Usually, fatigue crack propagation is experimentally characterized using cracks emanating from deep, throughspecimen-width notches that average the crack's propagation behavior over many microstructural features along the crack front. Such a deep and wide crack does not accurately represent the reality in most tools, where cracks typically emanate from small microstructural features, such as carbides or shallow notches at the surface. Therefore, the general applicability of conventionally obtained fatigue data on application-relevant short cracks was uncertain. Furthermore, by averaging local properties, the effect of microstructural features on the propagation behavior remained hidden. The current work addresses the open question of the validity of short crack fatigue data obtained on deep notches using a novel method to monitor the propagation of application-relevant, microstructurally small, and shallow cracks. The technique enables in situ measurement of microstructurally small cracks emanating from an artificial defect of application-relevant size near a small number of microstructural features by using the alternating-current potential drop method. The technique is demonstrated on the example of mu m-sized semi-elliptical notches introduced via focused ion beam milling. Measured potential increases were correlated to fatigue crack extensions for the utilized material, signal current, and signal frequency combination. The short crack propagation behavior of microstructurally small cracks resembled that observed for short cracks in single-edge notched bending specimens. The results indicate that the short crack propagation behavior obtained on physically short but macroscopically deep cracks is also valid for application-relevant defect sizes for high-strength materials.

The nickel-based superalloys are used in the field of hydrogen storage and transport where cryogenic temperatures are often required. The combined influence of hydrogen and cryogenic temperatures on their mechanical performance remains largely unexplored. In this work, the fracture behavior of alloy 718 was investigated using a slow strain rate testing (SSRT). The results at room temperature (RT) with and without previous gaseous hydrogen charging have been compared to the SSRT results obtained at 4.2 K using the same testing procedure. The findings show a pronounced embrittling effect due to hydrogen at room temperature, whereas no embrittlement was observed at cryogenic temperatures, which is attributed to limited hydrogen diffusivity at 4.2 K.

The fatigue strength of high-strength steels is significantly affected by residual stresses because of their pronounced mean stress sensitivity. Failing to consider the potential effect of residual stresses when evaluating fatigue data can lead to misinterpretations with serious implications for the design and reliability of components. Therefore, it should be standard practice to either remove residual stresses in fatigue test specimens – for instance, through electropolishing or stress-relief annealing – or quantify them appropriately. This is especially important in very high cycle fatigue (VHCF) studies, where failure beyond 107 cycles typically originates from interior defects. In such cases, the distribution of residual stresses within the material is of utmost importance.In the present study, VHCF data obtained with a martensitic stainless steel sheet are evaluated. Residual stress profiles were measured using X-ray diffraction, and the results were corrected to account for the incremental material layer removal during the measurement. It is demonstrated that VHCF failure originated solely at non-metallic inclusions located in a tensile residual stress field. This emphasises the importance of appropriate mean-stress corrections for reliable fatigue strength predictions.

State-of-the-art tool steels, such as high-speed steels, consist of a tempered martensitic matrix and other hard phases embedded within, forming a de facto metal-matrix composite of precipitated hard carbide phases embedded in a softer martensitic metal matrix. The precipitated carbide phases can be subdivided into primary carbides, which precipitate directly from the melt, and small secondary hardening carbides, which precipitate during tempering. During tooling application, most tool failures occur due to material fatigue, where microstructural effects, especially those related to the primary carbide architecture, are not yet fully understood. Therefore, three microstructural variants were developed from a single batch of the same high-speed steel grade to investigate the effect of their microstructure on fatigue crack propagation and fracture behavior. The variants were established solely by different heat treatments and featured i) a high-volume fraction of narrowly-spaced small primary carbides, ii) a high-volume fraction of widely-spaced large primary carbides, and iii) a low-volume fraction of widely-spaced small primary carbides. The primary carbide architecture most suited for long tool lifetimes was identified as featuring small, widely spaced carbides with a highly alloyed matrix.

