Homogeneous Plastic Deformation Research Articles

Exploring Refinement Characteristics in FeTi‐Cux Composites: A Study of Localization and Abrasion Constraints

FeTi‐Cu composites with varying Cu contents are subjected to high‐pressure torsion, and their deformation behavior is explored systematically using scanning electron microscopy, microhardness, and nanoindentation. The study identifies the limiting factors influencing the refinement during severe plastic deformation. The pronounced strength differences between phases lead to fragmentation primarily through hard‐hard (FeTi‐FeTi) contact points, promoted by homogeneous, i.e., non‐localized, and possibly turbulent material flow. These conditions are prevalent in Cu‐rich composites and during high‐temperature deformation. Conversely, Cu‐lean composites exhibit deformation localization, hindering the fragmentation process. Abrasion becomes an efficient refinement mechanism at the submicron‐/nano‐scale, particularly for composites containing higher concentrations of nanocrystalline FeTi and exhibiting homogeneous plastic deformation. Consequently, deformation localization in Cu‐lean composites inhibits both refinement mechanisms, while Cu‐rich compositions and higher temperatures result in efficient refinement but at the risk of coarsening at the nano‐scale. Refinement is localization‐limited in the former case and abrasion‐limited in the latter. Optimized processing conditions can overcome these constraints, yielding a uniform nanocomposite. This study sheds light on the intricate interplay of the mechanical properties of the respective phases in a composite, emphasizing the importance of tailored compositions and deformation conditions to optimize nanocomposites, particularly when dealing with challenging material pairings.This article is protected by copyright. All rights reserved.

Work hardening and X-ray diffraction studies on ASS 304 at high temperatures

Abstract One of the most common characteristics of metallic alloys is work hardening, which is most beneficial as it is the primary reason for the alloys’ tenacity to withstand loading even in the presence of internal flaws or geometrical errors. Thus, the work hardening coefficient gives the maximum amount of homogeneous plastic deformation in tensile straining. Thus, complex-forming operations are facilitated by a high coefficient without experiencing premature failure. Naturally, work hardening has a significant impact on the mechanical energy required to shape a material by plastic deformation, such as rolling, forming, etc. The quantity of energy that the material stores during plastic deformation is also managed by work hardening. As a result, it significantly influences how the metal behaves when it is subsequently softened during annealing. Finally, the hardening capacity and durability of the work hardened state are significant practical challenges because many high-volume stretch formed components are directly used. Typically, the current study begins, at homologous temperatures above 0.4 times melting point, with a description of work hardening at 700, 800, and 900°C temperatures in three different orientations with respect to rolling direction R 0, R 45, and R 90 and 10−1−10−3 s−1 strain rates, where thermally triggered processes exhibit a prominent role in work hardening. Three stages of behavior were identified by analyzing the tensile work hardening of ASS 304 steel. Dynamic strain aging is the cause of the anomalous fluctuation in the work hardening rate that is seen in hot working temperatures. X-ray diffraction examination is conducted to introspect any phase changes occurring in hot working regions improving plasticity of ASS 304.

High-temperature wear resistance improvement mechanism of TiAlCrNiSiNbx high-entropy alloy films through sliding wear-induced gradient nanostructure

TiAlCrNiSiNbx high-entropy alloy films (HEFs) with varying Nb concentrations were deposited using multi-target magnetron co-sputtering at a controlled sputtering power. At room temperature (RT), 300 ℃, 500 ℃ and 700 ℃, the average coefficients of friction (COF) and wear rates of TiAlCrNiSNbx HEFs were lower than those of TC4 alloy. TiAlCrNiSiNbx HEF with a Nb content of 12.36 at% exhibited the best wear resistance at 700 ℃. A nanocomposite layer (∼260 nm) and a plastic deformation region (∼1.2 µm) formed on the film during the wear at 700 ℃. The Al2O3, Cr2O3, SiO2, and Nb2O5 in the nanocomposite layer facilitated the formation of a dense oxide layer. The cooperative deformation effect of nanocrystalline metal oxides and amorphous phases is conducive for inhibiting strain localization and enhancing wear resistance. The grain size of the nanocomposite layer was significantly refined, with an average size of 3.98 nm. This was attributed to the grain refinement caused by the interaction of dislocations and deformation twins in the plastic deformation region, which improved the strength and hardness of the film. The gradient nanostructure of the nanocomposite layer and plastic deformation region led to a more homogeneous plastic deformation, effective inhibition of strain localization and suppression of the micro-crack.

