High Uniform Elongation Research Articles

Partially recrystallized heterostructure via Mo-doping endows CoCrNi medium-entropy alloy with exceptional strength and ductility

CoCrNi medium-entropy alloy (MEA) has excellent ductility, but the low strength at room temperature limits its practical usage. This study proposes an effective strategy to enhance the strength of CoCrNi MEA without apparent loss of ductility via partially recrystallized heterostructures resulted from doping of large atomic radius element, which was substantiated in a Mo-doping (CoCrNi)98Mo2 MEA. Compared with CoCrNi, the strength of (CoCrNi)98Mo2 is remarkably improved, maintaining the high uniform elongation of ∼24%. The yield strength and ultimate tensile strength increased from 950 MPa, 1287 MPa to 1131 MPa, 1564 MPa, respectively. The difference in deformation between the soft recrystallized region (SRR) and the hard non-recrystallized region (HNRR) leads to an excellent hetero-deformation induced (HDI) strengthening. Additionally, it is easier to reach the critical stress of twinning caused by high HDI stress at the interface of SRR/HNRR, which contributes to activation of high-density twins and produce dynamic Hall-Petch strengthening effect.

Improving the uniform elongation of ultrafine-grained pure titanium through judicious allocation of work hardening

Improving uniform elongation in metals typically involves enhancing the work hardening rate, as elevated work hardening can delay necking and fracture. However, our investigation into commercial pure titanium reveals a counterintuitive relationship between these properties. We find that high uniform elongation correlates with low work hardening capability, while a high work hardening rate results in reduced ductility. Two types of ultrafine-grained pure titanium, prepared by rotary swaging, subsequent rolling, and annealing, exhibit different mechanical properties. Microstructural and deformation mechanism analyses reveal that the difference arise from variations in texture. Specifically, extensive activation of <c+a> dislocations in the former sample leads to premature, intense work hardening that is quickly exhausted, while the latter sample shows a steady, uniform work hardening progression that delays necking. Our findings challenge the conventional understanding that high work hardening rates ensure high uniform elongation. Instead, we propose that optimizing ductility requires a strategic allocation of work hardening throughout the tensile deformation to delay necking. This study reveals the intrinsic relationship between work hardening and ductility, offering new strategies for designing stronger and tougher materials.

Tuning heterogeneous geometric microstructure of Cu-based materials via powder metallurgy for improving high synergistic strengthening and electrical conductivity

Designing heterogeneous structures (HS) is recognized as a promising method to achieve high performance for Cu-based materials. However, how to tune and control stably still remains challenging. In this work, a reproducible method based on powder metallurgy and heat treatment is proposed for designing controllable HS. Herein, spatial distribution of coarse and fine grains can be controlled via addition of different volume fraction of Cu powder. The microstructure evolution of the resultant HSed material was characterized in detail by utilizing electron backscattered diffraction (EBSD) combined with a transmission electron microscope (TEM). An optimal heterogeneous geometric microstructure consisting of individual isolated micron-sized grain completely surrounded and constrained by the hard ultrafine-grained Cu-Cr0.67 matrix is discovered, which produced an excellent combination of high yield strength (531.4 ± 10.3 MPa), uniform elongation (～22.1%)，and conductivity (～90.4% IACS) (International Annealed Copper Standard). The well-dispersed and high-density soft/hard zone interfaces in the resultant HS cause geometrically necessary dislocations (GNDs) piling ups, which produce extra strain hardening to enhance the strength and ductility. This reproducible method provides a feasible strategy to optimize the mechanical properties of metals via tailoring HS.

