Increased Strain Hardening Research Articles

Quantitative analysis of deformation characteristics and corrosion properties of high energy laser shock peened Ni-based superalloy

This study examines the influence of high-energy laser shock peening (LSP) using 7 J and 10 J pulse energies on the sub-surface deformation characteristics of Inconel 718 superalloy. High-magnitude compressive residual stresses were induced into the samples after LSP with large residual stress depths of the order of 2 mm – the experimental observations were in good agreement with finite element analyses of the LSP process. The propagation of intense shock waves led to an increased strain hardening and dislocation densities that were experimentally quantified by synchrotron diffraction and transmission electron microscopy. Microscopic analyses revealed highly refined grain structure only at the surface without much refinement observed in the residual depth region. Alongside high degree of strain hardening, a profuse amount of adiabatic shear bands was observed in the hardened depth, indicative of simultaneous strain localisation under such high laser pulse energy. These bands occurred along common slip planes in the Ni γ-matrix and could be potential areas of instability leading to failure. The LSP-treated samples exhibited improved corrosion resistance, with higher laser pulse energy peened samples performing better.

Plastic deformation and damage modeling of AA7075 synthetic 3D microstructure created using generative AI

3D microstructures provide valuable insight into material behavior which is essential in elucidating microstructural phenomena, such as particle morphology and void damage, and consequent macroscopic material response. However, creating 3D microstructures is extremely laborious and expensive, requiring complex microstructural characterization and imaging techniques such as Focussed-Ion Beam based Scanning Electron Microscopy (FIB-SEM) or X-ray Computed Tomography (XCT). To this end, synthetic 3D microstructures were rapidly generated from orthogonal 2D images using SliceGAN, which proved a practical and cost-effective method. In this study, multiple synthetic microstructures of AA7075-O, a complex microstructure of various strengthening precipitates within a softer aluminum matrix, were post-processed, meshed, and modeled for different damage behavior in FEA using advanced constitutive material models. Subsequently, the synthetic and real microstructures were qualitatively and quantitatively analyzed for their elastoplastic deformation and ductile void damage responses.This study illustrates the viability of an integrated AI-FE methodology in studying microstructural micromechanics, demonstrating that synthetic microstructures exhibited a very similar stress-strain response, especially when using a free boundary condition, and comparable stress distribution and void damage, albeit with some discrepancies. Also, it emphasizes the influence of particle morphology on strength and damage, where highly irregular particles play a dual role in increasing strain hardening by restricting matrix flow at the cost of increased ductile damage induced by decohered particles. Lastly, the more advanced FE models, with multiple voiding mechanisms, reduced the discrepancy between real and synthetic microstructures compared to simpler models

Optimizing the Rolling Process of Lightweight Materials

Conventional rolling is a plastic deformation process that uses compression between two rolls to reduce material thickness and produce sheet/plane geometries. This deformation process modifies the material structure by generating texture, reducing the grain size, and strengthening the material. The rolling process can enhance the strength and hardness of lightweight materials while still preserving their inherent lightness. Lightweight metals like magnesium alloys tend to lack mechanical strength and hardness in load-bearing applications. The general rolling process is controlled by the thickness reduction, velocity of the rolls, and temperature. When held at a constant thickness reduction, each pass through the rolls introduces an increase in strain hardening, which could ultimately result in cracking, spallation, and other defects. This study is designed to optimize the rolling process by evaluating the effects of the strain rate, rather than the thickness reduction, as a process control parameter.

Toughness behavior and deformation mechanisms in FCC-based Fe45Co30Cr10V10Ni5-xMnx high-entropy alloys: Insights from instrumented Charpy impact tests