Strong resistance to shear instability in multilayered metallic composites by nanoscale amorphous-BCC crystalline interfaces

Experiments have shown that high strength and high plastic stability are difficult to be simultaneously obtained in crystalline-amorphous layered composites. Here we report a complete suppression of plastic instability in a strong multilayered composite via nanoscale BCC Nb crystalline-amorphous CuNb interface architecting. Specifically, the size-dependent hardness and shear instability of the designed nanolayered composites are systematically investigated with individual layer thickness (h) ranging from 2 to 100 nm by nano/micro-indentation tests. A mechanics model is also proposed to quantify the shear instability. A non-monotonic shear instability behavior is revealed with decreasing h. When h > 40 nm, a cutting-like shear banding prevails in the composites and the shear instability reduces with decreasing h. Finally, a homogeneous plastic deformation without shear banding is achieved at h of 40 nm. For h < 40 nm, a kinking-like shear banding occurs in the composites and the shear instability increases with reducing h. Further theoretical analysis reveals a critical layer thickness for activating homogeneous deformation in the composites, i.e., 45 nm, above and below which the shear deformation would be dominated by cutting-like and kinking-like shear banding, respectively. The cutting-like shear banding is attributed to the propagation of the mature shear band formed in the amorphous phase that penetrates the crystalline layer. The homogeneous plastic deformation can be ascribed to the activation of the confined layer slip mechanism in the crystalline layer and the non-clustered shear transformation zones initiated in the amorphous layer. The kinking-like shear banding below 40 nm may originate from the cooperative kinking of the constituent layers with decreasing h. The potential advantage of the ductile BCC metals over FCC ones for suppression of shear instability in crystalline-amorphous composites is discussed based on the unique dislocation activities in nanoscale BCC crystals (e.g., self-multiplication and significant cross-slipping mechanisms). The present study sheds some light on designing strong and ductile crystalline-amorphous composites by selecting an appropriate BCC crystalline phase and an optimal interface spacing to eliminate plastic instability.

A novel dealloying assisted machining method to improve the machinability of NiTi alloy as a typical high toughness difficult-to-machine material

High quality and high efficiency cutting process is of great significance to the components manufacturing related to machinability of workpiece materials. As a typical difficult-to-machine material induced mainly by high toughness, NiTi alloy faces challenges such as high cutting force and difficult chip breaking during the cutting process. To solve these problems, this paper presents a novel surface dealloying assisted machining method, i.e., the preparation of nanoporous structures on the workpiece surface with dealloying treatment, to improve the machinability of NiTi alloy. Based on high speed in-situ imaging of material removal process, the presence of dealloyed layer leads to a segmented flow mode instead of homogeneous severe plastic deformation without dealloying treatment. The change in material removal mode is beneficial to decrease the cutting force and energy and improve the machined surface quality. The embrittlement effect of dealloyed layer inhibits material plastic flow and promotes periodic crack nucleation and propagation. An analytical model is developed to characterize the brittle crack propagation, which provides theoretical basis for explaining embrittlement effect of dealloyed layer. This work verifies the feasibility and effectiveness of the proposed method to modify the machinability of NiTi alloy and provides new ideas for the development of manufacturing processes.

Phase transformation and dislocation behavior dependence on cooling rate in novel refractory high entropy alloy