Comprehensive studies of hydrogen embrittlement in high-strength austenitic alloys under cryogenic conditions are scarce, leaving the combined effect of hydrogen charging and extreme temperatures largely unexplored. Given the demands of cryogenic applications such as hydrogen storage and transport, understanding material behavior under these conditions is crucial. Here, we present the first systematic study of hydrogen’s effect at liquid helium temperature (4.2 K) on the mechanical properties of precipitation hardened austenitic alloys, specifically the nickel-based Alloy 718 and austenitic stainless steel A286. Both materials were subjected to pressurized hydrogen charging at 473 K followed by slow strain rate tensile testing at room temperature and at 4.2 K. Hydrogen charging caused significant ductility loss at room temperature in both alloys. In contrast, testing at 4.2 K resulted in increased strength and no evidence of hydrogen embrittlement. Notably, materials pre-charged with hydrogen and tested at 4.2 K exhibited higher stress drop amplitudes and increased strain accumulation during serration events, suggesting persistent hydrogen–dislocation interactions and possible enhanced dislocation pinning by obstacles such as Lomer–Cottrell locks. These results indicate that while hydrogen influences plasticity mechanisms at cryogenic temperatures, embrittlement is suppressed, providing new insight into the safe development of austenitic alloys in cryogenic hydrogen environments.

The accurate determination of the cyclic R-curve of the threshold of stress intensity factor range is a demanding experimental task. The main challenge concerns the introduction of a pre-crack that is open under tension loading and free of residual stresses. When pre-cracks are generated by cyclic compression-compression loading, a zone of tensile residual stresses up to the material yield strength is generated ahead of the pre-crack tip. These stresses may affect crack propagation at the beginning of fatigue crack growth and should consequently be removed especially for high-strength materials. However, subsequent stress-relief annealing is impractical for numerous metallic material classes due to undesired microstructural alterations at the temperatures required for stress relief. In the present contribution, the above-described issue is addressed for a high speed steel using single-edge notched bending specimens pre-cracked at various stages of a typical high speed steel heat treatment process. It is shown that stress-relieving via a heat treatment route is a significant improvement compared to the conventional compression pre-cracking procedures to measure material-specific cyclic R-curves.

High-speed steels exhibit a good combination of high strength with reasonable toughness when compared to hardmetals. These mechanical properties enable the excellent performance of high-speed steels as tool materials in metalworking applications. The composite microstructure consisting of primary and secondary hardening carbides embedded in a martensitic matrix leads to this mechanical key feature. To increase efficiency parameters, such as tool life, high-speed steels are subject to continuous microstructural development by heat treatments. For tool steels, knowledge of the effect of primary carbide coarsening heat treatments of high-speed steel on primary carbide spacing is still incomplete. In this contribution, specimens made from commercially available high-speed steel were subjected to distinct heat treatments, such as long-term and high-temperature annealing. Specimens quenched and tempered to industrial standards were used for reference. The long-term austenitization resulted in a carbide coarsening to more than twice the carbide size than in the reference state with no associated change in primary carbide volume fraction. The high-temperature austenitization in contrast led to limited carbide coarsening, but a carbide content reduction of roughly 4 vol.

To predict the solidification and product properties of tool steels with complex chemical compositions, an understanding of the transformation behavior is crucial. Therefore, the quaternary Fe–C system with 10 wt% Cr and 3 wt% W (a subsystem of cold work steels, with M7C3 and M23C6 carbides) and the Fe–C system with 6 wt% W and 5 wt% Mo (simplified high‐speed steel, with M6C and MC carbides) are selected. The motivation for this study is to develop a methodology for the safe and fast production of model alloys and the close to equilibrium performance of differential scanning calorimetry (DSC) measurements. Regular diffusion annealing of as‐cast carbidic steels is time‐consuming, but with an additional heat treatment during the DSC measurement in the semisolid zone (30–50% liquid phase fraction), a status close to equilibrium can be achieved within minutes due to the high diffusion. To prove the potential of the equilibration by partial premelting in the DSC, additional equilibration and quenching experiments are performed in a Tammann furnace and investigated using a scanning electron microscope and X‐ray diffraction analysis. By combining these methods, carbide types and the transformation temperatures can be verified to evaluate and construct complete phase diagrams.

