High Solid Solution Strengthening Research Articles

Overcoming strength-ductility trade-off in Si-containing transformation-induced plasticity high-entropy alloys via metastability engineering

Benefiting from the transformation-induced plasticity effect has been recognized as a new approach to develop high-performance Si-containing high-entropy alloys with outstanding room-temperature strength-ductility balance. However, closely controlling the kinetics of deformation-induced α΄-martensite formation is essential to further boost ductility in addition to ultrahigh strength. In the present work, this aim was fulfilled by manipulating the chemical composition of the promising FeCoCrVMnSi system with the replacement of Mn with Ni and adjusting the Si content to fine-tune the stacking fault energy and the driving force of austenite to martensite transformation. The experimental results were obtained by EBSD, TEM, XRD, and tensile testing, and the results were supported by thermodynamic calculations, work-hardening analysis, and assessment of α΄-martensite formation kinetics. Accordingly, the single-phase Fe47Co30Cr10Ni5V8-xSix system was designed. In this system, the Fe47Co30Cr10Ni5V2Si6 alloy exhibited exceptional tensile strength (∼1.1 GPa) and total elongation (72 %), outperforming the competitive dual-phase Fe46Co30Cr10Mn5V2Si7 alloy. High strength was attributed to the high solid-solution strengthening effect as well as the volume fraction of α΄-martensite, while its ductility benefited from fine-tuned kinetics of martensitic phase transformation.

Microstructures and mechanical properties of novel MoTaVW refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs) containing multiple principal alloying elements have created a growing interest in exploring superalloys applications due to their high melting points and excellent softening resistance. The room-temperature brittleness and oxygen sensitivity greatly restrict their further processing and application. Here, utilizing the unique high strength of Vanadium (V) among alloy elements and cocktail effect of HEAs, we designed a MoTaVW RHEAs with equal atomic ratio, that exhibit multiphase phases (Ta-rich, Mo-rich and V-rich phase) with different composition at different sintering temperatures and the alloy elements in BCC solid solution tend to be uniformly distributed as the increased sintering temperature. Exploring the room-temperature compressive properties and underlying deformation mechanisms, the MoTaVW RHEAs exhibit the yield strength (σy) of 1420 MPa and the plastic strain (εp) of 18 %, that achieved good combination of strength and plasticity compared with other most known BCC RHEAs, which are attributed to high solid solution strengthening caused by lattice distortion and the cross slip of screw dislocations on the close-packed plane that can enhance the synergistic effect of strength and plasticity. These results reveal the MoTaVW RHEAs is a very promising materials for compositional space in RHEAs.

The microstructure and He+ ion irradiation behavior of novel low-activation W-Ta-Cr-V refractory high entropy alloy for nuclear applications

Microstructure and nanohardness of a nearly equimolar W-Ta-Cr-V high entropy alloy (HEA), as well as its irradiation response under He+ irradiation, were investigated. The single-phase body-centered cubic nanostructured alloy with a 1 µm thick layer was fabricated on a silicon substrate using a magnetron sputtering method. The HEA film has a complex microstructure consisting of micrometric domains that exhibit internal nanostructure controlled by their crystal orientation. The measured nanohardness of the W-Ta-Cr-V alloy is 13 ± 2 GPa, which significantly exceeds the hardness of nanocrystalline tungsten as a result of the high solid-solution strengthening effect. In order to evaluate the irradiation resistance of the HEA film, the material was irradiated with 200 keV He+ ions at room temperature, with two different ion fluences: 1 × 1016 and 5 × 1016 ions/cm2. Using transmission electron microscopy, a high density of extremely fine He bubbles is observed that were uniformly distributed in the matrix. The increase of He+ ion fluence increased the density of bubbles, whereas their size remained at a similar level, which indicates that the damage proceeds by the nucleation of additional He bubbles, not by their growth.

