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Improving the plasticity of bulk metallic glasses via pre-compression below the yield stress
Materials Science & Engineering A 602 (2014) 68–76
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Improving the plasticity of bulk metallic glasses via pre-compression below the yield stress Ji Gu a, Min Song a,n, Song Ni a, Xiaozhou Liao b, Shengfeng Guo c a
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia c Faculty of Materials and Energy, Southwest University, Chongqing 400715, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 29 December 2013 Received in revised form 19 February 2014 Accepted 19 February 2014 Available online 26 February 2014
The poor plasticity of bulk metallic glasses (BMGs) has severely limited their potential structural applications. In this paper, we reported that uniaxial pre-compression below the yield stress can be used to significantly improve the plasticity of BMGs. An as-cast Cu36Zr48Al8Ag8 BMG was pre-compressed under various pressures (400, 800 and 1600 MPa) and durations (30, 60 and 120 min). The pre-compression processes led to structural change that increases the free volume and produces nanocrystals in the BMG matrix, which subsequently improved the plasticity of the BMG. Detailed data analysis showed that increasing the applied pressure speeds up the structural relaxation process and nanocrystallization occurs only at high enough pressure. This investigation indicated that appropriate selection of the pressure and duration of a pre-compression process is critical for the improved plasticity of BMGs. & 2014 Elsevier B.V. All rights reserved.
Keywords: Bulk metallic glasses Pre-compression Plasticity Free volume Nanocrystallite
1. Introduction Bulk metallic glasses (BMGs) possess excellent mechanical properties including high hardness and strength, large elastic limit and excellent resistance to wear and corrosion [1–4]. However, the poor plasticity at room temperature severely restricts the applications of BMGs as structural materials. It has been well accepted that the poor plasticity is attributed to the shear localization and strain/thermal softening during deformation [3]. Significant efforts have been made to improve the plasticity of BMGs at room temperature. Three types of general strategies have been developed: (1) designing proper compositions of BMGs with high plasticity [5–8]; (2) producing composite structures that comprise amorphous matrix and second phase particles or dendrites. This can be achieved through externally added strengthening particles [9–11] and in-situ formation of second phase particles or dendrites [12–14]; and (3) introducing structural inhomogeneity in BMGs via pre-plastic deformation to produce residual stress [15,16], preexisting shear bands and excess free volume [17–20]. The first strategy is particularly sensitive to the BMG composition and even the heat treatment conditions. A slight change in stoichiometry can lead to dramatic ductile-to-brittle transition [6,21]. The second strategy usually requires good control of the casting and annealing conditions. In addition, the externally added strengthening
n
Corresponding author. Tel.: þ 86 731 88877677; fax: þ86 731 88710855. E-mail address: [email protected] (M. Song).
http://dx.doi.org/10.1016/j.msea.2014.02.065 0921-5093 & 2014 Elsevier B.V. All rights reserved.
particles may have a severe interface cohesion problem with the amorphous matrix, while annealing treatment that is used to introduce in-situ second phase particles or dendrites can lead to the annihilation of free volume and thus result in drastic brittleness of BMGs [22]. In recent years, the third strategy has been widely used due to its easy utilization and its independence on the composition of BMGs [15–20,23,24]. Techniques including high pressure treatment [17], quasi-constrained high-pressure torsion [18], lateral pre-compression treatment [23], equal channel angular pressing [24], shot peening [15], and cold rolling [19] have been used for the third strategy. It should be noted that the third strategy requires the application of plastic deformation that may not be suitable for some very brittle BMGs. Recent investigations [25,26] indicated that, different from crystalline materials in which elastic deformation does not lead to any permanent local structural change, irreversible structural changes take place in BMGs during inelastic deformation processes, i.e., by the application of a stress below the yield stress of the BMGs, which creates new shear transition zones (STZs) and excess free volume. The introduction of the excess free volume might substantially improve the plasticity of BMGs. In this paper, we presented a systematic investigation of the effects of precompression conditions (including both the pressure and compression duration) under pressures lower than the yield stress on the free volume and nanocrystallization, deformation behaviour and plasticity of BMGs. Our investigation indicates that precompression below the yield stress is a convenient and effective method to improve the plasticity of BMGs.
J. Gu et al. / Materials Science & Engineering A 602 (2014) 68–76
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Table 1 A list of the samples investigated in the present work. Sample
Pressure (MPa)
Duration (min)
Group P P1 P2 P3 (T2)
400 800 1600
60 60 60
Group T T1 T2 (P3) T3
1600 1600 1600
30 60 120
2. Experimental A Cu36Zr48Al8Ag8 (atomic percentage) BMG master ingot was prepared by arc melting high-purity copper, zirconium, aluminium, and silver (99.7–99.99%) in a Ti-gettered high-purity argon atmosphere using the method described in Ref. [27]. To ensure compositional homogeneity, the ingot was re-melted 4 times and stirred by a magnetic beater. The copper mould suction casting method was used to obtain cylindrical rods with a diameter of 3 mm and a length of 70 mm.The rods were cut into short rods with the length slightly longer than 4.5 mm using a low-speed diamond saw. The two ends of each short rod were carefully polished to acquire rod geometry with a length to diameter ratio of 1.5:1. Some rods were pre-compressed under stresses below the yield stress of the Cu36Zr48Al8Ag8 BMG at room temperature using an Instron 3369 testing machine. Because room temperature compression tests of the as-cast rods indicated an average yield strength of 1958 737 MPa, the highest applied stress for precompression process was chosen as 1600 MPa. Two groups of experiments, named Group P and Group T, were conducted, as shown in Table 1. In Group P, the rods were pre-compressed for 60 min under the stresses of 400 (P1), 800 (P2), and 1600 (P3) MPa, respectively. In Group T, the rods were pre-compressed under a stress of 1600 MPa for 30 (T1), 60 (T2, the same as P3), and 120 (T3) min, respectively. Hardened tool steel with the hardness and strength higher than those of the BMG was used to make anvils that prevent the deformation of the anvils during the deformation processes. Samples for Vickers hardness testing were cut from the middle height of the specimens and polished to acquire a mirror-like surface using diamond lapping films (1 μm). Hardness testing was carried out using a load of 5 kg and a dwell time of 30 s for each point. At least five Vickers hardness tests were conducted for each specimen. The indentation positions were selected randomly in the whole horizontal plane. Compressive mechanical testing was conducted under a strain rate of 10 5 s 1 using an Instron 3369 testing machine. An FEI Nova Nano230 scanning electron microscope (SEM) was used to characterize the indented areas, the side walls and fracture surfaces of the specimens. X-ray diffraction (XRD, Dmax 2500VB) measurement and transmission electron microscope (TEM, JEOL 2100F) characterization were carried out on both the as-cast and pre-compressed specimens. A NETZSCH STA 449C differential scanning calorimeter (DSC) was used for thermal analysis under pure argon atmosphere from room temperature to 1100 K at a heating rate of 20 K/min. 3. Results 3.1. Hardness and shear banding Fig. 1a shows the Vickers hardness of the Group P samples. The hardness of the as-cast material was 538 711 Hv. Applying precompression under a stress of 400 MPa led to the decrease of the
Fig. 1. Dependence of the Vickers hardness on (a) the pressure and (b) the duration of pre-compression processing. The duration in (a) is 60 min and the pressure in (b) is 1600 MPa.
