Papers by Gregory Gerstein
Materials Science Forum, 2021
Magnesium alloys are important engineering materials due to their good combination of strength an... more Magnesium alloys are important engineering materials due to their good combination of strength and very low densities. However, the low ductility imposed by the hcp-lattice has thus far limited the application of magnesium alloys as sheet material. The use of the electroplastic effect offers a route to increase formability of magnesium alloys while being more energy efficient than conventional hot forming. The underlying mechanism (s) of this effect have not yet been fully understood. This study investigates the impact of high current density electrical pulses on magnesium alloys. Special consideration was given to the effect of the orientation of the applied electric current relative to the mechanical loading of the specimens. The results show that the mechanical properties of coarse-grained materials are more strongly affected by the current pulses than finer grained material. Applying the current parallel to the compressive load shows a more pronounced softening of the material t...
Physica Scripta, 2021
Electrical transport properties were investigated under hydrostatic pressure in the Ni45.4Mn40In1... more Electrical transport properties were investigated under hydrostatic pressure in the Ni45.4Mn40In14.6 alloy. Temperature dependency of electrical resistivity was analysed to find dominant scattering mechanisms and to estimate the volume change during martensitic transformation. The main contributions into electrical resistivity in austenitic phase were found to be scattering by magnetic fluctuations and scattering due to structural disorder. The latter provides significantly higher residual resistivity in comparison with that of the stoichiometric alloy due to irregular positions occupied by excess Mn atoms. The value of volume change upon austenite to martensite transformation was −0.09%. It was found that alloy is characterised by hole conductivity and negative anomalous Hall resistivity, which doubles on application of hydrostatic pressure due to pressure-induced martensitic transformation and reaches the magnitude of 20 μΩ·cm. The unusual correlation between the Hall and the longitudinal resistivities in the martensitic phase is discussed.
HTM Journal of Heat Treatment and Materials, 2020
In this work, the influence of a cryogenic treatment on the microstructure, mechanical properties... more In this work, the influence of a cryogenic treatment on the microstructure, mechanical properties and wear resistance of the high-alloyed tool steels X38CrMoV5-3, X153CrMoV12 and ~X190CrVMo20-4 were investigated. Based on tempering curves of the steels, the heat treatment parameters were determined for the mechanical and wear specimens so that the conventionally heat-treated steels and the cryogenically treated steels featured similar hardness. The investigations showed that an almost complete transformation of retained austenite and a more homogeneous distribution of secondary carbides in the microstructure could be achieved by incorporating a cryogenic treatment. However, the cryogenic treatment does not show significantly positive effects on the investigated mechanical properties and wear resistance of the tool steels. The wear resistance of the samples was dominated by primary carbides. The cryogenic treatment would have a positive effect on large tool components with large wall...
Metals, 2020
A new sheet-bulk metal forming process for the production of bulk components out of a flat sheet ... more A new sheet-bulk metal forming process for the production of bulk components out of a flat sheet has been developed. Superimposed oscillation has been applied to the new process. By this means, process limits regarding better mould filling were expanded, and forming forces could be reduced. In order to investigate the effects of superimposed oscillation on material behaviour, plane strain and ring compression tests were carried out. The superimposed oscillated plane strain compression test showed a reduction in biaxial flow stress and thus in plastic work. Furthermore, reduced friction and roughness were verified in ring compression tests using superimposed oscillation.
IEEE Transactions on Magnetics, 2020
The existence of two Curie temperatures in the temperature interval of the martensitic phase of a... more The existence of two Curie temperatures in the temperature interval of the martensitic phase of a Ni51.9Mn27.0Ga21.1 alloy has been verified. This phenomenon was confirmed in both magnetic and electrical impedance measurements. Additionally, modified Arrott plot calculations were employed to determine order and temperatures of magnetic transitions occurring in a Ni51.9Mn27.0Ga21.1 alloy. The results of the calculations are in line with the ones obtained in the experiments. They indicate that the second-order transitions on cooling and heating in this alloy occur at different temperatures with thermal hysteresis of ~7 K. The measurements of the dependence of AC impedance on temperature allowed to distinguish these magnetic transitions from the nearby martensitic one.
