spots, indicating that a grain has broken down into sub-grains, which have smaller orientation difference. Fig. 4c shows the SAED patterns getting from smaller area as shown in Fig. 4a. They both have similar zone axis around [112], but the diffraction spots have very small difference, further indicating the formation of sub-grains. The sub-grains are formed in a process called polygonization cause by the incorporation of Al3BC. Because the pinning effect of Al3BC, the dislocations pile up around the particle and align with each other and form some dislocation cell wall. With the increase of dislocation density, these cell walls will finally transform into sub-grain boundaries (Fig. 4d). There exist both submicron α-Al grains and sub-grains caused by the incorporation of Al3BC. The nano-scale sub-grains are another important aspect contributing to the high strength of the composites. 4. Conclusions In summary, Al3BC/Al composites were fabricated and the deformation behavior as well as strengthening mechanisms of the composites was investigated via in-situ micropillar compression. With the addition of Al3BC, wrinkled slip bands instead of
large parallel local slip bands are observed and more uniform deformation caused by slip homogenization happens during micropillar compression. The interaction between dislocations and Al3BC particles during compression were investigated in this work. Microstructural characterizations reveal that the high density dislocations and ultrafine sub-grains induced by Al3BC determine its excellent strengthening effects on Al matrix. Acknowledgements The work was financially supported by the National Natural Science Foundation of China (No. 51731007) and the Key Foundation of Shandong Province (No. ZR2016QZ005). We also acknowledge the Center for 4D Materials Science at Arizona State University for support of this research work. We acknowledge the use of facilities within the Eyring Materials Center at Arizona State University supported in part by NNCI-ECCS-1542160. References [1] A. Mortensen, J. Llorca, Metal matrix composites, Annu. Rev. Mater. Res. 40 (2010) 243-270. [2] I.A. Ibrahim, F.A. Mohamed, E.J. Lavernia, Particulate reinforced metal matrix composites-a review, J. Mater. Sci. 26 (1991) 1137-1156. [3] C. Suryanarayana, N. Al-Aqeeli, Mechanically alloyed nanocomposites, Prog. Mater. Sci. 58 (2013) 383–502. [4] Y.F. Gao, Z.H. Huang, M.H. Fang, Y.G. Liu, S.F. Huang, X. Ouyang, Synthesis of Al8B4C7 ceramic powder from Al/B4C/C mixtures, Powder Technol. 226 (2012) 269-273. [5] S.H. Lee, H.D. Kim, S.C. Choi, T. Nishimra, J.S. Lee, H. Tanaka, Chemical composition and microstructure of Al3BC3 prepared by different densification
methods, J. Eur. Ceram. Soc. 30 (2010) 1015-1020. [6] A.J. Pyzik, D.R. Beaman, Al-B-C phase development and effects on mechanical properties of B4C/Al-derived composites, J. Am. Ceram. Soc. 78 (1995) 305-312. [7] D.C. Halverson, A.J. Pyzik, I.A. Aksay, Processing and Microstructural Characterization of B4C-Al Cermets, Ceram. Eng. Sci. Proc. 16 (1985) 736-744. [8] J.C. Viala, G. Gonzales, J. Bouix, Composition and lattice parameters of a new aluminium-rich borocarbide, J. Mater. Sci. Lett. 11 (1992) 711-714. [9] F.D. Meyer, H. Hillebrecht, Synthesis and crystal structure of Al3BC, the first boridecarbide of aluminum, J. Alloys Compd. 