Phase separation in Ni70Nb30−xYx glasses

Intermetallics 18 (2010) 1842e1845

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Phase separation in Ni70Nb30
xYx glasses N. Mattern a, *, U. Vainio b, B. Schwarz a, J.M. Park a, c, D.H. Kim c, J. Eckert a, d a

IFW Dresden, Institute for Complex Materials, P.O. Box 270116, D-01171 Dresden, Germany HASYLAB at DESY, Notkestr. 85, D-22603 Hamburg, Germany c Center for Non-crystalline Materials, Department of Metallurgical Engineering, Yonsei University, Seoul, Republic of Korea d TU Dresden, Institute of Materials Science, D-01062 Dresden, Germany b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 November 2009 Received in revised form 8 February 2010 Accepted 27 February 2010 Available online 25 March 2010

Phase-separated metallic glasses in the ternary NieNbeY system were prepared by rapid quenching of the melt. For Ni-rich alloys, an early stage of spinodal decomposition or an almost homogeneous glassy state is obtained due to a lowering of the critical temperature of the liquideliquid phase separation near to the glass transition temperature. In-situ small-angle X-ray scattering at elevated temperature gives evidence for ongoing phase separation of the glass prior to crystallization. The structural changes during isothermal heat treatment point to a spinodal mechanism of the decomposition. For glass with low Y-content (5 at%), no indication for phase separation is found in accordance with the composition dependence of the metastable miscibility gap of the supercooled liquid. Upon heating, the phase separated glass acts as a precursor and is the reason for the nanostructure of the Ni2Y-phase formed during the first stage of crystallization. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: A. ternary alloy systems B. thermal properties C. rapid solidification glass F. diffraction

1. Introduction Decomposition and phase separation in the liquid occur in several binary alloy systems due to a large positive enthalpy of mixing between elements like NbeY, AleIn, and PbeAl [1]. For binary systems with only a small positive of mixing like CoeCu [2] and FeeCu [3], a miscibility gap exists in the metastable undercooled liquid below the liquidus temperature. In bulk metallic glass-forming alloys the addition of elements with strong positive enthalpy of mixing leads to an improvement of ductility [4] or glass forming ability [5] in a certain composition range. Phase-separated metallic glasses have been obtained by rapid quenching of the melt in several alloy systems: ZreYeAleNi [6], ZreLaeAleNieCu [7], ZreYeCoeAl [8], NieNbeY [9], ZreNdeAleCo [10], Cue(Zr,Hf)e (Gd,Y)-Al [11], and Zre(Ce,Pr,Nd)eAleNi [12]. Phase separation of the liquid is determined by the thermodynamic properties of the alloy system. For the phase-separated metallic glasses reported in [6e12], the critical temperature of liquideliquid phase separation Tc is above the liquidus temperature. In this case, phase separation leads to a special microstructure with a length scale up to several micrometers and some self-similar features are observed by transmission electron microscopy [6,13,14]. The materials represent frozen-in late states, which have passed decomposition and growth

* Corresponding author. Tel.: þ49 351 4659 367; fax: þ49 351 4659 452. E-mail address: [email protected] (N. Mattern). 0966-9795/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.02.035

of the melt by diffusion and coalescence as well as secondary precipitation reactions. Recently, we succeeded in preparing phase separated glasses in the NieNbeY system with a fluctuation length in the nanometer range [15]. With increasing Ni-content the critical temperature TC drops near to the glass transition temperature, which enables freezing in the early stages of decomposition. In this paper, we report on the influence of the yttrium content on the formation and transformation of the early stages of phase separation in Ni70Nb30
xYx (x ¼ 5, 10, 15) glasses. In-situ small-angle X-ray scattering at elevated temperatures in combination with simultaneous X-ray diffraction was applied in order to analyze the temperature and time dependence of the decomposition. With regard to bulk metallic glasses the question viewed is to which extend phase separation occurs if the concentration of the element responsible for phase separation becomes low. 2. Experimental Pre-alloyed ingots were prepared by arc-melting elemental Ni, Nb and Y with purities of 99.9% or higher in a Ti-gettered argon atmosphere. To ensure homogeneity, the samples were remelted several times. From these pre-alloys, thin ribbons (3 mm in width and 30 mm in thickness) with nominal compositions Ni70Nb15Y15, Ni70Nb20Y10, and Ni70Nb25Y5 were prepared by single-roller melt spinning under argon atmosphere. The casting temperature was 1923 K. X-ray diffraction (XRD) patterns were recorded in Bragg

