The oxidation behavior of Cu₄₂Zr₄₂Al₈Ag₈ bulk metallic glasses

Parabolic plots of the oxidation kinetics over the temperature range of 330–460 °C are shown in Fig. 1. It is clear that the oxidation reaction kinetics could be well described by a parabolic rate law, W = kt ^1/2, where W is mass gain per unit area of the specimen, t the oxidation time in seconds, and k (g cm⁻² s^−1/2) is the parabolic rate constant. The oxidation kinetics followed a single parabolic rate law at low temperature (330–390 °C), but exhibited a two-stage parabolic rate law at high temperature (420–460 °C) which consists of a fast-growth stage oxidation and then a slow-growth stage. Values of k at different temperatures and stages were fitted and listed in Table 1. The duration of the first stage was also listed in Table 1. It is clear that the duration decreased with increasing temperature. Using value of k in Table 1, the activation energy Q for oxidation can be calculated with the Arrhenius equationwhere k is the parabolic rate constant, k ₀ is a constant, R is the universal gas constant, and T is the oxidation temperature in Kelvin scale. Figure 2a shows the Arrhenius plot between ln (k ₁) and 1000/T, where k ₁ is the slope measured from the initial linear portion of Fig. 1. All points lie on a straight line. The activation energy and pre-factor of the initial stage oxidation, related to the formation of the thin oxide layer, are calculated to be Q = 78 ± 3 kJ/mol, k ₀ = 3.58 g cm⁻² s^−1/2, respectively. However, the points in the second stage at 420–460 °C cannot be fitted with a line as shown in Fig. 2b, which indicates that the oxidation behavior in the temperature region cannot be simply described with a single activation energy.

Figure 3 shows the corresponding XRD patterns of oxide phases formed at different temperatures. At 330 °C, it can be seen that tetragonal ZrO₂ (t-ZrO₂) and metallic Ag are dominant in the surface layer. With increased temperature, minor amounts of Al₂O₃ and copper oxides appeared. The intensities of Cu₂O and CuO peaks become stronger with further increased temperature, which indicates the enrichment of copper oxides in the oxide layer. Minor amounts of m-ZrO₂ were observed at high temperatures (420–460 °C) but absent at low temperatures (330–390 °C). The formation of m-ZrO₂ was commonly seen in the high-temperature (above T _g) oxidation of Zr-based metallic glass [18, 19]. The structures of substrates were also examined. Crystal peaks were only observed above 450 °C, as shown in Fig. 4. The dominance of ZrO₂ in the oxide layer could be explained by the activation energy for diffusion in metal oxides. The activation energy (78 ± 3 kJ/mol) for oxidation of Cu₄₂Zr₄₂Al₈Ag₈ metallic glass obtained in this study is roughly close to the value for diffusion of oxygen in ZrO₂ (86.9 kJ/mol) [20], suggesting that the oxidation of Cu₄₂Zr₄₂Al₈Ag₈ metallic glass in the first stage might be controlled by the oxygen diffusion in ZrO₂. Similar results were also obtained in Cu₆₀Zr₃₀Ti₁₀ BMGs but at lower temperatures [21]. To investigate the oxidation behavior in the second stage at high temperatures, different short-term oxidation tests were performed at 450 °C. XRD analyses of the oxide scales produced by the exposure of the amorphous sample are shown in Fig. 5. It is obvious that, in the first 10 min, t-ZrO₂ and Ag were the main products formed on the surface, although minor Al₂O₃ was also detected. After the first stage oxidation (τ = 1764 s) for 30 min, little m-ZrO₂ and Cu₂O phases were detected. With further oxidation, the amount of Cu₂O phases increased and CuO phase appeared (Fig. 3). From the above results, it is concluded that the growth of CuO/Cu₂O is responsible for the second-stage oxidation behavior at T ≥ 420 °C.

Figure 6 shows the surface morphologies of specimens after oxidation at different temperatures. As seen in Fig. 6a, some white precipitates were formed along the grinding grooves on the oxidized sample surface at 330 °C. The particles might be Ag as predicted from the XRD spectra in Fig. 3. After further oxidation at 390 °C, many large hemispherical white precipitates were observed (Fig. 6b). EDS analysis showed that the white precipitates were almost pure silver (~92 %). The products around the silver precipitates are mainly composed of copper, corresponding to copper oxides. At 420 °C, mushroom-like precipitates which are similar to that formed on the ribbon counterpart were occasionally seen on the surface, as shown in Fig. 6c. Small Ag–metal channels through the oxide scale might exist, as observed in [17]. These channels could enhance the inward diffusion of oxygen. The high oxidation tendency of amorphous alloys with noble-metal-containing was also found in references [14‐16]. It was explained that in the noble-metal-containing amorphous alloy, the high oxidation rate is due to the large atomic size of noble metals which leads to an increase of the vacant space, facilitating higher oxygen diffusivity [14]. Meanwhile, the accelerated oxidation can also found in crystalline alloys with certain noble metals concentrations in which internal oxidation occurs [22, 23]. Although noble metals have low affinity for oxygen, the interface between the alloy and the internal oxide particles (internal oxidation zone) enhanced oxygen diffusion.

