Combination Of Strength Ductility Research Articles

Enhancing strength-ductility combination of Fe-Ni-Cr-Al-Ti high-entropy alloys via co-precipitation of sigma and L21 phases.

The sigma phase is a well-known harmful phase in transition metal alloys, leading to severe embrittlement due to its inherent brittleness. However, in this work, we develop a new strategy to obtain enhanced strength-ductility combination via the synergistic effects of interlocking co-precipitation of the sigma and L21 phases in Fe40Ni30Cr20Al5Ti5 (at. %) high-entropy alloy (HEA). This HEA exhibits a good strength-ductility combination after aging treatment (800°C for 4h), with a yield strength (YS) of about 920 MPa and an ultimate tensile strength (UTS) of about 1,300 MPa while maintaining a total elongation (TE) of 24%. It is attributed to the interlocking co-precipitation of the sigma and L21 phases with equal volume fractions into the face-centered cubic (FCC) matrix through thermomechanical treatment. During the tensile deformation, the softer L21 phase coordinates the deformation around the sigma phase, suppresses the local stress concentration, and achieves a stable high work hardening capability, as well as stacking faults (SFs) networks and density Lomer-Cottrell (L-C) locks, contributes to the excellent strain hardening capacity and thus obtains the enhanced strength-ductility combination in the HEA. This study will provide a new strategy to obtain enhanced strength-ductility combination for alloys containing brittle and hard topologically close-packed (TCP) phases, such as the sigma phases.

Heterogeneous structure and dual precipitates induced excellent strength-ductility combination in CoCrNiTi0.1 medium entropy alloy

Developing advanced structural materials with high strength and excellent ductility is crucial for meeting aerospace and military requirements. High/medium entropy alloys (H/MEAs) have garnered significant attention due to their superior mechanical properties. In this study, through severe deformation followed by heat treatment at temperatures in the range of 700–900 °C, we successfully obtained a fully recrystallized structure with double precipitates in non-equiatomic CoCrNiTi0.1. Tensile tests at room temperature revealed that the alloy annealed at 700 ° C for 1 h exhibited exceptional mechanical properties, with a yield strength of 1575 MPa, tensile strength of 1735 MPa, and ductility of 15 %. The ultrahigh yield strength is primarily attributed to the synergistic strengthening mechanisms including precipitation strengthening, grain-boundary strengthening, and HDI strengthening. The FCC/L12 interface facilitated dislocation transfer during deformation, reducing stress concentration and promoting uniform deformation. Additionally, deformation mechanisms such as stacking faults and trace twinning contributed to the alloy's excellent ductility, ensuring substantial work-hardening capability and achieving a remarkable strength-ductility combination. These findings not only offer an effective strategy for enhancing the mechanical performance of non-equiatomic MEAs but also deepen our understanding of synergistic strengthening and deformation mechanisms in alloys.

Effects of Annealing time on Microstructural Characteristics and Mechanical Properties of Cryo-Rolled CoCrFeMnNi High Entropy Alloys

High-entropy CoCrFeMnNi alloy was subjected to cryo-rolling with a reduction ratio of 25% and was then annealed at 1050 °C for 5, 25, and 45 min. The results of the recrystallization textures of the various annealed samples showed the presence of similar texture components, indicating that cryo-rolling had only a limited effect on the formation of the subsequent annealing texture. The volume fractions of the different texture components similarly showed no obvious dependence on the annealing temperature. The uniformity of the recrystallization texture suggests the absence of strong preferential nucleation and growth during annealing. The cryo-rolled samples processed with different holding times exhibited a remarkable ambient temperature strength-ductility combination.

