This paper systematically investigated the influence of microstructural characteristics such as grain size and morphology on the yielding behavior of a cold rolled medium-Mn Fe-6.4Mn-0.1C (wt%) ...steel. By intercritical annealing of heavily cold-worked and fully martensitic initial state, ultra-fine grained (UFG) microstructures with different morphologies could be obtained, namely globular in the former case and predominantly lath-like in the latter case. The influence of these initial microstructures and intercritical annealing temperature (TIA) on the final microstructure and resulting mechanical properties was presented in detail. Medium-Mn steels commonly exhibit large yield point elongations (YPE), easily exceeding 10%. Both low TIA and a globular microstructure remarkably supported the formation of large YPE. These YPE formed by localized deformation, which was analyzed by infrared (IR) thermography. Using interrupted tensile testing a vivid linear correlation between decreasing retained austenite (RA) stability and decreasing YPE could be manifested, while this dependency proved to be valid for several medium-Mn steel compositions. Besides the effect of the RA stability on YPE, STEM investigations on deformed tensile samples revealed an entirely different dislocation structure between the UFG globular and lath-like microstructure, suggesting different active deformation mechanisms depending on the overall grain size and morphology. Based on this investigation, it was recommended to provide a martensitic microstructure prior to intercritical annealing in order to limit YPE. Furthermore, special attention has to be paid to a careful design of the RA stability in order to adjust an appropriate balance between excellent mechanical properties and reduced YPE.
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In the field of power engineering, where materials are subjected to high pressures at elevated temperatures for many decades, creep-resistant steels are put to work. Their service life is still, ...however, finite, as the many changes in their microstructure can merely be mitigated and not avoided. Creep cavitation is one of those changes and, in many cases, ultimately causes failure by rupture. In this work, a model is proposed to simulate the nucleation and growth of cavities during creep. This exclusively physics-based model uses modified forms of Classical Nucleation Theory and the Onsager Extremum Principle in a newly developed Kampmann–Wagner framework. The model is validated on P23 steel which underwent creep rupture experiments at 600 °C and stresses of 50, 70, 80, 90 and 100 MPa for creep times up to 46000 hours. The model predicts qualitatively the shape and prevalence of cavities at different sites in the microstructure, and quantitatively the number density, size of cavities and their phase fraction contributing to a reduction in density. Finally, we find good agreement between the simulation and the experimental results especially at low stresses and longer creep times.
Hydrogen embrittlement (HE) of advanced high-strength steels is a crucial problem in the automotive industry, which may cause time-delayed failure of car body components. Practical approaches for ...evaluating the HE risk are often partially and contradictive in nature, because of hydrogen desorption during testing and inhomogenous hydrogen distributions in, e.g., notched samples. Therefore, the present work aims to provide fully parametrized and validated bulk diffusion models for three dual phase steels to simulate long-range chemical diffusion, trapping and hydrogen desorption from the surface. With one constant set of parameters, the models are able to predict the temperature dependency of measured Choo-Lee plots as well as the concentration dependency of measured effective diffusion coefficients. Finally, the parametrized and validated bulk diffusion models are applied for studying the role of the current density on the permeation time and the role of coatings as effective diffusion barriers.
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The number of fossil fueled power plants in electricity generation is still rising, making improvements to their efficiency essential. The development of new materials to withstand the higher service ...temperatures and pressures of newer, more efficient power plants is greatly aided by physics-based models, which can simulate the microstructural processes leading to their eventual failure. In this work, such a model is developed from classical nucleation theory and diffusion driven growth from vacancy condensation. This model predicts the shape and distribution of cavities which nucleate almost exclusively at grain boundaries during high temperature creep. Cavity radii, number density and phase fraction are validated quantitively against specimens of nickel-based alloys (617 and 625) tested at 700 °C and stresses between 160 and 185 MPa. The model's results agree well with the experimental results. However, they fail to represent the complex interlinking of cavities which occurs in tertiary creep.
The exact adjustment of the individual microstructural constituents, in particular the optimum volume fraction of metastable retained austenite (RA), is imperative in case of the 3rd generation ...advanced high strength steels (AHSS). Moreover, it is of vital importance to avoid the formation of fresh martensite (α’) upon final cooling, otherwise the resulting mechanical properties of these novel steel grades will be pronouncedly deteriorated. Therefore, an accurate estimation of the martensite start (Ms) temperature is essential for modelling and development of this steel group. In this context, the present contribution proposes a new relation for the Ms temperature prediction with a remarkably wide composition range based on the 3rd generation AHSS chemistries. Moreover, the new Ms formula allows for an accurate determination of the Ms temperature for both, extremely low and very high C contents, which is not the case in any Ms formula known from literature.
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The increasing use of light alloys owing to their high performance makes magnesium alloys very attractive for the use in automotive and biomedical applications. However, it is well known that ...magnesium and its alloys have poor corrosion resistance in different atmospheric and aqueous environments. As a means of improving corrosion resistance through the microstructure modification, electron beam processing (EBP) was applied on the as-cast AZ91 magnesium alloy. To evaluate the microstructure influence on the corrosion-resistant, the EB processed samples underwent a solution heat treatment and an artificial aging heat treatment. Four different obtained microstructures were investigated by standard microscopy and electrochemical corrosion tests to evaluate the microstructure and its effects on the corrosion resistance of AZ91 alloy. The EBPed specimens show a significant microstructure refinement and homogenous distribution of β-phase at the grain boundaries surrounded by supersaturated α-Mg which acts as a barrier against corrosion. The electrochemical corrosion test of the samples immersed in 3.5 wt% NaCl after 4 weeks indicates that the EBP improves the corrosion resistance of the alloy due to the nobler corrosion potential of supersaturated α-Mg and more stable protective hydroxide films compared to the heat-treated and as-cast conditions.
