Strength-ductility Trade-off Research Articles

Enhancement of the strength-ductility synergy of Al-Si-Mg alloys via C-doped TiB2 particles

• Novel C-doped TiB 2 particles are introduced into an Al-Si-Mg alloy. • The average grain size of α-Al can be reduced by 70% via C-doped TiB 2 particles. • The C-doped TiB 2 particles enhance the strength-ductility synergy of A357 alloy. We report novel C-doped TiB 2 particles introduced into an Al-Si-Mg alloy. The impact of C-doped TiB 2 particles on the α-Al grain and associated mechanical properties were systematically investigated. These particles were introduced via a master alloy (Al-TBC master alloy, or Al-TBC for short). The results show that the C-doped TiB 2 particles have a dramatical nucleation capability for the α-Al grain, thus eliminating the grain refinement problem of Si “poisoning” in the A357 alloy (one typical category in Al-Si-Mg alloy). As a result, the average grain size of α-Al can be refined to 140 μm from 490 μm by adding 0.5 wt.% Al-TBC. The C-doped TiB 2 particles can also enhance the strength-ductility synergy of A357 alloy at room temperature. At 0.5 wt.% Al-TBC, the ultimate tensile strength, yield strength, and elongation of T6-treated A357 alloy are 370.1 ± 1.2 MPa, 302.5 ± 7.5 MPa, 12.38 ± 1.2 %, respectively. This work provides a novel prospect for resolving the problem of Si “poisoning” in grain refinement and strength-ductility trade-off in hypoeutectic Al-Si alloys via C-doped TiB 2 particles.

In-situ laser shock peening for improved surface quality and mechanical properties of laser-directed energy-deposited AlSi10Mg alloy

In the recent decades, the laser-directed energy deposition (L-DED) of aluminium alloy has encountered significant challenges owing to the defects and tensile residual stress. In this study, the in-situ laser shock peening method is employed for the first time to improve the surface quality and mechanical properties of L-DED-fabricated AlSi10Mg alloy. The physical mechanism of the compressive pressure on the pore defects is accounted for using the finite element method. The numerical results indicate that severe plastic deformation driven by the shock wave contributes to defect healing, and the maximum pressure plays a crucial role. The effects of surface laser shock peening (SLSP) and layer-by-layer laser shock peening (LLSP) on the densification, surface quality, including the roughness and residual stress, microstructure, and mechanical properties of the samples are experimentally studied. After the LLSP treatment, the surface quality and mechanical properties of the samples exceed those of common aluminium alloys fabricated by laser powder bed fusion (L-PBF). After the SLSP treatment, a compressive residual stress with a maximum value of 30 ± 14 MPa is formed on the upper surface, and the LLSP treatment overcome the depth limitation of the affected zone. Furthermore, the LLSP treatment addresses the strength–ductility trade-off, and realizes an excellent combination of strength and ductility through grain refinement, dislocation strengthening, and compressive residual stress. Therefore, this work establishes the viability of in-situ laser shock peening for the rapid additive manufacturing of large-size aluminium alloys. The data used to support the findings of this study are available from the corresponding author upon request.

Effects of solid-state interfacial reaction on the mechanical properties of carbon nanotubes reinforced aluminum matrix composites with heterogeneous structure

To overcome strength-ductility trade-off in carbon nanotubes (CNTs) reinforced Al matrix composites, we developed a novel and controllable strategy via a combination of different length of CNTs and fine/coarse-grained matrices. Microstructure analysis showed that Al4C3 nanorods about 100 nm were distributed in the fine grain zone (FGZ), while CNTs of micron length were distributed in the coarse grain zone (CGZ). Different sizes of reinforcements play different strengthening roles, and the bimodal grains can effectively coordinate deformation, thereby achieving a good combination of strength and ductility. Heat treatment does not affect the microstructure of the FGZ, but significantly affect the CNT/Al interface bonding in the CGZ. Heat treatment at an appropriate temperature can further improve the strength of the composite by improving the interface bonding. When the heat treatment temperature is too high, the coarsening of Al4C3 in CGZ will deteriorate the mechanical properties of the composite.

