Institute of Materials Science

This paper describes an effective and simple procedure to derive information about the dislocation density distribution in metals by applying standard techniques of digital image processing on gray scaled microstructure images obtained from transmission-electron microscopy. In a representative transmission-electron microscopy image, two local dislocation density values were investigated by classical methods and were used as input parameter for further processing. A correlation between dislocations and image intensities is assumed such that dark areas in microstructural images are seen as a dense concentration of dislocations. Then, the contrast is increased for each transmission-electron microscopy photography. In the next step, posterization, a gradation of tone, is applied to these images. From this, a pixel weighted average distribution of gray level correlated dislocation densities is obtained as well as an average value for a given set of images. The implementation of the several processing steps is done in Matlab employing graphical user interfaces.

In the current work, the evolutions of grain and dislocation microstructures are investigated on the basis of plane strain tension and simple shear tests for an interstitial free steel (DC06) and a 6000 series aluminum alloy (AA6016-T4). Both materials are commonly-used materials in the automobile industry. The focus of this contribution is on the characterization and comparison of the microstructure formation in DC06 and AA6016-T4. Our observations shed light on the active mechanisms at the micro scale governing the macroscopic response. This knowledge is of great importance to understand the physical deformation mechanisms, allowing the control and design of new, tailor-made materials with the desired material behavior.

Zr-Nb alloys play the important role in the energy production being the main material for the cladding of nuclear fuel in the nuclear power plants. The thermo-mechanical treatment of these alloys proceeds in the (aZr) + (bZr, Nb) two-phase area of the Zr-Nb phase diagram. Therefore, the morphology and the mutual arrangement of the (Zr) and (Nb) phases play an extremely important role. The microstructure of binary Zr-Nb alloys with 2.5, 4, and 8 wt.% Nb after long anneals (720 h) was studied between 660 and 810°C in the two-phase (aZr) + (bZr, Nb) area of the Zr-Nb phase diagram. (bZr, Nb)/(bZr, Nb) grain boundaries (GBs) completely or incompletely wetted by the aZr phase were observed. The portion of the completely wetted (bZr, Nb)/(bZr, Nb) GBs increases from 10% (at 660°C) to 60% close to the upper border of the (aZr) + (bZr, Nb) two-phase area of the Zr-Nb phase diagram (850°C). The temperature of the beginning of the GB wetting phase transition of (bZr, Nb)/(bZr, Nb) GBs by the aZr phase is T ws = 630 ± 10°C. The aZr/aZr GBs completely wetted by a layer of (bZr, Nb) phase were not observed in the studied samples.

Interstitial free sheet steels show transient work hardening behavior, i.e., the Bauschinger effect and cross hardening, after changes in the loading path. This behavior affects sheet forming processes and the properties of the final part. The transient work hardening behavior is attributed to changes in the dislocation structure. In this work, the morphology of the dislocation microstructure is investigated for uniaxial and plane strain tension, monotonic and forward to reverse shear, and plane strain tension to shear. Characteristic features such as the thickness of cell walls and the shape of cells are used to distinguish microstructural patterns corresponding to different loading paths. The influence of the crystallographic texture on the dislocation structure is analyzed. Digital image processing is used to create a ''library'' of schematic representations of the dislocation microstructure. The dislocation microstructures corresponding to uniaxial tension, plane strain tension, monotonic shear, forward to reverse shear, and plane strain tension to shear can be distinguished from each other based on the thickness of cell walls and the shape of cells. A statistical analysis of the wall thickness distribution shows that the wall thickness decreases with increasing deformation and that there are differences between simple shear and uniaxial tension. A change in loading path leads to changes in the dislocation structure. The knowledge of the specific features of the dislocation structure corresponding to a loading path may be used for two purposes: (i) the analysis of the homogeneity of deformation in a test sample and (ii) the analysis of a formed part.

The wear properties of AISI 316 austenitic stainless steel implanted with Ti were investigated for ion doses in the range (2.3-5.4) x 10 i6 ions cm-2 and average ion energies of 60 and 90keV. The implanted layer was examined by Rutherford backscattering, from which the retained doses were determined, and glow discharge optical emission spectroscopy. Following implantation, the surface microhardness was observed to increase with the greatest change occurring at higher ion energy. Pin-ondisc wear tests and associated friction measurements were also performed under both dry and lubricated conditions using applied loads of 2 N and 10 N. In the absence of lubrication, breakthrough of the implanted layer occurred after a short sliding time; only for a dose of 5.1 x 1016 ions cm-2 implanted at an average energy of 90 keV was the onset of breakthrough appreciably delayed. In contrast, the results of tests with lubrication showed a more gradual variation, with the extent of wear decreasing with implant dose at both 2 N and 10 N loads. Finally, the influence of Ti implantation on possible wear mechanisms is discussed in the light of information provided by several surface characterization techniques.

This work focuses on the modeling of the evolution of anisotropy induced by the development of the dislocation microstructure. A model formulated at the engineering scale in the context of classical metal plasticity and a model formulated in the

The effect of precipitated disperse H phase particles on the thermoelastic B2-B19' martensitic transformation (MT) under compressive load has been studied on [001] , [236] , and [223] oriented single crystals of Ni 51.0 Ti 36.5 Hf 12.5 (at %) alloy in the initial (as grown) state. It is established that, in Ni 51.0 Ti 36.5 Hf 12.5 single crystals containing disperse H phase particles with dimensions within 125-150 nm at a volume fraction of ~30%, neither the critical stresses of martensite formation nor superelasticity strain depend on the orientation. Fully reversible B2-B19' MTs in Ni 51.0 Ti 36.5 Hf 12.5 single crystals have been observed in tests at external axial stresses up to 1700 MPa and temperatures up to T t ~ 373 K.

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.

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.

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 .

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.

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.

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.

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.

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.

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.

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.

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