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Metastable phases found in the Ni-Nb-Zr system
Materials Characterization 127 (2017) 60–63
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Metastable phases found in the Ni-Nb-Zr system Leonardo Pratavieira Deo a,⁎, Marcelo Falcão de Oliveira b a b
Materials Engineering, Engineering Department, Federal University of Lavras, Lavras, MG 37200-000, Brazil Department of Materials Engineering, São Carlos School of Engineering, University of São Paulo, São Carlos, SP 13563-120, Brazil
a r t i c l e
i n f o
Article history: Received 15 December 2016 Received in revised form 1 March 2017 Accepted 1 March 2017 Available online 03 March 2017 Keywords: Ni-Nb-Zr system Rapidly-quenched alloy Super-saturated solid solutions Metastable phases
a b s t r a c t The development of metastable phases in the microstructures from rapid solidification techniques could significantly improve the properties of metallic alloys, thus it is extremely important the characterization of these unusual structures. In this research, the microstructure of a rapidly-quenched alloy, namely Ni61.6Nb33.1Zr5.3 (at.%) was investigated in greater detail in order to determine the structures and compositions of their crystalline phases. These crystalline phases were characterized using a combination of the following techniques: scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy and x-ray diffraction. The phases were compared to the crystalline structures reported in the literature. Our results indicate a high solubility of niobium into Ni10Zr7 and Ni21Zr8 crystalline structures and consequently suggest the formation of two metastable phases in the Ni-Nb-Zr system. © 2017 Elsevier Inc. All rights reserved.
1. Introduction Traditional alloy development strategies typically involve the study of a known chemical composition cooled from the liquid in equilibrium conditions to form suitable microstructures predicted by the phase diagrams with specific properties [1]. The development of unique microstructures has been a central aim of the rapid solidification techniques for improving the properties of metals and alloys [2]. These microstructures have a large amount of metastable alloy phases, i.e. those which do not exist on equilibrium conditions or in the phase diagrams [3]. The properties of alloys containing metastable phases are related to the crystal structure and to the morphologies and distributions of these phases in the microstructures [4]. As a result, it is extremely important that these unusual structures be characterized as thoroughly as possible, as well as, the experimental conditions required to produce such microstructures [5]; however sometimes it is difficult to ascertain such experimental conditions precisely. Few studies have been detailed the characterization methods of the crystal structures and compositions of metastable alloy phases [6,7]. Consequently, there has been a considerable delay in the progress of this potentially exciting area. In this context, the present work presents the characterization of the post quenching crystalline microstructures that develop at intermediate cooling rates of one alloy, namely Ni61.6Nb33.1Zr5.3 (at.%), taken from our previous work [10]. This alloy presented both glassy and crystalline phases and the presence of these two different phase natures is attributed to the rapid solidification technique used for its production. In the
previous work, we focused only on the glassy nature of this alloy; however the crystalline phases which compete against the glass formation were not properly investigated. Thus, the present work aims to investigate in greater detail, the structures and compositions of crystalline phases produced through a non-equilibrium condition. The Ni-Nb-Zr system is considered a promising candidate for glassy alloys production over a broad composition range [9]; however only a limited amount of research has been developed on this ternary system about the crystalline solidification behavior. The most recent isothermal sections of this ternary system at 500 °C and 800 °C have also been reported by Tokunaga et al. over the entire composition range [8]; however, no data on the ternary compounds have been reported. Using thermodynamic parameters, Tokunaga et al. also calculated the liquidus projection diagram of the Nb-Ni-Zr system and indicated that there are 10 eutectic and peritectic invariant reactions [8]. According to the liquidus projection diagram calculated by Tokunaga et al., the expected equilibrium solidification products for the analyzed alloy are NbNi3 and Nb7Ni6, both with some minor zirconium solubility [8]; nevertheless our results indicate a different crystallization behavior with non-equilibrium crystalline structures. These structures are the same of that found in the equilibrium Ni10Zr7 and Ni21Zr8 phases, in addition with a large solubility of niobium in both structures. In the present investigation, Tokunaga et al. previous work [8] was used as a guide in order to obtain some understanding about the non-equilibrium behavior of the analyzed alloy. 2. Experimental
⁎ Corresponding author. E-mail addresses: [email protected]fla.br (L.P. Deo), [email protected] (M.F. de Oliveira).
http://dx.doi.org/10.1016/j.matchar.2017.03.001 1044-5803/© 2017 Elsevier Inc. All rights reserved.
