Enhanced Yield Strength Research Articles

Tailoring strength and ductility in novel Ni50Cr20Co15Al10V5 high entropy alloys by cold-rolling followed by aging treatment

In this work, novel Ni50Cr20Co15Al10V5 high entropy alloys were designed and subjected to aging treatment to achieve excellent strength-ductility synergy. The microstructures of the alloys were characterized using X-ray diffraction, electron backscattered diffraction, electron channeling contrast imaging, and transmission electron microscopy methods. The tensile properties and strengthening mechanism were studied and compared to the recrystallized sample. The results indicated that after aging treatment, L12 and BCC precipitates formed at the grain boundaries, enhancing both the yield strength and ultimate tensile strength while maintaining an acceptable elongation of 24.8 %. Dislocation slip dominated the deformation process, and precipitation strengthening contributed significantly to the enhanced yield strength. Molecular dynamics simulations indicated that Ni-Al, Cr-Cr, Cr-V, and Co-V elemental pairs formed the chemical short-range order (SRO) structures, which improved the yield strength of the alloys.

Achieving superior ductility with ultrahigh strength via deformation and strain hardening in the non-recrystallized regions of the heterogeneous-structured high-entropy alloy

Developing metallic structural materials with ultrahigh strength and exceptional ductility remains a significant challenge due to the trade-off between both properties. This study presents a heterogeneous-structured high-entropy alloy achieving a superior combination of strength and ductility compared to the reported heterogeneous-structured high entropy alloys through deformation and strain hardening in the non-recrystallized regions. The cold rolling followed by annealing at 760 °C resulted in a heterogeneous microstructure consisting of a small fraction of ultrafine recrystallized grains and extensive non-recrystallized regions, with a significant amount of L12 precipitates throughout the alloy. The architected microstructure led to a significant enhancement of yield strength through mechanisms including dislocation strengthening, L12 strengthening, and grain boundary strengthening. During the deformation, the non-recrystallized regions accommodated substantial strain through the reactivation of pre-existing deformation bands and the synergistic deformation of the FCC and L12 phases, thereby markedly enhancing ductility. Moreover, the metastable FCC matrix underwent FCC→BCC phase transformation, leading to the formation of numerous short-range BCC domains, which further contributed to the pronounced strain hardening. Consequently, the alloy annealing at 760 °C achieved a yield strength of 1.73 GPa, an ultimate strength of 2.05 GPa, and an elongation of 21.0 %. This study underscores a novel strategy for the concurrent enhancement of strength and ductility and provides valuable insights for the design of high-performance alloys.

Investigation on effect of L12 and B2 phases on tensile properties and mechanisms of Al0.3CoCrFeNi high entropy alloy through aging treatment

Al0.3CoCrFeNi high entropy alloys (HEAs) have excellent mechanical properties due to the large tunability of microstructures, which makes them one of the most promising candidates for engineering application. In the aspect of enhancing tensile properties, precipitation strengthening was verified effective in this HEA and more studies are needed to deepen relative understanding. In this work, Al0.3CoCrFeNi HEAs with coarse-grained single FCC (A) and fine-grained triplex FCC + B2 + σ (B) structures were aged at different temperatures and times to precipitate second phases, and the tensile properties and mechanisms of aged specimens were studied. The results showed that aging of A resulted in the main precipitation of L12 phases throughout FCC matrix and increasing aging temperature obviously increased L12 size, whereas increasing annealing time had minor effect on phase coarsening. At small L12 size of ∼2.0 nm, stacking fault shearing mechanism was activated during tension and planar dislocation configurations were formed, leading to simultaneous increase of strength and ductility. With increasing L12 size to ∼10.0 nm, the deformation mechanism was transformed into anti-phase boundary shearing and Orowan bypassing and wavy dislocation configurations predominated, which resulted in the significant increase of yield strength from ∼263 MPa to ∼433 MPa without sacrificing notable ductility. On the other hand, aging of B induced the formation of fine B2 phases at residual dislocations and enhancement of tensile yield strength from ∼705 MPa to ∼759 MPa. The ductility decreased slightly from ∼32 % to ∼28 %, which was related to the plastic deformation of B2 phases. However, it was found that the tensile strengths could not be continuously increased by further increasing aging temperature or time. These findings provide insights into the development of precipitation strengthened HEAs with good mechanical properties.

