Structural characterization of Ni-based refractory glassy metals

Journal of Non-Crystalline Solids 344 (2004) 110–118 www.elsevier.com/locate/jnoncrysol

Structural characterization of Ni-based refractory glassy metals Michelle L. Tokarz a

a,*

, John C. Bilello

a,b

Department of Materials Science and Engineering, Center for Nanomaterials Science, University of Michigan, Ann Arbor, MI 48109-2136, USA b Department of Materials Science, California Institute of Technology, Pasadena, CA 91125, USA Received 22 April 2004; received in revised form 9 July 2004

Abstract X-ray scattering patterns of several Ni-based refractory alloy glasses indicated short range atomic order to at least four nearest neighbor shells. Comparisons between diffraction results from a synchrotron source vs. a standard laboratory source showed the necessity for low divergence and high intensity incident radiation in order to distinguish a low concentration of crystallites as small as
15 nm, which were present within an amorphous matrix. The divergence of both sources was examined by comparing the diffraction patterns of the same LaB6 standard sample and using this as a reference standard to measure relative grain sizes. The crystallites in these glasses comprised between 0.0% and 7.5% by volume, and were relatively consistent amongst samples of the same composition. These crystalline peaks had potential matches with several metallic elements, compounds and oxides. It is seen that a very small composition range exists for near-perfect bulk metallic glass formers (as defined by a
0% crystallinity). Scattering electron microscopy was also performed to understand aspects of individual second phase particles such as size and distribution. Departure of radial distribution results from those predicted by hard sphere models indicated intermediate range order.
2004 Elsevier B.V. All rights reserved.

1. Introduction 1.1. Background Scientific interest in amorphous metals dates at least back to 1963 when ribbons of glassy metals were formed and found to have interesting properties [1]. These amorphous alloys were processed via a Ôsplat-coolingÕ method whereby the melted components were catapulted onto a cooled surface in order to bypass the critical cooling rate for glass formation (105 K/s) [1–6]. This method nominally produced ÔribbonsÕ of 40– 60 lm thicknesses. While of great educational interest, these materials were not practical for real applications

*

Corresponding author. Tel.: +1 734 6472643. E-mail address: [email protected] (M.L. Tokarz).

0022-3093/$ - see front matter
2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.07.060

due to their complicated processing techniques, and subsequent small dimensions. Recent breakthroughs in glassy metals included the formation of ÔbulkÕ (thicker than 1 mm) forms of these alloys, Vitreloy being one of the first of this particular class of materials [7]. These glasses were typically multicomponent alloys comprised of elements of widely varying atomic numbers and atomic radii. Individual components were chosen to approximate eutectic compositions of systems characterized by ÔdeepÕ eutectics. This design parameter, along with the judicious selection of additional appropriate elements, created an alloy with the ability to bypass the equilibrium eutectic structure upon cooling. Because of the steep nature of this eutectic, the resulting amorphous structure could be formed with reasonable cooling rates (on the order of 1 K/s). This allowed the opportunity for production of these materials with methods usually reserved for non-metals. Specifically injection molding of Vitreloy and other

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types of metallic glasses have been successfully accomplished and the resulting materials exhibited high dimensional detail and increased stiffness over traditional alloys [8–10].

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tionally, low-divergent, high brilliance, synchrotron Xray sources have the unique ability to discern small amounts and small sizes of particles, which may not be observable with standard laboratory sources.

