Elsevier

Journal of Alloys and Compounds

Volume 683, 25 October 2016, Pages 22-31
Journal of Alloys and Compounds

Effect of Nb substitution for Cu on glass formation and corrosion behavior of Zrsingle bondCusingle bondAgsingle bondAlsingle bondBe bulk metallic glass

https://doi.org/10.1016/j.jallcom.2016.05.077Get rights and content

Highlights

  • The addition of Nb deteriorates glass-forming ability.

  • Substitution of Nb for Cu improves corrosion resistance in various solutions.

  • Dissolve of Cu is the main contribution to corrosion in Cl-contained solution.

  • Substitution of Nb for Cu decreases the pitting nucleation site.

  • Nb2O5 is stable in Cl-contained solution restraining Cu dissolve.

Abstract

The influence of Nb substitution for Cu on glass forming ability and corrosion behavior in Zrsingle bondCusingle bondAgsingle bondAlsingle bondBe alloy was systematically studied. It was found that although Nb addition deteriorated glass forming ability and thermal stability, corrosion resistance, which was evaluated by both potentiodynamic polarization and immersion tests, was improved in 0.5 M NaCl, 0.1 M HCl and 0.5 M H2SO4 solutions. X-ray photoelectron spectroscopy depth profiles on the polished specimens revealed that Nb addition promoted the oxidation of Zr and resulted in thicker oxide film with high valence. Meanwhile, composition analyses before and after immersion in 0.5 M NaCl demonstrated that Cu had a strong affinity for reacting with chloride ions, causing the pitting susceptibility, and the substitution of Nb for Cu not only decreased the pitting nucleation site but also restrained the dissolve of Cu significantly through the passive film. As the immersion time was further prolonged, oxygen content in surface film increased remarkably, and only the fraction of Nb among all metallic elements rose, suggesting that the stability of Nb2O5 in chloride-containing solutions is crucial to improving corrosion resistance.

Introduction

Bulk metallic glasses (BMGs) exhibit an excellent combination of physical, magnetic, mechanical, and chemical properties due to the unique disordered atomic structure [1], [2], [3]. The corrosion-resistance behavior becomes crucial when these BMGs are considered for biomedical applications or used in hostile environments. BMGs have superior corrosion resistance in comparison with the crystalline counterparts due to the chemical homogeneity and the lack of crystal defects such as grain boundaries and dislocations, which could act as galvanic cells to initiate localized corrosion [4], [5]. Corrosion behavior of Cu- [6], [7], Fe- [8], Mg- [9], Ni- [10], [11], Ti- [12], [13] and Zr-based [14], [15], [16] BMGs were systematically investigated. Numerous attempts have been made to enhance the corrosion resistance by modifying the surface microstructure and composition, such as micro-arc oxidation and ion-implantation [17], [18]; decreasing the free-volume content through structural relaxation [19]; introducing second-phase into the amorphous matrix through either in-situ or ex-situ methods [20], [21]; adding valve elements with well-known passivating abilities [22], [23].

Since Zr-based BMGs have superior glass forming ability (GFA), improving their prospects for engineering applications [24], a number of investigations have been devoted to study their corrosion behaviors with especially much attention in the alloy systems, such as Zr–Cu–Al–Ni [14], Zr–Al–Co [25] and Zrsingle bondTisingle bondNisingle bondCu [26]. Recently, a series of Zrsingle bondCusingle bondAgsingle bondAlsingle bondBe BMGs were developed with the critical size of glassy rods up to 73 mm in diameter prepared by direct copper mold casting [27]. To have the opportunity of industrial applications, besides the high GFA, the corrosion resistance is also a critical factor for the consideration of their use in hostile environments. In previous corrosion work, it was found that the Zr-based BMGs were more resistant to localized corrosion in chloride-free solutions such as H2SO4, Na2SO4 and NaOH when compared to chloride-containing solutions, and component Cu were considered as weak point to corrosion resistance [14], [15], [28], [29]. Thus, it is important to enhance its localized corrosion resistance in solutions that contain chloride ions and understand its corrosion behavior and pitting mechanism. Unalloyed metal Zr [30], Ag [31] and Al [32] exhibited stable passivated when they were potentiodynamically polarized in NaCl aqueous solution, while Cu [33], [34] did not show obvious passive region, which means the corrosion resistance of Cu is inferior in NaCl aqueous solution. Numerous studies have revealed that the addition of valve metal elements such as Hf, Nb and Ti [25], [35], [36], can evidently improve the corrosion resistance of Zr-based BMGs to general corrosion and pitting corrosion, but the detailed mechanism is still far from being fully understood. In this work, we investigated the influence of substituting Nb for Cu on corrosion behavior in 0.5 M NaCl, 0.5 M H2SO4 and 0.1 M HCl solutions of Zrsingle bondCusingle bondAgsingle bondAlsingle bondBe BMG. Possible reasons for the improved corrosion resistance were also discussed.

Section snippets

Experimental procedure

The pre-alloyed ingots with nominally chemical compositions of Zr46Cu(30.14−x)NbxAg8.36Al8Be7.5 (at.%) with x = 0, 2, 5, 10 and 20% were prepared by arc-melting high purity Zr, Cu, Ag, Al and Nb with commercial Cu55.3Be44.7 alloy under a Ti-gettered purified argon atmosphere. Each ingot was remelted at least five times to ensure its chemical homogeneity. Plates with the thickness of 1.5 mm and the width of 10 mm and rods with the diameter of 3 mm were fabricated by suction-casting into

Influence of Nb addition on GFA and thermal stability

The as-cast Zr46Cu30.14−xNbxAg8.36Al8Be7.5 (x = 0, 2, 5, 10 and 20%) plate specimens with the thickness of 1.5 mm were examined by XRD, and all compositions except for x = 20% exhibit broad diffraction peaks, characteristic of amorphous structure, as showed in Fig. 1(a). It was reported that the composition with x = 0% possessed excellent GFA with the critical diameter of 73 mm prepared by direct copper mold casting [27]. When the addition of Nb reaches 10%, the XRD result indicates that BMG

Conclusions

The influence of Nb substitution for Cu on GFA and corrosion behavior in Zr46Cu30.14Ag8.36Al8Be7.5 alloy was systematically investigated. The main results allowed the following conclusions to be made:

  • (a)

    With the substitution of Nb for Cu up to 10%, the BMG still has a good GFA with the critical size of 1.5 mm in plate and 20 mm in diameter by cooper mold suction casting. Further increasing the Nb content to 20%, GFA was deteriorated with the 1.5 mm plate incapable of being fully amorphous. It was

Acknowledgements

Financial supports from the National Key Basic Research Program of China (2012CB825700), National Natural Science Foundation of China (grants 51371157 and U14321056), Natural Science Foundation of Zhejiang Province (grants Z1110196 and LY15E010003), and the Fundamental Research Funds for the Central Universities (2016FZA4006) are gratefully acknowledged.

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