252 (1997) 98-102. [10] V.L. Solozhenko, E.G. Solozhenko, C. Lathe, Equation of state and thermal stability of Al3BC, Solid State Commu.137 (2006) 533-535. [11] J.Y. Wang, Y.C. Zhou, T. Liao, Z.J. Lin, First-principles prediction of low shear-strain resistance of Al3BC3: A metal borocarbide containing short linear BC2 units, Appl. Phys. Lett. 89 (2006) 0219171-0219173. [12] Y.F. Zhao, Z. Qian, X.F. Liu, Identification of novel dual-scale Al3BC particles in Al based composites, Mater. Design 93 (2016) 283-290. [13] Y.F. Zhao, X. Ma, H.W. Chen, X.J. Zhao, X.F. Liu, Preferred orientation and interfacial structure in extruded nano-Al3BC/6061 Al, Mater. Design 131 (2017) 23-31. [14] Y.F. Zhao, Z. Qian, X. Ma, H.W. Chen, T. Gao, Y.Y Wu, X.F. Liu, Unveiling the semicoherent interface with definite orientation relationships between reinforcements and matrix in novel Al3BC/Al composites, ACS Appl. Mater. Interfaces 8 (2016), 28194-28201. [15] C.R. Mayer, L.W. Yang , S.S. Singh, J. Llorca, J.M. Molina-Aldareguia, Y.L. Shen, N. Chawla, Anisotropy, size, and aspect ratio effects on micropillar compression of Al-SiC nanolaminate composites, Acta Mater. 114 (2016) 25-32. [16] C. S. Kaira, C. Kantzos , J.J. Williams, V. D. Andrade , F. D. Carlo, N. Chawla, Microstructural evolution and deformation behavior of Al-Cu alloys: A Transmission X-ray Microscopy (TXM) and micropillar compression study, Acta Mater. 144 (2018) 419-431.
[17] K.S. Ng, A.H.W. Ngan, Stochastic nature of plasticity of aluminum micro-pillars, Acta Mater. 56 (2008) 1712-1720. [18] L.Y. Che, J.Q. Xu, H. Choi, M. Pozuelo, X.L. Ma, S. Bhowmick, J.M. Yang, S. Mathaudhu, Processing and properties of magnesium containing a dense uniform dispersion of nanoparticles, Nature 528 (2015) 539-543. [19] N. Chawla, C. Andres, J.W. Jones, J.E. Allison, Effect of SiC volume fraction and particle size on the fatigue resistance of a 2080 Al/SiCp composite, Metall. Mater. Trans. A 29 (1998) 2843-2854. [20] L. Wang, F. Qiu, Q.L. Zhao, H.Y. Wang, Q.C. Jiang, Simultaneously increasing the elevated-temperature tensile strength and plasticity of in situ nano-sized TiCx/Al-Cu-Mg composites, Mater. Charact. 125 (2017) 7-12. [21] Y.F. Li, J.T. Guo, L.Z. Zhou, H.Q. Ye, Effect of recrystallization on room-temperature mechanical properties of Zr-doped Ni3Al alloy, Mater. Lett.58 (2004) 1853-1856. [22] R.J. Arsenault, R.M. Fisher, Microstructure of fiber and particulate SiC in 6061 A1 composite, Scripta Metall. 17 (1983) 67-71. [23] M. Vogelsang, R.J. Arsenault, R.M. Fisher, An in situ HVEM study of dislocation generation at Al/SiC interfaces in metal matrix composites, Metall. Trans. A 17A (1986) 379-389. [24] R.J. Arsenault, N. Shi, Dislocation generation due to differences between the coefficients of thermal expansion, Mater. Sci. Eng. 81 (1986) 175-187. [25] G. Liu, G.J. Zhang, F. Jiang, X.D. Ding, Y.J. Sun, J. Sun1, E. Ma, Nanostructured high-strength molybdenum alloys with unprecedented tensile ductility, Nat. Mater. 12 (2013) 344-350. [26] I.A. Ovid’ko, R.Z. Valiev, Y.T. Zhu, Review on superior strength and enhanced ductility of metallic nanomaterials, Prog. Mater. Sci. 94 (2018) 462-540.
[27] Y. Cao, S. Ni, X.Z. Liao, M. Song, Y.T. Zhu, Structural evolutions of metallic materials processed by severe plastic deformation, Mater. Sci. Eng. R 133 (2018) 1-59.