Materials Science and Engineering: A, 2020
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Journal of Materials Engineering and Performance, 2019
A combined experimental and simulation investigation is carried out to characterize the high temp... more A combined experimental and simulation investigation is carried out to characterize the high temperature deformation behavior of a recently developed Al-Li alloy, AA2070, over a temperature range including typical forging temperatures. Focus is placed on providing explanations for the temperature-dependent plastic and fracture behavior observed for this material using macroscale tensile tests, microscale imaging and analysis, and physics-based micromechanical modeling. It is shown that AA2070 experiences nonmonotonic elongation to failure with a rise in temperature. Detailed fractography and microstructural analysis, including dynamic recrystallization analysis, provide insight into this interesting and practically useful behavior. In addition, a validated crystal plasticity model is called upon to explain the unique texture observed in the necked region of the ductile tensile specimen. The insights provided by this investigation will allow improved design of high temperature forming operations for AA2070 and similar alloys which can extend the application of third generation Al-Li alloys.
Metallurgical and Materials Transactions A, 2018
The alloying system CoAl -W-Ta is comprehensively investigated in the vicinity of the composition... more The alloying system CoAl -W-Ta is comprehensively investigated in the vicinity of the compositional point Co-9Al-10W-2Ta, at. pct. These investigations provided a large amount of quantitative information, which can be used for alloy development, namely, the compositional dependences of the c¢-solvus, solidus, and liquidus temperatures; fraction of the extrinsic phases after casting; the compositional dependence of the c/c¢-lattice misfit; the element partitioning between c-and c¢-phases; and the two phase compositional area c/c¢ in the Co-rich part of the CoAl -W-Ta phase diagram at 900°C. It is shown that additions of Ta elevate the c¢-solvus temperature and increase the c/c¢-lattice misfit, but adding more than about 3 at. pct Ta results in a large amount of undissolvable extrinsic phases. Additionally, two CoAl -W-Ta alloys with lower content of W were developed and solidified as [001] single crystals for mechanical testing in a temperature range between 20 and 1200°C. These tests included measurement of the Young modulus, tensile tests with constant strain rate, and stress rupture tests. It was found that at temperatures up to about 750°C the ultimate tensile strength of CoAl -Ta-W alloys can be at the same level or even higher than of Ni-based superalloys.
Journal of Magnetism and Magnetic Materials, 2018
The Displaced Aperture Method (DAM) of Transmission Electron Microscopy (TEM) is used to visualiz... more The Displaced Aperture Method (DAM) of Transmission Electron Microscopy (TEM) is used to visualize internal structure of magnetic domains in the Co 49 -Ni 21 -Ga 30 Ferromagnetic Shape Memory Alloy (FSMA). The preparation of the FSMA specimens and the details of DAM enabling visualization of these domains and their internal details are described. The magnetic domains of ca. 100-nm width with the domain walls of ca. 20 nm width are observed. The theoretical estimation supporting the possibility of appearance of such domains in the thin foil prepared for TEM observations is presented. The superfine crystallographic microstructure possessing the features, which are inherent to the quasiperiodic twinning of crystal lattice with period of ca. 6 nm, is observed inside the magnetic domains. The obtained TEM data may be useful for examination of the influence of external magnetic field on the magnetic and crystallographic microstructure of FSMAs.
