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Deformation and fracture behavior of hot extruded Mg alloys AZ31
M A TE RI A L S CH A R A CT ER IZ A TI O N 6 7 (2 0 1 2 ) 9 3–1 0 0
Available online at www.sciencedirect.com
www.elsevier.com/locate/matchar
Deformation and fracture behavior of hot extruded Mg alloys AZ31 Liwei Lua, b , Tianmo Liua, b,⁎, Yong Chena, b , Zhongchang Wangc,⁎⁎ a
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, PR China c WPI Research Center, Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b
AR TIC LE D ATA
ABSTR ACT
Article history:
Grains inside extruded Mg alloys AZ31 are usually elongated along the < 11 20 > crystal
Received 19 November 2011
direction with their c axis perpendicular to extrusion direction, which often affects
Received in revised form
mechanical properties of the AZ31 alloys significantly. Here, we conduct cold compressive
27 November 2011
and tensile deformation tests to the as-extruded AZ31 alloys, aimed at investigating the
Accepted 27 February 2012
role played by elongated grains in deformation and understanding the origin of fracture propagation. Using several analytic techniques, we characterize the fracture features
Keywords:
thoroughly and identify the fracture propagation mechanism as the dimpled rupture during
Mg alloys AZ31
cold tensile deformation. We also investigate the fundamental impact of the elongated
Cold deformation
grains, twins, inclusions, and secondary phase on the crack propagation, and propose an
Microstructure
effective way in modifying or even enhancing mechanical properties of the engineering
Fracture
important Mg alloys AZ31. © 2012 Elsevier Inc. All rights reserved.
1.
Introduction
Mg alloys find widespread applications in the aerospace and automotive fields for their high specific strength and low weight [1]. One of the critical issues currently limiting their potential uses is their low ductility at room temperature, which originates largely from the limited number of slip systems in the hexagonal close packed (HCP) structure of Mg alloys [2,3]. Since a large amount of twins have often been indentified once the Mg alloys are deformed at ambient temperature, it is generally believed that twinning plays a crucial role in accommodating the deformation of polycrystalline Mg alloys [4–7]. Indeed, it has been reported that twinning is able to coordinate the plastic deformation of Mg alloys so that grain misorientation is modulated, which as a result modifies the fracture behavior of Mg alloys [8].
From the engineering viewpoint, it is a prerequisite that the mechanical properties of Mg alloys should meet the fundamental demands on reliability and safety. However, for the materials with a poor plasticity such as Mg alloys, fracture often plays a crucial role in affecting textures and mechanical anisotropy, posing a significant hurdle to the practical applications of Mg alloys [9,10]. Surprisingly, little effort has been exerted on how the inhomogeneous microstructure can have an impact on plastic deformation and fracture behavior of Mg alloys in view of the relevance of the microstructure. In this work, we perform compressive and tensile tests to the as-extruded Mg alloys AZ31 with inhomogeneous microstructure, aimed at clarifying the fracture mechanism of the AZ31 alloys at room temperature. We demonstrate that the elongated grains in the as-extruded Mg alloys are not beneficial for their plasticity and the twinning plays a
⁎ Correspondence to: T. Liu, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China. ⁎⁎ Corresponding author. Tel.: + 81 22 217 5933; fax: +81 22 217 5930. E-mail addresses: [email protected] (T. Liu), [email protected] (Z. Wang). 1044-5803/$ – see front matter © 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2012.02.023
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M A TE RI A L S CH A R A CT ER IZ A TI O N 6 7 (2 0 1 2) 9 3–1 0 0
complicated role in the deformation. These findings help develop effective secondary techniques for processing Mg alloys.
2.
Experimental Procedures
The as-extruded Mg alloys AZ31 used in this study have a nominal composition (in mass %) of 3% Al, 1% Zn, 0.5% Mn, and Mg (balance). The samples were first sliced into discs with a thickness of 3 mm. Microstructures were observed using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Specimens for the OM observations were prepared by grinding the alloy discs with progressively finer grades of emery papers up to 2000 grit and etching in a mixed solution of acetic acid (10 ml), picric acid (4.2 g), H2O (10 ml), and ethanol (70 ml). For the TEM observations, discs were thinned to a thickness of ~50 μm using abrasive papers and further electro-polished to make electron transparent using twin-jet electro-polishing machine. The electro-polishing was operated at 12 mA and 110 V in a mixed solution of LiCl (5.3 g), Mg perchlorate (11.16 g), methanol (500 ml), and 2-butoxyethanol (100 ml). For mechanical measurements, the as-pressed alloys were machined into the samples with a cross-sectional dimension of Ф7 mm × 14 mm and a gauge section of 2 ×3 × 5 mm3 along the extrusion direction (ED). TEM images were taken using FETEM-200 electron microscope (Zeiss LIBRA) operated at an accelerating voltage of 200 kV. Grain orientations were studied by the electron backscattered diffraction using JEOL JSM-7600 field emission-gun scanning electron microscope, which was equipped with a Nordlys II detector. Scanning electron microscope was operated at a voltage of 20 kV and tilt angle of 70°. The HKL Channel 5 software was utilized to process the data obtained from the electron backscattered diffraction. Compressive and tensile experiments were conducted at room temperature under an initial strain rate of 1.8×10− 3 s− 1.
