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Phase transition, segregation and nanopore formation in high-energy heavy-ion-irradiated metallic glass
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Scripta Materialia 67 (2012) 887–890 www.elsevier.com/locate/scriptamat
Phase transition, segregation and nanopore formation in high-energy heavy-ion-irradiated metallic glass M.T. Myers,a,b,⇑ S. Charnvanichborikarn,a C.C. Wei,c Z.P. Luo,c,d G.Q. Xie,e S.O. Kucheyev,a D.A. Luccaf and L. Shaob a
Lawrence Livermore National Laboratory, Livermore, CA 94550, USA Department of Nuclear Engineering, Texas A&M University, College Station, TX 77843, USA c Materials Science and Engineering Program, Texas A&M University, College, Station, TX 77843, USA d Microscopy and Imaging Center, Texas A&M University, College Station, TX 77843, USA e Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan f School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK 74078, USA b
Received 10 July 2012; accepted 12 August 2012 Available online 17 August 2012
We report elemental segregation and the formation of a nanocomposite containing a secondary amorphous phase formed along the path of an incident ion in a metallic glass (MG) irradiated with high-energy heavy ions. Electropolishing with a solution of nitric acid preferentially attacks the damaged regions along ion trajectories leaving behind 50–500 nm diameter pores. No direct crystallization is observed as a result of damage induced by a single ion, further supporting the theory that irradiation-induced crystallization of the MG is a homogeneous process. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ion irradiation; Metallic glass; Nanocrystallization; Nanoporous
Various metallic glasses (MGs) have been shown to exhibit superior resistance to corrosion, making them ideal for harsh environments [1]. One particular MG, Ni52.5Nb10Zr15Ti15Pt7.5, has been shown to exhibit a large supercooled liquid region [2,3] and high stability [4]. On the other hand, a fully amorphous structure is brittle due to the ease of shear band formation and propagation under stress [5]. Enhanced ductility can be achieved by either introducing a secondary phase or nanocrystals, which provide a mechanism to release stress [6] by shear band deflection. When a MG is annealed at temperatures above the glass transition temperature and slowly cooled, partial crystallization can take place [7]. Mechanical deformation has also been used to induce, presumably athermally, small fractions of phase transformations [8–10]. Apart from these highly localized and destructive techniques, ion [11–15] and electron [16] irradiation has been shown to cause
⇑ Corresponding
author at: Lawrence Livermore National Laboratory, Livermore, CA 94550, USA. Tel.: +1 330 984 7996; fax: +1 979 845 6443; e-mail: [email protected]
the formation of nanometer-sized particles inside an amorphous matrix. During ion bombardment, local melting can be induced by the high-energy deposition rate in displacement cascades [17]. Previous results have, however, shown that nanocrystal formation is unlikely to be a consequence of localized heating of an MG irradiated at room temperature with 1 MeV Ni+ ions [13]. Although the cascade temperature probably exceeds the melting point of the MG, the quenching process occurs much faster than the critical cooling rate [2,18] required to form the MG [13]. Crystallization of MGs during irradiation is, therefore, more likely due to other processes such as atomic displacements that cause density fluctuations, leading to enhanced atomic mobilities and structural relaxation [19,20]. In the present study, we use high-energy heavy-ion irradiation to study the radiation response of Ni52.5Nb10Zr15Ti15Pt7.5. Unlike most of the previous studies using ions in the energy range of [1 MeV, energy loss of high-energy heavy ions is initially dominated by electronic stopping [21]. Although nuclei–nuclei scattering is a weak mechanism for displacement creation in this
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.08.015
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M. T. Myers et al. / Scripta Materialia 67 (2012) 887–890
