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Oxidation behavior of amorphous and nanoquasicrystalline Zr–Pd and Zr–Pt alloys
Journal of Alloys and Compounds 460 (2008) 172–181
Oxidation behavior of amorphous and nanoquasicrystalline Zr–Pd and Zr–Pt alloys K. Mondal a,1 , U.K. Chatterjee a , B.S. Murty b,∗ a
Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India b Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India Received 29 September 2006; received in revised form 15 May 2007; accepted 15 May 2007 Available online 18 May 2007
Abstract Oxidation behavior of amorphous and nanoquasicrystalline Zr70 Pd30 and Zr80 Pt20 alloys melt-spun at different wheel speeds has been studied in air by non-isothermal and isothermal techniques. Oxidation resistance of amorphous alloys has been found to be the lowest in comparison to the partially and fully crystallized Zr alloys. It has also been observed that oxidation does not induce crystallization of the amorphous phase. It has been shown that the oxygen diffusion rate increases gradually in the order of crystalline, nanoquasicrystalline, partially nanocrystalline and amorphous states of these alloys. Possible micromechanism of oxidation and the role of different grain/interface boundaries on the oxygen diffusion has been discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous materials; Quasicrystals; Nanostructures; X-ray diffraction; Thermal analysis
1. Introduction Since the seminal discovery of an amorphous phase in the Au–Si system by a rapid solidification technique in 1959 [1], a large number of amorphous alloys have been developed for the past three decades. Amorphous alloys including the recently developed bulk amorphous alloys as well as nanocrystalline alloys are of great research interest due to their promising properties such as superior mechanical, magnetic, corrosion and catalytic properties [2]. For high temperature applications, oxidation properties of amorphous as well as nanocrystalline alloys need to be studied. Very few investigations have so far been reported on oxidation of these alloys. Some recent works [4–7] have reported high sensitivity of some of these alloys to oxidation. Among the entire easy forming amorphous systems, Zr-based alloy systems are quite well known [3]. Depending
∗
Corresponding author. E-mail address: [email protected] (B.S. Murty). 1 Currently at National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan. 0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.05.058
upon the alloying elements present, such alloys exhibit either excellent or poor oxidation resistance [4], or even worse, such as Zr–Au alloy exhibits catastrophic oxidation which is attributed to weak attractive interaction between Zr and Au [8]. K¨oster and Triwikantoro [4] studied isothermal oxidation of Zr-based multi-component amorphous alloys and suggested that oxidation resistance could be improved either by reducing the driving force (i.e. in the nanocrystalline state), the mobility of the rate controlling elements or by adding small amount of Y, Mo, Sn and Si. Dhawan et al. [5] showed that oxidation kinetics of Zr-based multicomponent amorphous alloy obeyed the parabolic rate law, suggesting the diffusion-controlled growth of the oxide film. Shibata et al. [6] studied oxidation of Zr–Cu alloys in air at 500 K for 72 h observed the formation of crystalline Cu and tetragonal ZrO2 . Aoki et al. [7] reported that amorphous Zr67 Ni33 hardly oxidizes below its crystallization temperature. The early stages of oxidation of glassy Zr75 Pd25 have been studied by Kilo et al. [9] and auger depth profiling revealed a strong evidence for the formation of PdO even in the subsurface region. Jastrow and coworkers [10–13] studied isothermal oxidation of Zr70 Pd30 and Zr80 Pt20 alloys and showed that the former was highly prone to oxidation, while the latter has much
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better oxidation resistance. Kimura et al. [14] reported earlier that atomic size of the solute atom is the main factor in deciding the oxidation characteristics of Zr-based binary amorphous alloys (Zr–M; M: Pd, Cu, Ni, Co, Fe). They have felt that the mixing enthalpy of the element with Zr has insignificant effect on oxidation behavior of Zr–M alloys, where M is noble metals like Au and Pd. They also concluded that the oxidation tendency of the alloy is independent of the oxidation behavior of noble metals solutes. They explained the instantaneous oxidation of Zr–Au and Zr–Pd binary amorphous alloys on the basis of atomic size ratio arguing that bigger size atoms like Au and Pd lead to larger inter-atomic spacing facilitating higher oxygen diffusivity. K¨oster et al. [11] argued that though atomic size difference between solvent and solute atoms in case of Zr-based binary glasses is important in deciding their oxidation behavior, it is nevertheless not the only factor. Though atomic size of Pt and Pd are almost same (RPt = 0. 1373 nm; RPd = 0.1376 nm), Zr70 Pd30 gets oxidized rapidly, whereas Zr80 Pt20 alloy shows a reasonably good oxidation resistance. They attributed this to the possible difference in solubility of Pd or Pt in the ZrO2 layer being the possible reason for such behavior. In the present investigation, non-isothermal and isothermal oxidation of binary melt-spun Zr70 Pd30 and Zr80 Pt20 alloys have been carried out in air. Murty et al. [15,16] have shown that the Zr70 Pd30 alloy melt-spun at 10 and 20 m/s is totally amorphous, whereas Zr80 Pt20 alloy is partially crystallized (to nanoquasicrystalline phase) at the three different melt spinning speeds (20, 30 and 40 m/s). Detailed crystallization behavior of these alloys has also been reported [16]. The present study attempts to understand the possible reasons for the different oxidation behavior of amorphous and nanocrystalline Zr alloys and the possible oxidation mechanisms in these materials. 2. Experimental details Zr70 Pd30 and Zr80 Pt20 were prepared by arc melting of high purity metals. The alloy ingots were remelted several times in order to achieve homogeneity. The alloy compositions are close to the deep eutectic points in their respective phase diagrams. The alloys were rapidly solidified by single roller melt spinning at wheel speeds of 10, 20, 30 and 40 m/s for Zr80 Pt20 and at 10 and 20 m/s for Zr70 Pd30 . Detailed structural analysis of these alloys was reported elsewhere [15,16]. The alloys were heat treated at different temperatures in vacuum (10−5 mbar) to crystallization. The X-ray diffraction (XRD) of the melt-spun and heat-treated samples was carried out with Philips PW 1729 X-ray diffractometer using Cu K␣ radiation. Differential scanning calorimetric (DSC) analysis of Zr70 Pd30 and Zr80 Pt20 alloys was performed in a Netzsch STA409 DSC apparatus at a heating rate of 10 K/min in ultra pure argon. Both sides of the ribbons were polished and washed in acetone before oxidation tests. Non-isothermal oxidation studies of Zr70 Pd30 and Zr80 Pt20 alloys in air were carried out in a Shimadzu Thermal Analyzer (DT-40) up to 973 K over the heating rate range of 2.5–20 K/min. After oxidation, the surface films were examined by XRD and scanning electron microscopy (SEM) using a JEOLJSM5800. The fraction of the sample oxidized (α) was determined from the change of mass of the sample: α=
w W
where w is the increase in mass of the sample and W is the total amount of oxygen required for the complete conversion of alloy to its stable oxides. The total amount of oxygen required for complete oxidation was taken as the theoretical amount of oxygen to oxidize Zr to ZrO2 , Pd to PdO and Pt to PtO2 in the alloys
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studied. The ␣ parameter has been successfully used to understand oxidation mechanism by previous investigators [17,18]. Isothermal oxidation of Zr70 Pd30 alloy melt-spun at 10 and 20 m/s and Zr80 Pt20 alloy melt-spun at 10, 20, 30 and 40 m/s was carried out in a Netzsch STA409 DSC at a temperature below the crystallization temperature of the alloys. For checking the effect of the oxidation on the structure of underneath alloy after oxidation, samples of Zr70 Pd30 20 m/s and Zr80 Pt20 40 m/s were partially oxidized in air at 653 K in case of Zr70 Pd30 20 m/s and 673 K for Zr80 Pt20 40 m/s. The oxide layer was removed by gradual polishing. During the course of the oxide removal, XRD of the samples was taken after each removal step.
