Elevated temperature cyclic stress-strain behaviour in nickel based superalloys

A number of nickel superalloys which respond differently to cyclic stress-strain have been investigated by means of strain controlled low cycle fatigue testing. Fifteen tests have been conducted on cylindrical specimens of the powder metallurgy nickel disc alloy Udimet 720Li (U720Li) at temperatures of 650degreesC and 700degreesC. Comparisons are made to the behaviour seen in LCF tests on the commonly used gas turbine combustor material Nimonic alloy C263 which is seen to cyclically harden at 300degreesC and soften at 800degreesC. The test procedure and subsequent data analyses have been detailed, referring constantly to that recommended by the Code of Practice for the Determination and Interpretation of Cyclic Stress-Strain Data. A selection of hysteresis loop data is presented and details are provided for the method used to calculate elastic modulus. For U720Li, the evolution of maximum stress and total stress range throughout the tests is investigated and it is seen in general that neither hardening nor softening occurs during these tests, and it is concluded that this is due to the relatively small plastic strains applied. The C263 has been tested under conditions that show a change from cyclic plastic hardening to time-dependent softening behaviour, and this can be clearly seen in the cyclic stress-strain data.