As-cast Mg-Al-Ca alloys are among the most promising alloys for elevated temperature applications (≤200 °C) due to their superior creep properties when compared to conventional AZ or AM series Mg ...alloys. The microstructures of Mg-Al-Ca alloys consist of a soft α-Mg phase reinforced with hard interconnected Laves phases. These interconnected Laves phases are the main reason for the good creep resistance of these alloys as they impede creep deformation. The volume fraction, type and morphology of Laves phases can be controlled through the Ca/Al ratio. Consequently, the Ca/Al ratio can be used to manipulate the mechanical properties of this alloy system in order to achieve optimum creep resistance. We show here that a higher Ca/Al ratio results in i) higher volume fraction of intermetallic Laves phases in the microstructure, ii) improvement in the yield strength (YS), and iii) enhancement in creep resistance at a stress level of 50–70 MPa and a temperature of 170 °C of the as-cast alloys. Moreover, the local strain distribution and partitioning at the microstructural level occurring during high temperature tensile deformation (at ∼170 °C) was measured using quasi in-situ DIC in SEM revealing stress localisation at the α-Mg Laves phase interfaces.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The aim of this work is to determine the rolling contact fatigue (RCF) strength of steel AISI 52100 (DIN 100Cr6) as a common bearing steel in different heat treated conditions. In order to study and ...evaluate the steel cleanliness and its influence on the fatigue behavior the results are compared to AISI L3 (DIN 102Cr6) as a cold-working tool steel containing higher amounts of non-metallic inclusions as a consequence of a different metallurgical processing. In this work experimental results obtained from both materials with martensitic and bainitic microstructures are presented. In addition, the inclusion content in each material was analyzed using extreme value statistics, and the endurance limits are predicted based on the maximum expected inclusion in the maximum loaded volume as a critical crack initiating defect. The predicted RCF values are in good agreement with the experimental results. Irrespective of the microstructure, the hardness after heat treating dominates the endurable Hertzian pressure. The second important parameter, which determines the endurance limit, is the cleanliness of the steel. Furthermore, the crack pattern in the surface and microstructure of the RCF specimens were analyzed by optical microscopy. The results show no microstructural modifications such as dark etching regions or white etching bands. However, the formation of butterflies was detected around non-metallic inclusions in both steels.
•Systematical study of the classical rolling contact fatigue behavior of steel AISI 52100 and AISI L3.•Determination of the endurance limit at ultimate load cycles of NG = 108.•Investigation of the influence of steel cleanliness and heat treatment conditions on the fatigue behavior.•Prediction of the endurance limit based on the extreme value statistics of non-metallic inclusions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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In the first part of this two part study, the mechanical properties necessary for the simulation of tempering of an AISI H13 (DIN 1.2344, X40CrMoV5-1) tool steel was derived using ...physically based precipitation simulations and microstructure–property relationships. For this purpose, the precipitation of fine carbides were simulated using a thermo-kinetic software which allows prediction of the evolution of precipitation/dissolution reactions and the particle sizes. Then, those microstructural findings were coupled with physically based microstructure–property models to predict the yield stress, flow curve and creep properties. The predicted mechanical properties were verified with corresponding experiments and a good agreement was found. In the second part of this study, those properties were coupled with a Finite Element (FE) model in order to predict the relaxation of internal stresses and the evolution of deformations at the macroscopic scale.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
White etching cracks (WECs) have been studied over the last decades as a common cause for premature failure of roller bearings in various applications. As a consequence of the non-directional and ...branched crack networks associated with an altered microstructure named as white etching area (WEA) the damage pattern appears as cracking or flaking of the bearing components. In most published literature dealing with WEA/WEC, it is assumed that WEA/WEC formation should only occur under the condition of an additional load superposed to the bearing operation. The formation mechanisms of this damage are still under debate, though. In this work, the influence of diffusible hydrogen on the formation of WEA/WEC has been investigated systematically using hydrogen pre-charged rollers made from the bearing steel SAE 52100 (DIN 100Cr6) prior to testing. These rollers were tested on a four-wheel test rig using different hydrogen concentration, Hertzian pressure and number of load cycles. As a result of testing, thresholds for WEA/WEC formation were found for different hydrogen concentrations, Hertzian pressures and running time of the rollers. Further investigation with manually interrupted tests revealed the formation of the damage being beneath the raceway surface of the rollers. Detailed microstructure studies using comprehensive analysis in the scanning electron microscope including EDX and EBSD as well as plasma focused ion beam (PFIB) have been conducted to give detailed information of the altered microstructure.