The characterization of local and global fracture mechanical propertiesMechanical properties is carried out with destructive testing methods and is increasingly required in the specifications of forgingsForging. Especially in the case of alloy 718Alloy 718 aircraft parts, the numerical estimation of local material properties is essential for lightweight design, geometry optimization, and a significant reduction of development and experimental characterization costs. This leads to a demand for numerical models to capture initial microstructural inhomogeneities, describe the forming history, and reflect the local microstructureMicrostructures and properties of the final product. Therefore a digital twin for the complex forgingForging process was developed in order to reproduce and evaluate the resulting local microstructureMicrostructures across the complete process chain. Since the microstructureMicrostructures determines the mechanical propertiesMechanical properties like yield stress and fracture toughnessFracture toughness, a dedicated model was implemented to describe the local evolution of the relevant microstructural features.

The powder metallurgical incorporation of hard phases in steel matrices is an established concept to form a metal–matrix composite (MMC) with excellent mechanical properties. Especially, TiC has been extensively used in a wide variety of steel matrices in the past. While the stability of TiC particles is known to be low during liquid‐phase sintering, the thermodynamic stability during solid‐state consolidation is less investigated. As the reaction of TiC and the associated formation of reaction phases affect its bonding strength. The bonding strength is considered a crucial parameter for crack initiation in tools, it is important to understand the parameters that control this behavior. To this end, herein, the formation of a (Ti,M)C phase in a TiC–steel MMC is shown, modifying the interface of TiC to the steel matrix during solid‐state consolidation. This is corroborated by thermodynamic calculations and by scanning and transmission electron microscopy combined with energy‐dispersive X‐ray spectroscopy showing that (Ti,M)C is enriched with a significant amount of V, Mo, W, and Cr. High‐energy X‐ray diffraction measurements reveal that with larger normalized surface area of the TiC particles, the amount of (Ti,M)C increases in similar consolidation processes, while the M concentration decreases.

Coated hardmetals are commonly used tool materials for metal cutting applications. For these applications, the substrate- and the coating surface qualities are improved by grinding, polishing, or blasting. The focus of the current work was to study the influence of different combinations of substrate-coating surface processing techniques on the induced damage in Al2O3/TiCN hard-coated WC-12 wt% hardmetal substrates and the depth to which the damage reaches into the substrate. The substrate and coating surfaces were prepared by grinding, polishing, dry blasting, wet blasting, or remaining in the deposited state. Investigations were performed using cyclic indentation with a novel ball-in-cone test method in a vacuum at 700 degrees C, under conditions that imitate the combined shear-compression loads that occur at the cutting edges of metal cutting tools. Additionally, finite element simulations were performed to characterize the stress state arising during ball-in-cone testing. The kinetics of defect formation and accumulation in the hardmetal substrate as a function of the loading situation during the ball-in-cone test will also be discussed. The defects that formed in the substrate during cyclic loading were studied using scanning electron microscopy in cross-sections prepared by focused ion beam milling. A larger increase in defect frequency was observed for rougher coating surfaces and substrate-coating interfaces than for smoother ones. Furthermore, 6 mu m thick coatings exhibited a higher defect frequency after cyclic loading than 15 mu m thick coatings. Most of the observed defects were in the nm-size range close to the substrate-coating interfaces and tended to have increased in size after cyclic loading. The ball-in-cone test set-up enables the study of the damage mechanisms in coated hardmetal substrates with different treatments prior to and after deposition under tunable milling-like stress and temperature conditions.

The aim of the current study was the determination of the time-dependent visco-plastic material behavior of the molybdenum alloy MHC (Molybdenum-Hafnium-Carbon) in stress-relieved condition with cyclic strain -controlled low cycle fatigue experiments at room temperature, 800 degrees C and 1400 degrees C. To ensure high data qual-ity, a servohydraulic testing machine modified with a vacuum chamber was utilized to avoid material oxidation. The long-time strain-controlled experiments were conducted with a laser extensometer with high accuracy up to the highest applied test temperature of 1400 degrees C. The determined data were appropriate to generate cyclic stress -strain curves, strain-Wo center dot hler curves, and their descriptive parameters. Furthermore, the strain rate dependency of the cyclic stress response and the stress relaxation behavior of the cyclically stabilized material were also investigated at the mentioned temperatures. The current study indicates that MHC in stress-relieved condition has a softening ability during cyclic loading, especially at 800 degrees C and 1400 degrees C. The strain rate sensitivity of the stress amplitude for the case of cyclic stabilization shows similar behavior as described in the literature for the case of monotonously increasing loading conditions, with a minimum at 800 degrees C. An interesting effect was identified in relaxation tests with loading at different strain rates. A kind of rebound effect was observed at the highest utilized strain rate of 10-2 s- 1, which disappears with decreasing strain rate. Elastic strain proportions can be converted into plastic strain proportions during the application of the slow strain rate.