Vickers hardness to 506 77 Hv. The minimum hardness of 503 78 Hv was reached by applying pre-compression under a stress of 800 MPa. Further increasing the pre-compression pressure to 1600 MPa resulted in a slightly higher hardness of 513 73 Hv. Fig. 1b shows the Vickers hardness of the Group T samples. With the increase of the compression duration, the hardness first decreased from 538 711 Hv to 509 7 5 Hv at 30 min and then increased to 513 73 Hv and 5487 13 Hv at 60 min and 120 min, respectively. Previous investigation [20] reported that after pre-compression below the yield stress (it was called elastostatic deformation in Ref. [20]), the plastic strain of several BMGs increased due to the increased disorder, i.e., increased free volume. Because free volume usually acts as preferred shear band nucleation sites, the increased free volume leads to the increase of the shear bands that enhance plasticity [17]. Thus, the evolution of the hardness in Fig. 1 is likely due to the irreversible structural changes during the pre-compression and this point will be discussed later. While strain-induced hardening is commonly observed in crystalline metals and alloys, strain-induced softening is more common in BMGs [19,28,29] although some researches [30,31] reported the opposite phenomenon in BMGs. Note that both strain-induced hardening and softening were observed in the same BMGs recently [18], and in the present work we also observed both phenomena, depending on the pressure and duration applied during pre-compression deformation. To further understand the observed softening and hardening behaviours of the current BMGs, SEM analysis was conducted around the indented areas to see the shear bands evolution during the
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deformation. Fig. 2 shows that all indents were of a regular pyramidal shape. Shear bands were observed near the indented areas and some of the shear bands are indicated with white arrows in Fig. 2. However, these shear bands appear differently. Several shallow shear bands were observed in the as-cast sample (Fig. 2a). Many shear bands presented in samples P1, P2 and T1, as shown in Fig. 2b, c and d, respectively. Only one or two short shallow shear bands were seen in samples T2 and T3 (Fig. 2e and f). Note that plastic deformation in samples with a very high density of shear bands was better de-localized than that in samples having
only a few coarse shear bands [18]. It should be noted that the same topology for individual samples was always obtained during the indentation testing. 3.2. Compression tests Fig. 3a shows the stress–strain curves of the Group P samples. The ductility of the as-cast material was 2.8%. Applying precompression under stresses of 400 and 800 MPa led to the increase of the ductility to 4.3% and 4.6%, respectively. Further
Fig. 2. SEM images of indented areas of (a) the as-cast sample, and the samples pre-compressed under (b) 400 MPa for 60 min, (c) 800 MPa for 60 min, (d) 1600 MPa for 30 min, (e) 1600 MPa for 60 min and (f) 1600 MPa for 120 min.
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Fig. 3. The strain–stress curves of the samples pre-compressed (a) under different pressures for 60 min and (b) under 1600 MPa for different compression durations.
increasing the pre-compression pressure to 1600 MPa resulted in substantial decrease of the ductility to 1.9%. Fig. 3b shows the stress–strain curves of the Group T samples. With the increase of compression duration, the ductility first increased from 2.8% to 6.7% at 30 min and then decreased to 1.9% and 0.7% at 60 min and 120 min, respectively. All the pre-compressed samples have similar strength to the as-cast sample. From Fig. 3, it can be concluded that pre-compression with optimal pressure and duration can effectively improve the plasticity of BMGs. To further understand the deformation behaviour related to the pre-compression pressure and duration, SEM observation was conducted on the lateral surface of all fractured samples. In the as-cast sample, only a few straight shear bands parallel to the fracture surface and wavy shear bands were observed (Fig. 4a), while a high density of parallel primary shear bands, accompanied by branched secondary shear bands, were observed in samples P1, P2 (Fig. 4b) and T1 (Fig. 4c). For other samples (P3, T2 and T3), only a few shear bands were observed along the side surface. A typical SEM image for sample T3 is shown in Fig. 4d. 3.3. Free volume It has been shown that the application of a stress below yield stress can lead to irreversible structural changes that create excess free volume in BMGs [25,26] and the free volume affects significantly the plasticity of BMGs [18,32]. The change in the free volume can be reflected by enthalpy change (ΔH) in BMGs [33]. Hence, it is possible to evaluate the effect of pre-compression under a stress below the yield strength on the free volume through measuring the variation of ΔH using DSC analysis [22]. Fig. 5a and b compare the DSC traces of the as-cast sample and samples P2 and P3, and the as-cast sample and samples T1 and T2,
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respectively. It is clearly seen from Fig. 5 that all specimens have the same glass transition temperature (Tg) at 705.3 K and the same first and second crystallization onset temperatures (Tx1 and Tx2) at 764.7 K and 920.8 K, respectively. Compared to that of the as-cast sample, the endothermic enthalpy (ΔH) increases for sample P2 and decreases for sample P3 (as shown in Table 2). The variation of ΔH in Fig. 5a and Table 2 implies that the free volume increased via pre-compression under 800 MPa for 60 min, while it decreased via pre-compression under 1600 MPa for 60 min. Pre-compression duration has a similar effect on the variation of free volume. The free volume increased after precompression under 1600 MPa for 30 min and decreased after precompression under 1600 MPa for 60 min (see Table 2 for detailed information). Park et al. [20] indicated that excess free volume is created as a result of shear-induced atomic dilatation even under a deformation below the yield stress, and the amount of free volume reaches saturation on increasing the loading time. This does not agree with our present work, in which the free volume decreases after reaching the maximum value, on further increasing the preelastic compression pressure or duration. A few studies [29,34] reported that the free volume decreases during inhomogeneous plastic deformation. Mear et al. [34] observed free volume annihilation during shot-peening of a Pd-based BMG. Bhowmick et al. [29] reported the coalescence of the excess free volume condenses into nanovoids, which was confirmed via TEM observation [35,36]. Bhowmick et al. [29] also indicated that excess free volume promotes atomic diffusion and rearrangement, leading to structural heterogeneity and the formation of nanocrystallites in the BMG matrix that subsequently annihilates part of the free volume and therefore reduces overall free volume. This explains our observation that, instead of increasing the free volume, a very high pressure or very long compression duration actually reduces the free volume because of the increased structural heterogeneity and nucleation and growth of the nanocrystallites. 