3.
Results and Discussion
3.1.
Microstructure of the As-Extruded Mg Alloys AZ31
Fig. 1 presents microstructures of the as-extruded Mg alloys AZ31. The typical microstructure consists of α-Mg matrixes
a
and fine particles that distribute in them (less than 1%), as shown in Fig. 1(a). Grains are mainly classified as equiaxed recrystallized grains together with a small fraction of elongated coarse grains, both of which are not homogeneously distributed. To shed light on the elongated coarse grains and their corresponding boundaries, we carry out TEM observations, as shown in Fig. 1(b). A large amount of heterogeneously distributed dislocations turn up along the boundaries of the elongated coarse grains, while in their interiors there appear substructures. These coarse grains with the nonequilibrium boundaries could store sufficient energy, posing a significant hurdle to the movement of basal slip during deformation. Closer inspections using the energy-dispersive X-ray spectroscopy (EDS) identify the fine particles in the matrixes as Al–Mn phase (Fig. 2(a)). Moreover, inclusions and pores are observed (Fig. 2(b)), which break into pieces along ED after extrusion. These observed elongated coarse grains, inclusions, pores, and secondary phase should play a relevant role in affecting strength, ductility, and plasticity of Mg alloys. To gain insight into the textures, we present in Fig. 3(a) the crystal orientation of the elongated grains. Interestingly, we see that all the elongated grains are well aligned along the < 1120 > crystal direction and their c axis is perpendicular to the ED. This is due to the fact that the majority of the elongated grains in the as-extruded alloys have the (0001) <1120 > slip system [11], in which the {0001} planes are parallel to the ED (Fig. 3(b)). This prevents the elongated grains from rotating during the cold deformation.
3.2.
Characteristics of the Compressive Fracture
Fig. 4 shows metallographic microstructure of the compressive fractures. From Fig. 4(a), we note that cracks are propagated from the inclusions and secondary phase in between the metal strips. In fact, the metal strips are also one species of steps which could serve as the sinks for cracks. A large amount of recrystallized tiny grains are also observed along the grain boundaries due to the inhomogeneous microstructures in the as-extruded alloys. The originally coarse and elongated grains are very difficult to play their parts in coordinating plastic deformation once stress is concentrated on the grain boundaries, resulting in asymmetric deformation among grains during the compressive processing. In general, the basal slip system in Mg alloys cannot be activated once the {0001} plane turns vertical to the compressive
b
50µm
100nm
Fig. 1 – Microstructures of the hot extruded Mg alloys AZ31 showing (a) overall feature, and (b) dislocations and substructures.
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M A TE RI A L S CH A R A CT ER IZ A TI O N 6 7 (2 0 1 2 ) 9 3–1 0 0
a
b
5µm
50µm
Fig. 2 – Identification and distribution of the phase in the AZ31 alloy: (a) Al–Mn phase (circled), and (b) distribution of defects.
stress. However, stress could be released via the application of external force, which results in twin formation primarily at the elongated grains and the boundaries of the coarse grains (Fig. 4(b)). From Fig. 4(c), one can see a limited number of twins in the dynamic recrystallized tiny grains, which indicates that these grains tend to prevent twins and cracks from taking place. Further, we note that most grains are compressed along the direction approximately perpendicular to the c axis, while extended along the direction parallel to the c axis. Since the contraction twinning generally requires a much larger critical resolved shear stress (CRSS) (~112 MPa) than the tensile twinning (~3 MPa) [12,13], majority of twins can be classified as tensile twinning. However, the commonly occurring tensile twinning {10 12} <1011> exhibits a rotation orientation of 86° <1120> between the twin variants and untwined matrix, and the shear strain can reach a value of 0.131 [14]. This slip plane
a
50µm
ED
b {0001}
{10-10}
Fig. 3 – (a) EBSD map of the elongated grains and the crystal orientation, and (b) the plots of {0001} and {1010} pole for the alloys extruded along y axis.
