Plastic Instability Research Articles

Effect of SiC particle size and severe deformation on mechanical properties and thermal conductivity of Cu/Al/Ni/SiC composite fabricated by ARB process

SiC particles are among the most investigated types of reinforcing particles with high thermal conductivity in multilayered composites. This paper deals with the effect of the particle size and ARB rolled passes on tensile strength, elongation, and thermal conductivity of Cu/Al/Ni/SiC composite. Based upon the mechanical properties, the ultimate strength and the elongation of Cu/Al/Ni/SiC(2 μm) were more than that of Cu/Al/Ni/SiC(15 μm) at all passes, apart from the zero pass. This is because 2-μm particles not only tend to make smaller clusters than 15-μm particles but also are preferably trapped at the interfaces; however, 15-μm particles tend to be trapped in Al layers. Further, at each rolled pass, the values of thermal conductivity of Cu/Al/Ni/SiC (15 μm) were slightly higher compared to the other two composites. This is because of the larger cluster of 15-μm particles in the Al layers, providing better heat flux across the composite. In addition, the results of optical microscopy (OM) showed that applying more passes caused the discontinuity in all layers and also the non-uniform distribution of Ni and Al layers in the Cu matrix. Plus, due to the lower strength of Al layers in comparison to Cu and Ni layers, ceramic particles diffuse considerably in Al layers. The SEM images of the fracture surfaces indicated the ductile fracture with some uniaxial dimples for Al and Cu layers whereas for Ni layers both ductile and brittle cleavage fractures due to its higher work hardening exponent. • The fabrication of Cu/Al/Ni, Cu/Al/Ni/SiC(2 μm) and Cu/Al/Ni/SiC(15 μm) laminated composites was studied. • Effects of strain accumulation on all layers and the role of two sizes of SiC particles during four rolling passes of ARB process was investigated. • With increasing the strain accumulation, the ultimate strength of all three composites fluctuated due to the plastic instability of layers and the distribution of particles. • Reinforcing particles appeared in cluster shape at the interface of Al/Ni and Cu/Al because of being displaced by applied stress due to their small size and freedom of movement. Also, 15-μm particles were more trapped in Al layers than 2-μm trapped. • With increasing the strain accumulation, the elongations of composites decreased. • By increasing the number of ARB passes, thermal conductivity of composites dropped though at each pass Cu/Al/Ni/SiC(15 μm) had the highest values because of forming clusters at the interfaces.

Hot working behaviour and processing maps of duplex cast steel

Abstract In this paper the hot working behaviour of medium carbon duplex cast steel is studied using uniaxial hot compression tests over a temperature range varying from 700 ˚C to 1 000 °C and at different strain rates ranging from 10–4 to 10–1 s–1. A model based on a variant of a dynamic materials model was employed to construct processing maps. These maps delineate the safe and unsafe domains. The safe domains, associated with dynamic recrystallization and dynamic recovery, can be chosen to optimize the hot workability of the studied material. Whereas, the unsafe domain is to be avoided because it is associated with plastic deformation instabilities. The domain associated with dynamic recrystallization is centred at 1000 °C and 10–4 s–1 with a peak energy dissipation efficiency of about 40%, while the domain associated with dynamic recovery is centred at 700 °C and 10–4 s–1 with a peak energy dissipation efficiency of about 27%. The unsafe hot working domain, spread over the entire temperature range and moderate to high strain rates, predicts the appearance of flow instabilities, in the form of shear bands and intergranular cracks. To validate the obtained results, microstructural observations corresponding to different processing conditions are presented.

Significance of surface layer integrity for sustaining the ductility of gradient-structured nickel

Gradient-structured nickel (Ni) was processed by rotationally accelerated shot peening (RASP). The microstructures and mechanical properties of the RASP-processed samples were studied. Nanostructured layers with a thickness of ~50 μm were formed on the sample surfaces. Beneath the surface layer there is a transition region in the depth range of ~50–150 μm. The transition region contains high densities of dense dislocation walls (DDWs), microbands (MBs), sub-grain boundaries and dislocations, and is susceptible to plastic instability. High-speed RASP processing created some small cracks which deteriorate the integrity of the nanostructured surface layer. During tensile deformation, local stress concentrations are created beneath the surface cracks to trigger plastic instability in the nearby transition region. As a result, interior cracks nucleated and grew in the transition region, and eventually agglomerated with surface cracks to cause premature failure of the bulk material.