Deformation mechanism of a metastable medium entropy alloy strengthened by the synergy of heterostructure design and cryo-pre-straining

Face-centered cubic (FCC) medium entropy alloys (MEAs) have received considerable attention due to their impressive mechanical properties and responses. However, their practical application is limited by their modest yield strengths. The potential enhancement of the mechanical properties of single-phase MEAs was explored in this study through a synergistic approach combining heterogeneous structure design with subsequent cryo-pre-straining. A heterogeneous lamella structure was produced in a single-phase Fe55Mn20Cr15Ni10 MEA via two-step rolling and annealing. Cryo-pre-straining at varying degrees (6, 12, 21, and 36%) introduced hexagonal close-packed (HCP) phase, high-density dislocations, twins, and stacking faults, leveraging the reduced stacking fault energy at cryogenic temperatures. This process enhanced the alloy's yield strength from 353 MPa to 1.2 GPa (compared to the baseline uniform coarse-grained structure), while maintaining an acceptable total elongation of 8.4%. The impact of cryo-pre-straining on the microstructure and mechanical properties of the MEA was assessed using in-situ synchrotron X-ray diffraction analysis. Cryo-pre-straining (36%) achieved a higher dislocation density (6.1 × 1015m−2) compared to room-temperature straining (2.5 × 1015m−2). The stress contribution from HCP-martensite and the evolution of dislocation density during loading were quantified, along with observations of negative stacking fault probability and strain-induced HCP→FCC reverse transformation in cryo-pre-strained samples under loading conditions. Furthermore, the contributions of regulated microstructures to the enhancement of yield strength were quantitatively assessed.

Influence of laser power and scanning speed on microstructure evolutions and mechanical properties of the SLM fabricated Al-Mg-Mn-Sc-Zr alloy

The Al-Mg-Mn-Sc-Zr alloy is fabricated by selective laser melting (SLM). The influence of laser power and scanning speed on microstructure evolutions and mechanical properties is studied in this research. Results indicate that increasing laser power can refine grain structures and reduce anisotropy on the printing surface. However, the increase in laser power will enhance the residual stress inside the alloy, leading to a decrease in elongation. The scanning speed affects the molten pool energy input and cooling rate. When the scanning speed is between 750mm/s-1250mm/s, the alloy achieves a good grain refinement effect and weak texture strength. In addition, there exists a large number of nano-scaled Al3(Sc,Zr) particles precipitated during solidification and the subsequent annealing process, which can significantly enhance yield strength of the alloy. Overall, the Al-Mg-Mn-Sc-Zr alloy has a wide SLM forming window. When the laser power ranges from 200W to 250W, the scanning speed ranges from 750mm/s to 1250mm/s, and the volume energy density ranges from 80J/mm3 to 130J/mm3, the Al-Mg-Mn-Sc-Zr alloy can obtain good formability and mechanical property.

Effect of Ti–Mg–Ce Complex Deoxidation on the Inclusions and Mechanical Properties of S355 Steel

Oxide metallurgy technology not only refines the microstructure but also modifies the type, density, and size of inclusions. This study examines the impact of Ti–Mg–Ce composite deoxidation on the inclusions and mechanical properties of S355 steel used for heavy‐duty H‐beams. Compared to base alloy, Ti–Mg–Ce‐added alloy features a higher presence of inclusions such as MgO, TiN (containing Ce), TiOx, and TiS, except for SiO2, Al2O3, MnS, and MnO. Following Ti–Mg–Ce composite deoxidation, the number density of inclusions increases by 16.4%, and the equivalent diameter, length, and width of the inclusions decrease by 41.0%, 41.2%, and 37.9%, respectively. The length of large‐angle grain boundaries in the same area increases by 20%, and the average grain size reduces by 12.5%, resulting in an enhancement of yield strength by 23.6 MPa, tensile strength by 37 MPa, total elongation by 2.1%, and impact energy by 92 J. By employing Ti–Mg–Ce composite deoxidation to optimize inclusions and microstructure, the strength, plasticity, and toughness of the experimental steel are significantly improved.

Ultrahigh Elastic Energy Storage in Nanocrystalline Alloys with Martensite Nanodomains.

