The mechanical properties of Cu alloyed steels are influenced significantly by the Cu content and the respective state of Cu precipitations as well as the C content. In this context, the effect of an increased C content on the fatigue crack initiation and growth of differently aged Cu alloyed steels with 0.005 (X0.5CuNi2-2: X0.5) and 0.21 wt.-% C (X21CuNi2-2: X21) was investigated in this study. Notched specimens were examined via SEM in interrupted fatigue tests to detect the location of crack initiation and growth. The results showed that fatigue crack initiation and growth occurred for both steels at grain boundaries, and within ferrite grains. However, a higher C content increased the incidence of crack initiation and growth at grain boundaries. This is caused by the smaller grains of X21 and especially by the presence of cementite on the grain boundaries. This explains why, in contrast to X0.5, no influence of the Cu precipitation state on the defect-based failure was observed for X21, as the precipitates are located within the ferrite grains and, thus, only have a minor impact on the fatigue failure mechanisms of X21.

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.

The nanoscale Cu precipitation exhibits considerable potential for enhancing strength and cyclic hardening po-tential in steels. In the present work, Cu-alloyed ferritic steels with different C contents (X0.5: 0.005 wt.%, X21: 0.21 wt.%) were investigated to systematically manifest the influences of C contents on the microstructural evo-lution, the precipitation behavior of nano-sized Cu precipitates and the resulting effects on cyclic hardening behavior. The investigated materials were subjected to austenitization at 900 degrees C for 10 min and subsequently aged at 600 degrees C for different time durations (120, 720, 7200 s). The 3D atom probe tomography rendered the investigation of nanoscale Cu precipitates in terms of average size, local chemistry, number density and spatial distribution. In X21 steel, the Cu precipitates exhibited a large nucleation density upon the onset of aging and further grew into a larger precipitate size. Compared to X0.5 steel, the higher nucleation density and the inhomo-geneous precipitate distribution in X21 steel were attributed to the heterogeneous nucleation and high C content in the matrix. Cyclic indentation tests revealed a strong dependency of hardness and cyclic hardening potential on precipitation size and number density for both steels. Furthermore, for both steels aging had a similar effect on the mechanical properties. However, the increased C content induced a higher hardness and lower cyclic hardening exponentCHT of X21 steel.

Given the complex process condition, extruded aluminum (Al) alloy tubes show locally pronounced differences in microstructure and mechanical properties, which can be influenced by subsequent heat treatment. In the present study, cyclic indentation tests (CITs) were conducted on extruded Al alloy EN AW-6082 to locally determine hardness and cyclic hardening potential, which was complemented with light optical microscopy. To analyze the influence of extrusion process and subsequent heat treatment, the EN AW-6082 tubes investigated were manufactured with extrusion ratios Ψ of 13:1 and 22:1, both in as-extruded and T6 heat-treated conditions. The results obtained for the as-extruded state showed significant differences of the local mechanical properties and demonstrated that an increased Ψ leads to higher hardness, caused by more pronounced plastic deformation during the manufacturing process. Moreover, an increase of hardness and cyclic hardening potential was observed after a T6 heat treatment, which also reduced the difference in hardness between the different extrusion ratios. Additionally, the pronounced local differences in hardness and cyclic hardening potential correlated with the local microstructure. The results demonstrated that CITs enable the analysis of local mechanical properties of extruded EN AW-6082 profiles, resulting from different extrusions ratios as well as subsequent heat treatment.

From literature, it is well known that Cu precipitates in steels lead to an increase of hardness and quasi-static strength. However, the relation between Cu precipitates and cyclic mechanical properties has been investigated rather limited. Consequently, in the present work the evolution of Cu precipitates in a copper-alloyed low carbon, ferritic steel, aged at 550 degrees C, 600 degrees C and 650 degrees C for various times, and their influence on the resulting static and cyclic material properties were analyzed. Three-dimensional atom probe tomography (3DAPT) provided evidence that the ageing treatment led to the formation of Cu precipitates with different sizes and distribution. The existence of copper precipitates resulted in an increase of the ultimate tensile strength and yield strength of the investigated ferritic steel. Moreover, cyclic indentation tests were performed to investigate the influence of the ageing treatment and, hence, Cu precipitates, on hardness and especially cyclic properties of the material. The present results show a maximum of hardness and cyclic hardening potential at sizes of the precipitates of around 2.2 nm. Additionally, synchrotron X-ray diffraction (SYXRD) measurements were in a first approach proved to be viable for investigating the crystallographic structure of Cu precipitates within ferritic steels. Considering the evolution of Cu precipitates, determined with synchrotron as well as atom probe, a high dependency of quasi-static and cyclic properties on Cu precipitates could be demonstrated.