case, the energy of electronic excitation can be transferred to atomic motion through the following mechanisms: thermal spike [22], Coulomb explosion [23], and material instability due to lattice relaxation [24]. Understanding the effect of radiation damage due to high-energy heavy ions in MGs is of fundamental importance, and may pave the way for structural engineering of MGs with improved mechanical properties that could lead to novel applications. Ribbons of Ni52.5Nb10Zr15Ti15Pt7.5 MG (20 lm thick, 1.5 mm wide) were produced by rapid solidification of a liquid metal solution on a single copper roller at a surface velocity of 42 m s 1 [3]. Irradiation was performed at Lawrence Livermore National Laboratory with a 4 MV ion accelerator (National Electrostatics Corporation, model 4UH). The samples were irradiated at room temperature with 18 MeV Xe5+ ions to fluences of 5 109 and 5 1010 cm 2 resulting in an average lateral distance between ion impacts of 150 and 50 nm, respectively. During irradiation, the sample normal was tilted by 72° relative to the incident beam direction in order to reduce the projected ion range and to address the question of whether a single incident ion can lead to a direct amorphous–crystalline transition. In such a case, we expect to observe crystalline particles with rod-like shapes by plane-view transmission electron microscopy (PTEM). Following irradiation, specimens for PTEM were back-thinned until perforation by either conventional dimple polishing with 1 lm diamond paste or single-jet electropolishing (80/20 mixture of methanol/nitric acid at 40 °C). PTEM specimens were then examined in both a JEOL 2010 and a FEI Tecnai G2 F20 ST microscopes operated at 200 kV to perform PTEM, scanning TEM (STEM) and energy dispersive spectroscopy (EDS) analyses. Figure 1a shows a representative bright-field (BF) PTEM micrograph after 18 MeV Xe5+ irradiation at 72° off normal incidence to a fluence of 5 109 cm 2. This sample was prepared with the conventional dimple polishing technique. The region marked with a white arrow indicates the location of an individual ion incident on the sample surface. It is clear that the damage caused by an impinging ion at a tilted angle results in an elongated layered shell structure. The initial portion of the damage zone, probably the entry point of the ion on the sample surface, has a lighter contrast as opposed to the darker contrast surrounding the periphery of the damage zone. Across a number of regions imaged, such damage zones due to a single ion range from 50 to 100 nm in diameter. Localized crystallization was found near the sample edge, which might be caused by surface effects, i.e. enhanced atomic mobility in an ultra-thin membrane. In order to avoid this artifact of sample preparation, all reported data in the present study have been obtained from a relatively thicker region away from the specimen edge. Figure 1b shows a dark-field (DF) PTEM micrograph and the corresponding selected-area diffraction (SAD) pattern of the same region shown in Figure 1a. As evidenced by the bright diffuse rings in the SAD pattern, the ion impact does not cause crystallization—the
Figure 1. (a) BF PTEM micrograph of Ni52.5Nb10Zr15Ti15Pt7.5 MG irradiated at room temperature with 18 MeV Xe5+ ions incident at 72° off the sample surface normal to a fluence of 5 109 cm 2. (b) DF PTEM micrograph and the inset SAD pattern for the region seen in (a). The sample was prepared by the dimple polishing technique. Incoming ion beam direction is labeled with white arrows in (a) and (b).
sample has retained its amorphous state after irradiation. Therefore, the contrast variation observed in PTEM micrographs in Figure 1a and b could be attributed to a density variation. It is likely that irradiation leads to enhanced atomic mobilities, promoting the process of phase segregation. Figure 2a shows a high-angle annular dark field (HA ADF) PTEM micrograph of an individual damage zone. The micrograph shows a large contrast variation from the core of the damage zone (labeled as “2”) to the inner and outer periphery or shell (labeled as “1” and “3”, respectively). A clear difference is observed between damage that is created at the sample surface versus deeper in the sample bulk. A bright double-ring-like feature is seen on the right edge of the damage zone. The left side of the damage zone where the ion first penetrates the substrate shows much lower contrast. When the ion first penetrates the substrate, the cross-section for displacement creation due to nuclei–nuclei collisions is small. In fact, in the first few nanometers, the ratio of electronic to nuclear stopping is 10 [25]. As a result, the initial stage of the damaging process is probably dominated by electronic excitation. As the incoming ion loses its energy, the process of nuclei–nuclei scattering becomes the dominant mechanism for displacement creation. We also emphasize that the contrast that is observed in Figure 2a is not due to material crystallization as a SAD of the region (not shown) reveals bright diffuse rings without any evidence of crystallization. The shell of the damage zone, regions 1 and 3, has a brighter contrast, indicating a higher material density than that of both the central damage zone (region 2) and the nonirradiated amorphous matrix.