Elastic materials that store and release elastic energy play pivotal roles in both macro and micro mechanical systems. Uniting high elastic energy density and efficiency is crucial for emerging technologies such as artificial muscles, hopping robots, and unmanned aerial vehicle catapults, yet it remains a significant challenge. Here, a nanocrystalline structure embedded with elliptical martensite nanodomains in ferroelastic alloys wasutilized to enable high yield strength, large recoverable strain, and low energy dissipation simultaneously. As a result, the designed Ti-Ni-V alloys demonstrate ultrahigh energy density (>40 MJ m-3) with ultrahigh efficiency (>93%) and exceptional durability. This concept, which combines nano-sized embryos to minimize energy dissipation of psuedo-elasticity and employs a fine-grained structure to enhance yield strength, can be applied to other ferroelastic materials. Furthermore, it holds promise for the development of phase transformation-involved functionalities such as high-performance dielectric energy storage, ultralow-hysteresis magnetostrain, and high-efficiency solid-state caloriccooling.

Laser powder bed fusion of SiC particle-reinforced pre-alloyed TiB2/AlSi10Mg composite with high-strength and high-stiffness

Recently, laser powder bed fusion (LPBF) of particle-reinforced aluminum matrix composites (PAMCs) with high-strength and high-stiffness have attracted extensive attention in aviation and aerospace. However, performance improvement of single or dual PAMCs using traditional mechanical mixing method is still limited. Therefore, this study innovatively employed pre-alloyed ∼6.5 wt% TiB2/AlSi10Mg composite as the matrix and mechanically mixed SiC particles with different contents (5 vol% and 10 vol%) to fabricate dual PAMCs with high particles content through LPBF. The results indicated that the 5 vol% SiC+TiB2/AlSi10Mg composite revealed relatively weak agglomeration effect of SiC particle and highest relative density (∼99.1 %), thus exhibiting optimal processability. Using this composition material as the research object, it was found that the microstructure maintains the basic features of pre-alloyed TiB2/AlSi10Mg composite except for the slight grain coarsening. However, SiC particles react with α-Al matrix and Al3Ti. Then Al4C3 and TiC enhancement phase were formed, and micron-sized Si particles precipitated within the Al cells surrounded by the eutectic Al-Si. More importantly, due to novel preparation method of dual PAMCs powder, simultaneous enhancement in ultimate tensile strength (∼554.0 MPa), yield strength (∼376.0 MPa), and elastic modulus (∼97.4 GPa) was achieved. Total particle content (∼14.0 wt%) and tensile property were higher than those of reported other PAMCs processed by LPBF. Finally, expect for the fracture characteristics inherent to the pre-alloyed TiB2/AlSi10Mg composite, new fracture mechanism for the tearing of SiC particles was exhibited. This work provides new insights into the preparation of high-strength and high-stiffness PAMCs processed by LPBF.

Friction stir processing: A thermomechanical processing tool for high pressure die cast Al-alloys for vehicle light-weighting

This study uses friction stir processing (FSP) for thermomechanical processing of high-pressure die-casting (HPDC) to modify microstructure and improve mechanical properties. FSP is carried out on two different HPDC aluminum alloys: (a) general-purpose, high-iron, HPDC A380 alloy and (b) premium quality, low-iron HPDC Aural-5 alloy in thin wall, flat plate geometry. Subsequent mechanical testing shows ∼30 % and ∼65 % enhancement in yield strength and tensile ductility. In addition, FSP leads to ∼10 times improvement in fatigue life for A380 alloy and ∼70 % improvement in fracture toughness for Aural-5 alloy. These findings emphasize the capability of FSP to modify the microstructure of HPDC Al-alloys-based structural components so that they can demonstrate a good combination of strength, ductility, fracture toughness, and high fatigue properties for long-term durability and reliability.

