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A study of phase separated Ni66Nb17Y17 metallic glass using atom probe tomography
Ultramicroscopy 111 (2011) 1370–1374
Contents lists available at ScienceDirect
Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
A study of phase separated Ni66Nb17Y17 metallic glass using atom probe tomography A. Shariq a,n, N. Mattern b a b
Fraunhofer-Center Nanoelectronic Technologies, Koenigsbruecker Strasse 180, D-01099 Dresden, Germany Leibniz Institute IFW Dresden, Institute of Complex Materials, Dresden, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 August 2010 Received in revised form 15 April 2011 Accepted 8 May 2011 Available online 18 May 2011
Microstructural characterization of Ni66Nb17Y17 as spun metallic glass ribbon was carried out using atom probe tomography. A comparison of different experimental conditions for pulsed laser and pulsed voltage field evaporation reveal that the laser pulsing can be optimized to avoid preferential evaporation of yttrium. Atom probe tomography measurements illustrate that the sample undergoes phase separation resulting in two interconnected phases during the process of vitrification. The yttrium-enriched phase was depleted in niobium and yttrium-depleted phase was enriched in niobium. Moreover, detailed analyses of the roller-contact and non-contact sides of the melt-spun ribbon show different wavelength of phase separated regions revealing that the degree of phase separation is directly associated with the cooling rate. & 2011 Elsevier B.V. All rights reserved.
Keywords: Phase Separation Metallic glass Atom Probe Tomography (APT)
1. Introduction The unique set of properties offered by the metallic glasses triggered intensive microstructural investigations in the last decade [1]. Super-cooled liquids undergo crystallization and decomposition during the cooling cycle from the melt [2–4]. Phase separation requires diffusivity of the atomic species over several interatomic distances along with at least two local minima of the Gibbs free energy as a function of composition. In multi-component alloy systems, a strong positive enthalpy of mixing for the constituent elements may lead to phase separation already in the liquid state. Rapid quenching techniques can be utilized to vitrify the decomposed liquids into a phase separated glass for materials exhibiting high glass forming abilities e.g., Ni–Nb–Y system [5,6]. Phase separation in the aforementioned system may lead to elemental fluctuations on the length scale of a few nanometers, requiring an experimental technique capable of unambiguous separation of the constituent elements with a lateral resolution in the sub-nanometer range [7–9]. In this contribution, nano-scaled phase separated Ni66Nb17Y17 metallic glass has been analyzed using atom probe tomography (APT). For a quantitative analysis of a new alloy system with the constituent elements containing varying fields of evaporation, appropriate experimental conditions must be selected. It is necessary to minimize possible preferential evaporation or retention of one of the species from the sample surface during the field
n
Corresponding author. Tel.: þ49 351 2607 3060; fax: þ49 351 2607 3005. E-mail address: [email protected] (A. Shariq).
0304-3991/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2011.05.005
evaporation process. In addition to the characterization of the phase separated glass, the aim of this work is to present a comparison of the two atom probe analyzing methodologies i.e., voltage pulsing and laser pulsing.
2. Experimental Nickel, niobium and yttrium with purities of 99.9% or higher were arc-melted in a Ti-gettered argon atmosphere and pre-alloyed with a nominal composition of Ni66Nb17Y17. The samples were remelted several times to ensure homogeneity. The chemical composition was confirmed by the titration technique. Single-roller melt spinning was then used to vitrify the pre-alloyed melt from a temperature of 1923 K (in an argon atmosphere) leading to thin ribbons having width of 3 mm and 30 mm thickness. The glassy state of the ribbon was proven by X-ray diffraction (XRD) and transmission electron microscopy (TEM). No indications of heterogeneities were seen by TEM analysis. However, small angle X-ray scattering gave hints of chemical fluctuations with a correlation length in the range of 10 nm [5]. Focused ion beam/scanning electron microscopy (FIB/SEM) (FEI Strata 400) was used for the fabrication of the APT samples. Ion-beam deposited Pt was used as a sacrificial capping layer to avoid surface damage due to Ga ion-beam milling. The details of the sample preparation are described elsewhere [10]. High resolution scanning electron images were utilized to access the details of sample apex geometry. These specimens were then analyzed using the local electrode atom probe (LEAP 3000 X SiTM). Voltage pulsing was carried out at a frequency of 200 kHz, while the pulse fraction was varied from 16 to 24% of the standing voltage.
A. Shariq, N. Mattern / Ultramicroscopy 111 (2011) 1370–1374
The samples during pulsed voltage analyses were cooled to 50 K. Laser pulsing was done with a laser of wavelength 512 nm and o15 ps pulse duration, for two different frequencies i.e., 400 and 200 kHz. The specimen during pulsed laser atom probe analyses were cryogenically cooled to 38 K.
