The passivity and, hence, “stainlessness” as well as a very good combination of material strength and ductility was the main scope in the development of high alloyed chromium–nickel austenitic stainless steels. However, the chemical composition strongly influences not only the chemical passivity, but also the microstructural metastability, since deformation-induced martensite transformation takes place in a huge number of austenitic stainless steels. In this work, the phase transformation behavior was investigated under different loading conditions, using molecular dynamics simulation on the atomistic scale as well as phase field modeling on the microscale. To comprehensively assess the metastability of the investigated materials, a new method was developed and compared with the conventional metastability parameters in the literature. Furthermore, experimental work focusing on the detailed characterization of the surface morphology after different manufacturing processes, i.e., cryogenic turning, conventional milling, ultrasonic surface modification as well as micro-shot peening, is shown. The resulting surface morphologies and their influences on the fatigue properties were investigated from the low cycle fatigue, over the high cycle fatigue to the very high cycle fatigue regime, under uniaxial fully reversed tension–compression loading conditions. Additionally, the fatigue life of austenitic stainless steels with different metastabilities and surface morphologies was investigated in four-point bending fatigue tests.

The fatigue behavior of the two austenitic stainless steels AISI 904L and AISI 347 with different surface morphologies, (i) conventionally turned and finally polished, (ii) cryogenic turned using CO2 snow, as well as (iii) cryogenic turned and finally polished, was investigated using an ultrasonic fatigue testing system up to the very high cycle fatigue regime using an ultrasonic fatigue testing system. The AISI 904L is stable against deformation-induced phase formation while the AISI 347 is in the metastable state and shows martensite formation induced by cryogenic turning as well as mechanical loading. For the detailed characterization of the surface morphology, confocal microscopy, scanning electron microscopy, and X-ray diffraction methods were used. The specimens from stable austenite failed in the high cycle fatigue and very high cycle fatigue regime. Opposed to this, the metastable austenite achieved true fatigue limits up to load cycle N = 1 × 109 and failed only in the high cycle fatigue regime. Furthermore, due to surface modification, an increase of fatigue strength of metastable AISI 347 was observed.

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.

AbstractThin strips of electrochemically deposited nanocrystalline nickel-iron with thicknesses of 320 or 330 µm are modified by defined grinding. Small changes in the cutting depth and the variation of the grinding process, up cut or down cut, result in different surface states. X-ray diffraction provides the analyses of the microstructures and residual stresses on the surfaces. In the initial state, the grain sizes have an average value of 9.3 nm, the micro strains 0.74% and the residual stresses predominantly values in the low-pressure range. Up grinding with the smallest depth of cut 1 µm causes the lowest compressive residual stresses at workpiece surface due to cold plastic deformation. Larger cutting depths and surface temperatures reduce the mechanical effects. Then prevailing thermal effects cause tensile residual stresses through thermoplastic deformation and through changes in the microstructure, which can be observed by grain enlargements and decreases in micro strains. However, the recovery and recrystallization processes are only partial. Down grinding with a cutting depth of 3 µm thus leads to a maximum grain size increase to 23.4 nm and a maximum decrease in micro strain to 0.41% as well as to maximum residual stresses of 880 MPa.

Finishing processes result in changes of near‐surface morphology, which strongly influences the fatigue behavior of components. Especially, roller bearings show a high dependency of the lifetime on surface roughness and the residual stress state in the subsurface volume. To analyze the influence of different finishing processes on the near‐surface morphology, including the residual stress state, roller bearing rings made of AISI 52100 are finished in this work using hard turning, rough grinding, and fine grinding. In addition, fatigue specimens made of AISI 52100 and finished by cryogenic hard turning are investigated. For each condition, the residual stresses are determined at different distances from the surface, showing pronounced compressive stresses for all conditions. While the ground roller bearing rings show highest compressive residual stresses at the surface, the hard turned bearing ring and the cryogenic hard turned fatigue specimens reveal maximum compressive stresses in the subsurface volume. Moreover, cyclic indentation tests (CITs) are conducted in the different subsurface volumes, showing a higher cyclic plasticity in relation to the respective initial state, which is assumed to be caused by finishing‐induced compressive residual stresses. Thus, the presented results indicate a high potential of CITs to efficiently characterize the residual stress state.