M. T. Myers et al. / Scripta Materialia 67 (2012) 887–890
Figure 2. (a) HAADF PTEM micrograph of a different region of the specimen shown in Figure 1 showing an individual damage zone. The black dash line represents the length over which a STEM (EDS) line scan was performed. Three black vertical arrows, denoted 1–3, point to three regions of interest (ROIs), the leading edge periphery of the damage zone, the central damage zone, and the trailing edge periphery of the damage zone, respectively. (b) Depth dependence of integrated X-ray intensities (arbitrary units in linear scale) of Ni, Nb, Zr and Ti. The ROIs in each panel of (b), denoted 1–3, correspond with those of the STEM/EDS line scan performed across the line marked by a dash line in (a). The alternating lighter and darker background behind the spectra of the first three panels of (b) coincide with the ROIs described in (a) and are intended to serve as a visual guide.
Figure 2b shows integrated EDS intensities of Ni, Nb, Zr and Ti. Due to a highly metastable state of the MG, it is exceedingly difficult to perform a EDS line scan with a dwell time at each step that is long enough to provide a sufficient number of counts for a quantitative analysis and does not severely alter the structure of the MG. Hence, the results of Figure 2b will only be compared qualitatively. As seen in Figure 2b, a large variation in material composition exists across an individual damage zone, clearly deviating from the uniformity of the asspun MG. Ni, Nb and Zr exhibit a similar trend, whereas Ti has entirely different behavior. The profiles of Nb and Zr display broad maxima in regions 1 and 3. Their minima in region 2 are on the order of the level observed in the as-spun amorphous matrix. Unlike Nb and Zr, the profile of Ni in region 1 is split into two distinct peaks. In region 3, Ni, Nb and Zr all exhibit similar broad maxima. This suggests that radiation-induced
889
Figure 3. (a) BF PTEM micrograph of as-spun Ni52.5Nb10Zr15Ti15Pt7.5 MG after electropolishing. (b) DF PTEM micrograph with the inset SAD pattern of the same region shown in (a). (c) BF PTEM micrograph of Ni52.5Nb10Zr15Ti15Pt7.5 MG irradiated at room temperature with 18 MeV Xe5+ ions at 72° to a fluence of 5 1010 cm 2, prepared by electropolishing.
