Large Ductility Research Articles

Exploring Large Ductility in Cellulose Nanopaper Combining High Toughness and Strength.

Cellulose nanopaper is a strong lightweight material made from renewable resources with a wide range of potential applications, from membranes to electronic displays. Most studies on nanopaper target high mechanical strength, which compromises ductility and toughness. Herein, we demonstrate the fabrication of highly ductile and tough cellulose nanopaper via mechanical fibrillation of hemicellulose-rich wood fibers and dispersion of the obtained cellulose nanofibrils (CNFs) in an ionic liquid (IL)-water mixture. This treatment allows hemicellulose swelling, which leads to dissociation of CNF bundles into highly disordered long flexible fibrils and the formation of a nanonetwork as supported by cryogenic transmission electron microscopy (cryo-TEM) imaging. Rheology of the suspensions shows a 300-fold increase in storage and loss moduli of CNF-IL-water suspensions, compared to their CNF-water counterparts. The nanopaper prepared by removing the IL-water shows a combination of large elongation (up to 35%), high strength (260 MPa), and toughness as high as 51 MJ/m3, because of efficient interfibrillar slippage and energy dissipation in the highly disordered isotropic structure. This work provides a nanostructure-engineered strategy of making ductile and tough cellulose nanopaper.

Nano-Dual-Phase Metallic Glass Film Enhances Strength and Ductility of a Gradient Nanograined Magnesium Alloy.

Magnesium (Mg) alloys are good candidates for applications with requirement of energy saving, taking advantage of their low density. However, the fewer slip systems of the hexagonal‐close‐packed (hcp) structure restrict ductility of Mg alloys. Here, a hybrid nanostructure concept is presented by combining nano‐dual‐phase metallic glass (NDP‐MG) and gradient nanograin structure in Mg alloys to achieve a higher yield strength (230 MPa, 31% improvement compared with the reference base alloy) and larger ductility (20%, threefold higher than the SMAT‐H sample), which breaks the strength–ductility trade‐off dilemma. This hybrid nanostructure is realized by surface mechanical attrition treatment (SMAT) on the surface of a crystalline Mg alloy, and followed by physical vapor deposition of a Mg‐based NDP‐MG. The higher strength is provided by the nanograin layer generated by SMAT. The larger ductility is a synergistic effect of multiple shear bandings and nanocrystallization of the NDP‐MG, inhibition of crack propagation from the SMATed nanograined structure by the NDP‐MG, and strain‐induced grain growth in the SMATed nanograin layer. This hybrid nanostructure design provides a general route to render brittle alloys stronger and ductile, especially in hcp systems.

Use of GFRP as retrofit alternative for confined masonry walls with window opening subjected to in-plane lateral load

The results from an experimental campaign of four confined masonry walls subject to cyclic lateral load are presented. In every case, the wall included a central opening. The feasibility of using glass fiber reinforced plastic (GFRP) to reinforce the opening perimeter was studied. For this, the specimens had different reinforcement conditions in that zone. The first wall was not reinforced (control specimen), the second specimen was reinforced with tie-columns and tie-beams and the third specimen included GFRP surrounding the opening. In addition, the applicability of the GFRP on the rehabilitation of masonry walls with openings was studied. For this purpose, the nonreinforced wall after testing was repaired and tested again. The results indicate that the reinforced wall with GFRP composite had similar strength and stiffness and a larger ductility than the reinforced wall with the traditional system through tie-columns and tie-beams. The rehabilitated wall recovered the lateral strength, even though its stiffness was lower than the original wall.

A new class of lightweight, stainless steels with ultra-high strength and large ductility

Steel is the global backbone material of industrialized societies, with more than 1.8 billion tons produced per year. However, steel-containing structures decay due to corrosion, destroying annually 3.4% (2.5 trillion US$) of the global gross domestic product. Besides this huge loss in value, a solution to the corrosion problem at minimum environmental impact would also leverage enhanced product longevity, providing an immense contribution to sustainability. Here, we report a leap forward toward this aim through the development of a new family of low-density stainless steels with ultra-high strength (> 1 GPa) and high ductility (> 35%). The alloys are based on the Fe–(20–30)Mn–(11.5–12.0)Al–1.5C–5Cr (wt%) system and are strengthened by dispersions of nano-sized Fe3AlC-type κ-carbide. The alloying with Cr enhances the ductility without sacrificing strength, by suppressing the precipitation of κ-carbide and thus stabilizing the austenite matrix. The formation of a protective Al-rich oxide film on the surface lends the alloys outstanding resistance to pitting corrosion similar to ferritic stainless steels. The new alloy class has thus the potential to replace commercial stainless steels as it has much higher strength at similar formability, 17% lower mass density and lower environmental impact, qualifying it for demanding lightweight, corrosion resistant, high-strength structural parts.