Highlights: Micromechanical behavior of Al3BC/Al composites was investigated. Wrinkled slip bands and slip homogenization were observed and elucidated. Dislocation behavior and ultrafine sub-grains induced by Al3BC were unveiled
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(20)30205-X
DOI:
https://doi.org/10.1016/j.jallcom.2020.153842
Reference:
JALCOM 153842
To appear in:
Journal of Alloys and Compounds
Received Date: 12 November 2019 Revised Date:
6 January 2020
Accepted Date: 13 January 2020
Please cite this article as: Y. Zhao, A.S.S. Singaravelu, X. Ma, Q. Zhang, S.L.Y. Chang, X. Liu, N. Chawla, Unveiling the deformation behavior and strengthening mechanisms of Al3BC/Al composites via in-situ micropillar compression, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.153842. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Y. Zhao – writing – original draft, investigation A.S.S. Singaravelu, X. Ma, Q. Zhang, S.L.Y. Chang – investigation X. Liu, and N. Chawla – supervision, writing – reviewing and editing
Unveiling the deformation behavior and strengthening mechanisms of Al3BC/Al composites via in-situ micropillar compression Yongfeng Zhaoa,b, Arun Sundar S. Singaravelub, Xia Maa, Qingdong Zhangb, Shery L. Y. Changc, Xiangfa Liua, *, and Nikhilesh Chawlab, *
a
Key Laboratory for Liquid–Solid Structural Evolution & Processing of Materials,
Ministry of Education, Shandong University, Jinan 250061, China b
c
Center for 4D Materials Science, Arizona State University, Tempe, AZ 85287, USA
School of Molecular Sciences and Eyring Materials Center, Arizona State University,
Tempe, AZ 85287, USA
* Nikhilesh Chawla Tel.: +1 4809652402; fax: +1 4807279321; E-mail: [email protected]; 501 E. Tyler Mall, ECG 303. Arizona State University. Tempe, AZ 85287-6106, US * Xiangfa Liu Tel.: +86 531 88392006; fax: +86 531 88395414; E-mail: [email protected]; 17923 Jingshi Road, Jinan, Shandong Province, 250061, China
Submitted to: Journal of Alloys and Compounds Nov. 2019
Abstract Al3BC has been proved to be a promising candidate as the reinforcement for Al alloys. However the deformation behavior and strengthening mechanisms remain unclear hitherto. In this work, the deformation behavior and strengthening mechanisms of Al3BC/Al composites were investigated via in-situ micropillar compression. Wrinkled slip bands and slip homogenization were observed in the composites instead of large parallel slip bands in pure Al. The interaction between Al3BC
and
dislocations
was
investigated
in
this
work.
Microstructural
characterization reveals that the high density dislocations and Al3BC-induced ultrafine sub-grains determine its excellent strengthening effects on Al matrix. Keywords: Al3BC;
Al
composites;
micropillar compression;
strengthening
mechanism
1. Introduction Particulate reinforced metal matrix composites (PRMMCs) have been widely applied in many structural and functional parts due to their combination of high specific strength ， high stiffness and isotropic properties, etc. [1-3] Aluminum boron-carbide phases (such as Al3BC3, Al8B4C7, AlB24C4, etc.) in the Al–B–C system have been reported to be promising candidates as reinforcements of Al alloys [4-6]. The Al-rich phase，Al3BC, was discovered in B4C/Al composites by Halverson et al. in 1985 [7]. It has a hexagonal crystal structure with lattice parameters of a= 3.491(2) Å, c= 11.541(4) Å [8]. The crystal structure can be described as close packed Al atoms (layer sequence of ABACBC) with isolated boron atoms placed in all
octahedral voids between layers A and C while isolated carbon atoms occupy trigonal voids in the layer B [9]. One point need to be mentioned is that Al3BC has no B-C covalent bonds, which is different with some other ternary M/B/C compounds, resulting in its combination of high strength like ceramic particles but with more metallic characters [9]. Solozhenko et al. [10] reported its bulk modulus of 116 GPa and its thermal stability up to 1700 K under a pressure of 1.6–4.8 GPa. Wang et. al. [11] theoretically calculated its bulk modulus (152 GPa), shear modulus (140 GPa) and Young’s modulus (E along the a direction is 326 GPa, while E along the c direction is 294 GPa) through a first-principle calculation. With low density, excellent thermal stability and remarkable modulus, Al3BC is expected to be a competitive candidate as the reinforcement for Al composites. Therefore, a novel reinforcement strategy using Al3BC as reinforcement of Al alloys was presented in our previous work [12-14], and an excellent strengthening effect of Al3BC on Al matrix have been verified. However, the deformation behavior of Al3BC during deformation and strengthening mechanisms are still not clearly investigated yet. With the popularity of the focused ion beam (FIB), micropillar compression has been widely used to quantify uniaxial mechanical properties at small-length scales [15-16]. This technique is not only a good way to obtain micro-mechanical properties but also a way to understand the deformation mechanisms of the materials. In this work, the Al3BC/Al composites with Al3BC mass fraction of 25% (Al/Al3BC/25p) were in-situ fabricated. The deformation behavior and strengthening mechanisms of