3.4. TEM observations XRD patterns (not show here) of the BMG before and after preelastic compression exhibit a broad diffraction halo without detectable crystalline diffraction peaks, indicating an overall amorphous structure under each condition within the detection limit of XRD. Fig. 6a shows a typical high-resolution TEM (HRTEM) image of the as-cast sample and a corresponding selected area electron diffraction (SAED) pattern, presenting a typical uniform amorphous structure. Fig. 6b shows a typical HRTEM image of sample P2. An area with local contrast variation, which is usually caused by compositional heterogeneity, is identified and indicated using a white dashed square. The corresponding inverse fast Fourier transformation (IFFT) image of the area, as shown in the insert in Fig. 6b, showed that the area remains amorphous. Fig. 6c shows a typical HRTEM image of sample T1. Nanocrystallites with diameters of 5 nm were seen and indicated using white dashed circles/ellipses and a white dashed square. The IFFT image of the squared area is inserted in Fig. 6c and the crystalline features can clearly be observed. Fig. 6d shows a typical HRTEM image of sample T3, demonstrating clear evidence of partial crystallization (see the inserted corresponding IFFT of the squared area). All these results demonstrate that pre-compression under a stress below the yield strength produces structural heterogeneity, and the precompression pressure and duration might have significant effects on the degree of the structural heterogeneity. Fig. 6 clearly shows that deformation below yield stress (also called elastic deformation by some researchers) leads to irreversible local structural changes in BMGs [20,25,37,38], although the macroscopic shape change is reversible (which is the definition of elastic deformation). It is well known that the distribution of nanocrystallites in the amorphous matrix can significantly affect the mechanical properties of
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Fig. 4. SEM images of the shear bands for the samples after compression to failure: (a) an as-cast specimen and the specimens pre-compressed under (b) 800 MPa for 60 min, (c) 1600 MPa for 30 min and (d) 1600 MPa for 60 min.
BMGs. To understand how the nanocrystallites affect the plastic deformation behaviour of BMGs, HRTEM was conducted on the samples compressed to failure to reveal the microstructure of BMGs after deformation. Shear bands embedded in the amorphous matrix (as thick zones with the thickness range of 10–100 nm [21]) have a lower viscosity and density than the matrix [39]. Therefore, direct TEM observation of shear bands is possible [40]. Fig. 7 shows a straight shear band without any interaction with the nanocrystals in the fractured sample P2. The shear band appears as a bright line because of the relatively low density in the shear band compared with the surrounding matrix [40]. Fig. 8 shows a TEM image recorded from sample T2. It can be seen that the propagation of a shear band was effectively inhibited by a nanocrystal. Because shear banding is the main carrier of the plastic deformation in BMGs, shear band inhibition by nanocrystals would lead to poor global plasticity of sample T2 because both Fig. 5 and Table 2 indicated that the free volume decreased after pre-compression under 1600 MPa for 60 min (T2). Previous investigations show that a high density of nanocrystals can lead to poor plasticity of BMGs since the formation of nanocrystals is usually accompanied by the decrease of free volume [22,41]. On the other hand, if a BMG has high free volume and a high density of nanocrystals, this will lead to high ductility [18]. Nanocrystals with smaller sizes may not be able to effectively block the propagation of shear bands but they will still be able to detour the pathways of shear bands that prolong the propagation of the shear band and delay the catastrophe failure of the BMG. Fig. 9 shows a typical example of shear band detouring caused by the interaction between a shear band and nanocrystals with the size of 2–5 nm (indicated with white dashed ellipses) in fractured sample T1. The shear band propagation route is denoted
by the black dotted line. The red solid line denotes the straight route that the shear band would propagate through without the nanocrystals. The FFTs of areas marked with 1–4 in Fig. 9 are shown at the bottom of the figure, confirming the nanocrystalline nature of the areas. Evidence shown in Fig. 9 clearly indicates that suitable sized and distributed nanocrystallites can offset and branch shear bands, leading to an enhanced plasticity of BMGs [40].
4. Discussion In crystalline materials, elastic deformation leads to no permanent structural change, i.e., atoms return to their original positions once the applied stress is removed. In BMGs, however, molecular dynamic (MD) simulations suggested that atomic-scale permanent structural changes occur under a stress lower than the yield stress [38,42,43]. Previous experimental results indicated that deformation under a stress lower than the yield stress is homogeneous deformation that produces excess free volume caused by shearinduced atomic dilatation or atomic disordering [20,25]. In addition to atomic disordering, it also includes structural relaxation that leads to nanocrystallization [37] and structural heterogeneity at the expense of free volume, which was caused by local flow during pre-compression below the yield stress. The competition between atomic disordering and structural relaxation depends on the pre-compression conditions. It should be noted that stress field leads to atomic disordering and generates strain. Thus, stress is an important factor to improve the ductility. At low compression stress, e.g., 400 MPa and 800 MPa in this study, only atomic
J. Gu et al. / Materials Science & Engineering A 602 (2014) 68–76
Fig. 5. DSC traces corresponding to (a) the as-cast sample and the samples precompressed under 800 MPa and 1600 MPa for 60 min, and (b) the as-cast sample and the samples pre-compressed under 1600 MPa for 30 min and 60 min.
