- Email: [email protected]
Thermodynamic, corrosion and mechanical properties of Zr-based bulk metallic glasses in relation to heterogeneous structures
Materials Science and Engineering A 534 (2012) 157–162
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Thermodynamic, corrosion and mechanical properties of Zr-based bulk metallic glasses in relation to heterogeneous structures W.H. Li a,b , K.C. Chan a,∗ , L. Xia a , L. Liu c , Y.Z. He b a b c
Advanced Manufacturing Technology Research Centre, Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan 243002, People’s Republic of China Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 15 February 2011 Received in revised form 28 October 2011 Accepted 10 November 2011 Available online 30 November 2011 Keywords: Bulk amorphous alloys Thermal stability Corrosion resistance Plasticity
a b s t r a c t In this paper, Zr64.2 Ni16.2 Cu14.6 Al5 and Zr63.4 Ni16.2 Cu15.4 Al5 BMGs with heterogeneous structures were prepared through copper mold casting. The microstructure, thermal stability and corrosion behavior of the two Zr-based metallic glasses were investigated by X-ray diffraction (XRD), high-resolution transmission electron micrography (HRTEM), differential scanning calorimetry (DSC) and potentiodynamic polarization tests. In TEM micrographs, heterogeneous structures were revealed by the contrast between the bright and dark zones. Compared to the Zr65 A10 lNi10 Cu15 BMG with a homogenous structure, the heterogeneous structures of the two BMGs are shown to reduce their thermal stability and corrosion resistance. The two Zr-based BMGs however exhibit large plastic strain (>25%) and high yield strength (>1.6 GPa) under uniaxial compression tests. The low potential-energy barrier of shear transition zones (STZs) and the high resistance to the propagation of shear bands of the BMGs are related to the heterogeneous structures. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, a series of Pd- [1], Pt- [2], Zr- [3–5] and Cu-based [6] monolithic bulk metallic glasses (BMGs) with high plasticity at room temperature have been developed. In particular, a lot of attention has been paid to Zr-based BMGs due to their large glass forming ability and good combination of strength and plasticity, implying promising potential for structural applications. A number of approaches have been developed for the improvement of monolithic BMGs, which include (i) the achievement of a critical value of Poisson’s ratio [7]; (ii) the introduction of a compressive residual stress [8]; (iii) the design of specimens with small length–diameter ratio [9,10]; and (iv) the introduction of atomic-scale inhomogenities [1,11,12]. Much research work has also been carried out to understand the plastic deformation mechanisms, both spatially and temporally, of these disordered materials. It is well known that the plastic deformation of metallic glasses is inhomogeneous and highly localized into shear bands at room temperature, and the plasticity of BMGs strongly relies on the chemical and physical properties of the component elements [13].
∗ Corresponding author. Tel.: +852 27664981; fax: +852 23625267. E-mail addresses: [email protected], [email protected] (K.C. Chan). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.11.054
Heterogeneity at the atomic scale, such as medium range order, nanocrystallization and phase separation in BMGs, is found to be favorable for achieving good ductility [1,12,13]. However, the atomic structure of BMGs is still not fully understood, and there is also much discussion on characterizing micro-, or nano-meter scale structures in BMGs. The microstructure of BMGs is usually characterized through high resolution TEM, but artifacts, including heterogeneity, may arise during the sample preparation process, which may affect the results of the characterization [14]. Besides direct observation by TEM, and the study of the relationship between heterogeneity and plasticity, it is also of significance to investigate the effect of heterogeneous structures on other properties or behavior of BMGs, the results of which may help ascertain the heterogeneous microstructure in BMGs. It is interesting to point out that the strength and macroscopic plastic behavior of BMGs is related to their thermodynamic behavior, for example, the yield strength of BMGs is proportional to the glass transition temperature [15]. The energy of ˇ relaxation is equivalent to the potential-energy barrier of the dynamical shear transformation zone (STZ) [16], and pitting corrosion is also found to be sensitive to variations in the microstructure [17,18]. Although there have been extensive efforts to study the effect of heterogeneous structures on the mechanical properties, the effect on thermodynamic and corrosion properties of ductile BMGs have been less investigated. In this paper, by comparison to a homogeneous Zr-based BMG, the thermal stability, corrosion behavior and
158
W.H. Li et al. / Materials Science and Engineering A 534 (2012) 157–162
mechanical properties of two Zr-based BMGs, with heterogeneous structures, are investigated.
Ingots, with a nominal composition of Zr64.2 Ni16.2 Cu14.6 Al5 and Zr63.4 Ni16.2 Cu15.4 Al5 , were prepared by the arc-melting of elements Zr, Al, Ni and Cu, with a purity of 99.9%, in a titanium-gettered argon atmosphere. Cylindrical specimens of 3 mm and 2 mm diameter were prepared by suction casting into a copper mold. The structure of the samples was characterized by X-ray diffraction (XRD) in a Philips PW 1050 diffractometer using Cu K␣-radiation at a scanning speed of 2◦ /min. The microstructural observation of the samples was performed on a JEOL JEM-2010 high-resolution transmission electron microscope (HRTEM) operated at 200 kV. Thermal analysis was carried out in a Perkin-Elmer DSC-7 differential scanning calorimeter under an argon atmosphere. For continuous heating tests, a constant heating rate of 20 K/min was employed. A rapid heating rate of 100 K/min up to the annealing temperature was employed. The isothermal measurements were performed at two different temperatures T = 671 and 691 K. The corrosion behavior of the BMGs was examined by electrochemical polarization measurements. The specimens were mechanically polished to a mirror finished before electrochemical and immersion tests. The electrolytes used were a 3.5 wt.% NaOH aqueous solution which was prepared from reagent grade chemicals and distilled water. After immersing the specimens for about 20 min, at which time the open circuit potentials became almost steady, potentiodynamic polarization curves were measured at a potential sweep rate of 1 mV/s. Uniaxial compressive tests were performed in an MTS testing machine at room temperature on cylindrical samples 2 mm in diameter and 4 mm in length. The crosshead speed was constant with an initial strain rate of 2 × 10−4 s−1 . The compressive fracture surface analyses were performed using a JSM-6460 scanning electron microscope (SEM).