Examining deformation localization of irradiated tungsten under uniaxial compression with crystal plasticity

In this work, the deformation behavior of irradiated tungsten at elevated service temperatures is examined using mechanism-based crystal plasticity finite element framework by considering multiple mechanisms, including thermal softening and irradiation hardening. The previously developed mechanism-based crystal plasticity model is first utilized to assess the yield stress of unirradiated and irradiated tungsten upon low-temperature irradiation. The engineering stress-strain response under uniaxial compression is then predicted by considering the effect of irradiation-induced defect clusters on the motion of deformation-induced dislocations, and the predicted results agree well with the experimental measurements. More importantly, the formation and evolution of plastic strain localization zones in the form of shear bands are qualitatively captured, and the maximal principal strain distribution is quantitatively linked to the dose of irradiation damage. Finally, the deformation-induced shear band as a result of plastic instability in the compressed tungsten specimen is discussed.

Separation of Time Scales of Plastic Instabilities During Microscale Deformation of bcc Fe

Separation of Time Scales of Plastic Instabilities During Microscale Deformation of bcc Fe

Study on the Time-Dependent Mechanical Behavior and Springback of Magnesium Alloy Sheet (AZ31B) in Warm Conditions.

In this study, the time-dependent mechanical behavior of the magnesium alloy sheet (AZ31B) was investigated through the creep and stress relaxation tests with respect to the temperature and pre-strain. The microstructure changes during creep and stress relaxation were investigated. As the tensile deformation increased in the material, twinning and dynamic recrystallization occurred, especially after the plastic instability. As a result, AZ31B showed lower resistance to creep and stress relaxation due to dynamic recrystallization. Additionally, time-dependent springback characteristics in the V- and L-bending processes concerning the holding time and different forming conditions were investigated. We analyzed changes of microstructure at each forming temperature and process. The uniaxial tensile creep test was conducted to compare the microstructures in various pre-strain conditions with those at the secondary creep stage. For the bending process, the change of the microstructure after the forming was compared to that with punch holding maintained for 1000 s after forming. Due to recrystallization, with the holding time in the die set of 60 s, the springback angle decreased by nearly 70%. Increased holding time in the die set resulted in a reduced springback angle.

Understanding imprint formation, plastic instabilities and hardness evolutions in FCC, BCC and HCP metal surfaces

Nanoindentation experiments in metal surfaces are characterized by the onset of plastic instabilities along with the development of permanent nanoimprints and dense defect networks. This investigation concerns massive molecular dynamics simulations of nanoindentation experiments in FCC, BCC and HCP metals using blunted (spherical) tips of realistic size, and the detailed comparison of the results with experimental measurements. Our findings shed light on the defect processes which dictate the contact resistance to plastic deformation, the development of a transitional stage with abrupt plastic instabilities, and the evolution towards a self-similar steady-state characterized by the plateauing hardness p p at constant dislocation density ρ p . The onset of permanent nanoimprints is governed by stacking fault and nanotwin interlocking, the buildup of nanostructured regions and crystallites throughout the imprint, the cross-slip and cross-kinking of surfaced screw dislocations, and the occurrence of defect remobilization events within the plastic zone. As a result of these mechanisms, the ratio between the hardness p p and the Young's modulus E becomes higher in BCC Ta and Fe, followed by FCC Al, HCP Mg and large stacking fault width FCC Ni and Cu. Finally, when nanoimprint formation is correlated with the uniaxial response of the indented minuscule material volume, the hardness to yield strength ratio, p p / σ ys , varies from ≈ 7 to ≈ 10, which largely exceeds the continuum plasticity bound of ≈ 2.8. Our results have general implications to the understanding of indentation size-effects, where the onset of extreme nanoscale hardness values is associated with the occurrence of unique imprint-forming processes under large strain gradients.