A non-consumable arc melting was used to mix pure elemental pieces of the Ni (99.998%), Nb (99.8%) and Zr (99.98%). The samples
L.P. Deo, M.F. de OliveiraMaterials Characterization 127 (2017) 60–63
were produced by injection casting into a wedge shaped copper mold according to a procedure described in our previous work [10]. Only the thickest part of the wedge shaped sample was analyzed because that region had the lowest cooling rate during the solidification process and consequently the highest possibility of obtaining crystalline phases close to the equilibrium. The maximum thickness of the analyzed region was around 3 mm where the cooling rate was estimated to be ~250 K/s [11]. Initially the sample surface was sanded with 600, 800, and 1200 grit SiC papers followed by polishing with 15 μm, 5 μm, 1 μm, 0.3 μm and 0.05 μm alumina applied onto Leco Imperial cloths. In each step, a new cloth was used to prevent cross-contamination of the alumina particles. An aluminum stub with conductive carbon tape was used to mount the polished sample for Scanning Electron Microscopy (SEM) analysis. SEM was performed using an FEI Quanta 600i scanning electron microscope with tungsten source with 30 kV acceleration voltage coupled with an energy-dispersive X-ray spectroscopy (EDS) detector. Microstructural characterization and microchemistry analysis were made through the use of backscattered electron imaging (BSE) and EDS, respectively. Through the use of a FEI Helios 600i Focused Ion Beam (FIB)/Scanning Electron Microscope with the “lift-out” technique, a specimen for transmission electron microscopy (TEM) analysis was prepared. A brief summary of this technique is described by Giannuzzi et al. [12]. The FIB sectioning procedure involved first depositing a thin platinum coating on the region of interest. The estimated dimensions of the platinum deposits were around 5 μm wide by 20 μm long by 1 μm thick. A focused Ga+ ion beam was used to mill out a foil from the desired region of the bulk sample. The estimated foil dimensions were the same of the platinum coating. In order to extract the TEM specimen from the bulk material and position it on a standard copper half grid, a hydraulic nanomanipulator arm was used. The beam current was reduced and the milling was performed on alternate sides of the specimen in order to reduce its thickness. The milling was continued until the foil was thinned to ~100 nm or less. A Philips CM12 TEM with a tungsten source and an acceleration voltage of 120 kV coupled with an EDS detector was used to perform the TEM analysis. Selected area electron diffraction (SAED), bright field (BF) images and EDS analysis were used to investigate the microstructural characteristics of the analyzed alloy. From the unit cell parameters found in the literature, the d-spacings and interplanar angles were calculated for all expected stable phases established in the phase diagrams. When d-spacing and interplanar angles measured experimentally matched the calculations, the selected area microdiffraction patterns were simulated using JEMS Diffraction Simulation Software along various zone axes for each phase and compared with the experimental patterns. Polycrystalline bulk sample was ground with a steel mortar and pestle. The X-ray diffraction (XRD) analysis of powder samples were performed on a Phillips X'pert diffractometer operating at 45 kV and 40 mA using Cu Kα (1.5418 Å) radiation with a step time of 1 s and a step size of 0.02°. The 2-theta scans were run from 20 to 70°. The JCPDS database files were used to analyze and compare with diffraction peaks. XRD was used to confirm the phases previously identified by SAED in the TEM. 3. Results and Discussion A backscattered electron micrograph from the upper sections (~ 3 mm thickness) of the wedge castings of the analyzed alloy is shown in Fig. 1. The SEM-BSE image in Fig. 1 was the first attempt in order to characterize the crystalline phases present in the analyzed alloy; however due the little atomic number (Z) contrast in the micrograph, it is not possible to distinguish the boundaries between the phases in the SEM. The overall compositions (at.%) in the backscattered electron micrograph
61
Fig. 1. SEM-BSE image of analyzed alloy.