diffusion and the process of compositional changes differ substantially at/near the sample surface and deeper in the bulk. The profile of Ti exhibits a single broad peak, which is nearly uniform across the entire damage zone. Such a large difference between the behavior of Ti versus the other alloying components could be due to possible differences in their mobilities under irradiation. The thermal spike and accompanying melting process [13] could result in enhanced diffusion of Ti into locations previously occupied by other alloying components before melt solidification. Figure 3a shows a (BF) PTEM micrograph of as-spun Ni52.5Nb10Zr15Ti15Pt7.5 prepared by electropolishing. Alternating bright and dark contrast regions reveal that the material has undergone substantial microstructural changes. These features are very different from typical, featureless, TEM micrographs of an as-spun MG prepared by ion milling [13]. Figure 3b shows a (DF) PTEM micrograph and the corresponding SAD pattern from the region seen in Figure 3a. It is clear that electropolishing has partially crystallized the specimen. The crystallization appears uniform in both size and density of precipitates, and no pores or holes are observed in unirradiated
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M. T. Myers et al. / Scripta Materialia 67 (2012) 887–890
specimens subjected to electropolishing. Figure 3c shows a (BF) PTEM micrograph of Ni52.5Nb10Zr15Ti15Pt7.5 MG after 18 MeV Xe5+ irradiation at 72° to a fluence of 5 1010 cm 2 and electropolishing. Clearly resolved in Figure 3c are 50–500 nm pores to varying degrees of perforation. The pores appear to form in ion damage zones which have undergone substantial species diffusion, as observed in Figure 2b and discussed above. It is known that, in crystalline metallic alloys, the presence of secondary phases often results in preferential etching during electropolishing, usually along their interfaces with the matrix due to their different electrode potentials in the acid solution. Similarly, different electrochemical properties are not unexpected for a MG with different density and composition, even though the ion-induced secondary phase remains amorphous. This produces a porous MG membrane with pore sizes in the range of 50–500 nm after etching in the acid solution. Our finding suggests that high-energy heavy-ion irradiation is a new strategy to produce amorphous nanocomposites. Subsequent etching results in the formation of nanometer-sized pores in MGs, which may find great applications in sensors, detectors or waste management. MGs have superior resistance to corrosion, allowing the structure to be used in harsh environments. Although the pores obtained so far are still relatively large, both pore size and pore density can be further optimized by adjusting the ion-irradiation and etching parameters. In summary, we have studied the response of Ni52.5Nb10Zr15Ti15Pt7.5 metallic glass to 18 MeV Xe5+ ion irradiation. We have found that substantial elemental segregation and diffusion takes place in the direction perpendicular to the ion trajectory. This results in the formation of an amorphous phase with a composition deviating from that of the original as-spun MG. Electropolishing in nitric acid results in the preferential etching of the secondary amorphous phase, leaving behind 50– 500 nm holes in the MG. Finally, we have not observed direct crystallization along an individual ion path, further supporting the theory that irradiation-induced crystallization of MGs is a homogeneous process [19,20]. The study was supported in part by National Science Foundation (USA) through Grant No. 0846835. Work at LLNL was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344. The OSU team acknowledges support from NSF Grant No. CMMI-1130606. M.T.M. would like to acknowledge the LLNL Lawrence Scholar Program for funding.