1.2. Refractory alloy glasses (RAGs) 2. Experimental details More recent interest has shifted to refractory alloy glasses (RAGs) where at least one constituent is characterized by a high melting point. The intention was to create amorphous metals with similar improved mechanical properties as traditional bulk metallic glasses (BMGs), but for use in high temperature applications. For this study, a series of Ni-based ternaries were investigated. Table 1 lists the constituents and some of their properties. The Ni–Nb phase diagram has a eutectic at 51.9 wt% Nb and 1178 C, which translates into an at % Ni of approximately 60 (40% Nb). From a melting point of pure Nb of 2477 C, this translates into a slope of 21.7/at.%, indicating a deep eutectic [11]. A small amount of a large atom, Sn, was added to this approximate composition in an attempt to disrupt any chemical ordering between Ni and Nb. The specific compositions studied are shown in Table 2. The slight variation of these compositions allowed us to get a handle on the processing limits of these materials. Several samples of each composition also provided information about the consistency of the processing procedure. 1.3. Characterization While bulk metallic glasses have promising properties, their characterization has been somewhat limited. Characterization techniques such as X-ray scattering and scattering electron microscopy (SEM) have advantages in their availability and relative ease of use. AddiTable 1 Components of Ni-based refractory alloy glasses for this study ˚) Constituent Z Atomic radius (A Tm (C) Ni Nb Sn

28 41 50

2.492 2.936 3.246

1455 2477 232

The refractory alloys glasses (RAGs) used in this study were prepared by our collaborators at the California Institute of Technology via a melt and injection mold process, the details of which are described elsewhere [12]. Several samples of each of these compositions were supplied. Wide angle X-ray scattering experiments were performed with both standard laboratory equipment at our facilities at the University of Michigan, as well as with a synchrotron source at Stanford Synchrotron Radiation Laboratory (SSRL––Beamlines 2-1 and 22). Our laboratory source is an 18 kW Rigaku Mo Ka ˚ ), and the results quoted here were target (k = 0.711 A obtained with a 1 mm collimator in order to reduce the source divergence to approximately 5 mrad. The source divergence of Beamline 2-1 (SSRL) is 100 lrad and the ˚ . In both cases, a radiation was tuned to k = 1.54 A LaB6 standard was used to calibrate 2h positions as well as for use in deconvolution calculations to remove the effects of beam divergence and other instrumental broadening from experimental results. The percentage crystallinity was determined for each sample via Gaussian peak fitting. Further, each crystalline peak was deconvoluted from instrumental broadening and grain sizes were calculated according to Scherrer and described in Stokes [13], Scherrer [14] and Warren [15]. The schematic in Fig. 1 illustrates how the experimental scattering curve is affected by instrumental broadening. f(y) represents the true scattering data (with broadening effects only due to grain size), and h(x) represents the shape of a peak from a standard with infinitely sized grains (for example LaB6). Together these create the observed pattern, g(z). One is typically interested in the shape of the true peak such that further grain size analyses can be performed. Since, each set of data can be related to their Fourier transforms (and assuming an interval of a/2 to a/2): f ðyÞ ¼

Table 2 Compositions of RAG samples studied

gðzÞ ¼

Designation

Composition

hðxÞ ¼

RAG1 RAG3 RAG4 RAG5

Ni60Nb37Sn3 Ni60Nb35Sn5 Ni59.35Nb34.45Sn6.2 Ni59.5Nb33.6Sn6.9

X X X

F ðnÞe2piny=a ;

ð1aÞ

0

Gðn0 Þe2pin z=a ; 00 x=a

H ðn00 Þe2pin

ð1bÞ :

ð1cÞ

Knowing that the h(x) pattern of the standard is also a convolution of instrumental effects and the true scattering pattern, we can state the following for h(x):

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Fig. 1. Schematic of the convolution of instrumental and real data [15], all shown with arbitrary intensity units. (a) represents ÔtrueÕ scattering, (b) instrumental, and (c) is the experimental, showing the convolution of f(y) and h(x).

hðxÞ ¼ 1=A

Z X

0

Gðn0 Þe2pin z=a

X

F ðnÞe2pinðxzÞ=a dz: ð2Þ

Then the following solutions can be used to find the real and imaginary portions of the Fourier series F(n):   F r ðnÞ ¼ ½H r ðnÞGr ðnÞ þ H i ðnÞGi ðnÞ = G2r ðnÞ þ G2i ðnÞ ; ð3aÞ   F i ðnÞ ¼ ½H i ðnÞGr ðnÞ  H r ðnÞGi ðnÞ = G2r ðnÞ þ G2i ðnÞ : ð3bÞ Finally, the real and imaginary components are combined to obtain the overall f(y): X f ðyÞ ¼ fF r ðnÞ cosð2pny=aÞ þ F i ðnÞ sinð2pny=aÞg:

packing [17–20]. Thus hard-sphere models were created using random pair–pair distributions based upon chemical formulas and were compared to experimental results. While not intended to be an exhaustive modeling effort, it nonetheless allowed an initial assessment of the applicability of hard-sphere models to these materials. A Radial Distribution Analysis was performed for each scattering pattern according to WarrenÕs ÔApproximate and Exact Methods for Sample with More Than One Kind of AtomÕ [21]. Because of the relationship between the electron density function and X-ray scattering, a Radial Distribution Function (RDF) can be defined for any set of data according to: Z 4pr2 qðrÞ ¼ 4pr2 q0 þ 2r=p k iðkÞ sinðrkÞ dk; ð6Þ

ð4Þ In this way, the true scattering data was obtained for each crystalline peak, followed by an analysis of the grain size according to Scherrer [14,15]: D  k=2B cos h;

ð5Þ

where D is the average grain dimension, k is the source wavelength, B is the peak width (usually defined as the full width at half of the maximum intensity, FWHM), and h is half of the 2h angle at which the peak occurs in the scattering pattern. A list of over 30 potential crystalline compounds was identified (including various oxides) and the corresponding diffraction patterns were obtained from the accompanying diffraction files and potential peak fits were identified [16]. Traditional modeling efforts to describe metallic glasses have involved the concepts of dense random

where r is the distance from an average atom, q(r) is atomic density as a function of r, q0 is the overall average atomic density, k is the scattering vector, and i(k) is the intensity function defined as: ðI eu =N  f 2 Þ=f 2 ;

ð7Þ

Ieu is the scaled intensity in electron units, N is an average atomic number, and f is the atomic scattering factor. Actual calculations of the RDFs were performed with Matlab software using a code developed by Sean Brennan and co-workers and included the necessary scattering data corrections for incoherent scattering, absorption, multiple scattering, and scaling to electron units [22–24]. Scattering electron microscopy (SEM) images were obtained with a Phillips X30 SEM instrument at the

M.L. Tokarz, J.C. Bilello / Journal of Non-Crystalline Solids 344 (2004) 110–118

University of Michigan on several representative samples. Both as-cast sample surfaces and internal fracture surfaces were scrutinized. Finally, where possible, comparisons were made to a well-characterized standard bulk metallic glass, Vitreloy-106 (Zr57Nb5Cu15.4Ni12.6Al10).

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Table 3 Crystallinity percentages for all RAG compositions Designation

Composition

% Crystallinity

RAG1 RAG3 RAG4 RAG5

Ni60Nb37Sn3 Ni60Nb35Sn5 Ni59.35Nb34.45Sn6.2 Ni59.5Nb33.6Sn6.9

7.5 ± 0.1 0 1.1 ± 0.1 1.0 ± 0.2

Errors were found from the standard deviation of several samples of the same composition.

3. Results 3.1. X-ray scattering Laboratory source scattering patterns illustrated the presence of at least four local maxima in all four compositions as can be seen from the (ii) curves of Fig. 2(a) through Fig. 2(d). In addition, this data suggested fully amorphous character as evidenced by the lack of crystalline peaks. In contrast, the (i) data in Fig. 2(a) through Fig. 2(d) is that from a low-divergent synchrotron source (d, divergence, 0.1 mrad, as opposed to 5 mrad for labsource) on the exact same samples as that shown for the (ii) curves. This data revealed the presence of residual crystallites superimposed on an amorphous pattern. Via Gaussian fitting of these peaks, the percent crystallinities of all compositions was found and indicated in Table 3.