may not be strictly perpendicular to compressive stress, which forces the original basal slip system to be initiated again. From Fig. 4(d), we note that the crack propagation progress is altered: the original crack propagation is along the grain boundaries when the cracks come across the tiny grains and twins, while the new propagation process is symmetric. This demonstrates that twins and tiny grains play an important role in preventing cracks from taking place and propagating, which enhances the ductility of Mg alloys [15]. However, there are some newly formed narrow band twins at boundaries of elongated and coarse grains, which are identified as {1011}, {1011}–{1120}, and {3032} twins [16]. These twins have a high density of dislocations, which can serve as the nucleation sites for cracks (Fig. 4(b)). However, it remains quite difficult to index all of these twins in the EBSD patterns (Fig. 5(a)) [17]. The boundaries of the elongated coarse grains are curved due to the stress arising from the rotation of the grains neighboring the elongated ones. The {0001} planes of the elongated grains are hence vertical to the compression direction (CD). In this sense, the plasticity of AZ31 alloys can be improved via eliminating the elongated grains so as to obtain homogeneous microstructures and refined grains in the alloys. The {0001} and {10 10} poles in Fig. 5(b) differ remarkably from those shown in Fig. 3(b): majority of {0001} plane becomes normal to the ED and textures turn less pronounced. Twinning occurs on the {10 11} plane along the ED during compression at room temperature, which is followed by re-twinning and secondary twinning on the {10 12} plane. Ultimately, there appear rapid flow localization and failure. The {0001} plane can therefore be tuned to a more favorable orientation for the gliding until it is perpendicular to the CD [18]. Consequently, a large amount of stress is concentrated on the boundaries of the elongated grains, and the Schmid factor turns nil, which means that the {0001} plane is in a direction that hinders the basal slip. We therefore attribute the low ductility of the alloys to the boundaries of elongated grains with stress, which pose a hurdle to the initiation of easily activated slip or twinning systems in order to accommodate compression parallel to the c axis of the HCP structure. Fig. 6 shows SEM image of compression fracture. The compression fracture shares some features with the quasicleavage fracture: (i) they both are composed of ductile and brittle cracks, and (ii) there appear some cleavage planes and
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M A TE RI A L S CH A R A CT ER IZ A TI O N 6 7 (2 0 1 2) 9 3–1 0 0
a
b
50µm
50µm
c
d
50µm
50µm
Fig. 4 – Characteristic of the compressive fracture: (a) crack initiation and propagation, (b) elongated grains and twins, (c) fine grains, and (d) crack and twinning on fracture edge.
fine dimples in twins generated by atom shearing (Fig. 6(a)). The dimples are elongated and evolved to the wavy rock fractures. Moreover, the brittle cracks and intergranular fractures
are found to be generated along twin boundaries and {0001} plane of large grains when the deformation is severe (Fig. 6(b)). Fig. 6(c) shows the typical ductile and brittle fractures.
a ED
20µm
b {0001}
{10-10}
Fig. 5 – (a) EBSD map of the elongated grains and the crystal orientation, and (b) the pole plots of {0001} and {1010} for the alloys compressed along x axis.
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M A TE RI A L S CH A R A CT ER IZ A TI O N 6 7 (2 0 1 2 ) 9 3–1 0 0
a
b
100µm
c
100µm
d
50µm
25µm
Fig. 6 – SEM images of compressive fracture showing (a) mixed dimples and cleavage planes, (b) intergranular fracture, (c) ridges in between the ductile and brittle regions, and (d) secondary phases and shearing planes.
Interestingly, a large amount of ridges with a length of ~30 μm emerge in between the ductile and brittle regions. In addition, there also appear a large number of secondary phase and shearing planes on the surfaces of the fractures (Fig. 6(d)). This again indicates that the secondary phase plays an important role in influencing fracture behavior during the compressive tests at ambient temperature.
3.3.
Characteristics of the Tensile Fracture
Fig. 7 shows metallographic microstructure of the tensile fracture. One can first see a large amount of cracks present in the vicinity of fractures (Fig. 7(a)), which are initiated at the elongated grains and twin boundaries. The twins emerge primarily at the boundaries of elongated grains (Fig. 7(b)). We also see a great quantity of twins, which interact with cracks (Fig. 7(c)). Namely, the tip regions of cracks induce twins and the twin boundaries in turn promote nucleation of cracks [19]. The twins and fractures are produced in a fast fashion, and hence the tips of cracks suffer a large concentration of stress, which contributes to the formation of twins. Although the formations of twins and fractures both allow the release of stress, they two are actually competing [20], depending on their magnitude of CRSS [21]. Under the tensile stress, some grains are elongated and separated into many tiny grains as a direct result of drastic deformation (Fig. 7(b) and (c)). Fig. 8(a) and (b) illustrates representative microstructures of twins and cracks, where the species of twins are differentiated in color. The boundary frequency for the {1011}–{1012}, {1012} <1011>, {1012}–{1012}, {1011} <1012>, and {1013} <3032> twins is calculated to be 7.26%, 3.06%, 0.713%, 0.688%, and 0.464%,
respectively. It should be noted that it remains difficult to index all the EBSD patterns inside the contraction twins. A significant proportion of boundaries is associated with the {10 12} <1011> twinning, which is attributed to the abnormal stress induced by the deformation [22]. The presence of the {1011} <10 12> and {1013} <3032> contraction twinning and the {1011}–{10 12} double twinning poses a hurdle to further pursuing plastic deformation. Since the c axis of the elongated grains is perpendicular to the ED, rotation of the grains neighboring the elongated grains can impose a large stress on the elongated grains, which induces the formation of {1013} <3032> and {1011}–{1012} twinning. The twinning promotes the nucleation of cracks, which can penetrate grain boundaries and spread. Fig. 8(c) shows the {0001} and {1010} pole of the tensile fracture, where most of the basal poles subjected to shear stress are shifted by nearly 45° from the tensile direction (TD) (Fig. 3(b)). This explains the observed 45° tensile fractures and the fact that textures turn stronger after deformation. Fig. 9 shows microstructures of crack initiation and propagation, where one can see first many dimples on surfaces of the tensile fractures (Fig. 9(a)). The presence of dimples represents a typical characteristic of ductile cracks, indicating that the ductility is enhanced. However, due to the difficulty in coordinating deformation, micro-cavities emerge (Fig. 9(b)) and the stress turns three dimensional at the neck, which eventually result in rapid propagation and stretch of the micro-cavities. From Fig. 9(c), we can see that dimples are distributed in local fractures, at tips of sharp cracks, or even inside the tearing fractures. Moreover, we note in Fig. 9(d) that the stress accumulated at corner of deformed microcavities can trigger cracks under the tensile stress, which
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a
a
ED
A B
B 30µm
50µm
b
b {1011}-{1012} {1012}<1011> {1011}<1012>
A
{1012}-{1012} {1013}<3032>
B B
30µm
c 50µm
{0001}
{10-10}
c
50µm
Fig. 7 – Characteristic of the tensile fracture: (a) fracture edge with no microstructure background, (b) fracture edge with microstructure, and (c) microstructure of elongated grains and twins. The A and B mark the cracks from the boundaries of the elongated grains and the twins, respectively.
