Plastic Instability Research Articles

Plastic Flow Instability in Austenitic Stainless Steels at Room Temperature: Macroscopic Tests and Microstructural Analysis

Plastic Flow Instability in Austenitic Stainless Steels at Room Temperature: Macroscopic Tests and Microstructural Analysis

Ultrastrong and ductile superalloy joints bonded with a novel composite interlayer modified by high entropy alloy

Diffusion bonding (DB) with interlayers is sought-after for manufacturing high-performance turbine disks of powder metallurgy (PM) superalloys with precise and intricate inner cavity structures. Developing novel interlayer materials is challenging but crucial for enhancing bonding quality and joint properties. We designed a multi-interlayer composite bonding (MICB) method, employing sandwich-structured interlayers of “BNi2/high entropy alloy (HEA)/BNi2”, to join a PM superalloy FGH98. The MICB joint exhibited an ultrahigh shear strength of ∼1132 MPa and exceptional ductility, indicating a typical ductile fracture pattern with numerous dimples. Owing to the introduction of liquid BNi2 interlayer, initial bonding interfaces were eliminated and replaced by newborn grain boundaries (GBs), preventing brittle interfacial fracture. Due to the diffusion of Al/Ti/Ta from the base metals (BMs), massive ordered γ' nanoparticles also precipitated in the joint. Moreover, the addition of HEA foil reduced the stacking fault energy (SFE) of the joint and facilitated the formation of deformation twins (DTs). Thus, during the deformation process, the γ' nanoparticles, and multiple substructures like stacking faults (SFs), Lomer-Cottrell (L-C) locks, DTs, and 9R phases enhanced the work-hardening capability and strengthened the joint. Simultaneously, the multiplication and interaction of DTs induced a softening mechanism of dynamic recrystallization (DRX) during the entire deformation process and dominated when the plastic instability occurred, resulting in numerous adiabatic shear bands (ASBs) consisting of γ/γ' nano-bands, which indicates a significant improvement of the joint ductility.

Suppress defect-induced plastic instability to achieve superior strength-ductility combination of spray formed 7050 Al alloy

This research aimed to enhance the strength and ductility of spray formed (SFed) 7050 aluminum (Al) alloy through hot extrusion and subsequent heat treatment. Microstructure and mechanical properties evolution during these processes were investigated. It was found that the low densification of the SFed 7050 Al alloy due to various defects such as pores, with an average size of 7.6 μm and an area fraction of 3.7 % could be notably enhanced by hot extrusion, even achieving a level close to theoretical density. This process led to a transformation of grains from equiaxed to elongated shapes along extrusion direction and significant grain refinement, exhibiting a strong fiber texture. While the employed extrusion ratio exceeds 16 with proper heat treatment, the SFed 7050 Al alloys achieves superior strength-ductility combination compared with reported Al alloys. The achieved high performance should be attributed to the grain refinement, strong texture and high densification that suppress defect-induced plastic instability. Furthermore, the combination of hot extrusion and heat treatment, provides an effective way to enhance strength and ductility for SFed 7050 Al alloy.

Nonlinear Finite Element Simulation and Analysis of Steel Frame Structures Considering Nonlinear Behavior

This paper focuses on investigating the nonlinear behavior of steel frame structures in practical engineering applications. Utilizing finite element analysis, the study simulates the response of structures under extreme loads such as strong earthquakes. By incorporating nonlinear material properties and contact effects, the numerical simulation and analysis explore phenomena such as plastic deformation, yield, and instability in frame structures. The research outcomes contribute to a deeper understanding of the energy dissipation and safety performance of these structures.

Pursuing ultrahigh strength–ductility CoCrNi-based medium-entropy alloy by low-temperature pre-aging

Developing high-performance alloys with gigapascal strength and excellent ductility is crucial for modern engineering applications. The concept of multi-component high/medium entropy alloys (H/MEAs) provides an innovative approach to designing such alloys. In this work, we developed the Co1.5CrNi1.5Al0.2Ti0.2 MEA, which exhibits outstanding mechanical properties at room temperature through low-temperature pre-aging followed by annealing treatment. Tensile testing reveals that the MEA possesses an ultrahigh yield strength of 20±0785 MPa, an ultimate tensile strength of 2365 ± 70 MPa, and exceptional ductility of 15.8% ±1.7%. The superior tensile properties are attributed to the formation of fully recrystallized heterogeneous structures (HGS) composed of ultrafine grain (UFG) and fine grain (FG) regions, along with discontinuous precipitation of coherent nano-size lamellar L12 precipitates. The mechanical incompatibility between the UFG region and the FG regions during deformation induces the accumulation of a large number of geometrically necessary dislocations at the interface, resulting in strain distribution and hetero-deformation-induced (HDI) stress accumulation, contributing significantly to HDI strengthening. HDI strengthening, precipitation strengthening, and grain boundary strengthening are the primary mechanisms responsible for the ultra-high yield strength of the MEA. During deformation, the dominant deformation mechanisms include dislocation slip, deformation-induced stacking faults, and Lomer–Cottrell locks, with minor deformation twinning. The synergistic interaction of these multiple deformation modes provides the MEA with excellent work hardening capability, delaying plastic instability and achieving an excellent combination of strength and ductility. This study provides an effective strategy for synergistically strengthening MEAs by combining HDI strengthening with traditional strengthening mechanisms. These findings pave the way for the development of advanced structural materials with high performance tailored for demanding applications in engineering.