Composition design of a novel Ti-6Mo-3.5Cr-1Zr alloy with high-strength and ultrahigh-ductility

• A novel titanium alloy with high strength and ultrahigh ductility was designed to break through the longstanding strength-ductility trade-off. • The strong strain partitioning effect and the dislocation pile-up effect caused by SIω were found and quantitatively analyzed by in situ EBSD/DIC and in situ TEM tests, respectively. • The TRIP/TWIP/DPU coupling effect during plastic deformation was firstly proposed and deciphered. The strength-elongation to fracture ( ε f ) trade-off and low strain hardening rate have been a longstanding dilemma in titanium alloys. In this work, we innovatively manufactured a novel Ti-6Mo-3.5Cr-1Zr alloy via the introduction of a stress-induced strengthening phase into the material. Stress-induced ω (SI ω ) phase transformation was expected to replace the α'' martensitic transformation that resulted in the low yield strength of titanium alloys. The obtained alloy exhibited an extremely high strain hardening rate of up to ∼1820 MPa. The true peak tensile strength and ε f reached ∼1242 MPa and ∼40%, respectively. The intrinsic mechanisms underlying the simultaneous improvement of strength and ductility of the material were systematically investigated via in-situ and ex-situ characterizations. In-situ electron backscatter diffraction (EBSD)/digital image correlation (DIC) results showed that SI ω phase transformation dominated the early stage of plastic deformation (1.5%–3%) and promoted the strain partitioning between the stress-induced bands and β matrix. Subsequently, the formation of {332}<113> β twins and ω twins was observed via ex-situ EBSD. In-situ transmission electron microscopy results revealed that dislocation pile-up (DPU) occurred at the SI ω / β interface. The coupling effects associated with the transformation induced plasticity (TRIP), twinning induced plasticity (TWIP), and DPU mechanisms contributed to the enhanced strength and ε f of the designed titanium alloy.

Nano-twinning induced high strain hardening behavior in a metastable as-cast Co30Cr30Fe20Ni20 high entropy alloy at room temperature

High entropy alloys (HEAs) utilize multiple deformation mechanisms to overcome the strength-ductility trade-off by deformation treatment and low temperature conditions. To develop original engineering materials with strength-ductility synergy properties, we induced multiple deformation strengthening mechanisms into the as-cast face-centered cubic (FCC) HEAs at room temperature. A metastable as-cast Co 30 Cr 30 Fe 20 Ni 20 HEA was designed by adjusting composition and prepared by melting. The tensile properties and deformation microstructure evolution were investigated. Co 30 Cr 30 Fe 20 Ni 20 HEA has larger lattice distortion, stronger atomic binding force, and lower stacking fault energy (SFE) than equiatomic CoCrFeNi HEA. The low SFE and large grain size promote multiple slip systems (single slip, crossing slip, and blocked slip) activation and the formation of deformation twins. The as-cast Co 30 Cr 30 Fe 20 Ni 20 shows excellent properties at room temperature, including yield strength of 267 MPa, tensile strength of 613 MPa, and fracture strain of 96.2%, which increases by 92.2%, 47.9%, and 10.50% than that of equiatomic CoCrFeNi HEA. The enhancement of yield strength results from solid solution strengthening and strong atomic binding forces. The tensile strength-ductility synergy is attributed to a high strain hardening rate by utilizing multiple slip interactions of dislocations and twinning-induced plasticity (TWIP). • A metastable as-cast Co 30 Cr 30 Fe 20 Ni 20 HEA was designed by adjusting Co and Cr composition. • Co 30 Cr 30 Fe 20 Ni 20 HEA has large lattice distortion, strong atomic binding force, and low stacking fault energy. • As-cast Co 30 Cr 30 Fe 20 Ni 20 shows excellent strength-ductility synergy at room temperature, including tensile strength of 613 MPa and elongation of 96.2%. • Properties improvement is attributed to high strain hardening rate by utilizing multiple slip interactions of dislocations and twinning-induced plasticity.

