Role of grain structure, grain boundaries, crystallographic texture, precipitates, and porosity on fatigue behavior of Inconel 718 at room and elevated temperatures

This paper presents the main results from an investigation into the effect of initial microstructure on high cycle fatigue behavior of Inconel 718 superalloy. To create a variety of initial microstructures, the material was deposited at a 45 degrees (diagonal) and a 90 degrees (horizontal) angle with respect to the loading direction using direct metal laser melting (DMLM). In addition, a group of samples underwent hot isostatic pressing (HIP). Finally, a set of wrought Inconel 718 specimens was prepared. The samples underwent the same heat treatment per AMS 5663 after machining. The microstructure in the samples was characterized and found to vary in terms of grain structure, crystallographic texture, content of annealing twin boundaries, precipitates, and porosity. Stress controlled rotary bending fatigue tests were carried out to an endurance limit of 107 cycles at room temperature and at 500 degrees C with stress amplitudes ranging from 200 MPa to 1200 MPa. It was found that for low stress amplitudes, the fatigue strength is higher at elevated temperature than at room temperature for all the studied samples. For higher stress amplitudes, the materials showed a higher fatigue strength at room temperature than at elevated temperature. At room temperature, the wrought material was found to exhibit superior fatigue performance over the DMLM diagonal and DMLM horizontal materials. In contrast, at 500 degrees C, the wrought and DMLM materials behaved similarly, while the HIPed material performed notably worse than all other materials. The observed behaviors are rationalized in terms of the initial microstructure of the samples. Since crystallographic texture is weak in every material, effects of gran size, porosity, precipitates, and grain boundaries are discussed as the main competing microstructural features influencing the fatigue performances.