Excellent Combination Of Strength Research Articles

The effect of Si addition on the heterogeneous grain structure and mechanical properties of CrCoNi medium entropy alloy

In this study, CoCrNi MEAs added with different silicon contents (0, 0.1, 0.2 for molar ratio) were prepared by arc melting. The cold rolled and annealed microstructural evolution and tensile mechanical behavior of the alloys were systematically investigated. The results show that the distribution of elements in the alloys after homogenization annealing is uniform. As the rolling reduction increased from 20% to 60%, SEM-BSE characterization showed that the deformed microstructure of the alloys transformed from neatly aligned slip bands to tangled shear bands. TEM characterizations show that the alloys have typical cold rolling-induced microbands, dislocation cells, and high-density dislocation walls caused by plane slip after cold rolling for 60%. Also, these deformed microstructures are finer in size concerning silicon-alloyed MEAs compared to silicon-free MEAs. After annealing at 650 °C for 60 min, the deformed microstructure was replaced by a heterogeneous grain structure composed of ultrafine grains, micron-scale grains, high-density dislocation regions, and residual deformation regions. With the increase of Si content, the yield strength and tensile strength increased with the change of the heterogeneous grain structure. The tensile strength of the Si0.2 MEAs even exceeds 1.5 GPa, and still maintains a uniform elongation greater than 20% at 77 K. The evolution of the cold-rolled microstructure of silicon alloyed MEAs are mainly related to the reduction of stacking fault energy and a strong dependence on grain orientation and applied stress direction. The formation of nano-DTs and HCP phases resulted in the excellent combination of strength and ductility at 77 K. The typical heterogeneous grain structure realizes the strength–ductility synergy of the alloys.

Effect of κ Carbides on Deformation Behavior of Fe-27Mn-10Al-1C Low Density Steel

Fe-Mn-Al-C steel, which is a potential lightweight material for automobiles, has a variety of microstructures and good mechanical properties. The effect of κ carbides on the mechanical properties and strain hardening rate of Fe-27Mn-10Al-1C (wt.%) low density steel was studied by short-time heat treatment to control the precipitation behavior of κ carbides. Quenched specimens have an excellent combination of strength and plasticity and continuous high strain hardening rate, which is due to the uniform distribution of κ carbides with an average size of 1.6 nm in an austenite matrix. The fracture mode of the sample changed from ductile fracture to cleavage fracture, which was because the aging treatment promoted the precipitation of B2 phases and κ carbides at grain boundaries. The size and volume fraction of nanoscale κ carbides in austenite grains increase with the increase of aging temperature, and the yield strength increases but the density of slip bands decreases, resulting in the gradual decrease of strain hardening rate.

Effect of tempering on the stability of retained austenite in carbide-free bainitic steel

In this study, a C–Mn–Si alloyed bainitic steel was treated by bainitic austempering (BAT) process to form a multiphase of bainitic ferrite, filmy retained austenite (RA), and blocky martensite/austenite. Tempering was employed to optimize the microstructure and mechanical response of steels after BAT treatment. Microstructural alterations because of tempering that affects the mechanical stability of RA was discussed via X-ray diffractometer, high resolution transmission electron microscopy, and auger electron spectroscopy. Results showed that an excellent combination of strength and ductility was achieved after tempering (ultimate tensile strength: 1489 MPa; yield strength: 1014 MPa; total elongation: 33.2%), which was attributed to improved overall stability of RA and high fracture damage resistance. Interestingly, compared to normal BAT steel, the transformation fraction and rate of RA exhibit three stages with increase of strain in tempered BAT steel, leading to abnormally high stability of RA at strain of 2%–10%. It is suggested that the nano-sized η carbides precipitated after tempering could promote the dislocation multiplication in BCC matrix (i.e., bainitic ferrite and martensite) and obstruct the movement of dislocation from BCC matrix to RA, which results in a ductile but strong tempered matrix that provides a “shielding effect” on RA and improves RA's stability.

A Protein-Like Nanogel for Spinning Hierarchically Structured Artificial Spider Silk.

Spider dragline silk is draw-spun from soluble, β-sheet-crosslinked spidroin in aqueous solution. This spider silk has an excellent combination of strength and toughness, which originates from the hierarchical structure containing β-sheet crosslinking points, spiral nanoassemblies, a rigid sheath, and a soft core. Inspired by the spidroin structure and spider spinning process, a soluble and crosslinked nanogel is prepared and crosslinked fibers are drew spun with spider-silk-like hierarchical structures containing cross-links, aligned nanoassemblies, and sheath-core structures. Introducing nucleation seeds in the nanogel solution, and applying prestretch and a spiral architecture in the nanogel fiber, further tunes the alignment and assembly of the polymer chains, and enhances the breaking strength (1.27GPa) and toughness (383MJ m-3 ) to approach those of the best dragline silk. Theoretical modeling provides understanding for the dependence of the fiber's spinning capacity on the nanogel size. This work provides a new strategy for the direct spinning of tough fiber materials.

Development of Structural Alloys with Excellent Combination of Strength and Ductility <i>via</i> Utilizing Hydrogen as a Positive Constituent

Development of Structural Alloys with Excellent Combination of Strength and Ductility <i>via</i> Utilizing Hydrogen as a Positive Constituent

Excellent combination of strength and ductility in CoNiCr-based MP159 alloys at cryogenic temperature

In this study, a multi-component single-phase CoNiCr-based MP159 alloy with a high dislocation density is investigated when subjected to cold rolling. The alloy has an ultra-high yield strength of 1807 MPa and large ductility of more than 20% at 77 K, with a significant temperature dependence. The difference in mechanical properties between cryogenic and room temperatures is illustrated by the analysis of the deformation structures, and the deformation twinning is found to make an critical contribution to high strength and large ductility. It provides an alternative way to strengthening metallic materials in the current investigation.