Table 2 The endothermic enthalpy (ΔH) of the as-cast and pre-compressed samples. Sample
As-cast
P2
T1
P3 (T2)
ΔH (J/g)
5.883
6.141
6.197
5.449
disordering occurs and this leads to increased free volume. Increasing the compression pressure speeds up the atomic disordering process, i.e., less time is needed at higher pressure for achieving the same (or similar) amount of free volume, as evidenced by the free volume in samples P2 and T1, as shown in Table 2. At high compression stress, e.g., 1600 MPa in this study, both atomic disordering and structural relaxation occur and result in increased free volume and nanocrystallization in sample T1. However, sustained high pressure favours structural relaxation, leading to the formation of a significant number of over-sized nanocrystals and reduced free volume in sample T2. Therefore, appropriate selection of the pre-compression pressure and duration is critical for acquiring a BMG structure with high free volume and appropriate size and quantity of nanocrystals embedded in the BMG matrix. A semi-empirical relation on the evolution of nanocrystals and free volume as a function of pre-compression pressure and duration is shown in Fig. 10. It can be seen that the volume (number density) of the nanocrystals increases with the pre-compression pressure and duration, while the free volume
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initially increases with the pre-compression pressure and duration, and starts to decrease once it reaches the maximum value. In fact, the structural evolution of BMGs under precompression at stress below the yield stress is a complicated process. Recent investigation [23] indicated that the lateral precompression can induce micro-scale structural inhomogeneity in BMGs, and an appropriate inhomogeneous stress field can improve the macroscopic compressive plasticity. In this work, non-uniform stress distribution exists in the samples and local stress concentration exists near the contact edge between the sample's surface and the plateau of the plate, leading to local plastic deformation. Atomic packing density, which is inversely proportional to the free volume of BMGs [20,32,33], is a dominant structural factor that influences the mechanical properties of BMGs. Increasing the atomic packing density or reducing the free volume usually strengthens BMGs at the expense of their plasticity [20,32,33]. Our experimental results agree well with the literature reports on the effect of free volume or atomic packing density on the hardness of BMGs. Increasing free volume produces more shear band initiation sites and therefore makes the initiation of shear bands relatively easy, which decreases hardness. Although nanocrystals distributed in the BMG matrix can effectively block/hinder the propagation of shear bands, leading to strain hardening, they have little effect on the hardness. Comparison of samples P2 and T1 supports this conclusion. The two samples have similar free volume (see Table 2) and similar hardness (see Fig. 1) but only sample T1 contains nanocrystals, indicating that the nanocrystals play little role in hardness. The significant hardness increase in sample T3 is more likely caused by the reduced free volume and significantly increased volume fraction of the crystals in the sample, not by the interactions between shear bands and nanocrystals. While many materials show linear relationships between hardness and yield strength [44,45], i.e., the strength increases with the hardness, our experimental results show that the yield strength of BMG samples is independent of their hardness. The reason has not been clear and therefore further exploration is needed. Different from crystalline materials, in which plastic deformation is mainly controlled by dislocation mechanisms and plasticity is determined by the nucleation and motion of dislocations [46], room temperature plastic deformation of BMGs occurs via localized shear banding events [4,21]. Localized shear banding usually leads to strain softening [47,48] and catastrophic failure of BMGs with limited global plasticity [21]. To improve the plasticity of BMGs, it is necessary to introduce a high density of shear bands that spread the plastic deformation uniformly throughout the sample and to prevent catastrophic failure by blocking the propagation of shear bands. Shear bands are likely to nucleate at weak points such as free volume and the interface of the amorphous matrix and inclusions/nanocrystallites. The increase in free volume increases the number of shear band initiation sites that de-localizes the plastic deformation [17]. Nanocrystals are very effective in inhibiting the propagation of shear bands that prevents shear banding runaway failure and also in promoting the nucleation of multiple secondary shear bands [12,13,32,49]. Therefore, combined high free volume and a high density of nanocrystals in sample T1 leads to significantly improved plasticity. Note that, although thermal annealing can also result in the formation of nanocrystals in the amorphous matrix, the annealing process annihilates free volume that causes a complete loss of ductility in metallic glasses [50,51]. Our investigation suggests that free volume is the most important structural factor that determines the plasticity of BMGs. Increasing free volume leads to improved plasticity. Nanocrystals with an appropriate size range, which matches the widths of shear bands, in BMGs with high free volume can further improve the
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Fig. 6. HRTEM images and the corresponding SAED/IFFT patterns of (a) the as-cast sample, and the samples pre-compressed under (b) 800 MPa for 60 min, (c) 1600 MPa for 30 min, and (d) 1600 MPa for 120 min.
Fig. 8. A TEM image showing a shear band inhibited by a nanocrystallite in a sample pre-compressed under 1600 MPa for 60 min.
5. Conclusions Our experimental results demonstrated that pre-compression below the yield stress can be used as an easy method to significantly improve the plasticity of BMGs. This method can be used to process large-sized BMG samples. The following conclusions are drawn from this research:
Fig. 7. A TEM image showing a straight shear band in a sample pre-compressed under 800 MPa for 60 min.
plasticity of the materials. However, nanocrystals distributed in the BMG matrix do not benefit the plasticity if the free volume of the materials is low.
(1) During the pre-compression process, structural relaxation that leads to nanocrystallization, structural heterogeneity and atomic disordering that produces excess free volume in BMGs are concurrent. Under relative low pressure (400 and 800 MPa) atomic disordering is the dominant phenomenon, while under the high pressure (1600 MPa) structural relaxation becomes very significant that consumes free volume. (2) Free volume and nanocrystals embedded in the BMG matrix have significant effects on the mechanical behaviour of BMGs. The combined contribution of excess free volume and nanocrystals can effectively improve the plasticity of BMGs.
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(3) Appropriate combination of the pre-compression pressure and duration significantly increased the compression plasticity of the BMG from 2.8% to 6.7%.
Acknowledgements The financial supports from the National Natural Science Foundation of China (51328101), Hunan Provincial Natural Science Foundation of China (11JJ2024), the Program for New Century Excellent Talents in University (NCET-10–0842), the Fundamental Research Funds for Central Universities (2011JQ021), and Australian Research Council (LP100100566) are appreciated.