In-situ nanoscale precipitation behavior and strengthening mechanism of WC/IN718 composites manufactured by laser powder bed fusion

Nickel matrix composites (NMCs) have received much attention as they show excellent high temperature performances, which could meet the demand for hot-end components in severe environments. In this study, the duplex in-situ nanoscale carbides and oxides reinforcing IN718 were fabricated by laser powder bed fusion (LPBF) and addition of submicron WC particles. The microstructural features and mechanical properties were investigated with variations of WC content (0, 0.5, 2, and 4wt.%). The columnar dendrites with strong <001> texture that boundary distributed continuous Laves phase were found in the LPBF-processed IN718. Incorporating WC led to higher laser absorption and processability, whereas it also decreased the anisotropy of the matrix featured by weak texture index. With an optimized content of 2wt.%, the WC was dissolved into W and C atoms under laser irradiation, which promoted the homogeneous precipitation of intergranular cubic (Nb, Ti)C (∼50 nm) and intragranular spherical Al2O3 (∼20 nm). These precipitates would for heterogenous nucleation to refine grains (∼12.6 μm) and bring about high-density dislocations. The yield strength (YS) and ultimate tensile strength (UTS) increased by 28.8% and 26.5%, respectively, while maintaining high elongation compared with reported NMCs, demonstrating a favorable strength-ductility combination. Finally, the mechanism of microstructure evolution and strengthening was elucidated.

Deformation-induced multi-step TRIP/TWIP(fcc, hcp) behaviors in metastable Fe55Mn25Co10Cr10 high-entropy alloy at ambient and cryogenic temperatures

The mechanical performance and multi-step TRIP/TWIP behaviors in a metastable Fe55Mn25Co10Cr10 were systematically investigated at room temperature (RT) and 77 K. The deformation behaviors and mechanically induced ε-martensite, fcc twin and hcp twin bands in Fe55Mn25Co10Cr10 were studied based on the thermodynamics and nanostructural analyses. Deformation-induced nanostructural evolutions were found to vary greatly as the temperture decreases from RT to 77K. Deformation induced ε-martensite and fcc deformation twins (fcc) carrying more strain developed at RT. On the other hand, bi-directional ε-martensite bands were favored to accommodate the increased strain at 77K instead of fcc deformation twins because of the increased stability of hcp phase. Complicated hcp ε-martensite twins developed adjacent to the crossed region of two ε-martensitec bands at 77K. The tensile stress developed perpendicular to the original hcp ε-martensite band because of its intrinsic smaller c/a ratio (1.615-1.620) than the ideal ratio (1.633) in hcp ε-martensite, inducing {10Ī2} twins with (0001) basal plane approximately perpendicular (86.0o) to that of the original ε-martensite band. Hcp twinning in the ε-martensite band at 77K is suggested to be driven by the structural and electronic relaxations associated with the c/a distortion of the hcp ε-martensite bands. Excellent strength-ductility combinations were also obtained at RT (861MPa/73.2%) and 77 K (1.42GPa/61.8%) because of the enhanced hardening rates due to TRIP/TWIP(fcc) at RT and/or TRIP/TWIP(hcp) at 77 K in this study.

AlxHfTaTi (0≤x≤0.5) refractory medium entropy alloys with excellent room-temperature tensile properties

Refractory high entropy alloys are strong candidates for high-temperature structural materials, because of their high strength, excellent thermal stability and exceptional softening resistance under high-temperature. However, majority of the existing refractory high entropy alloys are seriously limited in widespread application, due to their severe brittleness at room temperature. In this study, we proposed a novel AlxHfTaTi body-centered cubic (BCC) refractory medium entropy alloy with remarkable strength-ductility combination at room temperature. Adjusting the atomic ratio of Al (from 0 to 0.5 at.%), we investigated the effect of Al on the structure-property characteristics of the alloy under as-cast as well as cold-rolled and annealed conditions. Compared with the as-cast samples, the rolled and annealed samples exhibit more excellent tensile properties due to the remarkable reduction of grain size resulted from recrystallization. The Al0.3HfTaTi sample annealed for 5 min exhibits outstanding comprehensive properties at room temperature, with a yield strength higher than 1000 MPa and a fracture elongation of up to 24 ± 1 %, respectively. Quasi in-situ EBSD analysis results reveal that, during tensile deformation, the grain size has a significant effect on the grain rotation and dislocation gliding. Small grains are more likely to initiate multiple slip bands, promoting the synergistic effect of grain rotation and dislocation gliding, thereby improving the ductility of the alloy. Our findings can provide new ideas for the development of refractory high entropy alloys with outstanding room-temperature tensile properties.

