Air‐Oxidation of a Cu₄₅Zr₄₅Al₅Ag₅ Bulk Metallic Glass

Abstract

The development of bulk-metallic glasses (BMGs) has been extensively studied over the past decades. Among those BMG systems developed, Zr-, Cu-, and Cu-Zr-based BMGs are generally the most popular amorphous alloys that possess the advantages of low material cost, superior mechanical properties, and good glass forming ability (GFA) against crystallization during their fabrication. [1][2][3][4][5][6] An example of the Cu 47.5 Zr 47.5 Al 5 -BMG (CZA3-BMG) had an excellent yield strength of 1.547 MPa and yield strain of 2.0%. In addition, this BMG also exhibited a work-hardening behavior when the applied stress level increased, which in turn had a larger plastic strain of 16%. [6][7][8] In addition, it was also found that small amounts of Ag can significantly improve the GFA and supercooled liquid region (DT x ) for both (Cu 0.5 Zr 0.5 ) 100Àx Ag x and Cu 45 Zr 45 Al 10Àx Ag x BMGs. [9][10] A typical example of the CZA4-BMG BMG exhibited a larger DT x of 61.3 8C with respect to $50.2 8C for the CZA3-BMG. An important aspect to use an amorphous alloy in ambient environments for practical applications is its capability of resisting the thermal degradation during oxidation. However, little work has been done on the oxidation behavior of the CZA4-BMG with the Ag addition although that of the Ag-free alloy (CZA3-BMG) has been studied previously. [11] Thus, the main goal of this work is to investigate the oxidation behavior of the CZA4-BMG, and, in particular, to understand the role of Ag on the oxidation kinetics with respect to those of the CZA3-BMG. ExperimentalThe CZA4-BMG was prepared by an injection-casting technique, described elsewhere [11] . The average composition in atomic percent for the CZA4-BMG, analyzed by X-ray wavelength-dispersive spectroscopy (WDS), contained 45.03% Cu, 45.10% Zr, and 4.94% Al, and 4.93% Ag. BMG rods prepared for oxidation tests were about 3 mm in diameter and 0.5 mm in length. Samples were ground and polished down to a 0.3 mm diamond paste, cleaned with acetone, and immediately dried before the tests.The thermal stability of the as-cast BMG samples was examined by the differential-scanning calorimetry (DSC) at a heating rate of 20 -C min S1 . Oxidation tests were carried out by means of the thermogravimetric analyzer (TGA) in dry air (>99.999 vol.% pure). For each experiment, the net flow rate of air was kept constant at 40 cm 3 min S1 , and the heating and cooling rates of the TGA furnace were set at 10 -C min S1 . The characterization of the BMGs and scales was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with the X-ray energy-dispersive spectrometry (EDS), electron probe microanalyzer (EPMA) equipped with WDS, and transmission electron microscopy (TEM) equipped with EDS and selected-area diffraction (SAD), as described previously [12] . Results and Discussion Alloy Constitution and Thermal StabilityThe XRD spectra of the as-cast CZA4-BMG are shown in Figure 1, revealing that this alloy only contains two widebroadening peaks near 2u ¼ 38.7 and 6...

The oxidation behavior of Cu42Zr42Al8Ag8 bulk metallic glasses

“…The large GFA, superior mechanical properties, and Ni-free nature, endowed such metallic glasses with potential applications in not only structural materials but also biomaterials. However, the investigations of the environmental behavior of metallic glasses in such systems are very limited [8,9]. It was reported that elemental metals such as nickel [10], copper [11][12][13][14], and noble metal (palladium, platinum, and gold) [14][15][16] could be separated or precipitated out during oxidation of zirconium-containing metallic glasses.…”

“…However, few investigations have been conducted on the oxidation behavior of silver containing glassy alloy. Recently, Kai et al [9] To improve the homogeneity of the alloy, re-melting was done three times. 3 mm diameter BMG samples were produced by copper mold casting technique.…”

Effects of Si addition on the oxidation behavior of a Cu–Zr-based bulk metallic alloy

“…BMG samples were directly sliced into about 1 mm in thickness, ground and polished with a 1-mm diamond paste, cleaned with acetone and methanol, immediately dried before the tests. Oxidation tests and characterization of the substrate and scales were similar to those described in the previous study [10].…”

“…The oxidation rate constants (k p values) at the steady-state stage of both alloys are summarized in Table 1. The k p values of CZ43-BMG progressively increase with increasing temperature when it is below 450 C, while a significant lower k p value is obtained at 500 C. For comparative purposes, the k p values of Cu 45 Zr 45 Al 5 Ag 5 BMG (CZ45-BMG) in the previous study are also tabulated in the same table [10]. In general, the CZ43-BMG exhibited slightly lower k p values at T 425 C, while the reverse condition was observed at higher temperatures.…”

“…For comparative purposes, it is useful to review the scale constitution and phases and the crystallization sequence of CZ45-BMG after air oxidation at the same temperature range [10]. The scales formed on CZ45-BMG were strongly temperature-dependent, consisting mostly of CuO and tetragonal-ZrO 2 (t-ZrO 2 ), minor amounts of monoclinic-and orthorhombic-ZrO 2 (m-and o-ZrO 2 ), and uncorroded Ag particles at T 450 C, while an additional aAl 2 O 3 phase was also detected at 500 C. In addition, the crystallization sequence for this BMG at 400 C underwent an initial selective oxidation of Zr to form t-and o-ZrO 2 (governed by inward diffusion of O), followed by the growth of CuO and m-ZrO 2 , while the substrate retained amorphous nature.…”