References
Fig. 9. An HRTEM image obtained from a sample pre-compressed under 1600 MPa for 30 min, showing the interaction between a shear band and nanocrystallites that detoured the shear band. Nanocrystallites are circled and marked with numbers 1–5. The FFTs of circled areas 1–4 are shown at the bottom of the figure and diffraction spots from crystals are indicated using arrows in the FFTs. The direction of the shear band propagation is denoted by the black dotted line, and the red solid line denotes the possible direction of the shear band propagation without the nanocrystallites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 10. A semi-empirical relation on the evolution of nanocrystals and free volume as a function of pre-compression (a) pressure and (b) duration.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Improving the plasticity of bulk metallic glasses via pre-compression below the yield stress Ji Gu a, Min Song a,n, Song Ni a, Xiaozhou Liao b, Shengfeng Guo c a
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia c Faculty of Materials and Energy, Southwest University, Chongqing 400715, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 29 December 2013 Received in revised form 19 February 2014 Accepted 19 February 2014 Available online 26 February 2014
The poor plasticity of bulk metallic glasses (BMGs) has severely limited their potential structural applications. In this paper, we reported that uniaxial pre-compression below the yield stress can be used to significantly improve the plasticity of BMGs. An as-cast Cu36Zr48Al8Ag8 BMG was pre-compressed under various pressures (400, 800 and 1600 MPa) and durations (30, 60 and 120 min). The pre-compression processes led to structural change that increases the free volume and produces nanocrystals in the BMG matrix, which subsequently improved the plasticity of the BMG. Detailed data analysis showed that increasing the applied pressure speeds up the structural relaxation process and nanocrystallization occurs only at high enough pressure. This investigation indicated that appropriate selection of the pressure and duration of a pre-compression process is critical for the improved plasticity of BMGs. & 2014 Elsevier B.V. All rights reserved.
Keywords: Bulk metallic glasses Pre-compression Plasticity Free volume Nanocrystallite
1. Introduction Bulk metallic glasses (BMGs) possess excellent mechanical properties including high hardness and strength, large elastic limit and excellent resistance to wear and corrosion [1–4]. However, the poor plasticity at room temperature severely restricts the applications of BMGs as structural materials. It has been well accepted that the poor plasticity is attributed to the shear localization and strain/thermal softening during deformation [3]. Significant efforts have been made to improve the plasticity of BMGs at room temperature. Three types of general strategies have been developed: (1) designing proper compositions of BMGs with high plasticity [5–8]; (2) producing composite structures that comprise amorphous matrix and second phase particles or dendrites. This can be achieved through externally added strengthening particles [9–11] and in-situ formation of second phase particles or dendrites [12–14]; and (3) introducing structural inhomogeneity in BMGs via pre-plastic deformation to produce residual stress [15,16], preexisting shear bands and excess free volume [17–20]. The first strategy is particularly sensitive to the BMG composition and even the heat treatment conditions. A slight change in stoichiometry can lead to dramatic ductile-to-brittle transition [6,21]. The second strategy usually requires good control of the casting and annealing conditions. In addition, the externally added strengthening
n
Corresponding author. Tel.: þ 86 731 88877677; fax: þ86 731 88710855. E-mail address: [email protected] (M. Song).
http://dx.doi.org/10.1016/j.msea.2014.02.065 0921-5093 & 2014 Elsevier B.V. All rights reserved.
particles may have a severe interface cohesion problem with the amorphous matrix, while annealing treatment that is used to introduce in-situ second phase particles or dendrites can lead to the annihilation of free volume and thus result in drastic brittleness of BMGs [22]. In recent years, the third strategy has been widely used due to its easy utilization and its independence on the composition of BMGs [15–20,23,24]. Techniques including high pressure treatment [17], quasi-constrained high-pressure torsion [18], lateral pre-compression treatment [23], equal channel angular pressing [24], shot peening [15], and cold rolling [19] have been used for the third strategy. It should be noted that the third strategy requires the application of plastic deformation that may not be suitable for some very brittle BMGs. Recent investigations [25,26] indicated that, different from crystalline materials in which elastic deformation does not lead to any permanent local structural change, irreversible structural changes take place in BMGs during inelastic deformation processes, i.e., by the application of a stress below the yield stress of the BMGs, which creates new shear transition zones (STZs) and excess free volume. The introduction of the excess free volume might substantially improve the plasticity of BMGs. In this paper, we presented a systematic investigation of the effects of precompression conditions (including both the pressure and compression duration) under pressures lower than the yield stress on the free volume and nanocrystallization, deformation behaviour and plasticity of BMGs. Our investigation indicates that precompression below the yield stress is a convenient and effective method to improve the plasticity of BMGs.
J. Gu et al. / Materials Science & Engineering A 602 (2014) 68–76
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Table 1 A list of the samples investigated in the present work. Sample
Pressure (MPa)
Duration (min)
Group P P1 P2 P3 (T2)
400 800 1600
60 60 60
Group T T1 T2 (P3) T3
1600 1600 1600
30 60 120
2. Experimental A Cu36Zr48Al8Ag8 (atomic percentage) BMG master ingot was prepared by arc melting high-purity copper, zirconium, aluminium, and silver (99.7–99.99%) in a Ti-gettered high-purity argon atmosphere using the method described in Ref. [27]. To ensure compositional homogeneity, the ingot was re-melted 4 times and stirred by a magnetic beater. The copper mould suction casting method was used to obtain cylindrical rods with a diameter of 3 mm and a length of 70 mm.The rods were cut into short rods with the length slightly longer than 4.5 mm using a low-speed diamond saw. The two ends of each short rod were carefully polished to acquire rod geometry with a length to diameter ratio of 1.5:1. Some rods were pre-compressed under stresses below the yield stress of the Cu36Zr48Al8Ag8 BMG at room temperature using an Instron 3369 testing machine. Because room temperature compression tests of the as-cast rods indicated an average yield strength of 1958 737 MPa, the highest applied stress for precompression process was chosen as 1600 MPa. Two groups of experiments, named Group P and Group T, were conducted, as shown in Table 1. In Group P, the rods were pre-compressed for 60 min under the stresses of 400 (P1), 800 (P2), and 1600 (P3) MPa, respectively. In Group T, the rods were pre-compressed under a stress of 1600 MPa for 30 (T1), 60 (T2, the same as P3), and 120 (T3) min, respectively. Hardened tool steel with the hardness and strength higher than those of the BMG was used to make anvils that prevent the deformation of the anvils during the deformation processes. Samples for Vickers hardness testing were cut from the middle height of the specimens and polished to acquire a mirror-like surface using diamond lapping films (1 μm). Hardness testing was carried out using a load of 5 kg and a dwell time of 30 s for each point. At least five Vickers hardness tests were conducted for each specimen. The indentation positions were selected randomly in the whole horizontal plane. Compressive mechanical testing was conducted under a strain rate of 10 5 s 1 using an Instron 3369 testing machine. An FEI Nova Nano230 scanning electron microscope (SEM) was used to characterize the indented areas, the side walls and fracture surfaces of the specimens. X-ray diffraction (XRD, Dmax 2500VB) measurement and transmission electron microscope (TEM, JEOL 2100F) characterization were carried out on both the as-cast and pre-compressed specimens. A NETZSCH STA 449C differential scanning calorimeter (DSC) was used for thermal analysis under pure argon atmosphere from room temperature to 1100 K at a heating rate of 20 K/min. 3. Results 3.1. Hardness and shear banding Fig. 1a shows the Vickers hardness of the Group P samples. The hardness of the as-cast material was 538 711 Hv. Applying precompression under a stress of 400 MPa led to the decrease of the
Fig. 1. Dependence of the Vickers hardness on (a) the pressure and (b) the duration of pre-compression processing. The duration in (a) is 60 min and the pressure in (b) is 1600 MPa.
