High Tensile Ductility Research Articles

Compressive behavior of steel spiral confined engineered cementitious composites in circular columns

Extensive research has been conducted on the uniaxial tensile and compressive behavior of engineered cementitious composites. Despite the high tensile ductility and high toughness of engineered cementitious composites, transverse steel reinforcement is still necessary for high-performance structural members made of engineered cementitious composites. However, very limited research has been concerned with the compressive behavior of steel-confined engineered cementitious composites. This article presents the results of axial compression tests on a series of circular engineered cementitious composite columns confined with steel spirals. The test variables included the engineered cementitious composite compressive strength, the spiral pitch, and the spiral yield stress. The test results show that steel-confined engineered cementitious composites in the test columns exhibited a very ductile behavior; the steel spiral confinement contributed effectively to the enhancement of both strength and ductility of engineered cementitious composites. The test results were then interpreted by comparing them with the predictions from some existing models. It was found that the existing models previously developed for confined concrete failed to predict the compressive strength of steel-confined engineered cementitious composites with sufficient accuracy. New fitting equations for the compressive properties of steel-confined engineered cementitious composites were then obtained on the basis of the test results of this study as well as those from an existing study.

Strain-Rate Sensitivity, Tension-Compression Asymmetry, r-Ratio, Twinning, and Texture Evolution of a Rolled Magnesium Alloy Mg-1.3Zn-0.4Ca-0.4Mn

In this work, the deformation response, texture evolution, and twinning development of a magnesium (Mg) alloy, Mg-1.3Zn-0.4Ca-0.4Mn, for biocompatible applications are investigated. Further, the alloy’s formability, by examining the instantaneous r-ratio and strain-rate sensitivity (SRS) as a function of strain and loading direction, is investigated. It is found that after rolling and peak aging, the alloy has a rolled texture of moderate intensity with the basal planes contained in the rolling plane and with a bimodal, fine-grained microstructure. The alloy shows both high room-temperature tensile strength (300 MPa) and ductility (25 pct) in the rolling direction (RD) and, remarkably, r-ratios saturating close to unity in all three in-plane testing directions. It is also found that the SRS is relatively high and uniform, with averages ranging from 0.015 to 0.025, depending on the in-plane testing directions. These are outstanding properties compared to pure Mg and most of its biocompatible alloys. Typical of Mg alloys, this alloy has a propensity for twinning by multiple twin modes, which leads to rapid texture evolution, anisotropy, and tension-compression (T-C) asymmetry in yield stress, with compression having the weaker response. These mechanical characteristics along with their microstructural origins are presented and discussed in this article.

The novel low cost as-casting Al-based composites with high strength and ductility

In this paper, three alloys Al81Cu13Si6, Al88Cu8Si4 and Al92Cu5.6Si2.4 were prepared by a simple solidification process. By controlling the alloy composition, different microstructures and mechanical properties have been achieved. The Al81Cu13Si6 alloy with micro-scale binary eutectic compositions embedded in an ultrafine ternary eutectic matrix exhibits the high yield strength (up to 750 MPa) but fails with a brittle fracture. While the Al92Cu5.6Si2.4 alloy with hypereutectic composite structure exhibits the optimum comprehensive mechanical properties with both high tensile strength and ductility. The present results provide a new perspective to prepare Al-based alloy with high strength and ductility by designing the some new composite with optimum microstructure.

High-temperature age-hardening of a novel cost-effective Fe45Ni25Cr25Mo5 high entropy alloy

Considering the high potential of age-hardening in metastable non-equiatomic high-entropy alloys (HEAs), the high-temperature aging behaviour of a newly developed cost-effective Fe45Ni25Cr25Mo5 HEA with a face-cantered cubic (FCC) matrix was investigated. The alloy showed high age-hardening response at temperatures up to 900 °C and the peak ageing was obtained at 900 °C for 48 h due to the precipitation of more monoclinic Cr5.5Mo1.5Fe6.5Ni1.5-type σ phase. Although the as-cast Fe-enriched HEA composed of FCC and σ phases possessed balanced high tensile strength and ductility compared with previously reported as-cast FCC-dominated HEAs, the peak ageing only led to substantial improvement in compression properties with compression yield strength of 543.7 MPa, ultimate strength of 1098.1 MPa and strain to fracture of 40%. The peak aged alloy exhibited tensile brittleness at room temperature, which was attributed to the restriction of dislocation movement by the precipitation of large needle-shaped intermetallics. Furthermore, the newly developed alloy showed high oxidation and corrosion resistance, which was superior to the 316L stainless steel and was comparable to the Inconel 625 super alloy. Thus, this HEA can be potentially used as a superalloy.

High oxygen-content titanium and titanium alloys made from powder

Oxygen remains to be a central issue in the manufacture of titanium (Ti) and its alloys from powder. The strong and short-range repulsion between interstitial oxygen and screw dislocation core strengthens titanium but compromises its resistance to cracking. It is therefore challenging to produce strong and ductile (tensile) Ti materials from high oxygen-content powder. This article reviews the tensile properties of high oxygen-content α-Ti, (α+β)-Ti alloys, and β-Ti alloys made from powder. A minimum tensile ductility value of 6% is used to define ductile Ti according to existing technical specifications for Ti-6Al-4V and other powder metallurgy materials. Two post-processing methods, solution heat treatment and thermomechanical processing, are identified to be effective remedies to convert non-ductile high oxygen-content Ti alloys into ductile ones. The underlying ductility-improvement mechanisms are discussed, which vary with alloy system. In particular, redistribution of oxygen between α and β phases by solution heat treatment offers a practical remedy to substantially improve tensile ductility, e.g., from 6% to 19% for the Ti-0.94 wt%O alloy. It is shown that appropriate post processing can enable a wide variety of Ti alloys made from powder to tolerate ≥0.5 wt%O for ≥6% tensile ductility for structural applications. • Redistribution of oxygen by solution heat treatment can reverse the detrimental effect of oxygen for Ti. • Thermomechanical processing can enable high oxygen-content Ti alloys to offer high tensile ductility (>15%). • Alloy composition affects exploitation of high oxygen Ti powder by PM, MIM and AM. • Alloy design plus post processing allows fabrication of strong and ductile Ti alloys from high oxygen Ti powder.

Numerical Study of Fracture Behavior of Gradient Nano-Grained Metals by Molecular Dynamics Simulations

Benefit from the gradient distribution of microstructure, gradient nanograined (GNG) metals have broad application prospect owing to their advantages of both high strength and good tensile ductility. Meanwhile, the fracture behavior of gradient nanograined metals is different from that of traditional homogeneous materials. Using molecular dynamics (MD) method, we simulated the propagation of a crack in a pre-cracked GNG Cu. Voronoi method was adopted to generate the polycrystalline topology with gradient grain size, and FCC copper atoms were filled into the topological structure. The crack was introduced by removing three layers of atoms. Then, the MD specimen was loaded to simulate the crack growth and/or blunting. The micro-defects were identified by the common neighbor analysis parameter. The effects of the grain size gradient and the crack tip initial position on the crack growth were also investigated.

Fracture of Void-Embedded High-Entropy-Alloy Films: A Comprehensive Atomistic Study

Comprehensive molecular dynamics (MD) simulations are performed to study the stress response and deformation mechanism in void-embedded, single-crystal and nanocrystalline, high-entropy-alloy (HEA) films under uniaxial tensile loading. Our results reveal that certain void-embedded HEA films can be, by far, superior to pure Ni in terms of tensile ductility and the resistance to crack propagation. The fracture strains of 10%Co CoCrFeMnNi and the equiatomic CoFeMnNi, respectively, double or triple that of the equiatomic CoCrFeMnNi, which still doubles that of pure Ni. Regarding the deformation mechanism, high tensile ductility of HEAs can be attributed to the formation of partial dislocations, nanotwinning and the impediment of the otherwise glissile dislocations due to the weak pinning effect. The ultimate tensile strength of HEA film shows better resistance against stress deterioration due to elliptical voids. The stress response of the void-embedded, nanocrystalline Ni films obeys the reverse Hall-Petch effect, while the void-embedded, equiatomic HEA film does not. The maximum stresses of nanocrystalline HEA films of different grain size are virtually the same. The void-embedded, finer-grain HEA film exhibits a better tensile ductility than the void-embedded, coarser-grain HEA film.

Elasticity, piezoelectricity, and mobility in two-dimensional BiTeI from a first-principles study

Through first-principle calculations, we investigate the stability, electronic structures, elasticity, piezoelectricity, and mobility of 2D BiTeI monolayer. Our calculations show that 2D BiTeI monolayer is energetically, mechanically, thermodynamically, and dynamically stable. In addition to its well-known large Rashba spin-splitting, the monolayer exhibits a high-tensile ductility and mechanical flexibility. Formation of BiTeI/BN van der Waals heterostructure makes it freestanding without perturbing the electronic structure. The broken mirror symmetry structure induces out-of-plane internal electric field of 0.289 eV Å−1, yielding out-of-plane piezoelectric coefficients of 0.556 pm V−1, which is larger than that of Janus group III and transition metal chalcogenide monolayers. Furthermore, the effective masses and mobility of electrons are 0.176 me and 392 cm2 V–1 s–1, respectively, which are half and six times of those of the MoS2 monolayer, respectively. These charateristics make 2D BiTeI a potential candidate for a wide variety of applications in nanoscale spin and electromechanical devices.

Effects of microstructure on the torsional properties of biodegradable WE43 Mg alloy

Torsional properties are important performance parameters for bone screw applications, but they are seldom studied, especially for newly developed biodegradable Mg alloys. In this study, WE43 Mg alloy with different microstructures was achieved by equal channel angular pressing (ECAP) and heat treatment, and their torsional properties were studied. In addition, tensile properties were also tested as a comparison. The results indicated that grain refinement led to higher torsional strength and ductility, while the second phases improved the torsional strength but reduced the ductility. The texture was strengthened after ECAP, as a result the tensile strength increased, but the torsional strength did not increase and even decreased, especially for 2-pass ECAP sample with a typical basal fiber texture. The basal plane orientation deviation from the extrusion direction after 4-pass ECAP resulted in higher torsional strength and lower torsional ductility, but lower tensile strength and higher tensile ductility were obtained. This implied that a strong fiber texture would reduce the torsional strength but improve the torsional ductility, which was different from its effect on tensile properties.

Shear band multiplication induced strong strain delocalization and high tensile ductility in amorphous thin films by metallic substrates

Metallic glass usually ruptures at a small strain by forming single dominant shear band under tension at room temperature. Experiments have shown that multiple shear bands can be induced in metallic glass film by a ductile metallic substrate, and the ductility of the film can be elevated significantly. However, the mechanism behind remains unsolved. Here we established a computational model to investigate the effect of a Ni substrate on the deformation of a Ni-P amorphous film. The shear band evolution in the film was described by a free volume-based constitutive theory, while the deformation of the Ni substrate was simulated by a dislocation density-based model. The film/substrate interface was modeled by a cohesive zone law. In the simulations, the shear band evolution and the stress in the film can be measured directly, which is difficult to achieve in experiments. Our simulations show that multiple shear bands have been formed in the substrate-supported metallic glass film if the film thickness hf is less than 10 µm. The thinner the film is, the more the shear bands the film has. The number of shear bands can reach as high as 16 as hf is refined down to 2.5 µm. The generation of the multiple shear bands alleviated the strong strain-softening as observed in free standing film, as clearly shown in the stress-strain response of the thin film in the film/substrate system. The suppressed strain-softening induces significantly enhanced tensile ductility in the amorphous film. The tensile ductility of the 2.5 µm-thick film on the Ni substrate can reach as high as 13.3%, which is much higher than that of the corresponding freestanding film, i.e., 3.1%. The proposed computational model has been validated by the coincident stress-strain responses between simulations and experiments. The findings revealed that the enhanced tensile ductility originates from the shear band multiplication-induced strain delocalization in the amorphous film due to the constraint of the metallic substrate.

The Dependence of Fracture Resistance on the Size and Distribution of Blocky Retained Austenite-Martensite Constituents

Although third-generation advanced high-strength steels have achieved a great combination of high ultimate tensile strength and ductility, edge cracking has been frequently reported during their cold forming of components. Following this issue, the effects of the stability, the morphology, and the amount of retained austenite (RA) on fracture resistance have already been investigated. However, the influence of the size and distribution of blocky RA and/or martensite (RA/M) islands on fracture resistance is absent, which is deliberately addressed in this study. After austenitization, the effect of holding temperatures between 278 °C and 400 °C on microstructure evolution in a Fe-0.3C-2.5Mn-1.5Si-0.8Cr (wt pct) steel is systematically and quantitatively investigated. Tensile properties and fracture resistance are characterized using uniaxial tension and double edge-notched tension tests, which interestingly show that an increased product of ultimate tensile strength and total elongation is accompanied with a decreased fracture resistance. This is because that tensile properties are mainly affected by the stability and amount of RA while the fracture resistance is also affected by the size and distribution of blocky RA/M islands. A coarse size of and a small interspacing between blocky RA/M islands are detrimental to the fracture resistance due to the promotion of crack nucleation and crack propagation.

Selective laser melting of low modulus Ti-Mo alloy: α/β heterogeneous conchoidal structure

The powder mixture, which used as feedstock materials in selective laser melting (SLM), always brings the undesirable problem in microstructure and then mechanical properties, due to its inhomogeneous chemical composition and stability. In this work, the in-situ SLMed Ti-7.5Mo alloy presents an heterogeneous conchoidal structure (hcp α-Ti and bcc β-Ti) from elemental powder mixture. During the SLM, the Mo, which possesses high melting temperature and density, was partial melted and then driven by Marangoni flow in molten pool. Thus, elemental micro-segregation appears with a result of special conchoidal structure. The SLMed sample shows low Young’s modulus (70 GPa) with high tensile strength (>700 MPa) and ductility (9%), given to the synergism effect of β channel and α island.

Fuzzy neural network analysis on the compacted graphite iron with improved tensile and heat transport properties

AbstractIn order to find out the most effective method for developing compacted graphite iron with a combination of high tensile strength, ductility and thermal conductivity, the superposed structural effects were investigated by experimental results and the relative significances were ranked on the basis of fuzzy neural network model. The concerned structural parameters consisted of graphite content, vermicularity and microhardness of the matrix. It was found that the relationships between properties and structural parameters become complex due to the mutual disturbances of various characteristics. Irregular and compossible corrections were both observed. The sensitivity level suggested that low microhardness of the matrix and low vermicularity are the optimal directions for improving simultaneously the tensile strength, thermal conductivity and elongation of compacted graphite iron.

Durability of engineered cementitious composite exposed to acid mine drainage

Acid mine drainage (AMD) is detrimental to concrete cut-off walls due to acid and sulfate attacks. As a recently developed fiber reinforced cement-based composite with high tensile ductility and intrinsically tight crack width, Engineered Cementitious Composites (ECC, also known as Strain-Hardening Cementitious Composites, or SHCC) has potential advantages in resisting transport of water and contaminant if applied to the mine cutoff walls. The durability of ECC exposed to AMD, however, has not been previously studied, and is the main objective of this research. Mechanical properties of ECC exposed to AMD wet-dry cycles were investigated. In addition, MgO was added in ECC to improve self-healing performance. It was found that AMD wet-dry cycles coarsened pore structure and led to ECC's mass loss and compressive strength reduction. The AMD exposure also reduced ECC's first cracking strength, ultimate tensile strength and tensile strain capacity by 23.7%, 17.8% and 36.5%, respectively. Addition of MgO was found effective in enhancing ECC's resistance to AMD attack, by moderating the reductions in compressive strength, tensile ductility and mass loss. Further, the addition of MgO densified ECC's microstructure and improved self-healing behavior. We conclude that ECC containing 6% MgO retains the desirable tensile ductility and possesses acceptable durability against AMD for cutoff wall applications at mining sites.

Influence of Wavy Bending on Microstructure and Mechanical Properties of a Rolled AZ31 Sheet

In the present work, cross-wavy bending at room temperature was carried out to tailor the microstructure and stretch formability of rolled AZ31 sheets. Wavy bending processing generates profuse {10–12} twins and a tilt basal texture. Subsequent recrystallization annealing causes grain coarsening and enhances the intensity of twin-orientation. The combined use of wavy bending and annealing can maintain high tensile ductility and remarkably enhances the stretch formability of rolled AZ31 sheet. It can be mainly attributed to the non-basal texture in the wavy bent sheet which increases the thinning capability during in-plane tension.

Effect of heat treatment on the precipitate behaviour, corrosion resistance and high temperature tensile properties of 7055 aluminum alloy synthesis by novel spray deposited followed by hot extrusion

The aim of current study is to observe the microstructure characterization, corrosion resistance and tensile behaviour of spray deposited 7055 aluminum alloy. The spray deposited aluminum alloy was subjected to extrusion at 703 K and then followed by solution treatment (743 K/1.6 h) and T76 aging treatment (393 K for 6 h + 433 K for 6 h). It was revealed that large fraction of fine η′ precipitates appeared after T76 treatment. The large fraction of fine precipitates was responsible for high tensile strength, ductility and corrosion resistance. The 7055-T76 heat-treated samples yield strength and ultimate tensile strength decreased gradually up to 498 K and decreased sharply above that temperature. On the other hand, the as-received 7055 Al alloy yield strength and ultimate tensile strength decreased dramatically after 398 K. The tensile fracture was occurred at elevated temperature owing to the growth of small voids and their agglomeration.

Static and fatigue behavior of basalt fiber-reinforced thermoplastic epoxy composites

The static, fatigue properties and their damage mechanism of basalt fiber-reinforced thermoplastic epoxy polymer composites are investigated. The stress–life curves and stiffness degradation under long-term cyclic loading were tested for basalt fiber-reinforced thermoplastic epoxy polymer. An advanced fatigue loading equipment combined with in situ scanning electron microscopy was used in the tests to track the damage propagations and analysis the fracture surfaces of all specimens. Results were also compared with those of thermosetting epoxy-based basalt fiber-reinforced polymer composites. The results show that the basalt fiber-reinforced thermoplastic epoxy polymer has good interface properties between the fiber and new thermoplastic epoxy, which results in high tensile strength and ductility. Different degradation rates of low- and high-cycle fatigue loads are observed for the basalt fiber-reinforced thermoplastic epoxy polymer composites. Under high fatigue stress levels, a high degradation rate of the fatigue life is found because the dominating damage pattern showed fiber fractures. At low and medium fatigue stress levels, the damage pattern is dominated by matrix cracking and interface debonding, which results in a low degradation rate of the fatigue life. A bilinear phenomenological fatigue model has a higher accuracy for fitting the stress–life data than linear fatigue models. In addition, 80–90% stiffness degradations are observed before failure for all stress levels at a stress ratio of 0.8. Furthermore, compared with thermosetting epoxy-based basalt fiber-reinforced thermoplastic epoxy polymer, the basalt fiber-reinforced thermoplastic epoxy polymer has similar static strength and similar fatigue life at high-stress levels. However, at low-stress levels, the fatigue life of the this polymer is much higher than that of thermosetting one.

Achieving high strength-ductility combination and negligible yield asymmetry in extruded AlN/AZ91 composite rods

The strength of conventional AZ91-based composites can be usually improved by the extrusion process, however the inadequate ductility is often induced, which limits the practical applications of AZ91-based composites. In this paper, the strength and ductility of AlN/AZ91 composites prepared by in situ synthetic method were significantly improved by a hot extrusion process. Microstructural analysis shows that in-situ AlN particles promoted the recrystallization nucleation and accelerated the recrystallization, resulting in a significant grain refinement in the extruded AlN/AZ91 composites. By the experimental observation, it was found that high density dislocations induced by AlN particles promoted the continuous precipitation of nano-sized γ-Mg17Al12 phases during the hot extrusion. By theoretical calculation and analysis, the contributions of different strengthening mechanisms on the yield strength increment were discussed, which confirmed that the grain boundary strengthening and precipitation strengthening were mainly strengthening mechanisms. By the EBSD-assisted slip trace analysis, it was also confirmed that the higher tensile ductility in the extruded AlN/AZ91 composites was mainly attributed to the activation of pyramidal <c+a> dislocation slip systems. Additionally, a weak basal texture was obtained in the extruded AlN/AZ91 composite rods due to the in-situ formation of AlN particles. Then a negligible tension-compression yield asymmetry in the extruded AlN/AZ91 composites was obtained.

On the origin of the high tensile strength and ductility of additively manufactured 316L stainless steel: Multiscale investigation

We report that 316L austenitic stainless steel fabricated by direct laser deposition (DLD), an additive manufacturing (AM) process, have a higher yield strength than that of conventional 316L while keeping high ductility. More interestingly, no clear anisotropy in tensile properties was observed between the building and the scanning direction of the 3D printed steel. Metallographic examination of the as-built parts shows a heterogeneous solidification cellular microstructure. Transmission electron microscopy observations coupled with Energy Dispersive X-ray Spectrometry (EDS) reveal the presence of chemical micro-segregation correlated with high dislocation density at cell boundaries as well as the in-situ formation of well-dispersed oxides and transition-metal-rich precipitates. The hierarchical heterogeneous microstructure in the AM parts induces excellent strength of the 316L stainless steel while the low staking fault energy of the as-built 316L promotes the occurrence of abundant deformation twinning, in the origin of the high ductility of the AM steel. Without additional post-process treatments, the AM 316L proves that it can be used as a structural material or component for repair in mechanical construction.

Role of isothermal omega phase precipitation on the mechanical behavior of a Ti-Mo-Al-Nb alloy

The formation and the evolution of the athermal and isothermal omega phase precipitates in a commercial metastable β–titanium alloy, β-21S, under controlled heat treatments, and its consequent impact on tensile properties has been presented in this paper. The structural and compositional changes of the omega precipitates have been tracked using SEM, TEM, and atom probe tomography (APT). Upon quenching from above the β-transus, the microstructure consisted of a single phase β without any discrete athermal omega precipitates and this condition exhibited the highest tensile ductility. Following β -solutionizing, when the samples were aged below the omega solvus temperature, there was an increase in the size scale of the omega precipitates with increasing time period. The larger and more solute-depleted omega precipitates, resulted in increased tensile strength and reduced ductility. The details of the deformation behavior were further analyzed using electron backscattered diffraction (EBSD) and TEM. Two different modes of deformation were noted in the system, depending on the initial microstructure. While the single phase condition (β solutionized sample) exhibited deformation occurring mainly via (110)β slip, the presence of larger isothermal omega precipitates in the aged condition (β solutionized + 350 °C/100H) resulted in the activation of not only (110) β slip but also (112) β slip. The reasonably high tensile ductility noted in this aged condition has been attributed to the activation of both these slip systems.