ultimately leads to the fractures in the AZ31 alloys [23]. Further propagation of cracks may either be obstructed by the secondary phase, fine grains and twins or continue as the cracks spread across the interfaces of the secondary phases, boundaries of fine grains, and twin boundaries (Fig. 9(e)). In this sense, the number of elongated grains, inclusions, and pores is lessened and in the meanwhile homogeneous and fine secondary phases are increased, thereby improving properties of Mg alloys AZ31.
4.
Fig. 8 – (a) EBSD map of the elongated grains, twins, and cracks, (b) demarcation of twin boundaries, and (c) plots of {0001} and {1010} poles. The x axis represents the tensile direction.
elongated coarse grains are found unable to coordinate plastic deformation during the cold compression owing to the large stress localized at grain boundaries, which arises from the asymmetric rotation among grains. We identify the fracture propagation mechanism as the dimpled rupture during the cold tensile deformation. Moreover, the contraction twins are found to emerge dominantly at the boundaries of the elongated grains because their c axis is almost perpendicular to the tensile direction. The twins and cracks interact mutually, namely, the cracks induce twins, and the {1013} <3032> and {1011}–{1012} twinning promote the nucleation of cracks. We therefore conclude that the elimination of the elongated grains, inclusions, and pores so as to obtain a homogeneous microstructure should serve as an effective way in modifying properties of the engineering important Mg alloys.
Conclusions Acknowledgments
We have applied compressive and tensile tests to the as-extruded Mg alloys AZ31 and systematically investigated microstructures and origin of fractures. We find that the elongated grains are harmful for plasticity of Mg alloys due to the discordant rotation among grains of varying sizes. The original
This work is supported in part by the National 973 Major Project of China, “The Key Fundamental Problem of Processing and Preparation for High Performance Magnesium Alloy”, under Grant no. 2007CB613700. Z.W. thanks supports from the Grant-
99
M A TE RI A L S CH A R A CT ER IZ A TI O N 6 7 (2 0 1 2 ) 9 3–1 0 0
a
b
100µm
c
50µm
d
25µm
50µm
e
20µm
Fig. 9 – SEM images of the tensile fracture showing (a) fracture, (b) cavity nucleation, (c) crack initiation, (d) and (e) crack propagation. in-Aid for Young Scientists (B) (Grant no. 22760500) and from IZUMI Science and Technology Foundation.
REFERENCES [1] Lou Y, Li LX, Zhou J, Na L. Deformation behavior of Mg–8Al magnesium alloy compressed at medium and high temperatures. Mater Charact 2011;62:346–53. [2] Chao HY, Sun HF, Chen WZ, Wang ED. Static recrystallization kinetics of a heavily cold drawn AZ31 magnesium alloy under annealing treatment. Mater Charact 2011;62:312–20. [3] Wang YN, Huang JC. Comparison of grain boundary sliding in fine grained Mg and Al alloys during superplastic deformation. Scr Mater 2003;48:1117–22. [4] Choi SH, Shin EJ, Seong BS. Simulation of deformation twins and deformation texture in an AZ31 Mg alloy under uniaxial compression. Acta Mater 2007;55:4181–92. [5] Chang LL, Wang YN, Zhao X, Qi M. Grain size and texture effect on compression behavior of hot-extruded Mg–3Al–1Zn alloys at room temperature. Mater Charact 2009;60:991–4.
[6] Jiang J, Godfrey A, Liu W, Liu Q. Microtexture evolution via deformation twinning and slip during compression of magnesium alloy AZ31. Mater Sci Eng A 2008;483–484:576–9. [7] Li WP, Zhou H, Lin PY, Zhao SZ. Microstructure and rolling capability of modified AZ31–Ce–Gd alloys. Mater Charact 2009;60:1298–301. [8] Masoudpanah SM, Mahmudi R. Effects of rare-earth elements and Ca additions on the microstructure and mechanical properties of AZ31 magnesium alloy processed by ECAP. Mater Sci Eng A 2009;526:22–30. [9] Somekawa H, Singh A, Mukai T. High fracture toughness of extruded Mg–Zn–Y alloy by the synergistic effect of grain refinement and dispersion of quasicrystalline phase. Scr Mater 2007;56:1091–4. [10] Yan H, Chen RS, Han EH. A comparative study of texture and ductility of Mg–1.2Zn–0.8Gd alloy fabricated by rolling and equal channel angular extrusion. Mater Charact 2011;62: 321–6. [11] Azeem MA, Tewari A, Mishra S, Gollapudi S, Ramamurty U. Development of novel grain morphology during hot extrusion of magnesium AZ21 alloy. Acta Mater 2010;58:1495–502. [12] Reed-Hill RE, Robertson WD. Additional modes of deformation twinning in magnesium. Acta Metall 1957;5:717–27.