Hard-coated high speed steels are commonly used as tool materials for metal cutting applications, where they are exposed to a complex load spectrum consisting of shear and compressive stresses near the interface to the hard coating. These steels consist of different microstructural components, which on the one hand increase the stiffness of the material and on the other hand can withstand damage such as wear and cyclic plastic deformation. Although MC carbides are essential for the wear resistance and in particular can significantly increase the strength of the interface to the hard coating, knowledge about the influence of the steel microstructure on the cyclic damage behaviour caused by application-oriented load spectra is incomplete for these systems. Hence, this study focuses on the influence of coarse carbides and the martensitic matrix on the cyclic damage behaviour of TiN-coated high speed steels. Using an inclined impact test, a combined shear-/compressive load is applied to the steel/TiN interface of two different specimens with systematically varied high speed steel microstructures. Scanning electron microscopy on cross sections placed in the remaining imprints prepared by means of focused ion beam milling reveal a strong cyclic plastic deformation of the substrate that occurred after surpassing a critical applied force. Scanning electron microscopy and nanoindentation measurements in the high-speed steel matrix suggest cyclic softening and cyclic plastification, which is assumed to induce cracking at the interface between MC carbides and TiN coating.

WC-Co hard metals are composite materials with extraordinary mechanical properties especially at elevated temperatures which make them common materials for metalworking tools. The current work investigates damage and fracture behavior of hourglass-shaped specimens made of a WC-Co hard metal with submicron-sized WC grains and 12 wt.% Co binder. All investigated specimens were isothermally heated inductively to 700 degrees C in a servo-hydraulic testing machine equipped with a vacuum chamber to avoid surface oxidation. The nature of origins of fracture and the evolution of bulk material damage features was studied via scanning electron microscopy for experiments under monotonically increasing load as well as cyclic loading under a stress ratio R = sigma min / sigma max = -1. A formation of small cavities that formed in the material bulk during loading was observed for all investigated loading conditions. For monotonically increasing load, the coalescence of these cavities was identified as the dominant damage mechanism. For cyclic loading, the number and size of the mentioned cavities did rise with the applied number of load cycles and rising stress amplitude. The coalescence of the cavities was found to be a main mechanism controlling the fatigue crack propagation process at elevated temperature. The found results imply that a certain combination of stress amplitude and temperature exists, at which a transition of failure control occurs from: (i) the size of material-inherent defects present prior to loading, to (ii) the kinetics of the coalescence of cavities that form due to plastic deformation and creep in the Co matrix during loading. (c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

WC-Co hardmetals are utilized as tool materials in metal cutting applications in which they are exposed to high mechanical cyclic loads and elevated temperatures. A better understanding of the failure mechanisms of WC-Co hardmetals under these application conditions and the ability to diagnose the damage evolution state are key factors to understand the limits of endurable cyclic load at a certain temperature. The aim of the current work was the experimental determination of stress-strain-hysteresis loops for the investigation of damage indicators in uniaxial cyclic tests at a stress ratio of R = sigma(min)/sigma(max) = -1 for two WC-10 wt% Co hardmetals at 700 degrees C and 800 degrees C in vacuum. An increase in the stress-strain-hysteresis loop area and tension-compression-strain asymmetry was recorded with increasing number of load cycles at 800 degrees C, with earlier failure than at 700 degrees C. The relationship between the stress-strain-hysteresis loop parameters and the damage evolution state at the microstructure level, as well as the deformation behavior of WC- and Co-phases with increasing number of load cycles, were analyzed. To this end, the microstructure for one WC-Co hardmetal grade was analyzed by scanning electron microscopy and electron backscatter diffraction after cyclic testing up to defined numbers of load cycles at 800 degrees C. It was observed that the hysteresis loop area and strain asymmetry coincide with the formation of nanopores at WC/WC interfaces and WC/Co phase boundaries, which enlarge to form larger cavities with increasing number of load cycles. Additionally, electron backscatter diffraction data showed that the fcc Co-phase partially transformed into hcp Co under cyclic loading. All specimens, in which an increase in the stress-strain hysteresis loop area or strain asymmetry was observed, ultimately failed when a sufficiently high number of load cycles was applied. Thus, these results indicate that the investigated parameters are reliable indicators for bulk material damage.