3. Results and discussion Fabricated APT samples were analyzed for different experimental conditions both in pulsed voltage and pulsed laser mode. The resultant compositions are summarized in Table 1. A mass spectrum obtained from the pulsed voltage analysis is shown in Fig. 1. There exists a mass overlap between 62Ni2 þ /93Nb3 þ . Peak deconvolution was done for the compositional analyses as described elsewhere [11]. Pulsed laser evaporation resulted in a different charge state ratio and niobium evaporates as 93Nb2 þ , instead, minimizing the chances of aforementioned overlap. Compositional details for different experimental parameters for pulsed laser atom probe analyses are also given in Table 1. The ratio of charge states of the field-ionized atoms from the surface of the specimen tip has been shown to have a correlation to the specimen apex temperature [12,13]. Therefore, to ensure that the specimens were not affected by undesirable artifacts, the charge state ratio was compared between voltage and laser modes [14] and low laser energies were employed for the qualitative and quantitative analysis of Ni–Nb–Y as spun ribbons. The experimental conditions on the specimen must be set so that there is an equal probability of evaporating the constituent elements during APT analysis. Failing to do so may lead to preferential evaporation of the elements with a low evaporation field and/or preferential retention of the elements that require relatively higher evaporation fields. Tsong [15] has calculated evaporation field from the single image hump model for nickel, niobium and yttrium as 35, 37 and 23.5 V/nm, respectively. Comparatively lower evaporation field of yttrium in this alloy system (assuming the same trend of evaporation field as present in the pure elemental form) is susceptible to the preferential evaporation during experiment with voltage pulse. This explains lower yttrium concentration than its nominal value as shown in Table 1. The evaporation field decreases with an increase in the applied temperature on APT specimen [16]. Thereby, field evaporation can be instigated at a lower DC voltage by applying a thermal pulse rather than a voltage pulse. A picosecond laser pulse leads to a momentarily increase in the specimen temperature from the bulk specimen temperature to a peak pulse temperature. It is desirable to apply a thermal pulse suitable for a uniform evaporation of all constituent elements during the pulse, keeping the DC voltage low enough leading to no evaporation between the pulses. This explains why preferential evaporation of yttrium is retarded during pulsed laser atom probe analysis. Contrarily higher laser energies can lead to undesirable artifacts, such as atomic diffusion
1371
on the apex [17]. Moreover, preferential DC field evaporation of certain elements can be induced by an increase in specimen temperature apart from possible deterioration of the mass resolution and signal to noise ratio due to excessive laser pulsing. Laser beam illuminating a tip apex from a side can lead to a non-uniform evaporation [14,18–19]. This can also be monitored during the analyses by two dimensional detector event histograms. Positions of the detected ions on the detector surface are binned and low atomic density regions are displayed in blue color and high atomic density regions are displayed in green. In conjunction with the charge state ratios such histograms can provide direct insights on the possible artifacts induced by the temperature differences. Examples of such maps for different laser pulsing conditions are shown in Fig. 2. The detector event histograms for a laser energy greater than 0.13 nJ when pulsed at a frequency of 400 kHz show a non-uniform evaporation from the sample apex resulting artifacts in data reconstruction. Intriguingly, if the pulsed frequency is decreased to 200 kHz, even a 0.23 nJ laser energy lead to a uniform evaporation. Owing to more accurate compositional measurements, only pulsed laser atom probe analyses are considered in the following sections. APT samples fabricated from the non-contact side of the as spun ribbon exhibit a fine scale percolated microstructure, illustrated by the Nb iso-concentration surface as shown in Fig. 3. The three-dimensional map of yttrium atoms from a thin slice depicts phase separated regions with low or higher concentrations of yttrium as illustrated in Fig. 4 for roller non-contact and contact
Fig. 1. Mass to charge state ratio for a pulsed voltage atom probe analysis for Ni66Nb17Y17 as spun ribbon exhibiting an overlap a mass overlap between 62 Ni2 þ /93Nb3 þ isotopes.
Table 1 A summary for different experimental conditions for pulsed laser and pulsed voltage atom probe analyses with corresponding nickel, niobium and yttrium concentrations. Where PF is pulse fraction to the standing voltage and DR is average detection rate for the corresponding pulse frequency.
Ni Nb Y
Voltage pulsing
Laser pulsing
50 K, 0.6% DR, 200 kHz
400 kHz, 38 K
200 kHz, 38 K
16% PF
20% PF
24% PF
0.13 nJ 0.6% DR
0.23nJ 0.6% DR
0.23 nJ 0.9% DR
0.23 nJ 1.2% DR
0.33 nJ 0.6% DR
0.23 nJ 0.6% DR
0.23 nJ 0.9% DR
0.23 nJ 1.2% DR
68.6 20.7 10.7
67.5 21 11.5
68.2 20.4 11.4
68 16.4 15.6
67.9 17.2 14.9
68 16.9 15.1
68 16.7 15.3
69.2 16.2 14.6
67 17.9 15.1
67.2 17.5 15.3
68.4 17.1 14.5
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Fig. 2. Detector event histograms for pulsed laser atom probe analysis of Ni66Nb17Y17 as spun ribbon for (a) 0.13 nJ laser energy and a pulse frequency of 400 kHz, (b) 0.23 nJ laser energy and a pulse frequency of 400 kHz, (c) 0.33 nJ laser energy and a pulse frequency of 400 kHz and (d) 0.23 nJ laser energy and a pulse frequency of 200 kHz. Arrow indicates the direction of the incident laser.