This study investigated toughness properties of FCC-based Fe45Co30Cr10V10Ni5-xMnx high entropy alloys (HEAs). Ductile-dimpled rupture mode in all alloys and test temperatures complicated precise explanations on Charpy impact energy, but the instrumented Charpy test provided a breakdown of total energy (ET) into initiation energy (EI) and propagation energy (EP), offering critical insights into Mn-content- and temperature-related energy variations. Pmax and TP, reflecting maximum flow stress and time, respectively, until initiation of crack propagation from the notch tip, involved a transition of deformation mechanisms from slip to TWIP, then to BCC-TRIP, thereby inducing increased strain hardening and delaying plastic instability at the notch tip. At 25 °C, an increase in Mn content prompted a transition in deformation behavior from slip to TWIP and subsequently to BCC-TRIP, correlating well with increased Pmax and TP and consequently ET due to enhanced strain hardenability. At −196 °C, predominant BCC-TRIP activity in all alloys led to increased formation of BCC martensite, intensifying strain hardening and requiring higher Pmax for crack initiation than at 25 °C. Due to minimal slip line field formation associated with reduced plastic deformation, however, cracks initiated directly from the notch-tip center, representing fast initiation of crack propagation and consequently reduction in EI and ET. Thus, utilizing parameters from instrumented Charpy tests, including Pmax, TP, EI, EP, and ET, provided insights into fracture phenomena and their interrelations in the present HEAs.

Modeling Elongational Rheology of Model Poly((±)-lactide) Graft Copolymer Bottlebrushes

The shear and elongational rheology of graft polymers with poly(norbornene) backbone and one poly((±)-lactide) side chain of length Nsc = 72 per two backbone repeat units (grafting density z=0.5) was investigated recently by Zografos et al. [Macromolecules 56, 2406–2417 (2023)]. Above the star-to-bottlebrush transition at backbone degrees of polymerization of Nbb>70, increasing strain hardening was observed with increasing Nbb, which was attributed to side-chain interdigitation resulting in enhanced friction in bottlebrush polymers. Here we show that the elongational rheology of the copolymers with entangled side chains and an unentangled backbone can be explained by the Hierarchical Multi-mode Molecular Stress Function (HMMSF) model, which takes into account hierarchical relaxation and dynamic dilution of the backbone by the side chains, leading to constrained Rouse relaxation. In nonlinear viscoelastic flows with larger Weissenberg numbers, the effect of dynamic dilution is increasingly reduced leading to stretch of the backbone chain caused by side chain constraints and resulting in strain hardening. If the backbone is sufficiently long, hyperstretching is observed at larger strain rates, i.e. the stress growth is greater than expected from affine stretch.

Superior strength–toughness synergy of heterogeneous TiB2p/AZ91 composites containing hard/soft zones via tailoring bimodal grain structure

Magnesium matrix composites show great prospects due to lightweight and high specific strength, but strength-toughness tradeoff has limited their engineering applications. Here, a novel strategy for strengthening and toughening TiB2p/AZ91 (TiB2p: TiB2 particles) Mg matrix composites via tailoring heterogeneous configuration containing hard/soft zones and bimodal grain structure is proposed. The results show that hot extrusion deformation can achieve heterogeneous structure of hard and soft zones resulting from alternating arrangement TiB2p, which induces the formation of coarse and fine grain zones. With the decrease of extrusion temperature, both the hard zone spacing and the number of coarse grains have a great decrease. In particular, TiB2p/AZ91 composites extruded at 400 °C and 350 °C exhibit obvious bimodal grain structure, while almost uniform fine grains are obtained in the composites extruded at 300 °C. This contributes to the excellent comprehensive mechanical properties of TiB2p/AZ91 composites at 400 °C and 350 °C, with UTS of 345 MPa, 388 MPa, and elongation of 12.1%, 10.1%, respectively. Compared with AZ91 alloy, the strength of composites extruded at 400 °C and 350 °C is significantly improved, while the ductility is not sacrificed, indicating that superior strength–toughness synergy is achieved. The improved toughness is ascribed to the increased strain hardening capacity, suppressing and deflecting effects of the hard zones on the crack propagation. Lower extrusion (300 °C), however, leads to the reduced toughness of the composites, which is due to the premature initiation of cracks caused by smaller hard zone spacing and large amount of precipitated phase.

