Plastic Instability Research Articles

Necking limit analysis of thin wall aerosol can

Abstract Because of thin wall thicknesses and closed bottom ends of the extruded aerosol can, the necking limit analysis needs intensive investigation. The numerical analysis of the necking process of 0.45 mm thickness pure aluminum aerosol can was carried out. The result indicated that the length of the aerosol can wall, which is not fixed by the bottom die and the angle of inclination of necking tools are important factors that affect the development of deformation boundary limits due to plastic instability of local buckling. The fraction of taper angle of tool becomes more series parameter while necking at larger free length and it needs more concentration. Instead, the ratio of necking tool displacement to the total free length to initiate buckling was increased while increasing free length.

Characterization and Analysis of Plastic Instability in an Ultrafine‐Grained Medium Mn TRIP Steel

Herein, the mechanical and magnetic behavior of an ultrafine‐grained (UFG) medium manganese (Mn) transformation‐induced plasticity (TRIP) steel is focused on in its plastic instability. The in situ methods of digital image correlation (DIC) and magnetic Barkhausen noise (MBN) are used to macroscopically characterize the propagation of the Lüders band (stretcher–strain marks) and the evolution of MBN activities during quasistatic tensile deformation. The evolution of microstructure during the plastic instability is investigated ex situ using X‐Ray diffraction (XRD) and transmission electron microscopy (TEM) for selected plastic strain states. It is showed in the results that the plastic instability of this steel is associated with an increase of hardness and enrichment of dislocation density, which can also amplify the MBN signal, while the derived coercivity behaves reversely on an overall trend due to work hardening. The different stress response of the medium Mn steel is closely related to the kinetic martensite microstructure, which in turn modifies the domain–structure response. Thus, the MBN can be used as a potential means for nondestructive evaluation (NDE) for the strengthening of the UFG medium Mn TRIP steel.

Microstructure characteristics and deformation behavior of tin bronze/1010 steel bimetal layered composite by continuous solid/liquid bonding

The tin bronze (TB) and steel (SAE 1010) bimetal layered composite (BLC) plate was prepared by continuous solid-liquid bonding. Microstructures and interdiffusion between Cu and Fe in the obtained BLC were characterized by optical microscope, electron probe micro-analysis and electron back scattering diffraction. Mechanical properties were analyzed by the uniaxial tensile and bending tests. Moreover, in order to study the deformation behavior of the two materials, the clad ratio and mechanical properties of the BLC after rolling were also performed. The results show that the TB/1010 steel BLC with excellent metallurgical bonding and homogeneous microstructure is obtained. The ultimate tensile strength of the BLC (512.3 MPa) is located between the TB (376.2 MPa) and 1010 steel (536.3 MPa), and it is 56.0 MPa larger than the calculated value by the rule of mixture, which is attributed to the synergetic strengthening of the interface. The plastic instability strain is calculated according to the long-wavelength prediction combining the strain hardening index n and work hardening coefficient k of TB and 1010 steel. The generation of twins in TB and the reduction in grain size caused by rolling result in an increase in yield strength of the BLC. The results of the bending tests further show that the strength of interface is larger than that of the TB because the crack propagation is located at the TB and the interface is always tightly bonded.

Axial buckling of imperfect cylindrical steel silos with isotropic walls under stored solids loads: FE analyses versus Eurocode provisions

Stored solids loads on silo vertical walls are composed of two orthogonal components: the horizontal component that internally pressurizes the silo and the vertical wall frictional component that causes axial compression. For an imperfect cylinder, the elastic axial buckling resistance may be considerably raised by a uniform internal pressure as it alleviates the detrimental effect of the geometrical imperfections. However, at high internal pressures, the combination of axial compression and circumferential tension may result into a plastic instability that is commonly termed elephant’s foot buckling. Both stabilizing and destabilizing effects of internal pressure are taken into account by EN 1993–4-1 in the buckling stress design of steel silos with isotropic walls under axial compression. In this paper, using non-linear Finite Element (FE) analyses, the effects of internal pressure on axial buckling of steel silos have been fully investigated and the Eurocode provisions on this issue have been thoroughly examined.