Plasticity in Amorphous Solids Is Mediated by Topological Defects in the Displacement Field

The microscopic mechanism by which amorphous solids yield plastically under an externally applied stress or deformation has remained elusive in spite of enormous research activity in recent years. Most approaches have attempted to identify atomic-scale structural "defects" or spatiotemporal correlations in the undeformed glass that may trigger plastic instability. In contrast, in this Letter we show that the topological defects that correlate with plastic instability can be identified, not in the static structure of the glass, but rather in the nonaffine displacement field under deformation. These dislocation-like topological defects (DTDs) can be quantitatively characterized in terms of Burgers circuits (and the resulting Burgers vectors) that are constructed on the microscopic nonaffine displacement field. We demonstrate that (i)DTDs are the manifestation of incompatibility of deformation in glasses as a result of violation of Cauchy-Born rules (nonaffinity); (ii)the resulting average Burgers vector displays peaks in correspondence of major plastic events, including a spectacular nonlocal peak at the yielding transition, which results from self-organization into shear bands due to the attractive interaction between antiparallel DTDs; and (iii)application of Schmid's law to the DTDs leads to prediction of shear bands at 45° for uniaxial deformations, as widely observed in experiments and simulations.

Mechanism of microweld formation and breakage during Cu–Cu wire bonding investigated by molecular dynamics simulation

Currently, wire bonding is the most popular first-level interconnection technology used between the die and package terminals, but even with its long-term and excessive usage, the mechanism of wire bonding has not been completely evaluated. Therefore, fundamental research is still needed. In this study, the mechanism of microweld formation and breakage during Cu–Cu wire bonding was investigated by using molecular dynamics simulation. The contact model for the nanoindentation process between the wire and substrate was developed to simulate the contact process of the Cu wire and Cu substrate. Elastic contact and plastic instability were investigated through the loading and unloading processes. Moreover,the evolution of the indentation morphology and distributions of the atomic stress were also investigated. It was shown that the loading and unloading curves do not coincide, and the unloading curve exhibited hysteresis. For the substrate, in the loading process, the main force changed from attractive to repulsive. The maximum von Mises stress increased and shifted from the center toward the edge of the contact area. During the unloading process, the main force changed from repulsive to attractive. The Mises stress reduced first and then increased. Stress concentration occurs around dislocations inthe middle area of the Cu wire.

Kinetics of strain-induced phase transformation in TRIP steel

This paper discusses the kinetics of the fronts of localized plastic deformation in TRIP steel using a combination of DIC methods and magnetization measurements. It is shown that the kinetics of deformation fronts, which is mainly determined by the stages of the hardening curve, is consistent with changes in the phase composition. A phase transformation starts with the propagation of Lüders bands and then proceeds through the formation and propagation of Portevin–Le Chatelier bands, but only in the region of increasing strain hardening coefficient. After the transition to the stage where the hardening coefficient decreases, the γ → αʹ phase transformation decays and is no longer the predominant mechanism of jerky flow. Both types of plastic instability in TRIP steel can be described using the autowave concept of localized plasticity.

DIC-based experimental study of fracture toughness through R-curve tests in a multi-layered Al-Mg (LZ91) composite fabricated by ARB