A yield strength prediction framework for refractory high-entropy alloys based on machine learning

Machine learning has been widely applied to materials research with the development of artificial intelligence. Here, a new framework mainly based on the LightGBM algorithm was proposed, which predicted the yield strength of refractory high-entropy alloys (RHEAs) in various temperatures. The features of T, D·B, μ, Smix, Gmix and r were recognized as the optimal feature set by several feature screening methods. The framework displayed good prediction results with a coefficient of determination (R2) of 0.9605 and a root mean square error (RMSE) of 111.99 MPa in the test set. A series of RHEA samples validated the generalization of this framework. SHAP with pearson correlation constant (PCC) and maximal information coefficient (MIC) interpreted the framework and analyzed the intrinsic mechanism of features on yield strength, discovering a novel μ-D·B-Gmix design strategy for obtaining RHEAs with enhanced yield strength. Both TiTaNbHfNi0.25 and TiTaNbHfNi0.5 alloys were fabricated as the experimental verification for this framework which showed 1230 and 1311 MPa yield strength with the predicted errors of 6.3 % and 3.7 %. The validations above demonstrated the excellent performance of the present framework and the effectiveness of such a strategy.

Influence of surface pre-deformation on the Portevin-Le Chatelier effect and the related multiscale complexity of plastic flow in an Al-Mg alloy

The influence of the surface pre-deformation on jerky flow caused by the Portevin-Le Chatelier (PLC) effect was investigated using flat tensile specimens of an Al-Mg alloy. Although jerky flow represents a macroscopic plastic instability, the underlying mechanisms stem from self-organization of dislocations, which pertains to deformation processes at mesoscopic scales. To provide a comprehensive approach, the investigation was carried out by coupling tensile tests, digital image correlation and acoustic emission techniques, each targeting a particular range of scales. Thin superficial layers were pre-deformed using surface mechanical attrition technique (SMAT). It was found that the observed effects depend on which surfaces are processed. Overall, the treated samples exhibited an enhanced yield strength without deterioration of ductility in comparison with the initial material. These changes in the general mechanical behavior are likely to be correlated with the changes in jerky flow. Using digital image correlation, a tendency to delocalization of bursts of plastic flow was found for both the PLC bands and smaller-scale strain heterogeneities outside the bands, the latter having been little studied so far. Unexpectedly, SMAT occurred to modify plastic flow drastically at this scale. In addition to clarification of the role played by the surface in the PLC effect, these findings provide new insights into relationships between deformation processes at distinct scales.

Enhanced compressive strength of graphene strengthened copper (G/Cu) composites

This study explores the compressive mechanical properties of copper composites reinforced with graphene. Graphene was synthesized on copper powders via plasma-enhanced chemical vapor deposition. Multilayer graphene formation has been substantiated by Raman analysis. Graphene-coated copper (G/Cu) powders were then subjected to pressing and sintering to fabricate G/Cu composites. The mechanical properties of G/Cu composites were investigated under compression from room temperature up to 400 °C in air. The results demonstrated a substantial improvement in the mechanical properties of G/Cu composites compared to monolithic copper. Specifically, the yield strength in compression of the G/Cu composite increased by 203% at room temperature and by 190% at 200 °C. At 400 °C, the yield strength enhancement exceeded 370%. Microstructural analysis suggests that the observed enhancements in G/Cu composites can be attributed to reduced porosity, smaller grain size, and inhibited dislocation motion at the increased grain boundary area (due to refined grain size) and graphene-copper interfaces.

Manipulation of the α-phase precipitation behavior through dual-aging treatment and its correlation with mechanical properties in a metastable β titanium alloy

The precipitation behavior of α-phase and its crucial impact on the mechanical properties have been investigated in a metastable β-type Ti-4Al-6V-5Mo-3Cr-1Zr (wt%, Ti-46531) alloy. Through dedicated design of dual-step aging conditions, the nucleation mechanism, number density and morphology of the secondary α-phase precipitates can be modulated. After the first-step aging treatment at 300 °C for 5−100 h, isothermal ω-phase (ωiso) particles precipitate and are uniformly dispersed in the β-phase matrix, with their number density gradually increasing with aging time. The subsequent second-step aging treatment at 600 °C for 2 h leads to the rapid dissolution of ωiso and the precipitation of α-phase through the classical nucleation process with a steady number density. Lowering the second-step aging temperature to 520 °C promotes the heterogeneous nucleation of the α-phase with the assistance of uniformly dispersed ωiso particles. This leads to a significant increase in the number density of α-phase precipitates. Consequently, the maximum yield strength increases from 1296 MPa for samples aged at 600 °C to 1623 MPa for samples aged at 520 °C. The precipitation of α-phase serves a dual role in enhancing the yield strength, both by providing precipitation strengthening and by increasing dislocation density. The contribution to yield strength enhancement from the former effect is considerably more substantial than the latter. This work underscores the crucial role of α-phase number density in determining the strength of metastable β titanium alloys, offering new insights into the tailoring of microstructures and mechanical properties through dual-aging treatments.

