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The discontinuous characteristics of the yielding behavior in a high Mn austenitic steel was examined in specimens with different grain sizes. In the stress–strain curves of the fine-grained specimens, both the stress drop and Lüders strain were observed, but as the grain size increases it becomes difficult to distinguish the signals related to the yield point phenomena. The effect of grain size on the local deformation behavior was quantitatively investigated using both the digital image correlation method and a simple displacement analysis method. While the velocity of the Lüders band was higher, the amount of Lüders strain was decreased with increasing the grain size. The results are mainly related to the degree of strain concentration during Lüders band propagation. Based on the interactions between the dislocations and grain boundaries, the effect of grain size on the overall discontinuous yielding behavior was discussed. [ABSTRACT FROM AUTHOR]

Plastic instability, including both the discontinuous yielding and stress serrations, has been frequently observed during the tensile deformation of medium-Mn steels (MMnS) and has been intensively studied in recent years. Unfortunately, research results are controversial, and no consensus has been achieved regarding the topic. Here, we first summarize all the possible factors that affect the yielding and flow stress serrations in MMnS, including the morphology and stability of austenite, the feature of the phase interface, and the deformation parameters. Then, we propose a universal mechanism to explain the conflicting experimental results. We conclude that the discontinuous yielding can be attributed to the lack of mobile dislocation before deformation and the rapid dislocation multiplication at the beginning of plastic deformation. Meanwhile, the results show that the stress serrations are formed due to the pinning and depinning between dislocations and interstitial atoms in austenite. Strain-induced martensitic transformation, influenced by the mechanical stability of austenite grain and deformation parameters, should not be the intrinsic cause of plastic instability. However, it can intensify or weaken the discontinuous yielding and the stress serrations by affecting the mobility and density of dislocations, as well as the interaction between the interstitial atoms and dislocations in austenite grains. [ABSTRACT FROM AUTHOR]

Traditional low-alloyed Mg–Al–Ca–Mn extrusion alloys always show a low strength and a high tension-compression yield asymmetry. In the present work, Mg–1Al–1Ca–0.2Mn (AXM1102, wt.%) alloys were extruded at 150–350 °C to produce different grain structure and mechanical properties. It is shown that AXM1102-150 (extruded at 150 °C) sample exhibits superhigh strength in both tension and compression, which exhibited a yield strength (YS) of 428 MPa in tension and 416 MPa in compression. Namely, ultrahigh strength and low tension-compression yield asymmetry were both obtained. The ultrahigh strength was found to be ascribed to the ultra-fine dynamically recrystallized (DRXed) grains (0.47 μm) together with grain boundary co-segregation of Ca and Al atoms. Submicron DRXed grains accounts for the improved tension-compression yield asymmetry through suppressing {10-12} twining during compression. Additionally, the discontinuous yielding was detected during tension of AXM1102-150 alloy, which is potentially related with the high energy barrier required for dislocation emission caused by grain boundary co-segregation of Al and Ca atoms. Once the tensile stress reaches the peak value, the mobile dislocation density increases immediately, thus the tensile stress-strain behavior of the AXM1102-150 alloy is dominated by strain softening. The results indicate that low-alloyed Mg–Al–Ca–Mn alloys demonstrate tremendous potential as next generation ultrahigh-strength and low tension-compression yield asymmetry wrought Mg alloys.

Currently, finite-element (FE) simulation is ubiquitous in engineering research and practice as a tool for solving a wide variety of problems. In steel structures, high-fidelity continuum FE (CFE) analysis is often used to characterize the mechanical behavior of structural members and geometrically complex components such as connections. Accurate description of the material constitutive behavior lies at the heart of these CFE simulations, and over the years, many phenomenologically-based rate-independent constitutive models have been proposed, and a select few have been incorporated in commercially available FE software. In this study, we examine the effect of constitutive model selection on the accuracy of FE analysis predictions of structural steel component responses. Models were selected to have a gradual progression in complexity; some are well-established, widely available in commercial FE software, and extensively used by researchers and practitioners, whereas others, despite being more advanced, have so far been limited in application. Given the difficulties in accurately accounting for extraneous variables common to full-scale experimental testing of realistic structural steel components such as, residual stresses, geometric imperfections, complex boundary conditions, and material inhomogeneities, a set of small-scale configured component experiments were designed to economically minimize these sources of uncertainty and establish a benchmark for critically examining the accuracy of model predictions. Data from these experiments, which included the accurate measurement of local strains that were as high as 75%, facilitated detailed comparisons between model predictions and experiment results, revealing both strengths and limitations of various models. [ABSTRACT FROM AUTHOR]