Study on the Bending Performance of High-Strength and High-Ductility CRE-Reinforced Concrete Beams

Constant resistance energy (CRE) steel reinforcement has a yield strength of up to 750 MPa and an ultimate elongation of more than 20%. CRE reinforcement overcomes the contradiction between high yield strength and high uniform elongation of ordinary high-strength bars. This paper explores the flexural performance and load-carrying mechanisms of CRE-reinforced concrete beams through a series of experiments, while also presenting a theoretical analytical method for such specimens. Flexural tests on six CRE-reinforced concrete beams and two control tests on hot-rolled ribbed bar 400 (HRB400)-reinforced concrete beams were conducted in this paper. The study examines the influence of the shear–span ratio and reinforcement type on the mechanical response of the beams, including cracking load, yield load, and ultimate load, while analyzing the variation patterns of concrete strain and reinforcement strain. The experimental results demonstrate that as the shear–span ratio decreases, the crack resistance and load-carrying capacity of CRE-reinforced concrete beams improve. Under equivalent conditions, CRE-reinforced concrete beams exhibit higher load-carrying capacity compared to HRB-reinforced concrete beams, surpassing the latter by approximately 43% in terms of ultimate load. Additionally, this paper proposes a calculation method for the mechanical response of NPR-reinforced concrete beams and compares the theoretical values with the experimental values. The differences between the two are within 13%, which proves the reliability of the calculation method.

Developing a Zn alloy with high strength and uniform elongation as a biomedical device

The equal channel angular pressing (ECAP) process was used to develop a Zn-1Mg alloy with a tensile strength of 440 MPa and uniform elongation of 11%. The uniform elongation of the ECAPed Zn-1Mg alloy is higher than that of other Zn alloys with strengths over 400 MPa. The microstructure of the ECAPed Zn-1Mg alloy evolved through dynamic recrystallization (DRX), resulting in a refined grain structure. Additionally, the lamellar eutectic structure was fragmented into sub-micrometer particles (∼0.9 μm). The high strength of the Zn-1Mg alloy is due to both grain boundary strengthening and second phase strengthening. The high uniform elongation is attributed to the presence of plate-shaped precipitates with a high density of 1014m−2. The in-vitro results indicate that ECAPed Zn-1Mg alloy has high cell viability (>100%). Meanwhile, the Zn-1Mg alloy processed by ECAP exhibited better ALP activity and alizarin red results than pure Zn. These results demonstrate that Zn-1Mg alloy is beneficial to the proliferation and differentiation of osteoblasts, and also promote blood vascular formation. The good osteogenic and angiogenic properties of the alloy are attributed to the release of Mg2+ and Zn2+ during the degradation process, which play a critical role in biochemical reactions in the human body. Therefore, the high uniform elongation and good biological properties make Zn-Mg based alloys a promising material for expanding applications in the orthopedic field.

Experimental assessment on cyclic flexural behavior of UHPC beam reinforced with negative Poisson’s ratio (NPR) steel rebar

The objective of this study is to experimentally explore the implication of an innovative steel rebar with negative Poisson’s ratio effect (termed as NPR steel rebar) on ultra high performance concrete (UHPC) beam under cyclic loading. Distinctly from conventional steel rebar, NPR steel rebar employed in this research inherently owns high ultimate tensile strength of approximately 1100 MPa, high uniform elongation of around 40% and superior durability. Four reinforced UHPC beams are conducted and tested under monotonic/cyclic loading through the four-point bending method. The structural characteristics of all specimens are discussed via failure mode, hysteretic curve, envelop response, stiffness degradation, energy-dissipating capacity, and steel strain evolution and etc. The results show that reinforced UHPC beam longitudinally configured with NPR steel rebar is available to exhibit satisfactory and superior monotonic/cyclic flexural performance. Firstly, the final failure mode of reinforced UHPC beam with NPR steel rebar exhibits multiple (7–8) closely-spacing localized cracks distributing within the pure bending section whether under monotonic or cyclic loading. Additionally, the superior mechanical performance of NPR steel rebar endows reinforced UHPC beam with high flexural capacity, high deformability, excellent stiffness degradation control, and outstanding energy dissipation capacity under cyclic loading. In which, the flexural capacity of UHPC beams reinforced with NPR steel rebar have been upgraded approximately by 47.8%, 52.3% under monotonic and cyclic loading respectively, compared with that of UHPC beams reinforced with HRB 400 steel rebar, meanwhile, the comparison for the energy dissipation capacity is emphasized in stages. Finally, the deflection prediction method for UHPC beam reinforced with NPR steel rebar at normal service stage under monotonic/cyclic loading is proposed. The results of this study provide valuable information for attaining a preliminarily comprehensive understanding of the structural behavior of reinforced UHPC members with NPR steel rebar under monotonic/cyclic loading, and the proposed combination of UHPC and NPR steel rebar provides a new idea for seismic toughness design of anti-bending structural components as well.