The aim of the current work was the experimental determination of limit stresses in the uniaxial cyclic compression test that lead to advancing ratcheting in different WC-Co hardmetal grades at elevated temperature. At stresses below the limit stress the plastic strain per cycle reduces and plastic strain accumulation stops after a characteristic number of load cycles. Special attention was paid to the microstructural influence on the onset of advancing ratcheting and the associated damage development at the microstructure level. WC-Co hardmetals are used in various areas such as forming and forging tools, where they are exposed to high temperatures and pressure loads. Their good high-temperature properties allow them to be used under these conditions, but these properties are influenced by the microstructure. Investigations of the mechanical properties of hardmetals had been carried out under monotonously increasing loads and cyclic tests at room temperature and elevated temperatures. In these tests, the effect of different stress ratios R = sigma(min)/sigma(max) on the fatigue behaviour of hardmetals were studied, However, no studies are known for WC-Co hardmetals and their limit stresses in relation to strain ratcheting under cyclic compressive loading at elevated temperature. Hence, in the current work the influence of increasing stresses as a function of microstructure and their effect on the evolution of the strain of six different WC-Co hardmetal grades are discussed. For this purpose, the materials were investigated by uniaxial cyclic compression tests at a stress ratio of R = -infinity at 700 degrees C and 800 degrees C in vacuum. The investigated hardmetal grades differ on the one hand in their WC grain size, which varies between 0.4 mu m and 2.0 mu m, and on the other hand in their Co-content, which varies between 6 wt% and 12 wt%. The residual strain value epsilon(res) at zero applied stress was observed to stabilize with increasing number of load cycles at low applied stress ranges. Strain ratcheting occurred above a critical stress range, referred to as the limit stress for strain ratcheting. Strain ratcheting is the accumulation of plastic strain with increasing number of load cycles in which no strain stabilization occurs. Further, for all investigated hardmetal grades, the limit stresses were observed to decrease with increasing temperature. In the following, the microstructure of one hardmetal grade was analysed after loading below and above the limit stress by scanning electron microscopy and electron backscatter diffraction (EBSD). The influence of strain stabilization and ratcheting was analysed with regard to damage development and deformation behaviour of the WC and Co-phases. Strain ratcheting was observed to result in the formation of cavities and nanopores at phase boundary triple points and WC/Co interfaces. Additionally, the EBSD data showed that the fcc Co phase was transformed into hcp Co. Therefore, it is assumed that on the one hand, a certain strain value needs to be exceeded for strain ratcheting to occur and, on the other hand, that besides dislocation movement, microdefect formation and phase transformation significantly contribute to the increase in strain.

WC-Co hardmetals are popular tool materials, which are used in applications such as metal milling or turning. In these applications, elevated temperatures occur in the tools during the machining process, although they are also cooled. This results in a complex interaction of thermal and mechanical loads in the tools. Within this current work, a strain asymmetry of a WC-10 wt% Co hardmetal after tensile and compression uniaxial step-loading creep tests is described. Two types of tests were performed: Firstly, specimens were deformed to certain strain limits at 700 ?C and 800 ?C. Strain asymmetry was observed for tensile and compression stresses above 600 MPa at 700 ?C and above 250 MPa at 800 ?C. In the second type of test, the specimens were stepwise loaded up to a stress of 300 MPa under tensile and compressive load at 800 ?C. The aim of test 2 was to identify the physical reason for the strain asymmetry from the first tests at 800 ?C. The material?s microstructure was analyzed for the specimens from test 2 by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The influence of the loading type was analyzed with regard to damage development and deformation behaviour of the WC- and Co-phase. SEM images showed that the faster increase in strain over time under tensile than under compressive loading was caused by the formation of cavities at WC-WC interfaces and at WC-Co phase boundaries. Due to the larger number of observed microdefects under tension than under compression, it was assumed that this was the physical reason for the strain asymmetry. In addition, EBSD data showed that during the compression and tension creep tests, the fcc Co-phase was partially transformed into the hcp Co-phase.