As the urgency for carbon-neutral fuels grows in response to global warming and environmental pollution, liquid hydrogen, with its high energy density, emerges as a promising candidate. Stored at temperatures below 20 K, liquid hydrogen's containment system requires materials resilient to such cryogenic temperatures. Austenitic stainless steel, including 304L grade, has been widely used due to its favorable properties. However, designing pressure vessels for these systems necessitates a deep understanding of fracture mechanics and accurate assessments of the material's fracture toughness at cryogenic temperatures. The mechanical behavior at these temperatures differs significantly from that at room temperature, making testing at 20 K a complex procedure that requires stringent facilities. This study examines the tensile behavior and fracture toughness of 304L stainless steel at cryogenic temperatures, comparing and analyzing the characteristics observed at 20 K with those at room temperature. The phenomenon of discontinuous yield, with abrupt stress drops and stepwise deformation at low temperatures, has been identified, resulting in more complex stress–strain curves. Limitations were found in the calculation of the crack length during the assessment of fracture toughness in stainless steel under extremely low-temperature environments through the J-integral compliance method. To address these constraints, a comparative analysis was carried out to determine potential corrective measures. [ABSTRACT FROM AUTHOR]

The behavior of the localized plasticity fronts corresponding to self-excited deformation instability, which repeatedly pass through the same region in semifinished NP2 nickel specimens (Ni ≥ 98.5 wt %), is investigated. Stress jumps appear sequentially in a plastic flow curve starting from the yield strength at room temperature, and their nature is caused by the formation of twins and dislocation slip bands reaching the material surface. As the strain rate decreases because of the elongation of a specimen, the shape of deformation jumps changes continuously. The stress jumps are associated with the formation of single macroscopic localized deformation bands, which are visualized using digital speckle-image correlation at a time step of 200 μs. The motion of a deformation front is accompanied by an increase in the deforming stress, i.e., strain hardening. The kinetic characteristics of localized plastic deformation bands are found. [ABSTRACT FROM AUTHOR]

The discontinuous yielding behaviour of Fe E420 steel was experimentally investigated through tensile tests conducted on laminated sheets. In order to assess the response under both quasi-static and dynamic loading conditions, uniaxial tensile tests were performed according to ASTM E8 standards, and dynamic tests were conducted on the Hopkinson bar using a specially designed flat specimen. Three nominal strain rates (2 × 10 − 4 , 500, and 1000) s − 1 were considered. The effect of heat treatment on the material was also considered. For this purpose, the proposed experimental campaign was carried out on both the as-is material and on one treated by water quenching. All dynamic tests were performed with the direct tensile split Hopkinson pressure bar and were recorded via a high frame rate camera. The post failure fracture surfaces were investigated using scanning electron microscopy technique. [ABSTRACT FROM AUTHOR]

Low-carbon hot-rolled steel generally undergoes a deformation process composed of four phases, i.e., elastic deformation, discontinuous yielding, work hardening, and macroscopic plastic-strain localization in a tension test. The evolution of the Poisson's ratio in terms of the average Poisson's ratio and the local Poisson's ratio in the deformation process from the non-load state to the onset point of specimen necking was investigated. The main results are as follows: (1) the average Poisson's ratio cannot accurately represent the local Poisson's ratio in the discontinuous-yielding phase; (2) the Poisson's ratio varied significantly within a plastic band in the discontinuous-yielding phase, and the maximum Poisson's ratio was reached within the plastic band; and (3) the strain rate greatly increased the Poisson's ratio. [ABSTRACT FROM AUTHOR]

Ultrafine-grained (UFG) commercially pure (Ti Grade 2) and high-purity (Ti 99.99%) titanium can be a good alternative to less biocompatible Ti alloys in many biomedical applications. Their severe plastic deformation may lead to a substantial increase of strength, but their highly refined microstructure show a lower thermal stability which may limit their range of applications. The purpose of this study was to investigate the effect of interstitial elements on the thermal stability of UFG Ti Grade 2 and high-purity Ti 99.99% processed by a multi-pass cold rolling to the total thickness reduction of 90%. The severely cold rolled Ti sheets were annealed at temperature in the range of 100–600 °C for 1 h and, subsequently, they were evaluated in terms of microstructure stability, mechanical performance as well as heat effects measured by differential scanning calorimetry (DSC). It was found that the microstructure and mechanical properties were relatively stable up to 200 and 400 °C in the case of UFG Ti 99.99% and Ti Grade 2, respectively. DSC measurements confirmed the aforementioned results about lower temperature of recovery and recrystallization processes in the high-purity titanium. Surprisingly, the discontinuous yielding phenomenon occurred in both investigated materials after annealing above their thermal stability range, which was further discussed based on their microstructural characteristics. Additionally, the so-called hardening by annealing effect was observed within their thermal stability range (i.e., at 100–400 °C for UFG Ti Grade 2 and 100 °C for UFG Ti 99.99%). [ABSTRACT FROM AUTHOR]