determined by SEM-EDS were reveled to be 62.4 ± 1.0 (Ni), 31.3 ± 1.3 (Nb) and 6.3 ± 1.1 (Zr). Because the identification of different phase morphologies in the analyzed alloy from SEM-BSE image was not possible due the absence of Z contrast, the BF imaging and diffraction in the TEM was needed in order to characterize the phases and their morphologies. Fig. 2 provides the BF image and some selected area microdiffraction patterns collected in the TEM as recorded for the different phases in the analyzed alloy. Only half of the microdiffraction patterns are shown along with the JEMS simulations (left side) and it can be seen that the simulation patterns are consistent with the experimental patterns. The variations in electron diffraction shown in Fig. 2 help to clearly distinguish the presence of two different phases in this alloy with the same crystalline structures as Ni10Zr7 and Ni21Zr8 found in the Ni-Zr equilibrium phase diagram [8]. The minority phase in Fig. 2 was identified with Ni21Zr8 crystalline structure and the majority phase was recognized with the Ni10Zr7 crystalline structure. The patterns in Fig. 2 were collected on multiple grains by tilting the samples to angles corresponding to the desired zone axes, following Kikuchi maps. These patterns matched very well with the mentioned crystalline structures from the Ni-Zr binary system, although the lattice parameters are significantly shorter. The crystallographic information about these phases was found in the literature and is presented as follows: patterns from analyzed alloy are shown in Fig. 2 and were indexed with respect to the Ni10Zr7 and Ni21Zr8 structures along the [010] and ½ 101 zone axes, respectively. The first structure, Ni10Zr7 has an orthorhombic unit cell with a = 9.216 Å, b = 9.154 Å, c = 12.385 Å and space group Aba2 [13]. The second structure, Ni21Zr8 has a triclinic unit cell with a = 6.476 Å, b = 8.064 Å, c = 8.594 Å, α = γ = 75.2° and β = 68.1 and space group P-1 [14]. In addition, although the identification of different phase morphologies from SEM-BSE images was difficult, as mentioned above, BF imaging and diffraction in the TEM could be used to identify the presence of these two distinct phases. The compositions of these phases that were measured using EDS in the TEM are presented in Table 1. Electron diffraction patterns were obtained along various zone axes for each phase. Some changes in the lattice parameters were observed in both phases when compared to literature data and this comparison is shown in Table 2. The X-ray diffraction pattern from the upper ground sections (~3 mm thickness) of the analyzed alloy is also shown in Fig. 3. The XRD diffraction pattern showed in Fig. 3 can be indexed as a mixture of the Ni10Zr7 (orthorhombic - JCPDS file number 00-047-
62
L.P. Deo, M.F. de OliveiraMaterials Characterization 127 (2017) 60–63
Fig. 2. (a) TEM BF image; (b) [010] selected area microdiffraction pattern from Nb10Ni7; (c) ½101 microdiffraction pattern from Ni21Zr8. Both patterns are together with the JEMS simulations for comparison.
1027) and Ni21Zr8 (triclinic - JCPDS file number 04-002-9932) crystalline structures, respectively. Thus, the XRD confirmed the identities of the crystalline structures previously identified in the TEM. The combination of TEM-EDS data and the changes of the lattice parameters determined from the selection area microdiffraction patterns, as shown in Tables 1 and 2 respectively, suggest the formation of two metastable super-saturated solid solutions in the crystalline phases Ni10Zr7 and Ni21Zr8. In other words, there is the formation of two metastable phases and their chemical compositions are about Ni55Nb40Zr5 and Ni70Nb25Zr5 (at.%) respectively. The experimental formation of these metastable phases is attributed to the rapid solidification process. Additional experimental evidence for this solid solution formation is provided by the peak shift in the XRD pattern for these phases in the analyzed alloy when compared to the JCPDS files. As shown in Table 2, the metastable phases have smaller unit cell volumes and lattice parameters calculated from the microdiffraction patterns compared to the literature data found for the binary Ni10Zr7 and Ni21Zr8 phases. Some decrease in unit cell volumes and lattice parameters is expected because the niobium atom has a smaller radius than the zirconium atom. According to Tokunaga et al., the solubility of niobium in Ni10Zr7 may reach about 20–25 at.% [8]. However, our results from the TEM-EDS microanalysis suggest a higher solubility (around 40 at.%) when the calculation is done assuming no changes in atomic radius of the elements. Although, the solubility of niobium in Ni21Zr8 has not been reported in the literature, our results indicate that this value is around 25 at.%. The poor contrast in the SEM-BSE image is an evidence of average atomic
Table 1 Phase compositions (at.%) of analyzed alloy determined by TEM-EDS. Phase
Ni
Nb
Zr
Gray dendrites, with Ni21Zr8 structure Gray matrix, with Ni10Zr7 structure
69.7 ± 0.7 53.9 ± 0.4
25.0 ± 1.5 41.0 ± 0.6
5.4 ± 3.5 5.1 ± 2.1
