Strength-ductility Trade-off Research Articles

Synthesis mechanism of pelleted heterostructure Ti64–TiB composites via an interdiffusion and self-organization strategy based on powder metallurgy

Titanium matrix composites (TMCs) are the most attractive next-generation structural materials for lightweight design due to their high specific modulus and specific strength. Designing heterogeneous grain structures has been a widespread strategy for overcoming the strength-ductility trade-off in powder metallurgical TMCs. However, achieving a customizable heterogeneous grain structure in TMCs remains a formidable challenge. Here, we propose a novel interdiffusion and self-organization strategy based on the powder metallurgy method to fabricate a pelleted heterostructure Ti6Al4V(Ti64)–TiB composites, which integrates a heterogeneous distribution of reinforcements and heterogeneous grain structures of the matrix. The study reveals that the formation of the pelleted heterostructures is primarily governed by the interdiffusion reaction process between Ti and Al, and V elements, coupled with the growth and distribution changes of TiB induced by the interdiffusion reaction. Moreover, the pellet size is tailored by the spherical Ti–TiB composite powder, allowing precise control over the heterogeneous grain regions. This research contributes valuable insights into the manufacturing process of pelleted heterostructure Ti64–TiB composites and provides a conceptual framework applicable to the design of similar structures in various metal matrix composite systems.

Sensitivity of reduction rate on the mechanical response of boron-doped Fe50Mn30Co10Cr10 high entropy alloy

Phase transformation induced plasticity (TRIP) effect is an effective technical solution to enhance the strength and ductility of alloys, represented by Fe50Mn30Co10Cr10 high entropy alloy (HEA). Introducing heterostructures into alloys is also an effective synergistic method for enhancing the strength and ductility of alloys. Combining element doping, heterostructures, and the TRIP effect of the alloy itself can enable the alloy to achieve both high strength and high ductility. In this work, the boron doped Fe50Mn30Co10Cr10 HEA were prepared by cold rolling with different reduction rates followed by recrystallization annealing processes. Different reduction rates result in different heterogeneous structures. The large rolling reduction weakens the advantage of heterogeneous structure for introducing large stress concentration, resulting in the low ductility of the alloy. The sample with small rolling reduction shows excellent strength and ductility, which comes from the synergistic effect of large-small grains matching and TRIP effect. In summary, the TRIP effect combined with the appropriate amount of reduction and low recrystallization temperature annealing induced heterostructure is an effective solution to overcome the strength ductility trade-off.

Exceptionally strong and ductile bulk metallic glass composite with bioinspired architecture mimicking porcupine fish spine

Introducing soft crystalline phases into the glassy matrix to produce bulk metallic glass composites (BMGCs) is an effective way to enhance the ductility of bulk metallic glasses (BMGs). However, the introduction of soft crystalline phases severely sacrifices the strength, resulting in the strength-ductility trade-off. To defeat this dilemma, here, we successfully fabricate a bioinspired BMGC with architecture mimicking a porcupine fish spine. The bioinspired BMGC shows a pronounced yield strength of ∼800 MPa with an excellent fracture strain of ∼35 %. The fabrication of the bioinspired BMGC is achieved through infiltration and vitrification of molten Zr50Ti5Cu27Ni10Al8(Zr50) melt into the crystalline Nb skeleton fabricated by laser additive manufacturing (LAM). Such enhanced strength-ductility synergy is attributed to the asynchronous deformation associated with the delicate bioinspired heterogeneous architecture. The bioinspired structural design motif, enabled by the combination of LAM and infiltration casting technologies, opens a new window to develop high-performance BMGCs on a large scale for structural applications.

Achieving well-balanced strength and ductility via dual nanoscale precipitate structures in Co-free high-entropy alloys

Recent studies confirmed that introducing nanoprecipitates as strengthening phase into high-entropy alloys (HEAs) is an extremely effective means to achieve the excellent strength-ductility synergy. In this work, we successfully created a dual nanoscale precipitate structure in a Co-free Ni2.5CrFeAl0.3Ti0.2 HEA by controlling thermo-mechanical process. Compared to the recrystallized alloy, homogeneous L12 precipitates in FCC matrix and heterogeneous L12/BCC precipitates near grain boundaries were formed in the aged alloys. The dual-nanoprecipitate structure induces a long-range back stress during deformation process, effectively contributing to a balance of high strength and good ductility at 298K. Especially, the mechanical properties of the recrystallized and aged alloys are dramatically improved without strength-ductility tradeoff, showing a multiple-stage strain-hardening behavior at 77 K. In the aged alloys, the nanoprecipitates shorten the mean free path of dislocation movement to effectively induce the precipitation strengthening appreciably depending on the precipitation size. Different from planar dislocations slip dominated by stacking faults at 298K, multiple stacking fault networks and deformation twins were activated sequentially with increasing the tensile strain, thus forming a dynamic Hall-Petch effect at 77K. The current work could help to understand the correlation between mechanical properties and deformation mechanisms in precipitation-strengthening HEAs, and the transformation of these deformation mechanisms provide an insight for developing outstanding-performance HEAs through trigging various hardening modes.

Evading the strength-ductility trade-off dilemma in AA2024 alloy by short-term natural re-aging after T351 temper

In this research, the influence of short-term natural re-aging after T351 temper on the microstructural evolution and mechanical behavior of AA2024 aluminum alloy was investigated. Grain growth occurred in the microstructures of the natural re-aged sample and a large number of Al7Cu2Fe particles were located inside the alpha grains. At the re-aging time of 1440 min, the peaks of XRD were shifted strongly to the right due to the formation of θ′′, S″, θ′, and S′. The results revealed that the precipitation rate was high in the AA2024 alloy during natural aging. With increasing the re-aging time, texture parameters remained almost unchanged. The hardness increased slowly within the first 60 min, then enhanced rapidly between 60 and 2880 min, and finally became stable at around 139 HV between 2880 and 11520 min. When the natural re-aging time increased from 240 to 2880 min, the strengthening trended speed up, viz, the yield strength increased from 226.6 to 357.3 MPa, and the ultimate tensile strength enhanced from 452.2 to 535.5 MPa. Compared to the as-received sample (T351 temper), the ultimate tensile strength of the re-aged sheet improved from 455.5 to 535.5 MPa, the ductility remained unchanged, and the hardness increased from 128.8 to 138.2 HV, which was owing to the acceleration of the precipitation caused by the presence of high-content Al7Cu2Fe particles in the interior of the alpha-aluminum grains in the natural re-aged sample. It was found that the Portevin-Le Chatelier instability of AA2024 alloy was effectively postponed after natural re-aging. With increasing the natural re-aging time, the strain hardening rate of the AA2024 sheet increased. The strengthening of the natural re-aged sample for 1440 and 2880 min was a result of a synergistic effect of precipitation hardening due to the formation of θ′′, S″, θ′, and S′ phases, elimination of Portevin Le-Chatelier instability, and highly efficient load transfer from alpha-aluminum to Al7Cu2Fe. Finally, to use the A2024 alloy produced by natural re-aging for 1440 or 2880 min, two methods were proposed.

Making high primary α phase content titanium alloy exceptional strength and ductility by designing the heterogeneous structure

As a challenge to avoid the strength-ductility trade-off for Ti alloys, strength and ductility are contradictory, especially at a high content of primary α phase. This study proposed a new strategy to evade this trade-off dilemma by designing an 81% primary α phase/ultrafine-grained (UFG) secondary α phase lamellar heterogeneous structure(LPSlow-HS) in near-α Ti alloys. Compared to the bimodal structure, the LPSlow-HS offers a remarkable increase in the ultimate tensile strength(1119 MPa) and yield strength(1069 MPa), respectively, while the elongation (15%) is not sacrificed. Owing to the strong heterogeneous deformation-induced(HDI) strengthening, more number of delamination of interfaces, UFG microstructures and strong strain hardening render the Ti alloys high strength. This strategy may provide a new approach for achieving higher HDI strengthening effects in heterogeneous structures (HS) under high soft phases, and offer a feasible method for producing high strength-ductility Ti alloys under high primary α phase.

Molecular dynamics simulation on the mechanical properties of Zr-Cu metallic nanoglasses with heterogeneous chemical compositions

The mechanical properties of metallic nanoglasses (NGs) strongly depend on the average size of glassy grains (Davg). Nevertheless, current knowledge on the effects of sizes of glassy grains is incomplete for the mechanical properties of NGs. Herein, ZrxCu100-x (25 ≤ x ≤ 75) nanoglasses containing glassy grains with different chemical compositions, i.e., the heterogeneous NGs (HNGs), are investigated by molecular dynamics simulation, and the relation between ultimate tensile strength (UTS) and Davg is determined. Specifically, the UTS decreases with decreasing Davg in Zr-Cu HNGs when Davg < 10 nm, mainly resulting from the increased volume fraction of glass-glass interfaces, while UTS would follow the Hall–Petch like relation for Zr-Cu HNGs when Davg > 10 nm, which is closely related to glassy grains with compositions dominated by Zr atoms. This study provides a deep insight into the mechanical property dependence on grain size in the HNGs, which could be a novel strategy in resolving the issue of strength-ductility tradeoff in NGs.

Stabilizing 3D-printed metal alloys

A design strategy overcomes the strength-ductility trade-off in alloy manufacturing

Improving density and strength-to-ductility ratio of a 3D-printed Al–Si alloy by high-pressure torsion

Good combination of strength and ductility in metallic materials is always desired. To this end, this study assesses the combination of two modern manufacturing processes, namely additive manufacturing (AM) and severe plastic deformation, for an AlSi11Cu alloy. Laser powder bed fusion (L-PBF) produced an alloy with spherical pores with an average size of 42 μm, representing a volume fraction lower than 0.15%. At the mesoscale, the alloy showed a cellular microstructure made up of Al cells and Si-rich boundaries with an average size of 0.69 µm, which were broken down by the high-pressure torsion (HPT) process into ultrafine particles smaller than 0.41 µm. The HPT process transformed the columnar grains of the as-built material into ultrafine-grained grains around the disk edges, while the central zone conserved the as-built characteristics for a number of HPT turns smaller than ¼. HPT processing at room and warm temperatures gave rise to strength–ductility improvements with yield strengths and elongations larger than 400 MPa and 10%, respectively. The good strength–ductility trade-off was related to the porosity decrease, the breakdown of the interconnected network into particles of ultrafine size, the grain size reduction due to the dislocation density increase, and the formation of precipitates and Si-rich particles of different sizes. Thus, AM and HPT improved the grain boundary and precipitation strengthening, giving rise to an Al–Si alloy with superior mechanical properties.Graphical abstract

Excellent strength-ductility combination of interstitial non-equiatomic middle-entropy alloy subjected to cold rotary swaging and post-deformation annealing

The effect of cold rotary swaging and subsequent annealing on the microstructure, texture and mechanical properties of a Fe49.5Mn30Co10Cr10C0.5 middle-entropy alloy was studied. Finite element modeling predicted inhomogeneous stress distribution and temperature gradient during deformation. Microstructure analysis indicated the development of deformation-induced γ→ε martensitic transformation during earlier steps of cold rotary swaging (20–40 % reduction). However, a single-phase face-centered cubic structure was attained after 60–90 % reduction due to reverse ε→γ transformation. Meanwhile, a twin-matrix microstructure was observed in the bar center, whereas an ultrafine microstructure was formed at the bar edge. Post-deformation annealing at 600 °C caused the onset of static recrystallization and thereby the formation of ultrafine dislocation-free grains. Apparently, after 60–90 % reduction, a ⟨100⟩- and ⟨111⟩-fiber texture gradient along the bar cross-section was developed. Besides, at the bar edge, a pronounced B/B‾ shear texture was obtained that transformed into a Cube texture after annealing at 700 °C. Compared to the as-swaged material (yield strength (YS) = 1250 MPa; ultimate tensile strength (UTS) = 1520 MPa; elongation to failure (EF) = 6.5 %), annealing at 500 °C resulted in additional strengthening (YS = 1655 MPa; UTS = 1660 MPa), but EF did not change noticeably. Yet, annealing at 600°С was accompanied by an essential increase in both EF (to 11 %) and YS (to 1500 MPa). The applied approach can be used for obtaining a material with an attractive strength-ductility combination due to overcoming the strength-ductility trade-off.

Multifunctional metallic nanocomposite for overcoming the strength–ductility trade-off

The actualization of high strength and ductility in alloys, in addition to providing strong, formable materials, can lead to reduced weights in practical applications. However, increasing strength typically comes at the cost of lowering the ductility and vice-versa, referred to as the strength–ductility trade-off. In this work, we investigate the thermo-mechanical response of a 3-element multifunctional NiTi–Nb nanocomposite material that overcomes this trade-off, as it exhibits a high strength of 980 MPa and an ultrahigh ductility of 58% at fracture. The remarkable properties are attributed to the underlying microstructure of Nb nanofibers dispersed in an NiTi matrix. Deformation is accommodated via the shape memory transformation of the active NiTi matrix in concert with elastoplastic deformation of Nb nanofibers embedded within the matrix. Consequently, the material exhibits multifunctionality and recovers deformation during heating via the reversion of the stress-induced martensitic transformation in the NiTi matrix. The high strength and high ductility of this 3-element nanocomposite material puts it amongst the best performing high-entropy alloys (HEAs) that are typically made up of five or more elements.

Breaking the strength-ductility trade-off via heterogeneous structure in FeCoCrNiMo0.2 high-entropy alloy

The synergistic effect between heterogeneous structures has been proved to enhance the strength and plasticity of alloys, especially in the context of single-phase face-centered cubic (FCC) high-entropy alloys with low stacking fault energy. This study explores the synergistic effects of heterogeneous structures on the strength and plasticity of single-phase FCC FeCoCrNiMo0.2 high-entropy alloys. Through a combination of cryogenic rolling and annealing at 1000 °C for 0.5 h (RA-1000), the alloy demonstrates exceptional strength-ductility synergy and work-hardening ability. The heterogeneous structure comprises fine grains, nano-scale rich- (Cr, Mo) σ phase, and high-density annealing twins. The interaction of σ precipitation and FCC matrix induces heterogeneous deformation-induced strengthening (HDI), while annealing twins and stacking faults act as barriers to dislocation movement, enhancing strength and ductility through a dynamic Hall-Petch effect. Additionally, chemically ordered structures, ordered L12 phase, and type 63 topologically close-packed phases in RA-1000 alloys contribute to improved strain hardening via anti-phase boundaries. This work provides valuable insights for designing multi-scale heterogeneous structures to strengthen high-entropy alloys.

Upgrading the strength-ductility trade-off and wear resistance of Al0.25CoCrFeNiCu and Al0.45CoCrFeNiSi0.45 high-entropy alloys through severe cold rolling process

In the present work, Al0.25CoCrFeNiCu and Al0.45CoCrFeNiSi0.45 high entropy alloys (HEAs) were fabricated by induction melting and then subjected to severe cold rolling (SCR) to induce an ultrafine lamellar microstructure. By using electron back scattered diffraction (EBSD) analysis, the microstructural features of the SCR processed HEAs were determined to be deformation bands, severely pancaked grains, and a refined lamellar microstructure. The intense cold rolling method did not result in phase changes, according to X-ray diffraction (XRD) analysis. The HEAs exhibited a decrease in elongation to fracture due to the development of new grain boundaries and dislocations strengthening effect, although an improvement in strength and microhardness was seen with increasing rolling deformation (reduction in thickness). With a tensile strength of 740 MPa and an elongation of 14%, Al0.25CoCrFeNiCu showed the best strength-ductility combination, while Al0.45CoCrFeNiSi0.45 showed a tensile strength of 890 MPa and ductility of 10%. Furthermore, it was discovered that the SCR process causes the alloys' fracture mechanisms to change from ductile to partially brittle. This was particularly noticeable in the Al0.45CoCrFeNiSi0.45 HEA. Lastly, the wear rate of 75% SCR Al0.45CoCrFeNiSi0.45 (1.2 ± 0.3 × 10–5 mm3 N–1 m–1) was less than that of 75% SCR Al0.25CoCrFeNiCu (1.8 ± 0.3 × 10–5 mm3 N–1 m–1). The predominant wear mechanism of both HEAs was abrasive wear with a certain amount of delamination.

Effect of deformation heterogeneity on development of geometrically necessary dislocation density in heterogeneous-structured CrMnFeCoNi high-entropy alloy

Heterogeneous-structured high-entropy alloys (HEAs) overcome strength-ductility trade-off, demonstrating potential for a wide range of applications in aerospace, defense and military fields. In this work, the evolution of geometrically necessary dislocation (GND) density in a heterogeneous-structured CrMnFeCoNi HEA is investigated at different strains. This HEA consists of fine- and coarse-grained domains. GND density measurements are performed using electron backscatter diffraction. GND density increases significantly in the fine-grained domains while increases slightly in the coarse-grained domains. The disparity of GND density between the coarse and fine grains indicates a distinct deformation gradient, which is related to back-stress strengthening. At 7% strain, the GND density in the fine-grained domains reaches parity with that in the coarse-grained domains, indicating a transition from single slip to multiple slip. The local misorientation distributions are described by a lognormal function.

Calphad-assisted design of high strength – ductility martensitic stainless-steels with reverted austenite

Introducing reverted austenite at lath martensite boundaries has been shown to be an effective method to increase the ductility of martensitic steels. However, significant strength reductions can also be introduced during the reversion process in martensitic stainless steels. To overcome the strength-ductility trade-off, we explore the effects of alloying elements in delaying the overaging of the precipitates and accelerating the austenite formation. To this end, we carry multi-objective optimization considering (1) lattice misfit and diffusivity for kinetics of precipitation, and (2) austenite fraction and thermodynamic driving force of austenite reversion for austenite formation. With the insights from calculations, model alloy compositions are then proposed, and mechanical testing and microstructural analysis are applied. The model alloy confirms that a relatively higher volume fraction of austenite can form prior to overaging, which doubles the total elongation without significant strength loss.

Pre-precipitating promoted by microshear bands effectively circumvents strength-ductility trade-off of RT-rolled Al–6Zn–1Mg alloy

The aging precipitated phases are of great significance for the strengthening and toughening of Al–Zn–Mg alloys. In this work, pre-precipitating was realized through equal channel angular pressing (ECAP) immediately followed by solid solution treatment at 475 °C for 6 h. Its corresponding strengthening and toughening effect on the room temperature (RT)-rolled Al–6Zn–1Mg alloy was systematically studied. The results indicate that pre-precipitating by ECAP can effectively improve the strength of the RT-rolled Al–6Zn–1Mg alloy and circumvent the strength-ductility trade-off. Microstructure characterization reveals that ECAP induces a large number of microshear bands, which can not only encourage recrystallization but also effectively promote the precipitation of the η′ phase. The synergistic effect of precipitation strengthening and dislocation strengthening contributes to the high strength of Al–6Zn–1Mg alloy, with UTS and YS increasing to ∼522 MPa and ∼400 MPa, respectively. Meanwhile, the ductility remains almost unchanged mainly due to the coordinated grain boundary sliding and the uniform and finely dispersed second phase particles. As a result, the strength-ductility trade-off is effectively avoided. The findings shed some light on optimizing the traditional aging treatment.

Simultaneously enhancing strength-ductility synergy in the Ti2AlNb diffusion bonding joint via heterogeneous high-entropy interface design

Brittle defects, such as continuous intermetallic compounds and straight bonding line composed of grain or phase boundaries, often form at the interface of dissimilar metals joints, resulting in the premature cracking at the interface. Therefore, bonding interfaces with excellent strength and ductility are attractive for application in welding structural components. However, increasing strength usually comes at the expense of ductility, which is known as strength-ductility trade-off. In this work, we explored a novel interface design strategy by introducing multi-scale grains into the bonding interface of the Ti2AlNb-based alloy using a heterogeneous refractory high entropy alloy. The interface was composed of the multi-sized grains, which successfully eliminated the brittle defects. Through this approach, the strength and ductility of the joint synergistically improved to 877 MPa and 11.54 %, respectively. Both strength and ductility exceeded 98 % of the mechanical performances of the base metal. The exceptional strength-ductility combination was benefiting from cooperative strain hardening mechanisms, which increased the interfacial hardening capability via in situ refinement of free path of dislocations motion. This hetero-interface strategy could expand the design ideas for the bonding interface of intermetallic compound-based alloys, and provides a new approach to synergistically increase the mechanical performance.

Enhancing the strength of Sc-containing Al-Cu-Li alloys by modifying the precipitation behavior through plastic deformation and heat treatment

Al-Cu-Li alloys are used extensively in the aerospace manufacturing industry for their outstanding properties, however, the addition of trace Sc elements will form high-temperature insoluble (Cu, Sc)-containing phases during homogenization leads to a decrease in age-strengthening, and there is a limited investigation on how to overcome this unfavorable factor. In this work, the effects of plastic deformation coupled with heat treatment on the microstructure and mechanical properties of Sc-containing Al-Cu-Li alloy were systematically investigated by using age-hardening curves and room-temperature tensile tests combined with characterization by TEM, EBSD, and SEM/EDS. The results show that plastic deformation before high-temperature treatment fragments and promotes the dissolution of coarse phases, reduces the development of (Cu, Sc)-containing enriched phases, and simultaneously increases the content of Cu atoms within the Al matrix. Among all precipitates (GPB zones, S phases, T1 phases) after T6 aging, T1 phases act as the dominant strengthening phase of the alloy. At the same time, we found that the decrease in the Cu-containing residual phase after solid solution treatment will result in a remarkably increasing number density and volume fraction of T1 phases at the peak aging. In a word, we obtain a good strength-ductility trade-off (the yield strength is ∼500 MPa while maintaining ∼10% elongation) Al-Cu-Li alloy with Sc element by optimizing the distribution of Cu elements and modulating the aging precipitation behavior. Our proposed process is expected to provide an experimental and theoretical guide toward the development of Al-Cu-Li alloys with superior properties.

A Crystal Plasticity Finite Element—Machine Learning Combined Approach for Phase Transformation Prediction in High Entropy Alloy

The mechanical properties of an alloy depend on its microstructure. The strength-ductility trade-off is a paradigm that existed for a long time. Advanced alloys, such as high entropy alloys (HEAs), utilize a dual-phase strengthening mechanism, which originates from the microstructural phenomena consisting of twinning and phase transformation, to significantly improve their mechanical properties. To understand the impact of phase transformation mechanism on stress–strain response, developments of crystal plasticity finite element models (CPFEM) and machine learning (ML) together with experimental methods have potential to capture the relationships between descriptive features and targeted phenomena. Here, ML models on local crystallography, local stresses, and energy-based driving forces are leveraged for phase transformation prediction in a HEA. The ML model (XGBoost classification model) uses a hybrid training data combining electron backscatter diffraction (EBSD) experimental data and CPFEM simulation results. This approach enhances prediction performance at optimum data sizes. This predictive model is implemented in multiple experimental measurements to validate our models and evaluates importance of different physical quantities on phase transformation phenomenon. The prediction accuracy reached over 95% compared to experimental data. The CPFEM-ML framework used in this study is expected to be applicable to other HEA systems to facilitate the understanding and prediction of the phase transformation.

Optimization of Aging Temperature and Heat-Treatment Pathways in Additively Manufactured 17-4PH Stainless Steel.

This study investigates the tensile behaviors of additively manufactured (AM) 17-4PH stainless steels heat-treated within various temperature ranges from 400 °C to 700 °C in order to identify the effective aging temperature. Despite an aging treatment of 400-460 °C increasing the retained austenite content, an enhancement of the tensile properties was achieved without a strength-ductility trade-off owing to precipitation hardening by the Cu particles. Due to the intricate evolution of the microstructure, aging treatments above 490 °C led to a loss in yield strength and ductility. A considerable rise in strength and a decrease in ductility were brought about by the increase in the fraction of precipitation-hardened martensitic matrix in aging treatments over 640 °C. The impact of heat-treatment pathways on aging effectiveness and tensile anisotropy was then examined. Direct aging at 482 °C for an hour had hardly any effect on wrought 17-4PH, but it increased the yield strength of AM counterparts from 436-457 to 588-604 MPa. A solid-solution treatment at 1038 °C for one hour resulted in a significant drop in the austenite fraction, which led to an increase in the yield (from 436-457 to 841-919 MPa) and tensile strengths (from 1106-1127 to 1254-1256 MPa) with a sacrifice in ductility. Improved strength and ductility were realized by a solid-solution followed by an aging treatment, achieving 1371-1399 MPa. The tensile behaviors of AM 17-4PH were isotropic both parallel and perpendicular to the building direction.