Lüders deformation is one type of discontinuous yielding in ferrite–pearlite steel. The yield-point phenomenon and localized plastic bands are two features of the Lüders phenomenon. It is believed that the yield-point phenomenon is related to the formation of plastic bands, but the correlation between them is unclear. In this study, this correlation was investigated by examining the global and local deformation behaviors in the tension processes of four ferrite–pearlite steels (carbon content, 0.05–0.3%; pearlite fraction, 1.2–32%) via an extensometer and digital image correlation (DIC) technique. The main obtained results are as follows: (1) the degree of yield drop decreased with an increase in the pearlite fraction (the magnitude of the yield stress drop was 8.6–0 MPa), and (2) a plastic band was formed at a certain stress level smaller than the upper yield stress; when the stress level was larger than 92% of the upper yield stress, the upper yield point disappeared. [ABSTRACT FROM AUTHOR]

As the urgency for carbon-neutral fuels grows in response to global warming and environmental pollution, liquid hydrogen, with its high energy density, emerges as a promising candidate. Stored at temperatures below 20 K, liquid hydrogen’s containment system requires materials resilient to such cryogenic temperatures. Austenitic stainless steel, including 304L grade, has been widely used due to its favorable properties. However, designing pressure vessels for these systems necessitates a deep understanding of fracture mechanics and accurate assessments of the material’s fracture toughness at cryogenic temperatures. The mechanical behavior at these temperatures differs significantly from that at room temperature, making testing at 20 K a complex procedure that requires stringent facilities. This study examines the tensile behavior and fracture toughness of 304L stainless steel at cryogenic temperatures, comparing and analyzing the characteristics observed at 20 K with those at room temperature. The phenomenon of discontinuous yield, with abrupt stress drops and stepwise deformation at low temperatures, has been identified, resulting in more complex stress–strain curves. Limitations were found in the calculation of the crack length during the assessment of fracture toughness in stainless steel under extremely low-temperature environments through the J-integral compliance method. To address these constraints, a comparative analysis was carried out to determine potential corrective measures.

The discontinuous yielding behaviour of Fe E420 steel was experimentally investigated through tensile tests conducted on laminated sheets. In order to assess the response under both quasi-static and dynamic loading conditions, uniaxial tensile tests were performed according to ASTM E8 standards, and dynamic tests were conducted on the Hopkinson bar using a specially designed flat specimen. Three nominal strain rates (2×10−4, 500, and 1000) s−1 were considered. The effect of heat treatment on the material was also considered. For this purpose, the proposed experimental campaign was carried out on both the as-is material and on one treated by water quenching. All dynamic tests were performed with the direct tensile split Hopkinson pressure bar and were recorded via a high frame rate camera. The post failure fracture surfaces were investigated using scanning electron microscopy technique.

Low-carbon hot-rolled steel generally undergoes a deformation process composed of four phases, i.e., elastic deformation, discontinuous yielding, work hardening, and macroscopic plastic-strain localization in a tension test. The evolution of the Poisson’s ratio in terms of the average Poisson’s ratio and the local Poisson’s ratio in the deformation process from the non-load state to the onset point of specimen necking was investigated. The main results are as follows: (1) the average Poisson’s ratio cannot accurately represent the local Poisson’s ratio in the discontinuous-yielding phase; (2) the Poisson’s ratio varied significantly within a plastic band in the discontinuous-yielding phase, and the maximum Poisson’s ratio was reached within the plastic band; and (3) the strain rate greatly increased the Poisson’s ratio.

Ultrafine-grained (UFG) commercially pure (Ti Grade 2) and high-purity (Ti 99.99%) titanium can be a good alternative to less biocompatible Ti alloys in many biomedical applications. Their severe plastic deformation may lead to a substantial increase of strength, but their highly refined microstructure show a lower thermal stability which may limit their range of applications. The purpose of this study was to investigate the effect of interstitial elements on the thermal stability of UFG Ti Grade 2 and high-purity Ti 99.99% processed by a multi-pass cold rolling to the total thickness reduction of 90%. The severely cold rolled Ti sheets were annealed at temperature in the range of 100–600 °C for 1 h and, subsequently, they were evaluated in terms of microstructure stability, mechanical performance as well as heat effects measured by differential scanning calorimetry (DSC). It was found that the microstructure and mechanical properties were relatively stable up to 200 and 400 °C in the case of UFG Ti 99.99% and Ti Grade 2, respectively. DSC measurements confirmed the aforementioned results about lower temperature of recovery and recrystallization processes in the high-purity titanium. Surprisingly, the discontinuous yielding phenomenon occurred in both investigated materials after annealing above their thermal stability range, which was further discussed based on their microstructural characteristics. Additionally, the so-called hardening by annealing effect was observed within their thermal stability range (i.e., at 100–400 °C for UFG Ti Grade 2 and 100 °C for UFG Ti 99.99%).