number similarities between the characterized metastable phases. This absence of Z contrast is attributed to the large niobium solubility in the Ni-Zr crystalline structures as evidenced by the combination of TEM-EDS data, changes in the lattice parameters and the peak shift in the XRD pattern as mentioned previously. Despite the higher amount of niobium than zirconium in the analyzed alloy, the crystallization of Ni10Zr7 and Ni21Zr8 is facilitated by the larger negative value of mixing enthalpy between Ni-Zr and Ni-Nb atomic pairs than it is between Nb-Zr atomic pair. The mixing enthalpies for these atomic pairs are −49 (Ni-Zr), −30 (Ni-Nb) and 4 (Nb-Zr) kJ/ mol [15]. According to the Hume-Rothery rules, several factors affect the formation of the solid solution [16]. The first is the size effects of component atoms. For alloys whose component atomic-size differences are less than 15%, it is most probable to form a substitutional solid solution. The second consideration is the chemical compatibility between components, such as the enthalpy of mixing. If the enthalpy of mixing between elements has positive or small negative values, it is also more probable the formation of a solid solution. The empirical radii of zirconium and niobium atoms are 155 and 145 pm respectively leading to an atomic size ratio of 1.07 (Zr/Nb) [17]. In addition, the mixing enthalpy for the Nb–Zr atomic pair has a positive value. Thus, the substitutional solid solution of niobium in Ni10Zr7 and Ni21Zr8 phases is favorable because the component atoms can easily substitute each other and have a similar probability to occupy the lattice sites. Part of the calculated Ni-Zr binary phase diagram according to Tokunaga et al. [8] is reproduced in Fig. 4 and includes a dashed vertical line to denote a binary alloy composition with the same amount of the majority element present as in the investigated ternary alloy. For this specific alloy composition, with around 62 at.% of nickel, the phase diagram suggests that the solidification should start with Ni10Zr7, as the primary phase, followed by an eutectic reaction producing Ni10Zr7 and Ni21Zr8. This agrees with the experimental results, once the metastable phases with crystal structures of Ni10Zr7 and Ni21Zr8 were successfully
Table 2 Measured vs. Literature values of the lattice parameters and cell volumes of the phases identified in this study. Phase
Ni21Zr8 Ni10Zr7
Measured lattice parameters (Å) a
b
c
6.0 9.0
7.5 8.9
8.0 11.9
Measured cell volume (Å3)
390.2 953.2
Reported lattice parameters (Å) [13,14] a
b
c
6.476 9.216
8.064 9.154
8.594 12.385
Reported cell volume (Å3)
396.128 1044.839
L.P. Deo, M.F. de OliveiraMaterials Characterization 127 (2017) 60–63
63
Acknowledgements The authors would like to thank FAPESP (2011/13221-0, 2013/ 18950-5) for financial research support, as well as, to Professor Michael J. Kaufman, Department of Metallurgical and Materials Engineering, Colorado School of Mines, USA, for his valuable support and helpful discussions in TEM analyses. References
Fig. 3. XRD patterns taken from the powder sample of analyzed alloy with indexed crystalline phases.
indexed in the analyzed alloy; however a large solubility of niobium into both phases were also detected as described previously. 4. Conclusion The indexed crystalline structures of the analyzed alloy Ni 61.6 Nb33.1Zr5.3 (at.%), are in good agreement with the calculated binary Ni-Zr phase diagram. From our EDS microanalyses, niobium has a solubility of around 40 at.% in the Ni 10 Zr 7 crystalline structure and according to literature, this solubility may reach only about 20–25 at.%. Still from our analyses, niobium also has a solubility of around 25 at.% in the Ni 21 Zr 8 crystalline structure; however there is no data in literature about this behavior. From the niobium solubility described above, our results also suggest the formation of two ternary metastable phases in the Ni-Nb-Zr system with similar crystalline structures as well as in the Ni10Zr7 and Ni21Zr8 phases. In addition, the high solubility of niobium into the Ni10Zr7 and Ni21Zr8 crystalline structures or the formation of two ternary metastable phases is attributed to the rapid solidification process.
Fig. 4. Part of the calculated binary phase diagram of the Ni-Zr system [8]. The alloy composition with the same amount of the majority element present in the experimental ternary alloy is represented by the dashed line.
[1] T.M. Butler, M.L. Weaver, investigation of the phase stabilities in AlNiCoCrFe high entropy alloys, J. Alloys Compd. (691) (2017) 119–129. [2] M.J. Kaufman, H.L. Fraser, Characterization of metastable crystalline phases in the Al-Ge alloy system, Acta Mater. 33 (2) (1985) 191–203. [3] Y.A. Chang, S.L. Chen, F. Zhang, X.Y. Yan, F.Y. Xie, R. Schmid-Fetzer, W.A. Oates, Phase diagram calculation: past, present and future, Prog. Mater. Sci. 49 (3–4) (2004) 313–345. [4] L.Q. Chen, Phase-field models for microstructure evolution, Annu. Rev. Mater. Res. (32) (2002) 113–140. [5] W.J. Boettinger, S.R. Coriell, A.L. Greer, A. Karma, W. Kurz, M. Rappaz, R. Trivedi, Solidification microstructures: recent developments, future directions, Acta Mater. 48 (1) (2000) 43–70. [6] J. Grobner, S.M. Zhu, J.F. Nie, M.A. Gibson, R. Schmid-Fetzer, Metastable phase formation in ternary Mg-Gd-Zn alloys, J. Alloys Compd. (675) (2016) 149–157. [7] X.H. Wang, C.Y. Chen, T.T. Guo, J.T. Zou, X.H. Yang, Microstructure and properties of ternary Cu-Ti-Sn alloy, J. Mater. Eng. Perform. 24 (7) (2015) 2738–2743. [8] T. Tokunaga, S. Matstnnoto, H. Ohtani, M. Hasebe, Thermodynamic analysis of the phase equilibria in the Nb-Ni-Zr system, Mater. Trans. 48 (9) (2007) 2263–2271. [9] Z.W. Zhu, H.F. Zhang, B.Z. Ding, Z.Q. Hu, Synthesis and properties of bulk metallic glasses in the ternary Ni-Nb-Zr alloy system, Mat. Sci. Eng. A Struct. 492 (1–2) (2008) 221–229. [10] L.P. Deo, M.F. de Oliveira, Accuracy of a selection criterion for glass forming ability in the Ni-Nb-Zr system, J. Alloys Compd. 615 (2014) S23–S28. [11] T. Mei-Bo, Z. De-Qian, P. Ming-Xiang, W. Wei-Hua, Binary Cu–Zr bulk metallic glasses, Chin. Phys. Lett. 21 (5) (2004) 901. [12] L.A. Giannuzzi, J.L. Drown, S.R. Brown, R.B. Irwin, F.A. Stevie, Focused ion beam milling and micromanipulation lift-out for site specific cross-section TEM specimen preparation, Symp. Specimen Preparation Transm. Electron Microsc. Mater. (4) (1997) 19–27. [13] J.L. Glimois, P. Forey, J. Feron, C. Becle, Structural investigations of the pseudo-binary compounds Ni10-XCuXZr7, J Less-Common Met. 78 (1) (1981) 45–50. [14] J.M. Joubert, R. Cerny, K. Yvon, M. Latroche, A. Percheron-Guegan, Refinement of the crystal structure of zirconium nickel, Zr8Ni21, Z Krist-New Cryst. St. 213 (1) (1998) 227–228. [15] S. Yamaura, M. Sakurai, M. Hasegawa, K. Wakoh, Y. Shimpo, M. Nishida, H. Kimura, E. Matsubara, A. Inoue, Hydrogen permeation and structural features of melt-spun Ni-Nb-Zr amorphous alloys, Acta Mater. 53 (13) (2005) 3703–3711. [16] Y. Zhang, Y.J. Zhou, J.P. Lin, G.L. Chen, P.K. Liaw, Solid-solution phase formation rules for multi-component alloys, Adv. Eng. Mater. 10 (6) (2008) 534–538. [17] J.C. Slater, Atomic radii in crystals, J. Chem. Phys. 41 (10) (1964) 3199.
