Microstructures and Mechanical Properties of in-situ V-V 3 B 2 Composites
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0980-II08-05
Microstructures and Mechanical Properties of in-situ V-V3B2 Composites Sujing Xie1 and Easo P. George1,2 1 Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN, 37996 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831
ABSTRACT A series of binary V-B alloys, with compositions spanning the eutectic, were produced by arc melting and drop casting. Microstructural examination revealed that the fully eutectic structure occurs at V-11B rather than V-15B as reported in the V-B phase diagram (all compositions in at.%). The V-11B eutectic was directionally solidified in an optical floating zone furnace, resulting in a composite microstructure consisting of a V matrix and flake or trigonal shaped V3B2 phase. The boride flake spacing (λ) decreases with increasing growth rate (R), following the relation λ2.56R=C, where C is a constant. TEM observations showed that the approximate orientation relationship between the V and V3B2 phases is given by: [001]V // [001]V3B2 and (100)V // (100)V3B2. The growth direction and the V/V3B2 interface are approximately parallel to the [001] direction and (100) planes in the two phases, respectively. Tensile tests were used to investigate the temperature dependence of the strength and ductility of the composite. At temperatures to 600°C, the yield strength of the eutectic is about 140 MPa higher than that of a commonly used vanadium solid-solution alloy, V-4Cr-4Ti. Surprisingly, the eutectic shows 5% tensile ductility at room temperature. INTRODUCTION V-based alloys have many attractive properties such as relatively low density (~6.11 g/cm3), high melting point (~1910°C), and high ductility at room temperature [1-3]. However, they exhibit relatively low yield strength (~200 MPa) at temperature below 800°C. Past efforts on vanadium alloy development have focused mainly on the solid-solution-strengthened V4Ti-4Cr alloy due to its potential applications in fusion systems [4, 5]. Alloys strengthened by second phases may provide higher strength if they contain high enough volume fractions of second phases. The limited results of vanadium alloys strengthened by second phases are mainly
Figure 1. The binary V-B phase diagram.
on the V-V3Si composite [6, 7]. Borides also have potential as reinforcements in metals due to their high melting point, high strength and hardness [8, 9]. As can be seen in the V-B phase diagram (Fig. 1) [1], a V–V3B2 eutectic exists at the composition V–15% B. Eutectic alloys can have a uniform dispersion of the reinforcing phase, significant volume fractions and high thermal stability, which are expected to lead to better mechanical properties. In the present study, V-V3B2 alloys around the eutectic composition were prepared. The eutectic microstructures, crystallographic orientation relation between the eutectic phases, and the effect of directional solidification conditions on microstructures were investigated. Mechanical properties of the individual phases (V and V3B2) were
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