Dr. J.C. Stinville will give a plenary talk at SuperAlloys2022 on Plastic localization and Fatigue Strength in Polycrystalline Nickel-Based Superalloys.
Correlations between fatigue strength and monotonic strengths such as yield strength and ultimate tensile strength exist for polycrystalline face-centered cubic (fcc) and hexagonal close-packed (hcp) alloys. It is observed that fatigue strength increases with increasing yield strength or ultimate tensile strength. However, plotting fatigue strength as a fraction of the alloy’s yield or ultimate tensile strength (fatigue ratio) shows that metallic materials with high strengths fail by fatigue at stresses as low as 40% of their yield strength, indicating markedly low fatigue efficiency. Nickel-based superalloys are no exception. Recent advances in accelerated fatigue testing, in-situ electron microscopy, digital image correlation methods, and multi-modal data analysis have been integrated to quantitatively and statistically characterize the evolution of plastic localization from the earliest stages of cycling. A direct relationship is observed between the localization of the plasticity by slip (slip localization) that occurs during the first cycle and the fatigue strength. Alloys with high yield strength exhibit intense slip localization during deformation, which leads to low fatigue ratios.
For polycrystalline superalloys with a minimum content of metallurgical defects, intense slip localization occurs near and parallel to twin boundaries, leading to early fatigue crack nucleation. It is demonstrated that the microstructure can be engineered to disrupt slip localization near twin boundaries, leading to a significant improvement in the fatigue resistance of nickel-based superalloys. While intense slip extends across the entire twin boundaries in conventional superalloys, microstructure control can be achieved to reduce slip localization near twin boundaries. Consequently, an increased fatigue resistance was reported. Design by targeting plastic slip delocalization is a significant deviation from conventional design approaches since it incorporates the effect of the microstructure and physical deformation mechanisms at the origin of the fatigue strength.