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Metastable phases found in the Ni-Nb-Zr system Leonardo Pratavieira Deo a,⁎, Marcelo Falcão de Oliveira b a b
Materials Engineering, Engineering Department, Federal University of Lavras, Lavras, MG 37200-000, Brazil Department of Materials Engineering, São Carlos School of Engineering, University of São Paulo, São Carlos, SP 13563-120, Brazil
a r t i c l e
i n f o
Article history: Received 15 December 2016 Received in revised form 1 March 2017 Accepted 1 March 2017 Available online 03 March 2017 Keywords: Ni-Nb-Zr system Rapidly-quenched alloy Super-saturated solid solutions Metastable phases
a b s t r a c t The development of metastable phases in the microstructures from rapid solidification techniques could significantly improve the properties of metallic alloys, thus it is extremely important the characterization of these unusual structures. In this research, the microstructure of a rapidly-quenched alloy, namely Ni61.6Nb33.1Zr5.3 (at.%) was investigated in greater detail in order to determine the structures and compositions of their crystalline phases. These crystalline phases were characterized using a combination of the following techniques: scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy and x-ray diffraction. The phases were compared to the crystalline structures reported in the literature. Our results indicate a high solubility of niobium into Ni10Zr7 and Ni21Zr8 crystalline structures and consequently suggest the formation of two metastable phases in the Ni-Nb-Zr system. © 2017 Elsevier Inc. All rights reserved.
1. Introduction Traditional alloy development strategies typically involve the study of a known chemical composition cooled from the liquid in equilibrium conditions to form suitable microstructures predicted by the phase diagrams with specific properties [1]. The development of unique microstructures has been a central aim of the rapid solidification techniques for improving the properties of metals and alloys [2]. These microstructures have a large amount of metastable alloy phases, i.e. those which do not exist on equilibrium conditions or in the phase diagrams [3]. The properties of alloys containing metastable phases are related to the crystal structure and to the morphologies and distributions of these phases in the microstructures [4]. As a result, it is extremely important that these unusual structures be characterized as thoroughly as possible, as well as, the experimental conditions required to produce such microstructures [5]; however sometimes it is difficult to ascertain such experimental conditions precisely. Few studies have been detailed the characterization methods of the crystal structures and compositions of metastable alloy phases [6,7]. Consequently, there has been a considerable delay in the progress of this potentially exciting area. In this context, the present work presents the characterization of the post quenching crystalline microstructures that develop at intermediate cooling rates of one alloy, namely Ni61.6Nb33.1Zr5.3 (at.%), taken from our previous work [10]. This alloy presented both glassy and crystalline phases and the presence of these two different phase natures is attributed to the rapid solidification technique used for its production. In the
previous work, we focused only on the glassy nature of this alloy; however the crystalline phases which compete against the glass formation were not properly investigated. Thus, the present work aims to investigate in greater detail, the structures and compositions of crystalline phases produced through a non-equilibrium condition. The Ni-Nb-Zr system is considered a promising candidate for glassy alloys production over a broad composition range [9]; however only a limited amount of research has been developed on this ternary system about the crystalline solidification behavior. The most recent isothermal sections of this ternary system at 500 °C and 800 °C have also been reported by Tokunaga et al. over the entire composition range [8]; however, no data on the ternary compounds have been reported. Using thermodynamic parameters, Tokunaga et al. also calculated the liquidus projection diagram of the Nb-Ni-Zr system and indicated that there are 10 eutectic and peritectic invariant reactions [8]. According to the liquidus projection diagram calculated by Tokunaga et al., the expected equilibrium solidification products for the analyzed alloy are NbNi3 and Nb7Ni6, both with some minor zirconium solubility [8]; nevertheless our results indicate a different crystallization behavior with non-equilibrium crystalline structures. These structures are the same of that found in the equilibrium Ni10Zr7 and Ni21Zr8 phases, in addition with a large solubility of niobium in both structures. In the present investigation, Tokunaga et al. previous work [8] was used as a guide in order to obtain some understanding about the non-equilibrium behavior of the analyzed alloy. 2. Experimental
⁎ Corresponding author. E-mail addresses: [email protected]fla.br (L.P. Deo), [email protected] (M.F. de Oliveira).