•Experimental study of the premature failure in SAE 52100 bearing using a rolling contact fatigue test rig.•Systematical investigation of the influence of diffusible hydrogen on White Etching Area and White Etching Crack formation.•Thresholds for WEA/WEC formation were found for different hydrogen concentrations, Hertzian pressures and number of cycles.•Detailed information about the microstructural change under rolling contact fatigue on manually stopped tests.•Combined SEM/3D PFIB analysis on manually stopped test showed microstructural altered regions without any link to a crack.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In future fusion reactors, tungsten is a main candidate material for plasma-facing components. However, the intrinsic brittleness of tungsten is an issue under the extreme fusion environment. To ...overcome this drawback, tungsten fiber-reinforced tungsten (Wf/W) composites are being developed relying on an extrinsic toughening principle. In this study Wf/W composites are produced by a Field-Assisted Sintering Technology (FAST) process with different fiber–matrix interfaces. The fracture behavior was studied by 3-point bending tests on notched samples. 4-point bending tests and tensile tests are performed to measure the flexural strength and tensile strength, respectively. Wf/W with a weak interface shows a typical pseudo-ductile fracture behavior, similar to ceramic matrix composites. A strong interface is beneficial to achieve higher flexural strength and tensile strength, but in turn, weakens the pseudo-ductile behavior.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Under high temperature and pressure during Hot Isostatic Pressing (HIP), various deformation and sinter mechanisms such as plastic yielding, power law creep, and diffusion occur. They contribute ...either simultaneously or consecutively to the densification process depending on temperature and pressure levels. Some published works have shown that neither the pure plastic model nor the pure viscoplastic model sufficiently models the densification process during HIP, therefore a combined model which includes both a time independent plasticity and a rate dependent plasticity (creep) should be used. An innovative aspect of this work in comparison with available models in literature is the accommodation for model inaccuracies in the initial creep stage. The densification model incorporates the initial creep mechanism which occurs for a short time with a higher rate as compared to the steady stage creep rate. In this study, a combined model which consists of the influence of plasticity, primary creep, and secondary creep mechanisms is used. The authors believe that this approach can improve the quality of “near-net-shape” simulation.
Innovative Aspects: The proposed model is more robust and predicts the final shape of HIP-ed components better which will support the production of Selective Net Shape or Net Shape HIP components. The reported results are of importance for companies producing big and complex shaped components through powder-HIP.
•A combined model which consists of the influence of plasticity, primary creep, and secondary creep mechanisms was developed.•The numerical model has been implemented as user defined subroutine (UMAT) in the FEM program Abaqus.•Results from this study show that the proposed model is more robust and predicts the final shape of HIP-ed components better.•This simulation model will support the production of Selective Net Shape or Net Shape HIP components.•The reported results are of particular importance for companies producing big and complex shaped components through powder-HIP.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
In P/M HIP, the capsule volume is reduced to 25–30% depending on the initial relative density of the powder after pre-consolidation. Under isostatic pressing conditions the capsule should have a ...uniform shrinkage, but in practice this is often not the case. This phenomenon is driven through many factors including, but not limited to, the temperature gradient, inhomogeneity of powder inside the capsule and capsule thickness of which the influence will be further examined in this study. Experimental Image Analysis is used to investigate the initial powder distribution which is subsequently implemented in a FEM model as initial condition to simulate the shape changes of hot isostatically pressed products. To analyze the effect of a temperature gradient in the capsule, the temperature is assumed to be inhomogeneously distributed on the surfaces of the capsules. Different temperature fields are then applied to a FEM model. Additionally, capsule thickness will be varied to study its influence on the final shape. Finally the simulation results are compared with experimental findings. Based on that the conclusion about the influence of inhomogeneous powder distributions, temperature gradients and capsule thickness on the final shape of powder HIP-ed products can be given.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The creep behavior of the Ni-base Alloy 617B was analyzed at 725°C with the focus on microstructural changes during temperature and stress exposure. High resolution electron microscopy of crept ...specimens reveals the precipitation behavior of secondary phases such as Cr-rich M23C6-carbides and the γ'-phase. Physical models are used to describe the Ostwald coarsening of the γ' particles and the evolution of the yield strength of the alloy. Together with the results from hot tensile tests and hardness measurements, a constitutive model for Alloy 617B has been developed. This model takes account of precipitation strengthening which is consistent with the microstructural observations, internal back stress due to dislocation hardening and material damage, all by evolutionary equations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Tungsten carbide hardmetals are widely used in cutting, tooling, and wear based applications. During the cooling stage of liquid phase sintering, severe internal micro stresses form between carbide ...and binder. A detailed microstructural FEM model, capable of predicting these residual stresses accurately, has been developed on the basis of EBSD images of a WC6wt.-%Co hardmetal. This approach yields an extruded 2.5D model that is generated using 2D geometry information on the grains of each phase and the corresponding 3D grain-orientation data. The orientations of the carbides and the binder were detected using samples prepared by ion beam etching. The simulation approach presented, utilizes a temperature-dependent, isotropic elasto-viscoplastic constitutive description of the binder and a temperature-dependent, orthotropic elastic description of the carbides. Using these constitutive models, the type-II residual-stress response is calculated for the hardmetal microstructure. The temperature-dependent phase-averaged hydrostatic stresses, which are predicted by the model, are in good agreement with experimental neutron-diffraction data.
•Temperature dependent simulation of residual stresses in cemented carbides.•Implementation of visco-plastic binder behavior.•Semi-automatic model generation based on EBSD-Data.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
A common challenge in multiple fields of the mechanical driveline technology is the premature failure of rolling bearings due to white etching cracks (WEC). This failure mode is characterized by ...three-dimensional cracks, bordered by regions of altered material microstructure, which eventually lead to cracking or spalling of the raceway. Ultrasonic analysis of cylindrical roller thrust bearings have already shown that WEC are mainly located in the region under negative slip, which agrees with a cumulative frictional energy criteria. In this study, WEC were reproduced on a two-disc test rig using inner rings from radial cylinder roller bearings without pre-charging the samples with hydrogen. The investigations revealed an influence of the sliding direction and the lubrication regime on the WEC formation. The tests showed that a WEC failure induced by sliding occurs under boundary lubrication regime. Furthermore, it was determined that while single axial cracks and small WEC networks formed in the sample under positive slip; large WEC networks formed in the sample under negative slip.
•Laboratory WEC tests successfully carried out on thrust bearing and two-disc test rigs.•Assessment of the influence of sliding direction and lubricating conditions on the formation of white etching crack.•Assessment of the influence of negative and positive slip on the type and extent of damage.•Metallographic characterization of the failed components.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