Dynamic competition between coarsening mechanisms of Ti2Cu phase during thermal cycling

The dynamic competition between the coarsening mechanisms activates various coarsening behavior of the precipitate in multi-phase metallic materials, leading to different mechanical performances of alloys. In this work, the morphological evolution and coarsening kinetic of Ti2Cu phase in Ti–14Cu alloy were investigated comprehensively during thermal cycling at 200–600 °C. Our results suggest that the coarsening kinetic showed a gradual transition from the Lifshitz-Slyozov-Wagner (LSW) mechanism to the trans-interface diffusion-controlled (TIDC) mechanism. Ti2Cu phases precipitated at the triple junctions of α-Ti matrix by discontinuous precipitation under the control of LSW mechanism at lower temperatures (200–400 °C), and grew into lamellae along the high-energy grain boundaries. While the low-angle grain boundaries within Ti2Cu phase accelerated solute atom diffusion during the coarsening governed by TIDC mechanism at higher temperatures (400–600 °C), which leads to the boundary splitting of lamellae. In addition, it was also found that the blocky Ti2Cu phase improved the yield strength of the alloy as its high solid solution strengthening. This work provides a clear understanding for the coarsening of Ti2Cu phase, it could provide theoretical evidence for the hot working process design to improve the damage tolerance of Ti–Cu alloys.

Experimental and Modeling Study of Phase-Specific Flow Stress Distribution in Intercritically Annealed Quenching and Partitioning Steels

To meet the increasing demand and stringent requirements of automotive structural steels, intercritically annealed quenching and partitioning (QP) steels are attracting significant attention owing to their excellent strength–plasticity balance. However, to date, limited reports have focused on the correlation between the microstructure and strength of intercritically annealed QP. In this study, the mechanical behaviors of QP steels with different Si contents were investigated by developing a physical-based mechanical model based on microstructural characterizations. In situ neutron diffraction was used to analyze the evolution of the phase constitution. Si content influenced the phase transformation behavior of the test steel. In the early stages of deformation, Si-strengthened steel exhibited lower retained austenite (RA) stability and faster transformation kinetics. The variation in the RA volume fraction with the deformation was fitted using a segmented exponential function. Based on the microstructure and strengthening mechanisms, a mechanical model considering grain refinement during phase transformation was proposed. The model was validated using intercritically annealed QP steels with different Si contents. The transformation-induced plasticity effect, that is, the contribution of RA to the strength, was discussed from two perspectives. Deformation-induced martensite (DIM) exhibited a significant work-hardening rate owing to the high solid solution strengthening by carbon and the high dislocation density. The residual RA after the DIM transformation exhibited a non-negligible stress distribution. Particularly, the grain boundary density and dislocations increased with strain, strengthening the remaining RA.

The Influence of Processing Parameters on the Al-Mn Enriched Nano-Precipitates Formation in a Novel Al-Mn-Cr-Zr Alloy Tailored for Power Bed Fusion-Laser Beam Process

Among the recently developed compositions tailored for the power bed fusion-laser beam process (PBF-LB), the novel Al-Mn-Cr-Zr alloy stands out. This composition exploits high solid solution strengthening, achieving a high hardness value in the as-built condition. The produced samples are inherently crack-free and have a good level of densification (~99.5%). The goal of this study is to investigate how this quaternary system is affected by the laser power while retaining a similar volumetric energy density. A comparison between the microstructural features and the mechanical performance was performed on a set of samples processed with power values ranging from 100 to 170 W. Microstructural features were investigated through optical microscopy, Electron Back Scattered Diffraction (EBSD) investigation and feature analysis using advanced microscopy to examine the amount, distribution, and shape of precipitates in the different process conditions. Although the quantitative feature analysis permitted analysis of more than 60 k precipitates for each power condition, all samples demonstrated a low level of precipitation (below 0.3%) with nanometric size (around 75 nm). The mechanical performances of this quaternary system as a function of the laser power value were evaluated with a microhardness test, recording very similar values for the different process conditions with a mean value of approximately 104 HV. The results suggested a very stable system over the tested range of process parameters. In addition, considering the low level of precipitation of nanometric phases enriched in Al-Mn, a supersaturated state could be established in each process condition.