The paper considers macroscopic plastic deformation localized in pure nickel NP2 (99.5%). Uniaxial tension of plane specimens is conducted at room temperature and loading velocities of 0.05, 0.2 and 0.8 mm/min. Stress-strain curves demonstrate a discontinuous yielding from the yield strength to necking. Digital image correlation and tracking is used to measure the total displacement and plastic strains in the conditions of the Portevin–Le Chatelier effect. It is shown that the loading velocity affects the band propagation velocity induced by localized deformation during tension. The inverse relationship is identified between the propagation velocity of localized deformation bands and the total displacement. [ABSTRACT FROM AUTHOR]

Lightweight materials are desired for energy saving and emission reduction of automobiles. A promising material for automobile parts is advanced high strength steel (AHSS). A recently developed material called medium-Mn steel, with excellent mechanical properties, has attracted increasing attention as the third-generation AHSS for automotive processing. However, medium-Mn steel is disadvantaged by plastic instability during tensile tests. This plastic instability is usually associated with localized and propagative bands on the material surface, which cause an unexpected surface roughening effect and premature failure in the most unfavorable cases. Therefore, plastic instability has severely impeded the commercialization of medium-Mn steels. The phenomenon manifests as discontinuous yielding followed by a yielding plateau (the Lüders strain), along with flow stress serrations (the Portevin-Le Chatelier (PLC) effect). Both effects are influenced by the composition, annealing process, and microstructure (phase morphology and constituents) of the steel. Both effects are also correlated with the austenite-to-martensite transformation during deformation to a greater or lesser extent, which is rarely observed in metallic materials. Consequently, the mechanisms of both effects are complicated and explainable by diverse theories. This paper reviewed the current research results on the influences of various factors on the Lüders strain and PLC effect, and discussed their corresponding mechanisms. This paper particularly emphasized the limitations of the existing theoretical explanations and proposed future researches to elucidate the existing disputes. Based on the current research and our preliminary experiment, this paper finally suggested ways of eliminating the plastic instability of medium-Mn steel, while guaranteeing ultrahigh strength, and excellent ductility. These improvements will drive the future development of this field.

Lüders deformation is one type of discontinuous yielding in ferrite–pearlite steel. The yield-point phenomenon and localized plastic bands are two features of the Lüders phenomenon. It is believed that the yield-point phenomenon is related to the formation of plastic bands, but the correlation between them is unclear. In this study, this correlation was investigated by examining the global and local deformation behaviors in the tension processes of four ferrite–pearlite steels (carbon content, 0.05–0.3%; pearlite fraction, 1.2–32%) via an extensometer and digital image correlation (DIC) technique. The main obtained results are as follows: (1) the degree of yield drop decreased with an increase in the pearlite fraction (the magnitude of the yield stress drop was 8.6–0 MPa), and (2) a plastic band was formed at a certain stress level smaller than the upper yield stress; when the stress level was larger than 92% of the upper yield stress, the upper yield point disappeared.

To investigate the changes in grain orientation resulting from dislocation slip systems and propose a novel approach to characterizing discontinuous yield behavior during hot deformation, Ti–6V–5Al–5Mo–5Cr–3Nb–2Zr-0.2Si alloy was melted using vacuum arc furnace and subjected to hot compressed at 740, 790, 840 and 890 °C, along with strain rates (ε ˙) of 0.001, 0.01, 0.1 and 0.5 s−1. The results indicate that discontinuous yield is observed at temperatures (T) of 740 or 790 °C and ε ˙ of 0.1 and 0.5 s−1. Discontinuous yielding occurs at all strain rates when T is 840 and 890 °C. The presence of discontinuous yield is only observed when the power dissipation value exceeds the critical value, which in this study is 0.2657. Notably, this is the first instance in which the power dissipation value has been utilized to characterize the presence of discontinuous yield. The microstructure of the discontinuous yield reveals the activation of dislocation slip in the β grain. The dislocation slip is initially activated in the <001> oriented β grains during hot compression, followed by activation in the adjacent <101> oriented β grains. When the power dissipation exceeds the critical value, the dislocations more readily spread to the <101> oriented β grains due to higher activation energy, which is an important reason for discontinuous yield. The <101> oriented β grains gradually deflect to <2−3−1> oriented and finally rotate to <001> oriented during the subsequent compression process, as a means of reducing deformation resistance. [ABSTRACT FROM AUTHOR]

The yielding of a high Mn twinning-induced plasticity steel was examined in three fine-grained specimens recrystallized at 700°C for 5 min with different cooling conditions. While the stress-strain curves of furnace-cooled and air-cooled specimens exhibit a stress drop at yielding, the drop was not observed in the water-quenched specimen. A simple analysis of the displacement data indicates the occurrence of localized deformation at the beginning of the plastic deformation in the three tensile specimens with different cooling conditions. The localized deformation of all three specimens was confirmed as Lüders strain by digital image correlation (DIC) analysis. Based on this observation, the role of yielding behavior on the strain hardening rate evolution at an early stage of the tensile deformation was discussed.