Together with the mechanical properties, the degradation rate is an important factor for biodegradable implants. The ZKX50 Mg alloy is a suitable candidate to be used as a biodegradable implant due ...to its favorable biocompatibility and mechanical properties. Current research investigates the degradation rate and corrosion behavior of the ZKX50 as a function of the microstructure constituents and their morphology. Since grain refinement is the main strengthening mechanism for the ZKX50, the effect of the microstructure refinement on the corrosion rate was studied by applying electron beam processing (EBP) and friction stir processing (FSP) on the ZKX50 cast alloy. To study the effect of the microstructure constituents and their morphology a subsequent solution heat treatment (HT) was applied to the processed samples. The results show that the EBP and FSP lead to a uniform and remarkably refined microstructure of the ZKX50 alloy and homogeneous distribution of the intermetallic phases. The results of electrochemical corrosion tests together with the microstructure characterization show that microgalvanic corrosion is the predominant mechanism that occurs between the Ca2Mg6Zn3 intermetallic phase and α-Mg matrix. According to the results attained through the electrochemical tests, the EBPed-HT ZKX50 alloy shows higher corrosion resistance compared to all other conditions immersed in 0.5 wt.% NaCl solution. The dissolution and spheroidizing of Ca2Mg6Zn3 particles during the solution heat treatment provides higher corrosion resistance mainly by decreasing the microgalvanic corrosion. The microstructure of the heat-treated samples does not show a significant grain coarsening which can degrade the enhancement of the mechanical properties achieved by the EBP and FSP.
The quality and characteristics of a powder in powder bed fusion processes play a vital role in the quality of additively manufactured components. Its characteristics may influence the process in ...various ways. This paper presents an investigation highlighting the influence of powder deterioration on the stability of a molten pool in a laser beam powder bed fusion (LB-PBF, selective laser melting) process and its consequences to the physical properties of the alloy, porosity of 3D-printed components and their mechanical properties. The intention in this was to understand powder reuse as a factor playing a role in the formation of porosity in 3D-printed components. Ti6Al4V (15 μm-45 μm) was used as a base material in the form of a fresh powder and a degraded one (reused 12 times). Alloy degradation is described by possible changes in the shape of particles, particle size distribution, chemical composition, surface tension, density and viscosity of the melt. An approach of 3D printing singular lines was applied in order to study the behavior of a molten pool at varying powder bed depths. Single-track cross-sections (STCSs) were described with shape parameters and compared. Furthermore, the influence of the molten pool stability on the final density and mechanical properties of a material was discussed. Electromagnetic levitation (EML) was used to measure surface tension and the density of the melt using pieces of printed samples. It was found that the powder degradation influences the mechanical properties of a printed material by destabilizing the pool of molten metal during printing operation by facilitating the axial flow on the melt along the melt track axis. Additionally, the observed axial flow was found to facilitate a localized lack of fusion between concurrent layers. It was also found that the surface tension and density of the melt are only impacted marginally or not at all by increased oxygen content, yet a difference in the temperature dependence of the surface tension was observed.
The current work presents the results of an investigation focused on the influence of process parameters on the melt-track stability and its consequence to the sample density printed out of NdFeB ...powder. Commercially available powder of Nd7.5Pr0.7Fe75.4Co2.5B8.8Zr2.6Ti2.5 alloy was investigated at the angle of application in selective laser melting of permanent magnets. Using single track printing the stability of the melt pool was investigated under changing process parameters. The influence of changing laser power, scanning speed, and powder layer thickness on density, porosity structure, microstructure, phase composition, and magnetic properties were investigated. The results showed that energy density coupled with powder layer thickness plays a crucial role in melt-track stability. It was possible to manufacture magnets of both high relative density and high magnetic properties. Magnetization tests showed a significant correlation between the shape of the demagnetization curve and the layer height. While small layer heights are beneficial for sufficient magnetic properties, the remaining main parameters tend to affect the magnetic properties less. A quasi-linear correlation between the layer height and the magnetic properties remanence (J
), coercivity (H
) and maximum energy product ((BH)
) was found.
Hybrid components made of aluminum alloys and high-strength steels are typically used in automotive lightweight applications. Dissimilar joining of these materials is quite challenging; however, it ...is mandatory in order to produce multimaterial car body structures. Since especially welding of tailored blanks is of utmost interest, single-sided Cold Metal Transfer butt welding of thin sheets of aluminum alloy EN AW 6014 T4 and galvanized dual-phase steel HCT 450 X + ZE 75/75 was experimentally investigated in this study. The influence of different filler alloy compositions and welding process parameters on the thickness of the intermetallic layer, which forms between the weld seam and the steel sheet, was studied. The microstructures of the weld seam and of the intermetallic layer were characterized using conventional optical light microscopy and scanning electron microscopy. The results reveal that increasing the heat input and decreasing the cooling intensity tend to increase the layer thickness. The silicon content of the filler alloy has the strongest influence on the thickness of the intermetallic layer, whereas the magnesium and scandium contents of the filler alloy influence the cracking tendency. The layer thickness is not uniform and shows spatial variations along the bonding interface. The thinnest intermetallic layer (mean thickness < 4
µ
m) is obtained using the silicon-rich filler Al-3Si-1Mn, but the layer is more than twice as thick when different low-silicon fillers are used.
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