The Langer, Bar-on, Miller (LBM) model has been utilized to quantify the compositional fluctuations. The LBM model considers nonlinear spinodal decomposition using a double Gaussian distribution with the probability distribution function PðcÞ ¼
Fig. 3. Spatial distribution of nickel atoms in green and interconnected niobium isosurface (for concentration of 17 at%) is presented in orange for Ni66Nb17Y17 as spun ribbon.
side. Fluctuations of the yttrium and niobium atoms are at a scale relatively smaller than found for the non-contact side. A two dimensional concentration color map in x and z image plane from a non-contacting side specimen shows that the regions enriched in the yttrium concentration are depleted in niobium and vice versa (see Fig. 5). A compositional depth profile is shown in Fig. 6, which quantitatively depicts niobium enriched and yttriumdepleted phase. The wavelength of the phase separation is estimated for the non-contact side to be in the range from 3–10 nm. The compositional depth profile in Fig. 7 shows that fluctuations of the yttrium and niobium atoms are smaller for the contact side of the as spun ribbon.
ð1Þ
One Gaussian is centered at m1, the other is centered at m2, and both have a width s. The LBM model has three independent parameters, m1, m2 and s. In implementations of the LBM model, the two parameters b1 ¼c m1 and b2 ¼ m2 c, where c is the mean concentration, are substituted into Eq. (1). The difference in the composition of the two decomposed phases is given by Dc¼b1 þ b2. The results of the statistical analysis with the LBM model are listed in Table 2 for the data sets collected. The Dc values in the LBM model show that the alloy primarily decomposes to Nb-enriched and Y-enriched regions. Further details on the LBM models can be found in Ref. [20]. w2-test was also utilized to check elemental homogeneities obtained from the atom probe data, where the analyzed volume is divided into small voxels within which the concentration of the alloying elements is determined. The concentration range of a certain element is split into N appropriate classes and w2 is evaluated by the following equation:
w2 ¼
Fig. 4. Spatial distribution of yttrium atoms showing phase separated yttriumenriched and yttrium-depleted regions for Ni66Nb17Y17 as spun ribbon (a) noncontacting side and (b) contacting side.
m2 exp½ðcm1 Þ2 =2s2 þ m1 exp½ðcm2 Þ2 =2s2 pffiffiffiffiffiffi ðm1 þ m2 Þs 2p
N X ½FðnÞBðnÞ2 BðnÞ n¼0
ð2Þ
where, F(n) is the concentration of an element in class n and B(n) is the corresponding value of the binominal distribution. Then according to Eq. (2), w2 is a measure of the deviation of experimentally determined concentration distributions from the binominal or homogeneous one. If this deviation is smaller than values tabulated for a certain significance parameter a, the distribution is called homogeneous. In the present work, a ¼ 0.05 was chosen and for w2 o w2a the distribution is considered to be homogeneous with an uncertainty of 5%. A comparison of the w2 values calculated for each constituent element via Eq. 2 and the w2 alpha values with 0.05 significance level of alpha, for both contact and non-contact sides are presented in Table 3. The w2 values exceeds remarkably as compared to the reference w2 alpha values for Nb and Y for both sides of the ribbon suggesting inhomogeneous distribution, which is also confirmed from the spatial distribution of the constituent elements and from the results of the LBM model on the APT data. The degree of the phase separation is higher for the slowly cooled side (non-contact side) of the as spun ribbon. These results suggest that the observed phase separation is associated with the cooling rate from the melt. The development of the phase separated microstructure in the super-cooled melt can be predicted from a phase diagram. A pseudo-binary section of the ternary phase diagram is shown in Fig. 8 [5]. The extrapolation of the critical temperature Tc for the Ni contents higher than 60 at% is depicted by
A. Shariq, N. Mattern / Ultramicroscopy 111 (2011) 1370–1374
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Fig. 5. Two dimensional concentration color maps in X - Z image plane for (left) niobium and (right) yttrium for the non-contact side of the Ni66Nb17Y17 as spun ribbon.
