Studies on high-temperature fatigue behaviour and mechanism for conventionally cast and directed energy deposited forms of Alloy 625

Abstract

The aim of the present work is to study the low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behaviour of two product forms of Alloy 625. Experiments were performed under total strain control mode using triangular and trapezoidal waveforms under complete reverse loading with R = −1, with a total strain amplitude of ±0.25 %, ±0.4 % and ±0.6 % employing a constant strain rate of 3 × 10−3 s−1 over a temperature range of 298 to 973 K. The additive manufacturing process using the directed energy deposition (DED) technique resulted in an improved fatigue life as compared with the conventional casting (CCA) route. Microstructural analysis using electron back scatter diffraction indicated that an alternating strain got generated within the microstructure and the damage was observed as a strain incompatibility within the partitioned regions of fine and neighbouring coarser grains. The preferred columnar growth occurred in the 〈001〉 direction. Inter-dendritic decohesions were attributed to the Mo, Nb and Ti enrichments. While at ambient temperature, an inter-dendritic de-cohesion and strain incompatibility was the cause for final failure, the occurrence of additional time dependent phenomena such as dynamic strain ageing (DSA), oxidation and creep triggered an earlier fatigue failure at elevated temperatures.