As stated earlier, several samples of each composition were supplied such that an assessment as to the processing stability could be made. Fig. 3(a)–(c) shows several representative comparisons between different samples of the same composition. 3.2. Particulate analysis These figures show the scattering results from the same LaB6 standard for both X-ray sources used in this study. The LaB6 standard, having relatively large grains, can be used to estimate the instrumental broadening, of which the source divergence can have a major contribution. Scherrer analysis data from these compositions are summarized in Table 4 (Panel A–C). Note the absence of a corresponding table for RAG3 due to its lack of crystalline components.

Fig. 2. Representative X-ray scattering data of all RAG compositions obtained from (i) synchrotron source and (ii) laboratory rotating anode ˚ ). source. Geometry and energy constraints of synchrotron source only allowed collection of data to k = 6.5 (maximum 2h angle = 120, l = 1.54 A The black arrows in (a) indicate typical crystalline peaks (horizontal offsets are 40 for each).

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M.L. Tokarz, J.C. Bilello / Journal of Non-Crystalline Solids 344 (2004) 110–118 Table 4 Grain size analysis results for RAG1, RAG4 and RAG5 ˚ 1) Grain size (nm) Peak position (k scattering vector, A RAG1 1.384 2.032 2.294 2.592 2.770 2.826 2.872 2.940 4.692 5.016 5.176

15 29 21 74 65 43 66 43 45 37 15

RAG4 1.346 2.017 2.784 4.760

>120 >120 >120 101

RAG5 1.346 2.017 2.784 4.760

>120 >120 >120 >120

3.3. Scattering electron microscopy

Fig. 3. Comparisons of scattering patterns from multiple samples of (a) RAG1, (b) RAG3, and (c) RAG4 illustrating the good sample-tosample variabiliity (horizontal offsets are 50).

Several SEM images were taken to observe precise distributions of larger particles that could be present (as opposed to the X-ray scattering data which gives an overall average). Some of these initial scans can be found elsewhere [25]. Fig. 6(a) through Fig. 6(d) show representative SEM images obtained for these RAG samples, indicating a presence of particulates up to micron dimensions. 3.4. Crystalline peak fits As described in the experimental section, peak identifications were attempted by comparisons to Powder Diffraction Files. The best fits for the crystalline portions of RAG1, RAG4 and RAG5 are indicated in the Table 5 (Panel A–C). 3.5. Radial distribution analysis

Fig. 4. Schematic of the effects of source divergence; I0 is the incident beam, x is the incident angle, a is the horizontal divergence, p and d are pathlengths, and d is the final divergence in the 2h measurement.

Fig. 7(a) through Fig. 7(d) compare the experimental reduced radial distribution functions (RRDFs) and the corresponding model for a typical sample of each RAG composition. Fig. 8 is the corresponding data for Vitrelog-106. Table 6 summarizes all the information for the first three shells. While both Vitreloy-106 and RAG3 samples diverge from the random model, the magnitude of this divergence is much larger for the RAG3. (This trend

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Fig. 5. LaB6 standard diffraction patterns on identical sample from (a) laboratory source and (b) synchrotron source. The 2h positions and relative intensities correlate well, however, the large difference in peak widths indicated that the Darwin width is insignificant when compared to the differences in instrumental broadening.

Fig. 6. SEM Images of (a) RAG1 surface at 50· magnification, (b) RAG4 surface at 500· magnification, (c) RAG1 surface at 2000· magnification, and (d) RAG5 sheared fracture surface at 2000· magnification. Table 5 Potential peak matches for RAG1, RAG4 and RAG5 Chemical composition

JCPDS file No.

RAG1 (SnO)16O (NbNi3)8O SnNb2O6 NiSnO3

13–111 15–101 23–592 28–711

RAG4 (NbNi3)8O Ni3Sn

15–101 35–1362

RAG5 Sn2O3 NiSnO3 SnO2 Nb Ni3Sn

25–1259 28–711 33–1374 34–372 35–1362

is continued for the 4th and 5th nearest neighbor shells, but is not shown for clarity and brevity.)