Experimental and numerical investigation of the deep rolling process focussing on 34CrNiMo6 railway axles

Deep rolling is a powerful tool to increase the service life or reduce the weight of railway axles. Three fatigue-resistant increasing effects are achieved in one treatment: lower surface roughness, strain hardening and compressive residual stresses near the surface. In this work, all measurable changes introduced by the deep rolling process are investigated. A partly deep-rolled railway axle made of high strength steel material 34CrNiMo6 is investigated experimentally. Microstructure analyses, hardness-, roughness-, FWHM- and residual stress measurements are performed. By the microstructure analyses a very local grain distortion, in the range < 5 µm, is proven in the deep rolled section. Stable hardness values, but increased strain hardening is detected by means of FWHM and the surface roughness is significantly reduced by the process application. Residual stresses were measured using the XRD and HD methods. Similar surface values are proven, but the determined depth profiles deviate. Residual stress measurements have generally limitations when measuring in depth, but especially their distribution is significant for increasing the durability of steel materials. Therefore, a numerical deep rolling simulation model is additionally built. Based on uniaxial tensile and cyclic test results, examined on specimen machined from the edge layer of the railway axle, an elastic–plastic Chaboche material model is parameterised. The material model is added to the simulation model and so the introduced residual stresses can be simulated. The comparison of the simulated residual stress in-depth profile, considering the electrochemical removal, shows good agreement to the measurement results. The so validated simulation model is able to determine the prevailing residual stress state near the surface after deep rolling the railway axle. Maximum compressive residual stresses up to about -1,000 MPa near the surface are achieved. The change from the induced compressive to the compensating tensile residual stress range occurs at a depth of 3.5 mm and maximum tensile residual stresses of + 100 MPa at a depth of 4 mm are introduced. In summary, the presented experimental and numerical results demonstrate the modifications induced by the deep rolling process application on a railway axle and lay the foundation for a further optimisation of the deep rolling process.

Processing and microstructure–property relations of Al-Mg-Si-Fe crossover alloys

This study introduces new alloys, which combine the age-hardening capability of Al-Mg-Si alloys with the microstructure-controlling effect on processing of primary Fe-rich intermetallic phases used in foil stock. In detail, the processing and microstructure–property relations in new crossover aluminum alloys derived from 6xxx and 8xxx foil stock alloys, is shown. A highly Fe-rich intermetallic phase content was deployed to conceptually mimic high scrap content. Fast and slow solidification rates were applied to represent thin strip and direct chill casting, respectively. The effects of adding Fe and Mn to alloy 6016 were examined, while the Si consumed in primary phases was partly adjusted to maintain age-hardening potential. It was shown that upon thermomechanical processing, primary intermetallic phases in the new alloys are finely fragmented and well dispersed, resulting in strong grain refinement and a uniform texture. Attractive combinations of strength and ductility were revealed, also in material processed under direct chill casting conditions. The new alloys’ high elongation values of up to 30%, and their age-hardening response, were similar to those seen in commercial alloy 6016, while their strain hardening capacity was significantly greater. This can be attributed mainly to the formation of geometrically necessary dislocations near primary Fe-rich intermetallic phases. The study discusses microstructure refinement on the basis of particle stimulated nucleation. It uses a simple model to describe the individual contributions to yield strength, including the effect of primary phases. It also models the effect of these particles on increased strain hardening and ductility.

Modelling mechanical behaviour of a gradient-microstructured material obtained by surface mechanical attrition treatment accounting for residual stresses

In this work, an austenitic 316 L steel was processed by surface mechanical attrition treatment (SMAT), and the induced gradient microstructure was highlighted by experimental measurements. A combined dislocation density-based and grain size-dependent constitutive model is developed to describe the mechanical behaviour of the gradient-microstructured material. In addition, this model also incorporates the grain size-dependent initial twin distribution and evolution of deformation twinning. Residual stress, initial dislocation density and twins are considered to reconstruct the depth-dependent residual fields induced by SMAT. Furthermore, the evolution of dislocation density and twin volume fraction is described during uniaxial tensile loading. Particular attention is devoted to investigate the effect of residual stress and deformation twinning on mechanical behaviour. Results of finite element simulation showed that the gradient microstructure enhances the yield strength, which is in agreement with previous experimental observations. It was revealed that residual stress significantly weakens the yield strength at the initial deformation stage, whereas it has little influence on the plastic behaviour at large deformation. The assumed grain size-dependent deformation twinning model allows describing the evolution of deformation twinning on gradient microstructure. The deformation twinning increases in relation to increased strain hardening during tensile loading. However, the evolution of twin volume fraction shows almost no twin and no variation of twinning within the nanocrystalline grain region.