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[13] Liu JZ, Liu TM, Yuan HQ, Shi XL, Wang ZC. Effect of cold forging and static recrystallization on microstructure and mechanical property of magnesium alloy AZ31. Mater Trans 2010;51:341–6. [14] Jiang S, Liu TM, Lu LW, Zeng W, Wang ZC. Atomic motion in Mg– 3Al–1Zn during twinning deformation. Scr Mater 2010;62:556–9. [15] Yoo MH. Slip, twinning, and fracture in hexagonal close-packed metals. Metallogr Trans A 1981;12:409–18. [16] Chino Y, Kimura K, Mabuchi M. Twinning behavior and deformation mechanisms of extruded AZ31 Mg alloy. Mater Sci Eng A 2008;486:481–8. [17] Cizek P, Barnett MR. Characteristics of the contraction twins formed close to the fracture surface in Mg–3Al–1Zn alloy deformed in tension. Scr Mater 2008;59:959–62. [18] Barnett MR, Keshavarz Z, Beer AG, Ma X. Non-Schmid behaviour during secondary twinning in a polycrystalline magnesium alloy. Acta Mater 2008;56:5–15.
[19] Bilby BA, Howard IC, Li ZH, Sbeikh MA. Some experience in the use of damage mechanics to simulate crack behavior in specimens and structures. Int J Press Vessels Piping 1995;64: 213–23. [20] Li RG, Fang DQ, An J, Lu Y, Cao ZY, Liu YB. Comparative studies on the microstructure evolution and fracture behavior between hot-rolled and as-cast Mg(96)ZnY(3) alloys. Mater Charact 2009;60:470–5. [21] Yoo MH, Wei CT. Slip modes of hexagonal-close-packed metals. J Appl Phys 1967;38:4317–22. [22] Nave MD, Barnett MR. Microstructures and textures of pure magnesium deformed in plane-strain compression. Scr Mater 2004;51:881–5. [23] Weiler JP, Wood JT, Klassen RJ, Maire E, Berkmortel R, Wang G. Relationship between internal porosity and fracture strength of die-cast magnesium AM60B alloy. Mater Sci Eng A 2005;395:315–22.
Available online at www.sciencedirect.com
www.elsevier.com/locate/matchar
Deformation and fracture behavior of hot extruded Mg alloys AZ31 Liwei Lua, b , Tianmo Liua, b,⁎, Yong Chena, b , Zhongchang Wangc,⁎⁎ a
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, PR China c WPI Research Center, Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b
AR TIC LE D ATA
ABSTR ACT
Article history:
Grains inside extruded Mg alloys AZ31 are usually elongated along the < 11 20 > crystal
Received 19 November 2011
direction with their c axis perpendicular to extrusion direction, which often affects
Received in revised form
mechanical properties of the AZ31 alloys significantly. Here, we conduct cold compressive
27 November 2011
and tensile deformation tests to the as-extruded AZ31 alloys, aimed at investigating the
Accepted 27 February 2012
role played by elongated grains in deformation and understanding the origin of fracture propagation. Using several analytic techniques, we characterize the fracture features
Keywords:
thoroughly and identify the fracture propagation mechanism as the dimpled rupture during
Mg alloys AZ31
cold tensile deformation. We also investigate the fundamental impact of the elongated
Cold deformation
grains, twins, inclusions, and secondary phase on the crack propagation, and propose an
Microstructure
effective way in modifying or even enhancing mechanical properties of the engineering
Fracture
important Mg alloys AZ31. © 2012 Elsevier Inc. All rights reserved.
1.
Introduction
Mg alloys find widespread applications in the aerospace and automotive fields for their high specific strength and low weight [1]. One of the critical issues currently limiting their potential uses is their low ductility at room temperature, which originates largely from the limited number of slip systems in the hexagonal close packed (HCP) structure of Mg alloys [2,3]. Since a large amount of twins have often been indentified once the Mg alloys are deformed at ambient temperature, it is generally believed that twinning plays a crucial role in accommodating the deformation of polycrystalline Mg alloys [4–7]. Indeed, it has been reported that twinning is able to coordinate the plastic deformation of Mg alloys so that grain misorientation is modulated, which as a result modifies the fracture behavior of Mg alloys [8].
