Microstructures and Mechanical Properties of NiAl-Mo Composites
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MICROSTRUCTURES AND MECHANICAL PROPERTIES OF NiAl-Mo COMPOSITES H. Bei1,2 and E. P. George1,2 1 The University of Tennessee, Department of Materials Science and Engineering, Knoxville, TN 37996 2 Oak Ridge National Laboratory, Metals and Ceramics Division, Oak Ridge, TN 37831 Abstract In-situ composites consisting of ~14 vol.% continuous Mo fibers embedded in a NiAl matrix were produced by directional solidification in a xenon-arc-lamp, floating-zone furnace. The fiber spacing and size were controlled in the range 1-2 µm and 400-800 nm, respectively, by varying the growth rate between 80 and 20 mm/h. Electron back-scatter diffraction patterns from the constituent phases revealed that the growth directions and interface boundaries exhibited the following orientation relationships: 100 NiAl // 100 Mo and {011}NiAl //{011}Mo . The temperature dependence of the tensile strength and ductility were investigated and the NiAl-Mo composite was found to be both stronger and have a lower ductile-brittle transition temperature than the unreinforced NiAl matrix. !
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Introduction The use of NiAl as a structural material suffers from two major drawbacks: poor ductility/fracture toughness at room temperature and low strength/creep resistance above 600ºC. Attempts have been made to toughen NiAl by combining it with a ductile metal [111]. For example, Johnson et al. [4] obtained a room-temperature fracture toughness of ~20 MPa√m in a composite alloy of composition NiAl – 34 at.% (Cr, Mo), and Misra et al. [11] obtained a fracture toughness of ~14 MPa√m in a NiAl – 9 at.% Mo eutectic alloy. Both these values are significantly higher than the room-temperature fracture toughness of monolithic NiAl single crystals (~6 MPa √m [12]). In this study, well-aligned microstructures of NiAl-Mo, devoid of any cellular or dendritic regions, were produced by directional solidification under carefully controlled conditions in a high-temperature optical floating zone furnace having a relatively steep temperature gradient (~30 K/mm). This furnace has been used by us previously to produce well-aligned eutectic microstructures of Cr-Cr3Si and V-V3Si alloys over a wide range of growth conditions [13-16]. Experimental Procedures Alloys having the nominal composition Ni – 45.5Al – 9Mo (at.%) were arc melted, drop cast, and directionally solidified in a high-temperature optical floating zone furnace in flowing argon gas at growth rates of 20-80 mm/h and a fixed rotation rate of 60 rpm. Additional details of the processing conditions are given elsewhere [17]. Total weight losses after melting and casting were less than 0.05%, so nominal (starting) compositions are used throughout this paper.
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Representative samples were cut from the directionally solidified (DS) rods along the transverse and longitudinal directions, polished using standard metallographic techniques, etched with a solution of 80% hydrochloric and 20% nitric acids, and examined by optical, electron, and orientation imaging microscopy. Dogbone-shaped specimens were machined f
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