- Email: [email protected]
Mechanical property evaluation of second phase particles in a Mg–8Al-0.5Zn alloy using micropillar compression
Materials Science & Engineering A 775 (2020) 138973
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Short communication
Mechanical property evaluation of second phase particles in a Mg–8Al-0.5Zn alloy using micropillar compression R. Sarvesha a, Ubaid-ur-Rehman Ghori b, Yu Lung Chiu b, Ian P. Jones b, S.S. Singh a, **, J. Jain b, c, * a b c
Department of Material Science and Engineering, Indian Institute of Technology Kanpur, Uttar Pradesh, 208016, India School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK Department of Materials Science and Engineering, Indian Institute of Technology Delhi, New Delhi, 110016, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Mg17Al12 Al8Mn5 Micropillar compression Magnesium alloys TEM
This study investigates the mechanical properties of the second phase particles and Mg-matrix in a Mg–8Al-0.5Zn cast alloy by compression of machined micropillars. The stress-strain behavior showed that the Mg17Al12 par ticles exhibited a sudden and unusual failure after a significant strain burst. The Al8Mn5 particles were much more ductile, but also much stronger.
1. Introduction Magnesium and its alloys have potential to be used in aerospace and automobile applications [1–6]. The AZ series (Mg–Al–Zn) alloys are a prominent class of magnesium alloys, which contain Mg–Al type and Mn–Al type second phase particles [7,8]. These second phase particles in the magnesium alloys have been found to greatly affect their deforma tion and fracture behavior [9–11]. The most commonly observed Mg–Al type particle is Mg17Al12, whereas different Mn–Al type particles have been observed, such as Al3Mn [12], Al2Mn [12], Al8Mn5 [13] and Al4Mn [9]. During external loading, cracks have been found to initiate at the location of these particles either by their fracture or by debonding of the particle/matrix interface [10,11]. Therefore, knowledge of the type of second phase particles and their distribution and individual mechanical properties will help in understanding the overall mechanical response of the alloy, and, ultimately, be helpful in future Mg alloy design. Due to the small size (typically a few micrometers) of the second phase particles, conventional techniques to evaluate their mechanical properties (Young’s modulus, compressive strength and ductility) cannot be utilized directly. Nanoindentation is an excellent tool to obtain the mechanical properties at a small length scale and a couple of studies have already been conducted to obtain the Young’s modulus and hardness of both Mg–Al type [14–17] and Mn–Al type particles [17].
Although these experiments provide a reasonable idea of elastic behavior (Young’s modulus), some of the questions concerning plastic behavior remain unanswered, such as the nature of the deformation (ductile or brittle), strength, ductility etc. Micropillar compression can be used to quantify uniaxial stress-strain behavior at a small length scale. In this work, micropillar compression has been used to quantify the deformation behavior of the second phase particles present in the AZ80 magnesium alloy. To the best of our knowledge, the stress-strain behavior of the second phase particles in the AZ series of magnesium alloys has not been obtained experimentally. 2. Experimental procedure As-received direct-chill cast AZ80 (Mg–8Al-0.5Zn-0.15Mn-0.025Si0.0072Fe, in wt. %) magnesium alloy was polished to a final finish using 0.05 μm alumina suspension on a vibratory polisher (Buehler, Vibro Met™ 2) to observe the microstructure. The composition was measured through optical emission spectroscopy (OES) and is in-line with ASTMB275-05. A combination of backscattered electron (BSE) imaging mode in SEM (scanning electron microscopy), EPMA (electron probe micro analysis), and EBSD (electron backscattered diffraction) was used to characterize the second phase particles present in the alloy. During EBSD, a hexagonal rhombohedral lattice (Space Group R3mH) was used
* Corresponding author. School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. ** Corresponding author. E-mail addresses: [email protected] (S.S. Singh), [email protected] (J. Jain). https://doi.org/10.1016/j.msea.2020.138973 Received 17 October 2019; Received in revised form 7 January 2020; Accepted 17 January 2020 Available online 20 January 2020 0921-5093/© 2020 Elsevier B.V. All rights reserved.
R. Sarvesha et al.
Materials Science & Engineering A 775 (2020) 138973
for indexing the Al8Mn5 particles. Circularly shaped micropillars of 1–2 μm diameter were fabricated from these second phase particles using a Xe plasma focused ion beam (Helios G4 PFIB CXe, FEI) maintaining an aspect ratio (height/diameter) of 3–4.2. For comparison, similar-sized micropillars were also fabricated from the matrix. A constant acceler ating voltage of 30 kV and currents varying from 15 nA to 0.1 nA were used to make pillars with a taper angle of less than 3� . The micropillars were compressed using a picoindenter (PI85, Hysitron) installed in a scanning electron microscope (Tescan Mira III SEM). A 20 μm diameter flat punch was used to compress the pillars at an average strain rate of ~10-3 s-1 in displacement control mode. The top diameter was used to calculate the stress and Sneddon’s correction was used to correct the displacement data of the pillars [18,19]. The deformed pillars were observed by SEM and transmission electron microscopy (TEM) to un derstand the deformation mechanism. A post mortem TEM sample was prepared from an Al8Mn5 compressed pillar to confirm the underlying deformation structure using a standard FIB lift-out method. For indexing selective area electron diffraction (SAED) patterns of the Al8Mn5 parti cle, a different lattice, body central rhombohedral, was used for con venience. The Xe Plasma focused ion beam (Helios G4 PFIB CXe, Thermo Fisher Scientific) was used to make a TEM lamella with a final thickness of ~300 nm using 30 kV and 0.1 nA as the final current. Note that the TEM sample was cut parallel to the loading direction. The TEM lamella
was then analyzed using an FEI Talos F200X Scanning/Transmission Electron Microscope at an operating voltage of 200 kV. 3. Results and discussion The backscattered electron image (Fig. 1a) shows the presence of two different second phase particles via a contrast difference: white and gray. The nominal compositions (in at. %) of these particles, obtained from EPMA, are listed in Fig. 1d. The ratio of Al to Mn in the white particles is ~1.22, suggesting the white particles to be Al8Mn5 based on the Al–Mn equilibrium phase diagram [20]. Similarly, the ratio of Mg to Al (~1.7) in the gray particles is close to the ratio in Mg17Al12, sug gesting the gray particles are Mg17Al12. These particles have also been observed in previous studies of AZ80 magnesium alloys [21,22]. Fig. 1b and c shows the IPF-Z (inverse pole figure) and phase map of the Al8Mn5 and Mg17Al12 particles, respectively. From the IPF maps, it is evident that both the particles are polycrystalline in nature. Further, it is also interesting to note that both Mg17Al12 and Al8Mn5 are randomly ori ented, with Mg17Al12 precipitates being predominantly along the grain boundaries. EBSD was performed on at least ten particles and in all cases, Mg17Al12 and Al8Mn5 grains were found to exhibit random orientations. SEM images of micropillars fabricated from the matrix and both
Fig. 1. Microstructural characterization of the AZ80 alloy (a) BSE image, (b) IPF and phase map of a Al8Mn5 particle, (c) IPF and phase map of α-Mg and Mg17Al12 precipitate, and (d) composition (in at. %) of particles using EPMA. 2