Numerical models of structural components that deteriorate primarily due to geometric instabilities under multiaxis cyclic loading are sensitive to both the assumed geometric imperfections and the nonlinear material model assumptions. Therefore, the accuracy of the constitutive model is a desirable feature in finite-element simulations. However, the classic Voce-Chaboche metal plasticity model, ubiquitous among commercial finite-element software, is found to underestimate the initial yield stress in structural steels by about 10%–30% when calibrated to minimize the overall difference in strain energy between the model and test data of load protocols representative of earthquake loading. This paper proposes a refined version of the Voce-Chaboche material model. When compared with the original model, the updated one improves the prediction of the initial yield stress, can simulate initial yield plateau behavior, and better estimates experimental cyclic stress-strain data. Constraints on the model parameters are established, a calibration procedure is developed, and model parameters are proposed for nine structural steels used worldwide. Source code for the material model is also made publicly available. A case study demonstrates that steel component behavior is sensitive to subtle differences in the material response that arise between the Voce-Chaboche and the proposed material models. [ABSTRACT FROM AUTHOR]

The flow behavior of Ti2AlNb-based alloy is critical to the research of hot forming due to its high sensitivity to the temperature and strain rate. The hot compression tests were conducted by using a Gleeble thermo-mechanical simulator in the temperature range of 950-1080 °C with strain rate from 0.001 to 10 s−1. The flow curves were modified firstly by taking into account the interface friction. In order to neutralize the thermal effect caused by high-speed deformation, the influence of strain rate on the deformation-induced temperature rise was evaluated, and the true temperature was obtained based on the collected stress–strain data. It was found that, under certain strain and strain rate, the true stress varied with the true temperature in an 'inverted S' format. Therefore, an 'inverted S' relation was adopted to calculate the flow stress at the given temperature. After the correction of flow stress, the constitutive model including the parabolic equation and modified Arrhenius equation in asymptotic form was established, and the discontinuous yielding and normal yielding behaviors were depicted, respectively. The constitutive model was integrated into the software DEFORM, by which the accuracy and predictability were verified. [ABSTRACT FROM AUTHOR]

The combination of different phase constituents to realize a mechanical composite effect for superior strength-ductility synergy has become an important strategy in microstructure design in advanced high-strength steels. Introducing multiple phases in the microstructure essentially produces a large number of phase boundaries. Such hetero-interfaces affect the materials in various aspects such as dislocation activity and damage formation. However, it remains a question whether the characteristics of phase boundaries, such as their chemical decoration states, would also have an impact on the mechanical behavior in multiphase steels. Here we reveal a phase boundary segregation-induced strengthening effect in ultrafine-grained duplex medium-Mn steels. We found that the carbon segregation at ferrite-austenite phase boundaries can be manipulated by adjusting the cooling conditions after intercritical annealing. Such phase boundary segregation in the investigated steels resulted in a yield strength enhancement by 100–120 MPa and simultaneously promoted discontinuous yielding. The sharp carbon segregation at the phase boundaries impeded interfacial dislocation emission, thus increasing the stress required to activate such dislocation nucleation process and initiate plastic deformation. This observation suggests that the enrichment of carbon at the phase boundaries can enhance the energy barrier for dislocation emission, which provides a favorable condition for plastic flow avalanches and thus discontinuous yielding. These findings extend the current understanding of the yielding behavior in medium-Mn steels, and more importantly, shed light on utilizing and manipulating phase boundary segregation to improve the mechanical performance of multiphase metallic materials. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Cold-rolled EA4T steel was heat-treated by inter-critical holding at 755 °C for 90, 120, 180, and 240 s, respectively, and then quenching in water. The tensile testing results of the EA4T specimens show an evident transition from the discontinuous yielding to the continuous yielding of the steel specimens by prolonging the holding time. A novel relationship between the discontinuous yielding behavior of tensile-deformed steel specimens and the carbide size was proposed based on experimental results and Cottrell’s theory. The model may provide a new clue for avoiding discontinuous yielding and improving mechanical properties of metals with static strain aging behaviors.