Novel BCC Ti-Al-Nb-Zr medium-entropy alloys with ultrahigh specific strength and ductility

Body-centered cubic (BCC) medium-entropy alloys (MEAs) are characterized by high strength, low ductility and often high density due to containing much refractory elements. In this paper, a series of BCC Ti-Al-Nb-Zr medium-entropy alloys were constructed by adding different contents of Al to Ti67Nb16Zr17 base alloy composed of α and β phases. It is shown that an enough addition of Al can depress the formation of α phase, resulting in single BCC structure. (Ti67Nb16Zr17)87Al13 alloy has the best comprehensive mechanical property as the strength-ductility trade-off is well overcome. Its specific strength is up to 230 MPa g−1 cm3 and its elongation is also as high as 18.5% in the as-cast state, better than most existing medium- and high-entropy alloy. It is revealed that abundant nanoscale chemical short-range order (CSRO) in the matrix phase plays an important role in the mechanical response of this alloy.

Simultaneously improving strength and ductility of NiAl-based composites through two-scale network structures and a core-shell-like structure

Simultaneously improving strength and ductility was achieved in a novel NiAl-based composite with two-scale network structures and a core-shell-like structure prepared by hot isostatic pressing (HIP). The HfO2 formed in situ along the original powder boundaries and HfRe2 transformed in situ along the grain boundaries within the original powders constituted the first-scale and second-scale network structures respectively. The nanoscale Hf21Re25 precipitation zone around the grain boundaries constituted the shell structure, and the region at the center of the grains constituted the core structure. The composite exhibited excellent comprehensive mechanical properties due to the multi-scale structures. The room-temperature ultimate compression deformation up to 0.471 could be attributed to the significant fine-grain effect due to the network structures, the hindering effect of the network structures on crack extension, and the shell structure preventing excessive concentration of dislocations at grain boundaries. The high level of ultimate compressive strength (207 MPa) at 1050 °C was mainly due to the hindrance of dislocations by the particles in the network structures and shell structure. The multi-scale structural design broke through the strength-ductility trade-off dilemma and provided a new idea for the structural design of composites.

The importance of interfacial stress-affected zone in evading the strength-ductility trade-off of heterogeneous multi-layered composites

The interfacial stress-affected zone (ISAZ), characterized by a stress gradient maximizing at the interface, is considered a key factor in determining the mechanical properties of heterogeneous multi-layered materials. In the past few decades, the ISAZ dimension has been routinely estimated by two-dimensional strain mapping or by computational modeling; accurate stress measurement remains technically challenging. In the present study, we confirmed the presence of the ISAZ and quantified the stress evolution process upon tension of Ti/Al multi-layered composites using in situ high-energy X-ray diffraction and in situ neutron diffraction. Our results demonstrated that the ISAZ spanned 15 μm and remained unchanged for the present model material, independent of the load and layer thickness. The length scale of 15 μm was closely correlated with the mechanical property in such a way that the model material exhibited the highest work hardening rate, the greatest uniform flow capacity, and the most dispersed crack distribution, only if the Al layer thickness was nearly twice the ISAZ size of 15 μm. Under that condition, the soft Al layer was almost fully occupied by two adjacent ISAZs, and such a perfectly-coated stress distribution may enable a more efficient interfacial constraint effect to achieve superior mechanical properties. The present study therefore opens a new window to optimize the mechanical properties by tailoring the layer thickness according to the ISAZ scale.