Cu Kα
Intensity (arb.units)
2. Experimental
Zr63.4Ni16.2Cu15.4Al5 Zr64.2Ni16.2Cu14.6Al5
20
30
40
50
60
70
80
2 Theta (degree) Fig. 1. The XRD patterns Zr63.4 Ni16.2 Cu15.4 Al5 BMGs.
of
as-cast
rods
of
Zr64.2 Ni16.2 Cu14.6 Al5
and
3. Results and discussion
investigations were undertaken for the Zr63.4 Ni16.2 Cu15.4 Al5 BMG. No obvious medium range order clusters, phase separation or nano-crystallization were found in the high resolution electronmicroscopy (HREM) image, as shown in Fig. 2(a). The halo ring in the selected area diffraction pattern indicates the glassy structure of the BMG. Fig. 2(b) shows the bright-field images of the Zr63.4 Ni16.2 Cu15.4 Al5 BMG. The heterogeneous structures were clearly revealed by the contrast between the bright and dark zones. The size of the bright zone is about 0.5–1 m. This result is similar to the microstructure of the Zr64.13 Cu15.75 Ni10.12 Al10 BMG with super plasticity [3]. Although the TEM observation can directly reveal the heterogeneous structure of the BMG, it is still not possible to exclude the likelihood that the heterogeneity is induced by the specimen preparation process. To better ascertain the material microstructures, the effect of heterogeneous structures on thermal stability, corrosion behavior and mechanical properties were investigated and are described in the following sections.
3.1. Microstructure
3.2. Thermal stability and heterogeneous structures
Fig. 1 shows the XRD patterns of the two Zr-based BMGs ascast 3 mm diameter rods. The typical broad diffraction maxima were observed in the XRD pattern illustrating fully amorphous characteristics. No crystalline peaks or the heterogeneous structures were found with the XRD detection limit. To further clarify the microstructure for nanometer-sized features, TEM
The thermal stability of two Zr-based BMGs was evaluated by DSC measurements at a heating rate of 20 K/min. The DSC curves of the two Zr-based BMGs exhibit an endothermic event characteristic of glass transition, followed by an exothermic event, characteristic of the crystallization process, as shown in Fig. 3(a). The temperature range of Tg , Tx and Tl in the two BMGs are 651–653 K, 720–722 K and
Fig. 2. Microstructure of Zr63.4 Ni16.2 Cu15.4 Al5 BMG (a) HREM image, with the inset related to the SEAD pattern. (b) The bright-field images of TEM.
W.H. Li et al. / Materials Science and Engineering A 534 (2012) 157–162
159
Tx
Zr63.4Ni16.2Cu15.4Al5 Zr64.2Ni16.2Cu14.6Al5 20 K/min
Zr63.4Ni16.2Cu15.4Al5 Zr64.2Ni16.2Cu14.6Al5 Zr65Ni10Cu15Al10
Tx
Endo. Up (a.u.)
Endo. Up (a.u.)
(a)
Tx Tg+40 K
Tg
10
20
30
40
Time (min) 550
600
650
700
750
800
Temperature (K)
Zr64.2Ni16.2Cu14.6Al5 Zr63.4Ni16.2Cu15.4Al5
Endo. Up (a.u.)
1119–1120 K, respectively. Thus the reduced glass transition temperature (Trg = Tg /Tl ), the supercooled liquid region (T = Tx − Tg ) and the GFA parameter [=Tx /(Tg + Tl )], are 0.58, 69 K and 0.41, respectively. The results show that the two Zr-based BMGs have high thermal stability and glass forming ability. Very often, continuous heating experiments alone are not sufficient for probing microstructure changes, such as relaxation and crystallization phenomena in inhomogeneous amorphous materials. There are two reasons for the disappearance of the thermodynamics phenomena. Firstly, the enthalpy of glass transition is too weak to be distinguished from the DSC curves. For example, no obvious glasses transition phenomena were found in iron-rich Nd-based BMGs [19,20]. Secondly, if the heating rate is rapid enough, it will restrain the BMG crystallization process. Under the condition of continuous heating, Fig. 3 only shows a single glass transition temperature and a single crystallization peak for the two BMGs, which seems that they exhibit homogenous structures. In order to better reveal the relationship between the microstructure and the thermodynamic properties, unambiguous identification of the microcrystalline structure is possible through the study of the isothermal crystallization kinetics. The study was carried out at temperatures between Tg and Tx . Fig. 4(a) shows the isothermal crystallization DSC curves of the three Zrbased BMGs at the annealing temperature (Tg +40 K). Only a single exothermic peak can be found for the Zr65 Al10 Ni10 Cu15 BMG, revealing its homogenous amorphous structure. For the Zr63.4 Ni16.2 Cu15.4 Al5 and Zr64.2 Ni16.2 Cu14.6 Al5 BMGs, two independent exothermic peaks resulting from the crystallization processes were observed. While the first exothermic enthalpy is found to be much less than the second enthalpy, the crystallization rate is much greater than in the latter. At the annealing temperature (Tg +20 K), Fig. 4(b) also shows two crystallization processes for the Zr63.4 Ni16.2 Cu15.4 Al5 BMG, although the peak height of the DSC curves is reduced and the peak width is increased. For the Zr63.4 Ni16.2 Cu15.4 Al5 BMG, one exothermic peak is observed within the timescale of the experiment. To better ascertain the microstructural changes during the annealing process, XRD tests were conducted. Fig. 5 shows the XRD patterns of as-cast and annealed Zr63.4 Ni16.2 Cu15.4 Al5 alloys at 671 K (Tg +20 K) holding for 5 min followed by cooling at a rate of 100 K/min. The annealed sample exhibits partial crystal phases within an amorphous matrix phase. The crystallized phases consist of Zr2 Cu and Zr2 Ni. This result reveals that the initial exothermic peak in Fig. 4(b) truly reflects the crystallization behavior. It is
(b)
Tx Tg+20 K
20
40
60
80
Time (min) Fig. 4. Isothermal crystallization DSC curves of Zr64.2 Ni16.2 Cu14.6 Al5 , Zr63.4 Ni16.2 Cu15.4 Al5 and Zr65 Al10 Ni10 Cu15 BMGs; (a) Tg +40 K, (b) Tg +20 K.
well known that the crystallization process is controlled by shortrange diffusion of the chemical elements. The two crystallization peaks indicate that there are two different structures with different thermal stabilities, and the results of the isothermal crystallization study illustrate the heterogeneous structures of the two Zr-based BMGs. These findings are consistent with the TEM observation, as shown in Fig. 2(a). 3.3. Corrosion and heterogeneous structures The corrosion of metallic alloys is significantly afffected by the microstructure and the phase interface. In general, amorphous alloys are known to exhibit improved corrosion resistance due
Zr2Cu Zr2Ni
Intensity (arb.units)
Fig. 3. The DSC curves of Zr64.2 Ni16.2 Cu14.6 Al5 and Zr63.4 Ni16.2 Cu15.4 Al5 BMGs at a heating rate of 20 K/min.