Effect of through-thickness microstructure inhomogeneity on mechanical properties and strain hardening behavior in heavy-wall X70 pipeline steels

The ever-increasing demand for oil & gas exploration from land to subsea necessitates using higher-strength and high-ductility heavy-wall thickness pipes, which may increase the microstructural inhomogeneity along the steel plate sections, ultimately impacting the mechanical properties. In this work, the differences in microstructure and mechanical properties between the through-thickness layers of a heavy-wall ferrite/bainite (F/B) dual-phase (DP) X70 pipeline steel plate were studied. The results showed that the size of the ferrite grain and the volume fraction of bainite increased from the surface layer to the mid-thickness layer of the DP X70 pipeline steel plate. The surface and mid-thickness layers had a lower uniform elongation and higher strength and hardness. In comparison, the quarter layer had higher uniform elongation and lower strength and hardness. The refined grain size and the strong {001}<110> texture were responsible for enhanced in strength and hardness. The higher γ-fiber texture at the quarter layer favored the higher plasticity. The analysis results of strain-hardening behaviors based on the modified C-J model showed that the mid-thickness layer had a higher strain-hardening capacity at the beginning of the plastic strain, leading to deformation instability and reduced material plasticity. The surface layer maintained a high hardening exponent even at high plastic strains, which facilitated the deformation properties of the material. The results provide a basis for evaluating the influence of microstructural factors on the strength and plasticity of heavy-wall X70 pipeline steel and the coordination mechanism of micro deformation in DP steels.

Strong work-hardenable body-centered-cubic high-entropy alloys at cryogenic temperature

Body-centered cubic (BCC) metals and alloys are usually brittle with limited strain hardening capability at cryogenic temperatures due to the restricted dislocation nucleation and mobility. Herein, we report that decrease of the Nb content in the TiZrHfNbTa0.2 high-entropy alloys (HEAs) can facilitate multiple deformation mechanisms, i.e., dislocation planar slip, strain-induced phase transformations and twinning, at the cryogenic temperature of 77 K due to the decreased phase stability. Particularly, the TiZrHfNb0.3Ta0.2 HEA showed pronounced strain hardening capability and exceptionally high uniform elongation of about 25% with no sign of ductile-brittle transition. Our findings are important not only for providing a prominent family of metallic materials for application at the extreme service conditions, but also for understanding the deformation mechanism of HEAs at cryogenic temperatures in general.

In-situ three-dimensional X-ray investigation on micro ductile fracture mechanism of a high-Mn steel with delayed necking effect

This study investigates the micro ductile fracture mechanism of a high-Mn steel with delayed necking effect (DN-HMn steel) under monotonic tension. A random-oriented micro-grain structure cooperating with intragranular twins in the DN-HMn steel contributes to its high tensile strength of 880 MPa and high uniform elongation of over 55%, accounting for 92.8% of the total elongation. Three-dimensional (3D) visualization of the ductile fracture process is presented, and characteristics of void evolution are identified. It was observed that voids distribute and grow uniformly during deformation, with larger voids forming a double-helix spatial distribution pattern at high strain levels, which fits the principal shear stress trajectory. The number and volume of voids in the DN-HMn steel showed a significant increase, with the porosity increasing from 0.071‰ (initial state) to 1.580‰ (before fracture). Intervoid necking was found to be the mode of void coalescence, which occurred close to the necking strain of the sample. The DN-HMn steel demonstrated a significant volume expansion of 4.58% throughout the elongation process, which breaks the volume conservation law of classical metal plasticity theory. This study opens new perspectives for understanding the ductile fracture mechanism of this steel.