Adding Li to Mg, dual-phase Mg-Li with very low density and high formability at room temperature was produced and used as a reinforcement phase for fabricating a three-layered metal matrix Al-Mg composite through accumulative roll bonding (ARB) process. In this study, due to the inherent nature of the ARB process, which is associated with high strain, and low strength of the magnesium-lithium alloy, the fracture toughness test and energy absorption were used in addition to the tensile test to observe the interaction effect of the increasing strength and the reducing elongation, which were caused by the ARB. By preparing the compact tension (CT) specimens and conducting R-curve tests, the fracture behavior of the processed composite in various ARB cycles (primary sandwich and four ARB cycles) was investigated. Meanwhile, a Digital Image Correlation (DIC) system was also employed to measure the variation of crack growth, crack opening, and corresponding forces and increase the accuracy and quality of the results. The surrounded area under the force-displacement of crack opening curves was measured and calculated as the toughness and absorbed energy. Tensile strength and total elongation of the processed composite improved gradually, 275.7% and 38%, respectively, compared to the primary phase as the number of cycles increased and the thickness of the layers decreased. Plastic instability was more prominent in the last cycles because of the increasing trend of imposed strains and the reduction in thickness which reached their highest amounts at the final step. Although obtained results confirmed the general improvement in fracture toughness of the ARBed composite, its value varied slightly from one cycle to another. The highest values were achieved for total energy absorption and crack opening at maximum force after the second ARB cycle. In contrast, the maximum amount of fracture toughness, 29.08 MPa m1/2, was belonged to the last cycle of the process. Dimples and micro-cavities, seen in both SEM images of Al and Mg layers, resulted in a ductile failure.

Universality of plastic instability and mechanical yield in metallic glasses

The generic response of a wide range of amorphous solids is the average increase of stress upon external loading until the yielding transition point, after which elasto-plastic steady state sets in. The stress–strain response comprises of a series of elastic branches interspersed with plastic drops. The ubiquitousness of these phenomena indicates universality, independent of material property, but the literature predominantly deals with specific materials. In pursuit of generality among different amorphous systems, we undertake a careful investigation in the mechanical response of metallic glasses using computer simulation. By comparing our results of multi-body metallic glass potentials to those obtained from pairwise Lennard-Jones glasses, we show that the mechanism of plastic instabilities is universal and independent of the details of the underlying potential. We also investigate the yielding transition in terms of the overlap parameter Q 12, which has been successfully used Lennard-Jones glasses. The yielding is unambiguously identified as a first-order phase transition. These observations conform the nature of plastic instabilities and mechanical yield as universal and independent of microscopic interactions.

Hot deformation behavior and related microstructure evolution in Au−Sn eutectic multilayers

Hot compression was performed on a multilayered Au−20Sn eutectic alloy to investigate the deformation behavior and microstructure evolution. During hot compression, microstructural spheroidization was initiated from plastic instability regions, and it was preferentially activated in vertical lamellae with a growth direction parallel to the compressive direction. Continuous dynamic recrystallization associated with lattice dislocations was the mechanism in both AuSn and Au5Sn multilayers. After spheroidization, strain accumulations were weakened in both of the equiaxed phases, and the deformation mechanism was substantially replaced by grain boundary sliding and migration. Based on these findings, hot rolling was conducted on an as-cast Au−20Sn alloy and a foil with a thickness of ~50 μm was successfully prepared. The present study can promote the development of Au−20Sn foils, and provide insights into the deformation behavior and microstructure evolution of multilayered eutectic alloys.

In-situ investigation of strain partitioning and microstructural strain path development up to and beyond necking

Multi-phase metallic materials exhibit significant levels of strain partitioning and localization when plastically deformed. Connecting these microstructural processes to macroscopic limits of uniform deformation, e.g., plastic instability and fracture, can reveal guidelines for damage-resistant microstructure design. This connection, however, is hindered due to the absence of strain partitioning and localization data from high strain levels, near and beyond necking. A parallel gap in our current understanding of the plasticity of multi-phase materials is regarding the spatial variations in strain path development at the micro-scale. Microscopic digital image correlation, the most powerful experimental method to provide such insights, is typically limited to low strain levels (< ~15%) due to crystal-plasticity- or damage-induced surface topography evolution. Here, we address these challenges by developing a practical method that relies on serial in-situ scanning electron microscope (SEM) mechanical tests of samples pre-strained to different levels. Applying this method to a dual-phase steel, we observe that ferrite and martensite on average exhibit a linear strain partitioning trend throughout the deformation. However, the highly-deformed ferrite regions (i.e., the top 4% most strained ferritic subsets) exhibit an exponential increase in the level of strain they accommodate, as well as sequential activation of the strain localization processes in different microscopic strain paths. Martensitic constituents play an important role in the strain localization processes and the resulting microscopic strain paths.