Ultrafine austenite-ferrite duplex medium Mn steels often show a discontinuous yielding phenomenon, which is not commonly observed in other composite-like multiphase materials. The underlying dislocation-based mechanisms are not understood. Here we show that medium Mn steels with an austenite matrix (austenite fraction ∼65 vol%) can exhibit pronounced discontinuous yielding. A combination of multiple in situ characterization techniques from macroscopic (a few millimeters) down to nanoscopic scale (below 100 nm) is utilized to investigate this phenomenon. We observe that both austenite and ferrite are plastically deformed before the macroscopic yield point. In this microplastic regime, plastic deformation starts in the austenite phase before ferrite yields. The austenite-ferrite interfaces act as preferable nucleation sites for new partial dislocations in austenite and for full dislocations in ferrite. The large total interface area, caused by the submicron grain size, can provide a high density of dislocation sources and lead to a rapid increase of mobile dislocations, which is believed to be the major reason accounting for discontinuous yielding in such steels. We simultaneously study the Lüders banding behavior and the local deformation-induced martensite forming inside the Lüders bands. We find that grain size and the austenite stability against deformation-driven martensite formation are two important microstructural factors controlling the Lüders band characteristics in terms of the number of band nucleation sites and their propagation velocity. These factors thus govern the early yielding stages of medium Mn steels, due to their crucial influence on mobile dislocation generations and local work hardening. Image 1 [ABSTRACT FROM AUTHOR]

A discontinuous yielding was observed in the as-extruded Mg-7Gd-2Y-1Zn-0.6Zr (wt. %) alloy under uniaxial tension along the extrusion direction, which was eliminated by the peak-aging treatment. The deformation mechanisms of the as-extruded and peak-aged alloys were studied by in-situ synchrotron diffraction measurement and elastic-viscoplastic self-consistent (EVPSC) modeling. The in-situ synchrotron diffraction results suggest that the unpinning of basal dislocations from solute atmospheres causes the stress relaxation in the basal slip favored grains. The EVPSC modeling results indicate that the discontinuous yielding in the as-extruded alloy is associated with the interaction between basal dislocations and solute atmospheres. After peak-aging treatment, the interaction between basal dislocations and solute atmospheres is weakened due to the consumption of a number of solute atoms. Consequently, the discontinuous yielding is eliminated in the peak-aged alloy. [ABSTRACT FROM AUTHOR]

Low‐carbon steel sheets are severely plastic deformed to strains of up to ≈3.48 and subsequently heat treated by conventional annealing followed by water‐quenching. Four temperatures are chosen for the annealing below and over the Ac1 and Ac3 transformation lines. The effects of post‐deformation heat treatment are investigated by evaluating the microstructure and mechanical properties, including strength, ductility, work hardening capability, and hardness. A maximum increase of 86% in the strength is obtained through intercritical annealing and quenching of the samples subjected to strain of 1.16. It is interesting that both the elongation and ultimate tensile strength values are higher after heat treatment at a variety of temperatures in comparison with those of the non‐treated samples; up to 197% for the former and up to 33% for the latter. These would be achieved in combination with yield ratios even lower than that of the as‐received material, demonstrating the higher deformation capability of the specimens. However, these beneficial outcomes are deteriorated toward higher amounts of strain. Despite the heat treated undeformed low‐carbon steels, the stress − strain curves for heat treated deformed samples show little or no discontinuous yielding correlated to variations of mobile dislocations density as a result of microstructural evolution. [ABSTRACT FROM AUTHOR]

Duplex stainless steels with ultrafine- or fine-grained microstructures showed pronounced discontinuous yielding and Lüders banding behavior during tensile deformation at 77 K. In particular, sample with high-stability "eye"-shaped structures and ultrafine grains exhibited a yield strength of 1750 MPa, a yield drop of 87 MPa and a Lüders strain of 0.23. [ABSTRACT FROM AUTHOR]

Discontinuous yielding behavior of TC11 alloy was found during hot compression tests at temperatures of 1000–1050 °C and the strain rate of 10 s −1 . The flow stress showed a rapid drop and subsequent oscillation after peak stress before true strain of 0.15. It was proved to be the result of the stress induced phase transformation ( α → β ), which introduced softening effect for the generation of new β phase. Moreover, the magnitude of the stress drop increased with the increasing of the fraction of stress induced phase transformation. In addition, these newly generated β phase were further deformed by subsequent compression strain. As a result, the phase transformation of β → α″ was found in final quenched microstructures. [ABSTRACT FROM AUTHOR]