3. Results Fig. 1(a) shows XRD patterns of the Zr70 Pd30 alloy melt-spun at 10, 20 m/s and annealed after melt spinning at 20 m/s. Both the melt-spun alloys are amorphous. The alloy melt-spun at 20 m/s and subsequently annealed at 623 K for 30 min remained amorphous, while the alloy annealed at 698 K for 15 min showed the formation of icosahedral nanoquasicrystalline phase. The XRD patterns of Zr80 Pt20 alloy melt-spun at 10, 20, 30 and 40 m/s are shown in Fig. 1(b). The alloy melt-spun at 40 and 30 m/s showed mostly amorphous nature. However, transmission electron microscopy has shown the presence of icosahedral nanoquasicrystalline phase with fivefold rotational symmetry in these samples [16]. The alloy melt-spun at 20 m/s is almost fully nanoquasicrystalline while that melt-spun at 10 m/s is fully crystalline and showed the presence of ␣-Zr, Zr5 Pt3 and ZrPt phases. The Zr–Pt alloy annealed at 623 K for 30 min after melt spinning at 20 and 40 m/s showed an increase in the amounts of the nanoquasicrystalline phase (Fig. 1(c)). Fig. 2(a) shows the DSC curves of Zr70 Pd30 alloy melt-spun at 10 and 20 m/s and Zr80 Pt20 melt-spun at 40 m/s at a heating rate of 10 K/min. Zr–Pd alloy showed two exothermic peaks depicting two-stage crystallization, the first crystallization peak temperature being 704 and 702 K, respectively, for the alloys melt-spun at 10 and 20 m/s. It is known that nanoquasicrystalline phase forms in the first stage of crystallization of amorphous phase in this alloy [15]. The DSC trace of Zr80 Pt20 alloy meltspun at 10 m/s exhibits no peak where as the ones melt-spun at 30 and 40 m/s show one strong peak (872 and 866 K, respectively), which depict crystallization of nanoquasicrystalline phase. 3.1. Non-isothermal oxidation Fig. 3(a) compares the extent of oxidation in Zr70 Pd30 meltspun at 10 and 20 m/s with pure bulk zirconium at a heating rate of 10 K/min. Pure Zr has shown very low values of α up to 973 K in comparison to the melt-spun amorphous Zr–Pd alloys. However, the Zr70 Pd30 alloy melt-spun at lower speed (10 m/s) has higher oxidation resistance than that prepared at higher wheel speed. The extent of oxidation is low for the alloy annealed at 698 K for 15 min in comparison to the meltspun alloy (Fig. 3(b)) indicating that its oxidation resistance has increased after nanocrystallization. Even the alloy annealed at 623 K for 30 min, which is amorphous from the XRD studies (Fig. 1(a)), has marginally better oxidation resistance than the melt-spun amorphous alloy.
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Fig. 2. DSC traces of (a) Zr70 Pd30 alloy and (b) Zr80 Pt20 alloy at a heating rate of 10 K/min.
Fig. 1. XRD patterns of (a) Zr70 Pd30 , (b) Zr80 Pt20 alloy melt-spun at different speeds and (c) heat treated Zr80 Pt20 alloys.
The degree of oxidation, α, for the Zr80 Pt20 alloy melt-spun at various wheel speeds is shown in Fig. 4(a) with that of pure zirconium up to 973 K of at the heating rate of 10 K/min. There is a gradual increase in oxidation resistance with increasing the melt spinning wheel speed. However, the oxidation resistance of pure crystalline Zr is better than that of all the melt-spun Zr–Pt alloys
as similar to the Zr70 Pd30 alloy. Crystallization enhanced oxidation resistance of the melt-spun Zr–Pt alloy (Fig. 4(b)). Fig. 4(c) shows that Zr70 Pd30 alloy melt-spun at 20 m/s has the highest degree of oxidation in comparison to the two melt-spun Zr–Pt alloys (20 and 40 m/s) even at a slow heating rate of 2.5 K/min, while Zr80 Pt20 alloy melt-spun at 20 m/s has the highest oxidation resistance among the three samples. The activation energy for the oxidation was calculated from the slope of the Kissinger plot (ln ϕ/Tp2 versus 1/Tp , where ϕ is the heating rate, Tp is the onset temperature of oxidation) in the heating rate range of 2.5–20 K/min. Fig. 5 shows the Kissinger plot for the nonisothermal oxidation of Zr80 Pt20 alloy melt-spun at 40 m/s as a representative case. Table 2 shows the values of activation energies calculated from Kissinger relation and the start temperatures of oxidation at a heating rate of 10 K/min. The XRD patterns of oxidized samples of Zr70 Pd30 alloy (20 m/s) and Zr80 Pt20 alloy (40 m/s) at 773 and 973 K (Fig. 6(a) and (b), respectively) show that the main constituent of the oxide scale is ZrO2 (both tetragonal and monoclinic) in both the cases. The peaks of cubic ZrO2 match exactly with those of tetragonal and hence it is difficult to conclude whether cubic phase is also present in the oxide layer. Peaks of PdO in Zr–Pd alloy and
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Fig. 3. (a) Variation of α with temperature for melt-spun Zr70 Pd30 alloys and pure Zr at the heating rate of 10 K/min and (b) variation of α with temperature for melt-spun Zr70 Pd30 alloy after subsequent annealing at the heating rate of 20 K/min.
Pt in Zr–Pt alloy were also detected. Presence of PdO has also been identified at the same angle as shown in Fig. 6(a) by previous investigators [11]. The crystallite size calculations from the XRD peak broadening show values of 21 and 55 nm for ZrO2 on Zr70 Pd30 alloy melt-spun at 20 and 10 m/s, respectively, and 69 nm for ZrO2 on Zr80 Pt20 alloy melt-spun at 40 m/s. The SEM micrographs of the surface of the oxidized samples of Zr70 Pd30 alloy melt-spun at 20 and 10 m/s, and Zr80 Pt20 alloy melt-spun at 40 m/s alloys is shown in Fig. 7(a)–(c), respectively. The micrographs correspond to the alloys heated up to 773 K at a heating rate of 20 K/min in case of Zr70 Pd30 alloy melt-spun at 20 m/s and 10 K/min in case of rest of the alloys. The oxidized surface layer of Zr70 Pd30 sample consists of wide cracks in case of the alloy melt-spun at 20 m/s, while fine cracks were observed in the same alloy melt-spun at 10 m/s. At the lower heating rate of 10 K/min, Zr–Pd alloy melt-spun at 20 m/s has become powder after heating to 773 K. Zr80 Pt20 alloy meltspun 40 m/s did not show any surface cracks even at the heating rate of 10 K/min suggesting higher stability of the surface oxide.
Fig. 4. Variation of α with temperature for (a) melt-spun Zr80 Pt20 alloys and pure Zr at the heating rate of 10 K/min; (b) melt-spun and annealed Zr80 Pt20 alloys at the heating rate of 10 K/min and (c) melt-spun Zr–Pd and Zr–Pt alloys at the heating rate of 2.5 K/min.
3.2. Isothermal oxidation Fig. 8(a) shows the isothermal oxidation behavior (α versus time (t)) of amorphous Zr70 Pd30 alloy melt-spun at 10 and 20 m/s at 623 K, which is about 70 K below the onset temperature of first crystallization (Table 1) of the alloy. Negligible amount of
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Fig. 5. Kissinger plot of Zr80 Pt20 alloy melt-spun at 40 m/s for non-isothermal oxidation at different heating rates (2.5–20 K/min).
Fig. 7. SEM micrographs of the surface of the oxidized samples of (a) Zr70 Pd30 20 m/s alloy at the heating rate of 20 K/min, (b) Zr70 Pd30 10 m/s alloy and (c) Zr80 Pt20 40 m/s alloy at the heating rate of 10 K/min.
Fig. 6. XRD patterns of oxidized samples of (a) Zr70 Pd30 20 m/s alloy heated up to 773 K and (b) Zr80 Pt20 40 m/s alloy heated up to 973 K at the heating rate of 10 K/min.
oxidation is observed while heating the alloy (10 K/min) to the isothermal oxidation temperature. The figure shows that oxidation is more in case of the amorphous alloy made at the higher wheel speed, similar to the case of non-isothermal oxidation studies (Fig. 3(a)). The oxidation of Zr70 Pd30 alloy melt-spun at 10 m/s is almost complete by 4000 s (Fig. 8(a)) and hence the
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Table 1 The onset of crystallization of the melt-spun alloys at a heating rate of 10 K/min
Fig. 8. (a) α vs. time (t) plot of isothermally oxidized samples of Zr70 Pd30 alloy melt-spun at 10 and 20 m/s at 623 K, (b) α2 vs. time (t) plot showing straight-line relationship for the oxidation of Zr80 Pd30 alloy melt-spun at 10 m/s at 623 K and (c) ln(k) vs. 1/T plot of Zr70 Pd30 alloy melt-spun at 10 m/s in the temperature range of 623–653 K.