First-principles design and experimental validation of β-Ti alloys with high solid-solution strengthening and low elasticities

β-Ti alloys with high strengths and low elasticities are suitable for application in biomaterials. However, to date, a clear and quantitative alloy design strategy to achieve alloys with these properties has not been developed. Herein, we designed β-Ti alloys with the required properties by first-principles calculations with experimental validations. Based on the design, N, O, Si, and Zr were selected as solute elements to realize β-Ti alloys with high solid-solution (SS) strengthenings and low Young's moduli. Compared to β-Ti alloys with other solute elements, β-type Ti–35 at% Ta alloys containing the abovementioned solute elements and fabricated by powder metallurgy demonstrated higher ratio of SS strengthening to modification in Young's modulus, validating the prediction by first-principles calculations. Among the β-Ti alloys with these solute atoms, the β-Ti alloy with N exhibited significant SS strengthening with the highest ratio of strength to Young's modulus. We believe that computational screening similar to the one in this study is effective in developing high-performance materials.

High solid-solution strengthening mechanism of a novel aluminum-lithium alloy fabricated by electromagnetic near-net shape technology

The microstructure evolution and precipitation strengthening behavior of Al–Li alloy manufactured by traditional casting, twin-roll casting (TRC) and electromagnetic twin-roll casting (E-TRC) were systematically investigated. The results show that the high cooling rate of the TRC process makes more alloying elements dissolve in the α(Al) matrix and improve the supersaturated solid solubility of the alloy. However, the application of the electromagnetic oscillation field (EOF) in the TRC process not only raises the undercooling of the alloy and increases the effective distribution coefficient Ke of the solute but also enhances the mixing capability and diffusion ability of the solute atoms and inhibit the recrystallization of deformed microstructure. On the one hand, the EOF reduces macro-segregation and refines the solidified microstructure of alloys. On the other hand, E-TRC increases the intragranular solid-solution capacity of the solute and effectively increases the intragranular solid-solution amount of solute elements in the α(Al) matrix. The higher the degree of supersaturated solid-solution, the greater the precipitation driving force of precipitates in the aging process, hence after the same heat treatment procedures, there are more volume fraction and finer strengthening phases such as T1 (Al2CuLi) phase and δ′ (Al3Li) phase precipitates in the E-TRC alloy, the aging strengthening effect is greatly improved, and the strength of the Al–Li alloy is significantly increased.

Design of near α-Ti alloys using δ-parameter

In this study, to obtain a high solid-solution strengthening effect, new Ti alloys with bimodal structures were designed using the δ-parameter, which represents the average atomic radius misfit of the constituent elements used in high-entropy alloys. The δ-parameter calculated by the alloy composition was used to estimate the average solid-solution strengthening of the alloys. In particular, to clarify the effect of Zr and Sn in Ti alloys, Ti alloys were designed with different amounts of Zr and/or Sn using the δ-parameter. The compressive strength and creep behavior of the designed alloys were investigated to determine whether the δ-parameter is useful for designing strengthened alloys. The compressive strength of the designed alloys was comparable to or higher than that of the commercial TIMETAL834 alloy up to 650 °C. This indicates that the δ-parameter is a useful parameter for estimating the average solid-solution strengthening of the alloys. The compressive strengths of the designed alloys were approximately proportional to the 4/3 power of the δ−parameter. An increase in the strength by precipitation strengthening by α2-Ti3Al was also observed in the Sn-added alloys, whereas Ti3Al was not formed in the Sn-free alloys. Microstructural factors, such as the volume fraction of the equiaxed α phase and lamellar spacing in the bimodal microstructure, significantly contributed to the minimum creep strain rate, in addition to solid-solution and precipitation strengthening.

Modeling solid solution strengthening in high entropy alloys using machine learning

Solid solution strengthening (SSS) influences the exceptional mechanical properties of single-phase high entropy alloys (HEAs). Thus, given the vast compositional space, identifying the underlying factors that control SSS to accelerate property-oriented design of HEAs is an outstanding challenge. In the present work, we demonstrate a relationship derived in terms of the electronegative difference of elements to characterize SSS for HEAs. We propose a model which shows superior performance in predicting solid-solution strength/hardness of HEAs compared to existing physics-based models. We discuss applications of our SSS model to HEA design and predict alloys with potentially high SSS in the four alloy systems AlCoCrFeNi, CoCrFeNiMn, HfNbTaTiZr and MoNbTaWV. Our findings are based on the use of machine learning (ML) methods involving feature construction and feature selection, which we employ to capture salient descriptors.