Recent reports of discontinuous yielding in the uniaxial compression of extruded magnesium suggest that twinning is accompanied by a significant stress decrease in the parent grain, leading to corresponding stress concentrations. We performed crystal plasticity finite element simulations using a simple twinning implementation that includes this “softening” effect for tensile twinning. With this method we were able to reproduce the experimentally observed yield point elongation and the Lüders-like propagation of strain and twinning over the sample. Twin variant selection was analysed in detail at various stages of deformation. Although lower rank variants were found to be activated, most of the twinning activity occurred on variants with the highest global Schmid factor. The stress fluctuations arising from the stress relaxation due to twinning did not appear to alter the dominance of the highest Schmid factor variant significantly. However, when twin softening is included, the twinning strain is dominated by a single variant. Despite giving rise to the highest stress fluctuations during yielding, simulations using the twin stress relaxation implementation actually showed less heterogeneity in stress after yielding. The results of our simple model suggest that “twin softening” could be key to the development of discontinuous yielding in extruded magnesium alloys. [ABSTRACT FROM AUTHOR]

We examined the hot deformation behaviors of Ti-40 at.% Al alloy via hot compression tests at temperatures in the range of 1423–1523 K (single α-hcp phase) at strain rates in the range of 1–0.001 s −1 . Specifically, we investigated the discontinuous yielding phenomenon and the strain rate dependence of the microstructural evolution of the alloy. Sharp drops in flow stresses occurred during hot compression at strain rates of 1 s −1 and 0.1 s −1 , which were attributed mainly to both the multiplication of dislocations and the formation of deformation twins. The twins were determined as being of the compound type with the following twin elements: K 1 : { 11 2 ¯ 1 } , K 2 : { 11 2 ¯ 2 ¯ } , η 1 : < 11 2 ¯ 6 ¯ > , η 2 : < 11 2 ¯ 3 > , s = 0.232. The grains of the α 2 -Ti 3 Al single phase were more significantly refined and homogenized by hot compression at a high strain rate of 1 s −1 than at a low strain rate of 0.001 s −1 , with the assistance of the deformation twins at the high strain rate. [ABSTRACT FROM AUTHOR]

Hot compression tests were carried out in temperature range of 680–880 °C and at strain rates of 0.001–1 s −1 on Ti–6V–6Mo–6Fe–3Al beta titanium alloy. The stress–strain curves showed yield point phenomena, especially at high strain rates and low temperatures. A polynomial equation was developed to relate the yield drop to the Zener–Hollomon parameter. It was found that the yield drop reaches a maximum and remanis nearly constant at high values of the Zener–Hollomon parameter (ln Z >30). The flow curves at high temperatures and low strain rates were typical of dynamic recovery. However, at low temperatures and high strain rates (27

Kinetic characteristics of discontinuous yielding at a temperature of 4 K as functions of a number of factors are obtained using numerical simulation and experimental data for austenitic steel and aluminum alloy. During the development of a strain jump, the deformation rate and acceleration are 19 s −1 and 5000 s −2 , respectively, for steel specimens and are much lower for aluminum alloy. The jump duration is mainly determined by the characteristics of the loading system. An equation relating the strain jump and the critical stress for low-temperature ductile materials is derived. The energy balance and the mechanism of low-temperature discontinuous yielding of metals are discussed. Its dynamic and thermally activated components are estimated taking into account the strain hardening of the material. [ABSTRACT FROM AUTHOR]

Discontinuous yielding in Ni-base superalloy during high-speed compression has been investigated. Flow stress curves of GH4049 can be divided into two types: the first type curves display common flow behavior revealing work hardening, stable, softening and steady stages; the second type curves present abnormal flow behavior revealing discontinuous yielding feature, which were characterized by a sharp peak stress ( σ P ), obvious upper yield point ( σ U ) and a lower yield point. Apparent activation energies for peak values and upper values were calculated to be Q P =1162 kJ mol −1 and Q U =1106 kJ mol −1 , respectively. Constitutive equations represent peak stress and upper stress as functions of strain rate and deformation temperature are described. First type curves present common work hardening behavior; however, second type curves present spiral hardening behavior since discontinuous softening during high-speed deformation. When GH4049 superalloys present first type flow behavior, volume fraction of dynamic recrystallization ( X DRX ) can be described in terms of normal S-curves revealing slow–rapid–slow property. However, when alloys present second type flow behavior, X DRX can be described in terms of double S-curves exhibiting sudden–steady–rapid–slow property. [ABSTRACT FROM AUTHOR]

Lüders deformation is one type of discontinuous yielding in ferrite-pearlite steel. The yield-point phenomenon and localized plastic bands are two features of the Lüders phenomenon. It is believed that the yield-point phenomenon is related to the formation of plastic bands, but the correlation between them is unclear. In this study, this correlation was investigated by examining the global and local deformation behaviors in the tension processes of four ferrite-pearlite steels (carbon content, 0.05-0.3%; pearlite fraction, 1.2-32%) via an extensometer and digital image correlation (DIC) technique. The main obtained results are as follows: (1) the degree of yield drop decreased with an increase in the pearlite fraction (the magnitude of the yield stress drop was 8.6-0 MPa), and (2) a plastic band was formed at a certain stress level smaller than the upper yield stress; when the stress level was larger than 92% of the upper yield stress, the upper yield point disappeared.