Simultaneous enhancement of strength and toughness of ultrafine-grained dual-phase steel with a bimodal lamellar microstructure

A novel heterogeneous lamellar microstructure with ultrafine grains was successfully introduced into a DP590 steel plate using a simple two-step controlled rolling process (TCR) described in this work. The ferrite and martensite are laminated, with an average size of 1.7 μm and 1.2 μm, respectively. The ferrite shows a bimodal grain size distribution and intensified α-fiber and rotated cubic texture. Compared to conventional controlled-roll (CCR) steel, TCR steel demonstrated 46.6 % higher tensile strength with a considerably higher work hardening rate while maintaining a reasonably high uniform elongation of 12.1 %. Furthermore, the impact energy at –60 °C was increased by almost 100 %. The ultrafine-grained lamellar microstructure in TCR steel was believed to be responsible for the synergistic deformation and delamination fracture toughening that resulted in the simultaneous improvement in tensile and impact properties.

Ultra-flash annealing constructed heterogeneous austenitic stainless steel with excellent strength-ductility

The application of austenitic stainless steel is seriously restricted due to its relatively low yield strength. To significantly improve the strength of austenitic stainless steel with an acceptable decrease in ductility, the as-received austenitic stainless steel is cold-rolled with 90% thickness reduction to obtain the hybrid structures including retained austenite, deformation-induced martensite, and subgrains. Subsequently, an ultra-flash annealing strategy is proposed to produce the heterostructured austenitic stainless steel consisting of recrystallized austenite grains and non-recrystallized austenite zones (diffusionless-reversion formation), and the hybrid microstructure formed by ultra-flash annealing shows superior strength-ductility synergy. The thorough investigation reveals that the origin of the high yield strength is not only grain boundary strengthening, but also the back-stress hardening induced by the constraint of hard domains to soft domains. The consequent high uniform elongation is provided by the synergistic effect of dislocation hardening, back-stress hardening and twinning, which is different from the coarse austenitic stainless steel with twinning-induced plasticity and transformation-induced plasticity.

Study on uniaxial tensile deformation of bainite/ferrite dual phase steel

The microstructure evolution of a ferrite/bainite dual-phase steel during uniaxial tension was observed in-situ technology. The effects of tensile rates on microstructure, mechanical properties and fracture mechanism were studied. The result shows that stress concentration is easy to form cracks at the interfaces between ferrite and bainite. The sample exhibits high uniform elongation and strength at low-strain rate, which is attributed to the effective interaction between dislocations. The microstructure undergoes orderly and continuous high hardening rate under ‘harmonious’ conditions. Dislocations are rapidly entangled at high-strain rate and dislocation cells forms. The increased strength of the ferrite brings a reduction in its ability to coordinate more plastic deformation, which eventually leads to local plastic instability and early fracture.

Investigating microstructural properties of ferrite/bainite dual-phase steel through simple process control

The ferrite/bainite dual-phase microstructure is obtained through controlling two-phase region temperature in the designed medium carbon steel. The influence of the two-phase region temperature on microstructure morphologies and volume fraction, as well as mechanical properties, is thoroughly studied. The microstructure at 795°C heat treatment process exhibits excellent strength and elongation. This is attributed to the increased proportion of large-angle misorientation, the better deformability between ferrite and bainite, and the TRIP effect of retained austenite. The strain hardening exponent curve of the sample subjected to the 795°C heat treatment process does not intersect with the straight-line n = e until the strain reaches the value of 0.1, which results in high tensile strength and high uniform elongation.

Optimizing the Electrical and Mechanical Properties of Cu-Cr Alloys by Hf Microalloying

Cu-0.4Cr (wt.%) alloys with the microalloying of Hf elements were subjected to a modified rolling–aging process to achieve high strength, high electrical conductivity and high ductility simultaneously. Transmission electron microscopy and X-ray line broadening analysis were conducted to characterize the microstructures of these alloys. Deformation twins and high-density dislocations were introduced into the copper alloys via the modified rolling–aging process and the microalloying of Hf, improving the mechanical properties of copper alloys. The Cu-Cr-Hf alloy with a reduced Hf content performed well in terms of strength, electrical conductivity and ductility. The microalloying of 0.4 wt.% Hf in the Cu-0.4Cr alloy was sufficient to achieve a good combination of high tensile strength (593 MPa), high uniform elongation (~5%) and high electrical conductivity (80.51% IACS).

