Microstructure, thermal stability and mechanical properties of slowly cooled Zr-based composites containing dendritic bc
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Microstructure, thermal stability and mechanical properties of slowly cooled Zr-based composites containing dendritic bcc phase precipitates Nicolle Radtke1, Jürgen Eckert2, Uta Kühn1, Mihai Stoica1, Ludwig Schultz1 1 IFW Dresden, Institute of Metallic Materials, P.O. Box 270016, D-01171 Dresden, Germany 2 TU Darmstadt, Institut of Material Science, Petersenstr. 23, D-64287 Darmstadt, Germany ABSTRACT We report on the microstructure, the thermal stability and the mechanical properties of slowly cooled Zr-Nb-Cu-Ni-Al alloys with ductile bcc phase precipitates embedded in a glassy or nanocrystalline matrix. The samples were prepared in form of rods by injection casting into a copper mold. The phase formation and the microstructure of the composite material were investigated by X-ray diffraction, EDX analysis and scanning and transmission electron microscopy. The thermal stability was examined by differential scanning calorimetry and the mechanical behavior was investigated by compression tests under quasistatic loading at room temperature. The formation of bcc phase dendrites and a glassy or nanocrystalline matrix is strongly governed by the alloy composition and the actual cooling rate during solidification. Besides, changes in composition and cooling rate lead to different volume fraction and size of the bcc phase precipitates and, hence, to different values of yield strength, elastic and plastic strain. The samples with nanocrystalline matrix show a homogeneous distribution of the bcc phase precipitates over the whole cross-section and exhibit higher yield strength and plastic strain than the samples containing an amorphous matrix. Illustrated by the presented results we show the possibility of obtaining tailored mechanical properties by control of composition and solidification conditions. INTRODUCTION Multicomponent Zr-based composites show a high glass-forming ability and a wide supercooled liquid region [1,2]. Zr-Ti-Cu-Ni-Be [2], Zr-Ti-Cu-Ni-Al [3] and Zr-Nb-Cu-Ni-Al [4] alloys can be produced as bulk samples because they are characterized by critical cooling rates as low as 1-10 K/s for bulk glass formation. Bulk metallic glasses are known for their high yield strength of up to 2 GPa and their large elastic strain limit of about 2 % [5], but they do not show yielding and strain hardening upon room temperature deformation [6,7] because of the formation of highly localized shear bands. Hays et al. developed a Zr-Ti-Nb-Cu-Ni-Be bulk metallic glass composite with a ductile dendritic bcc β-Ti phase [8]. The coupling of a high-strength glassy phase with a ductile dendritic bcc β-Ti phase improves the deformation behavior of the composite compared to monolithic glasses. This composite shows work hardening and an elastic strain of 3 % and plastic strain of around 5 % before failure [8]. Kühn et al. developed a Be-free Zr-based glass matrix composite, which also forms ductile dendritic bcc β-Ti precipitates [9]. The compressive stress-strain curves for a 5 mm diameter sample show high yield stress, work harden
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