Effect of Si and P on the Formation and Crystallization of Ti 25 Hf 50 Ni 25 Metallic Glasses
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Effect of Si and P on the Formation and Crystallization of Ti25Hf50Ni25 Metallic Glasses V. T. Huett and K. F. Kelton Department of Physics, Washington University, Saint Louis, MO 63130, U.S.A. ABSTRACT Ti25Hf50Ni25 metallic glasses, prepared by rapid quenching, are strongly metastable with a 65°C separation between the glass transition temperature, Tg = 335°C, and the onset temperature for primary crystallization. The deep metastability of the glass is likely due to sluggish nucleation and growth kinetics, limited by the Hf diffusion. The glass crystallizes to a nanoscale microstructure consisting of an icosahedrally symmetric ordered phase. This phase is metastable and transforms to a stable Ti2Ni-type phase with annealing at higher temperatures. The primary crystallization to a metastable icosahedrally-ordered phase suggests that the local structure of the glass contains a high degree of icosahedral short-range order. Here, glass formation and crystallization were studied in alloys containing 2 at.% of Si and P. INTRODUCTION Deeply metastable metallic glasses are interesting for a number of fundamental and technological reasons, allowing intricate bulk amorphous parts to be fabricated and allowing fundamental investigations of the amorphous state [1-5]. Many of these glasses crystallize to an extremely fine microstructure, producing consolidated nanostructured materials that have improved mechanical properties. In some glasses, the primary crystallizing phase is the icosahedral quasicrystal. The local structures of metallic glasses and undercooled liquids, believed to contain a high degree of icosahedral short-range order [6-9], could explain this, giving rise to a low nucleation barrier for icosahedrally-ordered phases. Metalloids, such as Si, Ge, and P, are known to enhance metallic glass formation. It is interesting to determine whether they also enhance icosahedral phase formation. Here, we present results of a study of the effects of small additions of Si and P in Ti25Hf50Ni25 alloys on glass formation and crystallization. EXPERIMENT Mixtures of Ti25Hf50Ni25 and Ti25Hf50Ni23X2 (X = Si or P) were prepared by combining Ti (99.995%), Hf (99.9%), Ni (99.995%), Si (99.999%), and Ni2P (99.5%) in the correct proportions. Alloy ingots were prepared by arc-melting these mixtures on a water-cooled Cu hearth in a high purity Ar atmosphere. The arc-melted ingots were broken and pieces were quenched in an Ar atmosphere onto a rotating Cu wheel with a tangential wheel speed of approximately 30 m/s. The ribbons obtained from quenching were approximately 1 mm wide, 30 mm thick, and were continuous for up to 30 cm. Differential scanning calorimetry (DSC) studies of the crystallization of the quenched ribbons were made using a Perkin Elmer DSC7 with a modified gas handling system to reduce sample oxidation. As-cast, as-quenched, and annealed ribbons were characterized by x-ray diffraction (XRD) using a Rigaku Geigerflex x-ray diffractometer using CuKa radiation. Bright-field and electron diffraction studies were mad
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