R. Sarvesha et al.
Materials Science & Engineering A 775 (2020) 138973
types of second phase particles are shown in Fig. 2 (a: matrix, b: Mg17Al12 and c: Al8Mn5). Fig. 3 shows the stress-strain behavior of all the microconstituents. It should be noted that the top diameter was used for the calculation of stress. As the micropillars have some taper, the calculated stress measured using the top diameter represents the upper bound value. The stress-strain curves (Fig. 3a) of the α-Mg matrix clearly show that the matrix exhibits plasticity with signatures of strain bursts. Strain bursts are commonly found in ductile metals/alloys and have been attributed to the formation of slip bands [23]. In this case, the micropillars were not fractured and were unloaded after a strain of ~3% was reached. The compressive yield stress of the matrix was calculated to be ~134 � 22 MPa. The yield stress value in this case represents the stress value at which the curves deviate from linearity. The stress-strain curves (Fig. 3b) for the Mg17Al12 particles are linear until a significant strain burst leading to a notable change in the shape of the pillar which here is interpreted as failure. The compressive failure strength of the Mg17Al12 particles was calculated to be ~814 � 141 MPa. Fig. 3c shows the stress-strain curves of the Al8Mn5 particles, where the micropillars were not strained up to fracture and the indenter was unloaded at different strain levels. It is evident that the particles exhibit both ductility and significant strength, with several load drops or strain bursts in the stress-strain curves due to dislocation activity. We believe that the first strain burst after the elastic portion corresponds to the formation of a slip band due to yielding. Therefore, the stress corre sponding to the first strain burst was taken as the compressive yield stress, which was calculated to be ~3084 � 630 MPa. For comparison purposes, the combined stress-strain curves of all micro-constituents are shown in Fig. 3d. It is clear that the Al8Mn5 phase exhibits the highest strength followed by the Mg17Al12 intermetallic phase and the α-Mg matrix. Nanoindentation studies have also shown that the highest hardness is exhibited by the Al8Mn5 particles followed by the Mg17Al12 particles and then the matrix [14–17], which agrees qualitatively with the observed trend of strength in this study, as strength and hardness can be directly correlated as a general case [24,25]. Fig. 4 (a: matrix, b: Mg17Al12 and c: Al8Mn5) shows SEM images of the deformed micropillars. The matrix and Al8Mn5 particle show the formation of slip lines (shown by arrows), which indicate ductile ma terials and involve dislocation activity. In contrast, the Mg17Al12 parti cles fractured during deformation (Fig. 4b) forming some visible cracks (shown by arrow). Fig. 4b suggests there has been considerable defor mation, but the nature of this is not currently known. As there was a large strain burst with fracture immediately following, it might be possible that localization of strain has caused sudden failure of the pillar. Somewhat similar behavior has been reported by Song et al. [26] for a quasi-crystalline approximant where there is strong evidence that strain localization did not proceed via dislocation movement. Fig. 4d shows the dislocation structure observed in a deformed Al8Mn5 micropillar. The inset to Fig. 4d shows a selected area diffraction (SAD) pattern taken along the [001] zone axis. As shown in the SAD pattern, the angle be tween (011) and (021) was 88.73� , which corresponds to the reported rhombohedral crystal structure [27]. Further, the dislocations are par allel to each other indicating that one type of dislocation is queued on
the slip plane. A more detailed analysis of dislocation activity is required to unravel the characteristics of slip in these inclusions. Nevertheless, the dislocation activity observed by TEM confirms the ductile behavior of the Al8Mn5 particles. To verify further the behavior of the Al8Mn5 particles, a qualitative analysis was performed by using nanoindentation followed by atomic force microscopy (AFM), as shown in Fig. 4(e, f). Indentations were performed on at least three Al8Mn5 particles to a maximum load of 30 mN and the line scan profiles along corner (C) to edge (E) of the resulting indents obtained from the AFM were used to observe the pile-up or sink-in phenomenon, as shown by three CE lines in one of the representative indents in Fig. 4e. It is known that plastically deforming materials tend to show pile-up characteristics during inden tation [28]. Fig. 4f shows the corresponding line profiles of the AFM scan and SEM image of the indent (in the inset). The formation of pile-ups in Al8Mn5 particles can be clearly visualized (along all three edges, E) in the AFM scan image (Fig. 4e) and corresponding SEM image of the indent (inset of Fig. 4f). The extent of pile-up was found to be at a level of ~100 nm, as shown in the obtained line profiles (near area ‘E’ in Fig. 4f), suggesting the particles to be plastically deformable and corroborating the results obtained from micropillar compression. Although a couple of studies have been performed to obtain the stress-strain behavior of Mg17Al12 [29,30], no study has been performed to quantify the