the composites were investigated and elucidated through an in-situ micropillar compression. 2. Experimental methods Al/Al3BC/25p composites were fabricated in situ through a liquid-solid reaction method at 750oC. Hot extrusion was conducted to make the material fully dense. More details on the synthesis methods have been documented in our previous work [12]. Commercial pure Al was also extruded in the same parameters as a reference sample in this work. No further heat treatment was done after the extrusion. To investigate the micromechanical properties of the composites, micropillars with size of 5×15 µm (Fig. 1c) were milled using a dual-beam focused ion beam (FIB) in a Zeiss Auriga FIB-SEM. For comparison, the micropillar compression tests on commercial pure Al were also conducted. Ex-situ micropillar compression tests were carried out on a nanoindenter XP system (MTS, Agilent Technologies, AZ) but using a flat diamond punch with a square cross-section. A CSM technique was used during the compression tests with nominal strain rate of 10-3 s-1. At least three pillars were tested to calculate the average properties. The in-situ compression tests were conducted using the Femto tools-NMT03 nanomechanical testing system in the Zeiss Auriga FIB-SEM. The pillar size was choose to be 4×8 µm according to the force limit and the size of the tungsten punch with a diameter of 7 µm. The compression process was videoed all the time, however we dwell to take high resolution images with an displacement interval of 0.2 µm. The post-compressed specimens for TEM characterizations were prepared through lift out
method using FIB. The aberration-corrected Titan E-TEM (Thermofisher), operated at 300kV, was used to acquire the data. 3. Results and Discussion The microstructure of the Al/Al3BC/25p composites was first characterized by SEM, as shown in Fig. 1a-b. The nano-scale platelet Al3BC particles uniformly distribute in the Al matrix, so the particles distribute both inside the α-Al grains and in the grain boundary area. Fig. 1d-f shows a representative stress-strain curve and microstructures of one post-compressed pillar of pure Al. Some large strain burst phenomena are observed on the stress-strain curve during plastic deformation (Fig. 1d), which can be attributed to the occurrence of slip bands during deformation. The large parallel slip bands observed on the post-compressed pillar (Fig. 1e-f) also verify this point. The number of slip bands on the pillar is the same with the number of strain bursts on the stress-strain curve. Without any reinforcements inside pure Al, the slip band propagates rapidly without any obstacle once the applied shear stress reach the critical resolved shear stress, thereby resulting in the large strain bursts on the stress-strain curve. A similar phenomenon has been also reported in some other Al alloys [16-17]. With the incorporation of Al3BC particles, the deformation behavior of the composite pillar is totally changed. Fig. 1g displays one representative stress-strain curve of the Al/Al3BC/25p composites. The compression strength of the composites is significantly improved to 773±11 MPa, about 14 times of that of pure Al (56±4 MPa). Excellent strengthening effects of Al3BC on Al matrix is verified. The strain burst
steps on the stress-strain curve are smaller and the strain at which the strain burst initiates is larger than that of pure Al. This can be attributed to the impeding effects of Al3BC on the initiation and propagation of slip bands. Al3BC particles are hard to be deformed for their strong elastic and strength, which will act as obstacles to the dislocation motion and delay the initiation of slip events. As a result, the initial strain of slip bands increases after the incorporation of Al3BC particles. Another point worth to note is that the strain hardening rate of the composites improve obviously in comparison with the pure Al, which can be attributed to the high dislocation density caused by Al3BC.