Orientation-dependent plasticity mechanisms control synergistic property improvement in dynamically deformed metals

We demonstrate the ability to synergistically enhance strength and toughness in metals by tailoring the dominant plasticity mechanisms from impact-induced heterogeneous nanostructures. Using a laser-induced projectile impact testing apparatus, we impact initially dislocation-free single crystal microcubes at high velocities to induce grain size and dislocation density gradients. These gradient nanostructures exhibit heterogeneous deformation under subsequent quasi-static loading leading to enhanced strength and toughness. Transmission Kikuchi diffraction (TKD) analyses show that the synergistic property improvement arises from the complimentary intragranular and intergranular plasticity mechanisms: the pronounced intragranular dislocation plasticity within coarse grained regions provides increased strain hardening and toughness while the nanograined regions with high dislocation densities provide high strength and supplemental toughness enhancement through cooperative grain rotation and grain boundary migration. These plasticity mechanisms are activated to different extents depending on the specific impact-induced nanostructures, which depend on the orientation of the single crystal upon impact. Controlled [100]-face, [110]-edge, and [111]-corner impact of single crystal microcubes, subsequent quasi-static mechanical testing, and pre- and post-compression nanostructural characterization provide a fundamental understanding of the comprehensive process-structure-property relations in heterogeneous nanostructured metals.

Characteristic material behaviors in conical indentation with friction

AbstractCharacteristic material behaviors in obtuse‐angled conical indentation for elasto‐viscoplastic solids are explored and discussed. Finite element calculations are carried out for indentation of a rigid sharp conical indenter into a cylindrical body that models a pseudo‐half space, considering the friction between the indenter and the solid. Under low‐friction conditions, it is assumed that breakage of the material by the sharp tip of the indenter occurs and a new surface emerges along the contact interface during indentation. It is revealed that the load required for indentation does not necessarily reach its maximum at the perfect sticking condition, which shows a nonlinear variation from the frictionless condition to the perfect sticking condition. The increase in strain hardening and the decrease in elastic modulus relative to yield strength both reduce the difference between the required loads for different friction conditions. For perfect sticking, the indenter tip is covered by an elastically deforming material cap during indentation similarly to the case of the plane strain wedge indentation. Similarity and discrepancy between the behavior in the conical indentation and that in the plane strain wedge indentation, which is well described by the classical slip‐line field theory, are clarified in detail.

Macro-Scale strain localization in highly irradiated stainless steel investigated using digital image correlation

Low-temperature radiation hardening and embrittlement is a major life-limiting radiation effect in austenitic stainless steels (AuSS). A strain-induced phase transformation to martensite is observed during plastic deformation of low-Ni AuSS, often increasing strain hardening and reducing ductility. However, in this paper we show an unexpectedly high ductility of 18–37% during room-temperature mechanical testing of a 0.12C–18Cr–10Ni–0.8Ti AuSS (AISI 321 analogue) cut from the hexagonal wrapper of a fuel assembly irradiated in the BN-350 sodium-cooled fast reactor located in Aktau, Kazakhstan. Using digital image correlation, we reveal two completely different deformation mechanisms in samples with the same chemical composition, irradiated to very similar doses. The roles of the martensitic γ→α′ transformation and neutron irradiation parameters explain the differences between plastic deformation mechanisms in the specimens.

A Comparative Study of Strain Rate Constitutive and Machine Learning Models for Flow Behavior of AZ31-0.5 Ca Mg Alloy during Hot Deformation

In this study, isothermal compression tests of highly ductile AZ31-0.5Ca Mg alloys were conducted at different strain rates (0.001–0.1 s−1) and temperatures (423–523 K) along with extruded direction. The flow stress characteristics were evaluated at elevated temperatures. In addition, a strain-dependent constitutive model based on the Arrhenius equation and machine learning (ML) were constructed to evaluate the stress–strain flow behavior. To build the ML model, experimental data containing temperature, strain, and strain rate were used to train various ML algorithms. The results show that under lower temperatures and higher strain rates, the curves exhibited strain hardening, which is due to the higher activation energy, while when increasing the temperature at a fixed strain rate, the strain hardening decreased and curves were divided into two regimes. In the first regime, a slight increase in strain hardening occurred, while in the second regime, dynamic recrystallization and dynamic recovery controlled the deformation mechanism. Our ML results demonstrate that the ML model outperformed the strain-dependent constitutive model.

