MECHANICAL PROPERTIES OF UNIDIRECTIONALLY SOLIDIFIED COBALT BASE - TANTALUM-CARBON CASTINGS

Cobalt-base, 15% Cr, 10% Ni-13% TaC castings approximately 5.5 cm x 5.5 cm x 8.2 cm long were rapidly unidirectionally solidified in air. The structure of the reinforcing TaC ranged from a regular lamellar eutectic, lamellar eutectic plus faceted primary TaC particles to a combination of irregular fibers plus clusters of primary TaC particles.;The creep properties of the rapidly solidified castings were inferior to the slowly solidified eutectics but were clearly superior to those of the unreinforced matrix. For the Larson-Miller Parameter of 51 x 10('3) corresponding approximately to a 1000 hour rupture life at 870(DEGREES)C, the stress-rupture strength of CoTac ;Neither of two different creep models worked well with the high temperature multiphase casting. The first model using a simple form of the Arrhenius type temperature dependence yielded an average value of the stress exponent, n, equal to 8.77 (+OR-) 0.43 and an apparent activation energy for creep of 363 (+OR-) 21 KJ deg('-1) mole('-1). The second model utilized the concept of a threshold stress. Using a measured value of the threshold stress equal to 84 (+OR-) 7 MPa (12.2 (+OR-) 1 ksi), the corresponding values for 'corrected' stress exponent and apparent activation energy for creep were 3.00 (+OR-) 0.21 and 468 (+OR-) 17 KJ deg('-1) mole('-1).;Only the hypereutectic rapidly unidirectionally solidified casting showed evidence of microstructural instability after an exposure of several hours in the temperature range of 700-1000(DEGREES)C.;Stress-rupture strengths in air were measured from 600(DEGREES)C to 1000(DEGREES)C at stress levels of 103, 138 and 172 MPa. The 0.2% offset yield strength of the composite casting at room temperature was in the range 520-750 MPa, about 20% less than the values of 800-900 MPa for slowly solidified eutectics but approximately 140-260% greater than the value of about 200 MPa for the matrix composition. At 900(DEGREES)C the 0.2% offset yield strength of the composite castings was approximately 200 MPa and was approximately only 40% of the value of 500 MPa for the slowly solidified eutectic. However the 900(DEGREES)C 0.2% yield strength of 200 MPa was approximately 170% higher than the value of 75 MPa for the unreinforced matrix.