Vickers hardness to 506 77 Hv. The minimum hardness of 503 78 Hv was reached by applying pre-compression under a stress of 800 MPa. Further increasing the pre-compression pressure to 1600 MPa resulted in a slightly higher hardness of 513 73 Hv. Fig. 1b shows the Vickers hardness of the Group T samples. With the increase of the compression duration, the hardness first decreased from 538 711 Hv to 509 7 5 Hv at 30 min and then increased to 513 73 Hv and 5487 13 Hv at 60 min and 120 min, respectively. Previous investigation [20] reported that after pre-compression below the yield stress (it was called elastostatic deformation in Ref. [20]), the plastic strain of several BMGs increased due to the increased disorder, i.e., increased free volume. Because free volume usually acts as preferred shear band nucleation sites, the increased free volume leads to the increase of the shear bands that enhance plasticity [17]. Thus, the evolution of the hardness in Fig. 1 is likely due to the irreversible structural changes during the pre-compression and this point will be discussed later. While strain-induced hardening is commonly observed in crystalline metals and alloys, strain-induced softening is more common in BMGs [19,28,29] although some researches [30,31] reported the opposite phenomenon in BMGs. Note that both strain-induced hardening and softening were observed in the same BMGs recently [18], and in the present work we also observed both phenomena, depending on the pressure and duration applied during pre-compression deformation. To further understand the observed softening and hardening behaviours of the current BMGs, SEM analysis was conducted around the indented areas to see the shear bands evolution during the
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deformation. Fig. 2 shows that all indents were of a regular pyramidal shape. Shear bands were observed near the indented areas and some of the shear bands are indicated with white arrows in Fig. 2. However, these shear bands appear differently. Several shallow shear bands were observed in the as-cast sample (Fig. 2a). Many shear bands presented in samples P1, P2 and T1, as shown in Fig. 2b, c and d, respectively. Only one or two short shallow shear bands were seen in samples T2 and T3 (Fig. 2e and f). Note that plastic deformation in samples with a very high density of shear bands was better de-localized than that in samples having
only a few coarse shear bands [18]. It should be noted that the same topology for individual samples was always obtained during the indentation testing. 3.2. Compression tests Fig. 3a shows the stress–strain curves of the Group P samples. The ductility of the as-cast material was 2.8%. Applying precompression under stresses of 400 and 800 MPa led to the increase of the ductility to 4.3% and 4.6%, respectively. Further
Fig. 2. SEM images of indented areas of (a) the as-cast sample, and the samples pre-compressed under (b) 400 MPa for 60 min, (c) 800 MPa for 60 min, (d) 1600 MPa for 30 min, (e) 1600 MPa for 60 min and (f) 1600 MPa for 120 min.
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Fig. 3. The strain–stress curves of the samples pre-compressed (a) under different pressures for 60 min and (b) under 1600 MPa for different compression durations.
increasing the pre-compression pressure to 1600 MPa resulted in substantial decrease of the ductility to 1.9%. Fig. 3b shows the stress–strain curves of the Group T samples. With the increase of compression duration, the ductility first increased from 2.8% to 6.7% at 30 min and then decreased to 1.9% and 0.7% at 60 min and 120 min, respectively. All the pre-compressed samples have similar strength to the as-cast sample. From Fig. 3, it can be concluded that pre-compression with optimal pressure and duration can effectively improve the plasticity of BMGs. To further understand the deformation behaviour related to the pre-compression pressure and duration, SEM observation was conducted on the lateral surface of all fractured samples. In the as-cast sample, only a few straight shear bands parallel to the fracture surface and wavy shear bands were observed (Fig. 4a), while a high density of parallel primary shear bands, accompanied by branched secondary shear bands, were observed in samples P1, P2 (Fig. 4b) and T1 (Fig. 4c). For other samples (P3, T2 and T3), only a few shear bands were observed along the side surface. A typical SEM image for sample T3 is shown in Fig. 4d. 3.3. Free volume It has been shown that the application of a stress below yield stress can lead to irreversible structural changes that create excess free volume in BMGs [25,26] and the free volume affects significantly the plasticity of BMGs [18,32]. The change in the free volume can be reflected by enthalpy change (ΔH) in BMGs [33]. Hence, it is possible to evaluate the effect of pre-compression under a stress below the yield strength on the free volume through measuring the variation of ΔH using DSC analysis [22]. Fig. 5a and b compare the DSC traces of the as-cast sample and samples P2 and P3, and the as-cast sample and samples T1 and T2,
71
respectively. It is clearly seen from Fig. 5 that all specimens have the same glass transition temperature (Tg) at 705.3 K and the same first and second crystallization onset temperatures (Tx1 and Tx2) at 764.7 K and 920.8 K, respectively. Compared to that of the as-cast sample, the endothermic enthalpy (ΔH) increases for sample P2 and decreases for sample P3 (as shown in Table 2). The variation of ΔH in Fig. 5a and Table 2 implies that the free volume increased via pre-compression under 800 MPa for 60 min, while it decreased via pre-compression under 1600 MPa for 60 min. Pre-compression duration has a similar effect on the variation of free volume. The free volume increased after precompression under 1600 MPa for 30 min and decreased after precompression under 1600 MPa for 60 min (see Table 2 for detailed information). Park et al. [20] indicated that excess free volume is created as a result of shear-induced atomic dilatation even under a deformation below the yield stress, and the amount of free volume reaches saturation on increasing the loading time. This does not agree with our present work, in which the free volume decreases after reaching the maximum value, on further increasing the preelastic compression pressure or duration. A few studies [29,34] reported that the free volume decreases during inhomogeneous plastic deformation. Mear et al. [34] observed free volume annihilation during shot-peening of a Pd-based BMG. Bhowmick et al. [29] reported the coalescence of the excess free volume condenses into nanovoids, which was confirmed via TEM observation [35,36]. Bhowmick et al. [29] also indicated that excess free volume promotes atomic diffusion and rearrangement, leading to structural heterogeneity and the formation of nanocrystallites in the BMG matrix that subsequently annihilates part of the free volume and therefore reduces overall free volume. This explains our observation that, instead of increasing the free volume, a very high pressure or very long compression duration actually reduces the free volume because of the increased structural heterogeneity and nucleation and growth of the nanocrystallites. 