Temperature dependent anisotropic mechanical behavior of TiAl based alloys

γ-based titanium aluminides exhibit remarkable mechanical properties at high temperatures and are deployed in jet engines. The microstructure of these alloys, consisting of alternating plates of the γ (TiAl) and α2 (Ti3Al) phases, exhibits highly anisotropic mechanical behavior. However, deep understanding of the associated micromechanics is lacking. The effect of differential strength of the slip/twin systems, structural length scales and temperature in determining the homogeneity of deformation at the intercolony and intracolony level has not been well understood. Hence, we establish structure–property relationships, under uniaxial compression and uniaxial tension, by crystal plasticity finite element techniques. The deviation from Hall–Petch behavior and the temperature-dependent yield stress anomaly are rationalized and included in the model. Our results reveal a complex interplay between lamellar orientation, structural length scale, temperature, and stress state. Key findings include: (1) Basal slip in the α2 phase can be activated by differential strengthening of slip systems at lamellar interfaces. Basal slip increases the propensity for crack nucleation due to coarse slip bands. (2) There is preferential activation of twinning in the γ phase under tension. These twins can improve ductility by increasing strain hardening. However, they can also promote crack nucleation. (3) Smaller lamellar widths at higher temperatures can improve strain homogeneity between the γ and α2 phases. (4) Yield stress anisotropy exhibited a non-monotonic variation with lamellar size and temperature. (5) Increasing the strength of slip/twin systems through solid solution or precipitation strengthening can improve intracolony deformation homogeneity, reducing crack nucleation. We suggest strategies for alloy and microstructure design to improve damage tolerance in these alloys.

Investigation of austenite decomposition behavior and relationship to mechanical properties in continuously cooled medium-Mn steel

The austenite decomposition behavior in medium-Mn steel was investigated here by combining thermo-mechanical controlled processing (TMCP) and continuous cooling processes. Microstructures were characterized by means of optical microscopy, electron probe micro analyzer, scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy. Furthermore, the effect of rolling reduction and cooling path on microstructural evolution and mechanical properties were studied. The results indicated that the austenite region was expanded by manganese, and the ferrite, pearlite, and bainite were not nucleated before martensite transformation even when the cooling rate was decreased to 0.07 °C/min. The approach to tailor yield ratio without compromising tensile strength depends on the control of martensite lath morphology and internal dislocation density. The resistance of lattice to shear transformation increases with the increasing strain hardening of prior austenite, which leads to decrease martensite-start (Ms) temperature. The martensite formed at lower transformation temperatures exhibited relatively high dislocation density and refined laths, resulting in high work-hardening capacity during the early stage of plastic deformation. The crack propagation can be effectively deviated or even terminated by high angle grain boundaries (HAGBs) and the nonparallel structures of martensite packets. These studies on the kinetics and thermodynamics of martensitic transformation confirmed the viability of processing continuously cooled medium-Mn steel in the hot rolling production line.

Texture dependent strain hardening in additively manufactured stainless steel 316L

Among the additive manufacturing processes, selective laser melting has gained wide popularity for manufacturing austenitic stainless steel (316L) due to the inherent advantages offered. The synergetic effect of non-equilibrium microstructure and crystallographic texture on mechanical properties has shown that its properties can be tuned by controlling scanning strategies such as interlayer hatch rotation, hatch strategies, etc. Recent studies have explored the mechanical properties and related responses during tensile related deformation. Accordingly, the present article highlights the evolution mechanism for microstructure and crystallographic texture as a function of hatch style. This manufacturing strategy provides an additional tuning parameter to control the morphology of grains and their crystallographic texture. Additionally, the relationship between the hatch style variation and the compressive deformation has been discussed. It was observed that in cases where the initial texture was <100> along compression direction has shown enhanced twin formation propensity, thereby, increased strain hardening response. The deformation asymmetry pertaining to tension and compression has also been discussed.

