Thermal Fatigue Characteristics of FSX-414, IN 939, IN 738LC and René 80 Superalloys by Fluidized Bed Technique

Thermal fatigue characteristics of superalloys for gas turbine components were investigated by a fluidized bed apparatus. Thermal fatigue tests were made using an improved disk-shaped specimen in heat cycles between 850°C and 300°C. The specimen was kept at 850°C with or without holding for 30 minutes. Thermal fatigue resistance was evaluated in terms of the crack initiation cycle number and the crack propagation rate from the standpoints of mechanical properties and precipitates morphology in the superalloys. The Ni-base superalloys (IN939, IN738LC, and René 80) exhibited no cracking even after 1200 heat cycles. This was related to the fact that the yield strength of these superalloys at high temperatutes was larger than that of the thermal stress generated during the heat cycle and no inelastic strain was produced. In the Co-base FSX-414, the yield strength and the creep rupture strength increased with increasing C content in the matrix. The crack initiation of this superalloy depended on the test condition. When a specimen was kept at 850°C for 30 minutes, the heat cycle number to crack initiation increased with increasing its high temperature strength. Without holding at 850°C, this cycle number of the alloy with higher C content tended to decrease. This was considered to result from the crack initiation acceleration due to the stress concentration produced at the tip of relatively large eutectic carbides. The crack propagation in the FSX-414 was significantly affected by the eutectic carbides morphology. With increasing C content in this alloy, the eutectic carbides precipitated in large quantities, forming network structure. When a crack was initiated in higher C content FSX-414 alloys, the crack propagated along this network structure carbides, thereby resulting in a remarkable increase in crack propagation rate.