Local Deformation Behavior Research Articles

Local deformation and transformation behavior of retained austenite in 18CrNiMo7-6 after high-carbon carburizing treatment

The objective of this investigation is to investigate the retained austenite deformation and transformation behavior during nanoindentation of 18CrNiMo7-6 steel after a high-carbon case-hardening process. We conducted a high-carbon carburizing treatment that led to approximately 1.2 mass% carbon at the surface. Further heat treatment consisted of austenitization, quenching in oil, and low-temperature annealing. To transform large amounts of retained austenite into martensite and to stabilize the existent retained austenite, some specimens were cryogenically treated in liquid nitrogen. The resulting microstructures were analyzed as a function of carbon content in the gradient by means of scanning electron microscopy with energy dispersive X-ray spectroscopy and electron backscatter diffraction analysis. Nanoindentation was used to study locally the mechanical behavior and stability of retained austenite at the carburized surface. To better analyze the deformation behavior during indentation, atomic force microscopy was applied to measure the topography of individual indentation imprints and the deformation pattern around the indents. The results show that nanoindentation is suitable for mechanically induced phase transformation of retained austenite into martensite, which leads to pronounced pop-ins in the load-displacement curves at specific critical loads. The occurrence of pop-ins, and, thus the phase transformation is highly dependent on the local microstructure of the carburized specimens. The main influencing factors are the local amount of retained austenite in the deformed region and, predominantly, the crystallographic orientation of retained austenite. (001)-Oriented retained austenite is highly metastable, whereas (111) orientations show mostly stable austenitic deformation behavior with only isolated mechanically induced retained austenite transformation during indentation. Cryogenic treatment transformed larger amounts of retained austenite into martensite. However, even after cryogenic treatment, (111)-oriented retained austenite could undergo mechanically induced transformation into martensite. Furthermore, we used the progression of hardness as a function of indentation depth and topography images to analyze the extent of retained austenite transformation and the complex compound deformation/transformation behavior of preexisting martensite and retained austenite.

Experimental methodology on the serviceability behaviour of reinforced ultra‐high performance fibre reinforced concrete tensile elements

AbstractDesign codes include serviceability limit state (SLS) provisions for stress, crack, and deflection control in concrete structures, which may limit the structural design. When drawing on reinforced ultra‐high performance fibre‐reinforced concrete (R‐UHPFRC), the process of cracking differs significantly from traditional concretes. Thus, it remains unclear whether the traditional provisions are applicable to R‐UHPFRC or should be reviewed. Uniaxial tensile tie test is an excellent option to analyse and review these criteria. This work proposes a novel test methodology to study the behaviour of R‐UHPFRC under serviceability conditions, which lets the study of the global and local deformation behaviour by using different measurement equipment. Two different types of R‐UHPFRC ties with variant fibre content were tested. The global average tensile stress–strain curve, cracking behaviour, number, and width of cracks were obtained. Promising preliminary results admitted that this methodology can be useful to propose design criteria of R‐UHPFRC under SLS.

Investigating the local deformation and transformation behavior of sintered X3CrMnNi16-7-6 TRIP steel using a calibrated crystal plasticity-based numerical simulation model

Abstract In this study, DAMASK was used to model and elucidate the microstructural deformation behavior of sintered X3CrMnNi16-7-6 TRIP steel. The recently developed TRIP-TWIP material model was used within the DAMASK framework. Material optimization was performed using the least computationally expensive method, which yielded the desired results. The physical parameters of the material model were identified and tuned to fit the experimental observations. This tuned material model was used to run simulations utilizing 2D EBSD data. The local deformation, transformation, and twinning behaviors of the material under quasi-static tensile and compressive loads were analyzed. The results of this are in good agreement with previous experimental observations. The phenomena of dislocation glide, twinning, martensitic transformation, stress evolution, and dislocation pinning in different deformation stages are discussed.