stress-strain properties of the Al8Mn5 particles. Mg17Al12 exhibits a body-centered cubic crystal structure and has a lattice parameter of 1.05 nm with 58 atoms per unit cell [14]. Ragani et al. [29] fabricated bulk Mg17Al12 by levitation melting and evaluated the compressive properties at four different temperatures: room tempera ture (25 � C), 200 � C, 250 � C, and 300 � C. The intermetallic phase was observed to exhibit brittle behavior up to 200 � C and ductile behavior above 250 � C. The room temperature fracture stress and strain were found to be ~325 MPa and ~2%, respectively. In another study by Fukuchi et al. [30], bulk Mg17Al12 intermetallic phase was also found to exhibit ductility at high temperatures (between 360 � C to 435 � C) during compression, but room temperature properties were not evaluated. It should be noted that all the above mentioned studies on Mg17Al12 have been carried out in bulk form. However, it is important to evaluate the stress-strain behavior at small length scale as the observed particle size in the magnesium alloys is small (micron level) [17]. Furthermore, the mechanical properties at large and small length scales might be different. In fact, it has been shown that materials exhibiting brittle behavior in bulk form can possess ductile behavior when deformed at small length scales [31–33]. Interestingly, our results show that Mg17Al12 exhibits distinct behavior at room temperature i.e., sudden failure after a significant strain burst, unlike the brittle nature reported by Ragani et al. [29] in the bulk samples. 4. Conclusions In summary, for the first time, we report the stress-strain behavior of the second phase particles in AZ80 magnesium alloy by micropillar compression. Al8Mn5 particles exhibited the highest strength (~3084 � 630 MPa) followed by Mg17Al12 particles (~814 � 141 MPa) and then
Fig. 2. SEM images of the micropillars of (a) matrix, (b) Mg17Al12, and (c) Al8Mn5. 3
R. Sarvesha et al.
Materials Science & Engineering A 775 (2020) 138973
Fig. 3. Compressive stress-strain curves of three individual tests of (a) matrix, (b) Mg17Al12 particles, (c) Al8Mn5 particles, and (d) comparison of all microconstituents.
Fig. 4. (a, b, c) are SEM images of the deformed pillars of the matrix, Mg17Al12 particles and Al8Mn5 particles, (yellow arrows in (b) and (c) show cracks and slip lines, respectively), (d) TEM micrograph of a deformed Al8Mn5 pillar; inset shows the corresponding SAD pattern taken along [100] zone axis, and (e, f) AFM image and corresponding line scan across an intent on an Al8Mn5 particle. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
4
R. Sarvesha et al.
Materials Science & Engineering A 775 (2020) 138973
the matrix (~134 � 22 MPa). Furthermore, Al8Mn5 particles showed ductile behavior which was also confirmed by the observed dislocation activities seen by TEM. The quantification of the mechanical properties of the second phase particles, as performed in this study, will be helpful to unravel the mechanical response of the alloy and in microstructurebased modeling.
[7] A. Zindal, J. Jain, R. Prasad, S.S. Singh, R. Sarvesha, P. Cizek, M.R. Barnett, Effect of heat treatment variables on the formation of precipitate free zones (PFZs) in Mg8Al-0.5Zn alloy, Mater. Char. 136 (2018) 175–182. [8] A. Zindal, J. Jain, R. Prasad, S.S. Singh, P. Cizek, Correlation of grain boundary precipitate characteristics with fracture and fracture toughness in an Mg-8Al-0.5 Zn alloy, Mater. Sci. Eng. A 706 (2017) 192–200. [12] M.C. Merino, A. Pardo, R. Arrabal, S. Merino, P. Casajús, M. Mohedano, Influence of chloride ion concentration and temperature on the corrosion of Mg–Al alloys in salt fog, Corros. Sci. 52 (2010) 1696–1704. [13] M. Ben-Haroush, G. Ben-Hamu, D. Eliezer, L. Wagner, The relation between microstructure and corrosion behavior of AZ80 Mg alloy following different extrusion temperatures, Corros. Sci. 50 (2008) 1766–1778. [9] I.A. Yakubtsov, B.J. Diak, C.A. Sager, B. Bhattacharya, W.D. MacDonald, M. Niewczas, Effects of heat treatment on microstructure and tensile deformation of Mg AZ80 alloy at room temperature, Mater. Sci. Eng. A 496 (2008) 247–255. [10] M. Lugo, M.A. Tschopp, J.B. Jordon, M.F. Horstemeyer, Microstructure and damage evolution during tensile loading in a wrought magnesium alloy, Scr. Mater. 64 (2011) 912–915. [11] Y.Z. Lü, Q.D. Wang, W.J. Ding, X.Q. Zeng, Y.P. Zhu, Fracture behavior of AZ91 magnesium alloy, Mater. Lett. 44 (2000) 265–268. [14] H.N. Mathur, V. Maier-Kiener, S. Korte-Kerzel, Deformation in the γ-Mg17Al12 phase at 25–278 � C, Acta Mater. 113 (2016) 221–229. [15] H.-W. Chang, M.