[1] H. Lin, J. Wu, C. Wang, P. Lee, Mater. Lett. 62 (2008) 2995. [2] A. Inoue, Acta Mater. 48 (2000) 279. [3] D.V. Louzguine-Luzgin, T. Shimada, A. Inoue, Intermetallics 13 (2005) 1166. [4] G.Q. Xie, D.V. Louzguine-Luzgin, H. Kimura, A. Inoue, Appl. Phys. Lett. 90 (2007) 241902. [5] W.H. Jiang, F.X. Liu, D.C. Qiao, H. Choo, P.K. Liaw, J. Mater. Res. 21 (2006) 1570. [6] C.A. Schuh, T.C. Hufnagel, U. Ramamurty, Acta Mater. 55 (2007) 4067. [7] T. Nagase, M. Nakamura, Y. Umakoshi, Intermetallics 15 (2007) 211. [8] J.J. Kim, Y. Choi, S. Suresh, A.S. Argon, Science 295 (2002) 654. [9] M.L. Trudeau, J.Y. Huot, R. Schulz, D. Dussault, A. Van Neste, G. L’Espe´rance, Phys. Rev. B 45 (1992) 4626. [10] H. Chen, Y. He, G.J. Shiflet, Nature 367 (1994) 541. [11] J. Carter, E. Fu, M. Martin, G.Q. Xie, X. Zhang, Y. Wang, D. Wijesundera, X. Wang, W.-K. Chu, S.M. McDeavitt, L. Shao, Nucl. Instr. Methods Phys. Res. B 267 (2009) 2827. [12] M. Iqbal, J.I. Akhter, Z.Q. Hu, H.F. Zhang, A. Qayyum, W.S. Sun, J. Non-Cryst. Solids 353 (2007) 2452. [13] M. Myers, E.G. Fu, M. Myers, H. Wang, G.Q. Xie, X. Wang, W-K Chu, L. Shao, Scripta Mater. 63 (2010) 1045. [14] S. Nagata, S. Higashi, B. Tsuchiya, K. Toh, T. Shikama, K. Takahiro, K. Ozaki, K. Kawatusra, S. Yamamoto, A. Inoue, Nucl. Instrum. Methods B 257 (2007) 420. [15] G. Rizza, A. Dunlop, M. Kopcewicz, Nucl. Instrum. Methods. B 245 (2006) 130. [16] G.Q. Xie, Q. Zhang, D.V. Louzguine-Luzgin, W. Zhang, A. Inoue, Mater. Trans. 47 (2006) 1930. [17] M. Nastasi, J.W. Mayer, J.K. Hirvonen, Ion–Solid Interactions, Cambridge University Press, Cambridge, 1996. [18] A. Inoue, Mater. Trans. 36 (1995) 866. [19] M.H. Cohen, D. Turnbull, J. Chem. Phys. 31 (1959) 1164. [20] A. Van Den Beukel, Acta Metall. Mater. 39 (1991) 2709. [21] P. Sigmund, Phys. Rev. A 56 (1997) 3781. [22] M. Toulemonde, C. Dufour, E. Paumier, Phys. Rev. B 46 (1992) 14362. [23] R.L. Fleischer, P.B. Price, R.M. Walker, E.L. Hubbard, Phys. Rev. 156 (1967) 353. [24] P. Stampfli, K.H. Bennemann, Phys. Rev. B 42 (1990) 7163. [25] J. Ziegler, J. Biersack, U. LittmarkThe Stopping and Range of Ions in Solids, Vol. 1, Pergamon Press, New York, 1985.
Scripta Materialia 67 (2012) 887–890 www.elsevier.com/locate/scriptamat
Phase transition, segregation and nanopore formation in high-energy heavy-ion-irradiated metallic glass M.T. Myers,a,b,⇑ S. Charnvanichborikarn,a C.C. Wei,c Z.P. Luo,c,d G.Q. Xie,e S.O. Kucheyev,a D.A. Luccaf and L. Shaob a
Lawrence Livermore National Laboratory, Livermore, CA 94550, USA Department of Nuclear Engineering, Texas A&M University, College Station, TX 77843, USA c Materials Science and Engineering Program, Texas A&M University, College, Station, TX 77843, USA d Microscopy and Imaging Center, Texas A&M University, College Station, TX 77843, USA e Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan f School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK 74078, USA b
Received 10 July 2012; accepted 12 August 2012 Available online 17 August 2012
We report elemental segregation and the formation of a nanocomposite containing a secondary amorphous phase formed along the path of an incident ion in a metallic glass (MG) irradiated with high-energy heavy ions. Electropolishing with a solution of nitric acid preferentially attacks the damaged regions along ion trajectories leaving behind 50–500 nm diameter pores. No direct crystallization is observed as a result of damage induced by a single ion, further supporting the theory that irradiation-induced crystallization of the MG is a homogeneous process. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ion irradiation; Metallic glass; Nanocrystallization; Nanoporous
Various metallic glasses (MGs) have been shown to exhibit superior resistance to corrosion, making them ideal for harsh environments [1]. One particular MG, Ni52.5Nb10Zr15Ti15Pt7.5, has been shown to exhibit a large supercooled liquid region [2,3] and high stability [4]. On the other hand, a fully amorphous structure is brittle due to the ease of shear band formation and propagation under stress [5]. Enhanced ductility can be achieved by either introducing a secondary phase or nanocrystals, which provide a mechanism to release stress [6] by shear band deflection. When a MG is annealed at temperatures above the glass transition temperature and slowly cooled, partial crystallization can take place [7]. Mechanical deformation has also been used to induce, presumably athermally, small fractions of phase transformations [8–10]. Apart from these highly localized and destructive techniques, ion [11–15] and electron [16] irradiation has been shown to cause
⇑ Corresponding
author at: Lawrence Livermore National Laboratory, Livermore, CA 94550, USA. Tel.: +1 330 984 7996; fax: +1 979 845 6443; e-mail: [email protected]