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Brentano geometry with CoKa radiation using a Panalytical X'Pert Pro diffractometer. Differential scanning calorimetry (DSC) experiments were performed employing a Netzsch DSC 404 calorimeter (heating rate: 20 K/min). Small angle X-ray scattering (SAXS) data were measured at the B1 synchrotron beam line of HASYLAB/DESY using an energy of 16 516 eV. For these experiments, the samples were mounted on a heating stage, which was used under vacuum for in-situ measurements. Intensity curves were registered by a PILATUS 300k area detector at a distance covering a q-range between 0.2 and 4.3 nm
1. The temperature was stepwise increased. For each temperature, the background was measured followed by a calibration standard (glassy carbon) and subsequent measurement of the sample. For isothermal analysis, the samples were heated at 100 K/min up to the corresponding temperature and the measurements were immediately started (5 min or 15 min per pattern) and repeated up to 10 h. Simultaneously, the wide-angle Xray Scattering (WAXS) was recorded by a linear position sensitive detector (MYTHEN). 3. Results The XRD patterns of as-quenched Ni70Nb15Y15 ribbons are shown in Fig. 1. The diffuse character of the scattering curves for all the as-quenched samples reveals the amorphous state. The amorphous structure was also confirmed by high resolution transmission electron microscopy (TEM), local electron diffraction, and DSC. TEM images of Ni70Nb15Y15 glass exhibit a “homogeneous” microstructure [14,15]. Fig. 2 shows the DSC scans of the glasses. Two crystallization events are visible for Ni70Nb15Y15 and Ni70Nb20Y10. XRD measurements of samples heated in the DSC up to the characteristic temperatures allowed to identify the first exothermic DSC event as crystallization of a Ni2Y nanoscale phase (about 5 nm in size, as indicated by broadened reflections and also confirmed by TEM [14]), and the second peak as the crystallization of NbNi3, respectively. Simultaneous with the formation of NbNi3 the reflections of Ni2Y become sharper indicating the growth of the former nanocrystals. In contrast, the Ni70Nb25Y5 glass exhibits only one single exothermic DSC event corresponding to an eutectic crystallization of the glass into Ni2Y þ NbNi3. In order to analyze the homogeneity of the Ni70Nb30
xYx glasses, SAXS measurements

Fig. 1. XRD patterns of rapidly quenched Ni70Nb30
xYx alloys (x ¼ 5, 10, 15).

Fig. 2. DSC scans (heating rate 20 K/min) of rapidly quenched Ni70Nb30
xYx alloys (x ¼ 5, 10, 15).

were done at different temperatures. Especially the simultaneous registration of WAXS patterns should reveal whether phase separation occurs before crystallization. SAXS provides integral information on existing inhomogeneities in electron density with a size ranging from the nanometer up to the micrometer range. The calibrated small-angle X-ray scattering curves depend on differences in electron density, but also on the volume fraction, size, and shape of the inhomogeneities built up from the different phases [16]:

ds ðqÞ ¼ dU

Z

~2 ðrÞ 4p$r 2 h

sinðqrÞ dr; qr

(1)

where ds/dU(q) is the differential cross section, q is the magnitude of the scattering vector (q ¼ 4p sin q/l, scattering angle 2q, wave~2 ðrÞ ¼ ~ rðrÞ2
V r20 is the square of the so-called electron length l), h density fluctuation, which is given by the difference of the electron scattering-length density r(r) and the average electron scatteringlength density r0 in the sample volume V. Figs. 3 and 4 show the obtained SAXS curves for two of the Ni70Nb30
xYx glasses at elevated temperatures. The simultaneously measured WAXS patterns are also given in the insets. The as-cast states are characterized by a very weak SAXS intensity pointing to an almost homogeneous structure. The intensity increase below q < 0.8 nm
1 probably originates from surface scattering. With rising temperatures a pronounced maximum becomes evident for the Ni70Nb15Y15 glass, which increases in height and shifts in position to lower qvalues. Such a maximum is indicative of a dominant correlation length. By using the relationship z ¼ 2p/qmax between the correlation length z and the position of the maximum one obtains values of z between 5 and 10 nm. The comparison with the XRD data reveals that the crystallization of the nanocrystalline Ni2Y phase is clearly reflected by the SAXS data. The shift of the interference maximum to lower q-values indicates growth, which is also seen by the reduction of the width of the reflections in the XRD patterns. The Ni70Nb20Y10 glass shows a similar development of the SAXS patterns with temperature. However, the Ni70Nb25Y5 glass exhibits a different behaviour. No change of the SAXS intensity (Fig. 4) is observed below the crystallization temperature. First, the eutectic crystallization of the amorphous phase at Tx ¼ 823 K leads to

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Fig. 3. In-situ SAXS and WAXS (inset) at different temperatures for glassy Ni70Nb15Y15.

a heterogeneous microstructure and to the increase of the SAXS intensity. Fig. 5 compares the temperature dependence of the integral (sum) of the SAXS intensities between q ¼ 0.2 and 3.0 nm
1 for the different glasses. For Ni70Nb15Y15 and Ni70Nb20Y10, an increase of the SAXS intensities is observed well below the crystallization temperature. The beginning crystallization is expressed by a change of the slope of the SAXS versus T curves. Obviously, for the Ni70Nb15Y15 and Ni70Nb20Y10 glasses phase separation occurs before crystallization. On the other hand, for the Ni70Nb25Y5 glass no indication for phase separation in the amorphous state can be detected from the SAXS measurements. To analyze the time dependence of the decomposition, isothermal measurements of Ni70Nb15Y15 and Ni70Nb20Y10 glasses were conducted at different temperatures. Fig. 6 shows the SAXS measurements of Ni70Nb20Y10 glass at T ¼ 753 K versus time. The WAXS patterns at the beginning and after a characteristic time, as well as the difference curve, are presented in the inset in Fig. 6. The SAXS intensity increases during

Fig. 4. In-situ SAXS and WAXS (inset) at different temperatures for glassy Ni70Nb25Y5.

Fig. 5. SAXS intensity of Ni70Nb30
xYx glasses (x ¼ 5, 10, 15) versus temperature during step-wise heating.

isothermal annealing indicating ongoing phase separation. The WAXS curve does not change, as can be seen by the difference curve DI ¼ It¼ 360 min
It¼ 15 min. For Ni70Nb20Y10 glass at T ¼ 773 K, the beginning formation of Ni2Y is visible in the XRD pattern already after 10 min. Fig. 7 compares the behaviour of the height I(qmax) and the position qmax of the SAXS intensity maximum versus time. For temperatures below the onset of crystallization (T  753 K), the integrated intensity increases and saturates for longer annealing times. For temperatures above the crystallization temperature (T  773 K), a much faster and stronger increase is observed due to the crystallization. The different mechanisms can clearly be distinguished by the way the interference maximum at qmax changes its position with time, staying constant for annealing below crystallization, but decreases with crystallization. 4. Discussion In order to understand the structure formation of the rapidly quenched alloys one has to analyze the phase diagram. In the

Fig. 6. In-situ SAXS and WAXS (inset) of glassy Ni70Nb20Y10 at T ¼ 753 K versus time.

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experimental SAXS data (Fig. 7), the position of the maximum of I (q) does not change below crystallization. This means that for the Ni70Nb15Y15 glass the fluctuation length remains constant and the amplitude of the fluctuation increases during the decomposition process. This behaviour points to a spinodal decomposition mechanism in the NieNbeY glasses. On the other hand, crystallization is accompanied by nucleation and growth, which is related to the shift of the maximum position qmax (Fig. 7). The continuous transition in the SAXS data demonstrates that the phase separated glass acts as a precursor for subsequent nanocrystallisation. Due to the ongoing decomposition, the Y-content increases in the Yenriched amorphous NieY(Nb) phase. The crystallization temperature of the NieNbeY glass decreases with Y-content (Fig. 2). Hence, nucleation of Ni2Y probably starts from the composition fluctuations with highest Y-content leading to a nanocrystalline microstructure in the first crystallization step. 5. Conclusions

Fig. 7. Intensity and position qmax of SAXS maximum of glassy Ni70Nb30
xYx versus time.

ternary NieNbeY system a miscibility gap exists in the equilibrium melt ranging from the binary Nb-Y liquid up to about 60 at.% Ni [17]. For higher Ni-contents, the critical temperature TC drops below the liquidus temperature resulting in a metastable miscibility gap. Fig. 8 shows a pseudo-binary section of the ternary NieNbeY phase diagram calculated by the CALPHAD method using the thermodynamic description given in [17]. The bimodal curve is also depicted. The critical temperature of liquideliquid phase separation TC is rather low and only slightly different from the glass transition temperature Tg. From this picture it is understandable that during rapid quenching early stages of spinodal decomposition or even an almost homogeneous glass can be frozen-in if crystallization is avoided. Subsequently annealing the glass at elevated temperatures below the crystallization temperature leads to phase separation by nucleation and growth or by spinodal mechanisms, or to further development of already frozen-in fluctuations [18,19]. The spinodal decomposition is initiated via the spontaneous formation and subsequent growth of coherent composition fluctuations. In the

Phase-separated metallic glasses can be prepared in the ternary NieNbeY system by rapid quenching of the melt. The microstructure formed is essentially determined by the critical temperature TC of the liquideliquid phase separation, which is a function of the chemical composition of the alloy. An almost homogeneous glassy state or early stages of spinodal decomposition are obtained if TC is near to the glass transition temperature. Small-angle X-ray scattering is a powerful tool for detecting fluctuations in the nanometer range. Especially in-situ measurements at elevated temperature provide evidence for phase separation prior to crystallization, and from the time dependence of the structural parameters during isothermal heat treatment a spinodal decomposition mechanism is concluded. Phase separation acts as a precursor for nanocrystallization. For the low Y-content, a complete solubility is found in agreement with the composition dependence of the miscibility gap. Acknowledgement The authors thank S. Donath, M. Frey and B. Opitz for sample preparation. Financial support of the Deutsche Forschungsgemeinschaft DFG (project Ma1531/10) and through the Global Research Laboratory Program of the Korea Ministry of Education, Science and Technology is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

Fig. 8. Pseudo-binary section of Ni70Nb30
xYx phase diagram.

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