-X. Zhang, A. Atrens, H. Huang, Nanomechanical properties of Mg–Al intermetallic compounds produced by packed powder diffusion coating (PPDC) on the surface of AZ91E, J. Alloy. Comp. 587 (2014) 527–532. [16] J.L. Hay, P. Agee, Mapping the mechanical properties of alloyed magnesium (AZ61), in: N. Hort, S.N. Mathaudhu, N.R. Neelameggham, M. Alderman (Eds.), Magnesium Technology, Springer, Cham, 2013, p. 329. [17] R. Sarvesha, W. Alam, A. Gokhale, T.S. Guruprasad, S. Bhagavath, S. Karagadde, J. Jain, S.S. Singh, Quantitative assessment of second phase particles characteristics and its role on the deformation response of a Mg-8Al-0.5 Zn alloy, Mater. Sci. Eng. A 759 (2019) 368–379. [18] C. Frick, B. Clark, S. Orso, A. Schneider, E. Arzt, Size effect on strength and strain hardening of small-scale [1 1 1] nickel compression pillars, Mater. Sci. Eng. A 489 (2008) 319–329. [19] H. Fei, A. Abraham, N. Chawla, H. Jiang, Evaluation of micro-pillar compression tests for accurate determination of elastic-plastic constitutive relations, J. Appl. Mech. 79 (2012), 061001-1:9. [20] A.J. McAlister, J.L. Murray, The (Al-Mn) aluminum-manganese system, J. Phase Equilibria 8 (1987) 438–447. [21] R. Arrabal, A. Pardo, M.C. Merino, S. Merino, M. Mohedano, P. Casajús, Corrosion behaviour of Mg/Al alloys in high humidity atmospheres, Mater. Corros. 62 (2011) 326–334. [22] K. Kadali, D. Dubey, R. Sarvesha, H. Kancharla, J. Jain, K. Mondal, S.S. Singh, Dissolution kinetics of Mg17Al12 eutectic phase and its effect on corrosion behavior of as-cast AZ80 magnesium alloy, J. Occup. Med. 71 (2019) 2209–2218. [23] A.S. Schneider, D. Kiener, C.M. Yakacki, H.J. Maier, P.A. Gruber, N. Tamura, M. Kunz, A.M. Minor, C.P. Frick, Influence of bulk pre-straining on the size effect in nickel compression pillars, Mater. Sci. Eng. A 559 (2013) 147–158. [24] P. Zhang, S. Li, Z. Zhang, General relationship between strength and hardness, Mater. Sci. Eng. A 529 (2011) 62–73. [25] L.J. Vandeperre, F. Giuliani, W.J. Clegg, Effect of elastic surface deformation on the relation between hardness and yield strength, J. Mater. Res. 19 (2004) 3704–3714. [26] G. Song, T. Kong, K.J. Dusoe, P.C. Canfield, S.-W. Lee, Shear localization and sizedependent strength of YCd6 quasicrystal approximant at the micrometer length scale”, J. Mater. Sci. 53 (2018) 6980–6990. [27] G. Zeng, J.W. Xian, C.M. Gourlay, Nucleation and growth crystallography of Al8Mn5 on B2-Al(Mn,Fe) in AZ91 magnesium alloys, Acta Mater. 153 (2018) 364–376. [28] C. Anthony, Fischer-Cripps, Nanoindentation, second ed., Springer-Verlag, New York, 2004. [29] J. Ragani, P. Donnadieu, C. Tassin, J. Blandin, High-temperature deformation of the c-Mg17Al12 complex metallic alloy, Scr. Mater. 65 (2011) 253–256. [30] M. Fukuchi, K. Watanabe, Tensile behavior of γ-phase in Al-Mg system at elevated temperatures, J. Jpn. Inst. Light Metals 30 (1980) 253–257. [31] J. Michler, K. Wasmer, S. Meier, F. Ostlund, K. Leifer, Plastic deformation of gallium arsenide micropillars under uniaxial compression at room temperature, Appl. Phys. Lett. 90 (2007), 043123-1:3. [28] S. Korte, W. Clegg, Phil Mag, 91 (2011) 1150-1162. [32] S. Korte, W. Clegg, Discussion of the dependence of the effect of size on the yield stress in hard materials studied by microcompression of MgO, Philos. Mag. 91 (2011) 1150–1162. [33] S.S. Singh, E. Guo, H. Xie, N. Chawla, Mechanical properties of intermetallic inclusions in Al 7075 alloys by micropillar compression, Intermetallics 62 (2015) 69–75.
Originality statement I write on behalf of myself and all co-authors to confirm that the results reported in the manuscript are original and neither the entire work, nor any of its parts have been previously published. The authors confirm that the article has not been submitted to peer review, nor has been accepted for publishing in another journal. The authors confirm that the research in their work is original, and that all the data given in the article are real and authentic. 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. CRediT authorship contribution statement R. Sarvesha: Conceptualization, Methodology, Writing - original draft. Ubaid-ur-Rehman Ghori: Methodology, Writing - review & editing. Yu Lung Chiu: Writing - review & editing. Ian P. Jones: Writing - review & editing. S.S. Singh: Conceptualization, Methodol ogy, Writing - review & editing. J. Jain: Conceptualization, Methodol ogy, Writing - review & editing. Acknowledgements Authors would like to thank Prof. Warren Poole of University of British Columbia for providing the AZ80 alloy. JJ also acknowledges the financial assistance provided by India-Birmingham institute fellowship during his stay at Birmingham. SSS acknowledges the financial support received from IIT Kanpur. We also acknowledge the support from the Electron Microscopy Facility at the Advanced Center of Materials Sci ence (ACMS) at IIT Kanpur. References [1] M.K. Kulekci, Magnesium and its alloys applications in automotive industry, Int. J. Adv. Manuf. Technol. 39 (2008) 851–865. [2] E. Aghion, B. Bronfin, Magnesium alloys development towards the 21st century, Mater. Sci. Forum 350–351 (2000) 19–30. [3] J.A. Yasi, L.G. Hector, D.R. Trinkle, First-principles data for solid-solution strengthening of magnesium: from geometry and chemistry to properties, Acta Mater. 58 (2010) 5704–5713. [4] J. Min, L.G. Hector Jr., J. Lin, J.T. Carter, A.K. Sachdev, Spatio-temporal characteristics of propagative plastic instabilities in a rare earth containing magnesium alloy, Int. J. Plast. 57 (2014) 52–76. [5] A.J. Carpenter, A.R. Antoniswamy, J.T. Carter, L.G. Hector Jr., E.M. Taleff, A mechanism-dependent material model for the effects of grain growth and anisotropy on plastic deformation of magnesium alloy AZ31 sheet at 450oC, Acta Mater. 68 (2014) 254–266. [6] G.P.M. Leyson, L.G. Hector Jr., W.A. Curtin, First-principles prediction of yield stress for basal slip in Mg-Al alloys, Acta Mater. 60 (2012) 5197–5203.
