Phase Separation in Ce-Based Metallic Glasses

*Dharmendra Singh, Kiran Mor, Devinder Singh and Radhey Shyam Tiwari*

## **Abstract**

In this chapter, the results of our recent studies on the role of Ga substitution in place of Al in Ce75Al25 − xGax (x = 0, 0.01, 0.1, 0.5, 1, 2, 4, and 6) metallic glasses (MGs) have been discussed with the aim to understand the genesis of phase separation. X-ray diffraction (XRD) study reveals two broad diffuse peaks corresponding to the coexistence of two amorphous phases. In order to see any change in the behavior of 4*f* electron of Ce, X-ray absorption spectroscopy (XAS) has been carried out for Ce75Al25 − xGax MGs. From the XAS results, it is evident that for x = 0, the spectrum exhibits only a 4*f* 1 component, which basically shows a pure localized configuration of electron. After the addition of Ga, 4*f* electrons of Ce atoms denoted by 4*f* 0 are getting delocalized. Thus, the phase separation in Ce75Al25 − xGax is taking place, owing to the formation of two types of amorphous phases having localized and delocalized 4*f* electrons of Ce atoms, respectively. It has been discussed how change in the electronic structure of Ce atoms may lead to phase separation in Ce75Al25 − xGax alloys. Extensive TEM investigations have been done to study the phase separation in these alloys. The microstructural features have been compared with those obtained by phase field modeling.

**Keywords:** metallic glass, phase separation, X-ray absorption spectroscopy, transmission electron microscopy, phase field modeling

### **1. Introduction**

In the past decades, considerable research attention has been given to rare-earth (RE)-based metallic glasses (MGs) due to their novel physical properties such as glass-forming ability [1] and mechanical [2, 3], magnetic [4], superplastic [5], and thermoplastic properties [6]. Thus, these MGs hold potential in many applications in the future. Many novel RE-based MGs, e.g., Ce-, La-, Y-, Er-, and Sm-based MGs, have been synthesized [7]. Among RE-based MGs, Ce-based MGs are of special interest due to their unusual behavior linked to 4*f* electrons [8]. Ce is the most abundant RE metal on earth. It is also one of the most reactive RE metal and oxidizes very readily even at room temperature. One of the key features of Ce is its variable valance states and electronic structure [9–11]. Thus to change the relative occupancy of the electronic levels, only a small amount of energy is required, e.g., a volume change of approximately 10% results when Ce is subjected to high pressure or low temperatures [9, 11]. Therefore, Ce-based MGs may possess structural and physical properties which are different from other known MGs [12].

Recently, a pressure-induced devitrification behavior of Ce75Al25 MG ribbon has been reported [13–15]. Prior to our study, only few studies have been done on the substitution and mechanical behavior of Ce75Al25 glassy alloy [1, 16].

Any approach to the description of the amorphous structure suggests that it is a homogeneous isotropic structure. In fact, it turned out that the structure of amorphous phase in alloys cannot always be uniform and isotropic. One situation occurs in the case when the amorphous phase contains two or more metals with comparable scattering amplitude. In such systems, the appearance of inhomogeneity areas or two types of amorphous phases is much more pronounced, since the formation of regions with different chemical compositions leads to the appearance of at least two types of shortest distances between atoms, which naturally results in the phase separation and also affects various properties. The first report by Chen and Turnbull [17] on phase separation in Pd-Au-Si alloy has attracted considerable attention due to their unique microstructural variation of amorphous phases at different length scales. Following this, the possibility of phase separation in MG compositions has been investigated by many authors [18–20]. However, such a phase separation is incompatible with the glass-forming criteria of negative heat of mixing [21]. The models of MGs based on the nature of geometrical clusters [22] may be helpful in comprehending phase separation in these alloys. According to this model, the MGs have geometry incompatibility in main clusters with long-range translational orders and are joined by the cementing cluster known as glue cluster [23–33]. Sohn et al. reported two general schemes for the design of phase-separating MGs [34]. The first scheme refers to the selection of atom pairs having positive enthalpy of mixing, and the second one refers to the selection of additional alloying element which can enhance glass-forming ability. In the case of ternary- and higher-component alloys, the opposite nature of enthalpy of mixing between the pairs of binaries is possible. In MG systems phase separation will be due to the complex interplay of positive and negative enthalpies of mixing, e.g., in Gd-Zr-Al-Ni Mg alloy system, the enthalpy of mixing is positive for Gd-Zr atom pairs, and other pairs consist of negative enthalpy of mixing [34]. That's why phase separation is shown by MG system in amorphous state. Phase separation is exhibited by many alloy systems such as La-Zr-Al-Cu-Ni [35], Zr-Ti-Ni-Cu-Be [36], Zr-Gd-Co-Al [37], Cu-(Zr,Hf)-(Gd,Y)-Al [38], Cu-Zr-Al-Nb [39], and Gd-Hf-Co-Al [40]. However, there are very few ternary systems reported in literature which show phase separation. Wu et al. have studied ternary Pd-Ni-P alloy system and observed phase separation through spinodal decomposition [41]. It is worthwhile to mention here that so far no report is available prior to our present study where very sparse atomic percent (~ 0.01 at.%) addition of an element leads to phase separation in a binary system.

In this chapter, we present extensive investigations of amorphous phase formation in Ce75Al25 − xGax alloys with a wide range of concentration of Ga (x = 0, 0.01, 0.1, 0.5, 1, 2, 4, and 6). Both Al and Ga are having the same valency (+3), comparable atomic radii (Ga, 1.41 Å; Al, 1.43 Å), and lying in the same group of the periodic table. Thus, the substitution of Al by Ga does not change the e/a ratio of Ce-Al alloy system (e/a = 1.39). It has been undertaken with a view to understanding the genesis of phase separation in this alloy system. The microstructural features arise due to phase separation which has been studied by transmission electron microscopy (TEM) and compared with those obtained by phase field modeling. The role of Ce electronic structure in phase separation has been discussed. It is important to mention that due to change in the electronic states of Ce, 4*f* electrons under high pressure, Ce75Al25 alloy undergoes polyamorphic transition [13, 42, 43]. One may expect that chemical pressure effect of Ga substitution in Ce75Al25 MG leads to change in the electronic structure of the Ce in this alloy [44]. Chemical pressure effect basically deals with the change in the electronic structure of atoms due to pressure,

**47**

20G2

**and 6) alloys**

*Phase Separation in Ce-Based Metallic Glasses DOI: http://dx.doi.org/10.5772/intechopen.88028*

this we refer the readers to reference [27].

**2. Materials and experimental procedure**

source (2.5 GeV, 100 mA), at RRCAT, India.

temperature, or alloying addition. Keeping these facts in view, extensive use of X-ray absorption spectroscopy (XAS) has been done to investigate Ce75Al25 − xGax alloys. Our investigations have clearly demonstrated that two types of short range order (SRO) may set in Ce75Al25 − xGax amorphous alloys [23]. This is due to delocalization of 4*f* electron with addition of Ga. The change in the electronic structure of Ce is considered as one of the important reasons for the phase separation in Ce-Al-Ga MG alloy system. The remarkable change in the behavior of glass transition with Ga substitution has been observed through DSC investigation [25–30]. The thermal stability of the studied materials has been discussed elsewhere, and for

In this chapter, the effect of Ga substitution (with x as low as 0.01 at.%) on the phase separation has been discussed. The substitution of Ga at place of Al in various alloy systems has been extensively studied by our group [45–50]. The Ce-Al [51] and Ce-Ga [52] binaries have negative heat of mixing, while Ga-Al pair has very low positive heat of mixing, i.e., 0.7 KJ/mol [53]. It seems unlikely that the phase separation is caused by Ga-Al which has a very small positive heat of mixing. Hence, the alternative explanation for this has been called for. One may thus expect that the substitution of Ga on Al sites may lead to change in the electronic behavior of Ce 4*f* electrons (owing to chemical pressure effect) [54]. We have also discussed the effect of Ga substitution on the formation of nanoamorphous domains as well as on the nature of Ce 4*f* electronic states. It should be pointed out that pressure-induced delocalization of 4*f* electron (using XAS studies) has also been reported by other researchers [13, 42]. However, the partial delocalization of 4*f* electron of Ce atoms in Ce75Al25 − xGax alloys due to Ga substitution has been pointed out for the first time based on XAS studies.

The details of the preparation methods of Ce75Al25 − xGax melt-spun alloys are reported elsewhere [2, 21]. The structural characterization has been carried out using X–ray diffractometer (X'Pert Pro PANalytical diffractometer) with CuK<sup>α</sup> radiation. The electrolyte with 70% methanol and 30% nitric acid at 253 K has been used to thin the ribbons for TEM characterization. The TEM using FEI: Tecnai

electron microscope has been used to observe the thinned samples. Energy-

at 200 keV using 100 seconds exposure time and 4 μA beam current. The X-ray absorption spectroscopy (XAS) measurements on these samples at Ce L3 edge were carried out in fluorescence mode with beamline (BL-9), INDUS-2 synchrotron

is obtained

dispersive X-ray analysis (EDX) attached to the TEM Tecnai 20 G2

**3. Investigation of Ce75Al25 − xGax (x = 0, 0.01, 0.1, 0.5, 1, 2, 4,** 

**3.1 A comparative X-ray diffraction investigation of Ce75Al25 − xGax alloys**

**Figures 1** and **2** show the XRD patterns of Ce75Al25 − xGax alloys at different Ga concentrations. For the alloy with x = 0, the broad halo peak is found within the angular range 28–35**°**. This indicates the formation of homogenous glassy phase in Ce75Al25 alloy. While for the alloys with x = 2–6, broad halo peak is found within the angular range 39–50**°**. The unusual effect was seen in the XRD pattern on substitution of 0.01 at.% Ga. The second diffuse peak with higher intensity can be seen at higher-angle side. With increase in the quantity of Ga (x = 0.1, 0.5, 1, 2, 4, and 6),

#### *Phase Separation in Ce-Based Metallic Glasses DOI: http://dx.doi.org/10.5772/intechopen.88028*

*Metallic Glasses*

Recently, a pressure-induced devitrification behavior of Ce75Al25 MG ribbon has been reported [13–15]. Prior to our study, only few studies have been done on the

Any approach to the description of the amorphous structure suggests that it is a homogeneous isotropic structure. In fact, it turned out that the structure of amorphous phase in alloys cannot always be uniform and isotropic. One situation occurs in the case when the amorphous phase contains two or more metals with comparable scattering amplitude. In such systems, the appearance of inhomogeneity areas or two types of amorphous phases is much more pronounced, since the formation of regions with different chemical compositions leads to the appearance of at least two types of shortest distances between atoms, which naturally results in the phase separation and also affects various properties. The first report by Chen and Turnbull [17] on phase separation in Pd-Au-Si alloy has attracted considerable attention due to their unique microstructural variation of amorphous phases at different length scales. Following this, the possibility of phase separation in MG compositions has been investigated by many authors [18–20]. However, such a phase separation is incompatible with the glass-forming criteria of negative heat of mixing [21]. The models of MGs based on the nature of geometrical clusters [22] may be helpful in comprehending phase separation in these alloys. According to this model, the MGs have geometry incompatibility in main clusters with long-range translational orders and are joined by the cementing cluster known as glue cluster [23–33]. Sohn et al. reported two general schemes for the design of phase-separating MGs [34]. The first scheme refers to the selection of atom pairs having positive enthalpy of mixing, and the second one refers to the selection of additional alloying element which can enhance glass-forming ability. In the case of ternary- and higher-component alloys, the opposite nature of enthalpy of mixing between the pairs of binaries is possible. In MG systems phase separation will be due to the complex interplay of positive and negative enthalpies of mixing, e.g., in Gd-Zr-Al-Ni Mg alloy system, the enthalpy of mixing is positive for Gd-Zr atom pairs, and other pairs consist of negative enthalpy of mixing [34]. That's why phase separation is shown by MG system in amorphous state. Phase separation is exhibited by many alloy systems such as La-Zr-Al-Cu-Ni [35], Zr-Ti-Ni-Cu-Be [36], Zr-Gd-Co-Al [37], Cu-(Zr,Hf)-(Gd,Y)-Al [38], Cu-Zr-Al-Nb [39], and Gd-Hf-Co-Al [40]. However, there are very few ternary systems reported in literature which show phase separation. Wu et al. have studied ternary Pd-Ni-P alloy system and observed phase separation through spinodal decomposition [41]. It is worthwhile to mention here that so far no report is available prior to our present study where very sparse atomic percent (~ 0.01 at.%) addition of an

substitution and mechanical behavior of Ce75Al25 glassy alloy [1, 16].

element leads to phase separation in a binary system.

In this chapter, we present extensive investigations of amorphous phase formation in Ce75Al25 − xGax alloys with a wide range of concentration of Ga (x = 0, 0.01, 0.1, 0.5, 1, 2, 4, and 6). Both Al and Ga are having the same valency (+3), comparable atomic radii (Ga, 1.41 Å; Al, 1.43 Å), and lying in the same group of the periodic table. Thus, the substitution of Al by Ga does not change the e/a ratio of Ce-Al alloy system (e/a = 1.39). It has been undertaken with a view to understanding the genesis of phase separation in this alloy system. The microstructural features arise due to phase separation which has been studied by transmission electron microscopy (TEM) and compared with those obtained by phase field modeling. The role of Ce electronic structure in phase separation has been discussed. It is important to mention that due to change in the electronic states of Ce, 4*f* electrons under high pressure, Ce75Al25 alloy undergoes polyamorphic transition [13, 42, 43]. One may expect that chemical pressure effect of Ga substitution in Ce75Al25 MG leads to change in the electronic structure of the Ce in this alloy [44]. Chemical pressure effect basically deals with the change in the electronic structure of atoms due to pressure,

**46**

temperature, or alloying addition. Keeping these facts in view, extensive use of X-ray absorption spectroscopy (XAS) has been done to investigate Ce75Al25 − xGax alloys. Our investigations have clearly demonstrated that two types of short range order (SRO) may set in Ce75Al25 − xGax amorphous alloys [23]. This is due to delocalization of 4*f* electron with addition of Ga. The change in the electronic structure of Ce is considered as one of the important reasons for the phase separation in Ce-Al-Ga MG alloy system. The remarkable change in the behavior of glass transition with Ga substitution has been observed through DSC investigation [25–30]. The thermal stability of the studied materials has been discussed elsewhere, and for this we refer the readers to reference [27].

In this chapter, the effect of Ga substitution (with x as low as 0.01 at.%) on the phase separation has been discussed. The substitution of Ga at place of Al in various alloy systems has been extensively studied by our group [45–50]. The Ce-Al [51] and Ce-Ga [52] binaries have negative heat of mixing, while Ga-Al pair has very low positive heat of mixing, i.e., 0.7 KJ/mol [53]. It seems unlikely that the phase separation is caused by Ga-Al which has a very small positive heat of mixing. Hence, the alternative explanation for this has been called for. One may thus expect that the substitution of Ga on Al sites may lead to change in the electronic behavior of Ce 4*f* electrons (owing to chemical pressure effect) [54]. We have also discussed the effect of Ga substitution on the formation of nanoamorphous domains as well as on the nature of Ce 4*f* electronic states. It should be pointed out that pressure-induced delocalization of 4*f* electron (using XAS studies) has also been reported by other researchers [13, 42]. However, the partial delocalization of 4*f* electron of Ce atoms in Ce75Al25 − xGax alloys due to Ga substitution has been pointed out for the first time based on XAS studies.