Simultaneous enhancement of strength and ductility of Al matrix composites enabled by submicron-sized high-entropy alloy phases

Our study investigated the reinforcing effect of submicron-sized (100–500 nm) Al0.5CoCrCuFeNi high-entropy alloy particles (HEAps) on the tensile properties of Al matrix composites. The HEAps were refined to the submicron-sized level via a thermal plasma process to maximize reinforcing efficiency by increasing specific surface (i.e., interface) area. The Al/HEA composites were produced by hot rolling of mechanically milled Al/HEA composite powder. The microstructural analysis demonstrated that the HEAps were uniformly dispersed within the Al matrix by forming a sound interface without interdiffusion brittle intermetallic layers or any noticeable voids. Moreover, the addition of 2 vol% HEAps resulted in a simultaneous increase in both yield strength from 355 to 403 MPa and tensile elongation from 1.3 to 3.3%. As a result, the composite experienced a synergistic effect due to the combination of intrinsic strengthening to promote uniform plastic deformation and extrinsic strengthening to mitigate plastic instability and crack propagation.

Comparison of copper-based composites fabricated by cross-accumulative roll bonding method for packaging design

Copper-based composites have interesting properties and they are widely used in electronics, packaging, and automotive applications. In the present work, the cross-accumulative roll bonding (CARB) process was used to fabricate Cu-based composites with Zn and Brass reinforcing layers. The experimental observations revealed a clear relationship between rolling strain and plastic instabilities including the necking and rupture of reinforcement. However, the grain refinement was observed on Cu, Zn, and Brass layers after seven CARB cycles, indicating the positive impact of rolling rotation on obtaining the ultrafine grains. Better ductility of Brass also resulted in finer grains. This also led to more significant improvement in mechanical properties. During the CARB process, the hardness of Zn and Brass increased from 59 HV and 250 HV to 89 HV and 310 HV, respectively.

Deformation mechanisms of additively manufactured Hf10Nb12Ti40V38 refractory high-entropy alloy: Dislocation channels and kink bands

To date, the reported refractory high-entropy alloys (RHEAs) with strength and ductility synergy all have up to 1 GPa strain hardening rate (SHR). Keeping the SHR of more than 1 GPa in the RHEAs can postpone the point of plastic instability, thus prolonging the uniform tensile ductility (UTD). In this paper, Hf10Nb12Ti40V38 RHEA with high strength and ductility is successfully manufactured by laser-directed energy deposition (L-DED). The yield strength of the alloy is about 1011 MPa with the tensile ductility of ∼12.6 %. Interestingly, the SHR of the alloy is very low, far below 1 GPa. The stress-strain curve of the alloy does not only show no premature dropout, but also exhibits a flat plateau-type curve. For this interesting phenomenon, the deformation mechanisms of Hf10Nb12Ti40V38 alloy are systematically investigated by investigating the deformation microstructures. The results indicate that the dislocation modes are diversifying, but the limited number of tangles generated by the intersection of dislocation channels are the main source of SHR. Various dislocation configurations such as single dislocation channel and cross-slip are stimulated sequentially, and although they have no positive effect on the SHR of the alloy, they allow alloy to accommodate plastic deformation. Combined with the kink bands as a strain softening mechanism, it improves the ductility while decreasing the SHR. Multiple dislocation channels interactions and kink bands are clearly identified as the two deformation mechanisms governing strain hardening and strain softening. The competition between two mechanisms generates the phenomenon mentioned above. This study does not only shed new insights on the unique deformation behavior of Hf10Nb12Ti40V38 RHEA but also complement the current general perception about the relationship between SHR and ductility of RHEAs.