5
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Short communication
Mechanical property evaluation of second phase particles in a Mg–8Al-0.5Zn alloy using micropillar compression R. Sarvesha a, Ubaid-ur-Rehman Ghori b, Yu Lung Chiu b, Ian P. Jones b, S.S. Singh a, **, J. Jain b, c, * a b c
Department of Material Science and Engineering, Indian Institute of Technology Kanpur, Uttar Pradesh, 208016, India School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK Department of Materials Science and Engineering, Indian Institute of Technology Delhi, New Delhi, 110016, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Mg17Al12 Al8Mn5 Micropillar compression Magnesium alloys TEM
This study investigates the mechanical properties of the second phase particles and Mg-matrix in a Mg–8Al-0.5Zn cast alloy by compression of machined micropillars. The stress-strain behavior showed that the Mg17Al12 par ticles exhibited a sudden and unusual failure after a significant strain burst. The Al8Mn5 particles were much more ductile, but also much stronger.
1. Introduction Magnesium and its alloys have potential to be used in aerospace and automobile applications [1–6]. The AZ series (Mg–Al–Zn) alloys are a prominent class of magnesium alloys, which contain Mg–Al type and Mn–Al type second phase particles [7,8]. These second phase particles in the magnesium alloys have been found to greatly affect their deforma tion and fracture behavior [9–11]. The most commonly observed Mg–Al type particle is Mg17Al12, whereas different Mn–Al type particles have been observed, such as Al3Mn [12], Al2Mn [12], Al8Mn5 [13] and Al4Mn [9]. During external loading, cracks have been found to initiate at the location of these particles either by their fracture or by debonding of the particle/matrix interface [10,11]. Therefore, knowledge of the type of second phase particles and their distribution and individual mechanical properties will help in understanding the overall mechanical response of the alloy, and, ultimately, be helpful in future Mg alloy design. Due to the small size (typically a few micrometers) of the second phase particles, conventional techniques to evaluate their mechanical properties (Young’s modulus, compressive strength and ductility) cannot be utilized directly. Nanoindentation is an excellent tool to obtain the mechanical properties at a small length scale and a couple of studies have already been conducted to obtain the Young’s modulus and hardness of both Mg–Al type [14–17] and Mn–Al type particles [17].
Although these experiments provide a reasonable idea of elastic behavior (Young’s modulus), some of the questions concerning plastic behavior remain unanswered, such as the nature of the deformation (ductile or brittle), strength, ductility etc. Micropillar compression can be used to quantify uniaxial stress-strain behavior at a small length scale. In this work, micropillar compression has been used to quantify the deformation behavior of the second phase particles present in the AZ80 magnesium alloy. To the best of our knowledge, the stress-strain behavior of the second phase particles in the AZ series of magnesium alloys has not been obtained experimentally. 2. Experimental procedure As-received direct-chill cast AZ80 (Mg–8Al-0.5Zn-0.15Mn-0.025Si0.0072Fe, in wt. %) magnesium alloy was polished to a final finish using 0.05 μm alumina suspension on a vibratory polisher (Buehler, Vibro Met™ 2) to observe the microstructure. The composition was measured through optical emission spectroscopy (OES) and is in-line with ASTMB275-05. A combination of backscattered electron (BSE) imaging mode in SEM (scanning electron microscopy), EPMA (electron probe micro analysis), and EBSD (electron backscattered diffraction) was used to characterize the second phase particles present in the alloy. During EBSD, a hexagonal rhombohedral lattice (Space Group R3mH) was used
* Corresponding author. School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. ** Corresponding author. E-mail addresses: [email protected] (S.S. Singh), [email protected] (J. Jain). https://doi.org/10.1016/j.msea.2020.138973 Received 17 October 2019; Received in revised form 7 January 2020; Accepted 17 January 2020 Available online 20 January 2020 0921-5093/© 2020 Elsevier B.V. All rights reserved.
R. Sarvesha et al.
Materials Science & Engineering A 775 (2020) 138973
for indexing the Al8Mn5 particles. Circularly shaped micropillars of 1–2 μm diameter were fabricated from these second phase particles using a Xe plasma focused ion beam (Helios G4 PFIB CXe, FEI) maintaining an aspect ratio (height/diameter) of 3–4.2. For comparison, similar-sized micropillars were also fabricated from the matrix. A constant acceler ating voltage of 30 kV and currents varying from 15 nA to 0.1 nA were used to make pillars with a taper angle of less than 3� . The micropillars were compressed using a picoindenter (PI85, Hysitron) installed in a scanning electron microscope (Tescan Mira III SEM). A 20 μm diameter flat punch was used to compress the pillars at an average strain rate of ~10-3 s-1 in displacement control mode. The top diameter was used to calculate the stress and Sneddon’s correction was used to correct the displacement data of the pillars [18,19]. The deformed pillars were observed by SEM and transmission electron microscopy (TEM) to un derstand the deformation mechanism. A post mortem TEM sample was prepared from an Al8Mn5 compressed pillar to confirm the underlying deformation structure using a standard FIB lift-out method. For indexing selective area electron diffraction (SAED) patterns of the Al8Mn5 parti cle, a different lattice, body central rhombohedral, was used for con venience. The Xe Plasma focused ion beam (Helios G4 PFIB CXe, Thermo Fisher Scientific) was used to make a TEM lamella with a final thickness of ~300 nm using 30 kV and 0.1 nA as the final current. Note that the TEM sample was cut parallel to the loading direction. The TEM lamella
was then analyzed using an FEI Talos F200X Scanning/Transmission Electron Microscope at an operating voltage of 200 kV. 3. Results and discussion The backscattered electron image (Fig. 1a) shows the presence of two different second phase particles via a contrast difference: white and gray. The nominal compositions (in at. %) of these particles, obtained from EPMA, are listed in Fig. 1d. The ratio of Al to Mn in the white particles is ~1.22, suggesting the white particles to be Al8Mn5 based on the Al–Mn equilibrium phase diagram [20]. Similarly, the ratio of Mg to Al (~1.7) in the gray particles is close to the ratio in Mg17Al12, sug gesting the gray particles are Mg17Al12. These particles have also been observed in previous studies of AZ80 magnesium alloys [21,22]. Fig. 1b and c shows the IPF-Z (inverse pole figure) and phase map of the Al8Mn5 and Mg17Al12 particles, respectively. From the IPF maps, it is evident that both the particles are polycrystalline in nature. Further, it is also interesting to note that both Mg17Al12 and Al8Mn5 are randomly ori ented, with Mg17Al12 precipitates being predominantly along the grain boundaries. EBSD was performed on at least ten particles and in all cases, Mg17Al12 and Al8Mn5 grains were found to exhibit random orientations. SEM images of micropillars fabricated from the matrix and both
Fig. 1. Microstructural characterization of the AZ80 alloy (a) BSE image, (b) IPF and phase map of a Al8Mn5 particle, (c) IPF and phase map of α-Mg and Mg17Al12 precipitate, and (d) composition (in at. %) of particles using EPMA. 2