4. Discussion The laboratory source allowed data collection to a k ˚ 1 and the scattering vector of approximately 6.5 A resulting scattering patterns illustrated the presence of at least four local maxima in all four compositions. This variance from the square of the average atomic scattering factor indicated the presence of at least this many nearest neighbor shells. In addition, this data suggested fully amorphous character. In contrast, the data from a low-divergent synchrotron source (d, divergence, 0.1 mrad, as opposed to

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Fig. 7. Reduced radial distribution functions (RRDF) for (a) RAG1, (b) RAG3, (c) RAG4 and (d) RAG5 comparing (i) hard-sphere model data and (ii) experimental data for each (horizontal offset = 12 for each).

nk ¼ 2d hkl sin h;

Fig. 8. Reduced radial distribution functions (RRDF) for Vitreloy-106 standard, comparing (i) hard-sphere model data and (ii) experimental data for each (horizontal offset = 10).

5 mrad for labsource) revealed the presence of residual crystallites superimposed on an amorphous pattern. This clearly illustrates the need for a high intensity and high resolution sychrotron source to detect such small amounts and sizes of crystallites, respectively. Only the RAG3 showed fully amorphous structure. Given the very similar compositions, this suggests a very narrow composition range necessary for producing ÔidealÕ metal glasses of these particular alloy components. The similarity between samples of the same composition in both the relative amounts of crystalline portions, and the crystalline peak positions, indicated good processing control. Fig. 4 depicts the effect of source divergence at high and low 2h angles. Classical X-ray scattering theory is based upon an assumption of a planar wavefront, giving rise to Bragg reflections at 2h values corresponding to the interplanar distances according to:

ð8Þ

where n is an integer, k is the source wavelength, dhkl is the interplanar spacing of hkl planes, and h is half the 2h scattering angle. However experimental setups are such that a small amount of source divergence is always present. This increases the ÔfootprintÕ, or the amount of the sample illuminated by the incident X-ray beam, and increases the spread of the reflected radiation that is detected for any give incident angle. This has the ultimate effect of peak broadening and is evident in Fig. 5(a) and (b). Note the relatively wider LaB6 peaks from the laboratory source due to the larger source divergence. The observed peaks from each RAG composition were modified to remove instrumental broadening via the Stokes deconvolution method described by Schwartz and Cohen [25]. Then a Scherrer analysis was performed to determine crystallite dimensions. The data from these compositions are summarized in Table 4. While scattering electron microscopy does not allow resolution of nanometer size particulates, it does indicate the presence of second phase particles throughout the material. This is additionally borne out by transmission X-ray diffraction studies, which indicate small amounts of nanocrystallites within an amorphous matrix [26]. Reasonably close agreement to JCPDS files were found for the crystalline portions of RAG1, RAG4 and RAG5 as indicated in the Table 5. As explained earlier, the X-ray scattering pattern is related to the Fourier transform of the electron density function, such a manipulation can give information about the degree of short-range order. Specifically Radial Distribution Functions (RDFs) were obtained for each data set, and nearest neighbor distances, and numbers of shell atoms were extracted from these functions.

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Table 6 Radial distribution information for RAG compositions and Vitreloy-106 for first three nearest neighbor shells Shell 1

Shell 2

# atoms

˚) r (A

RAG1 (Ni60Nb37Sn3) Beamline 2-1 CNS Model

10.6 ± 0.5 10.2 ± 0.5 11.5 ± 1.2

RAG3 (Ni60Nb35Sn5) Beamline 2-1 Model CNS

Shell 3

# atoms

˚) r (A

# atoms

˚) r (A

2.8 ± 0.2 2.7 ± 0.3 2.7 ± 0.3

27.4 ± 1.4 26.5 ± 1.3 33.9 ± 1.2

4.7 ± 0.2 4.6 ± 0.5 5.4 ± 0.3

48.8 ± 2.5 44.2 ± 2.3 62.4 ± 1.2

6.9 ± 0.2 6.8 ± 0.7 8.0 ± 0.3

11.0 ± 0.6 10.4 ± 0.5 11.6 ± 1.2

2.8 ± 0.2 2.7 ± 0.3 2.7 ± 0.3

27.8 ± 1.4 26.7 ± 1.4 33.6 ± 1.2

4.7 ± 0.2 4.7 ± 0.5 5.5 ± 0.3

49.0 ± 2.4 46.3 ± 2.3 63.6 ± 1.2

6.8 ± 0.2 6.8 ± 0.7 8.2 ± 0.3

RAG4 (Ni59.35Nb34.45Sn6.2) Beamline 2-1 10.6 ± 0.6 CNS 11.7 ± 0.6 Model 11.9 ± 1.2