Assessment of Small-Scale Deformation Characteristics and Stress-Strain Behavior of NiTi Based Shape Memory Alloy Using Nanoindentation

The study is focused on appreciating the micro-mechanisms that govern the deformation behavior of a Ni-rich NiTi alloy at the sub-micron scale. This alloy undergoes reversible stress-induced-martensitic-transformation leading to pseudoelasticity. Correspondingly, significant strain recoverability is noted. In this investigation, nanoindentation is utilized as the primary experimental tool. Various parameters including the indenter tip configuration, size and applied load levels are varied systematically. It is observed that compared to the most commonly used sharp Berkovich indenter tip, blunt spherical tip imposes reduced amount of strain and strain-gradient within the indentation volume. This opens up the window for systematically varying the sequential deformation modes in NiTi. Further in-depth analysis revealed that optimum combination of indenter tip configuration, size and applied load level is prerequisite to appreciate the pseudoelastic mechanism in NiTi. Most importantly, a tailored and simplified protocol is formulated for converting nanoindentation load-displacement response to corresponding indentation-stress-strain behavior for pseudoelastic alloy. These indentation-stress-strain curves featuring the signature deformation characteristics of NiTi alloy are realized without utilizing any specialized experimental modes. In a nutshell, this study highlights the importance of choosing adequate nanoindentation parameters for assessing the functional characteristics of pseudoelastic NiTi system at sub-micron scale. Accordingly, overall localized deformation behavior of this unique alloy is appreciated.

Dynamic Tensile Deformation of High Strength Aluminum Alloys Processed Following Novel Thermomechanical Treatment Strategies

Herein, the effects of a very recently introduced novel thermomechanical process route on the microstructural evolution and dynamic tensile deformation behavior of two different precipitation hardenable aluminum alloys, i.e., AA6082 and AA7075, are studied. The investigated materials are hot formed and quenched in differently tempered tools to reveal the influence of cooling rate. Microstructure analysis is conducted to study the influences of different cooling strategies on the microstructure evolution and prevailing strengthening mechanisms in the investigated conditions. Dynamic tensile tests at strain rates of 40, 200, and 400 s−1 coupled with digital image correlation are further conducted to study the mechanical performance and the local deformation behavior. Experimental results show a decrease in yield and tensile strength for the material quenched at higher tool temperature. With increasing strain rate, strength and elongation to failure of the investigated alloys increase for all conditions. The obtained mechanical properties can be rationalized based on the prevailing microstructural features. Both alloys show fine and well‐distributed precipitates upon fast quenching, whereas coarse precipitates prevail upon slow cooling.

Modelling high temperature deformation of lamellar TiAl crystal using strain-rate enhanced crystal plasticity

Multi-phase lamellar γ-TiAl alloys are highly anisotropic and rate sensitive. At high temperature, the strain rate sensitivity (SRS) increases, which eventually influences the yield behaviour of the alloy. With increasing temperature, the onset of yield changes anomalously showing an increasing or sometimes a decreasing trend. This anomalous behaviour depends on alloy chemistry, lamellar microstructure and anisotropy due to lamellar orientations. To capture the strain rate sensitive deformation behaviour over a wide temperature range and to understand distinct nature of yield anisotropy, a new formulation of the strain rate sensitivity (SRS) expressed by a temperature dependent exponent has been proposed within the framework of the classical power-law based visco-plastic Crystal Plasticity Finite Element Method (CPFEM). In the presented formulation, the evolution of SRS parameter was deduced from laws of thermally activated dislocation motion to obtain temperature-enhanced strain rate sensitivity. Moreover, a dislocation pile-up assisted mechanical threshold stress was incorporated to obtain an “anomalous yield” response. The model was validated for a lamellar PST-TiAl, which is a single crystal consisting of γ-TiAl and α2-Ti3Al lamellar phases. Detailed predictive numerical analysis was performed for an in-depth understanding of temperature enhanced strain-rate sensitivity during local deformation behaviour of TiAl alloy based on slip activities.

Investigating the local deformation and transformation behavior of sintered X3CrMnNi16-7-6 TRIP steel using a calibrated crystal plasticity-based numerical simulation model

Abstract In this study, DAMASK was used to model and elucidate the microstructural deformation behavior of sintered X3CrMnNi16-7-6 TRIP steel. The recently developed TRIP-TWIP material model was used within the DAMASK framework. Material optimization was performed using the least computationally expensive method, which yielded the desired results. The physical parameters of the material model were identified and tuned to fit the experimental observations. This tuned material model was used to run simulations utilizing 2D EBSD data. The local deformation, transformation, and twinning behaviors of the material under quasi-static tensile and compressive loads were analyzed. The results of this are in good agreement with previous experimental observations. The phenomena of dislocation glide, twinning, martensitic transformation, stress evolution, and dislocation pinning in different deformation stages are discussed.

