Exceptional Ductility Research Articles

Static and dynamic tests on steel joints equipped with novel structural details for progressive collapse mitigation

This paper presents static and dynamic tests on nominally-pinned steel joints equipped with novel structural details for progressive collapse mitigation. The proposed structural details utilise the exceptional ductility and strength of stainless steel pins to enhance both the tie force and the rotational capacity of a vulnerable steel joint. The stainless steel pins along with additional supporting elements are installed in the joint region without interfering with the design for gravity loads, and they can be used for both new designs and to retrofit existing steel buildings. A static test on a vulnerable industry-standard steel fin-plate connection is first presented followed by two static tests on the same connection retrofitted with the proposed structural details. The retrofitted connections were subsequently tested under dynamic conditions with increasing imposed loading using a test setup that simulates a sudden column loss scenario. The test results showed that nominally-pinned joints equipped with the proposed structural details can achieve the required tie force capacity while undergoing rotations larger than 0.2 rad. Analytical equations based on simple joint equilibrium are used to validate the results of the static tests. An analytical method based on the energy conservation principle is also proposed and comparison with the dynamic tests shows very good predictive capability when the assumed loss of energy is 22%.

Co-introduction of precipitate hardening and TRIP in a TWIP high-entropy alloy using friction stir alloying

Tuning deformation mechanisms is imperative to overcome the well-known strength-ductility paradigm. Twinning-induced plasticity (TWIP), transformation-induced plasticity (TRIP) and precipitate hardening have been investigated separately and have been altered to achieve exceptional strength or ductility in several alloy systems. In this study, we use a novel solid-state alloying method—friction stir alloying (FSA)—to tune the microstructure, and a composition of a TWIP high-entropy alloy by adding Ti, and thus activating site-specific deformation mechanisms that occur concomitantly in a single alloy. During the FSA process, grains of the as-cast face-centered cubic matrix were refined by high-temperature severe plastic deformation and, subsequently, a new alloy composition was obtained by dissolving Ti into the matrix. After annealing the FSA specimen at 900 °C, hard Ni–Ti rich precipitates formed to strengthen the alloy. An additional result was a Ni-depleted region in the vicinity of newly-formed precipitates. The reduction in Ni locally reduced the stacking fault energy, thus inducing TRIP-based deformation while the remaining matrix still deformed as a result of TWIP. Our current approach presents a novel microstructural architecture to design alloys, an approach that combines and optimizes local compositions such that multiple deformation mechanisms can be activated to enhance engineering properties.

Achieving an exceptional ductility at room temperature in a low SFE copper alloy fabricated by severe plastic deformation

Abstract Low stacking fault energy alloys often exhibit low ductility. However, sometimes these alloys show unusual mechanical behavior following specific processing parameters. In this study, a low stacking fault energy α+β brass was processed by equal channel angular pressing (ECAP) at 350 °C in ‘route C’ through three passes. Room temperature tensile testing showed that ductility, surprisingly, reached ~80%, whilst good tensile strength was maintained. Investigation of alloy microstructure revealed a combination of deformation mechanisms, including slip and twinning, accompanied by grain boundary serration and grain fragmentation. It is suggested that these deformation modes triggered these unexpected mechanical properties in this intrinsically brittle alloy. The required energy for discontinuous recrystallization was supplied after only one pass in the α phase, and three passes in the β phase, which then prompted good ductility. In order to study texture evolution, the macro-texture was measured. A high fraction of recrystallized grains and change in strain path fostered the development of Goss, Rotated Goss and Rotated Cube components in the α phase. While in the β phase {011} and Goss components were dominant. Despite the high Schmid factor, Hall-Petch effects and work hardening led to increase of strength and hardness after the final pass.

Multistage work hardening assisted by multi-type twinning in ultrafine-grained heterostructural eutectic high-entropy alloys

High strength of materials usually comes with low ductility due to the lost or short-lived strain hardening. Here, we uncover a sequentially-activated multistage strain hardening (SMSH) that allows for sustained and effective strain-hardening capability in strong ultrafine-grained eutectic high-entropy alloy (EHEA). Consequently, exceptional ductility is realized in an ultrafine-grained EHEA, accompanied with high ultimate strength. We demonstrate that the SMSH is derived from a coordinated three-level design on structural heterogeneity, grain-size control, and intragranular composition modification, which enables the sequential activation of stress-dependent multiple hardening mechanisms. Furthermore, despite the well-known low twinning propensity due to ultrafine grains and medium-to-high stacking fault energies of prototype EHEAs, our coordinated design sequentially activates three types of deformation twinning to assist this SMSH. This work sheds light on the SMSH effect assisted by multi-type twinning previously unexpected in ultrafine-grained EHEAs, and thereby represents a promising route for improving ductility of high-strength materials.

The role of minor alloying in the plasticity of bulk metallic glasses

Micro- or minor alloying of metallic glasses is of technological interest. An originally ductile Pd-based monolithic bulk metallic glass (Pd40Ni40P20) was selectively manipulated by additions of Fe or Co. The alloying effects were extreme, showing either exceptional ductility upon Co addition or immediate catastrophic failure upon Fe addition when tested under uniaxial compression or 3-point bending. The amorphous structure was characterized prior to deformation with respect to its medium-range order (MRO) using variable resolution fluctuation electron microscopy (VR-FEM). We observe striking differences in the MRO between the ductile and brittle metallic glasses, with the ductile glasses exhibiting a rich structural diversity and MRO correlation lengths up to 6 nm. The MRO diversity seems to enable easier shear banding and hence enhance the deformability.

Design of a high strength, high ductility 12 wt% Mn medium manganese steel with hierarchical deformation behaviour

A novel medium Mn steel of composition Fe-12Mn-4.8Al-2Si-0.32C-0.3V was manufactured with 1.09 GPa yield strength, 1.26 GPa tensile strength and 54% elongation. The thermomechanical process route was designed to be industrially translatable and consists of hot and then warm rolling followed by a 30 min intercritical anneal. The resulting microstructure comprised of coarse elongated austenite grains in the rolling direction surrounded by necklace layers of fine austenite and ferrite grains. The tensile behaviour was investigated by in-situ neutron diffraction and the evolution of microstructure studied with Electron Backscattered Diffraction (EBSD). It was found that the coarse austenite grains contributed to the first stage of strain hardening by transforming into martensite and the fine austenite necklace grains contributed to the second stage of strain hardening by a mixture of twinning and transformation induced plasticity (TWIP and TRIP) mechanisms. This hierarchical deformation behaviour contributed to the exceptional ductility of this steel.

Fabrication and Characterization of In Situ Zn-TiB2 Nanocomposite

Zinc (Zn) matrix composite has been newly discovered categories of biodegradable materials. With a combination of chemical stability, thermal stability and biocompatibility, ceramic nanoparticles outperformed intermetallics of zinc alloys with inherent advantages of retaining a proper corrosion rate and an exceptional ductility. Compared with Zn alloys, Zn matrix nanocomposites showed an unprecedented strengthening without sacrifices of corrosion rate, which were introduced by intermetallics. In this work, in situ titanium diboride (TiB2) reinforced Zn nanocomposite was prepared via a few cost-effective and economical methods: flux-assisted synthesis (FAS), ultrasound-assisted nanoparticle homogenization and hot rolling. 3 vol.% of TiB2 nanoparticles were synthesized with an average size of 454nm, followed by molten salt assisted ultrasound homogenization and hot rolling. Hot-rolled (HR) Zn-TiB2 performed high strength and high ductility, mostly due to precipitation strengthening (Orowan strengthening). Yield stress (YS) and ultimate tensile stress (UTS) increased by 90% and 45%, respectively, while the elongation to failure retained 23%. The mechanical performance of Zn-TiB2 made it promise to serve as an innovative biodegradable material for load-bearing applications.

Improving mechanical properties of an FCC high-entropy alloy by γ′ and B2 precipitates strengthening

The FCC-structured high entropy alloys (HEAs) possess exceptional ductility and fracture toughness, but they generally exhibit insufficient strength for engineering applications. In this work, a precipitation strengthening non-equimolar (FeCoNi)81Cr9Al8Ti1Nb1 HEA was fabricated via arc melting and subsequent two-step aging treatment. The microstructure, phase constitution and mechanical properties of this alloy during aging were investigated systematically. The results indicate that coherent γ′ and incoherent B2 are major precipitates for the alloy under the two-step aging. An excellent balanced tensile property is achieved at room temperature even with extensive B2 grain boundary coverage. Quantitative calculations of the individual strengthening effects demonstrate that particle (γ′) shearing mechanism is the predominant strengthening mechanism. The high work-hardening capability of the FCC matrix could greatly suppress the propagation of microcracks originated at these brittle B2 phases and promote a retention of ductility. In addition, this alloy exhibits outstanding high-temperature tensile properties up to 700 °C. It is attributed to the high thermal stability of the γ′ precipitates as well as the pinning effect of the grain-boundary B2 phases on the grain boundary. Present work will focus on optimizing of the alloy design of HEAs and the precipitation strengthening of HEAs for high-temperature structural applications.

Effect of Numbers of Turns of High‐Pressure Torsion on the Development of Exceptional Ductility in Pure Magnesium

The low ductility of magnesium at room temperature is usually attributed to an insufficient number of independent slip systems. Recent research has shown that refining the grain structure of pure magnesium promotes a breakdown in the Hall‐Petch relationship at low strain rates and may lead to the development of exceptional ductilities. This report describes the evolution of microstructure and the mechanical behaviour of pure magnesium using different amounts of imposed plastic deformation by high‐pressure torsion (HPT). It is shown that the initial coarse grains undergo twinning followed by a gradual grain refinement. The flow stress at low strain rates decreases as the grain size is reduced, thereby confirming an inverse Hall‐Petch behaviour. The elongation to failure increases with grain refinement and elongations over 300% are achieved after 1/2 turn of HPT. These experimental data agree with a model for the low temperature deformation behaviour of fine‐grained pure magnesium.

Processing Variants in Medium-Mn Steels

This paper highlights some recent efforts to extend the use of medium-Mn steels for applications other than intercritically batch-annealed steels with exceptional ductility (and strengths in the range of about 1000 MPa). These steels are shown to enable a range of promising properties. In hot-stamping application concepts, elevated Mn concentration helps to stabilize austenite and to provide a range of attractive property combinations, and also reduces the processing temperatures and likely eliminates the need for press quenching. The “double soaking” concept also provides a wide range of attractive mechanical property combinations that may be applicable in cold-forming applications, and could be implemented in continuous annealing and/or continuous galvanizing processes where Zn-coating would typically represent an additional austempering step. Quenching and partitioning of steels with elevated Mn concentrations have exhibited very high strengths, with attractive tensile ductility; and medium-Mn steels have been successfully designed for quenching and partitioning using room temperature as the quench temperature, thereby effectively decoupling the quenching and partitioning steps.

A promising CoFeNi2V0.5Mo0.2 high entropy alloy with exceptional ductility

A novel high entropy alloy with the composition of CoFeNi2V0.5Mo0.2, shows unusual strain hardening ability and an excellent combination of ductility and strength. Especially at 873 K, the elongation can reach 189.2%, almost three times that of the common high temperature alloys. Microstructural characterization suggests a coexistence of planar slip and cross slip. The resistance of planar slip serves to strengthen the alloy, while the cross slip effectively relieves strain concentration, substantially contributing to the uniform elongation. Also, the interaction of planar slip and cross slip will lead to the formation of two kinds of dislocation loops.

Processing Magnesium and Its Alloys by High‐Pressure Torsion: An Overview

Magnesium and its alloys have attracted significant attention in recent years because they display high strength‐to‐density ratios, they are biodegradable and they provide a potential for hydrogen storage. Many investigations have examined the effect of high‐pressure torsion processing on the microstructures and properties of these materials so that numerous reports are now available. This overview provides a summary of the observations reported to date on the structure and mechanical property evolution including the nature of grain refinement, the grain boundary misorientation distributions, texture evolution, and the minimum grain size. For convenience, the mechanical properties are separated into hardness, tensile behavior, and superplastic properties. It is shown that the mechanism of grain refinement differs from other metallic materials processed by severe plastic deformation but high strength may be achieved in magnesium alloys and exceptional ductility in pure magnesium. Hydrogen storage and corrosion behavior are also examined together with a discussion of recent attempts to produce magnesium‐based nanocomposites through processing by high‐pressure torsion.

Ultrahigh cryogenic strength and exceptional ductility in ultrafine-grained CoCrFeMnNi high-entropy alloy with fully recrystallized structure

Similar to the conventional alloys, attaining excellent combination of high strength and ductility is still challenging in the high-entropy alloys (HEAs). In this work, a fully recrystallized ultrafine-grained (UFG) microstructure was introduced into the single-phase CoCrFeMnNi HEA via cold rolling and annealing process. The UFG HEA displays ultrahigh yield and ultimate tensile strengths of 1.24 GPa and 1.46 GPa, respectively, and excellent ductility with a uniform elongation of 41% at 77 K. The exceptional strength-ductility combination in the UFG CoCrFeMnNi HEA surpasses those of all existing HEAs and other advanced engineering alloys. Such unparalleled mechanical properties stem from the fully recrystallized ultrafine grains that significantly strengthen the material and simultaneously provide high strain-hardening capability and the activations of multiple deformation mechanisms consisting of prominent dislocation activities, profuse stacking faults, and deformation twins. The fully recrystallized UFG microstructures do not only shed light on the design and development of HEAs with superior mechanical properties, but also enrich the potential applications of HEAs at the cryogenic environment.

Non-equiatomic FeNiCoAl-based high entropy alloys with multiscale heterogeneous lamella structure for strength and ductility

A new method based on conventional thermomechanical processing is proposed for design of non-equiatomic FeNiCoAlTaB (NCATB) high entropy alloy (HEA) with multiscale heterogeneous lamella (HL) structures to achieve outstanding mechanical properties. Two kinds of microstructures are produced in this study: 1) specimens annealed at 1050 °C exhibit structure composed of fine grains and ultrafine grains, which exhibits a combination of high strength (UTS ~1050 MPa) and good ductility (elongation ~23%); and 2) specimens annealed at 1200 °C which exhibit structure composed of fine grains and coarse grains, which demonstrates excellent synergy of good strength (UTS ~890 MPa) and exceptional ductility (elongation ~43%). It has been shown that shear bands formed in the cold rolling step are responsible for the fine-grain area in both specimens. Additionally, precipitation of NiAl (B2) particles also affects the formation of HL structures due to Zener pinning effect. The back-stress strengthening mechanism, responsible for high strength and ductility, is verified through combined “loading-unloading-reloading” (LUR) cyclic tensile tests and electron backscatter diffraction (EBSD) based geometrically necessary dislocation (GND) density analysis.

Experimental tests and design of rubberised concrete-filled double skin circular tubular short columns

The adequacy of rubberised concrete (RuC) for use in structural columns is, currently, investigated experimentally through the use of cold-formed double-skin circular steel tube confinement. The RuC is of particular interest because the aggregate can be sourced from recycled tyres, so it is a form of sustainable concrete, and it possesses superior mechanical properties to conventional concrete such as increased ductility and energy absorption. Rubberised concrete does have one major issue in that it has a low compressive strength compared to normal concrete, which limits its application. The aim of this study is to evaluate the effectiveness of confinement in overcoming this mechanical deficiency by using rubberised concrete-filled double-skin tubes (RuCFDST). The experimental program involves testing and measurements of key mechanical properties including compressive strength, hoop and axial strains, and compressive load-deflection curves. A total of 15 composite specimens were examined to ascertain the varying properties of single-skin, double-skin, confined, unconfined, standard and rubberised concrete. Recycled rubber particles ranging from two to seven millimeters in size were used to replace 15% and 30% of the fine and coarse concrete aggregate by weight. The rubber particles were treated with sodium hydroxide solution resulting in increased bonding strength to the concrete. Circular hollow section (CHS) configurations of different internal and external dimensions were also examined to further understand the mechanics of double-skin confinement. Confinement in RuCFDST showed significant improvements in strength and ductility properties. Experimental results proved to be in agreement with design ultimate axial strength predictions proposed by existing methods and design codes. Given the exceptional ductility, energy dissipation and improved strength of RuCFDST, this study shows the potential viability of RuCFDST as structural columns particularly in areas that are prone to seismic activity.

Stress corrosion cracking of laser alloyed 304L stainless steel with Ru in hot chloride solution

The exceptional ductility and good uniform corrosion resistance of austenitic stainless steels especially the commonly used 304L stainless steel is lost with its severe embrittling in aqueous conditions in the presence of chlorides at elevated temperatures. With the addition of ruthenium (Ru) the corrosion resistance of 304L stainless steel is enhanced, as well as the resistance to stress corrosion cracking. However, the cost of Ru is high and this limits the possibilities. Therefore, to lower the cost of Ru, a layer of approximately 950 ± 90 μm was laser cladded on the surface of 304L stainless steel. The purpose of this investigation is to evaluate the effectiveness of Ru additions to surface layer through laser cladding to mitigate cracking. Ru was applied to the surface of 304L stainless steel with a Nd:YAG laser as a metal powder with Ru concentrations of 1, 2, 5 and 10 wt%. With these methods, the cost of Ru is kept low while it can still cause inhibition of cracking. Three-point bend stressed samples were exposed to distilled water with 100 ppm sodium chloride at 200 °C with an initial dissolved oxygen concentration of 8 ppm at 25 °C. As expected the as-received 304L stainless steel was susceptible to stress corrosion cracking. With the addition of Ru to 304L stainless steel, the resistance to cracking improved markedly, the minimum and maximum crack propagation rate attained was 0.013 and 0.023 μm/s respectively. The crack propagation rate decreased as the Ru content was increased, and cracking was inhibited at 5 and 10 wt% Ru.

Warm ductility enhanced by austenite reversion in ultrafine-grained duplex steel

The current work investigated the relationship between microstructure and warm deformation properties in a strong but ductile Mn-rich steel. A cold-rolled Fe-11.3Mn-0.068C-0.3Si-1.1Al-0.25Mo-0.01P-0.01S-0.0003N (in wt. %) steel was deformed isothermally after inter-critical annealing at temperatures from 550 °C to 720 °C. Deformation at 650 °C led to exceptional ductility, corresponding to total elongation of over 100%. The microstructure was characterized by electron backscattered diffraction, transmission Kikuchi diffraction, and transmission electron microscopy. It was found that the rate of austenite reversion can be accelerated by deformation, and that the transformation makes strained austenite into equiaxed grains. Exceptional ductility can be achieved when warm deformation is accompanied by austenite reversion. This research will provide metallurgical principles for warm deformation of steel under reversed transformation.

Precipitation hardening in CoCrFeNi-based high entropy alloys

High entropy alloys (HEAs) with a face-centered cubic (fcc) structure have received an increasing attention in recent years due to their exceptional ductility and fracture toughness. On the other hand, they generally show lower strengths at ambient and elevated temperatures. In this review, we attempt to highlight recent advances in the design of computation-aided HEAs being alloyed with Al, Al/Ti, Nb and Mo additions on the precipitation hardening behavior of this fcc-type CoCrFeNi alloy, and to summarize the corresponding mechanical properties of the newly developed precipitation-strengthened HEAs under different thermomechanical processing. In particular, our emphasis is played on elucidating the correlation between precipitation behaviors and mechanical properties, which may serve as a demonstrative summary for the current progress in the scientific research of precipitation-strengthened HEAs. Finally, future research areas for these precipitation-strengthened HEAs are critically assessed.

High performance poly (lactic acid)/bio-polyamide11 through controlled chain mobility

This study shows that controlled PLA chain mobility in poly (lactic acid)/polyamide11 (PLA/PA11) blends can lead to high performance materials. The addition of an appropriate amount of polyethylene oxide (PEO) or polyethylene glycol (PEG) to a PLA/PA11 blend reduces the interfacial tension and increases the interfacial interactions due to an increase in the PLA chain mobility. The mechanical properties of (PLA-PEO)/PA11 50/50 systems are highly dependent on the PEO composition and at an optimal PEO concentration show a remarkable improvement in the Izod impact strength with specimens demonstrating 17.5 times the impact strength of the neat PLA and 3 times that of neat PA11. Also, these same blends show exceptional ductility with an elongation at break of 275% as compared to 5% for the neat PLA and 6% for PLA/PA11 without added PEO. Analysis of the fracture surface of the tough blends after impact indicates controlled interfacial debonding/cavitation at the continuous PLA/PA11 interface which leads to extensive shear yielding in the surrounding matrices. An excellent correlation is found between the optimal PEO composition for high performance properties and PEO limited miscibility in the PLA phase. When PEO phase separates in the PLA it tends towards the PLA/PA11 interface and provides a locus of failure for interfacial debonding. PEG is not as effective as PEO and it has virtually no effect on the mechanical properties of the PLA/PA11 blend since it phase separates at lower concentrations, thus limiting its effect on PLA mobility and ultimately providing insufficient plastic deformation to the PLA matrix. These results indicate that controlled PLA chain mobility through the use of an optimal concentration of an appropriate plasticizer is a highly effective approach towards the development of high performance PLA/PA11 blends.

Precipitation strengthening of ductile Cr15Fe20Co35Ni20Mo10 alloys

Non-equiatomic Cr15Fe20Co35Ni20Mo10 (Mo10) high-entropy alloys (HEAs) are produced by arc-melting and subsequent thermal-mechanical processing. The single-phase Mo10 HEAs with face centered cubic (FCC) structure exhibit an exceptional ductility and strain hardening capability associated with twinning. Tailoring thermal-mechanical processing condition enables the evolvement of single-phase Mo10 HEAs to multiphase microstructure that comprises simple FCC solid solution and nanoscale precipitates of Mo-rich μ phase. These precipitates primarily grow at grain boundaries and twin boundaries, strengthening Mo10 HEAs. This work sheds light on realizing a superb strength-ductility combination of HEAs by coupling solid solution hardening, nanotwinning and precipitation hardening mechanisms.