Fig. 6. Compositional depth profile for non-contacting side of the as spun ribbon showing yttrium and niobium enriched regions. Inset shows a magnified portion with arrows pointed to niobium and yttrium variations. Solid line indicates the nominal composition. The error bars for nickel, niobium and yttrium concentrations are 7.7, 2.5 and 2.3 at%, respectively.
the triangles in Fig. 8. The temperature difference between Tc and Tg strongly depends on the nickel contents. For Ni66Nb17Y17 system, the phase separation proceeds in the metastable super-cooled liquid at low temperature relatively close to Tg. The cooling rate through the cross-section of the ribbon plays an important role in determining the kinetics of the decomposition and growth of phase separated regions. The cooling rate is highest for the bottom side of the ribbon i.e., roller-contact side and slowest for the non-contact side [21]. For the contact side, due to faster cooling rate less time is required for the super-cooled melt to reach glass transition temperature Tg, forcing the melt to vitrify into the glass. Therefore, early stages of the decomposition are observed for the roller-contact side of the ribbon in comparison to the non-contact side. 4. Conclusions The effects of voltage pulsed and laser pulsed atom probe analyses for different experimental conditions on the compositional
Fig. 7. Compositional depth profile for contacting side of the as spun ribbon showing fluctuations of yttrium and niobium. The error bars for nickel, niobium and yttrium concentrations are 8.4, 3.0 and 2.8 at%, respectively.
Table 2 Results of the statistical analysis of Ni66Nb17Y17 APT data using the LBM model. The parameters b1, b2, s and Dc are given in atomic fraction. Ni66Nb17Y17
Contact side
Elements
Ni
Nb
Y
Ni
Nb
Y
b1 b2
0 0.01 0.01 25 33 0.01
0.03 0.01 0.01 34 28 0.04
0.01 0.02 0.01 32 26 0.03
0.01 0.01 0.01 33 34 0.02
0.04 0.02 0.02 54 31 0.06
0.02 0.03 0.02 54 30 0.05
s w2 (LBM) Degree of freedom Dc
Non-contact side
analyses of Ni66Nb17Y17 as spun metallic glass ribbon are presented in this contribution. Voltage pulsed atom probe analysis provides misleading composition of the sample due to preferential evaporation of yttrium, which has relatively low evaporation field. However, proper laser pulsed evaporation conditions need to be selected to avoid preferential evaporation of Y for reasonably correct quantitative analyses for this system.
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Table 3 Summary of the calculated w2 values for both contact and non-contact side of Ni66Nb17Y17 glassy ribbon. Value of w2 alpha for the stated degree of freedom is also given for a comparison, with 0.05 significance level of alpha. Ni66Nb17Y17
Contact side
Elements
Ni
Nb
Y
Ni
Nb
Y
w2 w2 alpha
65 51 35
843 41 27
538 40 26
88 50 34
7997 43 28
7602 40 26
Degree of freedom
Non-contact side
niobium and yttrium-depleted phase was enriched in niobium. The observed phase separation depends on the cooling rate from the super-cooled melt. The non-contact side of the ribbon exhibit larger wavelength of the phase separation in comparison to the contact side of the as spun ribbon.
Acknowledgments IFW gratefully acknowledges the financial support of the Deutsche Forschungsgemeinschaft DFG (Project Ma1531/10). AS acknowledges funding support by the Federal Ministry of Education and Research of the Federal Republic of Germany (Project no. 13N9432). The authors are responsible for the content of the paper. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Fig. 8. Section of a ternary phase diagram Ni75Nb12.5Y12.5- Ni50Nb25Y25 with extrapolation (triangles) of the critical temperature Tc for higher concentrations of nickel than 60 at%.
Atom probe tomography has revealed that Ni66Nb17Y17 as spun metallic glass ribbon has phase separated into two interconnected amorphous phases. The yttrium-enriched phase was depleted in
[15] [16] [17] [18] [19] [20] [21]
M.F. Ashby, A.L. Greer, Scr. Mater. 54 (2006) 321. L.E. Tanner, R. Ray, Scr. Metall. 14 (1980) 657. ¨ U. Koster, J. Meinhardt, Mater. Sci. Eng. A 178 (1994) 271. R. Suzuki, K. Shibue, K. Osamura, P.H. Shingu, Y. Murakami, J. Mater. Sci. Lett. 1 (1982) 127. N. Mattern, G. Goerigk, U. Vainio, M.K. Miller, T. Gemming, J. Eckert, Acta Mater. 57 (2009) 903. N. Mattern, T. Gemming, J. Thomas, G. Goerigk, H. Franz, J. Eckert, J. Alloys Compd. 495 (2) (2010) 299. F. Danoix, P. Auger, Mater. Charact. 44 (2000) 177. A. Shariq, M. Hattestrand, J.O. Nilsson, A. Gregori, J. Nanoscience Nanotechnol. 9 (2009) 3657. A. Shariq, T. Al-Kassab, R. Kirchheim, R.B. Schwarz, Ultramicroscopy 107 (2007) 773. K. Thompson, D. Lawrence, D.J. Larson, J.D. Olson, T.F. Kelly, B. Gorman, Ultramicroscopy 107 (2007) 131. M.K. Miller, A. Cerezo, M.G. Hetherington, G.D.W. Smith, Atom Probe FieldIon Microscopy, Oxford University Press, New York, 1996. G.L. Kellogg, J. Appl. Phys. 52 (1981) 5320. A. Cerezo, G.D.W. Smith, P.H. Clifton, Appl. Phys. Lett. 88 (2006) 154103. A. Shariq, S. Mutas, K. Wedderhoff, C. Klein, H. Hortenbach, S. Teichert, ¨ P. Kucher, S.S.A. Gerstl, Ultramicroscopy 109 (2009) 472. T.T. Tsong, Surf. Sci. 70 (1978) 211. G.L. Kellogg, T.T. Tsong, J. Appl. Phys. 51 (1980) 1184. A. Cerezo, P.H. Clifton, A. Gomberg, G.D.W. Smith, Ultramicroscopy 107 (2007) 720. G. Sha, A. Cerezo, G.D.W. Smith, Appl. Phys. Lett. 92 (2008) 043503. B. Gault, A.L. Fontaine, M.P. Moody, S.P. Ringer, E.A. Marquis, Ultramicroscopy 110 (2010) 1215. M.K. Miller, K.O. Bowman, A. Cerezo, J.M. Hyde, Appl. Surf. Sci. 67 (1993) 429. B. Cantor, W.T. Kim, B.P. Bewlay, A.G. Gillen, J. Mater. Sci. 26 (5) (1991) 1266.
