Strengths and failure mechanisms of a Co-15Cr-13TaC directionally solidified eutectic alloy

Abstract

The tensile and stress rupture properties of a Co(Cr)-TaC directionally solidified eutectic alloy have been investigated and compared to those of a single phase, directionally solidified Co(Cr) alloy corresponding in composition to that of the eutectic matrix. The temperature for 100 h stress rupture life at 20,000 psi (138 MN/m2) is about 200°F (111°C) better than that of any cast nickel-base superalloy now used in aircraft or land gas turbines. The degree of superiority becomes progressively less at higher stresses, and at 50,000 psi (345 MN/m2), the temperature for 100 h stress rupture life in the eutectic is about 150°F (83°C) less than for several high strength superalloys. This behavior is related to a bimodal stress rupture mechanism. A model predicts that at low stresses, failure is controlled by the stress rupture behavior of the matrix; and at high stresses failure occurs by a stress relaxation mechanism which causes early fiber failure. Fractographic observations are in agreement with the existence of two stress rupture mechanisms. It was also observed that both stress rupture mechanisms can occur at the same temperature, with specimens failing by the fiber-related mode having lives 1 to 2 orders of magnitude in time less than those which fail by the matrix-related mode at the same stress level.