Ultrastrong lightweight compositionally complex steels via dual-nanoprecipitation.

High-performance lightweight materials are urgently needed, given the pressing quest for weight reduction and the associated energy savings and emission reduction. Here, by incorporating the multi-principal element feature of compositionally complex alloys, we develop the concept of lightweight steels further and propose a new class of compositionally complex steels (CCSs). This approach allows us to use the high solid solution strengthening and shift the alloys' compositions into previously unattainable phase regions where both nanosized shearable κ-carbides and non-shearable B2 particles are simultaneously formed. The achievement of dual-nanoprecipitation in our CCSs leads to materials with ultrahigh specific tensile strength (up to 260 MPa·cm3 g-1) and excellent tensile elongation (13 to 38%), a combination outperforming all other high-strength high-entropy alloys and advanced lightweight steels. Our concept of CCSs is thus useful for guiding the design of ultrastrong lightweight metallic materials.

Microstructural changes in CoCrFeMnNi under mild tribological load

The lack of a principle element in high-entropy alloys (HEA) leads to unique and unexpected material properties. Tribological loading of metallic materials often results in deformed subsurface layers. As the microstructure feedbacks with friction forces, the microstructural evolution is highly dynamic and complex. The concept of HEAs promises high solid solution strengthening, which might decrease these microstructural changes. Here, we experimentally investigated the deformation behavior of CoCrFeMnNi in a dry, reciprocating tribological contact under a mild normal load. After only a single stroke, a surprisingly thick subsurface deformation layer was observed. This layer is characterized by nanocrystalline grains, twins and bands of localized dislocation motion. Twinning was found to be decisive for the overall thickness of this layer, and twin formation within the stress field of the moving sphere is analyzed. The localization of dislocation activity, caused by planar slip, results in a grain rotation. Fragmentation of twins and dislocation rearrangement lead to a nanocrystalline layer underneath the worn surface. In addition, oxide-rich layers were found after several sliding cycles. These oxides intermix with the nanocrystalline layer due to material transfer to the counter body and re-deposition to the wear track. Having revealed these fundamental mechanisms, the evolution of such deformation layers in CoCrFeMnNi under a tribological load might lead to other HEAs with compositions and properties specifically tailored to tribological applications in the future.

Engineering atomic-level complexity in high-entropy and complex concentrated alloys

Quantitative and well-targeted design of modern alloys is extremely challenging due to their immense compositional space. When considering only 50 elements for compositional blending the number of possible alloys is practically infinite, as is the associated unexplored property realm. In this paper, we present a simple property-targeted quantitative design approach for atomic-level complexity in complex concentrated and high-entropy alloys, based on quantum-mechanically derived atomic-level pressure approximation. It allows identification of the best suited element mix for high solid-solution strengthening using the simple electronegativity difference among the constituent elements. This approach can be used for designing alloys with customized properties, such as a simple binary NiV solid solution whose yield strength exceeds that of the Cantor high-entropy alloy by nearly a factor of two. This study provides general design rules that enable effective utilization of atomic level information to reduce the immense degrees of freedom in compositional space without sacrificing physics-related plausibility.

Low-temperature tensile ductility by V-alloying of high-nitrogen CrMn and CrNiMn steels: Characterization of deformation microstructure and fracture micromechanisms