Because components are usually not loaded with periodical waveforms and constant amplitudes, but with temporally variable, cyclic loadings, fatigue testing under near service loading conditions is essential to optimize the component's design. However, this kind of fatigue tests requires high efforts in material, time and costs, which can be significantly decreased by using efficient short-time procedures. In the present work, a modification of the physically based lifetime calculation method (PhyBaL(LIT)) for service loading conditions, i.e., PhyBaL(SL), was used to estimate the Gassner curve of ductile cast iron EN-GJV-400 resulting from cyclic loading based on the Car Loading Standard (CARLOS). For fatigue tests, the first 5 000 cycles of CARLOS were used for periodically repeated loading intervals (LIs) with defined maximum stress sigma(max), CARLOS. In the presented work single step tests (SSTs) with constant ermax, CARLOS as well as load increase tests (LITs), in which sigma(max), CARLOS was stepwise increased after each LI, were performed. Because for PhyBaL(SL) the determination of the material's cyclic deformation behavior is a prerequisite, measuring intervals (MI) with constant stress amplitude sigma(a), MI were performed after each LI. The influence of MIs on the fatigue lifetime and cyclic deformation behavior was analyzed by using different sigma(a), MI, showing no significant influence but higher resolution of fatigue-induced changes with increasing era, MI. Based on the measurements obtained in the LIT and two SSTs, the Gassner curve could be determined with PhyBaL(SL) showing a good correlation to the lifetime data obtained from additionally conducted SSTs. Furthermore, the results from LITs enable a rough estimation of the expected fatigue strength at 2 x 10(6) cycles. In addition to the results obtained from cyclic deformation behavior determined in MI, the change in electrical resistance Delta R was measured in the specimens' gauge length during the fatigue tests. This measurement does not require a constant stress amplitude sigma(a) and hence, can be used to characterize the fatigue behavior without using MIs. The presented results demonstrate that Delta R enables an adequate estimation of the Gassner curve by using PhyBaLSL.

Nanocrystalline nickel-iron layers are produced electrochemically on copper discs by varying the current density and then annealed in a vacuum furnace at a temperature range between 200 and 800 degrees C. Grain size, iron content, texture and microstrain of the microstructure are primarily characterized by X-ray diffraction (XRD). Instrumented indentation tests and microbending tests for mechanical characterization are carried out. The iron contents of the investigated layers are 5.7, 8.8, 13.5 and 17.7 wt.-%. By varying the annealing temperature, the reduction of the microstrains is initiated at 200 degrees C and ends at a temperature of about 280 degrees C. Primary recrystallization starts slightly higher at 220 degrees C and is completed at 300 degrees C. With higher iron content, the indicated temperatures shift to slightly higher values. Indentation modulus, Young's modulus, indentation hardness and strength change considerably after the annealing treatment. Fracture strain at the edge, as a measure of ductility, decreases immediately after annealing at 200 degrees C to 0 %. Low annealing temperatures occurring before the beginning of primary recrystallization lead to an increase in indentation hardness and 0.01-% offset bending yield strength R-p0.01(star) as compared to the electrochemically deposited initial state. After annealing at high temperatures, the mechanical parameters are mostly below the initial values for electrochemical deposition. Hall-Petch (HP) behavior is observed for R-p0.01(star), both for the electrochemically deposited specimens down to almost 6 nm and for the specimens annealed at high temperatures. Specimens annealed at low temperatures deviate from the HP straight line to higher values. In this case, an increase in strength is assumed to be due to the very small nanocrystalline (nc) grain sizes, segregation at the grain boundaries and a decrease in dislocation density. Indentation hardness measurements show almost no dependence on D-0.5 for the electrochemically deposited specimens and also for annealed specimens below 30 nm grain size. Above 30 nm, the indentation hardness values are considerably higher than for the HP straight line. Overall, the hardness and strength values of the nc specimens, electrochemically deposited or additionally annealed, are significantly higher than those of the microcrystalline (mc) specimens.

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.

In this paper we deal with the important problem of estimating the local strain tensor from a sequence of micro-structural images realized during deformation tests of engineering materials. Since the strain tensor is defined via the Jacobian of the displacement field, we propose to compute the displacement field by a variational model which takes care of properties of the Jacobian of the displacement field. In particular we are interested in areas of high strain. The data term of our variational model relies on the brightness invariance property of the image sequence. As prior we choose the second order total generalized variation of the displacement field. This prior splits the Jacobian of the displacement field into a smooth and a non-smooth part. The latter reflects the material cracks. An additional constraint is incorporated to handle physical properties of the non-smooth part for tensile tests. We prove that the resulting convex model has a minimizer and show how a primal-dual method can be applied to find a minimizer. The corresponding algorithm has the advantage that the strain tensor is directly computed within the iteration process. Our algorithm is further equipped with a coarse-to-fine strategy to cope with larger displacements. Numerical examples with simulated and experimental data demonstrate the very good performance of our algorithm. In comparison to state-of-the-art engineering software for strain analysis our method can resolve local phenomena much better.