From the engineering viewpoint, it is a prerequisite that the mechanical properties of Mg alloys should meet the fundamental demands on reliability and safety. However, for the materials with a poor plasticity such as Mg alloys, fracture often plays a crucial role in affecting textures and mechanical anisotropy, posing a significant hurdle to the practical applications of Mg alloys [9,10]. Surprisingly, little effort has been exerted on how the inhomogeneous microstructure can have an impact on plastic deformation and fracture behavior of Mg alloys in view of the relevance of the microstructure. In this work, we perform compressive and tensile tests to the as-extruded Mg alloys AZ31 with inhomogeneous microstructure, aimed at clarifying the fracture mechanism of the AZ31 alloys at room temperature. We demonstrate that the elongated grains in the as-extruded Mg alloys are not beneficial for their plasticity and the twinning plays a
⁎ Correspondence to: T. Liu, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China. ⁎⁎ Corresponding author. Tel.: + 81 22 217 5933; fax: +81 22 217 5930. E-mail addresses: [email protected] (T. Liu), [email protected] (Z. Wang). 1044-5803/$ – see front matter © 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2012.02.023
94
M A TE RI A L S CH A R A CT ER IZ A TI O N 6 7 (2 0 1 2) 9 3–1 0 0
complicated role in the deformation. These findings help develop effective secondary techniques for processing Mg alloys.
2.
Experimental Procedures
The as-extruded Mg alloys AZ31 used in this study have a nominal composition (in mass %) of 3% Al, 1% Zn, 0.5% Mn, and Mg (balance). The samples were first sliced into discs with a thickness of 3 mm. Microstructures were observed using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Specimens for the OM observations were prepared by grinding the alloy discs with progressively finer grades of emery papers up to 2000 grit and etching in a mixed solution of acetic acid (10 ml), picric acid (4.2 g), H2O (10 ml), and ethanol (70 ml). For the TEM observations, discs were thinned to a thickness of ~50 μm using abrasive papers and further electro-polished to make electron transparent using twin-jet electro-polishing machine. The electro-polishing was operated at 12 mA and 110 V in a mixed solution of LiCl (5.3 g), Mg perchlorate (11.16 g), methanol (500 ml), and 2-butoxyethanol (100 ml). For mechanical measurements, the as-pressed alloys were machined into the samples with a cross-sectional dimension of Ф7 mm × 14 mm and a gauge section of 2 ×3 × 5 mm3 along the extrusion direction (ED). TEM images were taken using FETEM-200 electron microscope (Zeiss LIBRA) operated at an accelerating voltage of 200 kV. Grain orientations were studied by the electron backscattered diffraction using JEOL JSM-7600 field emission-gun scanning electron microscope, which was equipped with a Nordlys II detector. Scanning electron microscope was operated at a voltage of 20 kV and tilt angle of 70°. The HKL Channel 5 software was utilized to process the data obtained from the electron backscattered diffraction. Compressive and tensile experiments were conducted at room temperature under an initial strain rate of 1.8×10− 3 s− 1.
3.
Results and Discussion
3.1.
Microstructure of the As-Extruded Mg Alloys AZ31
Fig. 1 presents microstructures of the as-extruded Mg alloys AZ31. The typical microstructure consists of α-Mg matrixes
a
and fine particles that distribute in them (less than 1%), as shown in Fig. 1(a). Grains are mainly classified as equiaxed recrystallized grains together with a small fraction of elongated coarse grains, both of which are not homogeneously distributed. To shed light on the elongated coarse grains and their corresponding boundaries, we carry out TEM observations, as shown in Fig. 1(b). A large amount of heterogeneously distributed dislocations turn up along the boundaries of the elongated coarse grains, while in their interiors there appear substructures. These coarse grains with the nonequilibrium boundaries could store sufficient energy, posing a significant hurdle to the movement of basal slip during deformation. Closer inspections using the energy-dispersive X-ray spectroscopy (EDS) identify the fine particles in the matrixes as Al–Mn phase (Fig. 2(a)). Moreover, inclusions and pores are observed (Fig. 2(b)), which break into pieces along ED after extrusion. These observed elongated coarse grains, inclusions, pores, and secondary phase should play a relevant role in affecting strength, ductility, and plasticity of Mg alloys. To gain insight into the textures, we present in Fig. 3(a) the crystal orientation of the elongated grains. Interestingly, we see that all the elongated grains are well aligned along the < 1120 > crystal direction and their c axis is perpendicular to the ED. This is due to the fact that the majority of the elongated grains in the as-extruded alloys have the (0001) <1120 > slip system [11], in which the {0001} planes are parallel to the ED (Fig. 3(b)). This prevents the elongated grains from rotating during the cold deformation.
3.2.
Characteristics of the Compressive Fracture
Fig. 4 shows metallographic microstructure of the compressive fractures. From Fig. 4(a), we note that cracks are propagated from the inclusions and secondary phase in between the metal strips. In fact, the metal strips are also one species of steps which could serve as the sinks for cracks. A large amount of recrystallized tiny grains are also observed along the grain boundaries due to the inhomogeneous microstructures in the as-extruded alloys. The originally coarse and elongated grains are very difficult to play their parts in coordinating plastic deformation once stress is concentrated on the grain boundaries, resulting in asymmetric deformation among grains during the compressive processing. In general, the basal slip system in Mg alloys cannot be activated once the {0001} plane turns vertical to the compressive
b
50µm
100nm
Fig. 1 – Microstructures of the hot extruded Mg alloys AZ31 showing (a) overall feature, and (b) dislocations and substructures.