R. Sarvesha et al.
Materials Science & Engineering A 775 (2020) 138973
types of second phase particles are shown in Fig. 2 (a: matrix, b: Mg17Al12 and c: Al8Mn5). Fig. 3 shows the stress-strain behavior of all the microconstituents. It should be noted that the top diameter was used for the calculation of stress. As the micropillars have some taper, the calculated stress measured using the top diameter represents the upper bound value. The stress-strain curves (Fig. 3a) of the α-Mg matrix clearly show that the matrix exhibits plasticity with signatures of strain bursts. Strain bursts are commonly found in ductile metals/alloys and have been attributed to the formation of slip bands [23]. In this case, the micropillars were not fractured and were unloaded after a strain of ~3% was reached. The compressive yield stress of the matrix was calculated to be ~134 � 22 MPa. The yield stress value in this case represents the stress value at which the curves deviate from linearity. The stress-strain curves (Fig. 3b) for the Mg17Al12 particles are linear until a significant strain burst leading to a notable change in the shape of the pillar which here is interpreted as failure. The compressive failure strength of the Mg17Al12 particles was calculated to be ~814 � 141 MPa. Fig. 3c shows the stress-strain curves of the Al8Mn5 particles, where the micropillars were not strained up to fracture and the indenter was unloaded at different strain levels. It is evident that the particles exhibit both ductility and significant strength, with several load drops or strain bursts in the stress-strain curves due to dislocation activity. We believe that the first strain burst after the elastic portion corresponds to the formation of a slip band due to yielding. Therefore, the stress corre sponding to the first strain burst was taken as the compressive yield stress, which was calculated to be ~3084 � 630 MPa. For comparison purposes, the combined stress-strain curves of all micro-constituents are shown in Fig. 3d. It is clear that the Al8Mn5 phase exhibits the highest strength followed by the Mg17Al12 intermetallic phase and the α-Mg matrix. Nanoindentation studies have also shown that the highest hardness is exhibited by the Al8Mn5 particles followed by the Mg17Al12 particles and then the matrix [14–17], which agrees qualitatively with the observed trend of strength in this study, as strength and hardness can be directly correlated as a general case [24,25]. Fig. 4 (a: matrix, b: Mg17Al12 and c: Al8Mn5) shows SEM images of the deformed micropillars. The matrix and Al8Mn5 particle show the formation of slip lines (shown by arrows), which indicate ductile ma terials and involve dislocation activity. In contrast, the Mg17Al12 parti cles fractured during deformation (Fig. 4b) forming some visible cracks (shown by arrow). Fig. 4b suggests there has been considerable defor mation, but the nature of this is not currently known. As there was a large strain burst with fracture immediately following, it might be possible that localization of strain has caused sudden failure of the pillar. Somewhat similar behavior has been reported by Song et al. [26] for a quasi-crystalline approximant where there is strong evidence that strain localization did not proceed via dislocation movement. Fig. 4d shows the dislocation structure observed in a deformed Al8Mn5 micropillar. The inset to Fig. 4d shows a selected area diffraction (SAD) pattern taken along the [001] zone axis. As shown in the SAD pattern, the angle be tween (011) and (021) was 88.73� , which corresponds to the reported rhombohedral crystal structure [27]. Further, the dislocations are par allel to each other indicating that one type of dislocation is queued on
the slip plane. A more detailed analysis of dislocation activity is required to unravel the characteristics of slip in these inclusions. Nevertheless, the dislocation activity observed by TEM confirms the ductile behavior of the Al8Mn5 particles. To verify further the behavior of the Al8Mn5 particles, a qualitative analysis was performed by using nanoindentation followed by atomic force microscopy (AFM), as shown in Fig. 4(e, f). Indentations were performed on at least three Al8Mn5 particles to a maximum load of 30 mN and the line scan profiles along corner (C) to edge (E) of the resulting indents obtained from the AFM were used to observe the pile-up or sink-in phenomenon, as shown by three CE lines in one of the representative indents in Fig. 4e. It is known that plastically deforming materials tend to show pile-up characteristics during inden tation [28]. Fig. 4f shows the corresponding line profiles of the AFM scan and SEM image of the indent (in the inset). The formation of pile-ups in Al8Mn5 particles can be clearly visualized (along all three edges, E) in the AFM scan image (Fig. 4e) and corresponding SEM image of the indent (inset of Fig. 4f). The extent of pile-up was found to be at a level of ~100 nm, as shown in the obtained line profiles (near area ‘E’ in Fig. 4f), suggesting the particles to be plastically deformable and corroborating the results obtained from micropillar compression. Although a couple of studies have been performed to obtain the stress-strain behavior of Mg17Al12 [29,30], no study has been performed to quantify the stress-strain properties of the Al8Mn5 particles. Mg17Al12 exhibits a body-centered cubic crystal structure and has a lattice parameter of 1.05 nm with 58 atoms per unit cell [14]. Ragani et al. [29] fabricated bulk Mg17Al12 by levitation melting and evaluated the compressive properties at four different temperatures: room tempera ture (25 � C), 200 � C, 250 � C, and 300 � C. The intermetallic phase was observed to exhibit brittle behavior up to 200 � C and ductile behavior above 250 � C. The room temperature fracture stress and strain were found to be ~325 MPa and ~2%, respectively. In another study by Fukuchi et al. [30], bulk Mg17Al12 intermetallic phase was also found to exhibit ductility at high temperatures (between 360 � C to 435 � C) during compression, but room temperature properties were not evaluated. It should be noted that all the above mentioned studies on Mg17Al12 have been carried out in bulk form. However, it is important to evaluate the stress-strain behavior at small length scale as the observed particle size in the magnesium alloys is small (micron level) [17]. Furthermore, the mechanical properties at large and small length scales might be different. In fact, it has been shown that materials exhibiting brittle behavior in bulk form can possess ductile behavior when deformed at small length scales [31–33]. Interestingly, our results show that Mg17Al12 exhibits distinct behavior at room temperature i.e., sudden failure after a significant strain burst, unlike the brittle nature reported by Ragani et al. [29] in the bulk samples. 4. Conclusions In summary, for the first time, we report the stress-strain behavior of the second phase particles in AZ80 magnesium alloy by micropillar compression. Al8Mn5 particles exhibited the highest strength (~3084 � 630 MPa) followed by Mg17Al12 particles (~814 � 141 MPa) and then
Fig. 2. SEM images of the micropillars of (a) matrix, (b) Mg17Al12, and (c) Al8Mn5. 3