2.8 ± 0.2 2.8 ± 0.3 2.7 ± 0.3

27.3 ± 1.4 31.5 ± 1.6 32.9 ± 1.2

4.7 ± 0.2 5.0 ± 0.5 5.4 ± 0.3

48.2 ± 2.4 53.4 ± 2.7 61.7 ± 1.2

6.9 ± 0.2 7.1 ± 0.7 8.1 ± 0.3

RAG5 (Ni59.5Nb33.6Sn6.9) Beamline 2-1 10.8 ± 0.5 CNS 10.6 ± 0.5 Model 11.8 ± 1.2

2.8 ± 0.2 2.8 ± 0.3 2.7 ± 0.3

25.9 ± 1.3 37.1 ± 1.9 31.7 ± 1.2

4.7 ± 0.2 4.8 ± 0.5 5.4 ± 0.3

48.0 ± 2.4 55.5 ± 2.8 63.1 ± 1.2

6.9 ± 0.2 7.0 ± 0.7 8.1 ± 0.3

Vit-106 (Zr57Nb5Cu15.4Ni12.6Al10) CNS 9.4 ± 0.5 Model 10.7 ± 1.2

2.9 ± 0.3 3.0 ± 0.3

26.0 ± 1.3 32.7 ± 1.2

4.9 ± 0.5 6.0 ± 0.3

50.2 ± 2.5 56.0 ± 1.2

7.2 ± 0.7 9.0 ± 0.3

Includes numbers of nearest neighbors and shell distances.

RAG3, with three components has six potential pair interactions (Ni–Ni, Nb–Nb, Ni–Nb, Ni–Sn, and Nb– Sn), as compared to the 15 potential pair interactions of the five-component Vitreloy-106, thus providing for a much simpler hard-sphere model. Yet the Vitreloy106 shows better agreement to its corresponding model.

complicated Vitreloy model was actually in better agreement with experimental data. Because this model was based upon an assumption of random pairing from chemical formulas, this suggests the presence of nonrandom pair ordering, which needs to be explored with more sophisticated techniques [27].

5. Conclusions

Acknowledgments

The presence of at least four local maxima for all Nibased RAG compositions was found via two X-ray scattering experiments from two different sources. Additionally, the low-divergent synchrotron source was able to demonstrate the existence of nano-crystalline residuals in all but one RAG composition, which was not observable using a laboratory X-ray source. Additionally these results were confirmed with scattering data from several samples of the same composition, illustrating the stability of the processing methods. These crystalline peaks also indicate the narrow composition range necessary for near-perfect glass formability (as defined by 0% crystallinity) for Ni-based Refractory Alloy Glasses (RAGs). The presence of 2nd phase particles within an amorphous matrix was confirmed by both transmission experiments and SEM (Scattering Electron Microscopy). Both of these methods were able to give Ôthrough-thicknessÕ characterization information. A random hard-sphere model was inadequate in describing the short-range order of both the RAG compositions and a standard Vitreloy; however, the more

Thanks to DARPA for support of this work under a subcontract form California Institute of Technology under #DAAD19-01-0525. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the US Department of Energy, Office of Basic Energy Sciences. Additionally, the technical assistance of Zofia Rek, Apurva Mehta, and Sean Brennan, was invaluable in the use of Beamlines 2-2, 21, and 7-2, respectively. Thanks also to Professor W.L. Johnson, Dr D. Conner, and Dr C.-Y. Haein for their insights into the processing of Bulk Metallic Glasses during a Sabbatical leave spent at California Institute of Technology (by John Bilello).

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