Investigation of effects of aluminum additions on microstructural evolution and mechanical properties of ultra-high strength and vanadium bearing dual-phase steels

The effects of aluminum (Al) additions (0.04, 0.4, and 0.8 wt%) on the microstructural evolution, tensile properties, and sheared-edge ductility of dual phase (DP) steels were investigated. The DP steels were processed with distinct coiling temperatures after finish rolling, cold rolled, and subjected to two continuous galvanizing line (CGL) simulations: standard galvanizing (GI) and supercooling process (SC). Al expanded both the ɑ-ferrite and (ɑ+γ) phase fields at the expense of the γ-austenite region. Continuous cooling transformation (CCT) diagrams showed that Al increased ferrite start temperatures, broadened the ferrite transformation zone, and reduced austenite stability. Austenite growth kinetics during intercritical annealing, quantified using a modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, revealed Avrami exponents ranging from 0.72 to 0.81 and apparent activation energies increasing from 422.9 to 496.7 kJ mol−1 with Al additions. Microstructural characterization revealed larger ferrite grain sizes and higher ferrite volume fractions with higher Al levels. Ultimate tensile strength (UTS) was influenced by Al contents, coiling temperatures, and CGL simulation methods. Global ductility (total elongation, TE) was highest for the 0.8Al Steel with a high coiling temperature and GI annealing, while local ductility (hole expansion ratio, HER) was maximized for the 0.04 Al Steel with a low coiling temperature and SC annealing. The majority of the CGL simulated DP steels exhibited good strength-ductility combinations, with the 0.8Al Steels demonstrating superior properties compared to current automotive advanced high-strength steel (AHSS) grades.

A novel Cu-containing TiZrNb-based medium entropy alloy with excellent mechanical properties, antibacterial activity and biocompatibility

Bacterial infection and stress shielding remain significant challenges in the field of orthopedic implant applications. Therefore, it is imperative to develop novel medical implant materials that possess exceptional mechanical properties, antibacterial efficacy, and biocompatibility. The current study reports a novel Ti1.2Zr1.2Nb0.6Cu0.15 medium entropy alloy (MEA) with an optimal combination of mechanical properties, antibacterial activity and biocompatibility. In the Ti1.2Zr1.2Nb0.6Cu0.15 MEA, bits of copper-enriched phases were uniformly dispersed within the body-centered cubic matrix phase. The MEA exhibited good strength-ductility combination with the yield strength of 906 MPa and the fracture elongation of 17.4 %. It is noteworthy that the MEA had a significantly lower elastic modulus (∼67 GPa) compared to pure titanium and Ti6Al4V (∼110 GPa) thus mitigating stress shielding. Meanwhile, the Cu ions released by the alloy resulted in remarkable antibacterial efficacy with an antibacterial rate of 99 % against Escherichia coli. Furthermore, this alloy displayed corrosion resistance and biocompatibility comparable to those of the Ti6Al4V alloy. These findings provide novel insights for designing antibacterial implant materials with excellent comprehensive performance.

Unveiling the Stacking Fault-Driven Phase Transition Delaying Cryogenic Fracture in Fe-Co-Cr-Ni-Mo-C-Based Medium-Entropy Alloy.

In this work, the tensile deformation mechanisms of the Fe55Co17.5Cr12.5Ni10Mo5-xCx-based medium-entropy alloy at room temperature (R.T.), 77 K, and 4.2 K are studied. The formation of micro-defects and martensitic transformation to delay the cryogenic fracture are observed. The results show that FeCoCrNiMo5-xCx-based alloys exhibit outstanding mechanical properties under cryogenic conditions. Under an R.T. condition, the primary contributing mechanism of strain hardening is twinning-induced plasticity (TWIP), whereas at 77 K and 4.2 K, the activation of martensitic transformation-induced plasticity (TRIP) becomes the main strengthening mechanism during cryogenic tensile deformation. Additionally, the carbide precipitation along with increased dislocation density can significantly improve yield and tensile strength. Furthermore, the marked reduction in stacking fault energy (SFE) at cryogenic temperatures can promote mechanisms such as twinning and martensitic transformations, which are pivotal for enhancing ductility under extreme conditions. The Mo4C1 alloy obtains the optimal strength-ductility combination at cryogenic-to-room temperatures. The tensile strength and elongation of the Mo4C1 alloy are 776 MPa and 50.5% at R.T., 1418 MPa and 71.2% in liquid nitrogen 77 K, 1670 MPa and 80.0% in liquid helium 4.2 K, respectively.

Grain growth kinetics, phase evolution during annealing and strain rate sensitivity of TRIP-assisted, dual-phase, non-equiatomic high entropy alloy

Transformation-induced plasticity-assisted dual-phase high-entropy alloys (TRIP-DP-HEAs) belong to a recently developed class of HEAs. TRIP-DP-HEAs have been reported to exhibit superior strength-ductility combination at room temperature as well as at high temperatures. Small amounts of carbon additions to TRIP-DP-HEAs are known to further improve the strength as a result of interstitial solid solution and nanoprecipitate strengthening effects. Current investigation compares the grain growth kinetics in a metastable, dual-phase (FCC and HCP), Fe49.5Mn30Co10Cr10C0.5, carbon-containing high entropy alloy (C-DP-HEA) with that in the corresponding carbon-free variant, Fe50Mn30Co10Cr10 HEA (DP-HEA). The alloys after being subjected to cold rolling, were annealed at 900 °C, 1000 °C and 1100 °C for 3, 30, 60 and 120 min, followed by water quenching. Slower grain growth kinetics was observed in C-DP-HEA due to Zener pinning by carbide particles. The activation energy for grain growth in C-DP-HEA was determined to be 339.71 kJ/mol, which is larger than that of DP-HEA (282.68 kJ/mol). The microstructures of the C-DP-HEA samples, after annealing at 900 °C for different durations of times and quenching, were investigated. The HCP phase fraction in the microstructure initially decreased with increase in annealing time/grain size and later increased with further increase in annealing time/grain size. This is explained based on the counteracting effect of grain boundary density and back stresses acting in samples having increased grain size. Strain rate sensitivity (SRS) parameter, ‘m’, was evaluated for both C-DP-HEA and DP-HEA. The strain rate jump tests were conducted at two quasi-static strain rates, 1 × 10−3 s−1 and 5 × 10−3 s−1, and at room temperature as well as at higher temperatures, 900 °C, 1000 °C and 1100 °C. The ‘m’ value was found to depend on the grain size, test temperature, and the presence of carbon in the alloy.

Achieving excellent strength–ductility combination of Al-aided Mg–Gd–Al–Zr–Zn alloys fabricated by cold metal transfer-based wire-arc directed energy deposition via uniform equiaxed grains and multiple precipitates

Limited ductility consistently poses a challenge for high-strength Mg–RE (rare earth) structures prepared by additive manufacturing. Particularly following heat treatment, while the strength of the Mg–RE alloy experiences a substantial increase, there is a noteworthy decrease in its ductility. Herein, a novel Mg–Gd–Al–Zr–Zn thin wall was successfully fabricated by cold metal transfer-based wire-arc directed energy deposition (CMT-WA-DED) and Al alloying. The microstructural evolution and mechanical properties of deposited and heat-treatment alloys were elucidated systematically. By adding Al element, Al2Gd and Al–Zr phases with thermal stability were precipitated in the deposited alloy. During the solidification or heat treatment stage, Al2Gd and Al–Zr phases could pin grain boundaries to restrict grain growth. The as-deposited alloys exhibited fully equiaxed grains at different heights and orientations. Thus, the thin wall exhibited good isotropic performance. After the heat treatment, the grain size only slightly grew and no abnormal grain growth was observed. Various morphologies of β precipitation phases were discovered along grain boundaries and within the grains. Additionally, a high density of β' and β1 phases within the grains and β phases along the grain boundary played pivotal roles in enhancing the mechanical strength of the Mg–Gd–Al–Zr–Zn alloy. The alloys exhibited an excellent combination of strength and ductility with ultimate tensile strength of 340 ± 7 MPa, yield strength of 212 ± 5 MPa, and elongation of 12.7 ± 0.9 %. This work provides theoretical and practical reference value for the Mg–RE structures obtaining an excellent combination of strength and ductility prepared by additive manufacturing.