Journal of the Mechanics and Physics of Solids, 2019
Material models for the thermoplastic material behaviour of a dual-phase steel on a microscopic a... more Material models for the thermoplastic material behaviour of a dual-phase steel on a microscopic and a macroscopic length scale,
Procedia Manufacturing, 2018
Under the concept of "Industry 4.0", production processes will be pushed to be increasingly inter... more Under the concept of "Industry 4.0", production processes will be pushed to be increasingly interconnected, information based on a real time basis and, necessarily, much more efficient. In this context, capacity optimization goes beyond the traditional aim of capacity maximization, contributing also for organization's profitability and value. Indeed, lean management and continuous improvement approaches suggest capacity optimization instead of maximization. The study of capacity optimization and costing models is an important research topic that deserves contributions from both the practical and theoretical perspectives. This paper presents and discusses a mathematical model for capacity management based on different costing models (ABC and TDABC). A generic model has been developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization's value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity optimization might hide operational inefficiency.
International Journal of Hydrogen Energy, 2018
The effect of the bimodal grain size distribution on the hydrogen susceptibility of a high-Mn ful... more The effect of the bimodal grain size distribution on the hydrogen susceptibility of a high-Mn fully austenitic twinning-induced plasticity (TWIP) steel was investigated by tensile testing under ongoing electrochemical hydrogen charging. Observation of the surface microstructure of the hydrogen-charged specimen yielded a correlation between the microstructure, crack initiation sites, and crack propagation path. The observed embrittlement arose from crack initiation/propagation along the grain and twin boundaries and delamination governed crack growth. In the present bimodal TWIP steel, the fine grained regions mostly showed intergranular cracking along the grain boundaries between the fine and coarse grains. By contrast, the coarse grained region exhibited transgranular cracking along the twin boundaries. The delamination cracking phenomena is rationalized by the evident nucleation, growth, and coalescence of microvoids in the tensile direction. The results reveal that the bimodal grain size distribution of TWIP steel plays a major role in hydrogen-assisted cracking and the evolution of delamination-related damage.
Practical Metallography, 2016
Ion beam polishing and ion beam etching have proven their worth in examinations for which mechani... more Ion beam polishing and ion beam etching have proven their worth in examinations for which mechanical and wet chemical methods can not be applied. The analysis of voids in steel microstructures requires a sample preparation which provides for a surface ablation without mechanical impact on the sample surface. Otherwise, smearing effects prevent the revealing of voids. These voids arise during cold forming. They provide information on the ductile damage which is characterized by the formation, growth, and the merging of cavities in the microstructure. Taking the example of deep drawing steels, modified preparation methods for the imaging of microvoids and nanovoids by means of electron microscopy are presented. The specific resulting surface topography, which is influenced by the ion beam's angle of incidence, the accelerating voltage, and the processing time, is examined in a parameter study. It will be analyzed to what extent the relief structures created by ion beam processing ...
Technical Physics Letters, 2017
The high-temperature superelasticity and temperature dependence of the yield stress of 14M and L1... more The high-temperature superelasticity and temperature dependence of the yield stress of 14M and L1 0 martensite in [001]-oriented single crystals of Ni 54 Fe 19 Ga 27 (at %) in compression alloy have been studied. As the temperature increases, the sequence of stress-induced martensitic transformations (MTs) changes from L2 1-14M to L2 1-L1 0. The yield stress of L1 0 martensite weakly depends on the temperature and is 1.7 times lower than that of 14M martensite. The temperature interval of superelasticity in [001]-oriented single crystals of Ni 54 Fe 19 Ga 27 under compression is determined by the growth of critical stresses with increasing temperature, the coefficient of strain-hardening, and the yield stress of L1 0 martensite.
Journal of Materials Science, 2017
Localized oxidation and corrosion behavior of a nickel-titanium (NiTi) shape memory alloy (SMA) w... more Localized oxidation and corrosion behavior of a nickel-titanium (NiTi) shape memory alloy (SMA) was investigated via static immersion experiments in a simulated body fluid solution. Detailed electron microscopy examinations on the sample surfaces revealed preferential formation of local oxide particles around dislocation networks, which constitute high-energy zones. Moreover, various intermediate phases were detected in addition to the parent NiTi phase around dislocation networks. These are also areas with enhanced diffusion, which promotes Ni release. These findings emphasize the significant role of fine microstructural features, such as dislocation networks, on the oxidation and Ni release, and thus, the biocompatibility of the NiTi SMAs.