http://dx.doi.org/10.1016/j.matchar.2017.03.001 1044-5803/© 2017 Elsevier Inc. All rights reserved.
A non-consumable arc melting was used to mix pure elemental pieces of the Ni (99.998%), Nb (99.8%) and Zr (99.98%). The samples
L.P. Deo, M.F. de OliveiraMaterials Characterization 127 (2017) 60–63
were produced by injection casting into a wedge shaped copper mold according to a procedure described in our previous work [10]. Only the thickest part of the wedge shaped sample was analyzed because that region had the lowest cooling rate during the solidification process and consequently the highest possibility of obtaining crystalline phases close to the equilibrium. The maximum thickness of the analyzed region was around 3 mm where the cooling rate was estimated to be ~250 K/s [11]. Initially the sample surface was sanded with 600, 800, and 1200 grit SiC papers followed by polishing with 15 μm, 5 μm, 1 μm, 0.3 μm and 0.05 μm alumina applied onto Leco Imperial cloths. In each step, a new cloth was used to prevent cross-contamination of the alumina particles. An aluminum stub with conductive carbon tape was used to mount the polished sample for Scanning Electron Microscopy (SEM) analysis. SEM was performed using an FEI Quanta 600i scanning electron microscope with tungsten source with 30 kV acceleration voltage coupled with an energy-dispersive X-ray spectroscopy (EDS) detector. Microstructural characterization and microchemistry analysis were made through the use of backscattered electron imaging (BSE) and EDS, respectively. Through the use of a FEI Helios 600i Focused Ion Beam (FIB)/Scanning Electron Microscope with the “lift-out” technique, a specimen for transmission electron microscopy (TEM) analysis was prepared. A brief summary of this technique is described by Giannuzzi et al. [12]. The FIB sectioning procedure involved first depositing a thin platinum coating on the region of interest. The estimated dimensions of the platinum deposits were around 5 μm wide by 20 μm long by 1 μm thick. A focused Ga+ ion beam was used to mill out a foil from the desired region of the bulk sample. The estimated foil dimensions were the same of the platinum coating. In order to extract the TEM specimen from the bulk material and position it on a standard copper half grid, a hydraulic nanomanipulator arm was used. The beam current was reduced and the milling was performed on alternate sides of the specimen in order to reduce its thickness. The milling was continued until the foil was thinned to ~100 nm or less. A Philips CM12 TEM with a tungsten source and an acceleration voltage of 120 kV coupled with an EDS detector was used to perform the TEM analysis. Selected area electron diffraction (SAED), bright field (BF) images and EDS analysis were used to investigate the microstructural characteristics of the analyzed alloy. From the unit cell parameters found in the literature, the d-spacings and interplanar angles were calculated for all expected stable phases established in the phase diagrams. When d-spacing and interplanar angles measured experimentally matched the calculations, the selected area microdiffraction patterns were simulated using JEMS Diffraction Simulation Software along various zone axes for each phase and compared with the experimental patterns. Polycrystalline bulk sample was ground with a steel mortar and pestle. The X-ray diffraction (XRD) analysis of powder samples were performed on a Phillips X'pert diffractometer operating at 45 kV and 40 mA using Cu Kα (1.5418 Å) radiation with a step time of 1 s and a step size of 0.02°. The 2-theta scans were run from 20 to 70°. The JCPDS database files were used to analyze and compare with diffraction peaks. XRD was used to confirm the phases previously identified by SAED in the TEM. 3. Results and Discussion A backscattered electron micrograph from the upper sections (~ 3 mm thickness) of the wedge castings of the analyzed alloy is shown in Fig. 1. The SEM-BSE image in Fig. 1 was the first attempt in order to characterize the crystalline phases present in the analyzed alloy; however due the little atomic number (Z) contrast in the micrograph, it is not possible to distinguish the boundaries between the phases in the SEM. The overall compositions (at.%) in the backscattered electron micrograph
61
Fig. 1. SEM-BSE image of analyzed alloy.