A newly named “NbSi-containing refractory high-entropy alloy (RHEA)” is proposed using the Nb–Si based alloy insight. Light-weight TiNbMo0.5Al0.225Six RHEAs were designed (6–6.67 g/cm3), then we studied the dependence of cooling rates on the phase transformation, and the deformation mechanism at the room, high temperatures. Two portions of phase transitions were observed, β-(Nb, Ti)5Si3 to γ-(Nb, Ti)5Si3, and BCC solid solution (BCCss) to γ’-(Nb, Ti)5Si3 precipitates. Elemental diffusion drives the splitting of the grain boundary grooving into sub-boundary, accompanied by phase transformation that occurs. An appropriate Si content favors an excellent strength-plasticity synergy, resulting in 1763.58 MPa compressive strength and 23.65% compressive strain at room temperature (RT), also exhibiting superior yield strength of 1444.82 MPa and relevant strain of 17.48% at elevated temperature (800 °C). The high-temperature dislocation behavior of quenched alloy, evolves from cross-slip (as-cast alloy) to climb-slip coupled deformation mechanism, which plays a crucial role in homogeneous plastic deformation. Therefore, the current work having designed novel alloys sheds new light on designing nanoprecipitates and diversifying dislocation modes.

Significant “smaller is softer” in amorphous silicon via irradiation-mediated surface modification

“Smaller is softer” is a reverse size dependence of strength, defying the “smaller is stronger” tenet. It usually results from surface-mediated displacive or diffusive deformation and is mainly found in some ultra-small-scale (below tens of nanometers) metallic materials. Here, making use of the surface modification via ion beam irradiation, we bring the “smaller is softer” into being in a covalently-bonded, hard, and brittle material-amorphous Si (a-Si) at a much larger size regime (< ∼500 nm). It is manifested as the transition from the quasi-brittle failure to the homogeneous plastic deformation as well as the decreasing yield stress with sample volume reduction at the submicron-scale regime. An analytical model of hard core/superplastic shell has been proposed to explain the artificially-controllable size-dependent softening. This surface engineering pathway via ion irradiation is not only of particular interest to tailor the strength and deformation behaviors in small-sized a-Si or other covalently-bonded amorphous solids but also of practical relevance to the utility of a-Si in microelectronics and microelectromechanical systems.

Mechanical stability and microstructural evolution during torsion in pristine and defect nickel nanowires of different orientations: a molecular dynamics simulation study

ABSTRACT In the present study, torsional deformation behaviour and microstructural evolution in perfect and defect nickel nanowires (NWs) of different orientations is investigated. The studies are performed using molecular dynamics (MD) simulations. The torsion test is carried out at temperatures of 300 and 1200 K by applying a torque load of 0.02° ps−1. At the temperature of 300 K, the orientation NW yields at a higher torsional angle (θ [010] = 55.6°); than the NWs with ( = 39.8°) and (θ [111] = 48.8°) orientations. The orientation NW shows homogeneous plastic deformation by emitting coaxial dislocations throughout the length of the NW, whereas it is found to be heterogeneous in the other orientations. Twist boundaries are formed in [010] and in [111] orientations which are a rectangular array of dislocations in the former and a hexagonal array of dislocations in the latter orientations. Defects lower the torsional angle (critical) at which yielding occurs and is significant in the NWs when the defects are nearer to the surface. Furthermore, defect NWs are found to show pronounced strain localisation. The size effect studies show smaller is stronger, and the critical angle decreases with an increase in the temperature of deformation.

Enhancement of tensile plastic stability of Ti-based metallic glass composite with substrate of titanium alloy

The tensile plastic stability of Ti-based metallic glass composites (MGCs) with substrates of titanium alloys was investigated. The findings have shown that the tensile plastic stability of MGC bimetals was significantly dependent on both the yield strength and geometrical dimension of the substrate. The substrate of high-strength Ti6Al4V (T64) alloy can enhance the ductility of the MGC greatly, while the MGC bimetal combined with low-strength pure titanium (PT) displayed worsen plastic stability. Further results showed that the plastic stability of the MGC was enhanced remarkably by increasing the thickness ration of the PT substrate. The enhancement of plasticity stability in the bimetals were explained and discussed from the perspective of normalized strain-hardening rate. These findings indicates that the plastic stability of MGCs can be enhanced by suitable reinforcer with optimized geometrical dimension. • The plastic deformation of Ti-based metallic glasses strongly depends on the mechanical properties of the substrates. • Homogeneous plastic deformation under tension can be realized in the MGC bimetals with substrate of titanium alloy. • The plastic stability of MGC bimetals can be explained by the concept of normalized strain-hardening rate.