oxidation curve has attained a plateau at this stage indicating complete oxidation (∼α = 1). The oxidation follows parabolic rate law, which is evident from the straight-line relationship between α2 and t (Fig. 8(b)). The rate constants (k) obtained from the slope of the α2 –t plot at 623 K for the oxidation of Zr70 Pd30 alloy melt-spun at 10 and 20 m/s is 1.39 × 10−4 and
Alloy
Melt-spun speed (m/s)
First crystallization (Tx 1)
Second crystallization (Tx 2)
Zr70 Pd30
10 20
691 692
744 749
Zr80 Pt20
10 20 30 40
– – – –
– – 867 861
3.32 × 10−4 s−1 , respectively. Activation energy for isothermal oxidation is calculated from ln(k) versus 1/T plot and one such plot for Zr70 Pd30 alloy melt-spun at 10 m/s is shown in Fig. 8(c). Table 3 shows the rate constants at 623 K and activation energy as calculated from isothermal results over the temperature range of 623–653 K for Zr–Pd alloys. The activation energy for oxidation of Zr70 Pd30 alloy melt-spun at 10 m/s is higher than that melt-spun at 20 m/s. In addition, the isothermal oxidation behavior of the meltspun Zr–Pt alloy at 843 K which is around 30 K below the onset temperature of crystallization for Zr–Pt alloy melt-spun at 30 and 40 m/s (Table 1) is shown in Fig. 9(a). The oxidation rate decreased with decreasing melt-spinning speed, similar to the non-isothermal oxidation behavior of the alloys (Fig. 4(a)). Table 3 shows the rate constants at 843 K and activation energy as calculated from isothermal results in the temperature range of 813–843 K for Zr–Pt alloys. The oxidation also follows parabolic rate law in this case (Fig. 9(b)). Here also it is evident that with increase in the melt-spun wheel speed the activation energy decreases. For Zr80 Pt20 alloy melt-spun at 10 m/s, the weight gain is so low that the activation energy for isothermal oxidation could not be calculated in the temperature range studied. Fig. 10(a) shows XRD patterns of the surfaces of partially oxidized the Zr70 Pd30 alloy melt-spun at 20 m/s at 653 K after removing the oxide layer by grinding. The alloy has been oxidized for 30 min at 653 K, which is below the first crystallization temperature of the alloy. As the oxide layer is gradually removed, the XRD pattern reveals the parent amorphous layer. This is also observed for the Zr80 Pt20 alloy melt-spun at 40 m/s (Fig. 10(b)). In case of Zr–Pt alloy, the oxidation has been carried out for 180 min at 673 K, which is also below the crystallization temperatures of the alloy. The oxide layer consists mainly of monoclinic and tetragonal ZrO2 and PdO in case of Zr–Pd alloys and tetragonal ZrO2 in case of Zr–Pt alloy. 4. Discussion The present results indicate that both Zr–Pd and Zr–Pt alloys have the highest oxidation tendency in the amorphous state. This is easy to understand as the amorphous structure is known to have large free volume in compared to crystalline counterpart [3] and hence diffusion of oxygen is expected to be faster through it. Both non-isothermal and isothermal studies (Fig. 3, Fig. 8(a) and Table 3) suggest that Zr70 Pd30 alloy melt-spun at 20 m/s
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Fig. 9. (a) α vs. time (t) plot of isothermally oxidized samples of Zr80 Pt20 alloy melt-spun at 10, 20, 30 and 40 m/s at 843 K and (b) α2 vs. time (t) plot showing straight-line relationship for the oxidation of Zr80 Pt20 alloy melt-spun at 40 m/s at 843 K.
has higher oxidation tendency than either the alloy melt-spun at lower wheel speed or the annealed alloy, though all three of them are amorphous as observed in their XRD patterns (Fig. 1(a)). This difference could be attributed to the possible differences in the structure of the alloy. The alloy melt-spun at slower wheel speed and that subjected to low temperature annealing (623 K to 30 min) are expected to be structurally relaxed and might contain less free volume. Earlier investigators have also shown that at lowering cooling rate and low temperature annealing, the amorphous alloy has lower free volume content because of structural relaxation [19–21]. This is expected to reduce the diffusivity of oxygen through the amorphous structure and thus improve the oxidation resistance of the alloy. Flege et al. [22] have shown that diffusion of tracer Cu is reduced as the Zr-based amorphous alloy gets relaxed. Similar result is expected for the diffusion of oxygen in amorphous alloy. The results of the Zr70 Pd30 alloy (Fig. 3(b)) also suggest that nanoquasicrystallization of glass improves the oxidation resistance of the alloy. Similar observations were made in case of Zr80 Pt20 alloy, wherein the oxidation resistance increases with increasing the volume fraction of nanoquasicrystalline phase in the amorphous matrix or, in other words, a decrease in the
Fig. 10. XRD patterns of partially oxidized samples and after removal of oxide layer for (a) Zr70 Pd30 alloy melt-spun at 20 m/s and (b) Zr80 Pt20 alloy melt-spun at 40 m/s.
amount of residual amorphous phase (Fig. 4(a)). The early studies of Murty et al. [16] indicate that the volume fraction of nanoquasicrystalline phase increases with decreasing the meltspun speed in the range of 40–20 m/s. With thermal treatment of Zr80 Pt20 alloy melt-spun at 40 m/s, more precipitation of nanoquasicrystalline phase has been observed as clear from the XRD pattern (Fig. 1(c)), which has lead to improved oxidation resistance (Fig. 4(b)). Isothermal oxidation of Zr80 Pt20 alloy (Fig. 9(a)) also showed similar results. Non-isothermal oxidation of Zr70 Pd30 alloy melt-spun at 10 and 20 m/s and Zr80 Pt20 alloy melt-spun at 30 and 40 m/s has revealed that oxidation rate suddenly increases at a temperature close to the crystallization temperature of the alloys, indicating possibly that oxidation is enhanced by crystallization. However, detailed investigations have shown that the sudden increase in the oxidation at a fixed temperature in Zr–Pd alloys is mainly due to a phase transformation in the structure of Zr oxide which leads to a volume expansion in the oxide layer leading to its cracking and not to the nanoquasicrystallization process [23,24]. In fact, studies on partial isothermal oxidation and subsequent oxide scale
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removal results (Fig. 10) prove that isothermal oxidation takes place in fully amorphous Zr70 Pd30 alloy melt-spun at 20 m/s, and in partially nanoquasicrystalline Zr80 Pt20 alloy melt-spun at 40 m/s. Preservation of the parent phase underneath the surface oxide layer also proves that surface oxidation has not led to crystallization in either of the cases. The previous work of the present authors [23] and in the present work clearly demonstrates that the stronger interaction between Zr and Pt in comparison to Zr–Pd leads to lower oxidation rate in the former alloys. In addition, the oxidation behavior of these two alloys mainly depends on the structure of the alloys and that of oxide scale, more specifically the tetragonal or monoclinic nature of ZrO2 [23]. The present results also confirm that oxidation resistance of the Zr alloys studied is highest if they are in the completely crystallized state (Zr80 Pt20 alloy meltspun at 10 m/s). The fact that the crystalline ␣-Zr has better oxidation resistance than the amorphous, nanoquasicrystalline states and the completely crystallized Zr–Pt alloy is also clear from Fig. 4(a). Thus the oxidation resistance of the different structures studied could be arranged in the following order:
lization of the amorphous phase in the Zr70 Pd30 and Zr80 Pt20 alloys are the amorphous-nanoquasicrystalline interfaces. With increased volume fraction of nanoquasicrystalline phase a second type of interface, namely the nanoquasicrystalline grain boundaries appears in the alloys. Further annealing leads to the formation of crystalline phases and thus the crystalline grain boundaries and crystalline interphase interfaces replace the nanoquasicrystalline grain boundaries. Finally, the highest oxidation resistance, among the structures studied, has been observed with crystalline ␣-Zr with crystalline high angle grain boundaries. Thus, the oxidation tendency of the alloy appears to increase with the presence of various interfaces/boundaries in the following order:
crystalline Zr > crystalline Zr alloy