A new crystal plasticity constitutive model for simulating precipitation-hardenable aluminum alloys

Age hardened aluminum alloys have superior strengths compared to non-age hardenable alloys due to the growth of a secondary phase of precipitates. This work presents a new crystal plasticity numerical framework to simulate the mechanical properties of precipitation-hardened aluminum alloys. A precipitation hardening constitutive law is implemented into the crystal plasticity finite element method (CPFEM) to simulate the localized deformation behavior of a commercially available AA6060-T6 extrusion. Experimental characterization in the form of uniaxial tension and cyclic simple shear is performed on the alloy of interest to validate the formulation. The framework uses information obtained from transmission electron microscopy (TEM) on a similar alloy for the simulation of the precipitate hardening response. This information is incorporated into an elastic-viscoplastic homogenization scheme to predict the behavior of the precipitate directly. The model is shown to capture the stress-strain response, precipitate anisotropy, and the observed Bauschinger effect with good agreement. A series of parametric analyses are then performed to show the effects of precipitation on the bulk and localized necking response. It is observed that the precipitate configuration heavily influences the polycrystalline behavior, and the localized behavior is also affected in a rather complex way. The model captures the general reduction of anisotropy in age-hardened alloys and serves as a good physics-based foundation for future investigative studies.

Orientation-Dependent Deformation Behavior of 316L Steel Manufactured by Laser Metal Deposition and Casting under Local Scratch and Indentation Load.

This study analyzes the local deformation behavior of austenitic stainless steel 316L, manufactured conventionally by casting and additively by laser metal deposition (LMD). We produced directionally solidified 316L specimens with most grains showing (001) orientations parallel to the longitudinal specimen axis. We conducted nanoindentation and scratch experiments for local mechanical characterization and topography measurements (atomic force microscopy and confocal laser scanning microscopy) of indentation imprints and residual scratch grooves for the analysis of the deformation behavior and, in particular, of the pile-up behavior. The local mechanical properties and deformation behavior were correlated to the local microstructure investigated by scanning electron microscopy with energy dispersive X-ray spectroscopy and electron backscatter diffraction analysis. The results show that the local mechanical properties, deformation behavior, and scratch resistance strongly depend on the crystallographic orientation. Nearly (001)-oriented grains parallel to the surface show the lowest hardness, followed by an increasing hardness of nearly (101)- and (111)-oriented grains. Consequently, scratch depth is the greatest for nearly (001)-oriented grains followed by (101) and (111) orientations. This tendency is seen independently of the analyzed manufacturing route, namely Bridgman solidification and laser metal deposition. In general, the laser metal deposition process leads to a higher strength and hardness, which is mainly attributed to a higher dislocation density. Under the investigated loading conditions, the cellular segregation substructure is not found to significantly and directly change the local deformation behavior during indentation and scratch testing.

Modeling the Local Deformation and Transformation Behavior of Cast X8CrMnNi16-6-6 TRIP Steel and 10% Mg-PSZ Composite Using a Continuum Mechanics-Based Crystal Plasticity Model

A Transformation-Induced Plasticity (TRIP) steel matrix reinforced with magnesium-partially stabilized zirconia (Mg-PSZ) particles depicts a superior energy absorbing capacity during deformation. In this research, the TRIP/TWIP material model already developed in the framework of the Düsseldorf Advanced Material Simulation Kit (DAMASK) is tuned for X8CrMnNi16-6-6 TRIP steel and 10% Mg-PSZ composite. A new method is explained to more accurately tune this material model by comparing the stress/strain, transformation, twinning, and dislocation glide obtained from simulations with respective experimental acoustic emission measurements. The optimized model with slight modification is assigned to the steel matrix in 10% Mg-PSZ composite material. In the simulation model, zirconia particles are assigned elastic properties with a perfect ceramic/matrix interface. Local deformation, transformation, and the twinning behavior of the steel matrix due to quasi-static tensile load were analyzed. The comparison of the simulation results with acoustic emission data shows good correlation and helps correlate acoustic events with physical attributes. The tuned material models are used to run full phase simulations using 2D Electron Backscatter Diffraction (EBSD) data from steel and 10% Mg-PSZ zirconia composites. Form these simulations, dislocation glide, martensitic transformation, stress evolution, and dislocation pinning in different stages of deformation are qualitatively discussed for the steel matrix and ceramic inclusions.