Designing ultrastrong maraging stainless steels with improved uniform plastic strain via controlled precipitation of coherent nanoparticles

The development of ultrastrong maraging stainless steels (MSSs) is always in high demand. However, traditional high-strength MSSs generally exhibit early plastic instability with a low uniform strain since the precipitated nanoparticles are non-coherent with the body-centered-cubic (BCC) lath martensitic matrix. Here, we design a novel ultrahigh strength MSS (Fe-5.30Cr-13.47Ni-3.10Al-1.22Mo-0.50W-0.23Nb-0.03C-0.005B, wt.%) using a cluster formula approach. A fabulous microstructure consisting of a uniform distribution of high-density coherent B2-NiAl nanoprecipitates (3−5 nm) in BCC martensitic matrix was successfully obtained. This alloy has not only an exceedingly high ultimate tensile strength of 2.0 GPa, but also a decent uniform elongation of 4.2%−5.1%, which is almost triple of the value observed in existing MSSs. We present an in-depth discussion on the origins of ultrahigh strength and uniform plastic strain in the new alloy to validate our design strategy and further offer a new pathway to exploit high-performance MSSs.

The Portevin-Le Chatelier (PLC) effect in an Al-Cu aluminum alloy fabricated by selective laser melting

An Al-Cu alloy fabricated by selective laser melting (SLM) was chosen in this study for the investigation of Portevin-Le Chatelier (PLC) effects. The effects of the process parameters, the building orientation, and the heat treatment were studied to reveal the correlation between the microstructure and the plastic instability. Serrated flow can be observed in all the as-built specimens due to the high concentration of the solute atoms and high dislocation density generated by the high cooling rate during the process. Strong anisotropy in PLC effects has been observed in the as-built samples, which is attributed to the grain structure and the texture. PLC effects changes a lot after the heat treatments. The solution treatment decreases the dislocation density. Hence, the PLC effects disappear. θ” nucleated at 446 K can delay the onset of the PLC effects. When aged at 466 K, with the absence of θ”, the onset strain of the PLC effect with the same duration is lower. As the aging duration increases from 1 h to 6 h, PLC effects become weaker and finally disappear. The concentration of the solute atom and the dislocation density decrease as the aging duration increases. Both aspects are responsible for the attenuation of PLC effects.

Prediction of Anisotropic Elastoplastic Instability with Rice’s Criterion in Plane Compression

Objectives: The main objective of this work is the development of a mechanically coherent framework for the implementation of numerical simulation tools for sheet metal forming processes. The work at hand is part of a study of the mechanical behaviour of metallic materials subjected to large elastoplastic deformations during channel-die testing and a study of the mechanisms that limit their formability and cause their instability. We analyse the effect of induced rotation on the appearance of plastic instability predicted by Rice’s criterion. Methods: At the beginning, an adequate modelling of the mechanical behaviour of metals in large elasto-plastic deformations using the formalism in rotational referential is made. Then, a prediction of plastic instabilities is developed using Rice’s bifurcation criterion. Indeed, the integration of the law of anisotropic elastoplastic behaviour is presented in a three-dimensional kinematics framework in order to write the deformation tensor as a superior triangular matrix, with Hill’s anisotropic yielding function, using Runge Kutta’s method to integrate the obtained equations. Findings: we were able to show that Rice’s criterion can be used for any kind of material and that this criterion is effective in predicting what happens in metals. It predicts shear-band bifurcation in a continuous plastic medium. Hill’s criterion is suitable for what is weakly anisotropic. The model described above represents a good mathematical framework for analysing the behaviour of materials and plastic instability. Novelty: This study allows to apply Rice’s criterion in the case of a channel-die compression test, so as to locate plastic instability and see the appearance of shear bands for all types of materials; it demonstrates that the initial rotation of the material influences the appearance of shear-bands during a test simulating rolling. Keywords: Elastoplastic; Large deformations; Anisotropy; Plastic instability; Localisation

