Fracture Behavior of Vanadium/Vanadium Silicide in-situ Composites
- PDF / 3,699,708 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 57 Downloads / 246 Views
FRACTURE BEHAVIOR OF VANADIUM/VANADIUM SILICIDE IN-SITU COMPOSITES M. J. Strum and G. A. Henshall Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94550.
ABSTRACT The fracture behavior of V-V 3 Si in-situ composite alloys has been evaluated for intermetallic phase fractions of 30, 50, and 70 volume percent. In addition, the mechanical properties of the ductile constituent in these composites have been evaluated from two alloys containing silicon at its solubility limit in vanadium at the eutectic temperature and at 1400 0 C. The fracture toughness was found to increase monotonically with increasing volume fraction of the ductile phase. Fractography has shown evidence of crack bridging by the vanadium solid solution phase and increased ductility within the composite relative to bulk measurements. The toughness results are compared with predictions based on an existing model of ductile-phase toughening using our measurements of component properties.
INTRODUCTION Refractory metal intermetallic compounds could satisfy needs for high strength and creep resistance at extremely high temperatures if they can be toughened by the dispersion of a ductile second phase [1]. The ductile phase may increase toughness by "bridging" the crack faces [2], thereby inhibiting crack opening, or by blunting the crack tip. In this investigation, "in-situ" methods are being explofed as a means of synthesizing V-V 3 Si composites, in which the two components of the composite form by phase separation during solidification. Using this system, the intermetallic phase is V3 Si (A15 structure) and the ductile phase is a solid solution of Si in V, V(Si), which appears to precipitate out small particles of V 3Si upon heat treatment. Both phases have densities significantly lower than Ni-based superalloys, which is important in aerospace structures, and vanadium has excellent low temperature ductility, perhaps making it a good toughening agent.
EXPERIMENTAL PROCEDURES The four alloy compositions produced in the series-1 castings were formulated to contain 30, 50, 70, and 100 percent ductile phase using an assessed V-Si phase diagram [3]. The compositions of the series-I alloys are listed in Table I. The alloys were arc melted in an argon atmosphere using procedures which attempted to minimize interstitial levels. The melting stock consisted of crushed boules of high-purity silicon and high-purity vanadium sheet sheared into small pieces and acid cleaned in an HNO 3/HF solution. Due to the relatively high hydrogen levels measured in the as-cast material, all test specimens were vacuum degassed at 800*C for I h. The level of measured hydrogen decreased substantially after degassing with only marginal increases in oxygen and nitrogen levels. To minimize hydrogen absorption from requisite acid cleaning and to attempt further minimization of interstitial levels from the melt stock, asreceived, high-purity vanadium chips were substituted as melting stock in the series-2 castings. The two additional compositions produced were a eutect
Data Loading...