This could be due to differences in the oxygen diffusivity along the various boundaries present in the alloy at the metal–oxide interface. Zr is known to dissolve up to 30 at.% of oxygen [27] at room temperature. Beyond this concentration, precipitation of ZrO2 is observed [28]. Thus, oxygen in the atmosphere first dissolves in Zr and, once the solubility limit is crossed, ZrO2 layer forms. Once the oxide layer forms, further oxidation occurs by diffusion of oxygen through the ZrO2 layer to the metal-oxide interface followed by its dissolution in the alloy and precipitation of ZrO2 once the solubility limit is crossed. The ZrO2 layer formed in the Zr70 Pd30 and Zr80 Pt20 alloys appears to be nanocrystalline in nature from its broad XRD peaks (Fig. 6(a) and (b)). The earlier studies [4,11] also indicate the formation of nanocrystalline ZrO2 . Diffusion of oxygen through nanocrystalline ZrO2 is expected to be faster [29]. The present calculations show that the crystallite size of ZrO2 in case of Zr70 Pd30 alloys is smaller than that in Zr80 Pt20 alloys, which means that diffusion through ZrO2 is faster in the former than in the latter. Jastrow and K¨oster [13] has suggested that the growth of oxide layer proceeds by inward oxygen diffusion probably along the interfaces of the lamellar microstructure towards the reaction zone at the interfaces between oxide and unoxidized alloy. Such diffusion of oxygen can be orders of magnitude faster the volume diffusion in the bulk of ZrO2 grains [29]. Once oxygen reaches the metal–oxide interface, the rate of further oxidation should depend on how fast oxygen will diffuse into the alloy so that the total dissolved oxygen reaches the solubility limit for ZrO2 to precipitate. The diffusivity of oxygen in Zr(O) phase is known to be faster than that in ZrO2 (4 × 10−21 and 5 × 10−22 m2 /s, respectively (at 530 K)) [28]. Volume and grain boundary diffusion of oxygen in undoped nanocrystalline monoclinic ZrO2 are recently calculated by Brossmann et al. [30] to be less than 10−22 and 10−18 m2 /s, respectively. However, the diffusion of oxygen in Zr(O) phase should depend on the microstructure of the Zr alloy. In case of an amorphous alloy
> nanoquasicrystalline Zr alloy > relaxed amorphous alloy > amorphous Zr alloy The present results are in agreement with those of K¨oster and his coworkers [4,11,12], who have shown that nanocrystalline and quasicrystalline alloys have better oxidation resistance than the amorphous alloys in case of Zr–Cu–Ni–Al and Zr–Pd alloys. Kai et al. [25] has also shown that the crystalline Z55 Cu30 Al10 N15 alloy has higher oxidation resistance than amorphous alloy. The present results also indicate that amorphous structure in the Zr–Pt alloy (the alloy melt-spun at 40 m/s, which has a large fraction of amorphous phase) has better oxidation resistance than that of the Zr70 Pd30 alloy (Fig. 4(c)). This could probably be due to slower diffusion of oxygen in Zt–Pt amorphous structure than in Zr–Pd. This could be attributed to the possible higher density of packing in Zr–Pt glass (stronger interaction between Zr and Pt) due to the larger enthalpy of mixing in the system (−100 kJ/mol) in comparison to the Zr–Pd alloy (−91 kJ/mol) [26]. It has also been shown earlier that transformation of tetragonal to monoclinic ZrO2 leading to difference in volume expansion is a strong reason for the difference in the oxidation behavior of Zr–Pd and Zr–Pt alloys [23]. However, Jastrow and co-workers [12,13] have proposed that the difference in the behavior of Zr70 Pd30 and Zr80 Pt20 might be due to different solubility of the noble metals into the ZrO2 . However, no data on the extent of solubility of Pd and Pt in ZrO2 is available in the literature. Oxidation in Zr alloys is usually driven by the diffusion of oxygen from the atmosphere to the oxide–metal interface through the oxide scale. The present results demonstrate that the microstructure and the various phase/grain boundaries in the alloy play a very important role in its oxidation resistance. The type of interfaces expected in the first stage of crystal-
grain boundaries in ␣-Zr >
grain boundaries interfaces in crystalline Zr alloy
> nanoquasicrystalline grain boundaries > amorphous-nanoquasicrystalline interfaces > amorphous structure
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Table 2 Activation energy for the start of oxidation in melt-spun amorphous alloys calculated from kissinger plot Alloy
Activation energy for the oxidation to start (kJ/mol)
Start temperature for oxidation (10 K/min)
Zr70 Pd30 10 m/s Zr70 Pd30 20 m/s Zr80 Pt20 40 m/s
115 111 299
683 K 67 8 K 842 K
Table 3 Rate constants and activation energies for isothermal oxidation of melt-spun alloys Alloy
Rate constant (s−1 ) At 623 K
Zr70 Pd30 10 m/s Zr70 Pd30 20 m/s Zr8 0Pt20 10 m/s Zr80 Pt20 20 m/s Zr80 Pt20 30 m/s Zr80 Pt20 40 m/s
Activation energy (kJ/mol) At 843 K
623–653 K
813–843 K
1.71 × 10−6 3.67 × 10−6 4.34 × 10−6 6.22 × 10−6
67.3 49.7 – – – –
– 217 175 109
2.03 × 10−4
3.32 × 10−4 – – – –
the diffusion is expected to be faster in comparison to all other microstructures studied in the present investigation and hence this type of microstructure shows the highest oxidation tendency. Once the nanoquasicrystalline phase forms in the amorphous matrix the oxidation tendency of the alloy gets reduced as the open structure of amorphous phase because of higher free volume content is replaced with amorphous-nanoquasicrystalline interfaces and nanoquasicrystalline grain boundaries. Diffusion of oxygen along these interfaces/boundaries is expected to be slower than that in the amorphous phase. Once the amorphous phase crystallizes completely, diffusion through the microcrystalline interphase interfaces is expected to be further lower in comparison to the nanoquasicrystalline interfaces/boundaries and hence the oxidation resistance of the alloy increases. This is also shown by the lowering of activation energy for oxidation with the progress in crystallization in melt-spun alloys (Table 3). Finally, in case of conventional pure Zr, which contains only high angle grain boundaries, the diffusivity of oxygen would be slower than that along either nanoquasicrystalline boundaries or interphase interfaces due to the less amount of grain boundary area. This explains the highest oxidation resistance of Zr. The effect of grain boundary on the isothermal oxidation behavior of binary Zr–Pd amorphous alloy and its crystalline counterpart has been discussed by Jastrow and K¨oster [13]. They showed that the large crystalline grains are very stable against oxidation, but oxidation proceeds very deep into the material preferentially along the grain boundaries associated with large stress fields. The stress field is more in case of large grained microstructure. From the total oxygen dissolution or total weight gain point of view, oxidation resistance of crystalline Zr is much better than the other alloys. Non-isothermal and isothermal oxidation of melt-spun binary alloys, are closely related, yet there are differences. First of all, non-isothermal oxidation is necessary to study the change in the oxidation behavior of any alloy with temperature. The present results indicate that there is a significant difference between activation energies found out from non-isothermal and isothermal
oxidation results (Tables 2 and 3). It appears that change in structure of the alloy during non-isothermal study plays an important role. Isothermal oxidation temperature is about 30–80 K less than the crystallization onset temperatures in both Zr–Pd and Zr–Pt alloys as observed in Tables 1 and 3. As a result, the as melt-spun structure remains more or less unchanged during isothermal oxidation studies, which is evident from Fig. 10. In case of non-isothermal oxidation, there is a possibility of crystallization of the amorphous phase (in both Zr–Pd and Zr–Pt alloys) and grain growth in the nanoquasicrystalline phase present in Zr–Pt alloy. Both these processes can lead to a decrease in the oxidation tendency of the alloy. Thus, the higher activation energies observed in case of non-isothermal oxidation in comparison to isothermal oxidation could be attributed to the possible crystallization and grain growth of the nanoquasicrystalline phase in the former case due to the higher temperatures experience. 5. Conclusions • The oxidation resistance of Zr–Pd and Zr–Pt alloys increases in the order of amorphous, nanoquasicrystalline and crystalline phases. • The microstructure of the alloy, in terms of different interfaces/boundaries, plays a significant role in the oxidation behavior of the alloy. The oxidation resistance appears to increase in the order of the amorphous phase, amorphous-nanocrystalline interfaces, nanoquasicrystalline grain boundaries, crystalline interphase interfaces/grain boundaries and grain boundaries in ␣-Zr. • A relaxed amorphous alloy shows better oxidation resistance than that of the unrelaxed one. References [1] W. Klement, R.H. Willens, P. Duwez, Nature 187 (1960) 869. [2] H. Warlimont, Mater. Sci. Eng. A 304–306 (2001) 61. [3] A. Inoue, Acta Mater. 48 (2000) 279.
K. Mondal et al. / Journal of Alloys and Compounds 460 (2008) 172–181 [4] U. K¨oster, Triwikantoro, Mater. Sci. Forum 360–362 (2001) 29. [5] A. Dhawan, K. Raetzke, F. Faupel, S.K. Sharma, Bull. Mater. Sci. 24 (2001) 281. [6] M. Shibata, N. Kawata, T. Masumoto, H.M. Kimura, J. Catal. 108 (1987) 263. [7] K. Aoki, T. Masumoto, C. Suryanarayana, J. Mater. Sci. 21 (1986) 793. [8] U. K¨oster, U. Sch¨unemann, in: H.H. Liebermann (Ed.), Rapidly Solidified Alloys, Marcel Dekker, New York, 1993, p. 303ff. [9] M. Kilo, M. Hund, G. Sauer, A. Baiker, A. Wokaun, J. Alloys Compd. 236 (1996) 137. [10] L. Jastrow, M. Meuris, U. K¨oster, N. Froumin, D. Eliezer, Mater. Sci. Forum 386–388 (2002) 627. [11] U. K¨oster, L. Jastrow, D. Zander, J. Metastable Nanocryst. Mater. 15–16 (2003) 49. [12] L. Jastrow, U. K¨oster, M. Meuris, Mater. Sci. Eng. A 375–377 (2004) 440. [13] L. Jastrow, U. K¨oster, Mater. Res. Soc. Symp. Proc. 806 (2004) 381. [14] H.M. Kimura, K. Asami, A. Inoue, T. Masumoto, Corros. Sci. 35 (1993) 909. [15] B.S. Murty, D.H. Ping, K. Hono, Appl. Phys. Lett. 77 (2000) 1102. [16] B.S. Murty, D.H. Ping, M. Ohnuma, K. Hono, Acta Mater. 49 (2001) 3453.