the formation of nanometer-sized particles inside an amorphous matrix. During ion bombardment, local melting can be induced by the high-energy deposition rate in displacement cascades [17]. Previous results have, however, shown that nanocrystal formation is unlikely to be a consequence of localized heating of an MG irradiated at room temperature with 1 MeV Ni+ ions [13]. Although the cascade temperature probably exceeds the melting point of the MG, the quenching process occurs much faster than the critical cooling rate [2,18] required to form the MG [13]. Crystallization of MGs during irradiation is, therefore, more likely due to other processes such as atomic displacements that cause density fluctuations, leading to enhanced atomic mobilities and structural relaxation [19,20]. In the present study, we use high-energy heavy-ion irradiation to study the radiation response of Ni52.5Nb10Zr15Ti15Pt7.5. Unlike most of the previous studies using ions in the energy range of [1 MeV, energy loss of high-energy heavy ions is initially dominated by electronic stopping [21]. Although nuclei–nuclei scattering is a weak mechanism for displacement creation in this
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.08.015
888
M. T. Myers et al. / Scripta Materialia 67 (2012) 887–890
case, the energy of electronic excitation can be transferred to atomic motion through the following mechanisms: thermal spike [22], Coulomb explosion [23], and material instability due to lattice relaxation [24]. Understanding the effect of radiation damage due to high-energy heavy ions in MGs is of fundamental importance, and may pave the way for structural engineering of MGs with improved mechanical properties that could lead to novel applications. Ribbons of Ni52.5Nb10Zr15Ti15Pt7.5 MG (20 lm thick, 1.5 mm wide) were produced by rapid solidification of a liquid metal solution on a single copper roller at a surface velocity of 42 m s 1 [3]. Irradiation was performed at Lawrence Livermore National Laboratory with a 4 MV ion accelerator (National Electrostatics Corporation, model 4UH). The samples were irradiated at room temperature with 18 MeV Xe5+ ions to fluences of 5 109 and 5 1010 cm 2 resulting in an average lateral distance between ion impacts of 150 and 50 nm, respectively. During irradiation, the sample normal was tilted by 72° relative to the incident beam direction in order to reduce the projected ion range and to address the question of whether a single incident ion can lead to a direct amorphous–crystalline transition. In such a case, we expect to observe crystalline particles with rod-like shapes by plane-view transmission electron microscopy (PTEM). Following irradiation, specimens for PTEM were back-thinned until perforation by either conventional dimple polishing with 1 lm diamond paste or single-jet electropolishing (80/20 mixture of methanol/nitric acid at 40 °C). PTEM specimens were then examined in both a JEOL 2010 and a FEI Tecnai G2 F20 ST microscopes operated at 200 kV to perform PTEM, scanning TEM (STEM) and energy dispersive spectroscopy (EDS) analyses. Figure 1a shows a representative bright-field (BF) PTEM micrograph after 18 MeV Xe5+ irradiation at 72° off normal incidence to a fluence of 5 109 cm 2. This sample was prepared with the conventional dimple polishing technique. The region marked with a white arrow indicates the location of an individual ion incident on the sample surface. It is clear that the damage caused by an impinging ion at a tilted angle results in an elongated layered shell structure. The initial portion of the damage zone, probably the entry point of the ion on the sample surface, has a lighter contrast as opposed to the darker contrast surrounding the periphery of the damage zone. Across a number of regions imaged, such damage zones due to a single ion range from 50 to 100 nm in diameter. Localized crystallization was found near the sample edge, which might be caused by surface effects, i.e. enhanced atomic mobility in an ultra-thin membrane. In order to avoid this artifact of sample preparation, all reported data in the present study have been obtained from a relatively thicker region away from the specimen edge. Figure 1b shows a dark-field (DF) PTEM micrograph and the corresponding selected-area diffraction (SAD) pattern of the same region shown in Figure 1a. As evidenced by the bright diffuse rings in the SAD pattern, the ion impact does not cause crystallization—the
Figure 1. (a) BF PTEM micrograph of Ni52.5Nb10Zr15Ti15Pt7.5 MG irradiated at room temperature with 18 MeV Xe5+ ions incident at 72° off the sample surface normal to a fluence of 5 109 cm 2. (b) DF PTEM micrograph and the inset SAD pattern for the region seen in (a). The sample was prepared by the dimple polishing technique. Incoming ion beam direction is labeled with white arrows in (a) and (b).