R. Sarvesha et al.
Materials Science & Engineering A 775 (2020) 138973
Fig. 3. Compressive stress-strain curves of three individual tests of (a) matrix, (b) Mg17Al12 particles, (c) Al8Mn5 particles, and (d) comparison of all microconstituents.
Fig. 4. (a, b, c) are SEM images of the deformed pillars of the matrix, Mg17Al12 particles and Al8Mn5 particles, (yellow arrows in (b) and (c) show cracks and slip lines, respectively), (d) TEM micrograph of a deformed Al8Mn5 pillar; inset shows the corresponding SAD pattern taken along [100] zone axis, and (e, f) AFM image and corresponding line scan across an intent on an Al8Mn5 particle. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
4
R. Sarvesha et al.
Materials Science & Engineering A 775 (2020) 138973
the matrix (~134 � 22 MPa). Furthermore, Al8Mn5 particles showed ductile behavior which was also confirmed by the observed dislocation activities seen by TEM. The quantification of the mechanical properties of the second phase particles, as performed in this study, will be helpful to unravel the mechanical response of the alloy and in microstructurebased modeling.
[7] A. Zindal, J. Jain, R. Prasad, S.S. Singh, R. Sarvesha, P. Cizek, M.R. Barnett, Effect of heat treatment variables on the formation of precipitate free zones (PFZs) in Mg8Al-0.5Zn alloy, Mater. Char. 136 (2018) 175–182. [8] A. Zindal, J. Jain, R. Prasad, S.S. Singh, P. Cizek, Correlation of grain boundary precipitate characteristics with fracture and fracture toughness in an Mg-8Al-0.5 Zn alloy, Mater. Sci. Eng. A 706 (2017) 192–200. [12] M.C. Merino, A. Pardo, R. Arrabal, S. Merino, P. Casajús, M. Mohedano, Influence of chloride ion concentration and temperature on the corrosion of Mg–Al alloys in salt fog, Corros. Sci. 52 (2010) 1696–1704. [13] M. Ben-Haroush, G. Ben-Hamu, D. Eliezer, L. Wagner, The relation between microstructure and corrosion behavior of AZ80 Mg alloy following different extrusion temperatures, Corros. Sci. 50 (2008) 1766–1778. [9] I.A. Yakubtsov, B.J. Diak, C.A. Sager, B. Bhattacharya, W.D. MacDonald, M. Niewczas, Effects of heat treatment on microstructure and tensile deformation of Mg AZ80 alloy at room temperature, Mater. Sci. Eng. A 496 (2008) 247–255. [10] M. Lugo, M.A. Tschopp, J.B. Jordon, M.F. Horstemeyer, Microstructure and damage evolution during tensile loading in a wrought magnesium alloy, Scr. Mater. 64 (2011) 912–915. [11] Y.Z. Lü, Q.D. Wang, W.J. Ding, X.Q. Zeng, Y.P. Zhu, Fracture behavior of AZ91 magnesium alloy, Mater. Lett. 44 (2000) 265–268. [14] H.N. Mathur, V. Maier-Kiener, S. Korte-Kerzel, Deformation in the γ-Mg17Al12 phase at 25–278 � C, Acta Mater. 113 (2016) 221–229. [15] H.-W. Chang, M.-X. Zhang, A. Atrens, H. Huang, Nanomechanical properties of Mg–Al intermetallic compounds produced by packed powder diffusion coating (PPDC) on the surface of AZ91E, J. Alloy. Comp. 587 (2014) 527–532. [16] J.L. Hay, P. Agee, Mapping the mechanical properties of alloyed magnesium (AZ61), in: N. Hort, S.N. Mathaudhu, N.R. Neelameggham, M. Alderman (Eds.), Magnesium Technology, Springer, Cham, 2013, p. 329. [17] R. Sarvesha, W. Alam, A. Gokhale, T.S. Guruprasad, S. Bhagavath, S. Karagadde, J. Jain, S.S. Singh, Quantitative assessment of second phase particles characteristics and its role on the deformation response of a Mg-8Al-0.5 Zn alloy, Mater. Sci. Eng. A 759 (2019) 368–379. [18] C. Frick, B. Clark, S. Orso, A. Schneider, E. Arzt, Size effect on strength and strain hardening of small-scale [1 1 1] nickel compression pillars, Mater. Sci. Eng. A 489 (2008) 319–329. [19] H. Fei, A. Abraham, N. Chawla, H. Jiang, Evaluation of micro-pillar compression tests for accurate determination of elastic-plastic constitutive relations, J. Appl. Mech. 79 (2012), 061001-1:9. [20] A.J. McAlister, J.L. Murray, The (Al-Mn) aluminum-manganese system, J. Phase Equilibria 8 (1987) 438–447. [21] R. Arrabal, A. Pardo, M.C. Merino, S. Merino, M. Mohedano, P. Casajús, Corrosion behaviour of Mg/Al alloys in high humidity atmospheres, Mater. Corros. 