Uncertainties in Finite Element Analysis of Yield Point Phenomena in Advanced High-Strength Steel Spot Welds

Effects of local plastic instabilities in the form of yield point phenomena (YPP) were investigated on local and global deformation and failure behavior of resistance spot-welded tension–shear (TS) samples made from M1700 advanced high-strength steel (AHSS) by means of finite element (FE) simulations. Material models with and without YPP were used in FE simulations, and sensitivity of results to YPP material model characteristics and failure criteria was investigated. Results from FE simulations revealed several stages of load oscillations in predicted global load–displacement curves with different amplitudes depending on material model with YPP. Results also showed the existence of heterogeneous distribution of equivalent stress in heat-affected zone (HAZ) of TS samples with strong sensitivity to the upper and the lower yield stress values in YPP material model. A stress-based failure criterion was added to the damage model to account for this stress heterogeneity, and results from this model showed that the loading and failure behavior of TS sample can significantly change from ductile to premature semi-brittle failure with significant reduction in strength of TS sample. Results from FE simulations were also compared with those from TS experiments and indicated the need for development of proper material and damage models suitable for materials with YPP.

Effects of strain rate on tensile deformation behaviour in Ti-6Al-4V at cryogenic temperature

In this study, we investigated the effects of strain rate on tensile deformation behaviour in Ti-6Al-4V sheet at cryogenic temperature. X-ray diffraction (XRD) was used to identify the crystallographic orientation of rolled Ti-6Al-4V. A series of tensile tests were performed by constant strain rate method (CRS) with variable strain rates (i.e., on the order of 1x10-2 to 10-4•s-1). Liquid nitrogen (LN2) was used to mimic cryogenic environment, and for the thermal equilibrium the specimens were immersed in the vessel containing liquid nitrogen for ~10 minutes before tensile testing, and the temperature condition was continuously maintained during the testing. Microstructure and fracture surface was analysed by polarised light microscopy and scanning electron microscope (SEM). Electron backscatter diffraction (EBSD) was further used to characterise local deformation behaviour. Deformation twinning is occurred at cryogenic tempearture, which is rather different to the deformation at room temperature. It is thought that the twinning induced deformation behaviour may lead to a strength enhancement and a rate dependent ductility improvement. Key words: Ti-6Al-4V, cryogenic, microstructure, deformation twinning, EBSD

Local deformation behavior of SHS in beam-to-column connection with exterior-diaphragm assembled by high-strength bolts

Local deformation behavior of SHS in beam-to-column connection with exterior-diaphragm assembled by high-strength bolts

The Influence of Microstrain Evolution by Tensile Straining on Localized Corrosion of Al-Li Alloys 2099 and 2196

The localized deformation behavior during tensile straining (0 to 9 pct) and its effect on the corrosion properties of two third-generation Al-Li alloys (2099 and 2196) were investigated. Both electron backscatter diffraction and 2D micro-digital image correlation were used to examine site-specific effects of orientation and microstrain evolution during tensile straining. After straining, in situ tapping mode atomic force microscopy was used to study galvanic corrosion as it occurred in artificial seawater (3.5 pct NaCl). Regions of high microstrain corroded preferentially, and large, recrystallized grains in mostly unrecrystallized microstructures were highly detrimental to corrosion properties. Recommendations for improved thermomechanical processing and/or alloying to promote corrosion resistance of 2XXX series Al-Li alloys were considered.

Toward a Better Understanding of Crack Growth in Nickel-Cast Alloys Under Creep-Fatigue and Thermo-Mechanical Fatigue Conditions

