Precipitation of Zr 2 Cu Stable Crystalline Phase from Zr 70 Cu 27.5 Rh 2.5 Amorphous Alloy
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Precipitation of Zr2Cu Stable Crystalline Phase from Zr70Cu27.5Rh2.5 Amorphous Alloy David Cushman1, Junji Saida2, and Chunfei Li1 1 Department of Physics, Portland State University, Portland, OR, 97207 2 Center for Interdisciplinary Research, Tohoku University, Sendai, 980-8578, Japan
ABSTRACT The crystallization process of Zr70Cu27.5Rh2.5 metallic glass was studied with Transmission Electron Microscopy (TEM). In contrast to previous studies where the precipitation of metastable icosahedral quasicrystalline (IQC) particles is of the interest, we designed the present work to focus on the nucleation process of the stable Zr2Cu crystalline phase. It has been found that the alloy consists of IQC particles distributed in amorphous matrix prior to the precipitation of the Zr2Cu stable crystalline phase and Zr2Cu nucleates from the amorphous matrix. The encounter of the IQC phase with Zr2Cu transforms the former into the latter so quickly that no interface between them was found in the present experiment. These insights provide the basis for a discussion of the stability of metallic glasses and the IQC particles. INTRODUCTION Compared to their crystalline counterparts, metallic glasses show some distinguished mechanical and physical characteristics, e.g., high corrosion resistivity and high strength [1]. The atomic structure of metallic glasses can be considered as obtained through freezing the molten alloy, where a rapid quenching process is involved. For most alloy systems, a cooling rate of approximately 106 K/s is required; otherwise the undesired crystalline phases precipitate. A process called single roller melt spinning is commonly used to ensure rapid cooling. The shape of the metallic glass is typically limited to a thin ribbon with a cross section of approximately 0.03×1 mm2, which makes the glass unsuitable for structural purposes in many cases. Naturally, there are many efforts to develop alloy systems that require relatively lower cooling rates for the conservation of the melt state. These works were rewarded in the early 1990’s, and were represented by reports regarding the preparation of so called bulk metallic glasses in Zr-, La-, and Pd-based alloys [2,3]. Among these, Zr-based alloys attract special attention because of the high glass forming ability and the excellent mechanical properties [4, 5]. Studies about the stability revealed the precipitation of an icosahedral quasicrystalline (IQC) phase in the initial crystallization process in some selected Zr-based alloys. These alloys were obtained by adding some noble metals such as Ag, Pd, Pt, Ir, and Rh to alloys that were reported to have high glass forming ability [6-9]. All the precipitated IQC phases are metastable, implying that further annealing leads to its disappearance and the precipitation of a stable crystalline phase(s). Intense studies have been devoted to eliminate questions regarding IQC phase precipitation, and determine from which metallic glasses they arise. It is important to understand the precipitation process of the c
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