Knowledge about the adhesion of protective hard coatings on tool materials is of great importance to understand their failure mechanisms in metalworking. Until now, common techniques such as scratch and indentation tests are used to establish a qualitative ranking of a coating’s adhesion on various substrate materials. Nevertheless, there is a lack of quantitative measures to describe the strength of the interfaces between individual microstructural constituents of substrate-coating composites. The current work investigates the interfacial strength and thus the adhesion of TiN deposited as a hard coating on an MC-type carbide, an M6C-type carbide and on martensite being constituents of high speed steels. Tensile stresses were introduced at the interface between TiN and the individual microstructural constituents of a high speed steel via micromechanical testing of a novel MSC specimen within a scanning electron microscope. The tested MSC specimens were subsequently investigated in detail by scanning electron microscopy. Evaluation of the interface stress at fracture via finite element analysis yielded a ranking in interface strength and therefore coating adhesion in a sequence from high to low strength values from MC/TiN over M6C/TiN to martensite/TiN.

At elevated temperatures, the material behaviour of WC-Co hardmetals shows differences under similar loading conditions depending on WC grain size and Co-content. Variations in the chemical composition and microstructure in hardmetals cause different material properties such as strength or creep resistance. In the current work, the influence of WC grain size on creep mechanism and creep resistance was investigated for WC-12 wt% Co hardmetals with 0.4 mu m, 0.7 mu m and 2.0 mu m average WC grain size. Specimens were tested in uniaxial tensile and compression step-loading creep tests at 700 degrees C and 800 degrees C under vacuum conditions. Time-dependent creep behaviour with steady-state secondary creep was observed for all hardmetal grades investigated, with specimens creeping faster under tensile than under compressive loading. At 700 degrees C, the medium-grained hardmetal grade exhibited the highest minimal creep rates epsilon min compared to the submicron and ultrafine-grained grades. In contrast, the ultrafine-grained hardmetal grade showed higher epsilon min at low stresses and 800 degrees C, because of the high amount of grain boundary area per unit volume, which is advantageous for vacancy diffusion at grain boundaries. Therefore, the epsilon min of the medium-grained hardmetal grade was less affected by temperature than that of the finer-grained grade. Also two stress exponent n-ranges were observed at 700 degrees C and 800 degrees C: At low stress levels, n was in the range of about 1. Above a critical stress level, n reached values between about 4 and 6. Beside the influence of the WC grain size on the creep mechanism and creep resistance, damage evolution with increasing stress levels was analysed for the ultrafine-grained grade at 800 degrees C. The microstructures of three compression step-loading creep tested specimens were examined after maximum stress levels of -350 MPa, -950 MPa and -1350 MPa. Microstructural investigations performed via scanning electron microscopy showed that more and larger cavities had formed at WC/WC interfaces and WC/Co phase boundaries in the specimen tested up to -1350 MPa compared to the ones tested up to -350 MPa and -950 MPa.

For long-term applications of components, such as in turbomachinery or automotive engineering, knowledge of creep behavior under increased load and temperature is of interest. Creep tests are commonly used to investigate the creep behavior of materials at a constant test temperature above room temperature under a constant force. The present work describes a so-called uniaxial step loading creep test setup and first results for a WC-Co hard metal under isothermal conditions at 700 degrees C in vacuum. Heating and temperature control within the tested specimen's gauge length were performed by induced eddy currents and a thermocouple, respectively. In contrast to conventional creep tests, the mechanical load is increased stepwise and the stress at each level is kept constant for 500 s. Displacement of the strain gauge markings was measured contactless with a laser extensometer. First tests were carried out for a WC-Co hard metal under compression and tensile loading. In order to avoid buckling of the high-strength material under compression, a special specimen geometry with non-constant specimen diameter was used. The minimum creep rate was determined for each applied tensile and compressive stress level. Under tensile load, minima of the creep rate were observed above a stress of 500 MPa that are interpreted as the secondary creep rates. Under compressive load, the respective creep rate minima were observed above a stress of -700 MPa.