determined by SEM-EDS were reveled to be 62.4 ± 1.0 (Ni), 31.3 ± 1.3 (Nb) and 6.3 ± 1.1 (Zr). Because the identification of different phase morphologies in the analyzed alloy from SEM-BSE image was not possible due the absence of Z contrast, the BF imaging and diffraction in the TEM was needed in order to characterize the phases and their morphologies. Fig. 2 provides the BF image and some selected area microdiffraction patterns collected in the TEM as recorded for the different phases in the analyzed alloy. Only half of the microdiffraction patterns are shown along with the JEMS simulations (left side) and it can be seen that the simulation patterns are consistent with the experimental patterns. The variations in electron diffraction shown in Fig. 2 help to clearly distinguish the presence of two different phases in this alloy with the same crystalline structures as Ni10Zr7 and Ni21Zr8 found in the Ni-Zr equilibrium phase diagram [8]. The minority phase in Fig. 2 was identified with Ni21Zr8 crystalline structure and the majority phase was recognized with the Ni10Zr7 crystalline structure. The patterns in Fig. 2 were collected on multiple grains by tilting the samples to angles corresponding to the desired zone axes, following Kikuchi maps. These patterns matched very well with the mentioned crystalline structures from the Ni-Zr binary system, although the lattice parameters are significantly shorter. The crystallographic information about these phases was found in the literature and is presented as follows: patterns from analyzed alloy are shown in Fig. 2 and were indexed with respect to the Ni10Zr7 and Ni21Zr8 structures along the [010] and ½ 101 zone axes, respectively. The first structure, Ni10Zr7 has an orthorhombic unit cell with a = 9.216 Å, b = 9.154 Å, c = 12.385 Å and space group Aba2 [13]. The second structure, Ni21Zr8 has a triclinic unit cell with a = 6.476 Å, b = 8.064 Å, c = 8.594 Å, α = γ = 75.2° and β = 68.1 and space group P-1 [14]. In addition, although the identification of different phase morphologies from SEM-BSE images was difficult, as mentioned above, BF imaging and diffraction in the TEM could be used to identify the presence of these two distinct phases. The compositions of these phases that were measured using EDS in the TEM are presented in Table 1. Electron diffraction patterns were obtained along various zone axes for each phase. Some changes in the lattice parameters were observed in both phases when compared to literature data and this comparison is shown in Table 2. The X-ray diffraction pattern from the upper ground sections (~3 mm thickness) of the analyzed alloy is also shown in Fig. 3. The XRD diffraction pattern showed in Fig. 3 can be indexed as a mixture of the Ni10Zr7 (orthorhombic - JCPDS file number 00-047-
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L.P. Deo, M.F. de OliveiraMaterials Characterization 127 (2017) 60–63
Fig. 2. (a) TEM BF image; (b) [010] selected area microdiffraction pattern from Nb10Ni7; (c) ½101 microdiffraction pattern from Ni21Zr8. Both patterns are together with the JEMS simulations for comparison.