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[17] S.K. Roy, A. Auddya, S.K. Bose, React. Solids 6 (1989) 301. [18] T.P. Bagchi, P.K. Sen, Thermochim. Acta 51 (1981) 175. [19] O.P. Borov, V.A. Khonik, K. Kitagawa, S.N. Laptev, J. Non-Cryst. Sol. 342 (2004) 152. [20] C. Nagel, K. Ratzke, E. Schmidtke, J. Wolff, U. Geyer, F. Faupel, Phys. Rev. B 57 (1998) 10224. [21] O. Haruami, Intermetallics 15 (2007) 659. [22] S. Flege, U. Fecher, H. Hahn, J. Non-Cryst. Sol. 270 (2000) 123. [23] K. Mondal, U.K. Chatterjee, B.S. Murty, J. Mater. Res. 21 (2006) 639. [24] M. Steinberg, D. Shamir, Thermochim. Acta 84 (1985) 357. [25] W. Kai, H.H. Hsieh, T.G. Nieh, Y. Kawamura, Intermetallics 10 (2002) 1265. [26] F.R. de Boer, R. Boom, W.C.M. Mattens, A. Miedema, A.K. Niessen, Cohesion in Metals-Transition Metal Alloys, Amsterdam, North-Holland, 1988. [27] T.B. Massalski (Ed.), Binary Alloy Phase Diagrams, ASM, Metals Park, 1986. [28] T. Tanabe, M. Tomita, Surf. Sci. 222 (1989) 84. [29] I.A. Ovid’ko, A.B. Reizis, Phys. Solid State 43 (2001) 35. [30] U. Brossmann, U. S¨odervall, R. W¨urschum, H.E. Schaefer, Nanostruct. Mater. 12 (1999) 871.
Oxidation behavior of amorphous and nanoquasicrystalline Zr–Pd and Zr–Pt alloys K. Mondal a,1 , U.K. Chatterjee a , B.S. Murty b,∗ a
Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India b Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India Received 29 September 2006; received in revised form 15 May 2007; accepted 15 May 2007 Available online 18 May 2007
Abstract Oxidation behavior of amorphous and nanoquasicrystalline Zr70 Pd30 and Zr80 Pt20 alloys melt-spun at different wheel speeds has been studied in air by non-isothermal and isothermal techniques. Oxidation resistance of amorphous alloys has been found to be the lowest in comparison to the partially and fully crystallized Zr alloys. It has also been observed that oxidation does not induce crystallization of the amorphous phase. It has been shown that the oxygen diffusion rate increases gradually in the order of crystalline, nanoquasicrystalline, partially nanocrystalline and amorphous states of these alloys. Possible micromechanism of oxidation and the role of different grain/interface boundaries on the oxygen diffusion has been discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous materials; Quasicrystals; Nanostructures; X-ray diffraction; Thermal analysis
1. Introduction Since the seminal discovery of an amorphous phase in the Au–Si system by a rapid solidification technique in 1959 [1], a large number of amorphous alloys have been developed for the past three decades. Amorphous alloys including the recently developed bulk amorphous alloys as well as nanocrystalline alloys are of great research interest due to their promising properties such as superior mechanical, magnetic, corrosion and catalytic properties [2]. For high temperature applications, oxidation properties of amorphous as well as nanocrystalline alloys need to be studied. Very few investigations have so far been reported on oxidation of these alloys. Some recent works [4–7] have reported high sensitivity of some of these alloys to oxidation. Among the entire easy forming amorphous systems, Zr-based alloy systems are quite well known [3]. Depending
∗
Corresponding author. E-mail address: [email protected] (B.S. Murty). 1 Currently at National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan. 0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.05.058
upon the alloying elements present, such alloys exhibit either excellent or poor oxidation resistance [4], or even worse, such as Zr–Au alloy exhibits catastrophic oxidation which is attributed to weak attractive interaction between Zr and Au [8]. K¨oster and Triwikantoro [4] studied isothermal oxidation of Zr-based multi-component amorphous alloys and suggested that oxidation resistance could be improved either by reducing the driving force (i.e. in the nanocrystalline state), the mobility of the rate controlling elements or by adding small amount of Y, Mo, Sn and Si. Dhawan et al. [5] showed that oxidation kinetics of Zr-based multicomponent amorphous alloy obeyed the parabolic rate law, suggesting the diffusion-controlled growth of the oxide film. Shibata et al. [6] studied oxidation of Zr–Cu alloys in air at 500 K for 72 h observed the formation of crystalline Cu and tetragonal ZrO2 . Aoki et al. [7] reported that amorphous Zr67 Ni33 hardly oxidizes below its crystallization temperature. The early stages of oxidation of glassy Zr75 Pd25 have been studied by Kilo et al. [9] and auger depth profiling revealed a strong evidence for the formation of PdO even in the subsurface region. Jastrow and coworkers [10–13] studied isothermal oxidation of Zr70 Pd30 and Zr80 Pt20 alloys and showed that the former was highly prone to oxidation, while the latter has much
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better oxidation resistance. Kimura et al. [14] reported earlier that atomic size of the solute atom is the main factor in deciding the oxidation characteristics of Zr-based binary amorphous alloys (Zr–M; M: Pd, Cu, Ni, Co, Fe). They have felt that the mixing enthalpy of the element with Zr has insignificant effect on oxidation behavior of Zr–M alloys, where M is noble metals like Au and Pd. They also concluded that the oxidation tendency of the alloy is independent of the oxidation behavior of noble metals solutes. They explained the instantaneous oxidation of Zr–Au and Zr–Pd binary amorphous alloys on the basis of atomic size ratio arguing that bigger size atoms like Au and Pd lead to larger inter-atomic spacing facilitating higher oxygen diffusivity. K¨oster et al. [11] argued that though atomic size difference between solvent and solute atoms in case of Zr-based binary glasses is important in deciding their oxidation behavior, it is nevertheless not the only factor. Though atomic size of Pt and Pd are almost same (RPt = 0. 1373 nm; RPd = 0.1376 nm), Zr70 Pd30 gets oxidized rapidly, whereas Zr80 Pt20 alloy shows a reasonably good oxidation resistance. They attributed this to the possible difference in solubility of Pd or Pt in the ZrO2 layer being the possible reason for such behavior. In the present investigation, non-isothermal and isothermal oxidation of binary melt-spun Zr70 Pd30 and Zr80 Pt20 alloys have been carried out in air. Murty et al. [15,16] have shown that the Zr70 Pd30 alloy melt-spun at 10 and 20 m/s is totally amorphous, whereas Zr80 Pt20 alloy is partially crystallized (to nanoquasicrystalline phase) at the three different melt spinning speeds (20, 30 and 40 m/s). Detailed crystallization behavior of these alloys has also been reported [16]. The present study attempts to understand the possible reasons for the different oxidation behavior of amorphous and nanocrystalline Zr alloys and the possible oxidation mechanisms in these materials. 2. Experimental details Zr70 Pd30 and Zr80 Pt20 were prepared by arc melting of high purity metals. The alloy ingots were remelted several times in order to achieve homogeneity. The alloy compositions are close to the deep eutectic points in their respective phase diagrams. The alloys were rapidly solidified by single roller melt spinning at wheel speeds of 10, 20, 30 and 40 m/s for Zr80 Pt20 and at 10 and 20 m/s for Zr70 Pd30 . Detailed structural analysis of these alloys was reported elsewhere [15,16]. The alloys were heat treated at different temperatures in vacuum (10−5 mbar) to crystallization. The X-ray diffraction (XRD) of the melt-spun and heat-treated samples was carried out with Philips PW 1729 X-ray diffractometer using Cu K␣ radiation. Differential scanning calorimetric (DSC) analysis of Zr70 Pd30 and Zr80 Pt20 alloys was performed in a Netzsch STA409 DSC apparatus at a heating rate of 10 K/min in ultra pure argon. Both sides of the ribbons were polished and washed in acetone before oxidation tests. Non-isothermal oxidation studies of Zr70 Pd30 and Zr80 Pt20 alloys in air were carried out in a Shimadzu Thermal Analyzer (DT-40) up to 973 K over the heating rate range of 2.5–20 K/min. After oxidation, the surface films were examined by XRD and scanning electron microscopy (SEM) using a JEOLJSM5800. The fraction of the sample oxidized (α) was determined from the change of mass of the sample: α=
w W
where w is the increase in mass of the sample and W is the total amount of oxygen required for the complete conversion of alloy to its stable oxides. The total amount of oxygen required for complete oxidation was taken as the theoretical amount of oxygen to oxidize Zr to ZrO2 , Pd to PdO and Pt to PtO2 in the alloys
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studied. The ␣ parameter has been successfully used to understand oxidation mechanism by previous investigators [17,18]. Isothermal oxidation of Zr70 Pd30 alloy melt-spun at 10 and 20 m/s and Zr80 Pt20 alloy melt-spun at 10, 20, 30 and 40 m/s was carried out in a Netzsch STA409 DSC at a temperature below the crystallization temperature of the alloys. For checking the effect of the oxidation on the structure of underneath alloy after oxidation, samples of Zr70 Pd30 20 m/s and Zr80 Pt20 40 m/s were partially oxidized in air at 653 K in case of Zr70 Pd30 20 m/s and 673 K for Zr80 Pt20 40 m/s. The oxide layer was removed by gradual polishing. During the course of the oxide removal, XRD of the samples was taken after each removal step.