Mechanical properties and deformation mechanisms of a Ni2Co1Fe1V0.5Mo0.2 medium-entropy alloy at elevated temperatures

A Ni2Co1Fe1V0.5Mo0.2 medium-entropy alloy (MEA) with a single-phase face-centered cubic (fcc) crystal structure are casted. The as-cast MEA possesses remarkable work hardening ability, yield strength (σy), ultimate tensile strength (σUTS) and uniform elongation (σu) at high temperatures up to 800 °C. Up-turn of strain hardening rate after elasto-plastic transition (secondary strain hardening) is observed for the MEA during tensile deformation in the temperature range of 25-800 °C. A high density of dislocation forests with solute atmospheres in concentrated solid solution is considered sufficient for back stress strengthening, thus causing the up-turn of strain hardening rate. Post-mortem microstructural analysis reveals that the temperature effects on mechanical properties of the MEA are closely related to dislocation structures, dislocation densities and dynamic-strain aging (DSA). With increasing deformation temperature from 25 °C to 800 °C, the dislocation density undulates and the fraction of screw dislocations increases. Strong solute pinning effect in conjunction with dominant forest strengthening mechanism (indicated by negative strain rate sensitivity and large activation volume) boost the strain hardening rate at the temperatures from 400 ℃ to 700 ℃. At 800 °C, the dominant deformation mechanism changes from the forest dislocation cutting mechanism to dislocation cross-slip with increasing strain, resulting in the sharp up-turn of strain hardening rate. On the other hand, DSA poses plastic instability to deteriorate the strain hardening of the MEA. These phenomena are considered to have material impacts on mechanical properties and fracture mechanisms of MEAs and high-entropy alloys at high temperatures.

Hot Deformation Constitutive Equation and Plastic Instability of 30Cr4MoNiV Ultra-High-Strength Steel

In this work, the hot deformation behavior of 30Cr4MoNiV ultra-high-strength steel is investigated by isothermal compression tests using the Gleeble-3500 thermal simulation machine (Dynamic Systems Inc., New York, NY, USA) at a temperature range of 1173–1373 K under the strain rate of 0.01–10 s−1. A constitutive equation with strain-dependent constants and processing maps suitable for 30Cr4MoNiV ultra-high-strength steel are established. The results show that the combination of the hyperbolic sine function and the Zener-Hollomon parameter can accurately represent the influences of deformation temperature, strain rate, and strain on the flow stress of the 30Cr4MoNiV ultra-high-strength steel. The applicability of plastic instability criteria such as , , , and the instability parameter ξ are analyzed, the stability and instability regions are clarified accurately, and the optimized processing regions are given in the stability regions. The optimized regions are verified by the uniform equiaxed grains, and the plastic instability is validated by dynamic strain aging and the microstructure of the voids.

Dynamic plastic instability of ring-shaped aluminum alloy with different interface behaviors

Unveiling the rate-dependent shear instability mechanism is essential in evaluating the reliability and safety of materials under dynamic loadings. Herein, the strain rate-dependent plastic instability behaviors of 2A12 aluminum alloy with ring-shaped structure and different surface states were systematically investigated and analyzed. The stress softening was displayed in the stress-strain curves as the loading rate increased to a high level. The lubricated interface will induce the prominent shear instability property, followed by the fine-polished interface and coarse-polished interface. It is revealed that the initial interface state between specimen and bars not only influences the onset strain and the yield strength, but also determines the duration of plastic instability stage. The simulation results also indicated that the shear band runs through the two surfaces with different interface friction coefficients. The minor disturbance under quasi-static loading will be accelerated under dynamic loading owing to the instantaneous rising of local temperature, which eventually causes the observably plastic shear instability. The critical state of shear band formation mechanism and the modeling of the stress-softening stage were then theoretically analyzed and discussed. This work on the dynamic instability behavior of ring-shaped aluminum alloy could provide an insight in understanding the formation of adiabatic shear band with different strain rates and interfaces.