Abstract To ensure the feasibility of gas turbines, despite rising commodity prices and emission restrictions, an enhancement of both their efficiency and flexibility is necessary. The consequential higher loading of components at high temperature conditions calls for an increased use of damage tolerant design approaches. To still guarantee a safe operation, a sound understanding and reliable estimations for crack growth under service conditions are indispensable. In this paper, the results from several projects in this field conducted at the TU Darmstadt and involved partners are summarized to identify and describe the various influences on crack growth under creep-fatigue and thermo-mechanical fatigue (TMF) loading. The activation of damage mechanisms under TMF loading and interactions between them is dependent on the temperature cycle and the respective load phasing. Depending on the type of loading (force versus strain control), contrary influences of the phase shift on the TMF crack growth rates are found. This can partially be attributed to the differences in mean stress evolution. Crack initiation and propagation under creep-fatigue and TMF conditions are also often connected with significant scattering of initiation sites and crack growth rates. One reason for this nonuniform behavior is the interaction of geometric discontinuities with the microstructure. To investigate the role of the local grain structure for crack initiation and propagation, in situ observation techniques for crack tip movement and local strain fields were applied. Harsh gradients in the local deformation behavior were identified as origins of secondary crack initiation. To describe crack growth under creep-fatigue and TMF conditions, the linear accumulation model “O.C.F.” was developed. It is based on the contributions of fatigue, creep, and oxidation to crack growth per load cycle. This model is capable of reproducing the effects of time-dependent damage, different load ratios, TMF phase shifts, as well as component geometry. Substantial advantages of this method are its independence from empiric correction factors to assess changing load cycle forms and the possibility to give analytic estimations without the need of extensive data processing. The model is currently validated for three nickel cast alloys, also including single-crystalline and directionally solidified cast variants, different creep-fatigue and TMF loading scenarios, and crack geometries. The model's linear formulation allows assessing the dominant driver of crack growth at each stage of an experiment. These predictions are compared with fractographic investigations and in situ observations of crack paths to identify the mechanisms of crack growth under different TMF load cycle forms.

Defect formation and prevention in directed energy deposition of high-manganese steels and the effect on mechanical properties

Laser beam directed energy deposition (DED-LB) differs from other metal additive manufacturing (AM) methods as it allows high building rates and manufacturing of multi-material components via in-situ alloying. This is especially compelling in combination with high-manganese steel (HMnS), as the mechanical properties can be influenced significantly by tailoring the chemistry-dependent stacking-fault energy (SFE). In DED, this can be used to design parts based on local deformation behavior. However, the increased affinity of HMnS to oxygen causes high amounts of oxide formation in the manufactured parts, ultimately deteriorating the mechanical properties due to premature cracking. To investigate the responsible mechanisms, two sets of processing parameters resulting in varying melt pool sizes were applied to produce X30Mn23 steel with up to 1 wt% in-situ alloyed Al. The melt pool was first modeled using a finite element method (FEM) approach and correlated with microstructure evolution (OM, SEM, EDS, EBSD, APT) and mechanical properties (tensile test). Interaction of the melt pool with the atmosphere and therefore oxide formation was successfully prevented by reducing the melt pool size, making the application of HMnS in DED feasible. The crack formation by oxides and its prevention are discussed in detail. Finally, the feasibility to manufacture HMnS with an in-situ alloying approach is critically evaluated.

Two spatial association–considered mathematical models for diagnosing the long-term balanced relationship and short-term fluctuation of the deformation behaviour of high concrete arch dams

The safety of a high concrete arch dam should be rapidly diagnosed from different angles. Displacement is an actual comprehensive reflection of the arch dam, and it is very important to diagnose the overall deformation behaviour by displacement-based mathematical monitoring models. In this article, based on the spatial association validation of the measured displacement of two high arch dams by the empirical orthogonal function decomposition and the Pearson correlation analysis, two spatial association–considered mathematical models were proposed for the dam displacement of multimonitoring points: one model for the long-term balanced relationship and one model for the short-term fluctuation. To diagnose the abnormality of the dam long-term spatial association, each displacement time series of the multimonitoring points on the dam body with strong spatial associations was decomposed by wavelet multiresolution analysis, and the decomposed high-frequency components, which had the same periodicity as the causal factors of the reservoir water level or air temperature, were determined to establish the cointegration monitoring model. The second model was a combination prediction model, with two sub-models established from the modelling angles of the hydraulic, seasonal and time causal factors and the adjacent point displacement factors, and this second model was mainly used for identifying dam short-term local abnormal deformation behaviour. Engineering examples show that the deformation behaviour of an arch dam under normal conditions has strong spatial associations. The two proposed models have high accuracy and interpreting ability and can effectively reduce the number of needed monitoring models.