As effective methods for characterization of temperature and load dependent fatigue behavior, the present paper describes (i) application of strain increase and temperature increase tests for comprehensive quantitative characterization of the dynamic strain aging influence on fatigue properties of cast iron and (ii) analysis of electromagnetic acoustic transducer signals for early detection of fatigue damage of AISI 347 metastable austenitic stainless steel. In case of cast iron, plastic strain and stress amplitude recorded in temperature increase tests allow identification of the onset and peak of dynamic strain aging at frequencies up to 92 Hz. Isothermal strain increase tests and application of Morrow's relationships between cyclic deformation behavior and slopes of a Wohler curve in the high cycle / low cycle fatigue-regime allow determining the total strain Wohler curve in excellent agreement with conventional constant amplitude experiments. In-situ characterization of fatigue processes in AISI 347 was performed by measurement of ultrasonic signal time-of-flight as well as amplitude decay by electromagnetic acoustic transducers which generate ultrasonic waves directly in electrically conductive materials without the use of a couplant. Especially the time-of-flight signal reflects the actual specimen loading as well as cyclic hardening / softening and early fatigue damage with excellent sensitivity.

Components of internal combustion engines not only undergo mechanical high and low-cycle fatigue but also thermally induced loadings during start-stop and load changes. Accordingly, an efficient lifetime calculation method of the ductile cast iron EN-GJS-600 for Out-of-Phase thermomechanical fatigue (OP TMF) loading was developed based on Morrow's relation between cyclic deformation and fatigue lifetime behavior. As an alternative to established fracture mechanics based approaches, the slope of the Wohler curve is calculated from data determined in at least one each of a TMF strain increase and TMF constant amplitude test. Based on the Physically Based Lifetime calculation (PhyBaL) method previously published for isothermal low cycle fatigue (LCF) and high cycle fatigue (HCF) loadings, and data taken from only one strain increase and at least one constant amplitude OP TMF test, the Wailer curve can be described as according well with constant amplitude OP TMF tests performed for validation. In the present work, triangular TMF cycles were applied at 5 mHz (0.005 Hz) with a temperature range from 50 to 350 degrees C. Cyclic deformation behavior as input data for calculation of the slope of the Wohler curve was determined by stepwise increases of mechanical strain amplitude in OP TMF tests. By constant amplitude tests at different mechanical strain amplitudes, the lifetime and cyclic deformation behavior at OP TMF loading were determined for validation. Moreover, applicability of measurement cycles at constant temperature and low stress amplitude applied in defined intervals of regular OP TMF cycles proved to match the OP TMF cyclic deformation behavior well. This opens up the prospect of characterizing cyclic deformation behavior in more complex, near service TMF cycles as a baseline for lifetime assessment using the PhyBaL approach outlined above.

In recent years, deformation induced surface hardening when turning was carried out to enhance the component performance of metastable austenitic steels. To induce such a phase transformation from austenite to martensite in the workpiece surface layer, high mechanical loads and low process temperatures are required. Therefore, cryogenic CO2-snow cooling is an appropriate method to assure low temperatures in the workpiece surface layer. In this context, the influence of the process parameters cutting speed, feed, depth of cut and tool cutting edge inclination on the surface morphology in terms of deformation induced surface hardening and resulting surface roughness was investigated. The results show that the deformation induced surface hardening when cryogenic turning is adjustable via a targeted variation of the investigated process parameters. Thus, the morphology of the surface layer and therefore the properties of the component can be adjusted according to the requirements of the application.

In the present paper, the physically based fatigue lifetime calculation approach "PhyBaL(SIT) was modified for isothermal low cycle fatigue of ductile cast iron EN-GJS-600 in the temperature range from ambient temperature up to 400 degrees C at a frequency of 0.005 Hz. One strain controlled temperature increase test is sufficient to determine the temperature for onset and most pronounced influence of dynamic strain aging, i.e. the reduction of dislocation mobility due to diffusing interstitial atoms. The comparison to prior results at 5 Hz and to results at 47 and 92 Hz reveals the significant influence of deformation rate on the temperature range of dynamic strain aging. With the results at 0.005, 47 and 92 Hz presented in this paper and at 5 Hz previously published, sound knowledge how elevated temperatures and deformation rate affect dynamic strain aging, cyclic deformation behavior and fatigue lifetime in the LCF- and HCF-regime is generated.

