Microstructure of Ti-Based, Dendrite/Nanostructured-Matrix Composites
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Microstructure of Ti-Based, Dendrite/Nanostructured-Matrix Composites Thomas G. Woodcock1, Sonia Mato1, Germán Alcalá1, Guo He2, Yinmin Wang3, En Ma3, Qunlong Dai4, Manling Sui4, Wolfgang Löser1, Jürgen Eckert1,5 and Ludwig Schultz1 1 IFW Dresden, Institute of Metallic Materials, Helmholtzstr. 20, D-01069 Dresden, Germany 2 Light Materials Group, National institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan 3 Department of Materials Science and Engineering, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA 4 Shenyang National Laboratory for Materials Science, Institute of Metal Research, CAS, 72Wenhua Road, Shenyang, 110016, P.R. China 5 present address: TU Darmstadt, Department of Materials and Earth Sciences, Physical Metallurgy Division, Petersenstr. 23, D-64287 Darmstadt, Germany
ABSTRACT Electron microscopy of the composite-forming alloys Ti60Cu14Ni12Sn4Nb10 and Ti60Cu14Ni12Sn4Ta10, shows that for both alloys, the microstructure consists of an array of micron-scale dendrites surrounded by a nanoscale binary eutectic. In the Ta-containing alloy, one of the eutectic phases has been identified as having a bcc crystal structure similar to that of the dendrites. In the Nb-containing alloy, one of the eutectic phases has been found to have a similar composition to that of the dendrites. Further detailed structural and compositional characterization is needed in order to understand the solidification behaviour of such materials. This knowledge may then be used to improve the casting conditions and subsequent mechanical properties of the materials.
INTRODUCTION The combination of high yield strength, high hardness, good wear resistance and excellent corrosion resistance in bulk metallic glasses (BMGs) has enabled consideration of many different applications [1][2]. Several of these are not possible in practice, however, because although the elastic strain limit is high in BMGs (~ 2% [1]), the plastic strain to failure tends to be almost zero [3]. The reason for this small plastic failure strain is the inhomogeneous deformation behaviour of such materials [1]. At stresses above the yield stress and temperatures below the glass transition temperature, localized shear bands form in BMGs and often failure can occur by the propagation of a single one of these [1][2][4][5][6]. Shear strain is localized in this way because of a combination of zero work hardening and thermal softening of the material within the band due to adiabatic heating [1]. This initiates a vicious circle of yielding, heating, softening and further yielding inside the band, leading to its propagation [1][7]. Several attempts have been made to increase the plastic failure strain of BMGs and nanocrystalline alloys by forming composites. These include particle [8-10] and fibre [11-14] reinforcement and precipitation of a ductile crystalline phase [15-18]. Recently, several promising results have been achieved using the last of these options. In the case of Be-containing, Zr-based BMG alloy syste
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