Fatigue studies of CoCrFeMnNi high entropy alloy films using nanoindentation dynamic mechanical analyses

Films in the microelectromechanical systems are frequently subjected to the fatigue damage under cyclic loading, and the fatigue properties of the films greatly influence the reliability and life of MEMS. However, because of the small dimension of films, characterization of their fatigue properties is a daunting challenge. Nanoindentation dynamic mechanical analysis (DMA) provides a possible solution. However, the setting of loading cycles is crucial to the fatigue test. Using CoCrFeMnNi high entropy alloy films, we investigated the fatigue properties extracted using two different types of loading: fixed load ratio with different mean loads and fixed mean load with different load ratios. Our results showed that both types of fatigue loading were reliable for the evaluation of the fatigue properties of films and it was demonstrated by the fatigue strength coefficients and exponents extracted from the test results. For comparison, DMA was also performed on CoCrFeMnNi high entropy alloys. Compared to the bulk counterpart, the film had much higher fatigue lifetime because of the thickness confinement, the presence of high-density nanotwins and the much smaller grain size in the film.