Weight-optimized component design as well as a reliable estimation of the lifetime of metallic materials and components require a comprehensive understanding of fatigue processes and a systematic investigation of the underlying fatigue behavior. This becomes even more important when designing highly loaded components such as wheels of high-speed passenger railway systems. Typically, mechanical stress-strain hysteresis measurements and increasingly different types of temperature and electrical resistance measurements are used to characterize the fatigue behavior and fatigue processes. Here, electrical resistance measurements provide significant information as they allow the detection of microstructural changes, e.g., through changes in dislocation density and structure. In addition, electrical resistance measurements can be considered in load increase and constant amplitude tests with inserted load-free sequences and in service load tests to characterize damage progress. In this paper, characteristic values of the change in electrical resistance were determined for ICE R7 wheel steel specimens and correlated with dislocation density, which was load- and cycle-dependent and determined through transmission electron microscopy.

Nanocrystalline nickel-iron microstructures, manufactured by means of an electrochemical deposition process via electrolyte solutions, were investigated to collect relevant information for the use of nickel-iron for micro-components. By varying the current density, nickel-iron coatings can be set to show specific grain sizes, iron content, lattice strains and textures. Uniform microstructures exist in each of the deposited nickel-iron coatings. The grain sizes determined using x-ray analysis (XRD) cover a range of 6 to 17 nm. XRD texture analyses parallel to the deposition plane resulted in {111} and {200} orientations. To characterize the material's mechanical properties indentation hardness measurements and micro-bending tests were performed. For a 0.01 %-offset bending yield strength (R-p0.01*), grain sizes of 6 to 17 nm clearly demonstrate Hall-Petch behavior. In addition, the investigations show lower work hardening and lower values for remaining edge strain at fracture for decreasing grain size. In contrast to R-p0(.)01*, the Young's modulus, indentation modulus, indentation hardness values and the bending strength, within their scatter bands, all remain largely unaffected by the different microstructures. Overall, all measured strength and hardness values of the considered nanocrystalline microstructures are very high in comparison to microcrystalline microstructures.

In the automotive and aerospace industry, the use of lightweight alloys is continuously increasing. However, classical aluminum alloys cannot fulfill all demands of improved future lightweight concepts. One possibility to overcome this issue is to reinforce a ductile metallic matrix with a second phase like fibers, whiskers or particles, i.e. producing metal matrix composites (MMC) or aluminum matrix composites (AMC) for innovative engineering applications. The main advantage of these multiphase materials is an increase of specific mechanical properties with respect to the matrix alloy, e.g. Young’s modulus and ultimate tensile strength (UTS). In addition, the composite mass density remains close to the matrix alloy due to relatively low volume contents of reinforcement like Al2O3, SiO2 or SiC. In contrast, the improved stiffness as well as the possibility to use further strengthening mechanisms of the metallic phase, such as precipitation hardening, leads to an increase of the global mechanical properties. The deformation capability and damage mechanisms are strongly affected by the matrix / reinforcement interaction. Especially local deformations caused by this pronounced heterogeneity are often not taken into account during the engineering design of AMC components. The present study focuses on AMC variants reinforced by SiC particles with a nominal volume content of 17 % and a varying size from nominal 0.3 μm to 3 μm. Micro-mechanical tests on notched samples were realized using an in situ tension-compression load frame mounted in a SEM. Micrograph sequences determined under increasing mechanical load up to final failure are the basis for computation of the strain tensor by mathematical image analysis. This procedure is used to monitor and analyze the arrangement of the multiphase microstructure and its changes at the surface during mechanical loading. Especially the size of the SiC particles as an influencing factor is discussed for monotonic properties of AMC.