The microstructure of one post-compressed pillar of
Al/Al3BC/25p composite is shown in Fig. 1h-i. Unlike the large parallel slip bands in the pure Al, some wrinkles which actually can be regarded as tortuous slip bands homogenously distributed on the whole pillar surface, indicating a more uniform deformation.
Fig. 1 Microstructure and micropillar compression tests: (a-b) microstructure of Al/Al3BC/25p composites; (c) morphology of one representative pillar of Al/Al3BC/25p composites before compression; (d-f) compression stress-strain curve and post-compressed pillar of pure Al; (g-i) compression stress-strain curve and post-compressed pillar of Al/Al3BC/25p composites In-situ micropillar compression tests on the Al/Al3BC/25p composite was conducted to further investigate how the wrinkled slip bands form and how the Al3BC particles contribute to the slip homogenization. Fig. 2a shows the force-displacement curve during the in-situ compression and the period force decrease is caused by the dwell to take an image. Fig. 2b-f shows the morphologies of the pillar with a variety of displacement. During the elastic deformation stage, nothing special happens. After 1.3 µm displacement, which is above the yield point, some slip behavior initiate on the surface of the pillar as shown in Fig. 2c. With a further increase of the displacement, more and more wrinkled slip bands occur (Fig. 2d-f, Supplementary
video). Comparatively speaking, the pillar of the composites was deformed more uniformly than the pure Al, indicating a slip homogenization effect caused by Al3BC. Fig. 2g clearly shows the wrinkled slip bands on the pillar surface. This wrinkled slip behavior is actually caused by the interaction between slip bands (dislocations) and Al3BC particles. When the slip bands (dislocations) propagate to Al3BC particles, they cannot shear the Al3BC particles and leave behind steps, that is, they have to bypass the particle and cause tortuous slip bands, finally resulting in the formation of the wrinkles described above. The interaction between Al3BC and dislocations during compression has been verified in the following TEM analyses (Fig. 3). The slip homogenization caused by Al3BC can be attributed into the following reasons. At the yield point, the sample usually tends to slip along the weakest planes, but the strong Al3BC nanoparticles can effectively block a further slip along those weak planes, thereby preventing a localized deformation along weak atomic planes and enabling the activation of slips along other atomic planes. In addition, as the inter-particle spacing gets smaller, there are less regions for the dislocations to move to. Thus, it is more likely to get large homogenous networks of dislocations and the slips become more homogenized [18-19]. Moreover, the α-Al grain structure will also contribute to the slip homogenization. Unlike the pillar of the pure Al matrix alloy which is single crystal (inset of Fig. 1d), the pillars of the composites are polycrystalline (inset of Fig. 1g). With the incorporation of Al3BC, the nucleation of α-Al grains is facilitated and the growth of them is retards by impeding the grain boundary movement during dynamic recrystallization in the extrusion process [20-21],
and
Fig. 2 In situ compression test on the Al/Al3BC/25p: (a) Force-displacement curve; (b-f) morphologies of the pillar with a variety of displacement; (g) high magnification images of the pillar after compression；(h-i) FIB cross-section of the post-compressed pillar; (j) a schematic showing the slip behavior of Al/Al3BC/25p composites.
hence the α-Al grain of the composites is significantly refined. For the polycrystalline Al, the slip behavior is separated into different grains and results in a more uniform deformation. Fig. 2h-i demonstrates the FIB cross-section of the pillar and no voids, interface debonding or cracked particles were observed, which also indicates a homogeneous deformation happening in this material. Fig. 2j shows the schematic which illustrates the slip behavior in Al/Al3BC/25p composites. Both the impeding
effects of Al3BC particles and the polycrystalline structure contribute to its slip homogenization.