Cu precipitates in steels can lead to significant changes of mechanical behavior. However, most investigations focus on the influence of these precipitates on hardness, whereas the effects on quasi-static deformation behavior and cyclic properties remain unclear. Therefore, in the present work the deformation behavior of Cu alloyed steels with two different carbon contents, 0.005 and 0.2 wt%, was analyzed in tensile as well as fatigue tests. To characterize the influence of different precipitation states, various heat treatments were performed. The heat treatment parameters were chosen based on results obtained with the short-time procedure PhyBaLCHT, which is based on cyclic indentation tests and enables the determination of microhardness and cyclic hardening potential. The results reveal that a higher C content increases hardness, decreases cyclic hardening potential and leads to generally higher tensile strength. However, only at a shorter aging time does a higher C content increase fatigue strength, whereas at a longer aging time both steel variants show comparable fatigue lifetimes, despite the significantly higher hardness and tensile strength of the steel with higher carbon content. This can be explained with the higher cyclic hardening potential of the lower carbon steel, leading to an improved fatigue lifetime. Additionally, it could be shown for both steels that an increase of cyclic hardening potential, caused by longer aging times and different precipitation states of Cu, can be associated with increased fatigue strength. However, for an overall assessment of a material’s mechanical properties, both, hardness and cyclic hardening potential have to be considered.

The 22 wt.% Cr, fully ferritic stainless steel Crofer®22 H has higher thermomechanical fatigue (TMF)- lifetime compared to advanced ferritic-martensitic P91, which is assumed to be caused by different damage tolerance, leading to differences in crack propagation and failure mechanisms. To analyze this, instrumented cyclic indentation tests (CITs) were used because the material’s cyclic hardening potential—which strongly correlates with damage tolerance, can be determined by analyzing the deformation behavior in CITs. In the presented work, CITs were performed for both materials at specimens loaded for different numbers of TMF-cycles. These investigations show higher damage tolerance for Crofer®22 H and demonstrate changes in damage tolerance during TMF-loading for both materials, which correlates with the cyclic deformation behavior observed in TMF-tests. Furthermore, the results obtained at Crofer®22 H indicate an increase of damage tolerance in the second half of TMF-lifetime, which cannot be observed for P91. Moreover, CITs were performed at Crofer®22 H in the vicinity of a fatigue crack, enabling to locally analyze the damage tolerance. These CITs show differences between crack edges and the crack tip. Conclusively, the presented results demonstrate that CITs can be utilized to analyze TMF-induced changes in damage tolerance.

Understanding the effects of microstructural defects, which can have a high impact on service life, is of great importance in the design of cyclically loaded components. For this, the size of the defect as well as the material's ability to counteract microstructural defects have to be considered. In this study, the defect tolerance of differently heat-treated Cu alloyed steels with two different C contents (0.005 and 0.21 wt% C) has been investigated by testing specimens with artificial defects. From the results of fatigue experiments, defect tolerance is assessed based on different approaches: (i) a comparison of stress intensities, derived from Murakami's root area approach, (ii) comparing fatigue lifetime for a given defect size and stress amplitude, (iii) the reduction of fatigue strength by a defect for a given defect size, and (iv) using the Kitagawa-Takahashi diagram, enabling the calculation of the critical defect size root area(0) as well as fatigue crack propagation threshold Delta K-th,K-calc. The results show that for the 0.005 wt% C steel an increase in aging time is accompanied by an increase in defect tolerance, which correlates with a higher cyclic hardening potential induced by the formation of Cu precipitates. However, the results show that strength and ductility also influence defect tolerance. By using the short-time procedure PhyBaLCHT, which is based on cyclic indentation tests, all these factors influencing defect tolerance can be determined efficiently. The 0.21 wt% C steel does not reveal an improved defect tolerance which is presumably caused by the dominating pearlite phase.

Short-time procedures for determining the fatigue properties of materials can provide significant improvements in cost efficiency and are therefore of great scientific and industrial interest (Jost et al., 2017). A promising short-time procedure is PhyBaL(CHT), which is based on cyclic indentation tests and can be used to determine the cyclic hardening potential of different materials at different conditions (Kramer et al., 2014; Bambach et al., 2016). In the present research work, this procedure was used to explore the cyclic hardening potential of 42CrMo4 steel (SAE 4140) in various heat treatment states as well as of 18CrNiMo7-6 steel variants with different chemical composition and heat treatments. A special focus of the investigation was the influence of maximum indentation force on the results determined with PhyBaL(CHT). As expected, a higher dependency on local effects is seen at lower indentation forces. However, consistent values of cyclic hardening potential were determined down to low indentation forces, and indent diagonals below 5 mu m. This gives perspective to describe e.g. gradients in material cyclic deformation behavior with high local resolution. Furthermore, the results of cyclic indentation tests were compared to the results of uniaxial cyclic compression tests, showing transferability of the cyclic indentation tests to the cyclic deformation behavior under uniaxial cyclic loading.