Two-temperature thermodynamics and a viscoplasticity model for FCC metals with grain boundary effect

A non-equilibrium thermodynamic route is pursued to model viscoplasticity, including the evolving grain boundary, in face-centered cubic (FCC) metals. The thermodynamic description includes two subsystems: the configurational subsystem describing the relatively slower motion of dislocations and grain boundaries, and a fast-evolving kinetic-vibrational (K-V) subsystem depicting atomic vibrations about equilibrium positions in the lattice. Grain boundary and dislocation densities as well as their interactions are incorporated within the model. The model is initially developed for homogeneous plastic deformation, which is then generalized to the inhomogeneous case via spatial variations in the thermodynamic states over a wide range of strain rate and temperature. The model is implemented as a user material subroutine (VUMAT) in ABAQUS to simulate a problem of industrial interest, viz. deep drawing on an AA 7075 aluminum alloy. Finally, the thermo-viscoplasticity model is used to predict negative strain rate sensitivity in the AA 2219 aluminum alloy at room temperature, for which an experiment is performed at a facility of the VSSC. The predictive quality of the model is demonstrated by a comparison of numerical simulations with the experimental evidence.

Study on the Fracture Mechanisms of a Grade 91 Steel at 600°C via Interrupted Miniature Tensile Testing

Abstract The study of the deformation and fracture mechanisms of power plant materials at elevated service temperature is of utmost importance in structural integrity assessment. In this work, interrupted miniature uniaxial tensile tests (MUTT) have been conducted to investigate the fracture evolution mechanisms for a Grade 91 steel at 600°C. The engineering stress–strain curves of the MUTT can be divided into six different stages, including the elastic stage, strain hardening stage, homogeneous plastic deformation stage, necking across the transverse and thickness direction stage, and finally, the fracture propagation stage. In addition to full history tests, MUTT were interrupted in each of the different stages to study the entire failure process evolution from the uniform deformation, necking to specimen separation, using optical microscopy and Scanning Electron Microscope (SEM). It has been shown that the necking of the interrupted testing specimens appears at Stage IV across the width direction prior to the thickness direction at Stage V. The thickness direction necking would lead to fracture rapidly. The void coalescence directly caused by damage has been observed in Stage VI.

History dependent plasticity of glass: A mapping between atomistic and elasto-plastic models

Mesoscale elasto-plastic models, with statistically distributed structural properties and elastic coupling between discrete blocks, have been shown to quantitatively reproduce the main phenomenology observed in the stationary flow state of glasses as modelled at the atomic scale [1]. In the present study, an extension of such approaches is proposed to describe the transient mechanical response of glasses from different off-equilibrium states in the athermal quasi-static limit. Equilibrated liquids are simulated using two-dimensional molecular dynamics, quenched instantaneously to zero temperature, and then sheared. The mechanical observables measured in atomistic and elasto-plastic models are compared at the same length scales to calibrate a state-dependent constitutive law. A physical mechanism is proposed where the structural properties’ evolution rate depends on the magnitude of local plastic deformation events, introducing an effective local memory of previous states in the system. This mechanism naturally leads to a brittle-ductile transition in the mechanical response of glasses, which depends exclusively on the quenched structure. Specifically, initially stable glasses exhibit strain-softening and localization, where the memory of the initial states is lost abruptly after the first plastic rearrangements. On the other hand, systems quenched from high-temperature liquids show a slow strain-hardening with statistically homogeneous plastic deformation. In these initially soft glasses, numerous plastic rearrangements are required to converge toward the stationary flow state. The elasto-plastic model successfully reproduces the stress-strain curves in the transient regime for the whole range of parent temperatures by including this local memory mechanism. The limitations of the model are finally discussed, together with possible improvements.