Nano Y2O3 particle enhanced IN718 nickel-based superalloy fabricated by laser powder bed fusion

In this study, 1 wt% Y2O3 nanoparticles were added into the IN718 superalloy fabricated by laser powder bed fusion (L-PBF). The results indicated a significant grain size refinement along the building direction and a substantial improvement in <100> texture. Room temperature tensile studies revealed that the addition of Y2O3 enhanced yield strength, ultimate tensile strength, and fracture elongation.

Fast shot speed induced microstructure and mechanical property evolution of high pressure die casting Mg-Al-Zn-RE alloys

In this study, the effects of fast shot speeds during high pressure die casting on the microstructure and mechanical properties of Mg-Al-Zn-RE (RE: rare earth) alloys were systematically investigated. The optimum mechanical performance was achieved at a fast shot speed of 6 m/s, and the ultimate tensile strength, yield strength, and elongation of the castings reached 290 MPa, 190 MPa, and 10.5 %, respectively. With an increase in fast shot speed, the yield strength, ultimate tensile strength, and elongation of the Mg-Al-Zn-RE alloy increased by 17 MPa, 82 MPa, and 8.2 %, respectively. These improvements are primarily attributed to the decreased area fraction of externally solidified crystals (ESCs) and the reduced volume fraction of pores. Furthermore, the enhancement in yield strength is due to grain boundary strengthening, solid-solution strengthening, and second-phase strengthening. The reduction in the area fraction of ESCs and porosity are the main contributors to the improvement in plasticity. The findings of this study provide valuable guidance for the intelligent design of novel high pressure die casting Mg alloys.

Multilayered W–Cu composites with enhanced strength, electrical conductivity and wear resistance

W–Cu composites have been widely applied in numerous critical applications due to their excellent comprehensive properties. However, the mechanical properties and physical properties of W–Cu composites can hardly be enhanced simultaneously. In this paper, multilayered W–Cu composites with various copper contents were fabricated by the electric field assisted fast hot-pressing technology. The prepared multilayered W–Cu composites demonstrate simultaneous enhancement in compressive yield strength, electrical conductivity, and wear resistance compared to those of the commercial counterparts. The improved strength can be attributed to both the enhanced stress partitioning and the exceptional strength of the tungsten layer. The combined lower friction coefficient and high strength of the composites contribute to the reduced wear rate. Moreover, the in-situ compression test reveals that the plastic strain of the multilayered W–Cu composites is primarily governed by the nucleation of kinking band, which differs it from the nanolaminated composites where plastic strain is also contributed by the propagation of kinking band. This work reveals the advantages of the multilayered structure and provides new insight for the structural design of high-performance W–Cu composites.

Strengthening a non-equiatomic quaternary high-entropy alloy via concurrent gradient structures of nanotwin and dislocation cell

One main drawback of FCC high-entropy alloys (HEAs) is their insufficient room temperature yield strength. In this work, concurrent gradient structures of nanotwins and dislocation cells have been introduced into a non-equiatomic quaternary Fe40Ni20Co20Cr20 (at. %) high-entropy alloy by ultrasonic shot peening to meet this challenge. The gradient alloy exhibits a significantly enhanced yield strength, i.e., from the original 204 MPa to 559 MPa, combining with a well-retained ductility of 29 % and a good work hardening ability. Strengthening mechanisms at different depths (5, 30, 200, 400 μm) are also carefully discussed. The current work presents a strategy for hardening FCC HEAs significantly while maintaining the single solid solution microstructure.