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a
b
5µm
50µm
Fig. 2 – Identification and distribution of the phase in the AZ31 alloy: (a) Al–Mn phase (circled), and (b) distribution of defects.
stress. However, stress could be released via the application of external force, which results in twin formation primarily at the elongated grains and the boundaries of the coarse grains (Fig. 4(b)). From Fig. 4(c), one can see a limited number of twins in the dynamic recrystallized tiny grains, which indicates that these grains tend to prevent twins and cracks from taking place. Further, we note that most grains are compressed along the direction approximately perpendicular to the c axis, while extended along the direction parallel to the c axis. Since the contraction twinning generally requires a much larger critical resolved shear stress (CRSS) (~112 MPa) than the tensile twinning (~3 MPa) [12,13], majority of twins can be classified as tensile twinning. However, the commonly occurring tensile twinning {10 12} <1011> exhibits a rotation orientation of 86° <1120> between the twin variants and untwined matrix, and the shear strain can reach a value of 0.131 [14]. This slip plane
a
50µm
ED
b {0001}
{10-10}
Fig. 3 – (a) EBSD map of the elongated grains and the crystal orientation, and (b) the plots of {0001} and {1010} pole for the alloys extruded along y axis.
may not be strictly perpendicular to compressive stress, which forces the original basal slip system to be initiated again. From Fig. 4(d), we note that the crack propagation progress is altered: the original crack propagation is along the grain boundaries when the cracks come across the tiny grains and twins, while the new propagation process is symmetric. This demonstrates that twins and tiny grains play an important role in preventing cracks from taking place and propagating, which enhances the ductility of Mg alloys [15]. However, there are some newly formed narrow band twins at boundaries of elongated and coarse grains, which are identified as {1011}, {1011}–{1120}, and {3032} twins [16]. These twins have a high density of dislocations, which can serve as the nucleation sites for cracks (Fig. 4(b)). However, it remains quite difficult to index all of these twins in the EBSD patterns (Fig. 5(a)) [17]. The boundaries of the elongated coarse grains are curved due to the stress arising from the rotation of the grains neighboring the elongated ones. The {0001} planes of the elongated grains are hence vertical to the compression direction (CD). In this sense, the plasticity of AZ31 alloys can be improved via eliminating the elongated grains so as to obtain homogeneous microstructures and refined grains in the alloys. The {0001} and {10 10} poles in Fig. 5(b) differ remarkably from those shown in Fig. 3(b): majority of {0001} plane becomes normal to the ED and textures turn less pronounced. Twinning occurs on the {10 11} plane along the ED during compression at room temperature, which is followed by re-twinning and secondary twinning on the {10 12} plane. Ultimately, there appear rapid flow localization and failure. The {0001} plane can therefore be tuned to a more favorable orientation for the gliding until it is perpendicular to the CD [18]. Consequently, a large amount of stress is concentrated on the boundaries of the elongated grains, and the Schmid factor turns nil, which means that the {0001} plane is in a direction that hinders the basal slip. We therefore attribute the low ductility of the alloys to the boundaries of elongated grains with stress, which pose a hurdle to the initiation of easily activated slip or twinning systems in order to accommodate compression parallel to the c axis of the HCP structure. Fig. 6 shows SEM image of compression fracture. The compression fracture shares some features with the quasicleavage fracture: (i) they both are composed of ductile and brittle cracks, and (ii) there appear some cleavage planes and
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a
b
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50µm
c
d
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Fig. 4 – Characteristic of the compressive fracture: (a) crack initiation and propagation, (b) elongated grains and twins, (c) fine grains, and (d) crack and twinning on fracture edge.
fine dimples in twins generated by atom shearing (Fig. 6(a)). The dimples are elongated and evolved to the wavy rock fractures. Moreover, the brittle cracks and intergranular fractures
are found to be generated along twin boundaries and {0001} plane of large grains when the deformation is severe (Fig. 6(b)). Fig. 6(c) shows the typical ductile and brittle fractures.
a ED
20µm
b {0001}
{10-10}
Fig. 5 – (a) EBSD map of the elongated grains and the crystal orientation, and (b) the pole plots of {0001} and {1010} for the alloys compressed along x axis.
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b
100µm
c
100µm
d
50µm
25µm
Fig. 6 – SEM images of compressive fracture showing (a) mixed dimples and cleavage planes, (b) intergranular fracture, (c) ridges in between the ductile and brittle regions, and (d) secondary phases and shearing planes.
Interestingly, a large amount of ridges with a length of ~30 μm emerge in between the ductile and brittle regions. In addition, there also appear a large number of secondary phase and shearing planes on the surfaces of the fractures (Fig. 6(d)). This again indicates that the secondary phase plays an important role in influencing fracture behavior during the compressive tests at ambient temperature.
3.3.
