Strength and toughness of a Nb/Nb 5 Si 3 composite
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Strength and Toughness of a Nb/Nb Si3 Composite M A D A N G. MENDIRATTA and DENNIS M . DIMIDUK Recent studies tl,2J have shown that within the Nb-Si system, the two-phase Nb/NbsSi3 microstructures exhibit excellent thermochemical stability and resistance to MADAN G. MEND/RATlrA, Director, Materials Research Division, is with UES, Inc., Dayton, OH 45432-1894. DENNIS M . D1MIDUK, Group Leader for Advanced Metallics, is with WL/MLLM, WrightPatterson AFB, OH 45433-6533. Manuscript submitted June 8 , 1992. METALLURGICAL TRANSACTIONS A
coarsening up to 1500 °C. In addition, these Nb/NbsSi3 "in situ composites" exhibit a good balance in mechanical properties, t3,4~ having a reasonably high strength up to 1500 °C and high fracture toughness at r o o m temperature. These combined properties o f the composites provide an attractive basis for developing high-temperature structural materials. Studies are currently underway on creep mechanisms and alloying to enhance the oxidation resistance o f these composites.iS] Most o f the published results on this system pertain to the wrought Nb-10 at. pct Si alloy in which the large primary Nb particles provide toughening while the intermetallic NbsSi3 phase in the eutectoid microconstituent provides high-temperature strength. The present work was undertaken to determine the strength and toughness o f a wrought alloy with neareutectic composition, Nb-16.5 at. pct Si, at temperatures from 25 °C to 1500 °C. The increased Si content is known to decrease the volume fraction o f the primary Nb particles and increase the overall volume fraction o f the NbsSi3 intermetallic phase.t 1.21 An alloy with a nominal eutectic composition o f Nb-18.7 at. pct Si was cast* and hot extruded (1800 °C, *Casting procured Pittsburgh, PA.
from
the
Westinghouse
Corporation,
3 . 5 : 1 extrusion ratio). The casting and extrusion procedures were similar to those reported earlier for the Nb-10 at. pct Si alloy.~4~ Chemical analysis (for details o f techniques used, see Reference 4) o f the extruded product revealed the Si content to b e 16.5 +-- 0.25 at. pct and carbon, nitrogen, and oxygen contents to be 0.032, 0.011, and 0.007 wt pct, respectively. Apparently, some Si was lost during melting; principally due to the large disparities in melting points and vapor pressures between the species. The major metallic impurity found in the material was a concentration o f 0.2 at. pct Ta. Smooth and single-edge notch bending specimens were machined using electrical discharge machining, wrapped in T a foils (to minimize oxygen pickup), and heat-treated in 10-6 Torr vacuum f o r 100 hours at 1500 °C. This heat treatment was chosen to ensure that the microstructure consisted o f the equilibrium Nb + NbsSi3 phases.t2] The mechanical properties, i . e . , strength and fracture toughness, were measured as a function o f test temperature by performing bending tests in vacuum. The specimen dimensions, including notch dimensions and details o f testing procedures, were similar to those given in a previous article, t4J Figur
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