Simultaneous improvement of strength and ductility in a P-doped CrCoNi medium-entropy alloy

A newly developed P-doped CrCoNi medium-entropy alloy (MEA) provides both higher yield strength and larger uniform elongation than the conventional CrCoNi MEA, even superior tensile ductility to the other-element-doped CrCoNi MEAs at similar yield strength levels. P segregation at grain boundaries (GBs) and dissolution inside grain interiors, together with the related lower stacking fault energy (SFE) are found in the P-doped CrCoNi MEA. Higher hetero-deformation-induced (HDI) hardening rate is observed in the P-doped CrCoNi MEA due to the grain-to-grain plastic deformation and the dynamic structural refinement by high-density stacking fault-walls (SFWs). The enhanced yield strength in the P-doped CoCrNi MEA can be attributed to the strong substitutional solid-solution strengthening by severer lattice distortion and the GB strengthening by phosphorus segregation at GBs. During the tensile deformation, the multiple SFW frames inundated with massive multi-orientational tiny planar stacking faults (SFs) between them, rather than deformation twins, are observed to induce dynamic structural refinement for forming parallelepiped domains in the P-doped CoCrNi MEA, due to the lower SFE and even lower atomically-local SFE. These nano-sized domains with domain boundary spacing at tens of nanometers can block dislocation movement for strengthening on one hand, and can accumulate defects in the interiors of domains for exceptionally high hardening rate on the other hand.

Influence of bismuth addition on the physical and mechanical properties of low silver/lead-free Sn-Ag-Cu solder

The Sn–Ag–Cu (SAC) solders with low Ag content have been identified as promising candidates to replace the traditional Sn–Pb solder for flip-chip interconnects. This study examines the impact of incorporating Bi element on the microstructure, thermal behavior, and mechanical performance of low-silver-content Sn-1.5Ag-0.7Cu (SAC157) solder alloy. Besides using the conventional cross-sectioned microstructure image, X-ray diffraction (XRD), differential scanning calorimetry (DSC) and tensile stress- strain techniques were used to investigate the microstructure, thermal and mechanical properties after the addition of Bi, respectively. The results demonstrate that adding of Bi could refine the microstructure, optimize the thermal properties, and improve the tensile strength. Meanwhile, the ductility of the solder alloys reduced evidently in comparison to the Sn-1.5Ag-0.7Cu solder. From the microstructural analysis, 0.5 wt % Bi addition to SAC157 significantly enhanced the solid solution effect of Bi and refined β-Sn dendrite, as well as extended the eutectic area. Moreover, as the Bi addition increases to 5 wt %, finely dispersed bismuth precipitates within the β-Sn matrix, along with an enlarged eutectic area, lead to a remarkable enhancement in both ultimate tensile strength and yield strength. The electrical resistivity was characterized by the four-point probe technique. The results show that the electrical resistivity of Bi-modified SAC157 solder alloy decrease from 13.9 to 9.8 μΩ cm with the addition of 5 wt % Bi. These improvements can be attributed to the solid-solution and precipitation strengthening effects induced by the presence of bismuth. The results confirmed that the developed material is applicable as a potential high strength solder material in the context of advanced interconnecting applications.

Tensile strength and immersion corrosion behavior of nano-ZnO, rare earth Gd reinforced Mg nanocomposites developed by novel stir ultrasonication process

This paper presents the tensile and immersion corrosion properties of Mg-Zn-0.2Gd-xZnO (x = 1, 1.5, 2 and 2.5 wt.%) alloy and nanocomposites developed by stir ultrasonication coupled with squeeze casting. The immersion corrosion tests were performed under the simulated body fluid (SBF) conditions. The microscopic measurements indicate that the examined alloys/nanocomposites exhibit a significant decrease in grain size when both alloying elements Gadolinium (Gd) and Zinc Oxide (ZnO) nanoparticles are added. The examination of the phase by XRD indicates the presence of α-Mg and Mg2Zn, Mg7Zn3 secondary phases, which act as barriers against corrosion. The study confirmed that the incorporation of nanoparticles ZnO content ranging from 1 to 2.5 wt. % leads to a notable enhancement in yield strength (140 to 230 MPa). From the immersion corrosion test, the nanocomposites with the inclusion of 2 wt. % ZnO ceramic nanoparticles resulted in the highest level of corrosion resistance with a corrosion rate of 0.049 mm per year. This nanocomposite also shows better wettability with improved hydrophilicity. The developed Mg based advanced nanocomposites have the potential to be used in load bearing implant applications as it exhibited high corrosion resistance characteristics.