After oxidization at 450 °C, little white mushroom-shaped particles were observed and many copper oxide islands were found on the surface (Fig. 6d). As seen from the inset in Fig. 6d, an area with the island-like topmost layer (copper oxides) peeled off and this shows that many precipitates were embedded beneath. According to the EDS results, the precipitates are mainly composed of silver. Besides, many openings around the silver precipitates were observed, which is consistent with the discrete distribution of the hemispherical silver shown in Fig. 6b. From the above results, it could be inferred that the oxidation process starts with the formation of ZrO₂ followed by the precipitation of silver particles. As explained by Zhang et al. [17] that silver mostly existed as solute in Zr, once Zr was oxidized to ZrO₂, pure metal silver will precipitate out due to the high enthalpy of formation for the silver oxide in the temperature range used in this study [24]. The weak boundaries or vacancies induced by oxidation are promising segregation sites for the expelled Ag. Meanwhile, the copper remaining in the matrix tended to move outward to form oxides along the grinding grooves. The internal stress induced by volume expansion squeezed the hemispherical silver precipitates to produce a mushroom-like morphology. With increasing temperature, copper ions move faster than silver due to its smaller atomic size, and finally, the copper oxides covered the silver precipitates underneath.

Figure 7 shows SEM micrographs in back-scattered-electron-image (BEI) mode of the cross sections of specimens oxidized from 360 to 460 °C. There are two oxide layers at 360 °C (Fig. 7a), 390 °C (Fig. 7b), and 420 °C (Fig. 7c) and the thickness of the scale increased with increasing temperature. The outer layer mainly contains Ag (with arrows below) and Cu, which is in agreement with the observation of the top surface. A rugged oxide–alloy interface between the matrix and the inner layer was clearly seen from 360 to 420 °C. It is probably due to the combined effect of outwards diffusion of the copper and the silver in the alloy [25]. The outward diffusion of Cu and Ag from the alloy to the oxide leads to open space in the glassy lattice. In order to fill the space, the alloy may deform plastically and move inward. Similar observations were also found in Zr–Cu–Al–Ni [26] and Cu–Zr–Al [27] metallic glasses.

After oxidation at 450 °C, a sandwich multilayer was observed, as shown in Fig. 7d. According to the results of EDS line profile (Fig. 7e), the island-like outermost layer is Cu-rich, the middle layer with white precipitates is silver-rich, and the inner layer is mainly Zr-rich. The inner layer can be further divided into two layers. The upper inner layer has lower copper content than that in the innermost layer. As the topmost layer is mainly composed of copper, we speculate that the lower content of copper in the upper inner layer is due to the fast outward diffusion of copper to the topmost layer. It is also the reason that silver precipitates were seldom observed on the top surface at high temperatures. The increased copper oxides at the topmost layer indicate that the rate-controlling step in the second-stage oxidation process at higher temperatures is the outward diffusion of copper. Meanwhile, the upper inner layer has a higher content of silver than that in the innermost layer as indicated by the appearance of a peak in the EDS line profile. This suggests that many silver particles may be embedded in the zirconium oxide. Similar observations were also found in Zr–Pd and Zr–Au metallic glasses [14‐16]. The average thickness of the oxide scale at temperature 450 and 460 °C is 7.6 ± 1 and 9.8 ± 1 μm, respectively. Selective oxidation with formation of oxide protrusions on the substrate was observed at 460 °C (Fig. 6f). It can be attributed to the coarse crystallized substrate as the oxidation is preferred in certain orientations of the grain or along grain boundaries [28].

As far as the difference in oxidation behavior between the glassy state and the supercooled liquid state was concerned, although the oxidation kinetics in both states follows the parabolic law and is controlled by the diffusion of O and Cu, the oxidation rates are quite different. Oxidation rate is much higher in the supercooled liquid state (in the initial period, see Fig. 1). This is due to not only the temperature effect but also a faster diffusion in the supercooled liquid state [29, 30]. The gradual slows down of oxidation rate at high temperatures can be attributed to two reasons. One is the thick oxide scale, which increased the diffusion distance between the oxygen ions and the metal ions. The other one is possibly related to the crystallization of metallic glasses [31]. This point was testified by many comparative studies that the mass gain of amorphous alloys is considerably larger than that of the crystallized alloy in the temperature region corresponding to the supercooled liquid state [21, 26].