Tx
20
30
40
50
60
70
80
2 Theta (degree) Fig. 5. The XRD patterns of as-cast and annealed Zr63.4 Ni16.2 Cu15.4 Al5 alloys at 671 K.
160
W.H. Li et al. / Materials Science and Engineering A 534 (2012) 157–162
-1 -2
Zr64.2Ni16.2Cu14.6Al5 Zr63.4Ni16.2Cu15.4Al5 Zr65Ni10Cu15Al10
log i (mA/cm2)
-3 -4 -5 -6 -7 -8 -9 0.0
-0.2
-0.4
-0.6
-0.8
E (V vs. SEC) Fig. 6. The polarization curves of Zr64.2 Ni16.2 Cu14.6 Al5 and Zr63.4 Ni16.2 Cu15.4 Al5 BMGs in a 3.5 wt.% NaCl solution.
to having fewer defects and optimized chemical homogeneity. With the aim of further clarifying the heterogeneous structures of the two Zr-based BMGs, pitting corrosion tests were conducted. Fig. 6 shows the polarization curves of the Zr64.2 Ni16.2 Cu14.6 Al5 and Zr63.4 Ni16.2 Cu15.4 Al5 BMGs in 3.5 wt.% NaCl solution at 298 K open to the atmosphere. For comparison, the corrosion behavior of the Zr65 Al10 Ni10 Cu15 BMG with a homogenous amorphous structure was also examined at the same conditions (see Fig. 6). There are significantly different corrosion and pitting potentials for the three Zr-based BMGs electrodes. Among the three Zr-based BMGs, the Zr65 Al10 Ni10 Cu15 BMG exhibits the highest corrosion and pitting potentials, and the lowest corrosion current density. However, the Zr63.4 Ni16.2 Cu15.4 Al5 BMG exhibits the lowest corrosion potential and pitting potentials, and the highest corrosion current density. It reveals that the Zr65 Al10 Ni10 Cu15 BMG has the best corrosion resistance in a 3.5 wt.% NaCl solution. It indicates that an amorphous alloy with a homogeneous structure is more corrosion resistant than in an heterogeneous structure under same corrosion conditions and with a similar composition. The low corrosion resistance of inhomogeneous BMGs may be caused by the increased number of interface zones. 3.4. Mechanical behavior and heterogeneous structures Fig. 7 shows the room-temperature true stress–strain curves of Zr63.4 Ni16.2 Cu15.4 Al5 and Zr64.2 Ni16.2 Cu14.6 Al5 glassy rods obtained
6
Zr
Ni
Cu
Al
True Stress (GPa)
5 4
Zr
Ni
Cu
Al
3 2
Zr63.4Ni16.2Cu15.4Al5 Zr64.2Ni16.2Cu14.6Al5
1 0
0
10
20
30
40
50
True Strain (%)
60
70
Fig. 7. The true stress–strain curves of Zr63.4 Ni16.2 Cu15.4 Al5 and Zr64.2 Ni16.2 Cu14.6 Al5 BMGs. The inset highlights the serrated flow segments.
by uniaxial compression tests at a strain rate of 2 × 10−4 s−1 . The yield stress and elastic modulus of the Zr64.2 Ni16.2 Cu14.6 Al5 BMG are 1.65 GPa and 76 GPa, respectively, and the corresponding values for the Zr63.4 Ni16.2 Cu15.4 Al5 BMG are 1.70 GPa and 78 GPa. These values are similar to those reported for nearly identical alloys [4]. Elastic strains of nearly 2% are achieved before yielding. Under these test conditions, the two Zr-based BMGs exhibit remarkable plastic deformation. The plasticity of the Zr64.2 Ni16.2 Cu14.6 Al5 BMG reaches up to 25% before failure. For the Zr63.4 Ni16.2 Cu15.4 Al5 BMG, it exhibits super plasticity, in which the plasticity reaches 70% without fracture, similar to that reported by Liu [3]. It should also be noted that a significant serration flow is revealed during the plastic deformation in the two BMGs. The inset of Fig. 7 highlights the serration behavior of these two BMGs. The stress and strain (or time) magnitudes or the stress drop of each serration in the Zr64.2 Ni16.2 Cu14.6 Al5 BMG are larger than those of the Zr63.4 Ni16.2 Cu15.4 Al5 BMG. Previous studies [21,22] have revealed that the stress drop in serrated flow originates from the rapid sliding of the shear bands. If the stress drop is too large, it will lead to plastic instability and fracture in the BMGs. The sample surface and fracture of two BMGs after uniaxial compressive tests are shown in Fig. 8. For the Zr63.4 Ni16.2 Cu15.4 Al5 BMG with super-plasticity, the 2 mm diameter rod was compressed into a flake without fracturing, as shown in Fig. 8(a). The diameter of the flake is more than 3.5 mm. A high density of shear bands is observed around the surface of the rod sample after plastic deformation (as shown in Fig. 8(b)), illustrating the formation of multiple shear bands. The primary shear bands, which are marked with white arrows, are approximately parallel to the direction of the shear stress. With careful examination, a number of secondary or tertiary shear bands perpendicular to the direction of the primary shear bands can be clearly observed. It is well known that plastic deformation of BMGs is highly localized into shear bands. The results of the SEM morphology are consistent with the stress–strain curve of the Zr63.4 Ni16.2 Cu15.4 Al5 BMG. The shear band pattern at the fracture surface was also investigated. A typical SEM morphology for the Zr64.2 Ni16.2 Cu14.6 Al5 BMG is shown in Fig. 8(c–f). The fracture angle between the stress axis and the shear plane is about 40◦ , as shown in Fig. 8(c). Fig. 8(d) further shows the intersection of the shear bands, and the density of the shear bands is less than for the Zr63.4 Ni16.2 Cu15.4 Al5 BMG. A well-developed vein-like pattern is observed on the fracture surface, in a direction along the shear plane, as indicated with the white arrow in Fig. 8(e). At a higher magnification, a significant viscous flow around the vein-like pattern is also revealed in Fig. 8(f). It is interesting to note that the Zr63.4 Ni16.2 Cu15.4 Al5 BMG with the poorest corrosion resistance has the largest plasticity. It is believed that the Zr63.4 Ni16.2 Cu15.4 Al5 BMG has more interfaces between the heterogeneous and homogeneous structures, and these can hinder the propagation of the shear bands and provide more origins for pitting corrosion. On the other hand, the shear transformation zone (STZ) model considers that the nucleation of the shear bands is located in a small cluster of closed-packed rearranged atoms under shear stress [23,24]. From the energy perspective, ductile BMGs have a low barrier energy density for the activation of STZs and thereby shear bands can easily be formed. Yu et al. [16] proposed that the potential-energy barrier of STZs is approximately equal to the activation energy of ˇ relaxation, and the intrinsic plasticity in metallic glass correlates with the structure heterogeneity. It is noted that the potential-energy barrier of STZs, or the activation energy of ˇ relaxation of the two Zr-based BMGs with the large plasticity, are low, facilitating the nucleation of the shear bands.