Corrosion and mechanical behavior of amorphous-nanocrystalline NiCrMo coatings

To develop a high-performance coating for marine engineering equipment, we successfully designed amorphous-nanocrystalline NiCr0.25Mox (x = 0, 0.02, 0.05, 0.1, 0.2, 0.4) alloy coatings via direct current (DC) magnetron sputtering. The compositional, microstructural, corrosion and mechanical properties, specifically the relationship between the coating structure and performance were discussed in detail. The results showed that the amorphous phase sharply alleviated grain boundary corrosion, and the addition of Cr and Mo was beneficial to improve repassivation ability with the formation of a dense passive film. In the nanoindentation and micro-scratch tests, the nanohardness and adhesion strength of the coatings first increased remarkably with Mo content and then declined due to the formation of an amorphous phase. The enhancement of strength was caused by nanocrystals hindering the formation of shear banding, while the addition of Mo improved the tendency of amorphous formation due to the difference in atomic size and lattice distortion. Most importantly, the NiCr0.25Mo0.1 coating exhibited excellent corrosion performance and mechanical behavior through the formation of a dense passivation film composed of Cr2O3 and Cr(OH)3. These ductile solid solution crystalline phases promoted homogeneous plastic deformation and decelerated crack propagation during the scratching process.

Shear band development under simple shear and the intrinsicality of strain softening of amorphous glassy polymers

The development of shear bands that induced by localized strain inhomogeneity is a characteristic deformation mechanism and determines ductility of amorphous glassy polymers. To investigate the essence of intrinsic strain softening and its influence on shear band evolution under shearing and reverse shearing, a full-network constitutive model based on the micro-mechanism is implemented into the material point method and finite element method respectively. The results show that the shear band nucleates and propagates in forward shearing, and gradually fades until disappears in following reverse shearing when the specimen regains its original geometry. As reverse shearing proceeds or there is a second forward shearing, the homogeneous plastic deformation occurs preferentially in the previously deformed band but it doesn't display any shear band like the initial forward shearing. It means the glassy polymers undergoing plastic deformation cannot be recovered by reverse straining. In further shearing, new shear bands tend to be initiated outside the existing band and then propagates to merge with the previous band. The initiation and propagation of shear band along the shearing direction is caused by the intrinsic strain softening, independent of loading histories and boundary conditions. The widening of shear band is related to the strain hardening. • Origin of intrinsic strain softening lies in the saturation of plastic shear strain. • Amount of intrinsic strain softening keeps fixed. • Glassy polymers undergoing plasticity cannot be recovered by reverse straining. • Full-network model can capture the shear band evolution under reverse shearing.

Role of Alloying Composition on the Nanomechanical Behavior of Amorphous Nanolaminates

Abstract In this work, the molecular dynamics (MD) simulation was performed to evaluate the nanomechanical properties and plasticity behavior of amorphous nanolaminates. For this purpose, two types of nanolaminates with the composition layers of Cu50Zr50/Cu54Zr46/Cu60Zr40 (ABC-type) and Cu50Zr50/Cu60Zr40 (AC-type) were firstly designed and the mechanical properties were then evaluated through the nanoindentation and tensile loading. The results indicated that the ABC-type nanolaminate exhibited higher strength and resisted the relaxation of stored energy in the holding stage of the indenting process. The Von-Mises strain distribution also showed that the plastic deformation initiated from the indenter/sample connection and gradually continued into the depth of the sample, while the strained regions in the AC-type considerably extended in the width of the sample perpendicular to the indenting force. The tensile loading results demonstrated that the intersection of the interface and free surface is the potential site for the initiation of shear events in both nanolaminates; however, the AC-type showed a sharp strain accumulation in the soft layer (Cu50Zr50) with the increase in the total strain value. On the other side, the gradual changes in the chemical compositions of layers and slight compositional variations at the interfaces led to more homogeneous plastic deformation in the ABC-type nanolaminate.

Discrete dislocation dynamics for crystal RVEs. Part 1: Periodic network kinematics

A novel implementation of the dislocation flux boundary condition in discrete dislocation dynamics is presented. The continuity of the individual dislocation loops in a periodic representative crystal volume (RVE) is enforced across the boundary of the RVE with the help of a dual topological description for representing dislocation line kinematics in two equivalent spaces representing the deforming crystal, the RVE and the unbounded crystal spaces. The former describes the motion of the dislocations in the simulated crystal RVE whereas the latter represents the motion of dislocations in an infinite space containing all replicas of the RVE. A mapping between the two spaces forms the basis of the implementation of flux boundary condition. The implementation details are discussed in the context of statistical homogeneity of bulk crystals undergoing macroscopically homogeneous plastic deformation. In this case, the boundary nodes associated with dislocation segments bear no relevance in the motion of the dislocations. Some test cases are presented and discussed to establish the proposed approach.