The high temperature discontinuous yielding behavior of Ti-7333 (Ti-7Mo-3Nb-3Cr-3Al), a new near β titanium alloy, has been examined between 770 and 845 °C over strain rates from 10-3 s-1 to 1 s-1. The results show that the discontinuous yielding of the alloy occurs when its strain rate is above 10-2 s-1 at all the experimental temperatures. The magnitude of the discontinuous yield drop for Ti-7333 alloy increases with decreasing of strain rate at a fixed temperature, and decreases with decreasing of temperature at a fixed strain rate. The magnitude is more sensitive to the strain rate than the temperature in this investigation. The discontinuous yield phenomena observed is associated with the mobile dislocation generated in grain boundaries during plastic deformation spreading to the grain interior with increasing of strain. The grain boundaries act as the sources of mobile dislocation generation. Further analysis suggests that the discontinuous yield drop is related to unlocking of dislocation sources through a localized dislocation climb, a boundary migration and/or a solute rearrangement. [ABSTRACT FROM AUTHOR]

Abstract: Effects of the processing parameters on the flow stress behavior and the microstructural revolution are investigated in the hot compression of as-cast Ti60 alloy. The flow stress behavior reveals greater flow softening in the two-phase field compared with that of single-phase field. In the two-phase field, flow softening is caused by break-up of lamellar α, deformation heating, flow localization and free-surface cracking. While in the single-phase field, flow softening is associated with dynamic recovery and recrystallization. In addition, the flow stress curves display discontinuous yielding, which is attributed to rapid dislocation generation and multiplication from the grain boundary. Moreover, the magnitude of yield drop is inversely proportional to the average grain size. In the α+β phase field, the distorted degree of lamellar α within the prior β grains increases with increasing strain rate and reduction, while flow instability including low localization and oxidation cracking occurs in higher strain rate (>1s−1) and larger reduction (>60%), a small amount of recrystallized grains are observed at the prior β grain boundaries in lower strain rates (≤0.1s−1) with reduction of 60%. In the β phase field, however, the concentration of recrystallized β grains at prior β grain boundaries is much higher than that observed during deformation in the α+β phase field. The fraction of recrystallized β grains increases with increasing temperature, reduction and decreasing strain rate. It indicates that the processing parameters have a significant influence on the deformation behavior and microstructural revolution of as-cast Ti60 titanium alloy. It is helpful for the controlling of the microstructure and the optimization of processing parameters. [Copyright &y& Elsevier]

This paper aims at assessing microstructure/processing route/mechanical property correlation of a high-strength TiB 2 /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. [ABSTRACT FROM AUTHOR]

[Display omitted] The development of crystalline materials with both high strength and ductility has been a challenging task in the fields of materials science and engineering. In this study, we demonstrate that the transformation of specific types of grain boundaries (GBs) associated with dislocation emission can serve as a novel work-hardening mechanism to achieve multilayered nanopillars with high ductility. We performed atomic simulations of tensile deformation tests of bicrystal models and multilayered nanopillar models consisting of the GBs. The bicrystal simulations showed that the critical stress for the dislocation emission from GBs progressively increased after the first emission of the dislocations owing to the transformation of the GB structure with a lower dislocation-source potential. Owing to this hardening of the dislocation sources, the plastic deformation propagated from grain to grain in the longitudinal direction of the multilayered nanopillar models, similar to the propagation of the Lüders band in discontinuous yielding, consequently attaining high ductility in the multilayered nanopillar models. This indicates that utilizing the untapped functions of GBs, that is, dislocation supplying and hardening capabilities, can facilitate high ductility in the multilayered nanopillars; hence, awakening the "dormant" functions of such lattice defects will facilitate the development of nanoscale materials with excellent mechanical properties, including high work-hardening capacity. The improved work-hardening capacity of nanoscale materials can promote the development of new strategies and approaches toward the advancement of nanoscale materials science research. [ABSTRACT FROM AUTHOR]

Abstract: Numerical results on the discontinuous yielding of austenitic steel and aluminum alloy at a temperature of 4K are presented. The method used allows qualitative and quantitative predictions of the influence of eight determining factors, including characteristics of the material and parameters of the loading system, on the low-temperature discontinuous deformation of metals. Recommendations are formulated on how to improve standards on tensile testing of metals in liquid helium as regards machine rigidity, standard specimen sizes, and new mechanical characteristics. [Copyright &y& Elsevier]