Strength-ductility Trade-off Research Articles

Stress–strain partitioning behavior and mechanical properties of dual-phase steel using finite element analysis

Various industrial applications require materials with both high strength and good ductility. However, strategies for enhancing material performance are usually trapped in a strength–ductility trade-off. In this study, the effect of martensite morphologies (including geometry, connectivity, and distribution direction) on the stress–strain partitioning behavior and mechanical properties of a ferrite-martensite dual phase steel was studied using the secant method and finite element analysis. The results demonstrated that the combination of rhombus and horizontal geometries provided a good balance of strength and ductility. Thus, a combination of 45° and 0° regarding the angle between the directions of martensite distribution and deformation was further determined to be beneficial to the strength–ductility balance. These results are expected to provide a general understanding of strengthening and toughening mechanisms and will promote the development of high-performance steels.

The Microstructural Difference and Its Influence on the Ballistic Impact Behavior of a Near β-Type Ti5.1Al2.5Cr0.5Fe4.5Mo1.1Sn1.8Zr2.9Zn Titanium Alloy.

In this work, a near β-type Ti5.1Al2.5Cr0.5Fe4.5Mo1.1Sn1.8Zr2.9Zn alloy was hot-rolled at the temperature of 800–880 °C with a thickness reduction of 87.5% and then heat-treated with the strategy of 880 °C/1 h/air cooling (AC) + 650 °C/3 h/AC. The microstructure difference between the hot-rolled and heat-treated titanium alloys and its influence on the ballistic impact behavior of the hot-rolled and heat-treated titanium alloys were analyzed. The microstructural investigation revealed that the average size of the acicular secondary α phase (αs) dropped from 75 to 42 nm, and the corresponding amount of this phase increased significantly after heat treatment. In addition, the dislocation density of the α and β phases decreased from 0.3340 × 1015/m2 and 4.6746 × 1015/m2 for the hot-rolled titanium alloy plate to 0.2806 × 1015/m2 and 1.8050 × 1015/m2 for the heat-treated one, respectively. The high strength of the heat-treated titanium alloy was maintained, owing to the positive contribution of the acicular secondary α phase. Furthermore, the critical fracture strain increased sharply from 19.9% for the hot-rolled titanium alloy plate to 23.1% for the heat-treated one, thereby overcoming (to some extent) the constraint of the strength–ductility trade-off. This is mainly attributed to the fact that the dislocation density and the difference between the dislocation densities of the α and β phases decreased substantially, and deformation localization was effectively suppressed after heat treatment. Damage to the hot-rolled and heat-treated titanium alloy plates after the penetration of a 7.62 mm ordinary steel core projectile at a distance of 100 m was assessed via industrial computer tomography and microstructure observation. The results revealed that a large crack (volume: 2.55 mm3) occurred on the rear face and propagated toward the interior of the hot-rolled titanium alloy plate. The crack tip was connected to a long adiabatic shear band with a depth of 3 mm along the thickness direction. However, good integrity of the heat-treated titanium alloy plate was maintained, owing to its excellent deformation capability. Ultimately, the failure mechanism of the hot-rolled and heat-treated titanium alloy plates was revealed by determining the crack-forming reasons in these materials.

Facile manufacturing of bimodal grained copper with nanoparticles

Bimodal grain structure has been demonstrated effective to overcome the strength-ductility tradeoff in nanostructured materials. Here we report a new and scalable method to fabricate bimodal grained metals, i.e., casting followed by regular hot rolling, when the metals are loaded with nanoparticles. The as-solidified copper (Cu) containing tungsten carbide (WC) nanoparticles displayed ultrafine grains. After hot rolling, as nanoparticles imposed restriction on Cu grain growth, the nanoparticle-rich zones retained ultrafine grains, while grains in the nanoparticle-sparse zones grew to microscale. The average grain size of the ultrafine and microscale grain regions is 230.6 ± 125.8 nm and 6.3 ± 3.5 μm, respectively. This new pathway has great potential to advance the scalable manufacturing of bimodal grained metals for various applications.

A powder-metallurgy-based fabrication route towards achieving high tensile strength with ultra-high ductility in high-entropy alloy

The strength-ductility trade-off dilemma is perennially problematic in the materials science community. In particular, the attainability of high tensile strength and large elongation is ambitious in alloys fabricated by powder metallurgy. Here, we demonstrate a powder-metallurgy-based fabrication route to achieve a high synergy of tensile strength and ductility through cold-consolidation of CoCrFeMnNi high-entropy alloy powder using high-pressure torsion followed by annealing. This approach has resulted in an exceptional synergy of high yield strength of 754 MPa with an ultra-high tensile elongation of 58%% which has never been achieved in alloys fabricated by powder metallurgy routes. Additionally, the microstructure can be tuned by annealing treatment to achieve a range of strength and ductility that are highly sought after in industries for a specific application. The present fabrication route can be applied for fabrication of high-entropy alloy-matrix composites using alloys, metals, and ceramic powders to achieve controllable microstructure and eminent tensile properties.

Aged metastable high-entropy alloys with heterogeneous lamella structure for superior strength-ductility synergy

High-entropy alloys containing multi-principal-element systems significantly expand the potential alloy design space, and offer the possibility of overcoming the strength-ductility trade-off in metallurgical research. However, the gain in ultra-high strength through traditional grain refinement and precipitation-strengthening mechanisms inevitably leads to a drastic loss of ductility. Here, we report on the design and fabrication of heterogeneous-lamella structured, aged bulk high-entropy alloy, which attains gigapascal tensile strength while retaining excellent ductility (UTS ~1.4 GPa, elongation ~30%; UTS ~1.7 GPa, elongation ~10%). Our work shows that the improved strength-ductility synergy arises due to various complementary strengthening mechanisms, including solid-solution, interfaces, precipitation and martensitic transformation, which influence the hardening and deformation processes at different strain levels. In particular, the hetero-deformation that is associated with the formation of microbands as well as the stress-induced martensite promotes additional hardening and hence high ductility. The strategy described here, that is leveraging the concept of heterogeneous microstructure design, provides a practical and novel method for fabricating high-performance structural materials.

A novel stress-induced martensitic transformation in a single-phase refractory high-entropy alloy

High-entropy alloys (HEAs) provide a new perspective to design metastable alloys with the stress-induced martensitic transformation (SIMT) for overcoming the strength-ductility trade-off. Here, we report a novel SIMT, orthorhombic to hexagonal close-packed martensite, in a single orthorhombic refractory HEA (Ti16Zr35Hf35Ta14 RHEA), showing a good yield strength-ductility matching. The analysis of the elastic distortion energy (∆Eels) of Ti16Zr35Hf35Ta14 and several other RHEAs reveals that severe lattice distortion is a key factor which causes this SIMT. Combined the “d-electron alloy design” approach with the ∆Eels, the phase configuration and SIMT path in RHEAs can be well predicted. Our work brings new insights between the lattice distortion and SIMT of RHEAs, benefiting the metastable alloy development.

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.

A mechanistic interpretation of the strength-ductility trade-off and synergy in lamellar microstructures

Among various mechanisms responsible for the strength-ductility trade-off in metallic materials, the leading strategy is to delay the onset of necking by improving the work hardening rate via a number of metallurgical approaches such as heterogeneous or gradient microstructures. Recent research activities on high-entropy alloys also witness a wide range of alloy design capabilities that permit these microstructural designs such as the dual-phase lamellar microstructures. This work addresses the contrasting strength-ductility behavior of equiaxed and lamellar microstructures when geometric features are the only tuning parameter. It is found that failures in lamellae are preceded by necking in the hard phase, the growth of which is significantly confined and suppressed by the surrounding soft phase. Detailed finite element simulations reveal various degrees of strength-ductility trade-off and synergy, which mainly depend on the microscopic processes that govern the delayed neck growth and ductile fracture. The upper limit of tensile ductility is theoretically believed to be determined by short-wavelength necking in the hard phase.

Overcoming the strength–ductility trade-off by tailoring grain-boundary metastable Si-containing phase in β-type titanium alloy

It is well accepted that grain-boundary phases in metallic alloys greatly deteriorate the mechanical properties. In our work, we report on a novel strategy to prepare high strength-ductility β-type (Ti69.71Nb23.72Zr4.83Ta1.74)97Si3 (at.%) (TNZTS) alloys by tailoring grain-boundary metastable Si-containing phase. Specifically, the thin shell-shaped metastable S1 phase surrounding the columnar β-Ti grain was intercepted successfully via nonequilibrium rapid solidification achieved by selective laser melting (SLM). Subsequently, the thin shell-shaped metastable (Ti, Nb, Zr)5Si3 (called S1) phase was transformed into globular (Ti, Nb, Zr)2Si (called S2) phase by the solution heat treatment. Interestingly, the globular S2 phases reinforced TNZTS alloy exhibits ultrahigh yield strength of 978 MPa, ultimate strength of 1010 MPa and large elongation of 10.4 %, overcoming the strength–ductility trade-off of TNZTS alloys by various methods. Especially, the reported yield strength herein is ∼55 % higher than that of conventionally forged TNZT alloys. Dynamic analysis indicates the globularization process of the metastable S1 phase is controlled by the model of termination migration. The quantitative analysis on strengthening mechanism demonstrates that the increase in yield strength of the heat-treated alloys is mainly ascribed to the strengthening of the precipitated silicide and the dislocations induced by high cooling rate. The obtained results provide some basis guidelines for designing and fabricating β-titanium alloys with excellent mechanical properties, and pave the way for biomedical application of TNZTS alloy by SLM.

A Powder-Metallurgy-Based Fabrication Route Towards Achieving High Tensile Strength with Ultra-High Ductility in High-Entropy Alloy

The strength-ductility trade-off dilemma is perennially problematic in the materials science community. In particular, the attainability of high tensile strength and large elongation is ambitious on alloys fabricated by powder metallurgy. Here we demonstrate a powder-metallurgy-based fabrication route to achieve high synergy of tensile strength and ductility through cold-consolidation of CoCrFeMnNi high-entropy alloy powder using high-pressure torsion followed by annealing. This approach has resulted in an exceptional synergy of high yield strength of 754 MPa with an ultra-high tensile elongation of 58% that has never been achieved in alloys fabricated by powder metallurgy routes. Additionally, the microstructure can be tuned by annealing treatment to achieve a range of strength and ductility that are highly sought after in industries for a specific application. The present fabrication route can be applied to fabricate high-entropy alloy-matrix composites using alloys, metals, and ceramic powders leading to a controllable microstructure and eminent tensile properties.

A HEA/MG composite breaks the strength-ductility trade-off by using ultrasonic technology

A HEA/MG composite breaks the strength-ductility trade-off by using ultrasonic technology

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.

Balancing strength and ductility of cylindrical-shaped Cu64Zr36 nanoglass via embedded Cu nanocrystals

Large-plasticity can be realized in nanoglass at the expense of strength, and such a loss can be compensated by embedding stronger nanocrystals. Here, the effects of average grain size (Daverage) and volume fraction of nanocrystalline (ϕ) on deformation behaviors of cylindrical-shaped Cu64Zr36 nanoglass/crystalline Cu composites (NGCCs) are investigated by molecular dynamics simulations of tensile loading. Our results reveal that the necking behavior of NGCCs persistently starts from the glass grains, followed by the glass fracture. In particular, there exists a critical Daverage of 7.96 nm, which is related to the diameter of nanowires. With Daverage larger than the critical value, NGCCs are more likely to achieve both high-strength and large-ductility in comparison with their NG counterparts. It also confirms the significance of the surface effect on stretching NGCC nanowires. As the volume fraction of the Cu nanograins increases, the peak stress of NGCCs increases while the ductility decreases. Therefore, the tweaking of appropriate Daverage and ϕ makes it possible to overcome the long-term strength-ductility trade-off dilemma.

Microstructure, mechanical and vacuum high temperature tribological properties of AlCoCrFeNi high entropy alloy based solid-lubricating composites

High-entropy alloys (HEAs) that have revolutionized the alloy design of strength-ductility trade-off, provide a pathway for the development of high strength solid-lubricating composites. Herein, a high performance AlCoCrFeNi HEA based composite reinforced with different contents of solid lubricant Ag was prepared by Spark Plasma Sintering technique. The composites retain the structure of nano-coupling network matrix and uniformly disperse Ag particles located at the grain boundaries, thereby exhibiting high strength of above 1917 MPa. Meanwhile, Ag particles render the composites excellent lubricity and significantly reduce the adhesion and wear of the frictional interface at temperature from room temperature to 800 °C in vacuum.

Breaking the strength-ductility paradox in advanced nanostructured Fe-based alloys through combined Cu and Mn additions

The strength-ductility paradox is a long-sought challenge for all engineering materials. In this study, we escaped the strength-ductility trade-off by engineering nano-scale heterogeneities carefully in the advanced nanostructured Fe-based alloys through alloying with Cu and Mn additions. We demonstrated a triple ductility enhancement by 20% together with a strength improvement of 100MPa compared to the alloys with sole Cu additions, overturning a common understanding of the strength-ductility trade-off. The strength-ductility enhancement is attributed to the complex interplay between the transformation induced plasticity (TRIP) and the coherent nano-scale Cu precipitates as well as the resultant heterogeneous stress–strain partitioning and dislocation interactions.

Achieving a unique combination of strength and ductility in CrCoNi medium-entropy alloy via heterogeneous gradient structure

Despite having otherwise outstanding mechanical properties, the practical application of CrCoNi medium-entropy alloy (MEA) is limited by its insufficient room temperature yield strength. Unfortunately, most traditional approaches for increasing strength usually lead to the degradation of ductility, a dilemma known as strength–ductility trade-off. Here, we report a heterogeneous gradient structure (HGS) in this MEA produced by ultrasonic surface rolling processing that can achieve a unique property combination: 3.7 times higher engineering yield strength compared to that of the fully recrystallized microstructure with homogeneous grain size, and at the same time preserving a very high elongation-to-failure of 54.1%. The HGS along depth from the treated surface is characterized by surface nanocrystalline, heterogeneous nanolaminate structure layers and deformed dislocation structure layer. The excellent combination of high strength and good ductility comes from extraordinary work-hardening ability arising from the heterogeneous gradient structure and the abundant extremely thin nanoscale deformation twins.

Achieving high strength and high ductility in WC-reinforced iron-based composites by laser additive manufacturing

Strategies for fabricating iron-based materials with high strength and ductility are rare despite intense research efforts within the last decades. This main challenge, which must be overcome in synthesizing such materials, is described by the strength-ductility trade-off dilemma. This study provides a novel approach to achieve the synthesis of highly strong and ductile iron-based composites reinforced with a high weight fraction of WC particles (20 wt%) utilizing laser powder bed fusion (LPBF) as processing technique. Thereby, the LPBF-fabricated composite material has a multi-phase microstructure consisting of ductile austenite (main phase), highly strong martensite and carbidic precipitations extending across different length-scales. The precipitation of (Fe, W)3C type carbide at the Fe/WC interface is well controlled. Thus, a very thin reaction layer (< 500 nm) forms between the WC particles and iron-based matrix. Additionally, nano-scaled precipitations evolve along sub-grain boundaries and within the sub-grains and they show a high coherency with the iron-based matrix. These iron-based composites synthesized by LPBF show an excellent compressive strength of about 2833 MPa and large fracture strain of about 32 %. The following mechanisms contribute to the improved mechanical properties: (1) multiphase material system, (2) grain refinement, (3) substructures, (4) coherent multiscale interfaces and (5) nano-precipitations.

Enhanced strength-plasticity combination in an Al–Cu–Mg alloy——atomic scale microstructure regulation and strengthening mechanisms

A new approach to the stimulation of atomic clustering reactions shows potential to address the 'strength-ductility trade-off' in Al–Cu–Mg alloys. The evolution of the alloy microstructure during this thermomechanical processing was followed carefully using electron microscopy, X-ray diffraction, and atom probe microscopy. The distributions of the Mg and Cu solute atoms, and the characteristic distributions of Mg–Cu co-clusters, were investigated in detail. The factors affecting the strength of the alloy were assessed semi-quantitatively and the results suggest that Mg–Cu atom co-clusters contribute significant strengthening in this Al–Cu–Mg alloy, when subjected to this thermomechanical processing. The novel introduction of asymmetric rolling into the thermomechanical process increases the overall strength and we propose that this is due to the synergistic effects of cluster strengthening, dislocation strengthening, and texture engineering. The significant improvement in plasticity is attributed to a combination of the shear texture, the relatively high ratio of low-angle grain boundaries, and the avoidance of microstructural stress and strain concentrations such as those often associated with precipitation and precipitate-free-zones.

Strength–Ductility Trade-Off in Dual-Phase Steel Tailored via Controlled Phase Transformation

The controlled phase transformation approach is a new generation of microstructure engineering for enhancing ductility in ultrafine-grained materials. The present study aims to extend this concept to dual-phase steel for enhanced tensile ductility at high strength. In this study, cold-rolled steel was subjected to annealing at different processing conditions to develop the core–shell microstructure constituting martensite core and ferrite as shell in the matrix. Controlled austenite decomposition was adopted in designing a core–shell-type structure. The evolved microstructure was characterized using a scanning electron microscope and electron backscatter diffraction technique. The results showed that the uniaxial tensile deformation of dual-phase structured steels had a remarkable strength–ductility trade-off. The ductility was increased anomalously at high martensite fractions. Further, the mechanism of damage activity leading to void nucleation and microcrack formation was studied in post-tensile fractured specimens to establish a correlation with tensile deformation characteristics. The possibility and outcomes of this approach are also reported here.

Enhanced strength and ductility synergy in aluminum composite with heterogeneous structure

A new heterogeneous aluminum composite was synthesized by one step mechanical milling followed by hot pressing sintering to evade the strength–ductility trade-off. The heterogeneous structure of the composite was composed of coarse-grained Al embedded in ultrafine-grained Al reinforced by Al2O3 nanoparticles. Compared to nanocrystalline Al and bimodal Al, an excellent combination of high strength and high ductility was obtained for the extruded composite specimen. The enhanced strength and ductility could be partially attributed to the induction of geometrically necessary dislocations resulting in back stress strengthening. The new strategy is expected to be applicable to other metal matrix composites and alloys.