Investigation of the Influence of a Superimposed Oscillated Forming Process on Forming Characteristics

The increasing demand for resource-efficient production methods is driving the development of new technologies. Sheet bulk metal forming (SBMF) offers the possibility to combine sheet metal and bulk forming operations. This allows the production of complex functional components with secondary forming elements from sheet metal. Compared to other production techniques such as machining, a more efficient use of material can be achieved. Further advantages are a near net shape production and increased strain hardening. SBMF processes are limited by forming technology boundaries. These include high forming forces, incomplete mould fillings and limited surface qualities. In this research, the possibility of enhancing the material flow, improving surface quality and reducing the tool loads in SBMF-processes is investigated by using a superimposed oscillation. The focus here is on achieving a high surface quality of components produced by forming technology and an enhanced material flow during forming. For this purpose, a forming process for ironing an axial gear geometry is superimposed with an oscillation in the main force flow.

On energetic and dissipative gradient effects within higher-order strain gradient plasticity: Size effect, passivation effect, and Bauschinger effect

The size effect, the passivation effect, and the plastic behavior under non-proportional loading in the context of bending and tension of thin foils, and a combination of compression and shear of constrained layers, are studied using Gudmundson's higher-order strain gradient visco-plasticity theory. Bending experiments on thin nickel foils with and without passivation are performed with a load-unload technique. The passivated layer increases the flow stress significantly. Finite element simulations are carried out on elasto-viscoplastic foils under bending and tension, and on constrained layers under combined compression and shear. The simulation results are generally in agreement with the experimental observations. Implications for higher-order boundary conditions and the roles of energetic and dissipative gradient terms are highlighted. The dissipative gradient terms contribute to the increased yield strength, whereas the energetic gradient terms lead to increased strain hardening and an anomalous Bauschinger effect. The modeling of the deformation of constrained layers suggests that compressive stress has a significant influence on the shear flow stress, reducing it compared with the shear flow stress under pure shear.

On a mixed energetic–dissipative constitutive law for non-proportional loading, with focus on small-scale plasticity

We analyse the mixed energetic–dissipative potential (MP) recently proposed by our group to predict, within higher-order strain gradient plasticity (SGP), reliable size-dependent responses under general loading histories. Such an MP follows former proposals by Chaboche, Ohno and co-workers for nonlinear kinematic hardening in the context of size-independent metal plasticity. The MP is given by M quadratic addends that each transitions, at a different threshold value, into a linear dissipative contribution. Hence, the MP involves 2 M positive material parameters, given by the M threshold values and the M moduli weighing each quadratic recoverable term. We analytically demonstrate that, under proportional loading, the MP limit for M → ∞ converges to a less-than-quadratic potential with well-defined properties. This result is of crucial importance for identifying the material parameters of any model adopting the MP. Moreover, our analysis provides a formula for the characterization of the energetic and dissipative parts of any possible MP limit, showing that, regarding the capability to describe the effect of diminishing size within SGP, the MP can be selected such that its contribution to the strengthening (i.e. an increase in yield point) is mostly dissipative, whereas its contribution to the increase in strain hardening is mostly recoverable.

Solid Rheological Properties of PBT-Based Vitrimers

We present an experimental investigation of the mechanical properties of solid vitrimer samples obtained by incorporating a diepoxide into commercial poly(butylene terephthalate) (PBT) in the presence of a Zn(II) catalyst using batch compounder and injection-molding machines. Tensile experiments were carried out at different temperatures (80, 120, and 160 °C) below the melting point (220 °C) and with various diepoxide concentrations (0, 1, and 2 wt %). We found that vitrimer plastic deformation mechanisms differ drastically from those of the PBT precursor and more generally from the regular behavior of other semi-crystalline polymers. In PBT vitrimers, strain localization (necking) is very weak and can even be suppressed. This type of behavior is due to the increase in strain hardening caused by vitrimer cross-linking. The creep resistance and stiffness of PBT vitrimers are significantly higher than those of pristine PBT. We found that vitrimer modification of PBT at moderate levels improves dimensional stability without inducing brittleness. These promising results highlight the need to find processes which enable significant production levels of these new materials. In the Perspective section, we briefly present a method to produce PBT-based vitrimers using continuous reactive extrusion.