An experimental assessment of ultra high performance concrete beam reinforced with negative Poisson’s ratio (NPR) steel rebar

An innovative meta steel rebar with negative Poisson’s ratio effect (termed as NPR steel rebar), which possesses both high strength (ultimate tensile strength is approximately 1100 MPa) and high uniform elongation (approximately 40%), is developed. This paper experimentally investigated the implications of NPR steel rebar on flexural behavior of ultra high performance concrete (UHPC) beams. Four UHPC beams reinforced with varying reinforcement ratio and longitudinal reinforcement type were prepared and tested. Traditional monotonic four-point bending test was performed. The test results showed that UHPC beams reinforced with NPR steel rebar could effectively improve the flexural strength by 47.8%-55.2%, and upgrade the deformability and ductility by 13.6%-180.9% respectively compared with UHPC beams reinforced with HRB 400 steel rebar under same reinforcement ratio, and the crack pattern of which exhibited multiple (6–8) uniformly main cracks located in pure bending region. Discussion included crack pattern, load-midspan deflection curve, ductility and load–strain response. The crack clustering mechanism of beams reinforced with NPR steel rebar was selectively emphasized in the discussion.

Ultra-high strength yet superplasticity in a hetero-grain-sized nanocrystalline Au nanowire

• Nanocrystalline nanowire always exhibits strength-ductility trade-off. Here, we provide in situ atomic-scale evidence that hetero-grain-sized nanocrystalline nanowires are simultaneously ultra-strong and ductile, with no strength-ductility trade-off. • The hetero-grain-sized nanocrystalline nanowire exhibits a super uniform elongation of ∼ 236% and high strength of ∼ 2.34 gigapascals at room temperature. • The in situ atomic-scale observations revealed that the dislocation activities, grain boundary plasticity, and surface atoms diffusion all contribute to the super elongation ability of hetero-grain-sized nanocrystalline nanowire. Nanocrystalline metals often display a high strength up to the gigapascal level, yet they suffer from poor plasticity. Previous studies have shown that the development of hetero-sized grains can efficiently overcome the strength-ductility trade-off of nanocrystalline metals. However, whether this strategy can lead to the fabrication of nanocrystalline nanowires exhibiting both high strength and superplasticity is unclear, similar to the atomistic deformation mechanism. In this paper, we show that ultra-small nanocrystalline Au nanowires comprising grains in both the Hall–Petch and inverse Hall–Petch grain-size regions can exhibit extremely high uniform elongation (236%) and high strength (2.34 gigapascals) at room temperature. In situ atomic-scale observations revealed that the plastic deformation underwent two stages. In the first stage, the super-elongation ability originated from the intergrain plasticity of small grains via mechanisms such as grain boundary migration and grain rotation. This intergrain plasticity caused the grains in the heterogeneous-structured nanowires to grow very large. In the second stage, the super-elongation ability originated from intragrain plasticity accompanied by the diffusion of surface atoms. Our results show that the hetero-grain-sized nanocrystalline nanowires, comprising grains with sizes both in the strongest Hall–Petch effect region and the inverse Hall–Petch effect region, were simultaneously ultra-strong and ductile. They displayed neither a strength-ductility trade-off nor plastic instability.

Design of tough, ductile direct quenched and partitioned advanced high-strength steel with tailored silicon content

The novel processing concept of direct quenching and partitioning (DQ&P) has been explored with a medium-carbon (0.4 wt.% C) steel to evaluate and optimize the processing route for excellent property combinations. New compositional design approach was based on physical simulation studies aiming to understand the influence of varying silicon contents (1.5, 0.75 and 0.25 wt.%) and Q&P processing parameters on microstructural development including carbide formation and retained austenite stabilization. Optimized Q&P parameters were selected to design a DQ&P processing route for laboratory hot-rolling trials based on the analyses of physical simulation data. The overall aim of the study was to produce ultrahigh-strength structural steels with yield strength ≥1100 MPa combined with high uniform and total elongation and impact toughness, achieved through designing a low-temperature quenching and partitioning route with effective carbon partitioning. The DQ&P steels gained an excellent combination of mechanical properties comprising of high yield strength of ∼1000–1200 MPa and tensile strength ∼2100–2300 MPa, good elongation (∼11–13%), and moderate impact toughness transition temperature T28J ∼ (−5 to +12 °C). Straining of austenite prior to DQ&P led to an extensive refinement of the final martensitic-austenitic nanostructure. Formation of nanoscale lath-martensite and fine film-like retained austenite structures enabled the observed improvement of mechanical properties. Besides the formation of nano-twinned martensite, inter-lath austenite and transitional carbides were comprehensively characterized. No adverse effect of prolonged partitioning during slow cooling, simulating coiling in actual industrial practice, has been noticed suggesting new possibilities for developing tough, ductile structural steels both for strip/plate products.