Performance of thin blast-loaded steel plates after ballistic impact from small-arms projectiles

It is known from real blast-load events that the combined effect of fragment impact and blast loading is more severe than the effect of the blast loading alone. The present study investigates this effect by comparing the blast response of thin steel plates pre-damaged by either ballistic impacts or idealized pre-cut circular holes under similar blast loading conditions. The ballistic impacts were performed with a rifle, firing 7.62 mm APM2 projectiles, and the subsequent blast loading was applied in a shock tube facility. During the blast tests, pressure measurements and two high-speed cameras were used to record the dynamic response of the target plates and allowed for a reliable experimental procedure. To investigate the strength-ductility trade-off dilemma of the material during extreme loading, three different steels with different levels of strength and ductility were used in the tests. Numerical simulations were conducted in an attempt to obtain more insight into the fracture characteristics of the plates.For similar loading conditions and material, the target plates exposed to ballistic impact showed a reduced resistance to fracture during blast loading compared with target plates containing pre-formed circular holes. As for the effect of material strength and ductility, the global deformation decreased, while the localization of plasticity increased, with an increased material strength. The fracture resistance was also observed to decrease with increased material strength. The numerical models were able to capture the main trends observed in the physical tests, i.e., a decrease in fracture resistance with initial ballistic impact, and a reduction in fracture resistance with increased material strength. The numerically predicted crack paths also showed a strong dependence upon the initial cracks caused by the petals from the ballistic impacts.

Hierarchically structured powder metallurgy austenitic stainless steel with exceptional strength and ductility

Powder metallurgy stainless steels have attracted much attention due to their wide applications. However, they often exhibit a low relative density (<96%) and a strength-ductility trade-off dilemma. We herein study the microstructure and mechanical property of a dense hierarchically structured stainless steel, which is synthesized by a high-pressure (4 GPa)/high-temperature (1200 °C) sintering technique and exhibits a relative density of approx. 99%. The hierarchically structured stainless steel is composed of 27.5% microcrystalline phase with an average grain size of 9 μm and 72.5% ultrafine-grained phase with an average grain size of 259 nm. These unique microstructures together with the high relative density enable us to achieve an exceptional combination of high yield strength of 1188 ± 59 MPa and a relatively high tensile elongation of 15 ± 2.6%, evading the strength-ductility dilemma observed in conventional powder metallurgy stainless steels synthesized by pressureless or low-pressure sintering. The high strength is mainly caused by the ultrafine-grained phase, which can be obtained even after sintering nanocrystalline stainless steel powders at a high temperature of 1200 °C (∼87% of melting temperature). The ultrafine-grained phase and the grain boundary strengthening factor in the ultrafine-grained phase contributes 91% and 48% to the total strength, respectively. Both experiments and calculations indicate that increasing the sintering pressure from 40 MPa to 4 GPa can significantly lower the grain growth rate by approx. 64%. The high ductility is mainly caused by the microcrystalline phase, which causes the formation of a large number of dislocations and the generation of nano-twins.

Effect of limited retained austenite on the strength–ductility trade-off in low-alloyed TRIP steel

A comprehensive experimental and numerical investigation of TRIP effect on mechanical behaviors, strain partitioning and fracture is carried out on a low-alloyed TRIP steel with and without subzero treatment. Load responses of individual phases and macroscopic mechanical properties are investigated through nanoindentation and uniaxial tensile tests. The increase of martensite volume fraction caused by subzero treatment improves tensile strength but appears detrimental to ductility. Two types of martensite transformation are observed: a twin-relationship variant selection to accommodate strain interior unstable austenite grains, and a variant selection based on the same crystal orientation as adjacent coarse ferrite grain reducing interfacial energy between ferrite and newly transformed martensite. A microstructure-based simulation model considering martensite transformation is developed with phenomenologically calibrated crystal plasticity parameters. The shift of strain localization from unstable austenite grain to neighboring ferrite grain is clarified to reduce the strain partitioning at interfaces. This effect is argued to be the origin of ductility enhancement in low-alloyed TRIP steel.