Contents lists available at ScienceDirect
Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
A study of phase separated Ni66Nb17Y17 metallic glass using atom probe tomography A. Shariq a,n, N. Mattern b a b
Fraunhofer-Center Nanoelectronic Technologies, Koenigsbruecker Strasse 180, D-01099 Dresden, Germany Leibniz Institute IFW Dresden, Institute of Complex Materials, Dresden, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 August 2010 Received in revised form 15 April 2011 Accepted 8 May 2011 Available online 18 May 2011
Microstructural characterization of Ni66Nb17Y17 as spun metallic glass ribbon was carried out using atom probe tomography. A comparison of different experimental conditions for pulsed laser and pulsed voltage field evaporation reveal that the laser pulsing can be optimized to avoid preferential evaporation of yttrium. Atom probe tomography measurements illustrate that the sample undergoes phase separation resulting in two interconnected phases during the process of vitrification. The yttrium-enriched phase was depleted in niobium and yttrium-depleted phase was enriched in niobium. Moreover, detailed analyses of the roller-contact and non-contact sides of the melt-spun ribbon show different wavelength of phase separated regions revealing that the degree of phase separation is directly associated with the cooling rate. & 2011 Elsevier B.V. All rights reserved.
Keywords: Phase Separation Metallic glass Atom Probe Tomography (APT)
1. Introduction The unique set of properties offered by the metallic glasses triggered intensive microstructural investigations in the last decade [1]. Super-cooled liquids undergo crystallization and decomposition during the cooling cycle from the melt [2–4]. Phase separation requires diffusivity of the atomic species over several interatomic distances along with at least two local minima of the Gibbs free energy as a function of composition. In multi-component alloy systems, a strong positive enthalpy of mixing for the constituent elements may lead to phase separation already in the liquid state. Rapid quenching techniques can be utilized to vitrify the decomposed liquids into a phase separated glass for materials exhibiting high glass forming abilities e.g., Ni–Nb–Y system [5,6]. Phase separation in the aforementioned system may lead to elemental fluctuations on the length scale of a few nanometers, requiring an experimental technique capable of unambiguous separation of the constituent elements with a lateral resolution in the sub-nanometer range [7–9]. In this contribution, nano-scaled phase separated Ni66Nb17Y17 metallic glass has been analyzed using atom probe tomography (APT). For a quantitative analysis of a new alloy system with the constituent elements containing varying fields of evaporation, appropriate experimental conditions must be selected. It is necessary to minimize possible preferential evaporation or retention of one of the species from the sample surface during the field
n
Corresponding author. Tel.: þ49 351 2607 3060; fax: þ49 351 2607 3005. E-mail address: [email protected] (A. Shariq).
0304-3991/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2011.05.005
evaporation process. In addition to the characterization of the phase separated glass, the aim of this work is to present a comparison of the two atom probe analyzing methodologies i.e., voltage pulsing and laser pulsing.
