The ferritic high chromium steel HiperFer-17Cr2 provides higher corrosion resistance than conventional 9-12 % Cr-steels and high strength at temperatures above 600 degrees C. This strength is mainly attributed to Laves phase precipitation, which is influenced by plastic deformation, temperature and loading duration. Previous work indicates a higher fatigue strength under low cycle (LCF) compared to high cycle fatigue (HCF) loads. Due to the incomplete understanding of the underlying microstructural changes, particularly how and when thermomechanically induced precipitates affect mechanical properties, this work examines the microstructural evolution of HiperFer-17Cr2 during LCF and HCF tests at 600 degrees C, 635 degrees C, and 650 degrees C. To provide a comparison with a benchmark material, similar tests were performed on the martensitic steel P91. The HiperFer-17Cr2 shows cyclic hardening and increasing hardness, initially driven by dislocation formation in the first cycles, while the further, more pronounced increase is attributed to precipitation formation. In contrast, P91 shows a reduction in hardness due to recovery processes. For HiperFer-17Cr2, a greater hardness increase was observed under LCF loads, which is attributed to stronger plastic deformation. This leads to a higher density of dislocations acting as nucleation sites for precipitates, enhancing precipitation and dislocation strengthening, and consequently improving fatigue strength.

As the fatigue strength of materials manufactured via lase-based powder bed fusion (PBF-LB) is highly influenced by process-induced defects and the “as-built” surface, in this work fatigue tests at specimens made of AlSi10Mg with “as-built” and polished surface condition were conducted. In this context also the defect tolerance of the material, and hence its ability to counteract process-induced notch effects, was analyzed. For this, specimens in a not heat-treated and an artificially aged (T6) condition were tested. For the not heat-treated specimens, a high reduction of the fatigue strength due to the “as-built” surface is observed, while the heat-treated specimens exhibit no influence of the surface condition. Using the √area approach, it is demonstrated that the smaller fatigue strength due to “as-built” surface observed for the not heat-treated condition results from process-related residual tensile stresses, which are removed by T6 heat treatment and polishing, respectively. Moreover, the results show that the “as-built” surface leads to similar stress concentrations than defects in the specimen volume, and thus, polishing had no influence on the fatigue performance after heat treatment. In addition to these findings, an increased fatigue strength after T6 heat treatment was observed, which is caused by an increased defect tolerance.

Additively manufactured structures reveal a poor surface quality and a high number of process‐induced defects in the surface‐near area, leading to a significant reduction of the fatigue strength. As laser‐based powder bed fusion (PBF‐LB) processes are used to produce topologically optimized lightweight structures with complex geometries, which cannot be fully machined, the influence of the process‐induced surface on the fatigue behavior needs to be analyzed. For this, also the interrelation of the surface‐induced notch effects with the surrounding material volume must be considered. As thermal treatments can also be applied to filigree components with complex geometries, in the presented work the influence of different heat treatments, i.e., stress relief annealing (SR) and artificial aging (T6), on the material properties, especially the defect tolerance, of AlSi10Mg manufactured via PBF‐LB is analyzed. Both heat treatments lead to a dissolution of the cellular Si‐rich network, resulting in decreased hardness and tensile strength, but higher fatigue strength. The increased fatigue strength results from a reduction of the process‐induced residual stresses, but mainly a strongly improved defect tolerance. Consequently, to evaluate the fatigue strength of additively manufactured materials, besides the materials strength and the process‐induced defects, also the defect tolerance must be considered.

For a reliable design of structural components, valid information about the fatigue strength of the material used is a prerequisite. As the determination of the fatigue properties, and especially the fatigue limit σw, requires a high experimental effort, efficient approaches to estimate the fatigue strength are of great interest. Available estimation approaches using monotonic properties, e.g., Vickers hardness (HV), and in some cases the cyclic yield strength, only allow a rough estimation of σw. The approaches solely based on monotonic properties lead to substantial deviations of the estimated σw in relation to the experimentally determined fatigue limit as they do not consider the cyclic deformation behavior. In this work, an estimation approach was developed, which is based on a correlation analysis of the fatigue limit σw, HV, and the cyclic hardening potential obtained in instrumented cyclic indentation tests (CIT). For this, eleven conditions from five different low-alloy steels were investigated. The CIT enable an efficient and quantitative determination of the cyclic hardening potential, i.e., the cyclic hardening exponentCHT eII, and thus, the consideration of the cyclic deformation behavior in an estimation approach. In this work, a strong correlation of σw with the product of HV and |eII| was observed. In relation to an existing estimation approach based solely on HV, considering the combination of HV and |eII| enables the estimation of σw with an enormously increased precision.