In the present work, specimens of the metastable austenitic stainless steel AISI 347 with different surface morphologies were investigated in stress-controlled fatigue tests in the high cycle fatigue (HCF) regime at ambient temperature. Specific surface morphologies were generated by cryogenic turning with CO2 snow cooling. As a result of the metastable austenite microstructure, phase changes from paramagnetic austenite to ferromagnetic martensite take place in the near-surface regime during cryogenic turning as well as in the whole specimen volume during monotonic and/or cyclic elastic–plastic deformation. The metastability of AISI 347 was characterized according to the MS-temperature determined from the chemical composition and by X-ray diffraction measurements with in situ cooling. Microhardness and strength of both phases were measured. Near-surface microstructure was analyzed by optical and scanning electron microscopy after focused ion beam preparation. Besides a partially martensitic surface layer, a thin nanocrystalline layer, both induced by cryogenic turning, was observed. In case of cyclic loading, the martensitic surface layer leads to a reduction of plastic strain amplitude as well as a retardation of crack initiation and consequently to an increase in fatigue life.

In the last two decades, focused ion beam (FIB) systems have been used for sample preparation. For example, the edges of a sample can be polished for analytical measurements or continuous cross-sections can be milled for three-dimensional (3D) tomography and reconstruction. One major challenge in both procedures is the so-called curtaining effect, i.e., increasing surface roughness in the direction of the milling depth. The roughness of the cut can influence the result of the measurement and the segmentation process. In the present study, the authors report on two different methods to reduce the curtaining effect, namely, a hardware- and a software-based solution. For instance, Tescan implemented the so-called “rocking stage” in its plasma FIB. However, this is not available for other FIB systems. Therefore, for our FEI gallium FIB, an inhouse-developed goniometer stage is installed, which can be adapted as necessary. With this relatively inexpensive solution, the sample can be rotated around an additional axis and tilted by ±8°. Different sample heights are adjustable, and the sample's edge can be polished and imaged without stage movement. However, for automated milling and imaging procedures such as 3D tomography, such a tilting stage is not feasible. Therefore, as a second option, an image processing method is proposed that can be applied after the milling procedure on a whole image stack. A novel variation of this method for mathematical image processing is developed to reduce milling artifacts. Besides the curtaining effect, additional artifacts such as discontinuities caused by redeposition of previously removed materials or charging effects can be removed. The method is applied to the entire 3D dataset, and distortions are reduced by using information of their particular structure and directional dependence. The resulting new image stack can then be used to compose a 3D volume reconstruction. As an example, the geometries of silicon carbide particles reinforcing an aluminum matrix can be measured with nearly no milling artifacts.

Low-cycle fatigue behavior, destabilization of austenite and emergence of deformation induced martensite (DIM) was investigated in wrought AISI 304 type (304, 304L, 348) austenitic stainless steels of diverse semi-product forms. The specimens fabricated from cylindrical bars, plates or thin sheets were cyclically strained with high strain amplitudes under total strain control in symmetrical push-pull at room and depressed temperatures to different stages of fatigue life. Quantitative and qualitative data on the formation, morphology and specific distribution of DIM in the volume of the material within the whole gauge part of the specimens are confronted with the respective cyclic stress-strain response and discussed in terms of characteristic chemical heterogeneity. This heterogeneity arises in the form of chemical banding due to the complex solidification behavior of AISI 304 type steels and its contemporary production route. Possible effect of additional solution treatment (various cooling rates due to quenching in different media) on DIM formation is discussed. (C) 2017 Elsevier Ltd. All rights reserved.

The effect of surface modification by cryogenic turning on fatigue behavior of metastable austenitic stainless steel AISI 347 was investigated in stress-controlled fatigue tests at ambient temperature (AT) and 300 °C in air. Five different surface morphologies were manufactured by the variation of turning parameters – with and without cryogenic CO2 snow cooling and feed velocity as well as by the application of polishing for reference surfaces with a very small surface roughness. For a comprehensive characterization of the surface and near surface morphology, X-ray diffraction investigations were performed. Three phases (γ-austenite, α-martensite and ε-martensite) were detected in the near-surface microstructure after cryogenic turning while after turning without cryogenic cooling the only microstructural constituent was γ-austenite. Moreover, residual stress state, micro hardness and surface roughness play an important role in surface morphology. The experimental data on the cyclic deformation behavior and stress-strain response of all surface morphologies are reported. Reference specimens with purely austenitic surface microstructure show the highest plastic strain amplitude during cyclic loading at both AT and 300°C. At elevated temperature these specimens achieved the shortest fatigue life. Martensitic surface layers induced by cryogenic turning result in the reduction of plastic strain amplitude during cyclic loading and significantly enhance fatigue life at both tested temperatures.