sample has retained its amorphous state after irradiation. Therefore, the contrast variation observed in PTEM micrographs in Figure 1a and b could be attributed to a density variation. It is likely that irradiation leads to enhanced atomic mobilities, promoting the process of phase segregation. Figure 2a shows a high-angle annular dark field (HA ADF) PTEM micrograph of an individual damage zone. The micrograph shows a large contrast variation from the core of the damage zone (labeled as “2”) to the inner and outer periphery or shell (labeled as “1” and “3”, respectively). A clear difference is observed between damage that is created at the sample surface versus deeper in the sample bulk. A bright double-ring-like feature is seen on the right edge of the damage zone. The left side of the damage zone where the ion first penetrates the substrate shows much lower contrast. When the ion first penetrates the substrate, the cross-section for displacement creation due to nuclei–nuclei collisions is small. In fact, in the first few nanometers, the ratio of electronic to nuclear stopping is 10 [25]. As a result, the initial stage of the damaging process is probably dominated by electronic excitation. As the incoming ion loses its energy, the process of nuclei–nuclei scattering becomes the dominant mechanism for displacement creation. We also emphasize that the contrast that is observed in Figure 2a is not due to material crystallization as a SAD of the region (not shown) reveals bright diffuse rings without any evidence of crystallization. The shell of the damage zone, regions 1 and 3, has a brighter contrast, indicating a higher material density than that of both the central damage zone (region 2) and the nonirradiated amorphous matrix.
M. T. Myers et al. / Scripta Materialia 67 (2012) 887–890
Figure 2. (a) HAADF PTEM micrograph of a different region of the specimen shown in Figure 1 showing an individual damage zone. The black dash line represents the length over which a STEM (EDS) line scan was performed. Three black vertical arrows, denoted 1–3, point to three regions of interest (ROIs), the leading edge periphery of the damage zone, the central damage zone, and the trailing edge periphery of the damage zone, respectively. (b) Depth dependence of integrated X-ray intensities (arbitrary units in linear scale) of Ni, Nb, Zr and Ti. The ROIs in each panel of (b), denoted 1–3, correspond with those of the STEM/EDS line scan performed across the line marked by a dash line in (a). The alternating lighter and darker background behind the spectra of the first three panels of (b) coincide with the ROIs described in (a) and are intended to serve as a visual guide.
Figure 2b shows integrated EDS intensities of Ni, Nb, Zr and Ti. Due to a highly metastable state of the MG, it is exceedingly difficult to perform a EDS line scan with a dwell time at each step that is long enough to provide a sufficient number of counts for a quantitative analysis and does not severely alter the structure of the MG. Hence, the results of Figure 2b will only be compared qualitatively. As seen in Figure 2b, a large variation in material composition exists across an individual damage zone, clearly deviating from the uniformity of the asspun MG. Ni, Nb and Zr exhibit a similar trend, whereas Ti has entirely different behavior. The profiles of Nb and Zr display broad maxima in regions 1 and 3. Their minima in region 2 are on the order of the level observed in the as-spun amorphous matrix. Unlike Nb and Zr, the profile of Ni in region 1 is split into two distinct peaks. In region 3, Ni, Nb and Zr all exhibit similar broad maxima. This suggests that radiation-induced
889
Figure 3. (a) BF PTEM micrograph of as-spun Ni52.5Nb10Zr15Ti15Pt7.5 MG after electropolishing. (b) DF PTEM micrograph with the inset SAD pattern of the same region shown in (a). (c) BF PTEM micrograph of Ni52.5Nb10Zr15Ti15Pt7.5 MG irradiated at room temperature with 18 MeV Xe5+ ions at 72° to a fluence of 5 1010 cm 2, prepared by electropolishing.