Nanoindentation of Polyethylene/Polypropylene Tri-Layer Separator Under Dry and Wet Conditions

Polymer separators play an important role in lithium ion batteries by preventing contact between the positive and negative electrodes while allowing the ions to pass through. It is, therefore, important to study not only the chemical properties, but also mechanical properties of separators. Although mechanical properties of separators have been measured using, for example, tensile tests or dynamic mechanical analysis (DMA), local mechanical properties, especially when the separators are immersed in the electrolyte, are not well known. The local deformation behavior under compression is important since the separator is usually in contact with the porous electrode layer which has micro- and nano-meter length scale surface roughness. To study the mechanical properties in a more realistic environment, a polyethylene/polypropylene tri-layer separator (Celgard 2325) was investigated, under both dry and wet conditions, using a nanoindentation system. The indentation measurements were preformed inside of an argon filled glovebox which allowed for indentation in an organic electrolyte. A piece of separator was first saturated with several drops of electrolyte before several more drops were added to create a thin electrolyte layer in which the indenter tip remained submerged during indentation to avoid the surface tension effect on the measured load vs. displacement curves. By analyzing these curves, the elastic modulus and hardness were obtained. A t-test for two population means was conducted to ascertain whether there is a statistical significance between the dry and wet conditions. Under wet conditions, the separator displayed a larger elastic modulus of 0.884 GPa and hardness of 0.0478 GPa compared to 0.801 GPa and 0.0418 GPa under dry conditions, respectively. The incompressibility of the liquid electrolyte may be responsible for the measured differences. This study provides a more representative set of mechanical properties that should be considered especially for use in computational modeling and design of battery cells.

Highly localized shear deformation in a Mg–Al–Mn alloy subjected to ballistic impact

Shear localization, which might result in the formation of shear bands in materials, is considered as a dominating deformation mechanism in materials under dynamic loading. In the present study, the microstructures of twin-induced ASBs in a Mg–Al–Mn alloy under ballistic impact were studied using scanning electron microscope and transmission electron microscope to investigate the localized shear deformation behavior of Mg alloys under high strain rate deformation. The results show that highly localized shear deformation mainly occurs at the boundary regions of deformed twins, resulting in the formation of narrow shear bands. The twin-induced ASBs composed of fine recrystallized grains and Mg17Al12 precipitates, while the internal part of twins shows the microstructure with elongated grains. The initiation of ASBs at the twin boundary regions is mainly attributed to the stress concentration at twin boundaries due to the dislocation pile-up. These results can contribute to further understanding of the shear deformation behavior of Mg alloys under dynamic loading.

Linking local and heterogeneous deformation behavior to global deformation of materials by in-situ experimental techniques

Plastic deformation of metallic materials is essentially heterogeneous and localized. In the case of dislocation slip, for example, slip of dislocations occurs on particular crystallographic planes and in particular directions, and dislocations are multiplied from dislocation sources located on limited slip planes. Heterogeneous deformation is also induced by the constraint of adjacent grains in polycrystalline materials and appears in different length scales in the form of different slip patterns within each grain, deformation bands, and so on. To achieve higher strengths, the microstructures of structural metallic materials have become more and more complicated and are often composed of different phases (or domains) having different strength levels. In such complicated microstructures, deformation becomes even more heterogeneous. Therefore, it is important, especially in high strength metals, to link local and heterogeneous deformation aspects to the global deformation behavior, in order to fundamentally understand the nature of deformation and then to design advanced materials for performing desired mechanical properties. Conventional material testing, such as tensile tests can determine the global deformation behavior of materials, but the obtained stress-strain curves show only the average material response, in which local information is hidden. Local and heterogeneous characteristics of deformation have been observed by various kinds of microscopy, but it is typically difficult to correlate such local and heterogeneous features with the global and average materials response, especially during deformation. Recently, significant progress has been made in various kinds of in-situ experimental techniques, and a breakthrough in deepening the fundamental understanding of deformation of materials is expected. Such techniques include in-situ neutron/synchrotron diffraction during deformation, in-situ deformation observations in scanning and transmission electron microscopy, nano-indentation, and digital image correlation (DIC) during deformation. In the current paper, some recent results obtained in the author’s group linking local and heterogeneous characteristics to the global deformation response for several metallic materials are introduced, illustrating the use of state-of-the-art techniques.