Shear instability in heterogeneous nanolayered Cu/Zr composites

Ultrastrong nanolayered metallic composites are usually subjected to low ductility due to plastic instability during deformation. Here we investigated the shear instability of a newly designed heterogeneous nanolayered Cu/Zr composites by microindentation. The heterogeneity in size was generated by inserting a few thin Cu-Zr bilayers with an individual layer thickness of 2.5 - 10 nm into the interface region of the Cu/Zr layered composites with an individual layer thickness of 100 nm. The microindentation tests showed that multiple shear bands appeared in the heterogeneous composite with one bilayer, whereas only a single shear band was formed in that with two or three bilayers. Most importantly, the layer strain in the multi-shear band region is much smaller than that in the single-shear band area. For example, the strain of the 100 nm layers within the shear band in the composite with one 10 nm bilayer could reach as low as 2.8, which was less than half of that in the composite with three 10 nm bilayers, i.e., 6.1. These findings demonstrated that strain delocalization can be achieved through shear band multiplication if an appropriate number of thin bilayers were used as interlayers in the 100 nm Cu/Zr composites. Besides, compared with the homogeneous composite with an individual layer thickness of 100 nm and the bimodal composite which is composed of alternating one 100 nm Cu-Zr bilayer and two 10 nm Cu-Zr bilayers, the heterogeneous composite with one bilayer displayed a higher strength (2.15 GPa) and a favorable resistance to strain localization.

Ductile failure under non-proportional loading

Ductile failure by void growth in an elasto-plastic material subjected to non-proportional loading, involving step changes in the stress triaxiality or the Lode parameter, is investigated using periodic unit cell model simulations. The equivalent strains to failure by the onset of void coalescence, defined as the localization of plasticity along a band of voids at the micro-scale, are determined as a function of the loading path parameters. The observed trends for the ductility under non-proportional loading are found to be inconsistent with the predictions of a continuum damage mechanics model based on the attainment of a constant critical damage variable, even if the model has been calibrated to predict accurate results for the ductility under proportional loading. It is shown that a recently developed failure criterion based on the onset of plastic instability in a porous material, coupled with a micromechanics-based void growth law, predicts the loading path dependence of failure under non-proportional loading, in better agreement with the cell model simulation results than the continuum damage model. In particular, the triaxiality dependence of the ductility is found to be primarily due to the hydrostatic stress dependence of void growth, while the Lode dependence is primarily due to the instability-based failure criterion with a relatively minor effect of the Lode parameter on void growth. Implications of these findings on the current modeling approaches to ductile failure under shear dominated loading are discussed.

Development of plasticity in vitrimers synthesized from a semi‐crystalline polymer using injection molding

AbstractThe main objective of this work is to analyze finely the influence of cross‐linking on the mechanical properties of polybutylene terephtalate‐based vitrimers. Tensile tests were carried out at four temperatures (80, 100, 130, and 160°C) on a series of vitrimer specimens made by injection molding for which the cross‐linker concentrations were changed by small increments of 0.25%, from 0% to 2%. The displacement and strain fields were measured on the specimen surfaces through 3D digital image correlation in order to analyze the strain localization/delocalization phenomena that occur during the successive stages of the deformation process. In particular, we measured the yield strain (onset of strain localization), and the strain at neck stabilization (beginning of the strain delocalization phase). When the degree of cross‐linking increases, we observed two complementary effects leading to the decrease of the strain range during which plastic instability develops. Firstly increases. Secondly, and this is the main cause of the plastic instability strain range reduction, decreases. This latter effect results from the limitation in extensibility of the macromolecular network in the solid state caused by cross‐linking. More specifically, cross‐linking leads to two distinct modifications of the macromolecular network that both contribute to the reduction of its extensibility. The first is the decrease of the chain length between the network nodes. The second is the pre‐orientation of the macromolecular network that occurs during injection molding in the case of vitrimers with high viscosities due to cross‐linking. Eventually, when , the suppression of the yield point on the tensile curve was observed for the most cross‐linked vitrimers. Furthermore, in the temperature range of investigation, was found to be independent of temperature.

Achieving ultrahigh strength with stable plasticity by stress-induced nanoscale martensitic transformation in Ti2448 sub-micron pillars

Stress induced nanoscale martensitic transformation (SINMT) as well as its effect on mechanical behaviors have been investigated for Ti2448 single crystal in compression along [100] orientation by decreasing sample size to micron-to nano scales which possess high stress due to “smaller, stronger” in metals to trigger SINMT. The transformation process of β (BCC)→O′ (Orthorhombic)→α" (Orthorhombic) involving {011}01‾1β shuffle followed by {21‾1}111‾β shear was directly observed by high resolution transmission electron microscopy (HRTEM). Real-time recording of phase transition by in-situ HRTEM in 90 nm pillar clearly reveals that this SINMT with a critical stress 1206 MPa is high-order-like (continuous) and reversible. Its competition and interaction with dislocation avalanche exhibited a strong size-dependence upon uniaxial compression, inducing a transition from dislocation avalanche to SINMT with decreasing of the pillar size from 2.5 μm to 90 nm, evidenced by scanning electron microscopy (SEM) and HRTEM. According to the results from uniaxial compression, scanning electron microscopy (SEM) and HRTEM, deformation behaviours and mechanical properties of Ti2448 pillars ranging from 2.5 μm to 90 nm exhibit strong size-dependence due to the competition and interaction between the dislocation avalanche and the SINMT. Owing to the “smaller, stronger” size effect, Ti2448 sub-micron pillars possess high stress to induce plenty of nanoscale α" martensites during loading which can effectively impede dislocation avalanche. Ti2448 sub-micron pillars (d < 1 μm) deform in homogenous mode and show an excellent combination of ultrahigh strength (1635 MPa) and plastic stability. By contrast, in micron scale (d ≥ 1 μm) dislocation avalanche dominates the deformation, leading to the plasticity instability.