W.H. Li et al. / Materials Science and Engineering A 534 (2012) 157–162
161
Fig. 8. The SEM images of the surface and fracture morphology. (a) Compressed flake of the Zr63.4 Ni16.2 Cu15.4 Al5 BMG at 70% true strain. (b) High density shear bands of surface morphology for the Zr63.4 Ni16.2 Cu15.4 Al5 BMG. (c)–(f) Fracture and underlying fracture surface morphology for the Zr64.2 Ni16.2 Cu14.6 Al5 BMG.
4. Conclusions Two Zr-based BMGs with high strength and large plasticity were prepared by copper mold casting. TEM observations reveal that both these two BMGs exhibit heterogeneous structures. Although the DSC curves of the two BMGs obtained under continuous heating cannot reveal the heterogeneous structure, the isothermal annealing studies clearly show that there are two independent exothermic peaks resulting from two crystallization processes. The potentiodynamic polarization curves further illustrate that, compared to the BMG with homogeneous structures, the two BMGs with heterogeneous structures exhibit poorer corrosion resistance. Multiple shear bands are observed on the surface of the compressed BMG specimens. The enhanced plasticity of the two BMGs is considered to be due to the low potential-energy barrier of STZs and the high resistance for the propagation of shear bands, which are related to their heterogeneous structures. Acknowledgements This work is fully supported by the Research Grants Council of Hong Kong Special Administration Region (under the project code PolyU511108). W.H. Li would also like to express his appreciation
for the support given by the National Nature Science Foundation of China (Grant Nos. 50701001, 51071001) and the Nature Science Foundation of Anhui Province (Grant No. KJ2010A052) References [1] K.F. Yao, F. Ruan, Y.Q. Yang, N. Chen, Appl. Phys. Lett. 88 (2006) 122106. [2] J. Schroers, W.L. Johnson, Phys. Rev. Lett. 93 (2004) 255506. [3] Y.H. Liu, G. Wang, R.J. Wang, D.Q. Zhao, M.X. Pan, W.H. Wang, Science 315 (2007) 1385–1388. [4] X.H. Du, J.C. Huang, K.C. Hsieh, Y.H. Lai, H.M. Chen, J.S.C. Jang, P.K. Liaw, Appl. Phys. Lett. 91 (2007) 131901. [5] L.Q. Xing, Y. Li, K.T. Ramesh, J. Li, T.C. Hufnagel, Phys. Rev. B 64 (2001) 180201. [6] J. Das, M.B. Tang, K.B. Kim, R. Theissmann, F. Baier, W.H. Wang, J. Eckert, Appl. Phys. Lett. 88 (2006) 051911. [7] J.J. Lewandowski, W.H. Wang, A.L. Greer, Philos. Mag. Lett. 85 (2005) 77–87. [8] Y. Zhang, W.H. Wang, A.L. Greer, Nat. Mater. 5 (2006) 857–860. [9] H. Bei, S. Xie, E.P. George, Phys. Rev. Lett. 96 (2006) 105503. [10] Z.F. Zhang, H. Zhang, X.F. Pan, J. Das, J. Eckert, Philos. Mag. Lett. 85 (2005) 513–521. [11] M.W. Chen, Annu. Rev. Mater. Res. 38 (2008) 445–469. [12] N. Van Sreenberge, A. Concustell, J. Sort, J. Das, N. Mattern, A. Gebert, S. Surinach, J. Eckert, M.D. BaÓ., Mater. Sci. Eng. A 491 (2008) 124–130. [13] C.A. Schuh, T.C. Hufnagel, U. Ramamurty, Acta Mater. 55 (2007) 4067–4109. [14] B.B. Sun, Y.B. Wang, J. Wen, H. Yang, M.L. Sui, J.Q. Wang, E. Ma, Scr. Mater. 53 (2005) 805–809. [15] B. Yang, T.G. Nieh, Appl. Phys. Lett. 88 (2006) 221911. [16] H.B. Yu, W.H. Wang, H.Y. Bai, Y. Wu, M.W. Chen, Phys. Rev. B 81 (2010) 220201.
162
W.H. Li et al. / Materials Science and Engineering A 534 (2012) 157–162
[17] H.B. Lu, L.C. Zhang, A. Gebert, L. Schultz, J. Alloys Compd. 462 (2008) 60–67. [18] B. Liu, L. Liu, Mater. Sci. Eng. A 415 (2006) 286–290. [19] B.C. Wei, Y. Zhang, Y.X. Zhuang, D.Q. Zhao, M.X. Pan, W.H. Wang, W.R. Hu, J. Appl. Phys. 89 (2001) 3529–3531. [20] R.J. Ortega-Hertogs, A. Inoue, K.V. Rao, Scr. Mater. 44 (2001) 1333–1336.