Twinning induced strain hardening and plasticity in a γ''-precipitated medium-entropy alloy with ultrahigh yield strength

Keeping adequate strain hardening to postpone plastic instability to a larger tensile strain is a stiff challenge in alloys with high yield strength. This study showed the feasibility of activating deformation twins (DTs) to ductilize an ultra-strong medium-entropy alloy (MEA) through microstructural design. High content (∼24 %) γ'' precipitates were introduced into a Ni49.9Fe33Cr10Nb4Ta3B0.1 (at.%) MEA to offer high yield stress. We found that increasing the γ'' precipitate size and spacing successfully activated DTs, contributing to a sustainable strain hardening of the alloy. Accordingly, an ultrahigh tensile yield stress (YS) of 1.55 GPa and ultimate tensile stress (UTS) of 1.7 GPa, along with a fracture elongation of 14.5 % were achieved in the MEA. We further demonstrated that compared to lowering the stacking-fault energy (SFE), increasing γ'' precipitate spacing significantly reduced the critical shear stress for triggering twinning partials in a nanoscale γ/γ'' system.

Simultaneously enhancing the strength and ductility of Fe–Mn–C twinning-induced plasticity steel via Cr/N alloying

The effect of Cr/N alloying on the tensile deformation behavior of Fe–Mn–C twinning-induced plasticity steel was investigated through monotonic tensile tests, X-ray diffraction, electron back-scattered diffraction, transmission electron microscopy, and digital image correlation technique. The results demonstrate that the addition of Cr/N significantly enhances both the strength and ductility of Fe–18Mn-0.6C steel while dynamic strain aging (DSA) behavior weakens or even disappears. Compared with that in the Fe–18Mn-0.6C steel, more densely-spaced, thinner deformation twins were formed in the Cr/N-alloyed steel. Moreover, the Cr/N additions promote deformation twins to form over a wider strain range. This unique twinning behavior leads to a sustained high strain-hardening rate at higher strains, thereby delaying plastic instability and concurrently enhancing strength and ductility.

Dynamics of Portevin-Le Chatelier (PLC) bands and fracture behavior of W-tempered 7075 aluminum alloys with different cooling methods

The relationship between Portevin-Le Chatelier (PLC) band propagation, plastic deformation, and failure mechanisms in solution heat-treated 7075-T6 aluminum alloys cooled by water quenching (7075-WQ) or air cooling (7075-AC) is compared using 5052-H32 alloy as a reference. Miniature tension tests, strain rate jump (SRJ) tests, and digital image correlation reveal that 7075-WQ shows diffused band configurations, 7075-AC exhibits localized patterns, and 5052-H32 has relatively stationary bands. These PLC band dynamics, related to geometry and band densities, are supported by fractography and microstructure analyses. The 7075-WQ and 7075-AC alloys exhibit mixed ductile and quasi-brittle fractures, while 5052-H32 shows ductile fractures with cup-and-cone surfaces. The findings provide new insights into the PLC bands and their formation mechanisms in W-tempered 7075 aluminum alloy concerning different cooling methods, supporting the development of thermal-mechanical processing routes for aluminum components with controllable plastic instability.

Influence of surface pre-deformation on the Portevin-Le Chatelier effect and the related multiscale complexity of plastic flow in an Al-Mg alloy

The influence of the surface pre-deformation on jerky flow caused by the Portevin-Le Chatelier (PLC) effect was investigated using flat tensile specimens of an Al-Mg alloy. Although jerky flow represents a macroscopic plastic instability, the underlying mechanisms stem from self-organization of dislocations, which pertains to deformation processes at mesoscopic scales. To provide a comprehensive approach, the investigation was carried out by coupling tensile tests, digital image correlation and acoustic emission techniques, each targeting a particular range of scales. Thin superficial layers were pre-deformed using surface mechanical attrition technique (SMAT). It was found that the observed effects depend on which surfaces are processed. Overall, the treated samples exhibited an enhanced yield strength without deterioration of ductility in comparison with the initial material. These changes in the general mechanical behavior are likely to be correlated with the changes in jerky flow. Using digital image correlation, a tendency to delocalization of bursts of plastic flow was found for both the PLC bands and smaller-scale strain heterogeneities outside the bands, the latter having been little studied so far. Unexpectedly, SMAT occurred to modify plastic flow drastically at this scale. In addition to clarification of the role played by the surface in the PLC effect, these findings provide new insights into relationships between deformation processes at distinct scales.

Effect of Cyclic Warm-Rolling Technique on Mechanical Properties of MoCu30 Thin Plates with Heterogeneous Structure.

