Spontaneous formation of filamentary Cd whiskers and degradation of CdMgYb icosahedral quasicrystal under ambient condit
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e whiskers spontaneously grown in Cd–Mg–Yb alloy containing icosahedral quasicrystal as dominant phase have been investigated by electron microscopy. It is found that: (i) the whiskers are mainly composed of Cd with growth direction along ,2110>Cd . CdO particles are frequently observed on surfaces of whiskers and the alloy, indicating the importance of the oxidation process during the whisker growth; (ii) there exist four types of orientation relationship between Cd and CdO. The interfaces between them are shown to accommodate large lattice misfit, which is well explained by near coincidence site lattice model; (iii) nanosized Cd particles are observed to aggregate around the whisker root, providing a convincing experimental evidence for the long-range atomic diffusion. Our study offers a unique opportunity to unveil the relationship between unstability of quasicrystal structure and whisker formation and may have some implications for oxidation process of metals.
I. INTRODUCTION
Cd-based alloy is one of the most significant groups in quasicrystal field. Inspired by the idea of replacing Zn by Cd in icosahedral quasicrystal (IQC) Zn–Mg–RE (RE 5 Rare Earth element),1,2 the stable quasicrystal of Cd–Mg–RE alloys were fabricated by Guo et al.3 Concerning that Cd and Mg can be replaced by each other,4 binary quasicrystal Cd5.7Yb eventually appeared.5,6 This thermodynamically stable binary quasicrystal was truly a landmark for that it offered a unique opportunity to provide a basis for the construction of crystallographic models.7 Unfortunately, the Cd-based alloys are very reactive and unstable, illustrated by the spontaneous whiskers growth on their surfaces in the air,8,9 leading to the degradation of the quasicrystal structure. Hence, a fundamental understanding of the detailed structure and nucleation mechanism of whisker is highly demanded. Since the first discovery of Cd whiskers in 1946,10 accumulated efforts have been dedicated into studying the spontaneous growth of low melting point metal whiskers including Sn, Cd, and Zn, etc., under ambient conditions. Despite the practical problems it may cause to the corresponding applications in electronic industry, whiskers were reported to possess remarkable mechanical properties such as high yielding strength.11 The whisker growth mechanism has consistently retained the attention of numerous researchers. In 1994, Tu12 put forward a theory that whiskers could grow at some localized stress relief a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.136 J. Mater. Res., Vol. 27, No. 14, Jul 28, 2012
centers where the surface oxide layer was broken. Later, Wu et al.13 suggested that oxygen was essential for the enhanced whisker growth kinetics observed after comparing the morphology, density, and growth rate of Cd islands and whiskers formed on surfaces of several Cd-based compounds. Now, it is widely accepted that whiskergrowth is a stress relief phenomenon.14–16 The strain gradient as a result of the mechanical (e.g., externally appl
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