2. Experimental Nickel, niobium and yttrium with purities of 99.9% or higher were arc-melted in a Ti-gettered argon atmosphere and pre-alloyed with a nominal composition of Ni66Nb17Y17. The samples were remelted several times to ensure homogeneity. The chemical composition was confirmed by the titration technique. Single-roller melt spinning was then used to vitrify the pre-alloyed melt from a temperature of 1923 K (in an argon atmosphere) leading to thin ribbons having width of 3 mm and 30 mm thickness. The glassy state of the ribbon was proven by X-ray diffraction (XRD) and transmission electron microscopy (TEM). No indications of heterogeneities were seen by TEM analysis. However, small angle X-ray scattering gave hints of chemical fluctuations with a correlation length in the range of 10 nm [5]. Focused ion beam/scanning electron microscopy (FIB/SEM) (FEI Strata 400) was used for the fabrication of the APT samples. Ion-beam deposited Pt was used as a sacrificial capping layer to avoid surface damage due to Ga ion-beam milling. The details of the sample preparation are described elsewhere [10]. High resolution scanning electron images were utilized to access the details of sample apex geometry. These specimens were then analyzed using the local electrode atom probe (LEAP 3000 X SiTM). Voltage pulsing was carried out at a frequency of 200 kHz, while the pulse fraction was varied from 16 to 24% of the standing voltage.
A. Shariq, N. Mattern / Ultramicroscopy 111 (2011) 1370–1374
The samples during pulsed voltage analyses were cooled to 50 K. Laser pulsing was done with a laser of wavelength 512 nm and o15 ps pulse duration, for two different frequencies i.e., 400 and 200 kHz. The specimen during pulsed laser atom probe analyses were cryogenically cooled to 38 K.
3. Results and discussion Fabricated APT samples were analyzed for different experimental conditions both in pulsed voltage and pulsed laser mode. The resultant compositions are summarized in Table 1. A mass spectrum obtained from the pulsed voltage analysis is shown in Fig. 1. There exists a mass overlap between 62Ni2 þ /93Nb3 þ . Peak deconvolution was done for the compositional analyses as described elsewhere [11]. Pulsed laser evaporation resulted in a different charge state ratio and niobium evaporates as 93Nb2 þ , instead, minimizing the chances of aforementioned overlap. Compositional details for different experimental parameters for pulsed laser atom probe analyses are also given in Table 1. The ratio of charge states of the field-ionized atoms from the surface of the specimen tip has been shown to have a correlation to the specimen apex temperature [12,13]. Therefore, to ensure that the specimens were not affected by undesirable artifacts, the charge state ratio was compared between voltage and laser modes [14] and low laser energies were employed for the qualitative and quantitative analysis of Ni–Nb–Y as spun ribbons. The experimental conditions on the specimen must be set so that there is an equal probability of evaporating the constituent elements during APT analysis. Failing to do so may lead to preferential evaporation of the elements with a low evaporation field and/or preferential retention of the elements that require relatively higher evaporation fields. Tsong [15] has calculated evaporation field from the single image hump model for nickel, niobium and yttrium as 35, 37 and 23.5 V/nm, respectively. Comparatively lower evaporation field of yttrium in this alloy system (assuming the same trend of evaporation field as present in the pure elemental form) is susceptible to the preferential evaporation during experiment with voltage pulse. This explains lower yttrium concentration than its nominal value as shown in Table 1. The evaporation field decreases with an increase in the applied temperature on APT specimen [16]. Thereby, field evaporation can be instigated at a lower DC voltage by applying a thermal pulse rather than a voltage pulse. A picosecond laser pulse leads to a momentarily increase in the specimen temperature from the bulk specimen temperature to a peak pulse temperature. It is desirable to apply a thermal pulse suitable for a uniform evaporation of all constituent elements during the pulse, keeping the DC voltage low enough leading to no evaporation between the pulses. This explains why preferential evaporation of yttrium is retarded during pulsed laser atom probe analysis. Contrarily higher laser energies can lead to undesirable artifacts, such as atomic diffusion
1371
on the apex [17]. Moreover, preferential DC field evaporation of certain elements can be induced by an increase in specimen temperature apart from possible deterioration of the mass resolution and signal to noise ratio due to excessive laser pulsing. Laser beam illuminating a tip apex from a side can lead to a non-uniform evaporation [14,18–19]. This can also be monitored during the analyses by two dimensional detector event histograms. Positions of the detected ions on the detector surface are binned and low atomic density regions are displayed in blue color and high atomic density regions are displayed in green. In conjunction with the charge state ratios such histograms can provide direct insights on the possible artifacts induced by the temperature differences. Examples of such maps for different laser pulsing conditions are shown in Fig. 2. The detector event histograms for a laser energy greater than 0.13 nJ when pulsed at a frequency of 400 kHz show a non-uniform evaporation from the sample apex resulting artifacts in data reconstruction. Intriguingly, if the pulsed frequency is decreased to 200 kHz, even a 0.23 nJ laser energy lead to a uniform evaporation. Owing to more accurate compositional measurements, only pulsed laser atom probe analyses are considered in the following sections. APT samples fabricated from the non-contact side of the as spun ribbon exhibit a fine scale percolated microstructure, illustrated by the Nb iso-concentration surface as shown in Fig. 3. The three-dimensional map of yttrium atoms from a thin slice depicts phase separated regions with low or higher concentrations of yttrium as illustrated in Fig. 4 for roller non-contact and contact
Fig. 1. Mass to charge state ratio for a pulsed voltage atom probe analysis for Ni66Nb17Y17 as spun ribbon exhibiting an overlap a mass overlap between 62 Ni2 þ /93Nb3 þ isotopes.