[21] [22] [23] [24]
C.A. Schuh, T.G. Nieh, Acta Mater. 51 (2003) 87–99. W.J. Wright, R. Saha, W.D. Nix, Mater. Trans. 42 (2001) 642–649. A.S. Argon, Acta Metall. 27 (1979) 47–58. M.L. Falk, J.S. Langer, Phys. Rev. E 57 (1998) 7192–7205.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Thermodynamic, corrosion and mechanical properties of Zr-based bulk metallic glasses in relation to heterogeneous structures W.H. Li a,b , K.C. Chan a,∗ , L. Xia a , L. Liu c , Y.Z. He b a b c
Advanced Manufacturing Technology Research Centre, Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan 243002, People’s Republic of China Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 15 February 2011 Received in revised form 28 October 2011 Accepted 10 November 2011 Available online 30 November 2011 Keywords: Bulk amorphous alloys Thermal stability Corrosion resistance Plasticity
a b s t r a c t In this paper, Zr64.2 Ni16.2 Cu14.6 Al5 and Zr63.4 Ni16.2 Cu15.4 Al5 BMGs with heterogeneous structures were prepared through copper mold casting. The microstructure, thermal stability and corrosion behavior of the two Zr-based metallic glasses were investigated by X-ray diffraction (XRD), high-resolution transmission electron micrography (HRTEM), differential scanning calorimetry (DSC) and potentiodynamic polarization tests. In TEM micrographs, heterogeneous structures were revealed by the contrast between the bright and dark zones. Compared to the Zr65 A10 lNi10 Cu15 BMG with a homogenous structure, the heterogeneous structures of the two BMGs are shown to reduce their thermal stability and corrosion resistance. The two Zr-based BMGs however exhibit large plastic strain (>25%) and high yield strength (>1.6 GPa) under uniaxial compression tests. The low potential-energy barrier of shear transition zones (STZs) and the high resistance to the propagation of shear bands of the BMGs are related to the heterogeneous structures. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, a series of Pd- [1], Pt- [2], Zr- [3–5] and Cu-based [6] monolithic bulk metallic glasses (BMGs) with high plasticity at room temperature have been developed. In particular, a lot of attention has been paid to Zr-based BMGs due to their large glass forming ability and good combination of strength and plasticity, implying promising potential for structural applications. A number of approaches have been developed for the improvement of monolithic BMGs, which include (i) the achievement of a critical value of Poisson’s ratio [7]; (ii) the introduction of a compressive residual stress [8]; (iii) the design of specimens with small length–diameter ratio [9,10]; and (iv) the introduction of atomic-scale inhomogenities [1,11,12]. Much research work has also been carried out to understand the plastic deformation mechanisms, both spatially and temporally, of these disordered materials. It is well known that the plastic deformation of metallic glasses is inhomogeneous and highly localized into shear bands at room temperature, and the plasticity of BMGs strongly relies on the chemical and physical properties of the component elements [13].
∗ Corresponding author. Tel.: +852 27664981; fax: +852 23625267. E-mail addresses: [email protected], [email protected] (K.C. Chan). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.11.054
Heterogeneity at the atomic scale, such as medium range order, nanocrystallization and phase separation in BMGs, is found to be favorable for achieving good ductility [1,12,13]. However, the atomic structure of BMGs is still not fully understood, and there is also much discussion on characterizing micro-, or nano-meter scale structures in BMGs. The microstructure of BMGs is usually characterized through high resolution TEM, but artifacts, including heterogeneity, may arise during the sample preparation process, which may affect the results of the characterization [14]. Besides direct observation by TEM, and the study of the relationship between heterogeneity and plasticity, it is also of significance to investigate the effect of heterogeneous structures on other properties or behavior of BMGs, the results of which may help ascertain the heterogeneous microstructure in BMGs. It is interesting to point out that the strength and macroscopic plastic behavior of BMGs is related to their thermodynamic behavior, for example, the yield strength of BMGs is proportional to the glass transition temperature [15]. The energy of ˇ relaxation is equivalent to the potential-energy barrier of the dynamical shear transformation zone (STZ) [16], and pitting corrosion is also found to be sensitive to variations in the microstructure [17,18]. Although there have been extensive efforts to study the effect of heterogeneous structures on the mechanical properties, the effect on thermodynamic and corrosion properties of ductile BMGs have been less investigated. In this paper, by comparison to a homogeneous Zr-based BMG, the thermal stability, corrosion behavior and
158
W.H. Li et al. / Materials Science and Engineering A 534 (2012) 157–162
mechanical properties of two Zr-based BMGs, with heterogeneous structures, are investigated.
Ingots, with a nominal composition of Zr64.2 Ni16.2 Cu14.6 Al5 and Zr63.4 Ni16.2 Cu15.4 Al5 , were prepared by the arc-melting of elements Zr, Al, Ni and Cu, with a purity of 99.9%, in a titanium-gettered argon atmosphere. Cylindrical specimens of 3 mm and 2 mm diameter were prepared by suction casting into a copper mold. The structure of the samples was characterized by X-ray diffraction (XRD) in a Philips PW 1050 diffractometer using Cu K␣-radiation at a scanning speed of 2◦ /min. The microstructural observation of the samples was performed on a JEOL JEM-2010 high-resolution transmission electron microscope (HRTEM) operated at 200 kV. Thermal analysis was carried out in a Perkin-Elmer DSC-7 differential scanning calorimeter under an argon atmosphere. For continuous heating tests, a constant heating rate of 20 K/min was employed. A rapid heating rate of 100 K/min up to the annealing temperature was employed. The isothermal measurements were performed at two different temperatures T = 671 and 691 K. The corrosion behavior of the BMGs was examined by electrochemical polarization measurements. The specimens were mechanically polished to a mirror finished before electrochemical and immersion tests. The electrolytes used were a 3.5 wt.% NaOH aqueous solution which was prepared from reagent grade chemicals and distilled water. After immersing the specimens for about 20 min, at which time the open circuit potentials became almost steady, potentiodynamic polarization curves were measured at a potential sweep rate of 1 mV/s. Uniaxial compressive tests were performed in an MTS testing machine at room temperature on cylindrical samples 2 mm in diameter and 4 mm in length. The crosshead speed was constant with an initial strain rate of 2 × 10−4 s−1 . The compressive fracture surface analyses were performed using a JSM-6460 scanning electron microscope (SEM).