Fig. 3 TEM analysis on the post-compressed pillar of Al/Al3BC/25p composites: (a) low magnification bright field image; (b) high magnification images of the area in (a), marked by the blue rectangle; (c) schematic showing the dislocation behavior with Al3BC particle in (b); (d) high magnification images of the area in (a), marked by the red rectangle; (e) SAED of the circled area in (d), which shows the crystallographic orientation of the α-Al; (f) Schematic showing the interaction between dislocations and Al3BC particle in (d).
To further verify how the Al3BC interact with the slip behavior of dislocations, TEM analyses on the post-compressed pillar of the composites were carried out. Fig. 3 displays the TEM results of Al/Al3BC/25p composites after 20% compression. Fig. 3a shows the low magnification bright field image of the post-compressed pillar. Nano Al3BC particles distribute inside Al grains and on the grain boundaries can be both observed. Large amount of dislocations in the matrix interact with Al3BC particles were observed. Numerous investigations have reported that the dislocation
density increase a lot with the incorporation of reinforcements [22-24]. The dislocation density in Al3BC/Al composites was also obviously increased, which has been verified in our previous work [13]. The dislocations with Burgers vector parallel to the slip direction will be impeded by other dislocations whose Burgers vector don’t parallel to the slip direction and form the dislocation tangles, thereby enhancing the strength of the composites. This can also explain the higher strain hardening rate in the Al3BC/Al composites (Fig. 1g). Some investigations [25-26] have revealed that higher strain hardening will also contribute to the uniform deformation of the material. Fig. 3b shows the high magnification image of the blue rectangle area in Fig. 3a. Selected area electric diffraction (SAED) was conducted and the [110] direction was determined to be the arrow direction. For FCC Al, the slip plane is {111} and slip direction is <110> [27]. The dislocations slip along the [110] direction and are then obstructed by the Al3BC particles. For further movement of dislocations, they will get curved, detour the particle and finally leave loops around the particles (Fig. 3c). Fig. 3d displays the high magnification image of another rectangle area in Fig. 3a, in which the dislocation lines are perpendicular to the paper surface and pile up when slip to the Al3BC particle. Here the α-Al was tilted to [111] zone axis as detected by SAED (Fig. 3e). The dislocations slip along the [101] direction and pile up around Al3BC. The pileup of dislocation will result in a stress concentration and more applied stress is need for further movement of the dislocations. For further slip, dislocation climb which requires more energy happens to get around the Al3BC particle (Fig. 3f).
One point need to be noted is that the diffraction spots in Fig. 3e is arced and some spots even separate into several small spots, which indicates the formation of sub-grain.
Fig. 4 Formation of sub-grains during compression in Al3BC/Al composites: (a) Bright field images of one Al grain (circled by dashed line) in Al/Al3BC/25p composite after compression; (b-c) SAED patterns obtained from the circled area in (a); (d) Schematic shown the formation of sub-grains in Al/Al3BC/25p.