Heterostructure enables anomalous improvement of cryogenic mechanical properties in titanium

The pursuit of strong and ductile structures for cryogenic applications has fueled a persistent interest in the microstructure design of metals and alloys. While heterostructure design has recently demonstrated efficacy in achieving superior strength-ductility combinations at room temperature, its potential remains less explored in the cryogenic regime. In this report, we unveil anomalously improved cryogenic mechanical properties of a pure titanium with harmonic heterostructure that matches or even surpasses those of commercial titanium alloys. Through detailed comparison to room-temperature deformation behavior, we reveal that the superior performance of heterostructure at cryogenic temperature is fundamentally underpinned by the amplified inter-zone mechanical incompatibility and the suppression of dislocation cross-slip. The former promotes the generation of abundant geometrically necessary dislocations, and the latter optimally configures them. Collectively, these two factors culminate in the hetero-deformation-induced effect, delivering the anomalous improvement of cryogenic mechanical properties in titanium. These findings not only significantly advance our understanding of cryo-deformation behaviors in heterostructured materials but also offer the potential for developing high-performance materials for cryogenic applications.

Revealing the Enhancement Mechanism of Laser Cutting on the Strength–Ductility Combination in Low Carbon Steel

The strength–ductility mechanism of the low-carbon steels processed by laser cutting is investigated in this paper. A typical gradient-phased structure can be obtained near the laser cutting surface, which consists of a laser-remelted layer (LRL, with the microstructure of lath bainite + granular bainite) and heat-affected zone (HAZ). As the distance from the laser cutting surface increases, the content of lath martensite decreases in the HAZ, which is accompanied by a rise in the content of ferrite. Considering that the microstructures of the LRL and HAZ are completely different from the base metal (BM, ferrite + pearlite), a significant strain gradient can be inevitably generated by the remarkable microhardness differences in the gradient-phased structure. The hetero-deformation-induced strengthening and hardening will be produced, which is related to the pileups of the geometrically necessary dislocations (GNDs) that are generated to accommodate the strain gradient near interfaces. Plural phases of the HAZ can also contribute to the increment of the hetero-deformation-induced strengthening and hardening during deformation. Due to the gradient-phased structure, the low carbon steels under the process of laser cutting have a superior combination of strength and ductility as yield strength of ~487 MPa, tensile strength of ~655 MPa, and total elongation of ~32.7%.

Good strength-ductility synergy achieved in a Fe62Ni16Co9Mn9Ti3Si1 Fe-high-entropy alloy with heterogeneous structures

Developing metal structural materials with a good combination of strength-ductility is of great significance for the rapid development of the aerospace industry and the energy industry. High-entropy alloys (HEAs) have attracted widespread attention due to their excellent mechanical properties, especially the excellent ductility and toughness of face-centered cubic (FCC) high-entropy alloys. However, similar to traditional alloys, there is a trade-off between strength and ductility in HEAs. In this paper, we report the successful preparation of Fe-HEA (Fe62Ni16Co9Mn9Ti3Si1 (atomic percentage, %)) with high strength-ductility synergy at room temperature by cryo-rolling and annealing at 600 °C for 10 min (CR600), which increases the yield strength by 59% compared to room temperature rolling (RR600), while the uniform elongation decreases by only 20%. The ultra-high yield strength and the not significant reduction in uniform elongation are mainly attributed to the formation of high-density dislocations, dual-phase bimodal ultra-fine grain heterostructures, (NiMn)3-xTix and (Fe, Ni)2SiTi composite nanoprecipitates, precipitation-induced chemical composition heterogeneity, and BCC→HCP phase transformation during tensile deformation during CR and subsequent annealing at a reduced temperature (600 °C). In addition, by increasing the annealing temperature of the alloy after CR to 800 °C, its uniform elongation can be increased to 53%, which is mainly due to the excellent strain hardening ability induced by changes in heterogeneous structures, nanoprecipitates, and martensitic phase transformation. Compared with other similar alloys, our CR alloy exhibits higher strength-ductility synergy. The results of this study provide a new perspective for developing HEAs with higher strength-ductility synergy.