Cu Kα
Intensity (arb.units)
2. Experimental
Zr63.4Ni16.2Cu15.4Al5 Zr64.2Ni16.2Cu14.6Al5
20
30
40
50
60
70
80
2 Theta (degree) Fig. 1. The XRD patterns Zr63.4 Ni16.2 Cu15.4 Al5 BMGs.
of
as-cast
rods
of
Zr64.2 Ni16.2 Cu14.6 Al5
and
3. Results and discussion
investigations were undertaken for the Zr63.4 Ni16.2 Cu15.4 Al5 BMG. No obvious medium range order clusters, phase separation or nano-crystallization were found in the high resolution electronmicroscopy (HREM) image, as shown in Fig. 2(a). The halo ring in the selected area diffraction pattern indicates the glassy structure of the BMG. Fig. 2(b) shows the bright-field images of the Zr63.4 Ni16.2 Cu15.4 Al5 BMG. The heterogeneous structures were clearly revealed by the contrast between the bright and dark zones. The size of the bright zone is about 0.5–1 m. This result is similar to the microstructure of the Zr64.13 Cu15.75 Ni10.12 Al10 BMG with super plasticity [3]. Although the TEM observation can directly reveal the heterogeneous structure of the BMG, it is still not possible to exclude the likelihood that the heterogeneity is induced by the specimen preparation process. To better ascertain the material microstructures, the effect of heterogeneous structures on thermal stability, corrosion behavior and mechanical properties were investigated and are described in the following sections.
3.1. Microstructure
3.2. Thermal stability and heterogeneous structures
Fig. 1 shows the XRD patterns of the two Zr-based BMGs ascast 3 mm diameter rods. The typical broad diffraction maxima were observed in the XRD pattern illustrating fully amorphous characteristics. No crystalline peaks or the heterogeneous structures were found with the XRD detection limit. To further clarify the microstructure for nanometer-sized features, TEM
The thermal stability of two Zr-based BMGs was evaluated by DSC measurements at a heating rate of 20 K/min. The DSC curves of the two Zr-based BMGs exhibit an endothermic event characteristic of glass transition, followed by an exothermic event, characteristic of the crystallization process, as shown in Fig. 3(a). The temperature range of Tg , Tx and Tl in the two BMGs are 651–653 K, 720–722 K and
Fig. 2. Microstructure of Zr63.4 Ni16.2 Cu15.4 Al5 BMG (a) HREM image, with the inset related to the SEAD pattern. (b) The bright-field images of TEM.
W.H. Li et al. / Materials Science and Engineering A 534 (2012) 157–162
159
Tx
Zr63.4Ni16.2Cu15.4Al5 Zr64.2Ni16.2Cu14.6Al5 20 K/min
Zr63.4Ni16.2Cu15.4Al5 Zr64.2Ni16.2Cu14.6Al5 Zr65Ni10Cu15Al10
Tx
Endo. Up (a.u.)
Endo. Up (a.u.)
(a)
Tx Tg+40 K
Tg
10
20
30
40
Time (min) 550
600
650
700
750
800
Temperature (K)
Zr64.2Ni16.2Cu14.6Al5 Zr63.4Ni16.2Cu15.4Al5
Endo. Up (a.u.)
1119–1120 K, respectively. Thus the reduced glass transition temperature (Trg = Tg /Tl ), the supercooled liquid region (T = Tx − Tg ) and the GFA parameter [=Tx /(Tg + Tl )], are 0.58, 69 K and 0.41, respectively. The results show that the two Zr-based BMGs have high thermal stability and glass forming ability. Very often, continuous heating experiments alone are not sufficient for probing microstructure changes, such as relaxation and crystallization phenomena in inhomogeneous amorphous materials. There are two reasons for the disappearance of the thermodynamics phenomena. Firstly, the enthalpy of glass transition is too weak to be distinguished from the DSC curves. For example, no obvious glasses transition phenomena were found in iron-rich Nd-based BMGs [19,20]. Secondly, if the heating rate is rapid enough, it will restrain the BMG crystallization process. Under the condition of continuous heating, Fig. 3 only shows a single glass transition temperature and a single crystallization peak for the two BMGs, which seems that they exhibit homogenous structures. In order to better reveal the relationship between the microstructure and the thermodynamic properties, unambiguous identification of the microcrystalline structure is possible through the study of the isothermal crystallization kinetics. The study was carried out at temperatures between Tg and Tx . Fig. 4(a) shows the isothermal crystallization DSC curves of the three Zrbased BMGs at the annealing temperature (Tg +40 K). Only a single exothermic peak can be found for the Zr65 Al10 Ni10 Cu15 BMG, revealing its homogenous amorphous structure. For the Zr63.4 Ni16.2 Cu15.4 Al5 and Zr64.2 Ni16.2 Cu14.6 Al5 BMGs, two independent exothermic peaks resulting from the crystallization processes were observed. While the first exothermic enthalpy is found to be much less than the second enthalpy, the crystallization rate is much greater than in the latter. At the annealing temperature (Tg +20 K), Fig. 4(b) also shows two crystallization processes for the Zr63.4 Ni16.2 Cu15.4 Al5 BMG, although the peak height of the DSC curves is reduced and the peak width is increased. For the Zr63.4 Ni16.2 Cu15.4 Al5 BMG, one exothermic peak is observed within the timescale of the experiment. To better ascertain the microstructural changes during the annealing process, XRD tests were conducted. Fig. 5 shows the XRD patterns of as-cast and annealed Zr63.4 Ni16.2 Cu15.4 Al5 alloys at 671 K (Tg +20 K) holding for 5 min followed by cooling at a rate of 100 K/min. The annealed sample exhibits partial crystal phases within an amorphous matrix phase. The crystallized phases consist of Zr2 Cu and Zr2 Ni. This result reveals that the initial exothermic peak in Fig. 4(b) truly reflects the crystallization behavior. It is
(b)
Tx Tg+20 K
20
40
60
80
Time (min) Fig. 4. Isothermal crystallization DSC curves of Zr64.2 Ni16.2 Cu14.6 Al5 , Zr63.4 Ni16.2 Cu15.4 Al5 and Zr65 Al10 Ni10 Cu15 BMGs; (a) Tg +40 K, (b) Tg +20 K.
well known that the crystallization process is controlled by shortrange diffusion of the chemical elements. The two crystallization peaks indicate that there are two different structures with different thermal stabilities, and the results of the isothermal crystallization study illustrate the heterogeneous structures of the two Zr-based BMGs. These findings are consistent with the TEM observation, as shown in Fig. 2(a). 3.3. Corrosion and heterogeneous structures The corrosion of metallic alloys is significantly afffected by the microstructure and the phase interface. In general, amorphous alloys are known to exhibit improved corrosion resistance due
Zr2Cu Zr2Ni
Intensity (arb.units)
Fig. 3. The DSC curves of Zr64.2 Ni16.2 Cu14.6 Al5 and Zr63.4 Ni16.2 Cu15.4 Al5 BMGs at a heating rate of 20 K/min.
