A review of low-cycle fatigue of Alloy 617 for use in VHTR components: Experimental outlook

The effect of strain range and temperature on the low-cycle fatigue behaviour and microstructure change during cyclic deformation of Alloy 617 for use in very high temperature gas-cooled reactor components were studied at elevated temperature starting from ambient condition. Increasing the strain range and the temperature was noticed to reduce the fatigue resistance of nickel-based Alloy 617 due to facilitating the transformation behavior of the carbides in the grain interior, precipitates along the grain boundary, and oxidation behavior inducing surface connected precipitates cracking. Initial hardening behavior was observed at room temperature condition during cyclic due to the pile-up dislocation of micro-precipitates. The grain size was also taking a role due to the formation of an obstacle in the matrix. In the high temperature regime, the alloy 617 was found to soften for its entire life due to the fast recovery deformation, proved by its higher plasticity compared with lower temperature. The deformation behavior also showing high environmentally assisted damage. Oxidation behavior was found to become the primary crack initiation, resulting in early intergranular surface cracking.