3. Results Fig. 1(a) shows XRD patterns of the Zr70 Pd30 alloy melt-spun at 10, 20 m/s and annealed after melt spinning at 20 m/s. Both the melt-spun alloys are amorphous. The alloy melt-spun at 20 m/s and subsequently annealed at 623 K for 30 min remained amorphous, while the alloy annealed at 698 K for 15 min showed the formation of icosahedral nanoquasicrystalline phase. The XRD patterns of Zr80 Pt20 alloy melt-spun at 10, 20, 30 and 40 m/s are shown in Fig. 1(b). The alloy melt-spun at 40 and 30 m/s showed mostly amorphous nature. However, transmission electron microscopy has shown the presence of icosahedral nanoquasicrystalline phase with fivefold rotational symmetry in these samples [16]. The alloy melt-spun at 20 m/s is almost fully nanoquasicrystalline while that melt-spun at 10 m/s is fully crystalline and showed the presence of ␣-Zr, Zr5 Pt3 and ZrPt phases. The Zr–Pt alloy annealed at 623 K for 30 min after melt spinning at 20 and 40 m/s showed an increase in the amounts of the nanoquasicrystalline phase (Fig. 1(c)). Fig. 2(a) shows the DSC curves of Zr70 Pd30 alloy melt-spun at 10 and 20 m/s and Zr80 Pt20 melt-spun at 40 m/s at a heating rate of 10 K/min. Zr–Pd alloy showed two exothermic peaks depicting two-stage crystallization, the first crystallization peak temperature being 704 and 702 K, respectively, for the alloys melt-spun at 10 and 20 m/s. It is known that nanoquasicrystalline phase forms in the first stage of crystallization of amorphous phase in this alloy [15]. The DSC trace of Zr80 Pt20 alloy meltspun at 10 m/s exhibits no peak where as the ones melt-spun at 30 and 40 m/s show one strong peak (872 and 866 K, respectively), which depict crystallization of nanoquasicrystalline phase. 3.1. Non-isothermal oxidation Fig. 3(a) compares the extent of oxidation in Zr70 Pd30 meltspun at 10 and 20 m/s with pure bulk zirconium at a heating rate of 10 K/min. Pure Zr has shown very low values of α up to 973 K in comparison to the melt-spun amorphous Zr–Pd alloys. However, the Zr70 Pd30 alloy melt-spun at lower speed (10 m/s) has higher oxidation resistance than that prepared at higher wheel speed. The extent of oxidation is low for the alloy annealed at 698 K for 15 min in comparison to the meltspun alloy (Fig. 3(b)) indicating that its oxidation resistance has increased after nanocrystallization. Even the alloy annealed at 623 K for 30 min, which is amorphous from the XRD studies (Fig. 1(a)), has marginally better oxidation resistance than the melt-spun amorphous alloy.
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Fig. 2. DSC traces of (a) Zr70 Pd30 alloy and (b) Zr80 Pt20 alloy at a heating rate of 10 K/min.
Fig. 1. XRD patterns of (a) Zr70 Pd30 , (b) Zr80 Pt20 alloy melt-spun at different speeds and (c) heat treated Zr80 Pt20 alloys.
The degree of oxidation, α, for the Zr80 Pt20 alloy melt-spun at various wheel speeds is shown in Fig. 4(a) with that of pure zirconium up to 973 K of at the heating rate of 10 K/min. There is a gradual increase in oxidation resistance with increasing the melt spinning wheel speed. However, the oxidation resistance of pure crystalline Zr is better than that of all the melt-spun Zr–Pt alloys
as similar to the Zr70 Pd30 alloy. Crystallization enhanced oxidation resistance of the melt-spun Zr–Pt alloy (Fig. 4(b)). Fig. 4(c) shows that Zr70 Pd30 alloy melt-spun at 20 m/s has the highest degree of oxidation in comparison to the two melt-spun Zr–Pt alloys (20 and 40 m/s) even at a slow heating rate of 2.5 K/min, while Zr80 Pt20 alloy melt-spun at 20 m/s has the highest oxidation resistance among the three samples. The activation energy for the oxidation was calculated from the slope of the Kissinger plot (ln ϕ/Tp2 versus 1/Tp , where ϕ is the heating rate, Tp is the onset temperature of oxidation) in the heating rate range of 2.5–20 K/min. Fig. 5 shows the Kissinger plot for the nonisothermal oxidation of Zr80 Pt20 alloy melt-spun at 40 m/s as a representative case. Table 2 shows the values of activation energies calculated from Kissinger relation and the start temperatures of oxidation at a heating rate of 10 K/min. The XRD patterns of oxidized samples of Zr70 Pd30 alloy (20 m/s) and Zr80 Pt20 alloy (40 m/s) at 773 and 973 K (Fig. 6(a) and (b), respectively) show that the main constituent of the oxide scale is ZrO2 (both tetragonal and monoclinic) in both the cases. The peaks of cubic ZrO2 match exactly with those of tetragonal and hence it is difficult to conclude whether cubic phase is also present in the oxide layer. Peaks of PdO in Zr–Pd alloy and
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Fig. 3. (a) Variation of α with temperature for melt-spun Zr70 Pd30 alloys and pure Zr at the heating rate of 10 K/min and (b) variation of α with temperature for melt-spun Zr70 Pd30 alloy after subsequent annealing at the heating rate of 20 K/min.
Pt in Zr–Pt alloy were also detected. Presence of PdO has also been identified at the same angle as shown in Fig. 6(a) by previous investigators [11]. The crystallite size calculations from the XRD peak broadening show values of 21 and 55 nm for ZrO2 on Zr70 Pd30 alloy melt-spun at 20 and 10 m/s, respectively, and 69 nm for ZrO2 on Zr80 Pt20 alloy melt-spun at 40 m/s. The SEM micrographs of the surface of the oxidized samples of Zr70 Pd30 alloy melt-spun at 20 and 10 m/s, and Zr80 Pt20 alloy melt-spun at 40 m/s alloys is shown in Fig. 7(a)–(c), respectively. The micrographs correspond to the alloys heated up to 773 K at a heating rate of 20 K/min in case of Zr70 Pd30 alloy melt-spun at 20 m/s and 10 K/min in case of rest of the alloys. The oxidized surface layer of Zr70 Pd30 sample consists of wide cracks in case of the alloy melt-spun at 20 m/s, while fine cracks were observed in the same alloy melt-spun at 10 m/s. At the lower heating rate of 10 K/min, Zr–Pd alloy melt-spun at 20 m/s has become powder after heating to 773 K. Zr80 Pt20 alloy meltspun 40 m/s did not show any surface cracks even at the heating rate of 10 K/min suggesting higher stability of the surface oxide.
Fig. 4. Variation of α with temperature for (a) melt-spun Zr80 Pt20 alloys and pure Zr at the heating rate of 10 K/min; (b) melt-spun and annealed Zr80 Pt20 alloys at the heating rate of 10 K/min and (c) melt-spun Zr–Pd and Zr–Pt alloys at the heating rate of 2.5 K/min.
3.2. Isothermal oxidation Fig. 8(a) shows the isothermal oxidation behavior (α versus time (t)) of amorphous Zr70 Pd30 alloy melt-spun at 10 and 20 m/s at 623 K, which is about 70 K below the onset temperature of first crystallization (Table 1) of the alloy. Negligible amount of
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Fig. 5. Kissinger plot of Zr80 Pt20 alloy melt-spun at 40 m/s for non-isothermal oxidation at different heating rates (2.5–20 K/min).
Fig. 7. SEM micrographs of the surface of the oxidized samples of (a) Zr70 Pd30 20 m/s alloy at the heating rate of 20 K/min, (b) Zr70 Pd30 10 m/s alloy and (c) Zr80 Pt20 40 m/s alloy at the heating rate of 10 K/min.