Tx
20
30
40
50
60
70
80
2 Theta (degree) Fig. 5. The XRD patterns of as-cast and annealed Zr63.4 Ni16.2 Cu15.4 Al5 alloys at 671 K.
160
W.H. Li et al. / Materials Science and Engineering A 534 (2012) 157–162
-1 -2
Zr64.2Ni16.2Cu14.6Al5 Zr63.4Ni16.2Cu15.4Al5 Zr65Ni10Cu15Al10
log i (mA/cm2)
-3 -4 -5 -6 -7 -8 -9 0.0
-0.2
-0.4
-0.6
-0.8
E (V vs. SEC) Fig. 6. The polarization curves of Zr64.2 Ni16.2 Cu14.6 Al5 and Zr63.4 Ni16.2 Cu15.4 Al5 BMGs in a 3.5 wt.% NaCl solution.
to having fewer defects and optimized chemical homogeneity. With the aim of further clarifying the heterogeneous structures of the two Zr-based BMGs, pitting corrosion tests were conducted. Fig. 6 shows the polarization curves of the Zr64.2 Ni16.2 Cu14.6 Al5 and Zr63.4 Ni16.2 Cu15.4 Al5 BMGs in 3.5 wt.% NaCl solution at 298 K open to the atmosphere. For comparison, the corrosion behavior of the Zr65 Al10 Ni10 Cu15 BMG with a homogenous amorphous structure was also examined at the same conditions (see Fig. 6). There are significantly different corrosion and pitting potentials for the three Zr-based BMGs electrodes. Among the three Zr-based BMGs, the Zr65 Al10 Ni10 Cu15 BMG exhibits the highest corrosion and pitting potentials, and the lowest corrosion current density. However, the Zr63.4 Ni16.2 Cu15.4 Al5 BMG exhibits the lowest corrosion potential and pitting potentials, and the highest corrosion current density. It reveals that the Zr65 Al10 Ni10 Cu15 BMG has the best corrosion resistance in a 3.5 wt.% NaCl solution. It indicates that an amorphous alloy with a homogeneous structure is more corrosion resistant than in an heterogeneous structure under same corrosion conditions and with a similar composition. The low corrosion resistance of inhomogeneous BMGs may be caused by the increased number of interface zones. 3.4. Mechanical behavior and heterogeneous structures Fig. 7 shows the room-temperature true stress–strain curves of Zr63.4 Ni16.2 Cu15.4 Al5 and Zr64.2 Ni16.2 Cu14.6 Al5 glassy rods obtained
6
Zr
Ni
Cu
Al
True Stress (GPa)
5 4
Zr
Ni
Cu
Al
3 2
Zr63.4Ni16.2Cu15.4Al5 Zr64.2Ni16.2Cu14.6Al5
1 0
0
10
20
30
40
50
True Strain (%)
60
70
Fig. 7. The true stress–strain curves of Zr63.4 Ni16.2 Cu15.4 Al5 and Zr64.2 Ni16.2 Cu14.6 Al5 BMGs. The inset highlights the serrated flow segments.
by uniaxial compression tests at a strain rate of 2 × 10−4 s−1 . The yield stress and elastic modulus of the Zr64.2 Ni16.2 Cu14.6 Al5 BMG are 1.65 GPa and 76 GPa, respectively, and the corresponding values for the Zr63.4 Ni16.2 Cu15.4 Al5 BMG are 1.70 GPa and 78 GPa. These values are similar to those reported for nearly identical alloys [4]. Elastic strains of nearly 2% are achieved before yielding. Under these test conditions, the two Zr-based BMGs exhibit remarkable plastic deformation. The plasticity of the Zr64.2 Ni16.2 Cu14.6 Al5 BMG reaches up to 25% before failure. For the Zr63.4 Ni16.2 Cu15.4 Al5 BMG, it exhibits super plasticity, in which the plasticity reaches 70% without fracture, similar to that reported by Liu [3]. It should also be noted that a significant serration flow is revealed during the plastic deformation in the two BMGs. The inset of Fig. 7 highlights the serration behavior of these two BMGs. The stress and strain (or time) magnitudes or the stress drop of each serration in the Zr64.2 Ni16.2 Cu14.6 Al5 BMG are larger than those of the Zr63.4 Ni16.2 Cu15.4 Al5 BMG. Previous studies [21,22] have revealed that the stress drop in serrated flow originates from the rapid sliding of the shear bands. If the stress drop is too large, it will lead to plastic instability and fracture in the BMGs. The sample surface and fracture of two BMGs after uniaxial compressive tests are shown in Fig. 8. For the Zr63.4 Ni16.2 Cu15.4 Al5 BMG with super-plasticity, the 2 mm diameter rod was compressed into a flake without fracturing, as shown in Fig. 8(a). The diameter of the flake is more than 3.5 mm. A high density of shear bands is observed around the surface of the rod sample after plastic deformation (as shown in Fig. 8(b)), illustrating the formation of multiple shear bands. The primary shear bands, which are marked with white arrows, are approximately parallel to the direction of the shear stress. With careful examination, a number of secondary or tertiary shear bands perpendicular to the direction of the primary shear bands can be clearly observed. It is well known that plastic deformation of BMGs is highly localized into shear bands. The results of the SEM morphology are consistent with the stress–strain curve of the Zr63.4 Ni16.2 Cu15.4 Al5 BMG. The shear band pattern at the fracture surface was also investigated. A typical SEM morphology for the Zr64.2 Ni16.2 Cu14.6 Al5 BMG is shown in Fig. 8(c–f). The fracture angle between the stress axis and the shear plane is about 40◦ , as shown in Fig. 8(c). Fig. 8(d) further shows the intersection of the shear bands, and the density of the shear bands is less than for the Zr63.4 Ni16.2 Cu15.4 Al5 BMG. A well-developed vein-like pattern is observed on the fracture surface, in a direction along the shear plane, as indicated with the white arrow in Fig. 8(e). At a higher magnification, a significant viscous flow around the vein-like pattern is also revealed in Fig. 8(f). It is interesting to note that the Zr63.4 Ni16.2 Cu15.4 Al5 BMG with the poorest corrosion resistance has the largest plasticity. It is believed that the Zr63.4 Ni16.2 Cu15.4 Al5 BMG has more interfaces between the heterogeneous and homogeneous structures, and these can hinder the propagation of the shear bands and provide more origins for pitting corrosion. On the other hand, the shear transformation zone (STZ) model considers that the nucleation of the shear bands is located in a small cluster of closed-packed rearranged atoms under shear stress [23,24]. From the energy perspective, ductile BMGs have a low barrier energy density for the activation of STZs and thereby shear bands can easily be formed. Yu et al. [16] proposed that the potential-energy barrier of STZs is approximately equal to the activation energy of ˇ relaxation, and the intrinsic plasticity in metallic glass correlates with the structure heterogeneity. It is noted that the potential-energy barrier of STZs, or the activation energy of ˇ relaxation of the two Zr-based BMGs with the large plasticity, are low, facilitating the nucleation of the shear bands.