Combined Effects of Annealing and Cyclic Loading on Structural Rejuvenation and Mechanical Properties of CuZr Metallic Glass: A Molecular Dynamics Study

Structural rejuvenation is one of the key topics in the field of metallic glasses (MGs). In this work, we evaluated the combined effects of annealing treatment and elastic cyclic loading to discover pathways for promoting structural rejuvenation and improving the mechanical properties in the MGs. Using molecular dynamics (MD) simulations, it was revealed that the sole cyclic loading led to the increase of rejuvenation degree; however, a saturated state was observed upon the 40th cycle. On the other side, the sample exposed to the combined treatment exhibited a slight relaxation at the first step of cycling process and then a sharp rejuvenation degree was detected in the material. The thorough analyses indicated that the change in the fraction of coordination polyhedrons at the relaxation stage was the main reason for the extra rejuvenation in the sample exposed to the combined treatments. The results also suggest that the combination of rejuvenation treatments improves the magnitude of rejuvenation in the amorphous alloys. It should be noted that the increase of rejuvenation in the alloying systems accompanied with a reduction in the tensile strength and an enhancement in the homogenous plastic deformation.

Напряженно-деформированное состояние континуума с очагом пластической деформации

The paper describes a method for constructing a shear plastic band in an elastic plane using the Crouch solution in the problem of a constant discontinuity of displacement on a finite segment of an unbounded plane. Analytical equations are obtained for calculating the stress-strain state (SSS) of a plane with a band of homogeneous plastic deformation for the cases of simple and pure shear. The analysis of SSS was carried out and the features of the field of internal stresses in an elastic plane with a band of plastic deformation shear were revealed. It is shown that a zone of homogeneous plastic shear deformation can be used as an element for constructing a zone of arbitrary geometric shape with a given distribution of plastic shear deformation. In the general case, the proposed approach can also be applied to construct zone with a homogeneous distribution of the normal components of plastic deformation.

Role of Glass Composition on Mechanical Properties of Shape Memory Alloy-Metallic Glass Composites

In this work, the molecular dynamics (MD) simulation was applied to design a laminated composite structure comprised of the shape memory alloy (SMA) and Cu-Zr metallic glasses (MGs). A wide range of MG compositions was considered to tune the mechanical features and improve the homogenous plastic deformation during the tension loading. The results indicated that the martensitic transformation in the SMA inhibited the sudden shear band propagation in the composite for all the samples. Moreover, it was revealed that the mechanism of plasticity was significantly affected by the change of MG composition. In the Cu-rich MGs, the formation and propagation of thick shear bands occurred at the end of the tension loading; however, the increase in Zr content induced the interaction of multiple shear bands with finer configurations in the system. Nevertheless, the excessive Zr addition in the MG composition facilitated the aggregation of nanopores at the interface of SMA and MGs, which may be due to the softening effect in the Zr-rich MGs. Finally, it is concluded that an optimized MG composition is required for the trade-off between the plasticity and the strength in the SMA-MG composites.

Low temperature deformation mechanisms of polycrystalline CoZr and Co39Ni11Zr50 B2-type intermetallic compounds

The B2-type intermetallic compounds CoZr and Co39Ni11Zr50 were deformed in tension at low temperatures. While CoZr is ductile down to 4 K, Co39Ni11Zr50 becomes brittle below 125 K due to a martensitic phase transformation. Thermal activation analysis shows that CoZr follows the Cottrell-Stokes law indicating forest dislocation cutting as the dominant rate-controlling deformation mechanism, similar to face-centered cubic metals. The moderate ductility of both intermetallic compounds at low temperatures may qualitatively be related to a significant metallic character of bonding giving rise to a low Peierls stress estimated for primary {110}<100> slip and most likely also leads to an easier activation of secondary {110}<110> slip which was proven by transmission electron microscopy. Secondary slip is necessary for the fulfillment of the von Mises criterion for homogeneous plastic deformation of polycrystalline materials. The present results generalize the findings made on the ductile rare earth intermetallics YAg and YCu and, therefore, may help to search for other ductile systems in the broad class of intermetallic compounds.