By employing a cyclic warm rolling technique, MoCu30 alloy sheets of different thicknesses were prepared to investigate the effects of various rolling reduction rates on the microstructure and mechanical properties of MoCu30 alloys. Additionally, the evolution of microscale heterogeneous deformation during the tensile process was observed based on DIC technology. This study reveals that Mo-Cu interfaces at different deformation rates exhibit an amorphous interlayer of 0.5-1.0 μm thickness, which contributes to enhancing the bond strength of Mo-Cu interfaces. As the rolling reduction rate increased, the grain size of the MoCu30 alloy gradually decreased, whereas the dislocation density and hardness increased. Furthermore, the yield strength and tensile strength of the MoCu30 alloy increased gradually, whereas the elongation decreased. At a deformation rate of 74% (2 mm), the yield strength, tensile strength, and elongation of the MoCu30 alloy were 647.9 MPa, 781.8 MPa, and 11.7%, respectively. During the tensile process of Mo-Cu dual-phase heterogeneous material, a unique hierarchical strain banding was formed, which helps to suppress strain localization and prevent premature plastic instability.

Unveiling the kinetics of interphase and random quaternary Nb[sbnd]Ti precipitations, and strengthening mechanism of HSLA steel

Traditional methods for analyzing multi-element precipitation are often time-consuming, labor-intensive, and expensive. Particularly, the initial nucleation stages of such precipitations remain poorly understood. A revised precipitation theory was employed to elucidate the nucleation and growth kinetic mechanisms of quaternary (Ti, Nb)(C, N) precipitation with two modes: interphase precipitation (IP) along the migrating α/γ interface and random precipitation (RP) along dislocations. The findings indicated that TiN preferentially nucleates at the ferrite side compared to that at the austenite side. The nucleation occurring at α/γ interface leads to a precipitation particle angle of approximately 5.3° within a single IP sheet. The interface provided more favorable nucleation sites than dislocations, significantly reducing the relative nucleation time. Additionally, the initial yielding behavior and strengthening mechanism were analyzed based on the two factors of the hot-rolling microstructure and precipitation. A favorable slip system was along the RD-ND plane as the texture index of α-{110} approximately 3.74, compared to 2.20 along the TD-ND plane, resulting in the lower upper yield strength. Once the number density of mobile dislocations increased during plastic deformation, the tensile stress decreased, leading to the local plastic instability. Finally, (Ti, Nb)(C, N) contributed to a strengthening increment of about 154 MPa. This study provides critical insights into the precipitation behavior in HSLA steels, offering significant implications for the design and development of advanced materials with enhanced mechanical properties.

Modeling for plastic instability in multi-directional rotary forging of thin-walled components with three ribs

Multi-directional rotary forging has the great potential to form thin-walled components with three ribs because of its incremental deformation characteristic and smaller forging load. However, for the components with thin-walled sterna and thick ribs, the plastic instability is easier to occur at the bottom of thick ribs in multi-directional rotary forging, resulting in the waste of components. Therefore, the aim of this paper is to establish the model for predicting plastic instability in multi-directional rotary forging of thin-walled components with three ribs. Firstly, the models for calculating actual and critical radial stresses on both sides of rib are established by analyzing complicated stress states caused by time-varying thinning deformation of sterna and thickening deformation of rib. It is found that the critical radial stress is mainly determined by ratio of deformation energy and external force work, while the actual radial stress is mainly determined by dissipated power and metal flow velocity. By combining actual and critical radial stresses, a unified model for simultaneously predicting plastic instability of ribs at different positions is proposed. Then, the influence laws of different technical parameters on critical conditions of plastic instability are revealed. It is found that the plastic instability is mainly determined by initial thickness of workpiece and width of rib, i.e., the critical deformation amount increases with increasing initial thickness of workpiece or decreasing width of rib, and vice versa. Finally, the experiments are conducted to verify that the proposed model for predicting plastic instability in multi-directional rotary forging of thin-walled components with three ribs is reliable. This paper provides a guideline to establish the model for predicting plastic instability under complicated stress states and metal flow modes.

Portevin-Le Chatelier behavior in AlMgScZr alloys: Effects of Al3(Sc,Zr) dispersoid distribution and grain structure

Plastic instability caused by the Portevin-Le Chatelier (PLC) effect remains a challenge for Al–Mg alloy sheets during shape-forming processes, as it causes unsightly stretcher-strain markings on the surface of final products. In this study, the PLC behavior in annealed AlMg and AlMgScZr alloy sheets with varying Al3(Sc,Zr) dispersoid distributions and grain structures were investigated by combining tensile tests, microstructural characterization, and DIC analysis. The results show that the presence of Al3(Sc,Zr) dispersoids can inhibit the PLC effect, which is manifested by a decrease in serration amplitude, PLC band velocity, and an increase in critical strain. The inhibition effect is shown to be highly related to the distribution of Al3(Sc,Zr) dispersoids and resultant grain structures. The alloy with the densest dispersoid distribution and finest subgrain structure showed the slightest PLC effect. The reduction in serration amplitude and PLC band velocity is ascribed to the augmented resistance to local strain softening, caused by inhibition of dislocation motion by Al3(Sc,Zr) dispersoids and subgrain boundaries. In addition, a high density of Al3(Sc,Zr) dispersoids can strongly trap vacancies, which is responsible for the increase in critical strain.