Correlation between tensile properties, microstructure, and processing routes of an Al–Cu–Mg–Ag–TiB2 (A205) alloy: Additive manufacturing and casting

This paper aims at assessing microstructure/processing route/mechanical property correlation of a high-strength TiB2/Al–Cu–Mg–Ag aluminum alloy, namely A205 alloy, additively manufactured via laser powder bed fusion (LPBF) versus the cast counter material. To this end, high magnification advanced microstructural characterization in connection with the tensile flow and strain hardening (i.e., work hardening) behavior of the materials were studied. Ambient temperature uniaxial tensile tests, based on ASTM E8/E8M, were conducted on the LPBF and cast as well as post-heat treated (T7: overaged and stabilized) materials were performed systematically. Results show a pronounced discontinuous yielding for the LPBF material along with extended ductility. The tensile curve of the LPBF T7 heat-treated material showed some plastic instabilities (Portevin–Le Chatelier effect) in the inelastic region. However, none of these phenomena were observed in the cast material, while ductility is limited. These responses were attributed to the very high cooling/solidification rates experienced by the LPBF materials, which result in a very fine equiaxed and supersaturated aluminum grain structure as compared with the coarse-grained structure of the cast counter material.

Plastic instabilities explored via acoustic emission during spherical nanoindentation

The plastic instabilities of a silicon iron single crystal (Fe single crystal doped with 3 wt% of Si) were explored via spherical nanoindentation coupled with an acoustic emission detection at nano/micro-scale. Three possible scenarios were identified (gradually developing plasticity without detectable AE activity, sudden onset of plasticity with strong AE event, and an elastic response with frequent AE activity both during loading and unloading). Beside the irreversible dislocation slip, the reversible microstructural changes were identified that in turn provokes to extend the common definition of acoustic emission from a phenomenon caused by the sudden irreversible microstructural changes by including also reversible atomic reorganization. AE activity at nano/micro-scale was for the first time observed during both the loading and the unloading for purely elastic indentations suggesting an action of reversible twinning. Critical indentation stresses representing the onset of AE activity during loading and its termination during unloading were identified. The unambiguous classification of AE events and irregularities on the indentation curve showed a direct correlation between the depth excursion and AE hit parameters. Acoustic emissions proved to be a very powerful and complementary method for the examination and classification of different deformation mechanisms (slip and twinning) observed during indentation loading.

Prediction of creep ductility for austenitic stainless steels and copper

ABSTRACT For modern creep-resistant steels, ductility is primarily controlled by cavitation, since brittle rupture is dominating during long-term service. In recent years, the present authors have formulated fundamental models for nucleation and growth of cavities that have been verified to be able to describe experiments for austenitic stainless steels. These equations, together with models for dislocation creep, are used in this paper to present basic modelling results for the creep ductility of austenitic stainless steels for the first time. New results are also presented for ductile rupture, where the elongation values are mainly governed by plastic instability and necking. The Hart criterion is used to identify the strain where the instability forms. Modelling shows that the instability grows very slowly and that its size is not significant until close to rupture. These facts are used to demonstrate that ductility during ductile rupture can be predicted from necking behaviour.

Modeling of temperature-dependent ultimate tensile strength for metallic materials

Quantitative evaluation of temperature effect on the mechanical properties of materials has always been the core issue concern. In this study, based on the Force-Heat Equivalence Energy Density Principle, a method to model the temperature-dependent ultimate tensile strength (UTS) for metallic materials was proposed. The critical plastic instability energy density related to the onset of plastic instability was put forward which is composed of the elastic-plastic strain energy and the corresponding heat energy. Subsequently, two temperature-dependent UTS models considering the effect of strain hardening behavior were theoretically derived. The models do not contain meaningless adjustable fitting parameters and each parameter has clear physical meanings. The intrinsic quantitative relationships between temperature, UTS, strain hardening exponent, strength coefficient, and specific heat capacity are revealed. Model predictions achieved good agreement with 31 groups of accessible metallic materials, including structural steels, high-strength steels, cold-formed steels, zirconium alloys, and aluminum alloys, which are commonly used in engineering. Especially, the models achieved a better agreement with the results of uniaxial tensile tests compared with the current indentation method. Furthermore, the quantitative influence of the strain hardening exponent and the strength coefficient on UTS at different temperatures was analyzed. This work will contribute to the evaluation of the properties of materials at elevated temperatures and the fire design of structures.