Strengthening complex concentrated alloy without ductility loss by 3D printed high-density coherent nanoparticles

Metallic 3D printing enables fast fabrication of net-shaped components for broad engineering applications, yet it restrains the use of most mechanical processing methods for strengthening alloys, e.g. forging, rolling, etc. Here, we proposed a new strategy for enhancing the strength of 3D printed complex concentrated alloys without losing ductility. This strategy relies on the rapid cooling of 3D printing to achieve a supersaturation state that is beyond conventional casting. Then, spinodal decomposition via aging is exploited to introduce high-density coherent nanoparticles for strengthening. The proposed strategy is demonstrated in a 3D printed Cu-based complex concentrated alloy. The rapid solidification during printing strongly inhibits elemental diffusion, leading to a high supersaturation state. High-density nanoparticles with coherent interface and size of ∼7 nm are introduced into the 3D printed samples through spinodal decomposition via simple aging treatment. The strength of the 3D printed alloy is increased by 30 % after aging with no ductility loss, leading to a strength-ductility combination superior to other Cu alloys. This strategy is readily applicable to other spinodal alloys fabricated by 3D printing for circumventing the strength-ductility trade-off dilemma.

In-situ EBSD investigation of excellent strength-ductility synergy mechanism achieved by thermal-mechanical processing of Al0·25FeCoNiV high-entropy alloy

The development of high-entropy alloys (HEAs) is plagued by a trade-off between strength and ductility. In this study, we investigated the effects of different cold rolling ratios and subsequent heat treatments on the microstructures and mechanical properties of the Al0·25FeCoNiV HEA. An efficient and economical thermal-mechanical processing route was successfully explored to prepare multiscale, multiphase heterogeneous structures consisting of heterogeneous grain structures and duplex structures. Yield strength, ultimate tensile strength, and fracture elongation of 1277.3 MPa, 1633.6 MPa, and 26.4 %, respectively, could be achieved. The strength-ductility trade-off was avoided. Detailed information on the evolution of grain boundary distribution, grain orientation changes, and grain rotation under stress and strain fields were obtained with in-situ electron backscatter diffraction (EBSD) tensile tests. The mechanism of coordinated plastic deformation and the intrinsic mechanism of excellent strength and ductility were investigated by quantitatively calculating the activation of the slip system, slip transfer at the phase boundaries, and grain rotation paths. It was found that the excellent mechanical properties can be attributed to multistage hetero-deformation-induced (HDI) strengthening and strain-hardening coupling mechanisms. In this study, an effective method for designing novel HEAs with excellent strength-ductility combinations and an analytical method based on in-situ tests are proposed.

Achieving high strength-ductility combination of Al–Mn–Mg-Sc-Zr alloy subjected by laser powder bed fusion and suitable post heat treatment

Post heat treatment is crucial for optimizing the performance of additive manufacturing components, yet it is still challenging to synergistically improve the strength and ductility through simple heat treatment. The present work investigated the microstructure evolution and mechanical properties of laser powder bed fusion (L-PBF) additive manufactured Al–Mn–Mg-Sc-Zr alloy after aging at 300 °C for different durations. All the as-built and aged samples exhibited a typical bi-modal microstructure consisting of coarse columnar and fine equiaxed grain regions. The as-built samples contained a number of primary Al6Mn precipitates distributed along grain boundaries; meanwhile, Mg solutes segregation was visible at grain boundaries, forming “Mg walls”. After aging for 4 h, the Al6Mn and a small content of Al3Sc were precipitated. Two types of Mg-rich precipitates evolved along grain boundaries simultaneously, increasing ultimate tensile strength from 401 MPa to 501 MPa, while the ductility was significantly reduced from 19.6 % to 4.6 %. When increased to 6 h, the Mg-rich precipitates disappeared, and the density of Al6Mn and Al3Sc precipitates reached its peak concurrently, providing a dual-precipitate strengthening effect. As a result, an excellent strength-ductility balance with an ultimate tensile strength of 528 MPa and ductility of 14.5 % was obtained. The Al6Mn precipitates became coarsening with increasing to 7 h, resulting in lower ductility. The strategy of modulating elemental distribution and precipitated phases by simple heat treatment is expected to be further developed for high strength and ductility Al alloys prepared by L-PBF.