Fig. 6. XRD patterns of oxidized samples of (a) Zr70 Pd30 20 m/s alloy heated up to 773 K and (b) Zr80 Pt20 40 m/s alloy heated up to 973 K at the heating rate of 10 K/min.
oxidation is observed while heating the alloy (10 K/min) to the isothermal oxidation temperature. The figure shows that oxidation is more in case of the amorphous alloy made at the higher wheel speed, similar to the case of non-isothermal oxidation studies (Fig. 3(a)). The oxidation of Zr70 Pd30 alloy melt-spun at 10 m/s is almost complete by 4000 s (Fig. 8(a)) and hence the
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Table 1 The onset of crystallization of the melt-spun alloys at a heating rate of 10 K/min
Fig. 8. (a) α vs. time (t) plot of isothermally oxidized samples of Zr70 Pd30 alloy melt-spun at 10 and 20 m/s at 623 K, (b) α2 vs. time (t) plot showing straight-line relationship for the oxidation of Zr80 Pd30 alloy melt-spun at 10 m/s at 623 K and (c) ln(k) vs. 1/T plot of Zr70 Pd30 alloy melt-spun at 10 m/s in the temperature range of 623–653 K.
oxidation curve has attained a plateau at this stage indicating complete oxidation (∼α = 1). The oxidation follows parabolic rate law, which is evident from the straight-line relationship between α2 and t (Fig. 8(b)). The rate constants (k) obtained from the slope of the α2 –t plot at 623 K for the oxidation of Zr70 Pd30 alloy melt-spun at 10 and 20 m/s is 1.39 × 10−4 and
Alloy
Melt-spun speed (m/s)
First crystallization (Tx 1)
Second crystallization (Tx 2)
Zr70 Pd30
10 20
691 692
744 749
Zr80 Pt20
10 20 30 40
– – – –
– – 867 861
3.32 × 10−4 s−1 , respectively. Activation energy for isothermal oxidation is calculated from ln(k) versus 1/T plot and one such plot for Zr70 Pd30 alloy melt-spun at 10 m/s is shown in Fig. 8(c). Table 3 shows the rate constants at 623 K and activation energy as calculated from isothermal results over the temperature range of 623–653 K for Zr–Pd alloys. The activation energy for oxidation of Zr70 Pd30 alloy melt-spun at 10 m/s is higher than that melt-spun at 20 m/s. In addition, the isothermal oxidation behavior of the meltspun Zr–Pt alloy at 843 K which is around 30 K below the onset temperature of crystallization for Zr–Pt alloy melt-spun at 30 and 40 m/s (Table 1) is shown in Fig. 9(a). The oxidation rate decreased with decreasing melt-spinning speed, similar to the non-isothermal oxidation behavior of the alloys (Fig. 4(a)). Table 3 shows the rate constants at 843 K and activation energy as calculated from isothermal results in the temperature range of 813–843 K for Zr–Pt alloys. The oxidation also follows parabolic rate law in this case (Fig. 9(b)). Here also it is evident that with increase in the melt-spun wheel speed the activation energy decreases. For Zr80 Pt20 alloy melt-spun at 10 m/s, the weight gain is so low that the activation energy for isothermal oxidation could not be calculated in the temperature range studied. Fig. 10(a) shows XRD patterns of the surfaces of partially oxidized the Zr70 Pd30 alloy melt-spun at 20 m/s at 653 K after removing the oxide layer by grinding. The alloy has been oxidized for 30 min at 653 K, which is below the first crystallization temperature of the alloy. As the oxide layer is gradually removed, the XRD pattern reveals the parent amorphous layer. This is also observed for the Zr80 Pt20 alloy melt-spun at 40 m/s (Fig. 10(b)). In case of Zr–Pt alloy, the oxidation has been carried out for 180 min at 673 K, which is also below the crystallization temperatures of the alloy. The oxide layer consists mainly of monoclinic and tetragonal ZrO2 and PdO in case of Zr–Pd alloys and tetragonal ZrO2 in case of Zr–Pt alloy. 4. Discussion The present results indicate that both Zr–Pd and Zr–Pt alloys have the highest oxidation tendency in the amorphous state. This is easy to understand as the amorphous structure is known to have large free volume in compared to crystalline counterpart [3] and hence diffusion of oxygen is expected to be faster through it. Both non-isothermal and isothermal studies (Fig. 3, Fig. 8(a) and Table 3) suggest that Zr70 Pd30 alloy melt-spun at 20 m/s
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Fig. 9. (a) α vs. time (t) plot of isothermally oxidized samples of Zr80 Pt20 alloy melt-spun at 10, 20, 30 and 40 m/s at 843 K and (b) α2 vs. time (t) plot showing straight-line relationship for the oxidation of Zr80 Pt20 alloy melt-spun at 40 m/s at 843 K.
has higher oxidation tendency than either the alloy melt-spun at lower wheel speed or the annealed alloy, though all three of them are amorphous as observed in their XRD patterns (Fig. 1(a)). This difference could be attributed to the possible differences in the structure of the alloy. The alloy melt-spun at slower wheel speed and that subjected to low temperature annealing (623 K to 30 min) are expected to be structurally relaxed and might contain less free volume. Earlier investigators have also shown that at lowering cooling rate and low temperature annealing, the amorphous alloy has lower free volume content because of structural relaxation [19–21]. This is expected to reduce the diffusivity of oxygen through the amorphous structure and thus improve the oxidation resistance of the alloy. Flege et al. [22] have shown that diffusion of tracer Cu is reduced as the Zr-based amorphous alloy gets relaxed. Similar result is expected for the diffusion of oxygen in amorphous alloy. The results of the Zr70 Pd30 alloy (Fig. 3(b)) also suggest that nanoquasicrystallization of glass improves the oxidation resistance of the alloy. Similar observations were made in case of Zr80 Pt20 alloy, wherein the oxidation resistance increases with increasing the volume fraction of nanoquasicrystalline phase in the amorphous matrix or, in other words, a decrease in the
Fig. 10. XRD patterns of partially oxidized samples and after removal of oxide layer for (a) Zr70 Pd30 alloy melt-spun at 20 m/s and (b) Zr80 Pt20 alloy melt-spun at 40 m/s.
amount of residual amorphous phase (Fig. 4(a)). The early studies of Murty et al. [16] indicate that the volume fraction of nanoquasicrystalline phase increases with decreasing the meltspun speed in the range of 40–20 m/s. With thermal treatment of Zr80 Pt20 alloy melt-spun at 40 m/s, more precipitation of nanoquasicrystalline phase has been observed as clear from the XRD pattern (Fig. 1(c)), which has lead to improved oxidation resistance (Fig. 4(b)). Isothermal oxidation of Zr80 Pt20 alloy (Fig. 9(a)) also showed similar results. Non-isothermal oxidation of Zr70 Pd30 alloy melt-spun at 10 and 20 m/s and Zr80 Pt20 alloy melt-spun at 30 and 40 m/s has revealed that oxidation rate suddenly increases at a temperature close to the crystallization temperature of the alloys, indicating possibly that oxidation is enhanced by crystallization. However, detailed investigations have shown that the sudden increase in the oxidation at a fixed temperature in Zr–Pd alloys is mainly due to a phase transformation in the structure of Zr oxide which leads to a volume expansion in the oxide layer leading to its cracking and not to the nanoquasicrystallization process [23,24]. In fact, studies on partial isothermal oxidation and subsequent oxide scale
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removal results (Fig. 10) prove that isothermal oxidation takes place in fully amorphous Zr70 Pd30 alloy melt-spun at 20 m/s, and in partially nanoquasicrystalline Zr80 Pt20 alloy melt-spun at 40 m/s. Preservation of the parent phase underneath the surface oxide layer also proves that surface oxidation has not led to crystallization in either of the cases. The previous work of the present authors [23] and in the present work clearly demonstrates that the stronger interaction between Zr and Pt in comparison to Zr–Pd leads to lower oxidation rate in the former alloys. In addition, the oxidation behavior of these two alloys mainly depends on the structure of the alloys and that of oxide scale, more specifically the tetragonal or monoclinic nature of ZrO2 [23]. The present results also confirm that oxidation resistance of the Zr alloys studied is highest if they are in the completely crystallized state (Zr80 Pt20 alloy meltspun at 10 m/s). The fact that the crystalline ␣-Zr has better oxidation resistance than the amorphous, nanoquasicrystalline states and the completely crystallized Zr–Pt alloy is also clear from Fig. 4(a). Thus the oxidation resistance of the different structures studied could be arranged in the following order:
lization of the amorphous phase in the Zr70 Pd30 and Zr80 Pt20 alloys are the amorphous-nanoquasicrystalline interfaces. With increased volume fraction of nanoquasicrystalline phase a second type of interface, namely the nanoquasicrystalline grain boundaries appears in the alloys. Further annealing leads to the formation of crystalline phases and thus the crystalline grain boundaries and crystalline interphase interfaces replace the nanoquasicrystalline grain boundaries. Finally, the highest oxidation resistance, among the structures studied, has been observed with crystalline ␣-Zr with crystalline high angle grain boundaries. Thus, the oxidation tendency of the alloy appears to increase with the presence of various interfaces/boundaries in the following order:
crystalline Zr > crystalline Zr alloy
This could be due to differences in the oxygen diffusivity along the various boundaries present in the alloy at the metal–oxide interface. Zr is known to dissolve up to 30 at.% of oxygen [27] at room temperature. Beyond this concentration, precipitation of ZrO2 is observed [28]. Thus, oxygen in the atmosphere first dissolves in Zr and, once the solubility limit is crossed, ZrO2 layer forms. Once the oxide layer forms, further oxidation occurs by diffusion of oxygen through the ZrO2 layer to the metal-oxide interface followed by its dissolution in the alloy and precipitation of ZrO2 once the solubility limit is crossed. The ZrO2 layer formed in the Zr70 Pd30 and Zr80 Pt20 alloys appears to be nanocrystalline in nature from its broad XRD peaks (Fig. 6(a) and (b)). The earlier studies [4,11] also indicate the formation of nanocrystalline ZrO2 . Diffusion of oxygen through nanocrystalline ZrO2 is expected to be faster [29]. The present calculations show that the crystallite size of ZrO2 in case of Zr70 Pd30 alloys is smaller than that in Zr80 Pt20 alloys, which means that diffusion through ZrO2 is faster in the former than in the latter. Jastrow and K¨oster [13] has suggested that the growth of oxide layer proceeds by inward oxygen diffusion probably along the interfaces of the lamellar microstructure towards the reaction zone at the interfaces between oxide and unoxidized alloy. Such diffusion of oxygen can be orders of magnitude faster the volume diffusion in the bulk of ZrO2 grains [29]. Once oxygen reaches the metal–oxide interface, the rate of further oxidation should depend on how fast oxygen will diffuse into the alloy so that the total dissolved oxygen reaches the solubility limit for ZrO2 to precipitate. The diffusivity of oxygen in Zr(O) phase is known to be faster than that in ZrO2 (4 × 10−21 and 5 × 10−22 m2 /s, respectively (at 530 K)) [28]. Volume and grain boundary diffusion of oxygen in undoped nanocrystalline monoclinic ZrO2 are recently calculated by Brossmann et al. [30] to be less than 10−22 and 10−18 m2 /s, respectively. However, the diffusion of oxygen in Zr(O) phase should depend on the microstructure of the Zr alloy. In case of an amorphous alloy
> nanoquasicrystalline Zr alloy > relaxed amorphous alloy > amorphous Zr alloy The present results are in agreement with those of K¨oster and his coworkers [4,11,12], who have shown that nanocrystalline and quasicrystalline alloys have better oxidation resistance than the amorphous alloys in case of Zr–Cu–Ni–Al and Zr–Pd alloys. Kai et al. [25] has also shown that the crystalline Z55 Cu30 Al10 N15 alloy has higher oxidation resistance than amorphous alloy. The present results also indicate that amorphous structure in the Zr–Pt alloy (the alloy melt-spun at 40 m/s, which has a large fraction of amorphous phase) has better oxidation resistance than that of the Zr70 Pd30 alloy (Fig. 4(c)). This could probably be due to slower diffusion of oxygen in Zt–Pt amorphous structure than in Zr–Pd. This could be attributed to the possible higher density of packing in Zr–Pt glass (stronger interaction between Zr and Pt) due to the larger enthalpy of mixing in the system (−100 kJ/mol) in comparison to the Zr–Pd alloy (−91 kJ/mol) [26]. It has also been shown earlier that transformation of tetragonal to monoclinic ZrO2 leading to difference in volume expansion is a strong reason for the difference in the oxidation behavior of Zr–Pd and Zr–Pt alloys [23]. However, Jastrow and co-workers [12,13] have proposed that the difference in the behavior of Zr70 Pd30 and Zr80 Pt20 might be due to different solubility of the noble metals into the ZrO2 . However, no data on the extent of solubility of Pd and Pt in ZrO2 is available in the literature. Oxidation in Zr alloys is usually driven by the diffusion of oxygen from the atmosphere to the oxide–metal interface through the oxide scale. The present results demonstrate that the microstructure and the various phase/grain boundaries in the alloy play a very important role in its oxidation resistance. The type of interfaces expected in the first stage of crystal-
grain boundaries in ␣-Zr >
grain boundaries interfaces in crystalline Zr alloy
> nanoquasicrystalline grain boundaries > amorphous-nanoquasicrystalline interfaces > amorphous structure
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Table 2 Activation energy for the start of oxidation in melt-spun amorphous alloys calculated from kissinger plot Alloy
Activation energy for the oxidation to start (kJ/mol)
Start temperature for oxidation (10 K/min)
Zr70 Pd30 10 m/s Zr70 Pd30 20 m/s Zr80 Pt20 40 m/s
115 111 299
683 K 67 8 K 842 K
Table 3 Rate constants and activation energies for isothermal oxidation of melt-spun alloys Alloy
Rate constant (s−1 ) At 623 K
Zr70 Pd30 10 m/s Zr70 Pd30 20 m/s Zr8 0Pt20 10 m/s Zr80 Pt20 20 m/s Zr80 Pt20 30 m/s Zr80 Pt20 40 m/s
Activation energy (kJ/mol) At 843 K
623–653 K
813–843 K
1.71 × 10−6 3.67 × 10−6 4.34 × 10−6 6.22 × 10−6
67.3 49.7 – – – –
– 217 175 109
2.03 × 10−4
3.32 × 10−4 – – – –
the diffusion is expected to be faster in comparison to all other microstructures studied in the present investigation and hence this type of microstructure shows the highest oxidation tendency. Once the nanoquasicrystalline phase forms in the amorphous matrix the oxidation tendency of the alloy gets reduced as the open structure of amorphous phase because of higher free volume content is replaced with amorphous-nanoquasicrystalline interfaces and nanoquasicrystalline grain boundaries. Diffusion of oxygen along these interfaces/boundaries is expected to be slower than that in the amorphous phase. Once the amorphous phase crystallizes completely, diffusion through the microcrystalline interphase interfaces is expected to be further lower in comparison to the nanoquasicrystalline interfaces/boundaries and hence the oxidation resistance of the alloy increases. This is also shown by the lowering of activation energy for oxidation with the progress in crystallization in melt-spun alloys (Table 3). Finally, in case of conventional pure Zr, which contains only high angle grain boundaries, the diffusivity of oxygen would be slower than that along either nanoquasicrystalline boundaries or interphase interfaces due to the less amount of grain boundary area. This explains the highest oxidation resistance of Zr. The effect of grain boundary on the isothermal oxidation behavior of binary Zr–Pd amorphous alloy and its crystalline counterpart has been discussed by Jastrow and K¨oster [13]. They showed that the large crystalline grains are very stable against oxidation, but oxidation proceeds very deep into the material preferentially along the grain boundaries associated with large stress fields. The stress field is more in case of large grained microstructure. From the total oxygen dissolution or total weight gain point of view, oxidation resistance of crystalline Zr is much better than the other alloys. Non-isothermal and isothermal oxidation of melt-spun binary alloys, are closely related, yet there are differences. First of all, non-isothermal oxidation is necessary to study the change in the oxidation behavior of any alloy with temperature. The present results indicate that there is a significant difference between activation energies found out from non-isothermal and isothermal
oxidation results (Tables 2 and 3). It appears that change in structure of the alloy during non-isothermal study plays an important role. Isothermal oxidation temperature is about 30–80 K less than the crystallization onset temperatures in both Zr–Pd and Zr–Pt alloys as observed in Tables 1 and 3. As a result, the as melt-spun structure remains more or less unchanged during isothermal oxidation studies, which is evident from Fig. 10. In case of non-isothermal oxidation, there is a possibility of crystallization of the amorphous phase (in both Zr–Pd and Zr–Pt alloys) and grain growth in the nanoquasicrystalline phase present in Zr–Pt alloy. Both these processes can lead to a decrease in the oxidation tendency of the alloy. Thus, the higher activation energies observed in case of non-isothermal oxidation in comparison to isothermal oxidation could be attributed to the possible crystallization and grain growth of the nanoquasicrystalline phase in the former case due to the higher temperatures experience. 5. Conclusions • The oxidation resistance of Zr–Pd and Zr–Pt alloys increases in the order of amorphous, nanoquasicrystalline and crystalline phases. • The microstructure of the alloy, in terms of different interfaces/boundaries, plays a significant role in the oxidation behavior of the alloy. The oxidation resistance appears to increase in the order of the amorphous phase, amorphous-nanocrystalline interfaces, nanoquasicrystalline grain boundaries, crystalline interphase interfaces/grain boundaries and grain boundaries in ␣-Zr. • A relaxed amorphous alloy shows better oxidation resistance than that of the unrelaxed one. References [1] W. Klement, R.H. Willens, P. Duwez, Nature 187 (1960) 869. [2] H. Warlimont, Mater. Sci. Eng. A 304–306 (2001) 61. [3] A. Inoue, Acta Mater. 48 (2000) 279.
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