As mentioned above, some clues which indicate the formation of sub-grains have been observed in the SAED patterns shown in Fig. 3e. More analyses have been done to further verify the formation of sub-grains caused by the incorporation of Al3BC during compression progress. Fig. 4a-c shows the bright field image and related
SAED patterns of the post-compressed Al/Al3BC/25p pillar, respectively. Through the EBSD result as shown in Fig. 1g, the grain size of α-Al in extruded Al/Al3BC/25p composites is refined to about 550 nm. Fig. 4a displays the bright field image of one α-Al grain of Al/Al3BC/25p composite after compression. Large amount of dislocations were observed and they tend to align with each other to form some substructures. SAED was conducted as shown in Fig. 3b to certify the orientation of the α-Al grain. It’ clearly shows that the diffraction spots break down into several smaller spots, indicating that a grain has broken down into sub-grains, which have smaller orientation difference. Fig. 4c shows the SAED patterns getting from smaller area as shown in Fig. 4a. They both have similar zone axis around [112], but the diffraction spots have very small difference, further indicating the formation of sub-grains. The sub-grains are formed in a process called polygonization cause by the incorporation of Al3BC. Because the pinning effect of Al3BC, the dislocations pile up around the particle and align with each other and form some dislocation cell wall. With the increase of dislocation density, these cell walls will finally transform into sub-grain boundaries (Fig. 4d). There exist both submicron α-Al grains and sub-grains caused by the incorporation of Al3BC. The nano-scale sub-grains are another important aspect contributing to the high strength of the composites. 4. Conclusions In summary, Al3BC/Al composites were fabricated and the deformation behavior as well as strengthening mechanisms of the composites was investigated via in-situ micropillar compression. With the addition of Al3BC, wrinkled slip bands instead of
large parallel local slip bands are observed and more uniform deformation caused by slip homogenization happens during micropillar compression. The interaction between dislocations and Al3BC particles during compression were investigated in this work. Microstructural characterizations reveal that the high density dislocations and ultrafine sub-grains induced by Al3BC determine its excellent strengthening effects on Al matrix. Acknowledgements The work was financially supported by the National Natural Science Foundation of China (No. 51731007) and the Key Foundation of Shandong Province (No. ZR2016QZ005). We also acknowledge the Center for 4D Materials Science at Arizona State University for support of this research work. We acknowledge the use of facilities within the Eyring Materials Center at Arizona State University supported in part by NNCI-ECCS-1542160. References [1] A. Mortensen, J. Llorca, Metal matrix composites, Annu. Rev. Mater. Res. 40 (2010) 243-270. [2] I.A. Ibrahim, F.A. Mohamed, E.J. Lavernia, Particulate reinforced metal matrix composites-a review, J. Mater. Sci. 26 (1991) 1137-1156. [3] C. Suryanarayana, N. Al-Aqeeli, Mechanically alloyed nanocomposites, Prog. Mater. Sci. 58 (2013) 383–502. [4] Y.F. Gao, Z.H. Huang, M.H. Fang, Y.G. Liu, S.F. Huang, X. Ouyang, Synthesis of Al8B4C7 ceramic powder from Al/B4C/C mixtures, Powder Technol. 226 (2012) 269-273. [5] S.H. Lee, H.D. Kim, S.C. Choi, T. Nishimra, J.S. Lee, H. Tanaka, Chemical composition and microstructure of Al3BC3 prepared by different densification