3.4. TEM observations XRD patterns (not show here) of the BMG before and after preelastic compression exhibit a broad diffraction halo without detectable crystalline diffraction peaks, indicating an overall amorphous structure under each condition within the detection limit of XRD. Fig. 6a shows a typical high-resolution TEM (HRTEM) image of the as-cast sample and a corresponding selected area electron diffraction (SAED) pattern, presenting a typical uniform amorphous structure. Fig. 6b shows a typical HRTEM image of sample P2. An area with local contrast variation, which is usually caused by compositional heterogeneity, is identified and indicated using a white dashed square. The corresponding inverse fast Fourier transformation (IFFT) image of the area, as shown in the insert in Fig. 6b, showed that the area remains amorphous. Fig. 6c shows a typical HRTEM image of sample T1. Nanocrystallites with diameters of 5 nm were seen and indicated using white dashed circles/ellipses and a white dashed square. The IFFT image of the squared area is inserted in Fig. 6c and the crystalline features can clearly be observed. Fig. 6d shows a typical HRTEM image of sample T3, demonstrating clear evidence of partial crystallization (see the inserted corresponding IFFT of the squared area). All these results demonstrate that pre-compression under a stress below the yield strength produces structural heterogeneity, and the precompression pressure and duration might have significant effects on the degree of the structural heterogeneity. Fig. 6 clearly shows that deformation below yield stress (also called elastic deformation by some researchers) leads to irreversible local structural changes in BMGs [20,25,37,38], although the macroscopic shape change is reversible (which is the definition of elastic deformation). It is well known that the distribution of nanocrystallites in the amorphous matrix can significantly affect the mechanical properties of
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Fig. 4. SEM images of the shear bands for the samples after compression to failure: (a) an as-cast specimen and the specimens pre-compressed under (b) 800 MPa for 60 min, (c) 1600 MPa for 30 min and (d) 1600 MPa for 60 min.
BMGs. To understand how the nanocrystallites affect the plastic deformation behaviour of BMGs, HRTEM was conducted on the samples compressed to failure to reveal the microstructure of BMGs after deformation. Shear bands embedded in the amorphous matrix (as thick zones with the thickness range of 10–100 nm [21]) have a lower viscosity and density than the matrix [39]. Therefore, direct TEM observation of shear bands is possible [40]. Fig. 7 shows a straight shear band without any interaction with the nanocrystals in the fractured sample P2. The shear band appears as a bright line because of the relatively low density in the shear band compared with the surrounding matrix [40]. Fig. 8 shows a TEM image recorded from sample T2. It can be seen that the propagation of a shear band was effectively inhibited by a nanocrystal. Because shear banding is the main carrier of the plastic deformation in BMGs, shear band inhibition by nanocrystals would lead to poor global plasticity of sample T2 because both Fig. 5 and Table 2 indicated that the free volume decreased after pre-compression under 1600 MPa for 60 min (T2). Previous investigations show that a high density of nanocrystals can lead to poor plasticity of BMGs since the formation of nanocrystals is usually accompanied by the decrease of free volume [22,41]. On the other hand, if a BMG has high free volume and a high density of nanocrystals, this will lead to high ductility [18]. Nanocrystals with smaller sizes may not be able to effectively block the propagation of shear bands but they will still be able to detour the pathways of shear bands that prolong the propagation of the shear band and delay the catastrophe failure of the BMG. Fig. 9 shows a typical example of shear band detouring caused by the interaction between a shear band and nanocrystals with the size of 2–5 nm (indicated with white dashed ellipses) in fractured sample T1. The shear band propagation route is denoted
by the black dotted line. The red solid line denotes the straight route that the shear band would propagate through without the nanocrystals. The FFTs of areas marked with 1–4 in Fig. 9 are shown at the bottom of the figure, confirming the nanocrystalline nature of the areas. Evidence shown in Fig. 9 clearly indicates that suitable sized and distributed nanocrystallites can offset and branch shear bands, leading to an enhanced plasticity of BMGs [40].
4. Discussion In crystalline materials, elastic deformation leads to no permanent structural change, i.e., atoms return to their original positions once the applied stress is removed. In BMGs, however, molecular dynamic (MD) simulations suggested that atomic-scale permanent structural changes occur under a stress lower than the yield stress [38,42,43]. Previous experimental results indicated that deformation under a stress lower than the yield stress is homogeneous deformation that produces excess free volume caused by shearinduced atomic dilatation or atomic disordering [20,25]. In addition to atomic disordering, it also includes structural relaxation that leads to nanocrystallization [37] and structural heterogeneity at the expense of free volume, which was caused by local flow during pre-compression below the yield stress. The competition between atomic disordering and structural relaxation depends on the pre-compression conditions. It should be noted that stress field leads to atomic disordering and generates strain. Thus, stress is an important factor to improve the ductility. At low compression stress, e.g., 400 MPa and 800 MPa in this study, only atomic
J. Gu et al. / Materials Science & Engineering A 602 (2014) 68–76
Fig. 5. DSC traces corresponding to (a) the as-cast sample and the samples precompressed under 800 MPa and 1600 MPa for 60 min, and (b) the as-cast sample and the samples pre-compressed under 1600 MPa for 30 min and 60 min.