High specific yield strength TiZrAlNbV high-entropy alloys via coherent nanoprecipitation strengthening

Developing lightweight high-entropy alloys (LHEAs) having considerable strength and ductility is a new perspective in the quest of novel lightweight alloys for potential engineering applications. The paper describes that by tailoring the aging treatment, abundant coherent B2 nanoprecipitates can be induced in body-centered-cubic (BCC) Ti–Zr–Al–Nb–V LHEAs. These nanoprecipitates can strengthen the alloys by around 20% based on the dislocation-shearing mechanism. Moreover, due to the effective inhibition of these precipitates on dislocation propagation, frequent cross-slip events appear, which benefit the deformation delocalization. Finally, based on the B2 precipitation strengthening, a LHEA having specific yield strength as high as ∼202 MPa m3/kg and considerable fracture strain of ∼25% is developed. Our results provide a solution to overcome strength-ductility trade-off for the alloy to be suitable for structural applications.

Strength and ductility enhancement of high-entropy FeCoNi2Al0.9 alloy by ultrasonically refining eutectic structures

The solidification process of high-entropy FeCoNi2Al0.9 alloy was modulated by 20 kHz power ultrasound with maximum 13 μm amplitude. The statically solidified eutectic structures were composed of L12 and B2 phases growing by K-S synergistic relationship. With the increase of ultrasound amplitude, the eutectic cell width and regular lamellar spacing were significantly refined, the proportion of strictly semi-coherent L12/B2 interfaces increased, and the solute concentration fluctuation in each eutectic phase was obviously suppressed. The steady-state cavitation and acoustic flow displayed remarkable influences on eutectic microstructure evolution. The yield strength, ultimate strength, and total elongation were increased by 23%, 21%, and 19% as compared with statically solidified alloy. This provided an effective approach to promoting the mechanical properties and overcoming the strength-ductility trade-off for those high-entropy alloys with eutectic structures.

Inherent and multiple strain hardening imparting synergistic ultrahigh strength and ductility in a low stacking faulted heterogeneous high-entropy alloy

Alloys with high yield strength and ductility are attractive for application because of their potential to offer mass reduction, energy savings, and enhanced structural reliability. However, increasing strength usually comes at the expense of ductility, which is commonly known as the strength-ductility trade-off for metal alloys. In this work, we explored a strategy of using a heterogeneous grain size structure and a reduced stacking fault energy in a CoCrFeNiMn FCC high entropy alloy to overcome the limitations of this trade-off. By this approach, the alloy achieved a yield strength of 980 MPa, a tensile strength of 1385 MPa, and tensile elongation to failure of 48% benefiting from cooperative strain hardening via multiple mechanisms, such as hetero-deformation induced (HDI) hardening, deformation twinning, Frank-Read dislocation sources and Lomer–Cottrell dislocation locks instigated by such structures. These micromechanisms of deformation help to harden the alloy via in situ refinement of the mean free paths for dislocations.

Hetero-deformation-induced strengthening behavior of as-annealed Co30Cr30Fe18Ni18Mo4 high entropy alloy by metastable σ phase

Multiple deformation mechanisms have been utilized to overcome the strength-ductility trade-off in high entropy alloys (HEAs), especially at low temperatures and deformation conditions. To trigger the multiple deformation mechanisms at room temperature, the multiphase heterostructure Co30Cr30Fe18Ni18Mo4 HEA was developed at moderate temperatures (600 °C–1100 °C) and short time (1 h–8 h) annealing treatment. The phase formation, microstructure evolution, and tensile properties of as-annealed Co30Cr30Fe18Ni18Mo4 HEA were systematically investigated. At 600 °C for 2 h (A600-2), the alloy is consisted of the face-centered-cubic (FCC) phase and rich (Cr, Mo) primary σ phase. The fine needle-shaped secondary σ phase precipitates at 800 °C. With the increase of annealing temperatures and times, the volume fraction of secondary σ phase increases. FCC matrix, metastable secondary σ phase, and primary σ phase are gradient heterostructure. The thermodynamic metastable of the FCC matrix and the sluggish diffusion kinetics of elements induce the formation of fine and metastable secondary σ phase at the thermal driving force. The as-annealed alloy exhibits high yield strength and applicable plasticity with the increase of the secondary σ phase. Hetero-deformation-induced (HDI) strengthening is induced by piled-up dislocations at the σ/FCC interface. The dominant roles in the improvement of global yield strength are precipitation strengthening and back-stress strengthening. The back stress, stacking faults (SFs), and twins are generated in the soft FCC matrix, and a few SFs are observed in the metastable secondary σ phase, which induces high strain hardening ability. The cracks initiated in the hard primary σ phase are passivated by the FCC matrix and metastable secondary σ phase. TWIP, SFs, and HDI strengthening multiple deformation mechanisms contribute to improving strength.