methods, J. Eur. Ceram. Soc. 30 (2010) 1015-1020. [6] A.J. Pyzik, D.R. Beaman, Al-B-C phase development and effects on mechanical properties of B4C/Al-derived composites, J. Am. Ceram. Soc. 78 (1995) 305-312. [7] D.C. Halverson, A.J. Pyzik, I.A. Aksay, Processing and Microstructural Characterization of B4C-Al Cermets, Ceram. Eng. Sci. Proc. 16 (1985) 736-744. [8] J.C. Viala, G. Gonzales, J. Bouix, Composition and lattice parameters of a new aluminium-rich borocarbide, J. Mater. Sci. Lett. 11 (1992) 711-714. [9] F.D. Meyer, H. Hillebrecht, Synthesis and crystal structure of Al3BC, the first boridecarbide of aluminum, J. Alloys Compd. 252 (1997) 98-102. [10] V.L. Solozhenko, E.G. Solozhenko, C. Lathe, Equation of state and thermal stability of Al3BC, Solid State Commu.137 (2006) 533-535. [11] J.Y. Wang, Y.C. Zhou, T. Liao, Z.J. Lin, First-principles prediction of low shear-strain resistance of Al3BC3: A metal borocarbide containing short linear BC2 units, Appl. Phys. Lett. 89 (2006) 0219171-0219173. [12] Y.F. Zhao, Z. Qian, X.F. Liu, Identification of novel dual-scale Al3BC particles in Al based composites, Mater. Design 93 (2016) 283-290. [13] Y.F. Zhao, X. Ma, H.W. Chen, X.J. Zhao, X.F. Liu, Preferred orientation and interfacial structure in extruded nano-Al3BC/6061 Al, Mater. Design 131 (2017) 23-31. [14] Y.F. Zhao, Z. Qian, X. Ma, H.W. Chen, T. Gao, Y.Y Wu, X.F. Liu, Unveiling the semicoherent interface with definite orientation relationships between reinforcements and matrix in novel Al3BC/Al composites, ACS Appl. Mater. Interfaces 8 (2016), 28194-28201. [15] C.R. Mayer, L.W. Yang , S.S. Singh, J. Llorca, J.M. Molina-Aldareguia, Y.L. Shen, N. Chawla, Anisotropy, size, and aspect ratio effects on micropillar compression of Al-SiC nanolaminate composites, Acta Mater. 114 (2016) 25-32. [16] C. S. Kaira, C. Kantzos , J.J. Williams, V. D. Andrade , F. D. Carlo, N. Chawla, Microstructural evolution and deformation behavior of Al-Cu alloys: A Transmission X-ray Microscopy (TXM) and micropillar compression study, Acta Mater. 144 (2018) 419-431.
[17] K.S. Ng, A.H.W. Ngan, Stochastic nature of plasticity of aluminum micro-pillars, Acta Mater. 56 (2008) 1712-1720. [18] L.Y. Che, J.Q. Xu, H. Choi, M. Pozuelo, X.L. Ma, S. Bhowmick, J.M. Yang, S. Mathaudhu, Processing and properties of magnesium containing a dense uniform dispersion of nanoparticles, Nature 528 (2015) 539-543. [19] N. Chawla, C. Andres, J.W. Jones, J.E. Allison, Effect of SiC volume fraction and particle size on the fatigue resistance of a 2080 Al/SiCp composite, Metall. Mater. Trans. A 29 (1998) 2843-2854. [20] L. Wang, F. Qiu, Q.L. Zhao, H.Y. Wang, Q.C. Jiang, Simultaneously increasing the elevated-temperature tensile strength and plasticity of in situ nano-sized TiCx/Al-Cu-Mg composites, Mater. Charact. 125 (2017) 7-12. [21] Y.F. Li, J.T. Guo, L.Z. Zhou, H.Q. Ye, Effect of recrystallization on room-temperature mechanical properties of Zr-doped Ni3Al alloy, Mater. Lett.58 (2004) 1853-1856. [22] R.J. Arsenault, R.M. Fisher, Microstructure of fiber and particulate SiC in 6061 A1 composite, Scripta Metall. 17 (1983) 67-71. [23] M. Vogelsang, R.J. Arsenault, R.M. Fisher, An in situ HVEM study of dislocation generation at Al/SiC interfaces in metal matrix composites, Metall. Trans. A 17A (1986) 379-389. [24] R.J. Arsenault, N. Shi, Dislocation generation due to differences between the coefficients of thermal expansion, Mater. Sci. Eng. 81 (1986) 175-187. [25] G. Liu, G.J. Zhang, F. Jiang, X.D. Ding, Y.J. Sun, J. Sun1, E. Ma, Nanostructured high-strength molybdenum alloys with unprecedented tensile ductility, Nat. Mater. 12 (2013) 344-350. [26] I.A. Ovid’ko, R.Z. Valiev, Y.T. Zhu, Review on superior strength and enhanced ductility of metallic nanomaterials, Prog. Mater. Sci. 94 (2018) 462-540.
[27] Y. Cao, S. Ni, X.Z. Liao, M. Song, Y.T. Zhu, Structural evolutions of metallic materials processed by severe plastic deformation, Mater. Sci. Eng. R 133 (2018) 1-59.
Highlights: Micromechanical behavior of Al3BC/Al composites was investigated. Wrinkled slip bands and slip homogenization were observed and elucidated. Dislocation behavior and ultrafine sub-grains induced by Al3BC were unveiled
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: