Effect of yttrium on thermal stability and crystallization behavior of Nd60Fe20Al10Ni10 amorphous alloys

JOURNAL OF RARE EARTHS, Vol. 26, No. 5, Oct. 2008, p. 735

Effect of yttrium on thermal stability and crystallization behavior of Nd60Fe20Al10Ni10 amorphous alloys ZHANG Shasha (), TIAN Xuelei (), KONG Fanli ( ) (Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, China) Received 18 December 2007; revised 10 April 2008

Abstract: The effect of yttrium on the thermal stability and crystallization behavior of Nd-Fe-Al-Ni amorphous alloys was investigated using X-ray diffraction (XRD), differential scanning calorimeter (DSC), and transmission electron microscopy (TEM).The results indicated that the as-cast Nd60Fe20Al10Ni10–xYx(x=0, 2) amorphous alloys were fabricated with some quenched-in crystals, which could be restrained by Y. With the effect of yttrium, both the crystallization temperature and exothermic peak shifted to higher temperatures, illustrating that the thermal stability could be improved. The addition of Y changed the crystallization process and final crystallization results. Moreover, the crystallites in the amorphous matrix became more homogeneous and smaller. Meanwhile, Y was useful for the passivation of oxygen in chemistry and restrained the negative effect of oxygen. The activation energies of the start of crystallization and peaking were 1.21 and 1.16 eV, respectively, according to the Kissinger equation. Keywords: Nd-based amorphous alloy; quenched-in crystals; thermal stability; crystallization behavior; activation energy; rare earths

Magnetic alloys prepared at room temperature can be subdivided into two categories: one is the Fe-based alloys exhibiting soft magnetic properties and the other is the Nd-based and Pr-based systems showing hard magnetic properties[1]. During recent years, much attention has been paid to the multicomponent bulk amorphous Nd-based alloys, which are considered as certain types of magnetic materials with potential applications, because they possess unique properties such as no apparent glass transition in the DSC curve and good, hard magnetic properties at room temperature[2–7]. Nd-Fe-Al alloys can be prepared as bulk glasses at a wide range of composition (0–93at.%Al, 0–90 at.%Fe)[8]. Recent investigations on Nd60Fe30−xNixAl10 bulk metallic glasses illustrate that proper Ni addition can improve the glass-forming ability (GFA) of Nd-Fe-Al alloys[9,10]. In this study, Y was partly substituted for Ni based on Nd60Fe20Al10Ni10. According to the three empirical rules[11] and phase competition effects proposed by Johnson[12], Y addition may increase disordering in the system. Moreover, phase-competition stimulated the formation of intermetallic compounds. As a result, the GFA ability can be improved. In this study, based on the investigation and the effect of Y on Nd60Fe20Al10Ni10 amorphous alloys, the relationship between the amorphous and crystalline structure was revealed and the mechanism of thermal stability for Nd-based amorphous al-

loys was discussed.

1 Experimental Prealloyed Nd60Fe20Al10Ni10–xYx(x=0, 2) ingots were prepared by arc-melting a mixture of Nd, Fe, Al, Ni, and Y elements with a purity of at least 99.9 wt.%. Ribbons were produced by melt-spun using a quartz crucible and a copper wheel of 350 mm diameter with surface velocity of 11 m/s, under argon atmosphere. The amorphous state of the as-quenched ribbons was assessed by XRD (D/max-rB), with Cu Kα radiation and TEM analysis (HITACHI-800). Thermal properties were examined by differential scanning calorimetry (Netzsch DSC404) at the heating rates of 5, 10, 20, 40 °C/min, respectively. The melt-spun ribbon was heated in the same calorimeter up to each peak crystallization temperature. Microstructural changes during heat treatment of the amorphous ribbons were observed by XRD after annealing for 10 min at 300, 400, 470, and 510 °C, respectively. TEM was also applied to study the microstructure of the as-quenched samples, which were annealed at 300 °C. The samples were polished and etched for 20 s at room temperature in a solution of 2% hydrofluoric acid and 98% distilled water in a volume ratio before metallographic study.

Foundation item: Project supported by the National Natural Science Foundation of China (50571052) Corresponding author: TIAN Xuelei (E-mail: [email protected]; Tel.: +86-21-54749949)

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2 Results and discussion The structure of the as-quenched Nd60Fe20Al10Ni10–xYx (x=0, 2) amorphous alloys was determined by XRD. Two diffraction hump peaks around 2θ=32° and 2θ=55° could be observed in the XRD patterns shown in Fig.1, suggesting the formation of a kind of nearly full amorphous structure in the alloys. During the synthesis process of amorphous alloys, however, a small number of crystalline phases sometimes exist in the matrix because of the effect of melting and atmosphere. These phases are called quenched-in crystals, if the number and volume fraction are too small to be detected by XRD[13]. To further confirm the amorphous traits of the alloys, the microstructure of the Nd60Fe20Al10Ni10–xYx(x=0, 2) amorphous alloy was observed by TEM. The TEM image of asquenched Nd60Fe20Al10Ni10 amorphous alloy is shown in Fig.2. It shows a kind of almost featureless structure with small quenched-in crystals with a size of 4–6 nm, as shown in Fig.2 (a). Fig.2 (b) shows the corresponding SAED pat-

terns. A diffuse halo indicative of the amorphous state of the alloy, with a small number of diffraction spots observed, confirms the fact that quenched-in crystals exist in the amorphous matrix. The TEM image of the as-quenched Nd60Fe20Al10Ni8Y2 amorphous alloy is shown in Fig.3. Compared with Fig.2(a), the quenched-in crystals in the amorphous matrix become smaller and more homogenous, with a size of 1–2 nm. The corresponding SAED is shown in Fig.3(b). There is also a diffuse halo. Meanwhile, a small number of diffraction spots appear,illustrating the existence of quenched-in crystals again. According to XRD and TEM analysis, it can be concluded that the microstructure of the Nd60Fe20Al10Ni10–xYx (x=0, 2) alloy is almost amorphous with few quenched-in crystals. The addition of Y can restrict the precipitation of the quenched-in crystals and make their distribution more homogeneous. 2.1 Effect of Y on thermal ability of amorphous alloys Fig.4 demonstrates DSC curves of the Nd60Fe20Al10 Ni10–xYx (x=0, 2) amorphous alloys at a heating rate of 20 °C/min. It reveals that: (1) There is no appreciable glass transition or undercooled liquid zone; (2) Both the crystallization temperature and exothermic peak shift to higher temperatures with 2at.%Y, illustrating that the thermal stability is improved. The characteristic temperatures are given in Table 1.

Fig.1 X-ray diffraction patterns of as-cast Nd60Fe20Al10Ni10–xYx (x=0, 2) amorphous alloys

Fig.3 TEM image of Nd60Fe20Al10Ni8Y2 amorphous alloy (a) Bright field image; (b) SAED pattern Table 1 Thermodynamic parameters of Nd60Fe20Al10Ni8Y2 amorphous alloys at a heating rate of 20 °C/min

Fig.2 TEM image of Nd60Fe20Al10Ni10 amorphous alloy (a) Bright field image; (b) SAED pattern

Alloys

Tx/°C

Tp/°C

Nd60Fe20Al10Ni10

490.0

506.4

Nd60Fe20Al10Ni8Y2

512.5

530.2

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It can be seen that the crystallization peak shifts toward higher temperatures with the increase of heating rate, indicating the obvious kinetic behavior of crystallization. The activation energy can be interpreted as additional energy that an atom must acquire to be a part of an activated cluster. The high values of activation energy for crystallization imply good thermal stability. The crystallization kinetics of the amorphous alloys is evaluated with Kissinger’s equation in the following form:

Fig.4 DSC curves of Nd60Fe20Al10Ni10–xYx(x=0, 2) amorphous alloys

It is known that the quenched-in crystals can decrease the thermal stability of alloys[14]. By comparing the bright field image in Fig.2 with that in Fig.3, it can be learned that the difference of the distribution and amount of the quenched-in crystals in Nd60Fe20Al10Ni10–xYx(x=0, 2) amorphous alloys are one reason why thermal stability is improved by Y addition. 2.2 Kinetic effect and activation energy calculation of the crystallization behavior of Nd60Fe20Al10Ni8Y2 Fig.5 shows the DSC curves at different heating rates for Nd60Fe20Al10Ni8Y2 alloy, and its thermodynamic parameters are listed in Table 2.

where φ is the heating rate, T is the onset temperature of glass transition or crystallization, kB is the Boltzmann constant (kB=8.6173585×10−5 eV/K), k0 is a frequency factor, and E is the apparent activation energy of crystallization. By using the values of T and φ listed in Table 2, the relationship between ln(T2/φ) and 1/T is linear, as shown in Fig.6. According to the slopes of these straight lines, the activation energies for crystallization that start at (Ex) and peak (Ep) are 1.21 and 1.16 eV, respectively. The higher value of Ex, compared to Ep, implies that it is not easy for the crystallization to start to occur, as it requires larger additional energy. 2.3 Crystallization of Nd60Fe20Al10Ni10–xYx(x=0, 2) amorphous alloys To shed more light on the amorphous structure and crystallization of amorphous alloys, Nd60Fe20Al10Ni10–xYx(x=0, 2) ribbons, which were heated to temperatures of different stages of crystallization according to their DSC curves, were investigated by XRD and TEM. Fig.7 shows XRD patterns of Nd60Fe20Al10Ni10 alloys annealed for 10 min at different temperatures. The alloy is amorphous at room temperature. After annealing at 300 °C, the hcp-Nd phase and a kind of unknown phase 1 are formed. Fig.8 shows the TEM image of Nd60Fe20Al10Ni10 annealed at 300 °C. There are a number of

Fig.5 DSC curves of Nd60Fe20Al10Ni8Y2 amorphous alloys at different heating rates Table 2 Thermodynamic parameters of Nd60Fe20Al10Ni8Y2 amorphous alloys at different heating rates Heating rates/(K/min)

Tx/°C

Tp/°C

5

509.2

524.6

10

512.5

530.2

20

529.4

546.9

40

543.6

564.2

Fig.6 Relationship between ln(T2/φ) and 1/T of Nd60Fe20Al10Ni8Y2 BMG

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grains with a size of 20–60 nm at random. The selected-area electron diffraction (SAED) pattern can be considered as the cooperation of the hcp-Nd phase and unknown phase 1, which agrees well with the results in Fig.7. The unknown phase 1 disappears when the sample is annealed at 400 °C, implying that it is a kind of metastable phase that is first formed during the continuous heating process. Thus, to get bulk Nd60Fe20Al10Ni10 bulk metallic glasses, the cooling rates should be controlled to restrain the

Fig.7 X-ray diffraction patterns of Nd60Fe20Al10Ni10 amorphous alloys annealed at various temperatures

Fig.8 TEM image of Nd60Fe20Al10Ni10 amorphous alloy annealed for 10 min at 300 °C (a) Bright field image; (b) SAED pattern

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precipitation of the metastable phase. By continuously increasing the annealing temperature, more hcp-Nd crystals form as indicated by stronger peaks. After annealing at 470 °C, Nd3AlN,Nd2O, and another unknown phase 2 are formed. With the increase of temperature to 510 °C, a microstructure containing hcp-Nd+Nd3AlN+Nd2O+unknown phase 2 are formed. XRD patterns of Nd60Fe20Al10Ni8Y2 alloys annealed for 10 min at different temperatures are shown in Fig.9. It can be found that many changes occur during crystallization. Unlike that of the sample Nd60Fe20Al10Ni10, the hcp-Nd crystals are the only formed phase without the appearance of the metastable phase. Bright-field TEM image with corresponding SAED patterns of Nd60Fe20Al10Ni8Y2 ribbons are shown in Fig.10. As compared with Fig.8, improvement in the microstructural homogeneity with reduction in the average grain size is observed. The average grain size is 10–20 nm, which contributes to the enhancement of the thermal stability, by restricting the precipitation and growth of the crystals. The hcp-Nd phase is formed according to the SAED patterns, which agree well with the result in Fig.9. The hcp-Y and additional unknown phases can be detected by XRD only after they are heated up to 400 °C. A small account of Bcc-Nd crystals are formed at 470 °C. By continuously increasing the temperature to 510 °C, the alloy is found to have crystalline phases, such as, hcp-Nd, hcp-Y, bcc-Nd, and some unknown phases. Comparing Fig.7 with Fig.9, the crystallization processes of these two kinds of alloys are obviously different. The crystallization process of Nd60Fe20Al10Ni10 alloy under continuous heating conditions can be expressed as follows: amorphous → amorphous + hcp-Nd + unknown phase 1 amorphous+hcp-Nd amorphous+hcp-Nd+Nd3AlN+Nd2O+ unknown phase 2 hcp-Nd+Nd3AlN+Nd2O+unknown phase 2, whereas, Nd60Fe20Al10Ni8Y2 alloy exhibits the fol-

Fig.9 X-ray diffraction patterns of Nd60Fe20Al10Ni8Y2 amorphous alloy annealed at various temperatures

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quenched-in crystals and make their distribution more homogenous. 2. The crystallization processes of amorphous Nd60Fe20Al10Ni8Y2 and Nd60Fe20Al10Ni10, when observed, were obviously different. The crystallites in the amorphous matrix became more homogeneous and smaller and in turn restrained the formation and further growth of grains, resulting in the improvement of thermal stability. 3. According to the Kissinger equation, the value of activation energy required for a crystallization start and peak of the Nd60Fe20Al10Ni8Y2 alloy were 1.21 and 1.16 eV, respectively. The higher value of Ex, as compared with Ep, implied that it was not easy for the crystallization to start to occur, as additional energy was needed.

References:

Fig.10 TEM image of the Nd60Fe20Al10Ni8Y2 amorphous alloy annealed for 10 min at 300 °C (a) Bright field image; (b) SAED pattern

lowing processes: amorphous amorphous+Hcp-Nd amorphous+hcp-Nd+hcp-Y+unknown phase amorphous+ hcp-Nd+hcp-Y+bcc-Nd+unknown phase hcp-Nd+hcp-Y+ bcc-Nd+unknown phase. Moreover, because of the high affinity of the oxygen element to Nd, the detrimental effect of oxygen is unavoidable during the fabrication process of the amorphous. However, based on the fact that Nd2O is reduced abruptly during the crystallization of the Nd60Fe20Al10Ni8Y2 alloy, a conclusion can be reached that Y is useful for the passivation of oxygen in the early stage of the solidification process. It has been reported that the existence of oxygen may have significantly adverse effects on the amorphous properties, such as, thermal stability and nucleation of crystalline phases, during solidification. Y can be introduced to obtain oxygen and in turn improve the alloy performance. Similar reports have appeared in some articles[15–18].

3 Conclusions 1. The a Addition of Y improved the thermal stability of Nd60Fe20Al10Ni10, as Y could restrict the precipitation of

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