W.H. Li et al. / Materials Science and Engineering A 534 (2012) 157–162
161
Fig. 8. The SEM images of the surface and fracture morphology. (a) Compressed flake of the Zr63.4 Ni16.2 Cu15.4 Al5 BMG at 70% true strain. (b) High density shear bands of surface morphology for the Zr63.4 Ni16.2 Cu15.4 Al5 BMG. (c)–(f) Fracture and underlying fracture surface morphology for the Zr64.2 Ni16.2 Cu14.6 Al5 BMG.
4. Conclusions Two Zr-based BMGs with high strength and large plasticity were prepared by copper mold casting. TEM observations reveal that both these two BMGs exhibit heterogeneous structures. Although the DSC curves of the two BMGs obtained under continuous heating cannot reveal the heterogeneous structure, the isothermal annealing studies clearly show that there are two independent exothermic peaks resulting from two crystallization processes. The potentiodynamic polarization curves further illustrate that, compared to the BMG with homogeneous structures, the two BMGs with heterogeneous structures exhibit poorer corrosion resistance. Multiple shear bands are observed on the surface of the compressed BMG specimens. The enhanced plasticity of the two BMGs is considered to be due to the low potential-energy barrier of STZs and the high resistance for the propagation of shear bands, which are related to their heterogeneous structures. Acknowledgements This work is fully supported by the Research Grants Council of Hong Kong Special Administration Region (under the project code PolyU511108). W.H. Li would also like to express his appreciation
for the support given by the National Nature Science Foundation of China (Grant Nos. 50701001, 51071001) and the Nature Science Foundation of Anhui Province (Grant No. KJ2010A052) References [1] K.F. Yao, F. Ruan, Y.Q. Yang, N. Chen, Appl. Phys. Lett. 88 (2006) 122106. [2] J. Schroers, W.L. Johnson, Phys. Rev. Lett. 93 (2004) 255506. [3] Y.H. Liu, G. Wang, R.J. Wang, D.Q. Zhao, M.X. Pan, W.H. Wang, Science 315 (2007) 1385–1388. [4] X.H. Du, J.C. Huang, K.C. Hsieh, Y.H. Lai, H.M. Chen, J.S.C. Jang, P.K. Liaw, Appl. Phys. Lett. 91 (2007) 131901. [5] L.Q. Xing, Y. Li, K.T. Ramesh, J. Li, T.C. Hufnagel, Phys. Rev. B 64 (2001) 180201. [6] J. Das, M.B. Tang, K.B. Kim, R. Theissmann, F. Baier, W.H. Wang, J. Eckert, Appl. Phys. Lett. 88 (2006) 051911. [7] J.J. Lewandowski, W.H. Wang, A.L. Greer, Philos. Mag. Lett. 85 (2005) 77–87. [8] Y. Zhang, W.H. Wang, A.L. Greer, Nat. Mater. 5 (2006) 857–860. [9] H. Bei, S. Xie, E.P. George, Phys. Rev. Lett. 96 (2006) 105503. [10] Z.F. Zhang, H. Zhang, X.F. Pan, J. Das, J. Eckert, Philos. Mag. Lett. 85 (2005) 513–521. [11] M.W. Chen, Annu. Rev. Mater. Res. 38 (2008) 445–469. [12] N. Van Sreenberge, A. Concustell, J. Sort, J. Das, N. Mattern, A. Gebert, S. Surinach, J. Eckert, M.D. BaÓ., Mater. Sci. Eng. A 491 (2008) 124–130. [13] C.A. Schuh, T.C. Hufnagel, U. Ramamurty, Acta Mater. 55 (2007) 4067–4109. [14] B.B. Sun, Y.B. Wang, J. Wen, H. Yang, M.L. Sui, J.Q. Wang, E. Ma, Scr. Mater. 53 (2005) 805–809. [15] B. Yang, T.G. Nieh, Appl. Phys. Lett. 88 (2006) 221911. [16] H.B. Yu, W.H. Wang, H.Y. Bai, Y. Wu, M.W. Chen, Phys. Rev. B 81 (2010) 220201.
162
W.H. Li et al. / Materials Science and Engineering A 534 (2012) 157–162
[17] H.B. Lu, L.C. Zhang, A. Gebert, L. Schultz, J. Alloys Compd. 462 (2008) 60–67. [18] B. Liu, L. Liu, Mater. Sci. Eng. A 415 (2006) 286–290. [19] B.C. Wei, Y. Zhang, Y.X. Zhuang, D.Q. Zhao, M.X. Pan, W.H. Wang, W.R. Hu, J. Appl. Phys. 89 (2001) 3529–3531. [20] R.J. Ortega-Hertogs, A. Inoue, K.V. Rao, Scr. Mater. 44 (2001) 1333–1336.
[21] [22] [23] [24]
C.A. Schuh, T.G. Nieh, Acta Mater. 51 (2003) 87–99. W.J. Wright, R. Saha, W.D. Nix, Mater. Trans. 42 (2001) 642–649. A.S. Argon, Acta Metall. 27 (1979) 47–58. M.L. Falk, J.S. Langer, Phys. Rev. E 57 (1998) 7192–7205.