Table 1 A summary for different experimental conditions for pulsed laser and pulsed voltage atom probe analyses with corresponding nickel, niobium and yttrium concentrations. Where PF is pulse fraction to the standing voltage and DR is average detection rate for the corresponding pulse frequency.
Ni Nb Y
Voltage pulsing
Laser pulsing
50 K, 0.6% DR, 200 kHz
400 kHz, 38 K
200 kHz, 38 K
16% PF
20% PF
24% PF
0.13 nJ 0.6% DR
0.23nJ 0.6% DR
0.23 nJ 0.9% DR
0.23 nJ 1.2% DR
0.33 nJ 0.6% DR
0.23 nJ 0.6% DR
0.23 nJ 0.9% DR
0.23 nJ 1.2% DR
68.6 20.7 10.7
67.5 21 11.5
68.2 20.4 11.4
68 16.4 15.6
67.9 17.2 14.9
68 16.9 15.1
68 16.7 15.3
69.2 16.2 14.6
67 17.9 15.1
67.2 17.5 15.3
68.4 17.1 14.5
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A. Shariq, N. Mattern / Ultramicroscopy 111 (2011) 1370–1374
Fig. 2. Detector event histograms for pulsed laser atom probe analysis of Ni66Nb17Y17 as spun ribbon for (a) 0.13 nJ laser energy and a pulse frequency of 400 kHz, (b) 0.23 nJ laser energy and a pulse frequency of 400 kHz, (c) 0.33 nJ laser energy and a pulse frequency of 400 kHz and (d) 0.23 nJ laser energy and a pulse frequency of 200 kHz. Arrow indicates the direction of the incident laser.
The Langer, Bar-on, Miller (LBM) model has been utilized to quantify the compositional fluctuations. The LBM model considers nonlinear spinodal decomposition using a double Gaussian distribution with the probability distribution function PðcÞ ¼
Fig. 3. Spatial distribution of nickel atoms in green and interconnected niobium isosurface (for concentration of 17 at%) is presented in orange for Ni66Nb17Y17 as spun ribbon.
side. Fluctuations of the yttrium and niobium atoms are at a scale relatively smaller than found for the non-contact side. A two dimensional concentration color map in x and z image plane from a non-contacting side specimen shows that the regions enriched in the yttrium concentration are depleted in niobium and vice versa (see Fig. 5). A compositional depth profile is shown in Fig. 6, which quantitatively depicts niobium enriched and yttriumdepleted phase. The wavelength of the phase separation is estimated for the non-contact side to be in the range from 3–10 nm. The compositional depth profile in Fig. 7 shows that fluctuations of the yttrium and niobium atoms are smaller for the contact side of the as spun ribbon.
ð1Þ
One Gaussian is centered at m1, the other is centered at m2, and both have a width s. The LBM model has three independent parameters, m1, m2 and s. In implementations of the LBM model, the two parameters b1 ¼c m1 and b2 ¼ m2 c, where c is the mean concentration, are substituted into Eq. (1). The difference in the composition of the two decomposed phases is given by Dc¼b1 þ b2. The results of the statistical analysis with the LBM model are listed in Table 2 for the data sets collected. The Dc values in the LBM model show that the alloy primarily decomposes to Nb-enriched and Y-enriched regions. Further details on the LBM models can be found in Ref. [20]. w2-test was also utilized to check elemental homogeneities obtained from the atom probe data, where the analyzed volume is divided into small voxels within which the concentration of the alloying elements is determined. The concentration range of a certain element is split into N appropriate classes and w2 is evaluated by the following equation:
w2 ¼
Fig. 4. Spatial distribution of yttrium atoms showing phase separated yttriumenriched and yttrium-depleted regions for Ni66Nb17Y17 as spun ribbon (a) noncontacting side and (b) contacting side.