Dependence of mechanical behavior on grain size of metastable Ti–Nb–O titanium alloy

Cold-rolled metastable β-type Ti–38Nb-0.2O alloy was subjected to annealing treatment to obtain different precipitates and grain sizes. The influence of annealing on microstructure and mechanical properties was investigated. The alloy annealed at 673 ​K or 773 ​K exhibited a single-stage yielding with high strength and low uniform elongation, due to the residual work hardening and the precipitation of ω or α phases. The alloy annealed at above 873 ​K exhibited an obvious double yielding behavior resulting from the stress-induced martensitic transformation. The grain growth kinetics of single β phase alloy is sensitive to temperature, and it is suggested that the existence of oxygen decreases the grain growth exponent and increases the required activation energy for grain growth. The critical stress for slip decreased monotonously with the increase of grain size, following the classic Hall-Petch relationship. However, the critical stress for martensitic transformation decreased to a minimum and then increased again, as the grain size increased. The results are worth for design of the heat-treatment parameters of the Ti–38Nb-0.2O alloy for engineering applications.

Improved Mechanical Properties of Two Twinning‐Induced Plasticity Steels with Novel Grain Morphology

The mechanical properties and microstructures of two high Mn austenitic steels with novel grain morphology prepared by directional solidification and warm forging are investigated and compared with common twinning‐induced plasticity (TWIP) steel with equiaxed grains (EGs). The directional solidified sample (DS) is composed of columnar grains with simplified grain boundaries mainly characterized by low‐angle boundaries (LABs) with a fraction of 0.886. The warm forged sample (WF) contains numerous fibrous‐liked grains and fine nearly equiaxed grains featured with dominate LABs and complicated dislocation substructure. The significantly improved elongation and energy absorption capability of DS compared with EG are ascribed to the larger dislocation mean free path together with better plastic deformation compatibility and synergy between and within grains of DS, respectively. The largest energy absorption efficiency with enhanced yield strength of WF is mainly attributed to numerous fine grains with dominated LABs and complicated dislocation substructure. Inhomogeneous deformation characterized by well‐developed twins structure with dense dislocation pileup occurs in some regions of WF. It will contribute to enhancing work hardening, leading to a relatively high uniform elongation and significant increased strength. On the contrary, it may easily cause severe stress concentration and promote the occurrence of necking followed by plastic instability.

Enhancing strength while preserving elongation: A study on copper after accumulative skin pass rolling

Current surface mechanical treatment techniques that produce gradient microstructures and return a combination of high strength and high uniform elongation are limited in terms of their industrial suitability. This study applies a modified surface rolling strategy, namely accumulative skin pass rolling (ASPR), to produce a gradient microstructure in copper and return an increase in yield strength while preserving high uniform elongation. Uniaxial tension combined with in-situ electron back-scattering diffraction compared the variations in mechanical properties and microstructure after annealing and ASPR. The increase in yield strength after ASPR is ascribed to the higher dislocation density at subgrain and grain boundaries, junctions and interiors whereas the preservation of uniform elongation is due to the gradient microstructure comprising a range of grain sizes across the sheet thickness. Finite element analysis of ASPR samples revealed strain heterogeneity along the gauge length and a transfer of load from softer grains in the sheet interior to harder grains closer to the sheet surface during tension. The results indicate that ASPR may be a promising new strategy for the continuous fabrication of sheets with gradient microstructures.