62 (2011) 326–334. [22] K. Kadali, D. Dubey, R. Sarvesha, H. Kancharla, J. Jain, K. Mondal, S.S. Singh, Dissolution kinetics of Mg17Al12 eutectic phase and its effect on corrosion behavior of as-cast AZ80 magnesium alloy, J. Occup. Med. 71 (2019) 2209–2218. [23] A.S. Schneider, D. Kiener, C.M. Yakacki, H.J. Maier, P.A. Gruber, N. Tamura, M. Kunz, A.M. Minor, C.P. Frick, Influence of bulk pre-straining on the size effect in nickel compression pillars, Mater. Sci. Eng. A 559 (2013) 147–158. [24] P. Zhang, S. Li, Z. Zhang, General relationship between strength and hardness, Mater. Sci. Eng. A 529 (2011) 62–73. [25] L.J. Vandeperre, F. Giuliani, W.J. Clegg, Effect of elastic surface deformation on the relation between hardness and yield strength, J. Mater. Res. 19 (2004) 3704–3714. [26] G. Song, T. Kong, K.J. Dusoe, P.C. Canfield, S.-W. Lee, Shear localization and sizedependent strength of YCd6 quasicrystal approximant at the micrometer length scale”, J. Mater. Sci. 53 (2018) 6980–6990. [27] G. Zeng, J.W. Xian, C.M. Gourlay, Nucleation and growth crystallography of Al8Mn5 on B2-Al(Mn,Fe) in AZ91 magnesium alloys, Acta Mater. 153 (2018) 364–376. [28] C. Anthony, Fischer-Cripps, Nanoindentation, second ed., Springer-Verlag, New York, 2004. [29] J. Ragani, P. Donnadieu, C. Tassin, J. Blandin, High-temperature deformation of the c-Mg17Al12 complex metallic alloy, Scr. Mater. 65 (2011) 253–256. [30] M. Fukuchi, K. Watanabe, Tensile behavior of γ-phase in Al-Mg system at elevated temperatures, J. Jpn. Inst. Light Metals 30 (1980) 253–257. [31] J. Michler, K. Wasmer, S. Meier, F. Ostlund, K. Leifer, Plastic deformation of gallium arsenide micropillars under uniaxial compression at room temperature, Appl. Phys. Lett. 90 (2007), 043123-1:3. [28] S. Korte, W. Clegg, Phil Mag, 91 (2011) 1150-1162. [32] S. Korte, W. Clegg, Discussion of the dependence of the effect of size on the yield stress in hard materials studied by microcompression of MgO, Philos. Mag. 91 (2011) 1150–1162. [33] S.S. Singh, E. Guo, H. Xie, N. Chawla, Mechanical properties of intermetallic inclusions in Al 7075 alloys by micropillar compression, Intermetallics 62 (2015) 69–75.
Originality statement I write on behalf of myself and all co-authors to confirm that the results reported in the manuscript are original and neither the entire work, nor any of its parts have been previously published. The authors confirm that the article has not been submitted to peer review, nor has been accepted for publishing in another journal. The authors confirm that the research in their work is original, and that all the data given in the article are real and authentic. 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. CRediT authorship contribution statement R. Sarvesha: Conceptualization, Methodology, Writing - original draft. Ubaid-ur-Rehman Ghori: Methodology, Writing - review & editing. Yu Lung Chiu: Writing - review & editing. Ian P. Jones: Writing - review & editing. S.S. Singh: Conceptualization, Methodol ogy, Writing - review & editing. J. Jain: Conceptualization, Methodol ogy, Writing - review & editing. Acknowledgements Authors would like to thank Prof. Warren Poole of University of British Columbia for providing the AZ80 alloy. JJ also acknowledges the financial assistance provided by India-Birmingham institute fellowship during his stay at Birmingham. SSS acknowledges the financial support received from IIT Kanpur. We also acknowledge the support from the Electron Microscopy Facility at the Advanced Center of Materials Sci ence (ACMS) at IIT Kanpur. References [1] M.K. Kulekci, Magnesium and its alloys applications in automotive industry, Int. J. Adv. Manuf. Technol. 39 (2008) 851–865. [2] E. Aghion, B. Bronfin, Magnesium alloys development towards the 21st century, Mater. Sci. Forum 350–351 (2000) 19–30. [3] J.A. Yasi, L.G. Hector, D.R. Trinkle, First-principles data for solid-solution strengthening of magnesium: from geometry and chemistry to properties, Acta Mater. 58 (2010) 5704–5713. [4] J. Min, L.G. Hector Jr., J. Lin, J.T. Carter, A.K. Sachdev, Spatio-temporal characteristics of propagative plastic instabilities in a rare earth containing magnesium alloy, Int. J. Plast. 57 (2014) 52–76. [5] A.J. Carpenter, A.R. Antoniswamy, J.T. Carter, L.G. Hector Jr., E.M. Taleff, A mechanism-dependent material model for the effects of grain growth and anisotropy on plastic deformation of magnesium alloy AZ31 sheet at 450oC, Acta Mater. 68 (2014) 254–266. [6] G.P.M. Leyson, L.G. Hector Jr., W.A. Curtin, First-principles prediction of yield stress for basal slip in Mg-Al alloys, Acta Mater. 60 (2012) 5197–5203.
5