The Effect of Hypoxia and Hypoxia-Associated Pathways in the Regulation of Antitumor Response: Friends or Foes?

Hypoxia is an environmental stressor that is instigated by low oxygen availability. It fuels the progression of solid tumors by driving tumor plasticity, heterogeneity, stemness and genomic instability. Hypoxia metabolically reprograms the tumor microenvironment (TME), adding insult to injury to the acidic, nutrient deprived and poorly vascularized conditions that act to dampen immune cell function. Through its impact on key cancer hallmarks and by creating a physical barrier conducive to tumor survival, hypoxia modulates tumor cell escape from the mounted immune response. The tumor cell-immune cell crosstalk in the context of a hypoxic TME tips the balance towards a cold and immunosuppressed microenvironment that is resistant to immune checkpoint inhibitors (ICI). Nonetheless, evidence is emerging that could make hypoxia an asset for improving response to ICI. Tackling the tumor immune contexture has taken on an in silico, digitalized approach with an increasing number of studies applying bioinformatics to deconvolute the cellular and non-cellular elements of the TME. Such approaches have additionally been combined with signature-based proxies of hypoxia to further dissect the turbulent hypoxia-immune relationship. In this review we will be highlighting the mechanisms by which hypoxia impacts immune cell functions and how that could translate to predicting response to immunotherapy in an era of machine learning and computational biology.

Ultra-high strength yet superplasticity in a hetero-grain-sized nanocrystalline Au nanowire

• Nanocrystalline nanowire always exhibits strength-ductility trade-off. Here, we provide in situ atomic-scale evidence that hetero-grain-sized nanocrystalline nanowires are simultaneously ultra-strong and ductile, with no strength-ductility trade-off. • The hetero-grain-sized nanocrystalline nanowire exhibits a super uniform elongation of ∼ 236% and high strength of ∼ 2.34 gigapascals at room temperature. • The in situ atomic-scale observations revealed that the dislocation activities, grain boundary plasticity, and surface atoms diffusion all contribute to the super elongation ability of hetero-grain-sized nanocrystalline nanowire. Nanocrystalline metals often display a high strength up to the gigapascal level, yet they suffer from poor plasticity. Previous studies have shown that the development of hetero-sized grains can efficiently overcome the strength-ductility trade-off of nanocrystalline metals. However, whether this strategy can lead to the fabrication of nanocrystalline nanowires exhibiting both high strength and superplasticity is unclear, similar to the atomistic deformation mechanism. In this paper, we show that ultra-small nanocrystalline Au nanowires comprising grains in both the Hall–Petch and inverse Hall–Petch grain-size regions can exhibit extremely high uniform elongation (236%) and high strength (2.34 gigapascals) at room temperature. In situ atomic-scale observations revealed that the plastic deformation underwent two stages. In the first stage, the super-elongation ability originated from the intergrain plasticity of small grains via mechanisms such as grain boundary migration and grain rotation. This intergrain plasticity caused the grains in the heterogeneous-structured nanowires to grow very large. In the second stage, the super-elongation ability originated from intragrain plasticity accompanied by the diffusion of surface atoms. Our results show that the hetero-grain-sized nanocrystalline nanowires, comprising grains with sizes both in the strongest Hall–Petch effect region and the inverse Hall–Petch effect region, were simultaneously ultra-strong and ductile. They displayed neither a strength-ductility trade-off nor plastic instability.

Heterogeneous evolution of lattice defects leading to high strength and high ductility in harmonic structure materials through atomic and dislocation simulations

Core–shell harmonic structure materials, in which coarse grains (core regions) are surrounded by many fine grains (shell region), have excellent mechanical properties, particularly, the coexistence of high strength and ductility. We use atomic and dislocation simulations to investigate the mechanism of the excellent mechanical properties. The harmonic structures are modeled simply while maintaining the three main characteristics of harmonic structure materials: heterogeneous distributed strength, interfaces between the core and shell regions, and shell region network. Based on the synergistic effect of the coexistence of core and shell regions, which is caused by the heterogeneous evolution of lattice defects near the core–shell interface as obtained in atomic and dislocation simulations, a possible mechanism providing high strength and ductility of the harmonic structure materials is discussed. Plastic deformation starts from the core regions with lower strength and progresses to the shell region with higher strength. The core–shell interfaces act as a strong barrier for the plastic deformation propagation from the core to shell regions because of the back stress caused by the transmitted dislocations in the shell regions, which increase the dislocation density of the core regions in contact with the interface, resulting in increased strength and work-hardening rate in the core regions. The former contributes to the high strength of the harmonic structure materials and the latter can suppress the plastic instability of the shell regions, resulting in high ductility of the harmonic structure materials.