Ultra-high strength and excellent ductility high entropy alloy induced by nano-lamellar precipitates and ultrafine grain structure

We describe the development of a non-equiatomic CoCrFeNi2Al0.3Ti0.25 high entropy alloy (HEA). Both high-density nano-lamellar precipitates and ultrafine grain structure are simultaneously introduced by cold rolling then annealing at 600–800 °C for 1 h. The HEA annealing at 700 °C (CR-700) exhibits ultra-high yield strength (σy) of 1900 ± 15 MPa, ultimate tensile stress of 2074 ± 17 MPa and excellent total elongation (εte) of 16.7 ± 1.5. This succeeds in overcoming the strength-ductility trade-off that often hampers the development of high-performance alloys. Compared with the as-cast alloy, σy of the CR-700 HEA increases by 171% and εte decreases by 54%. The ultra-high strength is mainly attributed to the synergistic strengthening effects of both nano-lamellar precipitates and ultrafine grains. The high coherent FCC/L12 interface facilitates the transmission of dislocations, which eliminates the stress concentration during deformation and effectively maintains uniform deformation. Meanwhile, high-density stacking faults and Lomer-Cottrell locks are formed during the deformation process. These promote the multiplication of dislocations and further improves the work-hardening ability. The combination of high-density nano-lamellar precipitates and ultrafine grain structure can potentially be applied to other conventional alloys. This work provides a new strategy for fabricating materials with ultra-high strength and excellent ductility.

Spatially heterogeneous microstructure in in-situ TiO-reinforced Ti6Al4V/316L functionally graded material fabricated via directed energy deposition

The strength–ductility trade-off is challenging for metal materials fabricated using conventional techniques. Directed energy deposition (DED), which can manufacture functionally graded materials (FGMs) with spatially heterogeneous microstructure, offers a potential solution to this problem. Here, a spatially heterogeneous microstructure which consisted of equiaxed, irregular, and columnar grains from top to bottom regions was realized in a DED-fabricated Ti6Al4V/316 L FGM. In building direction, the steep temperature gradient in the bottom region promoted the epitaxial growth of columnar grains. The columnar grains in the middle and top regions were restrained by the heterogeneous nucleation of in-situ formed TiO particles and the increasing constitutional supercooling, ultimately inducing the irregular and equiaxed grains. The equiaxed grains in the top region contributed to superior strength and wear resistance, while the irregular and columnar grains in the middle and bottom regions correlated with large elongation. Hence, FGM integrates the high strength, superior ductility, and excellent wear resistance. Furthermore, the strength–ductility synergistic mechanism and heterogeneous microstructure formation mechanism, including heterogeneous nucleation of the in-situ formed TiO particles and columnar-to-equiaxed transition, were unveiled. • Spatially heterogeneous microstructure was obtained in Ti6Al4V/316 L FGM. • The TiO particles for heterogeneous microstructure formation were in-situ formed. • Ti6Al4V/316 L FGM shows the trade-off between strength and ductility. • The refined grains in the middle region contribute to the sustained good ductility.

Thin-Copper-Layer-Induced Early Fracture in Graphene-Nanosheets (GNSs)-Reinforced Copper-Matrix-Laminated Composites.