In situ investigation of the deformation behaviors of Fe20Co30Cr25Ni25 and Fe20Co30Cr30Ni20 high entropy alloys by high-energy X-ray diffraction

In situ synchrotron-based high-energy X-ray diffraction (HE-XRD) technique was employed to investigate the mechanical behaviors and microstructural evolution of face-centered cubic (FCC) Fe20Co30Cr25Ni25 and Fe20Co30Cr30Ni20 high entropy alloys (HEAs) during tensile deformation. Fe20Co30Cr30Ni20 HEA has a good combination of strength (ultimate tensile strength of 864 ± 35 MPa) and ductility (elongation of 0.627 ± 0.021). The HE-XRD investigation reveals that Fe20Co30Cr30Ni20 HEA has the transformation-induced plasticity (TRIP) effect, which starts at a critical stress of ~555 MPa. Transmission electron microscopy confirmed this deformation-induced new phase is hexagonal-close-packed structured ε-martensite, which follows an orientation relationship of {111}γ//(0001)ε, and ⟨110⟩γ//[112¯0]ε with the FCC γ matrix. The observation of deformation twins in the deformed samples of the two studied HEAs proves that twinning-induced plasticity (TWIP) effect occurs in both HEAs. The combination of TRIP and TWIP effects lead to the high strength, large ductility and improved strain hardening behavior of Fe20Co30Cr30Ni20 HEA.

In-situ microstructural investigations of the TRIP-to-TWIP evolution in Ti-Mo-Zr alloys as a function of Zr concentration

Aiming at overcoming the strength-ductility trade-off in structural Ti-alloys, a new family of TRIP/TWIP Ti-alloys was developed in the past decade (TWIP: twinning-induced plasticity; TRIP: transformation-induced plasticity). Herein, we study the tunable nature of deformation mechanisms with various TWIP and TRIP contributions by fine adjustment of the Zr content on ternary Ti-12Mo-xZr (x = 3, 6, 10) alloys. The microstructure and deformation mechanisms of the Ti-Mo-Zr alloys are explored by using in-situ electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results show that a transition of the dominant deformation mode occurred, going from TRIP to TWIP major mechanism with increasing Zr content. In the Ti-12Mo-3Zr alloy, the stress-induced martensitic transformation (SIM) is the major deformation mode which accommodates the plastic flow. Regarding the Ti-12Mo-6Zr alloy, the combined deformation twinning (DT) and SIM modes both contribute to the overall plasticity with enhanced strain-hardening rate and subsequent large uniform ductility. Further increase of the Zr content in Ti-12Mo-10Zr alloy leads to an improved yield stress involving single DT mode as a dominant deformation mechanism throughout the plastic regime. In the present work, a set of comprehensive in-situ and ex-situ microstructural investigations clarify the evolution of deformation microstructures during tensile loading and unloading processes.

Grain boundaries dependence of plastic deformation in nanocrystalline Cu film investigated by phase field and molecular dynamics methods

The phase filed model was used to create the nanocrystalline (NC) Cu film with more natural grain boundaries (GBs) for molecular dynamics (MD) simulation. The results obtained by using these methods are in good agreement with previous experimental results. In the NC Cu film, the plastic deformation is governed by the stacking faults (SFs) and deformation twinnings (DTs). DTs in NC Cu film have a great influence on the deformation behaviors of grains, which can make two adjacent grains merge into one larger grain, generate a new grain, and twist a grain. The model with average grain size (d) of 17.84 nm exhibits large flow stress and ductility because of the large fraction of hexagonal-close-packed (HCP) structures including SFs and twinning boundaries (TBs). Besides, the deformation of GBs in the 17.84 nm model is large, which can reduce the stress-concentration on the GBs and then results in delaying the generation of DTs. The deformation behavior of curved GBs in the initial tensile stage is that atoms migrate from face-centered cubic (FCC) structures to curved GBs along (111) surface, which is different from the shuffling motion of the flat GBs.

Effect of silicon addition on the microstructures, mechanical properties and helium irradiation resistance of NiCoCr-based medium-entropy alloys

A series of NiCoCrSix (x = 0, 0.1, 0.2, 0.3) medium-entropy alloys were synthesized to investigate the effect of Si addition on the microstructures, mechanical properties and helium irradiation resistance of the NiCoCr-based alloys. The microstructures and the phase evolution of these alloys were examined using electron microscopy as well as the existing models. The results show that the alloys exhibit a single face-centered-cubic structure when the Si content is in the range of 0–6.25 at.%, whereas phase decomposition occurs as the Si content further increases (∼9 at.%). Comparing with NiCoCr, NiCoCrSi0.2 exhibited improved strength without sacrificing the ductility. The yield strength and ultimate tensile strength increased from 438 to 599 MPa and 921 to 1032 MPa, respectively, whilst the ductility kept as high as ∼50%. Our analysis indicates that the increased strength is mainly ascribed to the grain boundary strengthening. The large ductility is primarily due to the low stacking fault energy and shear modulus of the NiCoCrSi0.2 alloy, which makes mechanical twinning in small grains accessible and provides a steady strain-hardening rate in a wide strain regime. Besides, the enhanced compositional complexity and local lattice distortion induced by the introduction of Si can improve the helium irradiation resistance of NiCoCr, resulting in reduced average bubble size.

Exceptional tensile properties under cryogenic temperature in heterogeneous laminates induced by non-uniform martensite transformation and strain delocalization

In the present study, the heterogeneous low C steel/304 austenite stainless steel (SS) laminates were observed to have exceptional tensile properties under cryogenic temperature, i.e., the yield strength and the uniform elongation were found to be significantly elevated simultaneously compared to these under room temperature. The underlying deformation mechanisms have been revealed by a novel tensile testing method coupled with in-situ digital image correlation imaging under cryogenic environment, and subsequent microstructure observations. Strain localization was found to initiate from low C steel, propagate across the interface and then towards 304 SS side. While the formed localized strain zone (LSZ) was observed to be delocalized at larger tensile strain due to the propagation of LSZ towards the un-deformed region along the gage length. Martensite transformation was found to be concentrated in the LSZ of 304 SS to regain strain hardening ability and reduce severity of strain concentration. Strain partitioning between 304 SS and low C steel was found to be more significant in the LSZ than that out of the LSZ. The non-uniform martensite transformation along the gage length should be the origin for the strain delocalization in the LSZ, resulting in large ductility in the laminates under cryogenic temperature.

Electric Current Dependence on Superplastic Tensile Flow in Tetragonal Zirconia Polycrystal Under a DC Field

Electric DC current dependence on the superplastic flow in densified, fine-grained tetragonal ZrO2 polycrystal was systematically investigated. An optimal current density for large tensile ductility existed; the maximum elongation of 135% was achieved under the current density of 250 mA∙mm-2 at a furnace temperature of 1000 oC and an initial strain rate of 1×10-3 s-1. Application of DC current decreased the flow stress, while that simultaneously accelerated the grain growth in tetragonal ZrO2 polycrystal. The reduced flow stress and improved ductility can be explained not only by Joule heating but also by enhanced atomic diffusion or accelerated grain boundary sliding.

Probabilistic seismic performance and loss evaluation of a multi-story steel building equipped with butterfly-shaped fuses

Shear fuses are structural elements that protect surrounding members from damages by undergoing substantial yielding, and then are easily replaced after a major earthquake event. Butterfly-shaped shear fuse is a promising type of structural system, which can effectively align member's flexural capacity to the imposed moment demand due to its unique geometry. Although recent studies suggest that butterfly-shaped fuses exhibit substantial ductility and energy dissipation, their impact on the global performance of multi-story buildings requires further investigation. This study presents a comprehensive risk-based evaluation of a six-story eccentrically braced steel frame retrofitted with butterfly-shaped fuses. Two nonlinear finite element models of the original prototype building and retrofitted building with butterfly-shaped fuses are developed in OpenSees and incremental dynamic analysis is conducted. The results are used to derive global and story-based fragility and seismic demand hazard curves. Furthermore, earthquake-induced losses associated with structural and non-structural assemblies are quantified and the impact of butterfly-shaped fuses on the distribution of story acceleration and drift demands are evaluated.The results show that butterfly-shaped fuses significantly improve the structure's performance in terms of all drift-related damage states and the improvement is more pronounced at severe damage states. In particular, the risk of exceeding complete damage state in the retrofitted building's lifetime is reduced to approximately one-fourth of the original building's values. Furthermore, shear fuses effectively mitigate weak story formation at lower stories due to their large energy dissipation and ductility. The improved drift-related performance reduces the drift-induced loss of structural and non-structural assemblies, resulting in 44.64% smaller total annual loss for the studied building. In addition, although butterfly-shaped fuses reduce the probability of exceeding slight damage state for the floor acceleration, their impact is negligible at higher acceleration-related damage states.

Snoek-type damping performance in strong and ductile high-entropy alloys.

Noise and mechanical vibrations not only cause damage to devices, but also present major public health hazards. High-damping alloys that eliminate noise and mechanical vibrations are therefore required. Yet, low operating temperatures and insufficient strength/ductility ratios in currently available high-damping alloys limit their applicability. Using the concept of high-entropy alloy (HEA), we present a class of high-damping materials. The design is based on refractory HEAs, solid-solutions doped with either 2.0 atomic % oxygen or nitrogen, (Ta0.5Nb0.5HfZrTi)98O2 and (Ta0.5Nb0.5HfZrTi)98N2. Via Snoek relaxation and ordered interstitial complexes mediated strain hardening, the damping capacity of these HEAs is as high as 0.030, and the damping peak reaches up to 800 K. The model HEAs also exhibit a high tensile yield strength of ~1400 MPa combined with a large ductility of ~20%. The high-temperature damping properties, together with superb mechanical properties make these HEAs attractive for applications where noise and vibrations must be reduced.

Ductility and strain hardening in gradient and lamellar structured materials

Low ductility has long been the bottleneck especially at high strength in metallic structured materials due to conventional forest hardening having little to no effect. Hetero-structuring is an emerging strategy producing superior strength and ductility combination. This Viewpoint article delineates mechanisms for strain hardening and plastic deformation in gradient and lamellar structured materials. Both have typical trans-scale grain hierarchy, leading to sharp mechanical incompatibility and consequent strain gradient at hetero-interfaces during plastic deformation. This induces heterogeneous deformation-induced hardening, along with recovered forest hardening, jointly to improve ductility. The heterogeneous deformation-related deformation physics sheds lights on the path to designing novel heterostructures particularly for large ductility at high strength.

Designing Concrete with Self-healing Properties Using Engineered Cementitious Composites as a Model

Engineered Cementitious Composites (ECC), also known as Strain Hardening Cement-based Composites, are an easily molded mortar-based composites reinforced with polymer fibers. ECCs are designed based on micromechanics and fracture mechanics theory, to feature large tensile ductility and a variety of unique properties, including tensile properties, superior to other fiber-reinforced composites. The properties of ECCs can be custom-tailored through micromechanics design due to the interaction between the fibers and cement matrix. A structural deterioration of ECC is avoided because the fibers do not allow cracks with large widths to form, unlike conventional concrete. ECCs have the capacity to bend, generating a flexible material. Ductile properties rather than brittle had increased, unlike ordinary concrete, leading to a wide variety of applications. Obtaining superior characteristics for ECC, both in fresh and hardened state, a transition from ECC paste to concrete with self-healing properties was made. To obtain self-healing concrete, with same ECC paste behavior and characteristics, the mix-design of the paste was optimized. The aim of this article is to present the experimental results regarding the transition from ECC paste to self-healing concrete and to analyze the results in order to establish a mix-design pattern for concrete with self-healing properties.

Implications of Bond-Slip in Inelastic Dynamic Responses of Degrading Structural Systems

The influence of bond-slip behavior, shown by pinched hysteretic curves in the inelastic response of degrading structural systems, is investigated in this paper. Inelastic response spectra are calculated and compared for deteriorating structures with and without manifestation of bond-slip. The response spectra are generated by using a smooth hysteretic model with distinct degrading parameters that are calibrated to the generic inelastic envelopes. In addition to the ductility demands, degraded residual strengths and stiffness of structural systems after an earthquake are considered as the relevant response measures. These measures can serve as design parameters for structures that should remain functional after an extreme event. Based on the response spectra analyses, it is found that nonlinear responses of degrading structural systems with bond-slip develop larger ductility demands and greater loss of strength and stiffness than the responses without bond-slip. Numerical results show that, ductility demand is increased up to 25–100% with consideration of slip in the model, as well as residual strength and stiffness decreased up to almost half.

High strength and large ductility of a fine-grained Al–Mg alloy processed by high strain rate hot rolling and cold rolling

Achieving high strength and large ductility simultaneously has been a long-standing goal in Al alloy. In the present study, a superior combination of ultimate strength (354 ± 1 MPa) and elongation (34 ± 0.9%) are achieved in a fine-grained (4.7 μm) Al-6.5 Mg alloy prepared by high strain rate hot rolling (HSRR) at 400 °C. The ultimate strength can be further improved to 454 ± 2 MPa after the subsequent cold rolling (CR), accompanied by an acceptable good ductility of 9.2 ± 1.1%. The microstructural evolution is characterized by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD). It has been shown that the high ductility of HSRRed alloy is attributed primarily to the high content of solute Mg atoms, high fraction of fine grains with high-angle grain boundaries (HAGBs) and the weak texture, while the high strength is mainly due to the combination strengthening effect from strain hardening, grain refinement and high content of solute Mg atoms. This study provides a possible strategy to gain high strength and large ductility, i.e. obtaining fine grains by enhanced recrystallization resulted from high strain rate and high content of solute Mg atoms, and then followed by cold working hardening.

Theory of designing the gradient microstructured metals for overcoming strength-ductility trade-off

Gradient microstructure design has been found to be an effective approach to significantly produce the utmost advanced structural metallic materials, which have a signature in mechanical characteristic of strength-ductility-hardenability combination. In this work, a physical theory and mechanism-based numerical model are developed for sophisticatedly designing the gradient microstructured metals. It considers the gradient distribution of grain size, twin, martensite and austenite in a metallic alloy. The numerical results are validated through experiments. The mechanical performance of high strength, large ductility and considerable hardenability is achieved and their trade-off is successfully overcome.

Temperature Dependence of the Static and Dynamic Behaviour in a Quenching and Partitioning Processed Low-Si Steel

Because of their excellent combination of strength and ductility, quenching and partitioning (Q & P) steels have a great chance of being added to the third generation of advanced high strength steels. The large ductility of Q & P steels arises from the presence of 10% to 15% of retained austenite which postpones necking due to the transformation induced plasticity (TRIP) effect. Moreover, Q & P steels show promising forming properties with favourable Lankford coefficients, while their planar anisotropy is low due to a weak texture. The stability of the metastable austenite is the key to obtain tailored properties for these steels. To become part of the newest generation of advanced high strength steels, Q & P steels have to preserve their mechanical properties at dynamic strain rates and over a wide range of temperatures. Therefore, in the present study, a low-Si Q & P steel was tested at temperatures from −40 °C to 80 °C and strain rates from 0.001 s−1 to 500 s−1. Results show that the mechanical properties are well-preserved at the lowest temperatures. Indeed, at −40 °C and room temperature, no significant loss of the deformation capacity is observed even at dynamic strain rates. This is attributed to the presence of a large fraction of austenite that is so (thermally) stable that it does not transform in the absence of deformation. In addition, the high stability of the austenite decreases the elongation at high test temperatures (80 °C). The additional adiabatic heating in the dynamic tests causes the largest reduction of the uniform strain for the samples tested at 80 °C. Quantification of the retained austenite fraction in the samples after testing confirmed that, at the highest temperature and strain rate, the TRIP effect is suppressed.

Bimodal microstructure dispersed with nanosized precipitates makes strong aluminum alloy with large ductility

A bimodal microstructure composed of large primary α-Al phases (PPs) and small secondarily solidified grains (SSGs) is fabricated in 6061 aluminum alloy by powder thixoforming (PTF) method after solution treatment. Artificial aging treatment introduces nanosized precipitates into the bimodal microstructure, in which the number and growth rate of precipitates are different: the number in PPs is larger than that in SSGs due to the higher density of dislocations and lattice distortions in PPs, while the growth rate in SSGs is higher than that in PPs owing to the higher solute concentration in SSGs. As the number and size of precipitates are optimized in the bimodal microstructure, i.e., in peak-aged state, remarkable increments of 56.1% and 130.3% in ultimate tensile strength and yield strength while a moderate increase of 6.3% in ductility are achieved compared with the as-fabricated state, which overcomes the trade-off in strength and ductility of conventional aging-treated 6061 alloys with unimodal microstructure. The strengthening comes from the geometrically necessary dislocations induced by the deformation incompatibility of bimodal microstructure and from precipitate strengthening by nanosized precipitates, while the toughening is from the enhanced dislocation accumulation in this alloy.

Ductility of an ultrastrong glass-crystal nano-dual-phase alloy in sub-micron

We report an excellent tensile property of the Mg-based nano-dual-phase glass-crystal alloy in sub-micro size. It shows room temperature large ductility (>50% true strain, resulted from the localized necking deformation) with ultrahigh tensile strength (2.4 GPa), compared with 1.4 GPa strength and limited ductility of its metallic glass counterpart. To unraveling the underlying reasons, we have made transmission electron microscope (TEM) in-situ tensile investigation and molecular dynamic (MD) simulation for the materials, showing that homogenous plastic flow of nano-sized metallic glass phase activated by strain non-localization mechanism is responsible for the large ductility.