Thermo-mechanical fatigue testing of superalloys using miniature specimens

Thermo-mechanical fatigue (TMF) test is the only way to simulate the true service conditions experienced by many safety critical components in laboratory. However, conventional TMF testing using large specimens is a time consuming and expensive. In this work TMF tests were carried out on miniature size specimen, with a very thin cross-section of 1 × 1 mm, and the results were compared with those of full-scale tests. The test was carried out using a 1 kN Electro-Thermo Mechanical Testing machine (ETMT) at a temperature range of 400–850 °C with a heating and cooling rate of 5 °C/s. The specimens were prepared from well-characterized Nimonic 90 superalloy. The TMF tests were carried out at four phase angles: out of phase (OP), in phase (IP), clockwise (CD), and contour clockwise (CCD) Diamond shape tests at a mechanical displacement range of about ±50 μm. The displacement was chosen to match the stress levels of the full-scale tests. The results showed that there is good agreement between the TMF lives for the miniature and conventional specimens over several different TMF cycles used. The failure mechanisms were the same in both types of the specimens. Early changes in the resistivity were used to estimate the apparent strain on the specimen (mainly plastic for Nimonic 90). Sharp changes in the resistivity at longer cycles were reflected crack propagation on the specimen.