The strength-ductility trade-off has been a long-standing challenge when designing and fabricating a novel metal matrix composite. In this study, graphene-nanosheets (GNSs)-reinforced copper (Cu)-matrix-laminated composites were fabricated through two methods, i.e., the alternating electrodeposition technique followed by spark plasma sintering (SPS) and direct electrodeposition followed by hot-press sintering. As a result, a Cu-GNS-Cu layered structure formed in the composites with various Cu layer thicknesses. Compared with the pure Cu, the yield strength of the GNS/Cu composites increased. However, the mechanical performance of the GNS/Cu composites was strongly Cu-layer-thickness-dependent, and the GNS/Cu composite possessed a brittle fracture mode when the Cu layer was thin (≤10 μm). The fracture mechanism of the GNS/Cu composites was thoroughly investigated and the results showed that the premature failure of the GNS/Cu composites with a thin Cu layer may be due to the lack of Cu matrix, which can relax the excessive stress intensity triggered by GNSs and delay the crack connection between neighboring GNS layers. This study highlights the soft Cu matrix in balancing the strength and ductility of the GNS/Cu-laminated composites and provides new technical and theoretical support for the preparation and optimization of other laminated metal matrix composites.

Control of dislocation density maximizing precipitation strengthening effect

The strength-ductility trade-off has been the most challenging problem for structural metals for centuries. Nanoprecipitation strengthening is an ideal approach to enhance the strength without significantly sacrificing the ductility. Stable nanoprecipitates have been successfully acquired by nanostructural design, but the number density of nanoprecipitates cannot be further increased. Researchers attempted to enhance number density by introducing highly potent nucleation sites (e.g., dislocations). However, there remains controversy over the influence of dislocations on the nucleation and growth of coherent nanoprecipitates with minimized nucleation barrier. Here, Cu-rich nanoprecipitates in an HSLA steel, as a typical type of coherent nanoprecipitates, are investigated. By combining analytical calculation and experiments, we show that dislocations are harmful for the formation of large numbered Cu-rich nanoprecipitates in a certain density range. Insufficient dislocations deprive solute atoms which decrease homogenous precipitation that cannot be compensated by the increase in heterogeneous precipitation. Under such circumstance, Cu-rich nanoprecipitates have smaller number density but larger size and higher fraction of incoherent structures due to rapid Ostwald ripening. As a result, by controlling dislocation density, the yield strength is increased by 24% without obvious loss in ductility as compared with traditional solution-quench-age process. Our work would help to optimize composition and processing routes that fully exploit the nanoprecipitation strengthening effect.

Strength-plasticity regulation via nanoscale precipitation and coprecipitation in cobalt-free medium-entropy alloys

Nanoprecipitate-strengthened Co-free medium- and high-entropy alloys represent a new candidate for nuclear applications because of their low costs and excellent radiation resistance. However, the premature stress concentration around the nanoprecipitates during deformation tends to greatly reduce the tensile ductility. In this work, L1 2 and D0 22 nanoprecipitates were introduced for the primary purpose of strengthening Co-free Cr 25 Fe 32 Ni 35 Mo 3 Al 5- x Nb x alloys ( x = 1, 2, or 3). The Al/Nb content can influence both the secondary phase formation and the type of nanoprecipitates. Calculations show that order and coherent strengthening play a major role in the D0 22 nanoprecipitation strengthening. In contrast, the strengthening effect of the L1 2 nanoprecipitates was dominated by an order strengthening mechanism. Precipitation not only increases the strength, but also controls the deformation mechanism, which can activate an abnormal deformation mechanism under the condition of low stacking fault energies. This abnormal deformation mechanism originates from the regulation of nanoprecipitates, which determines whether to induce stacking faults or suppress stacking faults to induce microbands, thereby contributing a large ductility. These discoveries provide new insights into the development of alloys with excellent mechanical properties. • D0 22 /L1 2 nanoprecipitates-strengthened Co-free MEAs were designed. • The Al/Nb content affects the phase composition and the type of the nanoprecipitates. • Precipitation not only increases the strength, but also controls the deformation mechanism. • SFs and microbands improve the ductility and strain hardening rate to overcome the strength-ductility tradeoff.