m2 exp½ðcm1 Þ2 =2s2 þ m1 exp½ðcm2 Þ2 =2s2 pffiffiffiffiffiffi ðm1 þ m2 Þs 2p
N X ½FðnÞBðnÞ2 BðnÞ n¼0
ð2Þ
where, F(n) is the concentration of an element in class n and B(n) is the corresponding value of the binominal distribution. Then according to Eq. (2), w2 is a measure of the deviation of experimentally determined concentration distributions from the binominal or homogeneous one. If this deviation is smaller than values tabulated for a certain significance parameter a, the distribution is called homogeneous. In the present work, a ¼ 0.05 was chosen and for w2 o w2a the distribution is considered to be homogeneous with an uncertainty of 5%. A comparison of the w2 values calculated for each constituent element via Eq. 2 and the w2 alpha values with 0.05 significance level of alpha, for both contact and non-contact sides are presented in Table 3. The w2 values exceeds remarkably as compared to the reference w2 alpha values for Nb and Y for both sides of the ribbon suggesting inhomogeneous distribution, which is also confirmed from the spatial distribution of the constituent elements and from the results of the LBM model on the APT data. The degree of the phase separation is higher for the slowly cooled side (non-contact side) of the as spun ribbon. These results suggest that the observed phase separation is associated with the cooling rate from the melt. The development of the phase separated microstructure in the super-cooled melt can be predicted from a phase diagram. A pseudo-binary section of the ternary phase diagram is shown in Fig. 8 [5]. The extrapolation of the critical temperature Tc for the Ni contents higher than 60 at% is depicted by
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Fig. 5. Two dimensional concentration color maps in X - Z image plane for (left) niobium and (right) yttrium for the non-contact side of the Ni66Nb17Y17 as spun ribbon.
Fig. 6. Compositional depth profile for non-contacting side of the as spun ribbon showing yttrium and niobium enriched regions. Inset shows a magnified portion with arrows pointed to niobium and yttrium variations. Solid line indicates the nominal composition. The error bars for nickel, niobium and yttrium concentrations are 7.7, 2.5 and 2.3 at%, respectively.
the triangles in Fig. 8. The temperature difference between Tc and Tg strongly depends on the nickel contents. For Ni66Nb17Y17 system, the phase separation proceeds in the metastable super-cooled liquid at low temperature relatively close to Tg. The cooling rate through the cross-section of the ribbon plays an important role in determining the kinetics of the decomposition and growth of phase separated regions. The cooling rate is highest for the bottom side of the ribbon i.e., roller-contact side and slowest for the non-contact side [21]. For the contact side, due to faster cooling rate less time is required for the super-cooled melt to reach glass transition temperature Tg, forcing the melt to vitrify into the glass. Therefore, early stages of the decomposition are observed for the roller-contact side of the ribbon in comparison to the non-contact side. 4. Conclusions The effects of voltage pulsed and laser pulsed atom probe analyses for different experimental conditions on the compositional
Fig. 7. Compositional depth profile for contacting side of the as spun ribbon showing fluctuations of yttrium and niobium. The error bars for nickel, niobium and yttrium concentrations are 8.4, 3.0 and 2.8 at%, respectively.
Table 2 Results of the statistical analysis of Ni66Nb17Y17 APT data using the LBM model. The parameters b1, b2, s and Dc are given in atomic fraction. Ni66Nb17Y17
Contact side
Elements
Ni
Nb
Y
Ni
Nb
Y
b1 b2
0 0.01 0.01 25 33 0.01
0.03 0.01 0.01 34 28 0.04
0.01 0.02 0.01 32 26 0.03
0.01 0.01 0.01 33 34 0.02
0.04 0.02 0.02 54 31 0.06
0.02 0.03 0.02 54 30 0.05
s w2 (LBM) Degree of freedom Dc
Non-contact side
analyses of Ni66Nb17Y17 as spun metallic glass ribbon are presented in this contribution. Voltage pulsed atom probe analysis provides misleading composition of the sample due to preferential evaporation of yttrium, which has relatively low evaporation field. However, proper laser pulsed evaporation conditions need to be selected to avoid preferential evaporation of Y for reasonably correct quantitative analyses for this system.
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Table 3 Summary of the calculated w2 values for both contact and non-contact side of Ni66Nb17Y17 glassy ribbon. Value of w2 alpha for the stated degree of freedom is also given for a comparison, with 0.05 significance level of alpha. Ni66Nb17Y17
Contact side
Elements
Ni
Nb
Y
Ni
Nb
Y
w2 w2 alpha
65 51 35
843 41 27
538 40 26
88 50 34
7997 43 28
7602 40 26
Degree of freedom
Non-contact side
niobium and yttrium-depleted phase was enriched in niobium. The observed phase separation depends on the cooling rate from the super-cooled melt. The non-contact side of the ribbon exhibit larger wavelength of the phase separation in comparison to the contact side of the as spun ribbon.
Acknowledgments IFW gratefully acknowledges the financial support of the Deutsche Forschungsgemeinschaft DFG (Project Ma1531/10). AS acknowledges funding support by the Federal Ministry of Education and Research of the Federal Republic of Germany (Project no. 13N9432). The authors are responsible for the content of the paper. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Fig. 8. Section of a ternary phase diagram Ni75Nb12.5Y12.5- Ni50Nb25Y25 with extrapolation (triangles) of the critical temperature Tc for higher concentrations of nickel than 60 at%.
Atom probe tomography has revealed that Ni66Nb17Y17 as spun metallic glass ribbon has phase separated into two interconnected amorphous phases. The yttrium-enriched phase was depleted in
[15] [16] [17] [18] [19] [20] [21]
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