diffusion and the process of compositional changes differ substantially at/near the sample surface and deeper in the bulk. The profile of Ti exhibits a single broad peak, which is nearly uniform across the entire damage zone. Such a large difference between the behavior of Ti versus the other alloying components could be due to possible differences in their mobilities under irradiation. The thermal spike and accompanying melting process [13] could result in enhanced diffusion of Ti into locations previously occupied by other alloying components before melt solidification. Figure 3a shows a (BF) PTEM micrograph of as-spun Ni52.5Nb10Zr15Ti15Pt7.5 prepared by electropolishing. Alternating bright and dark contrast regions reveal that the material has undergone substantial microstructural changes. These features are very different from typical, featureless, TEM micrographs of an as-spun MG prepared by ion milling [13]. Figure 3b shows a (DF) PTEM micrograph and the corresponding SAD pattern from the region seen in Figure 3a. It is clear that electropolishing has partially crystallized the specimen. The crystallization appears uniform in both size and density of precipitates, and no pores or holes are observed in unirradiated
890
M. T. Myers et al. / Scripta Materialia 67 (2012) 887–890
specimens subjected to electropolishing. Figure 3c shows a (BF) PTEM micrograph of Ni52.5Nb10Zr15Ti15Pt7.5 MG after 18 MeV Xe5+ irradiation at 72° to a fluence of 5 1010 cm 2 and electropolishing. Clearly resolved in Figure 3c are 50–500 nm pores to varying degrees of perforation. The pores appear to form in ion damage zones which have undergone substantial species diffusion, as observed in Figure 2b and discussed above. It is known that, in crystalline metallic alloys, the presence of secondary phases often results in preferential etching during electropolishing, usually along their interfaces with the matrix due to their different electrode potentials in the acid solution. Similarly, different electrochemical properties are not unexpected for a MG with different density and composition, even though the ion-induced secondary phase remains amorphous. This produces a porous MG membrane with pore sizes in the range of 50–500 nm after etching in the acid solution. Our finding suggests that high-energy heavy-ion irradiation is a new strategy to produce amorphous nanocomposites. Subsequent etching results in the formation of nanometer-sized pores in MGs, which may find great applications in sensors, detectors or waste management. MGs have superior resistance to corrosion, allowing the structure to be used in harsh environments. Although the pores obtained so far are still relatively large, both pore size and pore density can be further optimized by adjusting the ion-irradiation and etching parameters. In summary, we have studied the response of Ni52.5Nb10Zr15Ti15Pt7.5 metallic glass to 18 MeV Xe5+ ion irradiation. We have found that substantial elemental segregation and diffusion takes place in the direction perpendicular to the ion trajectory. This results in the formation of an amorphous phase with a composition deviating from that of the original as-spun MG. Electropolishing in nitric acid results in the preferential etching of the secondary amorphous phase, leaving behind 50– 500 nm holes in the MG. Finally, we have not observed direct crystallization along an individual ion path, further supporting the theory that irradiation-induced crystallization of MGs is a homogeneous process [19,20]. The study was supported in part by National Science Foundation (USA) through Grant No. 0846835. Work at LLNL was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344. The OSU team acknowledges support from NSF Grant No. CMMI-1130606. M.T.M. would like to acknowledge the LLNL Lawrence Scholar Program for funding.
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