Influence of thickness and strength on plastic instability in tailored steel structures

A mathematical model was intricately devised to explore the influence of continuous variations in thickness and mechanical properties on the performance of tailor rolled blanks (TRB) and tailor rolled tubes (TRT). Through the integration of analytical and numerical techniques, it was discerned that these variations play a pivotal role in modulating stress distribution and strain localization, thereby inducing a spectrum of plastic instability behaviors within the structures. The introduction of an ‘equivalent strength’ metric as a novel means to quantify structural performance shed light on strategic material distribution to enhance durability and mechanical efficiency. Moreover, the insights garnered from this research deepen the understanding of the mechanical responses of tailor-rolled constructs under varying loads, offering valuable perspectives for the development and fabrication of engineered materials with bespoke properties. This study not only contributes to bridging a knowledge gap in the realm of tailored material engineering but also fosters the advancement of design methodologies in the construction of high-performance engineered structures.

Electric pulse-induced suppression of the Portevin – Le Chatelier behavior of 5086 aluminum alloy under uniaxial tension

The electrical-assisted tensile tests of 5086 Al alloy sheets with different current parameters were carried out at room temperature with a frequency of 150 Hz and voltages of 50 V, 60 V and 70 V. It was found that the serrated plastic instability (named PLC effect) was eliminated as the pulse current increased to 150 Hz/60 V, but the PLC effect was observed with 150 Hz/50 V. To distinguish the thermal effect and athermal effects of pulse current on PLC effect, the comparative heating tensile (HT) test in furnace with a temperature rising trend of 150 Hz/60 V was conducted. The obtained curves still showed serrated flow behavior, indicating that the thermal effect was insufficient to suppress the PLC effect. The influence of pulse current on the microstructure and texture evolution was investigated through TEM and EBSD. According to the microscopic analysis, the pulse current accelerated the solute atoms' diffusion and dislocation movement. S orientation {123}<634> of 150 Hz/60 V presented more obvious transformation compared with that of HT specimen. It was illustrated that the athermal effect of pulse current plays an important role in the crystal orientation and suppressing the PLC effect.

Serrated flow stress in the CoCrFeNi multi-principal element alloys with different grain sizes and added Re

Multi-principal element alloys have attracted extensive attention as a new alloy design concept. In particular, the CoCrFeNi alloys with low stacking fault energy have excellent tensile plasticity and fracture toughness at both low and room temperature, and good phase stability at high temperatures, considered a new type of structural material with great potential. Multi-principal element alloys exhibit plastic instability termed serrated flow, at certain strain rates and temperature ranges. The serrated flow stress is usually closely related to dynamic strain aging, which reflects dislocation and barrier processes and impacts the mechanical properties of alloys. In the present study, CoCrFeNi alloys were selected as the research object, and the impact of grain size and the addition of Re elements on the serrated flow stress behavior was investigated. The results showed that in the tensile tests at 600 °C, the serrated flow stress behavior occurred in the fine-grain specimen only at a lower strain rate. The tensile curves of the coarse-grain specimen at different strain rates all show serration. Overall, the serrated type changes with the decreasing grain size, the increasing strain rate, and the addition of Re element, all of which result in a decrease in the magnitude and number of serrations and a weakening of the serrated behavior.

Revealing the Superior Post-Necking Elongation in the Fine-Grained Ti-6Al-4V ELI at Cryogenic Temperature

The mechanical properties of a fine-grained (FG) Ti-6Al-4V extra-low interstitial (ELI) alloy were investigated by tensile tests at 298 K and 77 K. The experimental results indicated that, at 77 K, the alloy exhibits a small uniform elongation of 2.65%, but has a fracture elongation of 19.2%, showing superior post-necking elongation. At 298 K, the alloy displays a single dislocation slipping, β→α″ phase transformation occurred, and 6.35% uniform elongation was obtained, whereas the coupling of dislocation slipping and twinning deformation behaviors dominated at 77 K. The limited uniform elongation is attributed to the absence of martensite phase transformation at 77 K, whereas the decent fracture elongation is ascribed to the resistance offered by twinning against plastic instability.