Materials Letters, 2018
Stress-strain loops illustrating the superelastic behaviour of shape memory alloys (SMAs) were co... more Stress-strain loops illustrating the superelastic behaviour of shape memory alloys (SMAs) were computed based on the theory of ferroelastic phase transitions. The predictions of the theory demonstrate the possibility of drastic changes in the stress-strain dependences due to the expansion of the SMA upon heating. Specifically, the computations were carried out taking into account the characteristics of Co-Ni-Ga alloys, which exhibit a high-temperature superelasticity. It is shown that the expansion of crystal lattice, which can be caused by the appearance of small particles and crystal defects, or change of chemical order in SMA, can induce (i) an extension of the temperature range of superelastic behaviour of SMA to high temperatures; (ii) an increase of the superelastic strain at elevated temperatures; (iii) an increase of the stress needed to reach the superelastic strain plateau and (iv) a widening of the hysteresis of stressinduced martensitic transformation. Theoretical results are in a qualitative agreement with experimental data obtained for Co-Ni-Ga alloys. Ó 2017 Elsevier B.V. All rights reserved. * Value reported for Co 49 Ni 21 Ga 30 in Ref. [10]. ** This value is close to the superelastic strain of-0.045 measured in Ref. [2] at the austenite finish temperature. *** This follows from the minimum conditions for the Gibbs potential. **** These values must be of the order of u À1 3 ðT MF Þ [8]. The values given provide for a widening of the hysteresis upon heating from 400 K to 500 K; moreover, they result in the reasonable value of the volume change during MT of DV/V = 1.7 Â 10-3 .
Procedia Structural Integrity, 2016
During their operation, modern aircraft engine components are subjected to increasingly demanding... more During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
Microscopy Research and Technique, 2016
To investigate ductile damage in parts made by cold sheet-bulk metal forming a suited specimen pr... more To investigate ductile damage in parts made by cold sheet-bulk metal forming a suited specimen preparation is required to observe the microstructure and defects such as voids by electron microscopy. By means of ion beam slope cutting both a targeted material removal can be applied and mechanical or thermal influences during preparation avoided. In combination with scanning electron microscopy this method allows to examine voids in the submicron range and thus to analyze early stages of ductile damage. In addition, a relief structure is formed by the selectivity of the ion bombardment, which depends on grain orientation and microstructural defects. The formation of these relief structures is studied using scanning electron microscopy and electron backscatter diffraction and the use of this side effect to interpret the microstructural mechanisms of voids formation by plastic deformation is discussed. A comprehensive investigation of the suitability of ion beam milling to analyze ductile damage is given at the examples of a ferritic deep drawing steel and a dual phase steel.
steel research international, 2016
This paper presents investigations on the characterization of ductile damage and identification o... more This paper presents investigations on the characterization of ductile damage and identification of the porosity-related material model parameters in a dual-phase steel DP600. As a modeling reference for the damage evolution, a variant from the Gurson model family is taken. The micromechanical investigations related to nucleation and growth of voids have been carried out. In order to show the void-volume evolution during the deformation, post-mortem scanning electron microscope (SEM) analysis of a notched tensile test is used. Using the ion beam slope cutting methodology to prepare the specimens for SEM analysis, the microstructure can be observed in 2D including the voids. In this way, for the dual-phase steel, characteristic damage behavior upon deformation due to interaction of martensite and ferrite can be investigated. The minimum void size (areal) that can be measured is 0.05 µm2. This resolution of the measurements provides the detection of the newly nucleated voids. For the related material parameters, void-size relevant criterion is applied to determine the newly nucleated voids at a certain plastic strain.
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Papers by Gregory Gerstein