1027) and Ni21Zr8 (triclinic - JCPDS file number 04-002-9932) crystalline structures, respectively. Thus, the XRD confirmed the identities of the crystalline structures previously identified in the TEM. The combination of TEM-EDS data and the changes of the lattice parameters determined from the selection area microdiffraction patterns, as shown in Tables 1 and 2 respectively, suggest the formation of two metastable super-saturated solid solutions in the crystalline phases Ni10Zr7 and Ni21Zr8. In other words, there is the formation of two metastable phases and their chemical compositions are about Ni55Nb40Zr5 and Ni70Nb25Zr5 (at.%) respectively. The experimental formation of these metastable phases is attributed to the rapid solidification process. Additional experimental evidence for this solid solution formation is provided by the peak shift in the XRD pattern for these phases in the analyzed alloy when compared to the JCPDS files. As shown in Table 2, the metastable phases have smaller unit cell volumes and lattice parameters calculated from the microdiffraction patterns compared to the literature data found for the binary Ni10Zr7 and Ni21Zr8 phases. Some decrease in unit cell volumes and lattice parameters is expected because the niobium atom has a smaller radius than the zirconium atom. According to Tokunaga et al., the solubility of niobium in Ni10Zr7 may reach about 20–25 at.% [8]. However, our results from the TEM-EDS microanalysis suggest a higher solubility (around 40 at.%) when the calculation is done assuming no changes in atomic radius of the elements. Although, the solubility of niobium in Ni21Zr8 has not been reported in the literature, our results indicate that this value is around 25 at.%. The poor contrast in the SEM-BSE image is an evidence of average atomic
Table 1 Phase compositions (at.%) of analyzed alloy determined by TEM-EDS. Phase
Ni
Nb
Zr
Gray dendrites, with Ni21Zr8 structure Gray matrix, with Ni10Zr7 structure
69.7 ± 0.7 53.9 ± 0.4
25.0 ± 1.5 41.0 ± 0.6
5.4 ± 3.5 5.1 ± 2.1
number similarities between the characterized metastable phases. This absence of Z contrast is attributed to the large niobium solubility in the Ni-Zr crystalline structures as evidenced by the combination of TEM-EDS data, changes in the lattice parameters and the peak shift in the XRD pattern as mentioned previously. Despite the higher amount of niobium than zirconium in the analyzed alloy, the crystallization of Ni10Zr7 and Ni21Zr8 is facilitated by the larger negative value of mixing enthalpy between Ni-Zr and Ni-Nb atomic pairs than it is between Nb-Zr atomic pair. The mixing enthalpies for these atomic pairs are −49 (Ni-Zr), −30 (Ni-Nb) and 4 (Nb-Zr) kJ/ mol [15]. According to the Hume-Rothery rules, several factors affect the formation of the solid solution [16]. The first is the size effects of component atoms. For alloys whose component atomic-size differences are less than 15%, it is most probable to form a substitutional solid solution. The second consideration is the chemical compatibility between components, such as the enthalpy of mixing. If the enthalpy of mixing between elements has positive or small negative values, it is also more probable the formation of a solid solution. The empirical radii of zirconium and niobium atoms are 155 and 145 pm respectively leading to an atomic size ratio of 1.07 (Zr/Nb) [17]. In addition, the mixing enthalpy for the Nb–Zr atomic pair has a positive value. Thus, the substitutional solid solution of niobium in Ni10Zr7 and Ni21Zr8 phases is favorable because the component atoms can easily substitute each other and have a similar probability to occupy the lattice sites. Part of the calculated Ni-Zr binary phase diagram according to Tokunaga et al. [8] is reproduced in Fig. 4 and includes a dashed vertical line to denote a binary alloy composition with the same amount of the majority element present as in the investigated ternary alloy. For this specific alloy composition, with around 62 at.% of nickel, the phase diagram suggests that the solidification should start with Ni10Zr7, as the primary phase, followed by an eutectic reaction producing Ni10Zr7 and Ni21Zr8. This agrees with the experimental results, once the metastable phases with crystal structures of Ni10Zr7 and Ni21Zr8 were successfully
Table 2 Measured vs. Literature values of the lattice parameters and cell volumes of the phases identified in this study. Phase
Ni21Zr8 Ni10Zr7
Measured lattice parameters (Å) a
b
c
6.0 9.0
7.5 8.9
8.0 11.9
Measured cell volume (Å3)
390.2 953.2
Reported lattice parameters (Å) [13,14] a
b
c
6.476 9.216
8.064 9.154
8.594 12.385
Reported cell volume (Å3)
396.128 1044.839
L.P. Deo, M.F. de OliveiraMaterials Characterization 127 (2017) 60–63
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Acknowledgements The authors would like to thank FAPESP (2011/13221-0, 2013/ 18950-5) for financial research support, as well as, to Professor Michael J. Kaufman, Department of Metallurgical and Materials Engineering, Colorado School of Mines, USA, for his valuable support and helpful discussions in TEM analyses. References
Fig. 3. XRD patterns taken from the powder sample of analyzed alloy with indexed crystalline phases.
indexed in the analyzed alloy; however a large solubility of niobium into both phases were also detected as described previously. 4. Conclusion The indexed crystalline structures of the analyzed alloy Ni 61.6 Nb33.1Zr5.3 (at.%), are in good agreement with the calculated binary Ni-Zr phase diagram. From our EDS microanalyses, niobium has a solubility of around 40 at.% in the Ni 10 Zr 7 crystalline structure and according to literature, this solubility may reach only about 20–25 at.%. Still from our analyses, niobium also has a solubility of around 25 at.% in the Ni 21 Zr 8 crystalline structure; however there is no data in literature about this behavior. From the niobium solubility described above, our results also suggest the formation of two ternary metastable phases in the Ni-Nb-Zr system with similar crystalline structures as well as in the Ni10Zr7 and Ni21Zr8 phases. In addition, the high solubility of niobium into the Ni10Zr7 and Ni21Zr8 crystalline structures or the formation of two ternary metastable phases is attributed to the rapid solidification process.
Fig. 4. Part of the calculated binary phase diagram of the Ni-Zr system [8]. The alloy composition with the same amount of the majority element present in the experimental ternary alloy is represented by the dashed line.
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