Effect of solute clusters on plastic instability in magnesium alloys

The interaction between solute atoms and matrix dislocations during plastic deformation of materials strongly influences the flow behavior and can lead to plastic instability at the macroscopic scale. In this study, Lüders band formation and dynamic strain aging phenomena were investigated in a Mg–1%Mn-1%Nd (wt.%) alloy, deformed in uniaxial tension at ambient temperature and 150 °C. The investigated alloy was studied in the initial as-extruded state and after a subsequent heat treatment at 275 °C for 120 min, which allowed for a variation of solute concentration in the microstructure, and thus for a different type/level of solute – dislocation interaction. Microstructure investigations at near-atomic resolution using three dimensional atom probe tomography were carried out in order to visualize the spatial distribution of alloying elements and reveal potential solute clustering trends in connection with the occurrence of plastic instability. Identified nanoscale solute clusters were analyzed on the basis of their size distribution, chemical composition, and statistical significance. Results demonstrate that the number, as well as the types of solute clusters, play an essential role in triggering plastic instability by interacting with deformation carriers at the atomic scale.

Chemical composition dependence of the strength and ductility enhancement by solute hydrogen in Fe–Cr–Ni-based austenitic alloys

Tensile tests of five commercial Fe–Cr–Ni-based austenitic alloys were conducted after thermal hydrogen precharging in a pressurized gaseous environment. The divergence in Cr and Ni concentrations affected the hydrogen solubility significantly as well as the impacts of dissolved hydrogen on the mechanical performance of the alloy. Hydrogen solubility increased with increasing Cr content and Cr/Ni compositional ratio, bringing about an escalating solid-solution hardening with a magnitude of ≈ G/1000 (G: shear modulus) per atomic percent of solute hydrogen. Furthermore, hydrogen facilitated deformation twinning in alloys with relatively low stacking fault energy (lower Ni content), in which deformation twinning occurred even in a nonhydrogenated state. Augmenting the twin density enhanced the work-hardening capability at the later deformation stage, giving rise to the improvement of uniform elongation via retarded onset of plastic instability. Consolidating the experimental results and hitherto-understood hypothesis on the response to hydrogen of other austenitic materials, the essential conditions for promoting hydrogen-related strengthening and ductilization were deduced.

Plastic instability and texture modification in extruded Mg-Mn-Nd alloy

Even though Mg alloys containing Mn and rare earth elements lead to higher ductility and lower yield asymmetry due to the weak texture after extrusion, plastic instability, commonly known as the Portevin–Le Chatelier (PLC) effect, causes unexpected fragility in the service environment. In the present study, the PLC phenomenon and texture development during the deformation of Mg-Mn and Mg-Mn-Nd extruded alloys were investigated under various temperatures and strain rates. The addition of Nd causes not only texture weakening but also severe PLC phenomenon. The PLC phenomenon was significantly affected by the temperatures and the strain rates, which causes a difference in mechanical properties and development of texture. In the conditions of high temperature and low strain rate, the strength increased while the elongation decreased significantly, and obvious PLC phenomenon with severe serration and negative strain rate sensitivity. The initial texture was maintained even after deformation only under severe PLC conditions, and this is due to the restriction of basal slip and suppression of lattice rotation in PLC conditions. The series of results indicate that the PLC phenomenon causes a reduction of formability even at high temperature.

Effects of Extrusion Temperature on Microstructure, Mechanical Properties and Plastic Instability of Ultrahigh-Strength Low-Alloy Mg-0.7al-0.3ca-0.4mn (Wt.%) Alloy

Effects of Extrusion Temperature on Microstructure, Mechanical Properties and Plastic Instability of Ultrahigh-Strength Low-Alloy Mg-0.7al-0.3ca-0.4mn (Wt.%) Alloy

Plastic Instability for a Plastic Orthotropic Spherical Pressure Vessel under Internal Impulsive Loading

Plastic Instability for a Plastic Orthotropic Spherical Pressure Vessel under Internal Impulsive Loading