Ductile Materials Research Articles

Achieving exceptional strength-ductility combination in a novel L12 nanoparticles-strengthened high-entropy alloy via conventional casting

A novel L12 precipitation-strengthened Co51.3Ni19.5Cr19Al6.4Ti3.4Nb0.4 medium-entropy alloy (MEA) with excellent combination of large ductility and high strength was prepared via direct casting. The coherent ultrafine L12 precipitates particles (∼30 nm) with a volume fraction of 65% formed uniformly in the FCC matrix after heat-treatment, which endows this newly-designed MEA with superior tensile properties, i.e., a yield strength of ∼710 MPa, an ultimate tensile strength of ∼1045 MPa and a uniform elongation of ∼31.3%, respectively. The blocking effect of ultra-fine L12 precipitates significantly contributes to the high strength of the Co51.3Ni19.5Cr19Al6.4Ti3.4Nb0.4 MEA. In addition, deformation-induced nano-spaced stacking-fault networks, nano-twins and high-density Lomer-Cottrell locks facilitate the high work-hardening capacity, leading to excellent tensile ductility. This alloy design strategy can potentially be applied to many other alloys and open new avenues for designing strong yet ductile materials for engineering applications.

Tensile behaviour of WAAM high strength steel material and members

Wire arc additive manufacturing (WAAM), a method of metal 3D printing, has the capacity to create large scale elements suitable for construction applications with a high degree of design freedom and structural efficiency. There is currently however a lack of fundamental experimental data on the material and structural performance of such elements. Towards addressing this limitation, the tensile behaviour of WAAM high strength steel produced using different printing strategies is the focus of the present study. WAAM steel plates and tubular tension members manufactured with different interpass temperatures and toolpaths using ER110S-G welding wire were examined. A total of 60 tensile coupons, consisting of 40 as-built and 20 machined specimens, and 8 as-built circular hollow section (CHS) tension members, were tested. The examined WAAM materials were found to exhibit very little anisotropy, corroborated by a nearly homogeneous crystallographic texture observed by microstructural analysis, while the inherent surface undulations were shown to result in a varying degree of reduction in the material stiffness, strength and ductility at different angles to the print layer orientation. The different printing strategies led to varying surface geometries; combined with different interpass temperatures, they also resulted in different thermal histories and thus different mechanical properties. The tension members showed good structural resistance, but a considerable reduction in ductility compared to the coupon tests, due to the greater geometric variability and manufacturing defects.

Effect of T6 heat treatment on microstructure and mechanical properties of Al–18Si–10Cu–10Ni–5Mn–5Mg alloy prepared by laser-directed energy deposition

The new aluminum alloy Al–18Si–10Cu–10Ni–5Mn–5Mg (wt.%) prepared by laser-directed energy deposition (L-DED) process has excellent mechanical strength but poor ductility. In this paper, a T6 heat treatment is applied to it, aiming at the improvement of the material's ductility. The results show that after the T6 heat treatment, the original fine microstructure of the material is destroyed, and the phase composition changes. However, it is found that the formation of the nano-strengthened θʹ phase and the clear crystal orientation relationship between the θʹ phase and the α-Al matrix make the θ′ phase form a continuous grain boundary in the Al matrix, thus enhancing the strength and hardness of the alloy. It is worth noting that the microhardness is reduced slightly, but the residual stress is almost completely eliminated. The experimental results of mechanical properties show that the tensile strength and compressive strength of the material are reduced by 12 % and 17 % respectively after T6 heat treatment, but the ductility of the material is increased by 1.3 times, which is mainly attributed to the change of strengthening mechanism and the large and uniform second phase.

Effects of Thermomechanical Treatments on Tensile Properties of Pure Titanium

In order to increase strength while maintaining the ductility of material, pure titanium was improved through the thermomechanical treatment that combines rolling and heat treatment. The tensile properties of pure titanium treated by rolling and heating were investigated. Test material was JIS Grade 2. This material has a higher corrosion resistance. However, the strength of JIS Grade 2 is lower than that of JIS Grade 3. JIS Grade 2 with high strength while maintaining corrosion resistance is being developed. Techniques for improving the properties of materials with simple compositions are important. Thermomechanical treatment is used as a method for improving material properties. In the present study, the effect of thermomechanical treatment on the material properties of JIS Grade 2 was investigated. Rolling was performed at room temperature and the reduction ratio ranged from 70 to 90 %. The heating temperature was in the range of 300 to 700 °C. Heat treatment from 400 to 500 °C showed an increase in tensile strength while maintaining ductility. When the heat treatment temperature was 450 °C, the strength and elongation were approximately 600 MPa and 25 %. Tensile stress of JIS Grade 4 and the tensile strain of JIS Grade 1 were exhibited.

Strength-ductility synergy in twinned titanium fabricated via dynamic equal channel angular pressing and heat treatment

Dynamic equal channel angular pressing (DECAP) and heat treatment are applied to pure Ti to achieve a strength-ductility synergy. DECAP induces a complex multiscale twin structure in Ti including four primary twin types, secondary twins and tertiary twins with multiple variants, and an increased yield strength (58% higher than the as-received Ti). Subsequent annealing at 200 ℃ and 400 ℃ leads to improved ductility and strength. The yield strength (σy) and total elongation (ɛt) for the Ti sample processed with DECAP and 400 ℃ annealing are 555 MPa and 30%, respectively, and its σyɛt product is ∼45% higher than the as-received Ti. DECAP and subsequent annealing lead to a superior strength-ductility synergy in twinned Ti, providing a novel approach to produce high strength and ductility materials via dynamic deformation and subsequent heat treatment.

Effect of deformation temperature on strain localization phenomena in an austenitic Fe-30Mn-6.5Al-0.3C low-density steel

We have investigated the influence of the deformation temperature from 25 °C (RT) to -196 °C on the dislocation structures associated with strain localization phenomena in an austenitic Fe-30Mn-6.5Al-0.3C (wt.%) low-density steel by electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and bright-field transmitted forescattered electron imaging ((BF) t-FSEI) techniques. The characteristics of the dislocation structures were evaluated on the main texture components, i.e. <111>//tensile axis, <112>//tensile axis, and <001>//tensile axis directions. The inhomogeneous character of the plastic behavior is promoted upon cryogenic deformation due to the formation of dislocation structures associated with strain localization, namely microbands (MBs) and deformation bands (DBs). ECCI analysis of the dislocation structure reveals that the deformation temperature has a strong influence on the thermal-assisted dislocation processes controlling the dislocation configurations and MB formation mechanisms. The MB nucleation mechanism evolves from a cross-slip-assisted mechanism at RT deformation conditions to a slip band-assisted mechanism at cryogenic deformation temperatures. This effect has a profound effect on the grain orientation dependence of the MB structure but not on its crystallographic alignment. On the other hand, cryogenic deformation temperatures (-196 °C) enhance the material's mechanical strength and ductility due to the activation of deformation twinning, which is associated with the reduction of the stacking fault energy. We find that MBs have a small contribution to strain-hardening and ductility due to the small mechanical resistance of these dislocation structures against the advance of deformation twins and dense dislocation layers, and the comparatively small plastic strain accommodated by them, respectively. These findings provide new insights into the microband-induced plasticity (MBIP) effect.

Behavior of Different β Stabilizers on the Microstructure and Properties of Ternary Ti‐3Sn‐X Alloys

The manufacturing of Ti alloys via powder metallurgy and the development of novel compositions are two strategies to reduce the cost of Ti, which is still the primary factor deterring its wider use in engineering applications. In this study, new Ti alloys based on the combined addition of Sn with Nb, Mo, or Mn are manufactured via powder metallurgy to gain an understanding of the role of these β stabilizers on the performance achievable. It is found that the designed alloys have a fully homogeneous chemistry regardless of their actual composition and a lamellar or β‐type microstructure depending on the actual β stabilizer used. This study confirms that the β‐stabilizing power effect decreases from Mn to Mo and, eventually, Nb. The compressibility and sinterability of the alloys increase with the progressive addition of the selected powders, generally leading to stronger and more ductile materials. It is also found that the proposed Ti‐3Sn‐Mo alloys are characterized by the best strength/ductility pairs compared to a variety of sintered or cast binary/ternary Ti alloys bearing the alloying elements considered in this work.

Numerical Investigation of the Seismic Performance of an Innovative Type of Buckling-Restrained Brace (BRB)

Previous studies have demonstrated that the inclusion of tire-derived aggregate (TDA) enhances the damping, ductility, and toughness of concrete mixtures. The effectiveness of tire-derived aggregate as a ductile material with a higher damping ratio and lower density in buckling-restrained braces has been examined at California State University’s Structures Laboratory (CSU). Through experimental and theoretical investigations, this study compares the structural application of buckling-restrained braces with TDA and with conventional concrete infill subjected to various ground motions as well as artificial excitations. The evaluations include modeling a full-scale experimental setup equipped with a single-leg BRB utilizing ETABS 2016 and OpenSees 2000 software. The effectiveness of the application is demonstrated through a comparison of accelerations, displacements, stiffness, and damping ratios between TDA and concrete filling. Additionally, a design guideline for TDA-filled buckling-restrained braced frames is provided.

Compressive and tensile behaviour of alkali‐activated slag‐based concrete incorporating single hooked‐end steel fibres

AbstractThe effect of single hooked‐end steel fibres, namely Dramix® 3D, on the mechanical performance of alkali‐activated slag‐based concrete (AASC) and Portland cement concrete (PCC) has been investigated. Compressive strength, modulus of elasticity, stress‐strain response under uniaxial compression and tension, splitting tensile strength and flexural strength have been evaluated. The experimental results show that AASC incorporating 3D fibres in a volume fraction of 0.75% exhibits an enhanced behaviour, under both compression and tension, in comparison to PCC incorporating the same fibre type and dosage. Although the reference mixes show similar compressive strength, 3D fibres enhance the modulus of elasticity, splitting tensile strength and flexural strength of AASC of 8.6%, 61.7% and 12.8%, respectively, while for PCC 1.1%, 42.2% and 16.1%, respectively. Three‐point bending tests show the effect of 3D fibres on the response of AASC and PCC under flexural loading. Although fibres have a limited effect on the strength corresponding to the limit of proportionality (LOP), they enhance the post‐peak behaviour, increasing the residual flexural strength and the material ductility. Finite element analysis has been performed to predict the flexural behaviour of steel fibre‐reinforced AASC (FRAASC) under flexural loading. The Concrete Damage Plasticity (CDP) model implemented in ABAQUS software can predict the flexural response of FRAASC quite accurately, although additional experimental data are needed to calibrate the model for different alkali‐activated matrix types.

Damage and failure of semi-rigid steel joints during progressive collapse

Ductile metal materials during a ductile failure will suffer from micro-void damage, simulation of which can contribute to the prediction of the macro fracture of steel structure. In this paper, the Gurson-Tvergaard-Needleman (GTN) constitutive model which describes the aggregate failure of metal voids is selected as the constitutive relation to investigating the damage evolution and fracture of semi-rigid joints in steel structures during a progressive collapse. During the process, specific parameters of the GTN model of steel are confirmed based on the combined results of the test and numerical simulation. The progressive collapse of two kinds of semi-rigid joints of steel pipe-to-H shaped steel were simulated by a nonlinear finite element method. A comparison between the simulation and test results verified that the GTN constitutive model can accurately simulate the fracture failure of the steel semi-rigid joints. Besides, a simplified joint failure model based on concentrated plastic hinge theory is proposed. Considering the damage and fracture of semi-rigid joints and the collision contact among different components, the progressive collapse of the semi-rigid steel frame structure is simulated by explicit dynamic analysis based on a finite particle method. Afterwards, the dynamic response of the structural progressive collapse can be predicted more accurately.

Comparison of Single-Pass Differential Speed Rolling (DSR) and Conventional Rolling (CR) on the Microstructure and Mechanical Properties of Mg5Zn

Background: The primary hot rolling method implemented is differential speed rolling (DSR). The material is rolled and grains are strained, producing fine dynamic recrystallization (DRX) grains that improve material strength and ductility. Objective: The material introduced and under investigation in this paper is an Mg-based alloy, Mg5Zn (wt. %), whose microstructure is enhanced through a combination of heat treatments with proper temperature and holding time and subsequent plastic deformation through hot rolling to evaluate the effect on mechanical properties Methods: The method involves preheating the material to various temperatures in a range from 250ºC to 350ºC and rolling to various thickness reductions to analyze the effect of single-pass differential speed rolling (DSR) and conventional rolling (CR) on the DRX process and its influence on mechanical properties. Results: The effect of single-pass differential speed rolling (DSR) and conventional rolling (CR) on the DRX process shows that the process produces increasing amounts of finer DRX grains at higher rolling reductions, thereby improving the strength and ductility of the material. Conclusion: This investigation demonstrated that single-pass DSR can improve the mechanical properties and formability of Mg5Zn more effectively than CR in terms of grain refinement analyzed through OM, SEM, and EBSD resulting in enhanced tensile strength and ductility [1].

Functionally Graded Steels Obtained via Electron Beam Powder Bed Fusion

Electron-Beam Powder Bed Fusion (EB-PBF) is one of the most important metal additive manufacturing (AM) technologies. In EB-PBF, a focused electron beam is used to melt metal powders in a layer by layer approach. In this investigation two pre-alloyed steel-based powders, stainless steel 316L and V4E, a tool steel developed by Uddeholm, were used to manufacture functionally graded materials. In the proposed approach two powders are loaded into the feeding container, V4E powder on top of 316L one, preventing their mixing. Such type of feeding yields components with two distinct materials separated by a zone with gradual transition from 316L to V4E. Microstructure and local mechanical properties were evaluated in the manufactured samples. Optical Microscopy, Scanning Electron Microscopy and EDX on the polished cross-sections show a gradual microstructural and compositional transition from characteristic 316L at the bottom of the specimens to the tool steel towards the top. Nanoindentation experiments confirmed a consequent gradient in hardness and elastic modulus, which gradually increase towards the top surface of the samples. The achieved results provide great possibilities to tailor the composition, microstructure, mechanical properties, and wear resistance by combining different powders in the powder bed AM technology. Potential applications include the tooling industry, where hard and wear-resistant materials are demanded on the surface with tougher and more ductile materials in the core of the tool.

Tailoring of residual stress by ultrasonic vibration-assisted abrasive peening in liquid cavitation of metallic alloys

The present study proposes a novel method of ultrasonic vibration assisted-abrasive peening for the enhancement of residual stress on the surface of metals and their alloys. The system employs a vibrating sonotrode that drives the formation and collapse of bubbles within a fluid medium. The imploding bubbles produce pressure waves which transfer momentum to the abrasives which are uniformly distributed in the fluid medium. The abrasives bombard a targeted surface along with intense pressure waves. This induces compressive residual stress through local plastic deformation in a short period. The capability of the ultrasonic-assisted abrasive peening setup is analysed in terms of residual stress by altering the abrasive concentration, peening time, and stand-of-distance between the bottom of the sonotrode and the exposed surface to be treated. The process is able to induce significant residual stress at around 67 % of yield strength for hard material Ti–6Al–4V and more than 80 % of yield strength for ductile materials, Al-6061 and OFHC-Cu. A numerical method coupled with a finite element model is employed to predict the dynamics of the process from cavitation of the bubble to the plastic deformation of the work material. At first, the model estimates the magnitudes of high-pressure waves at the bubble implosion near the solid surface, micro-jet velocity, and abrasive velocity. This information is then fed to Abaqus for numerical modelling of the deformation of work material. The impact of high-speed abrasives in the range of 100 m/s, pressure waves and microjets at the material surface are simulated through the FE model. The simulated results are verified with experimental findings in terms of surface residual stress for different materials, deviating within 10 %.

Wear Evolution on PVD Coated Cutting Tool Flank and Rake Explained Considering Stress, Strain and Strain-Rate Dependent Material Properties

Impact loads developed on a tool cutting edge when milling into a workpiece material are prevailing metrics for explaining coating fatigue failure and the subsequent tool-wear evolution. For predicting related stress and strain fields in the compound coating-substrate, stress, strain, and strain-rate, dependent material properties are required. The attainment of such data is briefly described in the paper. Considering these data, the occurring strains in the cutting edge at various entry impact durations, i.e., strain rates, were calculated and compared with fatigue-critical strains. In this way, the wear phenomena causing the coating failure on the flank and rake during milling were clarified. The attained results were also correlated to corresponding ones in turning, where the dynamic loads of the cutting edge are comparably negligible. The conducted investigations showed that the fatigue-critical strains strongly diminish, when the relevant strain rates increase; thus, leading to a remarkable tool-life reduction. This happens, because the increase of the strain-rate restricts the time for the dislocations movements; thus, regions with stress concentrations occur, deteriorating the material ductility, increasing its brittleness, and diminishing the fatigue critical strains. In cutting operations, where the coating fatigue is the main wear factor, the tool-life can be predicted considering these phenomena. In the paper, relevant experimental analytical procedures are introduced.

Improving the interfacial properties of PVA fiber and cementitious composite: Design and characterization

Poly vinyl alcohol (PVA) fiber has been widely applied to improve the ductility of cementitious materials, and the overall performance of fiber reinforced cementitious materials largely depends on the interfacial properties of the fiber and cementitious matrix. In this work, a novel design of graphene oxide (GO) modified PVA fiber is proposed to improve the interfacial properties of PVA fiber and cementitious matrix. It is found that the interaction between GO and PVA fiber forms the hydrogen bond and enables the GO to form a tight three-dimensional bonding on the surface of the PVA fiber. Meanwhile, the oxygen-containing functional groups of GO could form chemical bond with the hydration products and promote the formation of denser microstructure at the interface. Therefore, the introduction of GO prominently improves the interfacial properties of PVA fiber and cementitious matrix, which is verified by the characterizations on the microstructures and morphologies. Furthermore, it is found that GO remarkedly improves the interfacial mechanical properties and results that a zone with the enhanced mechanical properties appears near the interface, verified by the characterizations on the micro-mechanical properties including fracture toughness near the interface.

Erosion resistant effects of protective films for wind turbine blades

Over the course of many years of use, impingement wear from dust, sand, and other materials can damage wind turbine blades, necessitating repairs and other maintenance work. Recently, wind turbine operators are turning to protective films, which allow such work to be completed more efficiently, as an alternative to the conventional approach of using paint to repair wind turbine blades. However, the erosion resistance characteristics of repaired blades remain unclear. In this study, we create paint- and protective film-coated samples to reproduce repairs, measure their erosion resistance, and study underlying factors in an effort to verify the erosion resistance of repaired materials and associated mechanisms. The low erosion resistance of GFRP can be significantly improved by applying a protective film made of a ductile material. Such material effectively protects the surface of the GFRP. Moreover, the erosion resistance of protective films made from polyurethane material is superior to that of paint. We recommend use of protective film with wind turbine blades when manufacturing blades and regular maintenance.

Pinning cracks by microstructure design in brittle materials

In this work, we explore the possibility of controlling toughening mechanisms in brittle materials through microstructure design to enhance their resistance to crack growth. First, a computational framework is established using the distinct element method (DEM) with J integral implemented to simulate complex crack propagation and characterize the effective fracture energy of brittle materials. The fracture behavior of a soda-lime glass plate containing pre-existing voids under tensile loading is simulated, and toughening mechanisms associated with the existence of voids, including blunting-induced pinning and deflection-induced pinning, are identified in brittle materials. Then we seek to modulate the fracture toughness of brittle materials by introducing either randomly distributed voids or sinusoidally distributed voids and controlling the pinning events. Our findings reveal that randomly distributed voids with different volume fractions induce only modest toughening in brittle materials. Sinusoidally distributed voids with specific amplitude and wavelength trigger both crack blunting and deflection in the vicinity of voids, increasing the probability of crack pinning and leading to a significant increase in crack growth resistance. Moreover, we identify the critical conditions for the “embrittlement-toughening transition” and “maximized toughening”. Finally, we discuss the difference between void toughening for brittle materials and ductile materials due to distinct toughening mechanisms activated, and also extend the toughening strategy to nacre-like materials. The stairwise herringbone structure is demonstrated to be a promising candidate for structure optimization due to unique stiffness, strength, and toughness.

Hierarchically ordered coherent interfaces-driven ultrahigh specific-strength and toughness in a nano-martensite titanium alloy

Precipitates in alloys are traditionally considered as dislocation obstacles that often lead to high stress concentration and even microcracks owing to the strain incompatibility of semi-coherent interfaces, a cause of progressive strain localization and the origin of the strength-toughness conflict in engineering materials, such as titanium (Ti) alloys. Based on the metastability engineering, here we architect hierarchically ordered coherent interfaces for strength-toughness optimization through densely dispersed nanomartensites in a ductile Ti-Cr-Zr-Al alloy with ultrahigh specific-strength and superior fracture toughness. It is unveiled that these ordered coherent interfaces simultaneously serve as dislocation obstacles and sources, leading to a sustainable and self-hardening deformation mechanism via hierarchical nanomartensite-dislocation interactions for ultra-high strength and toughness of Ti alloys. These nanomartensites are thermally stable at elevated temperature less than 400 °C, above which tempering-induced ductile-to-brittle transition occurs due to the decomposition of hierarchically ordered nanomartensites and the spheroidization of prior β lamellae. The design strategy of hierarchically ordered coherent interfaces confers our cost-effective nanomartensite Ti alloys an unprecedented combination of strength, ductility and toughness, which provides a new pathway in the microstructural design for strong and ductile structural materials with superior fracture resistance.

Lightweight Fe47Mn25Al13Cr7Ni5C3 medium-entropy alloy with enhanced mechanical properties

In this study, we designed a lightweight Fe47Mn25Al13Cr7Ni5C3 medium-entropy alloy (MEA) (calculated density of 6.803 g/cm3) with enhanced mechanical properties. The MEA samples were produced in three conditions with varying microstructures via thermomechanical processing. They achieved excellent tensile properties through strengthening mechanisms such as partial recrystallization, ultrafine grains, and M23C6 and B2 precipitates. Different strengthening mechanisms were applied depending on the annealing heat treatment conditions, resulting in three different strength-elongation combinations. Furthermore, the MEA, under all designed conditions, exhibited superior specific yield strength-uniform elongation and specific ultimate tensile strength-uniform elongation combinations compared to previously studied lightweight high-entropy alloys (HEAs) and lightweight steel. This was primarily attributed to the combination of benefits obtained from the low proportion of iron (47 at%) as a principal element and the large amount of aluminum addition (13 at%). The proposed MEA and its design strategy can satisfy the requirements for lightweight, cost-effective, strong, and ductile metallic materials, making a great contribution to the automotive industry in terms of crash resistance and fuel efficiency.

Uniqueness of Finite Element Limit Analysis solutions based on weak form lower and upper bound methods

Finite Element Limit Analysis (FELA) is increasingly used for calculating the ultimate bearing capacity of structures made of ductile materials. Within FELA for reinforced concrete structures the elements have been based on rigorous lower bound or boundary mixed formulations. The lower bound element formulation may be overly constrained for certain meshes and the dual displacement interpretation may contain spurious modes, moreover the boundary mixed element formulation has an internal equilibrium node with no associated displacement field. Here a consistent and general weak formulation based on virtual work is presented specifically for both the lower and the upper bound problem, and it is shown that they are each others dual ensuring uniqueness of the optimal solution. As a consequence there is no difference between the solutions based on the weak upper and lower bound methods. Here a plane element is presented, with a linear stress variation and a quadratic displacement field, optionally including a concentrated bar element with a linear variation of the normal force. These elements are applied in a verification example and two reinforced concrete examples where they show very good results for both load level, stress distribution and collapse mechanism even for coarse meshes.