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.

Nanoindentation is a widely used material characterization technique. However, the interaction of different phases in multi-phase materials and their influence on measurement results is often only insufficiently considered. In particular, the question of how, and to what degree, hard phases embedded invisibly beneath the surface of a softer matrix affect the load–displacement curves during nanoindentation was unexamined. To address this issue, nanoindentation mapping was performed near the edge of cubic samples made from high-speed steel composed of a soft metal matrix and hard primary carbides. The high-speed steels’ heat treatment was varied to achieve two microstructurally distinctive material states featuring: I.) small, tightly-spaced, and II.) large, widely-spaced primary carbides. The microstructures below the indents were exposed on cross-sections prepared by focused ion beam milling. The experimental set was complemented by a parameterized two-dimensional axisymmetric finite element model that replicated load–displacement curves for indentations with hard carbide phases located at well-defined depths below the indenter. The ratio of the indenter tip’s contact depth to the carbide’s depth was used to quantify the effect of sub-surface carbides on apparent material properties. Above ratios of 0.2 and 0.3, the reduced elastic modulus, respectively hardness, were significantly higher than pure matrix values.

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.

When using man-made devices of everyday life such as cars, planes, or smartphones, the general public commonly does not consider the relevance of material characterization techniques. When pushing down the “gas” paddle in an electrically driven vehicle or turning up the heating in our flats or offices, and asking ourselves whether or not this will be possible next winter or how much it will cost — do we think of the relevance of material characterization techniques? Many of us may answer with “no” to these questions. Therefore, this contribution wants to showcase material characterization techniques developed at the Materials Center Leoben Forschung GmbH (MCL). The contribution aims at giving a brief impression of the described techniques’ basic ideas and main benefits in the cost- and resource-efficient production of goods of everyday life.

Tool wear monitoring is crucial for quality control and cost reduction in manufacturing processes, of which drilling applications are one example. Identification of the wear area in images of cutting inserts is important to building a reliable ground truth for the development of indirect monitoring approaches. Therefore, we present a semantic image segmentation pipeline for wear detection on microscopy images of cutting inserts. A broadly used convolutional neural net, namely a U-Net, is trained with different preprocessing and optimisation task configurations: On the one hand the problem is considered as binary problem, and on the other hand as multiclass problem by differentiating the wear into two different types. By comparing these two problem formulations we investigate whether the separation of the two wear structures improves the performance of the recognition of the wear types. For both problem formulations three loss functions, i. e., Cross Entropy, Focal Cross Entropy, and a loss based on the Intersection over Union (IoU), are investigated.The use of different augmentation intensities during training suggests adequate but not too excessive augmentation, and that with optimal augmentation the choice of loss function gets less important. Furthermore, models are trained on image tiles of different sizes, which has an impact on producing artefacts on the whole image predictions performed by the overlap-tile strategy. In summary, the best performing models are binary models, trained on data with moderate augmentation and an IoU-based loss function.

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.

Monitoring of the end milling tool wear in real-time is very crucial for the quality control of a surface finish. In the model-based approaches, usually the tool wear is monitored by tracking the force model coefficients during the milling operation. In this work, we explore a monitoring approach based on two metrics derived from the goodness-of-fit of the milling mechanistic model. The cumulative sum control charts are constructed to monitor changes occurring to the tool condition during a milling operation. Compared to previously suggested methods based solely on the values of force model coefficients, our approach facilitates a preciser and less conservative identification of the point of transition from homogeneous wear to onset of breakout formation at the cutting edges. The proposed strategy is an alternative approach for real time monitoring of tool condition based on mechanistic models.

In the metalworking industry, hard-coated WC-Co hardmetal tools are often used in machining applications. Generally, tool life is limited by pre-existing defects in the hardmetal substrate and defects that form during tool application. The complex, multi-axial loading situations and the high temperatures present in the cutting-edge area of machining tools are among the main reasons for the formation and growth of defects in operation. At present, there is a lack in experimental setups that can replicate these conditions. In the current work a novel material testing method is presented, which uses a spherical indenter and inclined specimen surfaces to apply multi-axial loads at 700 C. The local stress state in the specimen was calculated using finite element simulation implementing an experimentally parameterized material model considering ratchetting and creep of the substrate material. Stresses ranging from mainly compressive to tensile-compressive were predicted. Initiation and accumulation kinetics of defects in the nm-to mu m size regime were studied quantitatively. The comparison of stress calculations and damage development shows that positions with tensile-compressive stresses exhibited significantly higher defect formation rates than those with mainly compressive stresses.

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 adhesion of protective hard coatings to the substrate is one of the most important parameters that influence the performance of metal cutting tools. By a combination of high-resolution transmission electron microscopy and first principles modelling we determine the interface adhesion between a TiN hard coating and microstructural constituents of a high speed steel on an atomic level for the first time. Based on experimentally observed orientation relationships, multiple structures are studied for each interface and both interface cohesion and resistance to sliding are evaluated for MC and M6C carbides as well as the martensitic matrix to TiN coating. We find that TiN coatings have high adhesion to MC carbides, while lower adhesion is present for M6C and the martensitic matrix. Close agreement with experimental measurements validates this approach and suggests new strategies for developing steels with increased coating adhesion.

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.

Milling tools are subjected to severe loading conditions causing different wear mechanisms. Among others, the dominating tool wear mechanism depends on the combination of workpiece material and tool material, cutting parameters and the mode of operation. Usually, in industrial milling processes, the mode of operation is a combination of up- and down-milling. The present work is devoted to the question how up-milling and down-milling processes differ with respect to the thermomechanical loading and the tool wear, particularly in the case of milling titanium alloy Ti-6Al-4V. To this end, cutting tests for both modes of operation have been performed. The cutting inserts have been evaluated in certain intervals via optical and electron microscopy. Finite element simulations provide the corresponding thermal and mechanical loading at the cutting edge during service. Experiment and simulation consistently show the detrimental effect of up-milling in comparison to down-milling. However, the wear analysis suggests the same mechanism for both modes of operation but with higher rate of wear in up-milling. The cutting experiments show that wear is driven by the growth of fatigue cracks and thus indicate a mechanical reason for tool wear. From the simulations, it is clear that the specific interaction between thermal and mechanical loading in up-milling is the reason for accelerated tool wear in up-milling compared to down-milling. The higher rate of wear in up-milling is attributed to its specific thermomechanical loading.

The current paper presents a new computational approach to detect wear and damage to milling tools' cutting edges. The proposed approach is independent from exact information on tool-workpiece interaction conditions and only requires that they remain constant for compared milling operations. Additionally, the approach was thoroughly tested on time-series data obtained from an industrial-scale milling process, instrumented by commercially available instrumentation equipment, during which 18 identical parts were milled. The time-series data contains the bending moments in the x and y directions as well as the torque and tension acting on the milling tool. Some measures used are systematic in nature, based on shape, rotation and work needed for milling, whereas others are statistical in nature, describing the change in the distribution of the data. All of the measures proposed in the current work are relative and mutually invariant, meaning they address different information content of the data independently. A comparison of the mentioned measures with the real-world damage evo-lution of the milling tool's cutting edges for multiple produced parts yielded consistent results and suggests a high potential for practical tool damage detection in industrial production.

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.