Table 2 The endothermic enthalpy (ΔH) of the as-cast and pre-compressed samples. Sample
As-cast
P2
T1
P3 (T2)
ΔH (J/g)
5.883
6.141
6.197
5.449
disordering occurs and this leads to increased free volume. Increasing the compression pressure speeds up the atomic disordering process, i.e., less time is needed at higher pressure for achieving the same (or similar) amount of free volume, as evidenced by the free volume in samples P2 and T1, as shown in Table 2. At high compression stress, e.g., 1600 MPa in this study, both atomic disordering and structural relaxation occur and result in increased free volume and nanocrystallization in sample T1. However, sustained high pressure favours structural relaxation, leading to the formation of a significant number of over-sized nanocrystals and reduced free volume in sample T2. Therefore, appropriate selection of the pre-compression pressure and duration is critical for acquiring a BMG structure with high free volume and appropriate size and quantity of nanocrystals embedded in the BMG matrix. A semi-empirical relation on the evolution of nanocrystals and free volume as a function of pre-compression pressure and duration is shown in Fig. 10. It can be seen that the volume (number density) of the nanocrystals increases with the pre-compression pressure and duration, while the free volume
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initially increases with the pre-compression pressure and duration, and starts to decrease once it reaches the maximum value. In fact, the structural evolution of BMGs under precompression at stress below the yield stress is a complicated process. Recent investigation [23] indicated that the lateral precompression can induce micro-scale structural inhomogeneity in BMGs, and an appropriate inhomogeneous stress field can improve the macroscopic compressive plasticity. In this work, non-uniform stress distribution exists in the samples and local stress concentration exists near the contact edge between the sample's surface and the plateau of the plate, leading to local plastic deformation. Atomic packing density, which is inversely proportional to the free volume of BMGs [20,32,33], is a dominant structural factor that influences the mechanical properties of BMGs. Increasing the atomic packing density or reducing the free volume usually strengthens BMGs at the expense of their plasticity [20,32,33]. Our experimental results agree well with the literature reports on the effect of free volume or atomic packing density on the hardness of BMGs. Increasing free volume produces more shear band initiation sites and therefore makes the initiation of shear bands relatively easy, which decreases hardness. Although nanocrystals distributed in the BMG matrix can effectively block/hinder the propagation of shear bands, leading to strain hardening, they have little effect on the hardness. Comparison of samples P2 and T1 supports this conclusion. The two samples have similar free volume (see Table 2) and similar hardness (see Fig. 1) but only sample T1 contains nanocrystals, indicating that the nanocrystals play little role in hardness. The significant hardness increase in sample T3 is more likely caused by the reduced free volume and significantly increased volume fraction of the crystals in the sample, not by the interactions between shear bands and nanocrystals. While many materials show linear relationships between hardness and yield strength [44,45], i.e., the strength increases with the hardness, our experimental results show that the yield strength of BMG samples is independent of their hardness. The reason has not been clear and therefore further exploration is needed. Different from crystalline materials, in which plastic deformation is mainly controlled by dislocation mechanisms and plasticity is determined by the nucleation and motion of dislocations [46], room temperature plastic deformation of BMGs occurs via localized shear banding events [4,21]. Localized shear banding usually leads to strain softening [47,48] and catastrophic failure of BMGs with limited global plasticity [21]. To improve the plasticity of BMGs, it is necessary to introduce a high density of shear bands that spread the plastic deformation uniformly throughout the sample and to prevent catastrophic failure by blocking the propagation of shear bands. Shear bands are likely to nucleate at weak points such as free volume and the interface of the amorphous matrix and inclusions/nanocrystallites. The increase in free volume increases the number of shear band initiation sites that de-localizes the plastic deformation [17]. Nanocrystals are very effective in inhibiting the propagation of shear bands that prevents shear banding runaway failure and also in promoting the nucleation of multiple secondary shear bands [12,13,32,49]. Therefore, combined high free volume and a high density of nanocrystals in sample T1 leads to significantly improved plasticity. Note that, although thermal annealing can also result in the formation of nanocrystals in the amorphous matrix, the annealing process annihilates free volume that causes a complete loss of ductility in metallic glasses [50,51]. Our investigation suggests that free volume is the most important structural factor that determines the plasticity of BMGs. Increasing free volume leads to improved plasticity. Nanocrystals with an appropriate size range, which matches the widths of shear bands, in BMGs with high free volume can further improve the
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Fig. 6. HRTEM images and the corresponding SAED/IFFT patterns of (a) the as-cast sample, and the samples pre-compressed under (b) 800 MPa for 60 min, (c) 1600 MPa for 30 min, and (d) 1600 MPa for 120 min.
Fig. 8. A TEM image showing a shear band inhibited by a nanocrystallite in a sample pre-compressed under 1600 MPa for 60 min.
5. Conclusions Our experimental results demonstrated that pre-compression below the yield stress can be used as an easy method to significantly improve the plasticity of BMGs. This method can be used to process large-sized BMG samples. The following conclusions are drawn from this research:
Fig. 7. A TEM image showing a straight shear band in a sample pre-compressed under 800 MPa for 60 min.
plasticity of the materials. However, nanocrystals distributed in the BMG matrix do not benefit the plasticity if the free volume of the materials is low.
(1) During the pre-compression process, structural relaxation that leads to nanocrystallization, structural heterogeneity and atomic disordering that produces excess free volume in BMGs are concurrent. Under relative low pressure (400 and 800 MPa) atomic disordering is the dominant phenomenon, while under the high pressure (1600 MPa) structural relaxation becomes very significant that consumes free volume. (2) Free volume and nanocrystals embedded in the BMG matrix have significant effects on the mechanical behaviour of BMGs. The combined contribution of excess free volume and nanocrystals can effectively improve the plasticity of BMGs.
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(3) Appropriate combination of the pre-compression pressure and duration significantly increased the compression plasticity of the BMG from 2.8% to 6.7%.
Acknowledgements The financial supports from the National Natural Science Foundation of China (51328101), Hunan Provincial Natural Science Foundation of China (11JJ2024), the Program for New Century Excellent Talents in University (NCET-10–0842), the Fundamental Research Funds for Central Universities (2011JQ021), and Australian Research Council (LP100100566) are appreciated.
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Fig. 9. An HRTEM image obtained from a sample pre-compressed under 1600 MPa for 30 min, showing the interaction between a shear band and nanocrystallites that detoured the shear band. Nanocrystallites are circled and marked with numbers 1–5. The FFTs of circled areas 1–4 are shown at the bottom of the figure and diffraction spots from crystals are indicated using arrows in the FFTs. The direction of the shear band propagation is denoted by the black dotted line, and the red solid line denotes the possible direction of the shear band propagation without the nanocrystallites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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