Combination Of Strength Ductility Research Articles

Enhanced strength of additively manufactured Ti–6Al–4V alloy through multistage strain hardening

Titanium alloys fabricated by wire and arc additive manufacturing (WAAM) are prone to produce defects with coarse intracrystalline α lamellae, continuous grain boundary of α phase (GB-α), and chemical segregation. The synergistic combination of the factors leads to high strength at the expense of plasticity. Adding activating flux of CaF2 during WAAM can provide an effective way to achieve a remarkable strength-ductility combination of Ti–6Al–4V alloy. Adding CaF2 produced refined intragranular α lamellae, discontinuous GB-α with dowel-like α lamellae (DL-α, primary α phase penetrated through the GB-α and embedded within adjacent β grains), as well as achieved concentration modulations. The tailored microstructure induced multistage strain hardening behavior (sequentially activated multiple strain hardening) during tensile plastic deformation, which allows WAAM-fabricated Ti–6Al–4V alloy a high tensile strength with good plasticity. The good strength-ductility balance is mainly attributed to the utilization of multiple strengthening mechanisms in titanium alloys. This study provides a novel method to improve the mechanical properties of WAAM-fabricated titanium alloys.

Enhanced strength-ductility synergy of laser additive manufactured stainless steel/Ni-based superalloy dissimilar materials characterized by bionic mechanical interlocking structures

The unique mechanism of toughness enhancement found in natural materials has served as inspiration for the development of a new generation of dissimilar metal materials (DMMs). Here we present a bionic mechanical interlocking structure strategy that stainless steel 316L and Inconel 625 DMMs with an enhanced strength-conductivity trade-off processed by laser additive manufacturing (AM), possessing an ultimate tensile strength of 675.64 MPa, and a uniform elongation of 33.6%. The incompatible deformation between SS316L (plastic deformation) and IN625 (non-deformation) contributes to overcoming the strength-ductility trade-off. The integration of alternating soft and hard non-uniform structures plays a pivotal role in effectively absorbing and dissipating the energy introduced by external loads during crack propagation. This geometric arrangement and multilevel structure at the interface notably reduces the effective stress intensity factor along the crack path, leading to added resistance against loading. The multi-strengthening mechanism coupled with the heterogeneous deformability gives the material excellent strength-ductility synergy, especially the bionic mechanical interlocking structure extending the crack expansion path, which helps to avoid premature failure of weak interface layers. These findings will provide new insights into the controlled design of high-performance DMMs, which highlights the potential to inherit merits from constitutive materials for better strength-ductility combinations.

Ta-doping induces heterogeneous microstructures in CoCrNi medium-entropy alloy to achieve excellent strength-ductility combination

Face-centered cubic high-entropy alloys and medium-entropy alloys (MEAs) are limited by their mediocre yield strength. This work proposes a simple yet efficacious strategy that capitalizes on the advantages of heterogeneous structure and doping large-size atoms. Using this strategy, various types of heterogeneous structures can be introduced simultaneously into Ta-doped CoCrNi MEAs, resulting in significant enhancements in their mechanical properties. After annealing at 800 °C, the (CoCrNi)100-xTax (x = 1, 2, and 3) MEAs show heterogeneous, dual heterogenous, and partially recrystallized structures, respectively. Notably, compared to the CoCrNi MEA (546 MPa) processed under the same conditions, the yield strength of these alloys is increased to 696 MPa, 1180 MPa, and 1470 MPa, respectively, which indicates increases of 27%, 116%, and 169%. Precipitation morphology and distribution significantly impact the formation of heterogeneous structures. During recrystallization, primary coarse precipitation and secondary fine precipitation give rise to the particle-stimulated nucleation and Zener pinning effects, respectively, inducing the refinement of surrounding grains. Furthermore, a detailed investigation was conducted to elucidate the effect of Ta doping on grain growth kinetics and deformation mechanisms in the (CoCrNi)98Ta2 alloy.

Effects of soaking temperature on the microstructure and mechanical properties of a medium Mn steel for hot/warm forming

The increasing demands for better crashworthiness of vehicles require structural components with excellent energy absorption performance and anti-intrusion resistance. Here, we explored the potential of using a medium Mn steel with a 10 wt.% Mn content for hot/warm forming with the aim to produce automotive components with improved strength–ductility combination. The soaking temperature was varied from 670 to 870 °C for tuning the microstructure and the resultant mechanical properties. It is found that a duplex microstructure consisting of ferrite with a high dislocation density and retained austenite with a volume fraction of 53.5% is formed at 720 °C, leading to an exceptional strength–ductility combination (yield strength: 1016 MPa; ultimate tensile strength: 1407 MPa; total elongation: 32.7%). The energy absorbed before fracture is 39.8 GPa·%, which is four times more than that of the widely-used 22MnB5 boron steel. On basis of the experimental results, the effects of soaking temperature on austenite reverse transformation, ferrite recrystallization and martensitic transformation are discussed in depth to provide guidance for optimizing the strength–ductility combination of medium Mn steels processed by hot/warm forming.

High Cycle Fatigue Testing of Lap Shear RSW Joints from Martensitic MS1400 Steel Sheets

Resistance spot welding (RSW) is one of the most common welding methods for steel sheets, as it is mainly used to join the automotive body structure parts. Different types of ultra-high strength steels (UHSS) have become widely used in the automotive body to obtain the required demands such as lower car weight, improving crashworthiness behavior, and enhancing strength–ductility combination. Martensitic UHSS belong to the highest grades width their tensile strength above 1000 MPa. During the lifetime of the vehicle cyclic loading generally occurs, therefore the optimization of welding technology should be performed considering the fatigue resistance of the welded joints. In our research 1 mm thick standardized lap shear sheets of martensitic MS1400 steel were welded by a TECNA 8007 RSW equipment with two different welding parameter combinations. The idea was to analyze the effect of welding and pulsation parameters on joint properties under static and cyclic loading. The welding parameters have been calibrated to produce the same weld nugget size for both technological combinations. Macroscopic, hardness, and tensile-shear tests were carried out to determine the fundamental mechanical characteristics of the RSW joints. The relation between the weld nugget microstructure and mechanical properties was explored. The high cycle fatigue (HCF) tests were performed on an MTS 810.23 universal electro-hydraulic materials testing system. A statistical approach was applied during the preparation and evaluation of the investigations, which increased their reliability. Measured and analyzed data of the lap shear welded joints, prepared by different technological parameters, were compared and discussed. The parameters of the HCF experiments were calculated considering the Japanese testing method (JSME S 002-1981). In most of the samples it was observed from both welding parameter combinations that the fatigue cracks initiate and grow in curvature shape in the softened part of the heat-affected zone towards the base metals in both directions symmetrically. A slight difference was observed in the HCF resistance of the welded joints prepared by different welding parameters.

A multi-scale study on the effect of interfacial microstructure on interfacial strength and fracture behaviour in Fe/Ni bonded interfaces

Multilayered Steel (MLS) enables impressive strength-ductility combinations by controlling the interfaces. Previous studies indicated that the interface is affected by various factors at multiple length scales, whereas their mechanisms had not been fully clarified. In this study, we focused on the effects of interfacial microstructure on interfacial strength of Fe/Ni interface. At macro scale, the pre-exist grain boundaries (GBs) were controlled to fabricate bicrystal and polycrystal interfaces. Uniaxial tensile test results showed that the bicrystal interfaces exhibit relatively higher interfacial crack initiation strength compared to the polycrystal interfaces. In-situ observation revealed that the decreased strength was owing to the unique crack initiation mechanism: the pre-exist GB-interface junctions behaved as weak spots in tensile separation. At atomic scale, interfacial atomic behaviour at the junctions was investigated by the molecular dynamics (MD) simulation method. The result indicated that the disordered High-Angle GBs (HAGBs) blocked the growth of interfacial dislocations and formed local dislocation pile-ups, where subsequent crack initiation was observed. This is attributed to the relatively lower intrinsic bonding strength at the GB-interface junctions.

Ultrastrong High‐Ductility Ni35Co35Fe10Al10Ti8B2 High‐Entropy Alloy Strengthened with Super‐High Concentration L12 Precipitates

L12 phase hardening alloys with excellent mechanical properties are of great significance for structural applications. However, low volume fractions of L12 precipitates in conventional alloys (nearly lower than 60%) tend to limit their practical usage, while the strengths of the alloys generally increase with L12 precipitation contents. Herein, a novel high‐entropy alloy (HEA) Ni35Co35Fe10Al8Ti10B2 with ultrahigh concentration L12 precipitates is successfully designed aided by the calculation of phase diagrams (CALPHAD). The volume fraction of L12 precipitates in this HEA is up to 75% and outperforms that of most of traditional superalloys. The novel L12‐strengthened Ni35Co35Fe10Al8Ti10B2 has an ultrahigh tensile yield strength of ≈1.45 GPa, ultimate tensile strength of ≈1.9 GPa, and great ductility of ≈23% at room temperature. The desirable strength–ductility combination is superior to most of conventional superalloys and reported HEAs, mainly due to the presence of ultrahigh concentration L12 precipitates that act as dislocation obstacles and the formation of numerous stacking faults and deformation twining. This work is expected to provide guidance for developing new high‐performance HEAs with an excellent combination of strength and ductility.

On the Strength of a 316L-Type Austenitic Stainless Steel Produced by Selective Laser Melting

The developed microstructure and the tensile behavior of a 316L-type steel produced by selective laser melting were studied. This paper particularly aims to clarify the dislocation substructures in the developed steel, focusing on the density of dislocations, their arrangement in cells/subgrains, related internal distortions, and specific strengthening. The experimental samples were obtained using a 3D selective laser melting system ProX200 (laser power of 240 W, beam speed of 1070 mm/s, distance between tracks of 80 µm, and layer thickness of 30 µm) in a nitrogen atmosphere. The steel microstructure was characterized by a grain size of 20 μm and a high dislocation density of 5 × 1014 m−2 in the grain/subgrain interiors. The rather strong fiber texture of <012> along the building direction resulted in different Taylor factors of 2.89 and 3.30 for tension along the building direction and the side direction, respectively. The yield strength of 645 ± 5 MPa, the ultimate tensile strength of 750 ± 10 MPa, and an elongation of 40 ± 5% were obtained with a tensile test along the side direction. The rough calculation of the strengthening mechanisms suggested that the solid solution strengthening of 273 MPa and the dislocation strengthening of 262 MPa were the main contributors to the yield strength. Such a combination of strengthening from solid solution and homogeneously distributed numerous dislocations provides the processed steel with sufficient strengthening ability, leading to an outstanding strength–ductility combination.

Microstructural Analysis of the Improved Strength–Ductility Combination in Titanium Alloy with Bi-modal Structure

Microstructural Analysis of the Improved Strength–Ductility Combination in Titanium Alloy with Bi-modal Structure

Optimizing strength-ductility of laser powder bed fusion-fabricated Ti–6Al–4V via twinning and phase transformation dominated interface engineering

Titanium alloys produced by laser powder bed fusion (LPBF) are known to possess fine microstructures and multi-scale interfaces. Here, we introduce a high density of α'/β, α/β phase interfaces, {10 1‾1} twin boundaries, and basal stacking faults (BSFs) in a Ti–6Al–4V alloy through LPBF and annealing. The LPBF-fabricated alloy consists of fine acicular α′ martensite with numerous {10 1‾1} twins and BSFs, which results in ultrahigh strength (>1300 MPa), but very low ductility (<5%) due to the massive interface strengthening. Subsequent annealing treatments decrease the strength while increasing the ductility, as the α′ martensite partially or fully decomposes into a lamellar (α+β) structure. The 955 °C-annealed alloy possesses a good strength-ductility combination (yield strength of 1000 MPa, tensile strength of 1078 MPa, and total elongation of 20%). During the stages of uniform plastic strain (up to ∼12%), deformation occurs by dislocation slips, leading to significant dislocation accumulations around the α/β interfaces. At later stages (∼20% strain), when necking occurs, a high density of fcc-γ bands is observed in the hcp-α phase, indicating the onset of deformation-induced martensitic transformation (DIMT) from hcp-α to fcc-γ. The occurrence of DIMT may be attributed to the decreased stacking fault energy and the cohesive energy difference between the hcp and fcc phases, due to the segregation of Al elements in the hcp-α phase. Low-loss electron energy loss spectroscopy reveals that the deformation-induced fcc phase is not Ti-hydride, but a new allotrope of Ti.

Deformation behaviors and microstructure evolution of a novel multiphase medium Mn high Al lightweight steels with excellent strength-ductility combinations

The effects of phase constituents on the deformation behavior and the correlated mechanical properties at different solution temperatures were investigated in medium Mn high Al lightweight steels with different Al contents (Fe-12Mn-(6,8)-Al-1C). This study indicates that the mechanical properties of medium Mn high Al lightweight steels can be significantly enhanced though changing the content of Al and solution treatment. 6Al steel shows lower yield strength (YS) and tensile elongation (TEL) values than those of 8Al steel at the same temperature due to the unitary austenite phase. When Al is added to 8 wt%, both the precipitation of intergranular and intragranular κ-carbide are promoted and the constituents of multiple phases (austenite + ferrite +κ-carbide) result in ultra-high strengthening. The apparent strain partitioning causes by the different characteristics between different phases leads to the high initial strain hardening. During the deformation process, the sufficient formation of slip bands in austenite grains keeps the high strain hardening exponent and postponed the plastic stability, which also contribute to a pronounced ductility. The existence of multiple phases owing to the addition of Al is the key feature of excellent properties in the investigated lightweight steel.

Ultrafine lamellar microstructures for enhancing strength-ductility synergy in high-entropy alloys via severe cold rolling process

In the present study, the effect of severe cold rolling (SCR) on the microstructure and mechanical characteristics of X0.25FeCoNiMnV (X: Al, Ti) high-entropy alloys (HEAs) specimens was investigated. It was observed that the as-cast and homogenized specimens exhibited a two-phase microstructure consisting of face-centered cubic (FCC) and a minor body-centered cubic (BCC) structure. Electron backscattered diffraction (EBSD) analysis indicated that SCR could effectively induce a strong rolling texture with stretched grains along the rolling direction and lamellar deformation bands as the major microstructural features for all the HEAs specimens. Moreover, a large number of nano-scale grains are formed within the coarse-stretched microstructure of Al0.25FeCoNiMnV and Ti0.25FeCoNiMnV HEAs specimens due to local fragmentation, resulting in a bimodal grain structure. After 85 % cold rolling, the fraction of high angle grain boundaries and mean misorientation angle of the boundaries are 54 % and 22.15° in the FeCoNiMnV specimen, while they are 61 % and 26.42° for Al0.25FeCoNiMnV, and 57 % and 28.53° for Ti0.25FeCoNiMnV. Ti0.25FeCoNiMnV specimen exhibited the best combination of strength-ductility (tensile strength of 1225 MPa and elongation of 26 %) compared to FeCoNiMnV (720 MPa and 15 %) and Al0.25FeCoNiMnV (970 MPa and 18 %). Observations revealed that the failure mode of HEAs specimens was a ductile type fracture with a combination of deep and shallow dimples. The findings revealed that SCR can effectively improve the strength and microhardness of HEAs, while it highly retains their elongation owing to the formation of a bimodal structure consisting of new grain boundaries and dislocation substructures.

Achieving a strength-ductility combination in VCoNi medium-entropy alloy via N alloying

Single-phase FCC-structured medium-entropy alloys (MEAs) have excellent ductility, but the low strength limits their applications in engineering. In this study, non-equiatomic V32Co33Ni33Cr1N1 MEA was prepared by vacuum arc melting, and its comprehensive mechanical properties were improved by nitrogen alloying and subsequent thermo-mechanical processing. Compared with the VCoNi MEA, it was observed that nitrogen alloying inhibited the recrystallization process and contributed to the formation of nitride precipitates at the grain boundaries, significantly refining the grains. Among them, the alloy subjected to cold rolling and annealing at 900 ℃ exhibited a high yield stress of ∼ 1.1 GPa, an ultimate tensile stress of ∼ 1.5 GPa, and possessed a high fracture elongation of ∼ 25.3 %. The outstanding mechanical properties were mainly attributed to the grain refinement strengthening resulting from the nitrogen alloying and multiple twins strengthening containing annealing twins, deformation twins and multiple twinning systems. This work was expected to aid in understanding the effect of nonmetallic elements alloying on the microstructures and mechanical properties of MEAs, high-entropy alloys and traditional alloys.

Dependence of micromechanical behaviours on grain orientation and size in a nanoscale C11b precipitate-strengthened Ni-23Cr-16Mo pillar via micro-pillar compression

Long-range order (LRO) precipitate-strengthening is an important strategy that endows a superior strength-ductility combination in Ni-based alloys. However, the micromechanical behaviour of the LRO phase-strengthened single-crystals and its dependence on size and orientation remain unclear. In this study, the micromechanical behaviour of LRO-C11b phase-strengthened Ni-Cr-Mo pillars was investigated as a function of the pillar's size and orientation. The stress-strain (σ-ε) curve feature in the [20 2 9]-oriented pillars is independent of their sizes, but the yield strength monotonically decreases with increasing pillar sizes due to the size effect. In contrast, the σ-ε curve exhibits quite diversity when the [20 2 9]-oriented pillar is replaced by other oriented pillars. Slip is responsible for the plastic deformation in all [20 2 9]-oriented pillars, regardless of the pillar's size, while the deformation modes exhibit diversity with the change of orientation of the pillars, i.e., slip combined with twinning in the [411]-oriented pillar, twinning in the [001]-oriented pillar, and initial twinning followed by multiple slips in the [111]-oriented pillar. The diversity of deformation modes is related to the activation of different slip systems. (1—11)[110] slip is predominantly activated in the [20 2 9]- and [411]-oriented pillars, while (—111)[110] is activated in the [001]-oriented pillar, and both (1—11)[110] and (—111)[110] are activated simultaneously for the [111]-oriented pillar. After deformation, multiple substructures appear in the post-deformed pillars, including shearing of LRO-C11b by slip bands or deformation twins, double twinning of both C11b precipitates and disordered γ phases, and disordering transformation.

An Overview of the Effect of Grain Size on Mechanical Properties of Magnesium and Its Alloys

Recent studies show significant advances in improving the mechanical properties of magnesium and its alloys. While many papers deal with different alloy compositions, it is apparent that grain size plays a key role in the mechanical behavior of these materials. The ability to produce samples with very fine grain sizes leads to observations of high strength and/or high elongations. There are recent reports of exceptional elongations of over 100% in pure magnesium and a few alloys. These recent findings are critically reviewed in the present study. The experimental data from over 300 papers are collected, and trends between flow stress, elongation, strain rate sensitivity, and grain size are identified. The role of alloy content is examined. The data clearly shows a transition in the flow stress vs. grain size relationship which is attributed to a change in deformation mechanism from twinning controlled in coarse grained to slip controlled in fine and ultrafine grained samples. The slip controlled deformation agrees with the model of grain boundary sliding, which has shown good agreement with multiple metallic materials. It is shown that the elongations display a maximum in the grain size range in which there is a transition in the deformation mechanism. Three strategies are described for achieving high strength, high ductility, and good strength-ductility combination.

Low-density nano-precipitation hardened Ni-based medium entropy alloy with excellent strength-ductility synergy

The development of advanced structural materials with a synergistic combination of strength and ductility is currently one of the thrust areas of research for material scientists. An alloy design strategy for such materials takes into consideration the ease of processing, density reduction, and economic viability along with unprecedented mechanical properties. The present study reports a low-density, low-cost medium entropy alloy, designed based on the CALPHAD approach, for structural applications. The designed Ni-Fe-Cr-Al based alloy was cast, homogenized and cold rolled to 75% thickness reduction. The cold-rolled full hard material was isothermally annealed at 900°C for varying duration with a quest for an in-depth understanding of static recrystallization mechanisms. The specimens annealed at 900°C for varying durations were aged at 600°C for 5 h. Based on the systematic understanding of recrystallization and precipitation mechanisms involved, a suitable thermo-mechanical process route was judicially selected for the designed alloy, which contributed to an excellent combination of high tensile strength of > 1 GPa accompanied by an appreciable total elongation of > 50%. This superior strength-ductility synergy surpassed the strength-ductility combination of many existing medium and high entropy alloys and can be investigated further to assess its potential applications in challenging existing solid solution (SS) strengthened superalloys.

Evading the strength and ductility trade-off dilemma in titanium matrix composites through designing bimodal grains and micro-nano reinforcements

Synchronous improvement of strength and ductility is a challenging and longstanding problem in the further development and engineering applications of titanium matrix composites (TMCs). In this study, we designed a novel heterogeneously optimized TMC for the first time, which features fine grain/coarse grain (FG/CG) regions embedding in-situ interfacial submicron-TiB/micron-TiC and intragranular nano-TiB whiskers. The TMCs achieved a strength-ductility combination, representing a nearly 30% increase without any reduction in ductility compared with the matrix alloy. The bimodal structures of the TMCs, as well as the micro-nano reinforcements and their interfacial/intragranular distribution, play a predominant role on the strengthening-toughening effect. Besides, the various dislocation behaviors including multiple slips, slip transmission across grain boundaries and activation of more 〈c + a〉 dislocations near the reinforcements, contribute to the desirable ductility. These findings highlight the untapped potential for improving mechanical properties of metal matrix composites.

Designing NbTiTa-Cr/Al refractory complex concentrated alloys with balanced strength-ductility-oxidation resistance properties: Insights into oxidation mechanisms

Limited tensile ductility, poor oxidation resistance, and the ductility-oxidation resistance trade-off are key bottlenecks besetting refractory complex concentrated alloys (RCCAs). Here, in this study, by alloying ductile NbTiTa RCCAs with low content of Cr or Al, we design two novel single-phase RCCAs Nb40Ti40Ta10Cr10 (Cr10) and Nb45Ti30Ta15Al10 (Al10). These two alloys demonstrate excellent tensile strength-ductility combination (∼800 MPa, >20%) and the best oxidation resistance at 800 °C (<7.2 mg/cm2 after 20 h exposure) among all tensile-ductile RCCAs reported by far. Cr10 exhibits much slower oxidation kinetics compared with Al10, attributed to i) the dominant rutile-type Ti1–2x(Nb,Ta)xCrxO2 oxide that has lower volume mismatch with the alloy matrix compared with (Nb,Ta)2O5, as indicated by low Pilling-Bedworth ratio (PBR) values of 1.03–1.76; and ii) the alternative layer structure consisting of dense rutile and porous mixed oxides that is conducive to prohibiting stress cumulation and cracking. A new thermodynamic model connecting the Gibbs free energy of formation of a related oxide to the dynamically varied alloy compositions was proposed and used to unveil the dynamic formation mechanisms of oxides in Cr10 and Al10. The special alternative layer structure, mixed-oxides structure, voids, and/or cracks featured by oxide scales of Cr10 and/or Al10 are well-understood by combining detailed scanning transmission electron microscopy (STEM) studies, thermodynamic simulations, and stress analyses based on PBR evaluations.

Deformation mechanism and mechanical properties of a CoCrFeNi high-entropy alloy via room-temperature rolling, cryorolling, and asymmetric cryorolling

The deformation mechanism and mechanical properties of CoCrFeNi high-entropy alloys (HEA) processed by room-temperature rolling (RTR), cryorolling (CR) and asymmetric cryorolling (ACR) were studied. As the rolling reduction increased from 20 % to 80 %, a transition in the dislocation and twinning deformation mechanisms occurred. The cryogenic environment in rolling process can promote the activation of the twins in the HEA at a lower strain, improving the deformation efficiency. The introduction of differential speed ratio led to the finer micro-shear bands with a more uniform distribution, so that the grains were refined to a greater extent. In the ACR HEA after large deformation, the stacking faults accumulated inside the deformation twins further refined the twins along the direction of long axis. This increased the strength of ACR HEA together with Lomer-Cottrell locks and FCC/HCP boundaries, reaching the maximum value compared with CR and RTR. The ultimate tensile strength exceeded 1450 MPa, and the yield strength of ACR HEA increased by nearly 1200 MPa compared to the initial material, while it still maintained the same elongation as RTR and CR. ACR showed the best strength-ductility combination during the whole rolling deformation process. The microstructure evolution and the strengthening contribution mechanism of different rolling were discussed and calculated, and the specific mechanism of how the introduced differential speed ratio further improves the strength-ductility synergistic strengthening in CR was clarified.

The role of trace Nb in enhancing the strength-ductility combination of a Ni2CoCrFe-based high-entropy alloy via thermo-mechanical processing

The role of trace Nb in enhancing the strength-ductility combination of a Ni2CoCrFe-based high-entropy alloy via thermo-mechanical processing