Unveiling the impact of heat treatment on powder-metallurgy α+β titanium alloy for achieving a superior strength-ductility combination

The present study investigated the effects of heat treatment (solid solution and aging treatments) on the microstructure and mechanical properties of a hot-rolled α+β Ti–3Al–5Mo–4.5V (TC16) produced through blended elemental powder metallurgy (BEPM) process using master alloy hydrogenation (MAH) approach. After solid solution treatment, metastable martensite α'' phase and athermal ω phase were formed. Aging treatment facilitated decomposition of metastable phases and precipitation of αs particles. Dispersion strengthening, resulting from the formation of a significant amount of α/β interfaces between fine secondary αs particles and β matrix, played a decisive role in enhancing the strength of the TC16 alloy. However, elevated aging temperatures promoted the coarsening of αs particles, leading to a reduction in the strengthening effect. A systematic investigation of the effects of the solid solution and aging treatments enabled the evolution of a bimodal microstructure with the precipitation of fine αs particles, contributing to an outstanding combination of strength and ductility. The ultimate tensile strength reached 1424.6 MPa with an elongation at break of 7.7%, while the hardness reached 380.2HV.

A novel cast Co68Al18.2Fe6.5V4.75Cr2.55 dual-phase medium entropy alloy with superior high-temperature performance

A newly-designed Co68Al18.2Fe6.5V4.75Cr2.55 medium entropy alloy (MEA) with FCC + B2 structure exhibits not only an excellent strength-ductility combination at room temperature, but also an outstanding high-temperature mechanical properties in particular. The phase formation, microstructure and deformation mechanism of this designed dual-phase MEA at room and high temperatures were investigated and discussed in detail. Based on experimental results (XRD, SEM and TEM) and theoretical analysis (calculated diagram and empirical thermodynamic parameters), it is demonstrated that this newly-designed MEA consists of a dual-phase structure of a face-centered-cubic (FCC) phase (70.3 vol %) plus chemically ordered body-centered cubic phase (B2) (29.7 vol %). Meanwhile, the FCC phase is enriched with Co, Fe and Cr elements, while the B2 phase is enriched with Al and Co elements. Furthermore, compared with some HEAs/MEAs and commercial alloys, this newly-designed dual-phase MEA exhibits excellent strength-ductility combination with a high compressive yield strength of ∼1254 MPa, a fracture strength of ∼2657 MPa and a large compressive plasticity of ∼34.6 % at room temperature. In particular, the studied MEA also shows an outstanding high-temperature performance with an ultrahigh yield strength of ∼1213 MPa at 600 °C and ∼626 MPa at 800 °C, respectively. The excellent comprehensive mechanical properties of this designed dual-phase MEA at both room and temperatures mainly originate from synergy effect of alternate FCC + B2 structure and strengthening contributions of B2 phase. These important findings not only reveal the deformation and strengthening mechanisms of the newly-designed dual-phase MEA, but also provides an effective alloy design strategy for developing advanced structural materials serving as a wide range of temperature.

A bimodal microstructure to improve the strength-ductility properties of Ti-Cu alloys by using eutectoid reactive laser depositing

A novel bimodal structure was achieved in Ti-6wt%Cu by laser metal deposition. The microstructures and tensile behaviors of this heterostructure, comparing with typical colony structure, were examined to understand the mechanisms of strengthening and deformation. The bimodal structure consisting of nano-scale eutectoid lamellae and fine Widmanstätten-α exhibits the strength-ductility combination and excellent strain hardening ability. The shear deformation mechanism in ultrafine lamellae and hetero-deformation induced hardening in bimodal structure contribute to the high strain hardening. In contrast, the colony structure with coarse laths and Ti2Cu precipitates show inferior dislocation strengthening leading to a lower tensile strength, while the formation of deformation twins ensures a higher ductility.