Microstructure and mechanical properties of slowly cooled Zr 66.4 Nb 6.4 Cu 10.5 Ni 8.7 Al 8.0 with ductile bcc phase
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Microstructure and mechanical properties of slowly cooled Zr66.4Nb6.4Cu10.5Ni8.7Al8.0 with ductile bcc phase Uta Kühn, Jürgen Eckert, Norbert Mattern, Nicolle Radtke and Ludwig Schultz IFW Dresden, P.O. Box 270016, D-01171 Dresden, Germany ABSTRACT We report about the preparation and properties of a Be-free Zr66.4Nb6.4Cu10.5Ni8.7Al8.0 alloy with a glassy or a nanocrystalline matrix and ductile bcc precipitates, which were developed with the aim to improve the mechanical properties. The samples were prepared in form of rods by injection casting into a copper mold. The phase formation as well as the resulting microstructure and the mechanical properties of the different samples have been investigated upon cooling from the melt at different quenching rates. The formation of the bcc phase embedded in a glassy matrix is strongly governed by the alloy composition and the actual cooling rate during solidification, because the glass forming ability is much lower compared to Zr-based alloys containing Be. Already small reductions in cooling rate lead to precipitation of additional crystalline phases. Compression tests reveal that the in-situ glassmatrix composite undergoes work hardening and plastic deformation prior to failure. Surprisingly, also a nanocrystalline matrix leads to high elastic strain values. These features significantly improve the mechanical behavior of the composites compared to the monolithic glass.
INTRODUCTION Multicomponent Zr-based alloys present a high glass-forming ability and a wide supercooled liquid region [1,2]. Some optimized compositions of these bulk metallic glasses are characterized by cooling rates as low as 1-10 K/s for the alloy systems Zr-Ti-Cu-Ni-Be [2], Zr-Ti-Cu-Ni-Al [3] and Zr-Nb-Cu-Ni-Al [4]. Their physical [5] and chemical [6] properties have been the subject of a series of experimental studies in the last decade. Monolithic glasses are known for their large elastic strain limit of about 2 % combined with a high yield strength of up to 2 GPa [7], but they do not exhibit yielding and strain hardening upon room temperature deformation [8,9] because of the formation of highly localized shear bands. Recently, Hays et al. presented a Zr-Ti-Nb-Cu-Ni-Be alloy [10], with a ductile bcc βTi phase embedded in a glassy matrix. This in-situ bulk metallic glass composite has significant advantages compared to monolithic glass. As a result of the ductile dendritic precipitates, the initiation and propagation of shear bands is controlled [10] and the composite reveals work hardening, enhanced plasticity, and toughness [11]. Afterwards, a Be-free alloy, Zr66.4Nb6.4Cu10.5Ni8.7Al8, containing bcc-type dendritic precipitates, has been developed [12]. This composition also reveals strain hardening and plastic deformation prior to failure, but the values of these mechanical parameters are lower compared to the Be-containing composite. Due to the higher critical cooling rate of the Be-free Zr-based metallic glasses, the dimensions to cast such alloys into a fully glassy state are relatively
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