Tensile properties, work-hardening, microstructure and fracture mechanisms were studied in two vanadium-containing high-nitrogen austenitic steels, Fe-19Cr-21Mn-1.3V-0.22C-0.81N and Fe-17Cr-10Mn-7Ni-1.0V-0.09C-0.65N, in low-temperature deformation regime. Vanadium-alloying provides several concurrent effects – decreases a solid solution hardening in the steels (softening effect) due to formation of nitrides(carbonitrides), causes precipitate hardening by vanadium-based particles (hardening effect) and suppresses grain growth during solid solution treatment (hardening effect). The yield stresses at room temperature deformation and 77 K-deformation are analyzed taking into account the contributions from these softening/hardening mechanisms. As a result of complex hardening and softening effects, both steels possess high tensile properties, show striking temperature dependences of the yield strength, an ultimate tensile strength and an elongation (the ultimate tensile strength and elongation at 77 K reaches 2600 MPa and 14% for Fe-19Cr-21Mn-1.3V-0.22C-0.81N steel) in low-temperature deformation regime. Striking temperature dependence of the yield strength for particle-containing steels is in accordance with deformation behavior of high-nitrogen particle-free steels with high solid-solution strengthening. In spite of similarities in temperature dependence of strength properties and deformation mechanisms with precipitate-free high-nitrogen steels, V-containing steels do not undergo ductile-to-brittle transition during deformation at cryogenic temperatures and they fracture in ductile manner even during tensile deformation at 77 K. Deformation microstructures at different strain levels and test temperatures are studied in order to reveal the dominating structural parameters responsible for fracture micromechanisms. Despite the activation of mechanical twinning and planar dislocation slip as dominating deformation mechanisms during tension at 77 K, precipitate-hardened high-nitrogen steels show rather high elongation values of 14–15% and ductile fracture with dimple micromechanism. Vanadium-based precipitates of ≈ 300 nm in diameter do not cause strong precipitate hardening, but strongly influence dislocation arrangement and suppress low-temperature brittle fracture in high-nitrogen austenitic steels.

Preparation, mechanical and degradation properties of Mg–Y-based microwire

Mg–Y-based microwire which offers high strength in combination of low degradation rate has been prepared for the first time by a modified melt extraction technique. A circular Mg–Y-based microwire is achieved with an extraction rate of 40m/s, which is composed of Mg matrix and an amorphous phase, and exhibits higher basal texture than that of as-cast sample. The outstanding tensile strength accompanying with an acceptable elongation is obtained with an extraction rate of 40m/s. The improved strength is mainly attributed to high solid solution strengthening, fine grain and the presence of an amorphous phase. In addition, the reduction of secondary phase and homogenous microstructure after melt extraction eliminate both pitting corrosion and micro-galvanic corrosion. A low degradation rate of 0.366mm/y is attained in a simulated body fluid, which is less than 1/10 of that of as-cast sample. These excellent mechanical properties and low degradation rate provide some prerequisites to develop bio-Mg implants. It reveals that this modified extraction technique is one of effective approaches to prepare microwire, which can be directly used for Mg-based stent self-assembly.

Precipitation in cold-rolled Al–Sc–Zr and Al–Mn–Sc–Zr alloys prepared by powder metallurgy

The effects of cold-rolling on thermal, mechanical and electrical properties, microstructure and recrystallization behaviour of the AlScZr and AlMnScZr alloys prepared by powder metallurgy were studied. The powder was produced by atomising in argon with 1% oxygen and then consolidated by hot extrusion at 350°C. The electrical resistometry and microhardness together with differential scanning calorimetry measurements were compared with microstructure development observed by transmission and scanning electron microscopy, X-ray diffraction and electron backscatter diffraction. Fine (sub)grain structure developed and fine coherent Al3Sc and/or Al3(Sc,Zr) particles precipitated during extrusion at 350°C in the alloys studied. Additional precipitation of the Al3Sc and/or Al3(Sc,Zr) particles and/or their coarsening was slightly facilitated by the previous cold rolling. The presence of Sc,Zr-containing particles has a significant antirecrystallization effect that prevents recrystallization at temperatures minimally up to 420°C. The precipitation of the Al6Mn- and/or Al6(Mn,Fe) particles of a size ~1.0μm at subgrain boundaries has also an essential antirecrystallization effect and totally suppresses recrystallization during 32h long annealing at 550°C. The texture development of the alloys seems to be affected by high solid solution strengthening by Mn. The precipitation of the Mn-containing alloy is highly enhanced by a cold rolling. The apparent activation energy of the Al3Sc particles formation and/or coarsening and that of the Al6Mn and/or Al6(Mn,Fe) particle precipitation in the powder and in the compacted alloys were determined. The cold deformation has no effect on the apparent activation energy values of the Al3Sc-phase and the Al6Mn-phase precipitation.