Characteristics of the Tensile Fracture
Fig. 7 shows metallographic microstructure of the tensile fracture. One can first see a large amount of cracks present in the vicinity of fractures (Fig. 7(a)), which are initiated at the elongated grains and twin boundaries. The twins emerge primarily at the boundaries of elongated grains (Fig. 7(b)). We also see a great quantity of twins, which interact with cracks (Fig. 7(c)). Namely, the tip regions of cracks induce twins and the twin boundaries in turn promote nucleation of cracks [19]. The twins and fractures are produced in a fast fashion, and hence the tips of cracks suffer a large concentration of stress, which contributes to the formation of twins. Although the formations of twins and fractures both allow the release of stress, they two are actually competing [20], depending on their magnitude of CRSS [21]. Under the tensile stress, some grains are elongated and separated into many tiny grains as a direct result of drastic deformation (Fig. 7(b) and (c)). Fig. 8(a) and (b) illustrates representative microstructures of twins and cracks, where the species of twins are differentiated in color. The boundary frequency for the {1011}–{1012}, {1012} <1011>, {1012}–{1012}, {1011} <1012>, and {1013} <3032> twins is calculated to be 7.26%, 3.06%, 0.713%, 0.688%, and 0.464%,
respectively. It should be noted that it remains difficult to index all the EBSD patterns inside the contraction twins. A significant proportion of boundaries is associated with the {10 12} <1011> twinning, which is attributed to the abnormal stress induced by the deformation [22]. The presence of the {1011} <10 12> and {1013} <3032> contraction twinning and the {1011}–{10 12} double twinning poses a hurdle to further pursuing plastic deformation. Since the c axis of the elongated grains is perpendicular to the ED, rotation of the grains neighboring the elongated grains can impose a large stress on the elongated grains, which induces the formation of {1013} <3032> and {1011}–{1012} twinning. The twinning promotes the nucleation of cracks, which can penetrate grain boundaries and spread. Fig. 8(c) shows the {0001} and {1010} pole of the tensile fracture, where most of the basal poles subjected to shear stress are shifted by nearly 45° from the tensile direction (TD) (Fig. 3(b)). This explains the observed 45° tensile fractures and the fact that textures turn stronger after deformation. Fig. 9 shows microstructures of crack initiation and propagation, where one can see first many dimples on surfaces of the tensile fractures (Fig. 9(a)). The presence of dimples represents a typical characteristic of ductile cracks, indicating that the ductility is enhanced. However, due to the difficulty in coordinating deformation, micro-cavities emerge (Fig. 9(b)) and the stress turns three dimensional at the neck, which eventually result in rapid propagation and stretch of the micro-cavities. From Fig. 9(c), we can see that dimples are distributed in local fractures, at tips of sharp cracks, or even inside the tearing fractures. Moreover, we note in Fig. 9(d) that the stress accumulated at corner of deformed microcavities can trigger cracks under the tensile stress, which
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a
a
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A B
B 30µm
50µm
b
b {1011}-{1012} {1012}<1011> {1011}<1012>
A
{1012}-{1012} {1013}<3032>
B B
30µm
c 50µm
{0001}
{10-10}
c
50µm
Fig. 7 – Characteristic of the tensile fracture: (a) fracture edge with no microstructure background, (b) fracture edge with microstructure, and (c) microstructure of elongated grains and twins. The A and B mark the cracks from the boundaries of the elongated grains and the twins, respectively.
ultimately leads to the fractures in the AZ31 alloys [23]. Further propagation of cracks may either be obstructed by the secondary phase, fine grains and twins or continue as the cracks spread across the interfaces of the secondary phases, boundaries of fine grains, and twin boundaries (Fig. 9(e)). In this sense, the number of elongated grains, inclusions, and pores is lessened and in the meanwhile homogeneous and fine secondary phases are increased, thereby improving properties of Mg alloys AZ31.
4.
Fig. 8 – (a) EBSD map of the elongated grains, twins, and cracks, (b) demarcation of twin boundaries, and (c) plots of {0001} and {1010} poles. The x axis represents the tensile direction.
elongated coarse grains are found unable to coordinate plastic deformation during the cold compression owing to the large stress localized at grain boundaries, which arises from the asymmetric rotation among grains. We identify the fracture propagation mechanism as the dimpled rupture during the cold tensile deformation. Moreover, the contraction twins are found to emerge dominantly at the boundaries of the elongated grains because their c axis is almost perpendicular to the tensile direction. The twins and cracks interact mutually, namely, the cracks induce twins, and the {1013} <3032> and {1011}–{1012} twinning promote the nucleation of cracks. We therefore conclude that the elimination of the elongated grains, inclusions, and pores so as to obtain a homogeneous microstructure should serve as an effective way in modifying properties of the engineering important Mg alloys.
Conclusions Acknowledgments
We have applied compressive and tensile tests to the as-extruded Mg alloys AZ31 and systematically investigated microstructures and origin of fractures. We find that the elongated grains are harmful for plasticity of Mg alloys due to the discordant rotation among grains of varying sizes. The original
This work is supported in part by the National 973 Major Project of China, “The Key Fundamental Problem of Processing and Preparation for High Performance Magnesium Alloy”, under Grant no. 2007CB613700. Z.W. thanks supports from the Grant-
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b
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c
50µm
d
25µm
50µm
e
20µm
Fig. 9 – SEM images of the tensile fracture showing (a) fracture, (b) cavity nucleation, (c) crack initiation, (d) and (e) crack propagation. in-Aid for Young Scientists (B) (Grant no. 22760500) and from IZUMI Science and Technology Foundation.
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