Effect of extrusion ratios on microstructure evolution and strengthening mechanisms of a novel P/M nickel-based superalloy

A novel P/M nickel-based superalloy is developed with an excellent combination of strength and ductility for use in turbine disks. This superalloy is prepared through hot isostatic pressing (HIP) and subsequent hot extrusion (HEX) with different extrusion ratios (λ). The effects of λ on the evolution and mechanism of γ′ precipitation behavior, grain refinement, microtexture, and mechanical properties are systematically studied. As λ increases, the primary γ′ phase fraction decreases from 17.22% to 8.47%, and the precipitation morphology changes from an irregular to a spherical shape. Increasing λ is beneficial to dynamic recrystallization. Fully recrystallized fine grains with a diameter of 4.47 μm are obtained at an optimal λ = 5.0, a decrease of 76% compared with the HIPed alloy. Abnormal grain coarsening was observed at a higher λ with 8.5 owing to the lack of γ′ pinning and a stronger driving force for grain boundary migration induced by deformation heat. The yield strength (YS) and ductility of the HEXed alloy are over 30% higher than those of the HIPed alloy, thanks to refined grains and the γ′ phase. The alloy extruded with λ of 5.0 exhibits a high YS of 1376 MPa with a ductility of 22.6% at room temperature and a YS of 1208 MPa with a ductility of 18.3% at 700 °C, which are better than those of other superalloys. A multi-mechanistic strengthening model is constructed by correlating the chemical composition, size and fraction of γ′, grain size, and YS. The calculations suggest that precipitation strengthening contributes more than 60% of the YS. Extrusion can dramatically enhance the YS of the superalloy owing to the increment of grain boundary strengthening and precipitation strengthening. This work provides a practical route and theoretical support for optimizing microstructure and enhancing the mechanical properties of a HIPed P/M nickel-based superalloy.

Mechanical performance and deformation mechanisms at cryogenic temperatures of 316L stainless steel processed by laser powder bed fusion: In situ neutron diffraction

Manufacturing austenitic stainless steels (ASSs) using additive manufacturing is of great interest for cryogenic applications. Here, the mechanical and microstructural responses of a 316L ASS built by laser powder bed fusion were revealed by performing in situ neutron diffraction tensile tests at the low-temperature range (from 373 to 10 K). The stacking fault energy almost linearly decreased from 29.2 ± 3.1 mJm−2 at 373 K to 7.5 ± 1.7 mJm−2 at 10 K, with a slope of 0.06 mJm−2K−1, leading to the transition of the dominant deformation mechanism from strain-induced twinning to martensite formation. As a result, excellent combinations of strength and ductility were achieved at the low-temperature range.

Thermochemical post-processing of additively manufactured austenitic stainless steel

The effects of different thermochemical post-processing treatments on the microstructure and properties of additively manufactured austenitic stainless steel were investigated. If the nitrogen content in the as-built condition is high, austenitization in vacuum causes a reduction in nitrogen content near the surface. This can be remedied by applying a small amount of nitrogen in the gas during austenitization. Using high temperature solution nitriding, the surface hardness could be effectively raised by deliberate nitrogen ingress, while maintaining a very fine structure inside the primary austenite grains. The excellent combination of strength and ductility of the as-built condition is accompanied by a low degree of work-hardening. This condition showed elastic and plastic anisotropy. The vertically built condition exhibits a lower strength and an early initiation of yielding as compared to the horizontally built conditions. High temperature treatments were able to efficiently reduce the mechanical anisotropy exhibited in the as-built condition. A combination of high- and low temperature surface hardening was investigated using nitriding, carburizing and nitrocarburizing. In all cases expanded austenite developed on the surface, which created a strong hardness increase.

The determining role of carbon addition on mechanical performance of a non-equiatomic high-entropy alloy

• The role of C on the microstructure and mechanical properties are discussed. • The microstructure of the tensile specimen interrupted at different tensile strains are examined by EBSD and TEM. The deformation mechanism of a C-doped iHEA with excellent mechanical properties is discussed. • The contributions of different strengthening mechanism to yield strength are calculated and the comprehensive strengthening mechanism is discussed. The mechanical properties and deformation mechanism of a C-doped interstitial high-entropy alloy (iHEA) with a nominal composition of Fe 49.5 Mn 29.7 Co 9.9 Cr 9.9 C 1 (at.%) were investigated. An excellent combination of strength and ductility was obtained by cold rolling and annealing. The structure of the alloy is consisted of FCC matrix and randomly distributed Cr 23 C 6 . For gaining a better understanding of deformation mechanism, EBSD and TEM were conducted to characterize the microstructure of tensile specimens interrupted at different strains. At low strain (2%), deformation is dominated by dislocations and their partial slip. With the strain increase to 20%, deformation-driven athermal phase transformation and dislocations slip are the main deformation mechanism. While at high strain of 35% before necking, deformation twins have been observed besides the HCP phase. The simultaneous effect of phase transformation (TRIP effect) and mechanical twins (TWIP effect) delay the shrinkage, and improve the tensile strength and plasticity. What's more, compared with the HEA without C addition, the yield strength of the C-doped iHEA has been improved, which can be attributed to the grain refinement strengthening and precipitation hardening. Together with the lattice friction and solid solution strengthening, the theoretical calculated values of yield strength match well with the experimental results.

Simulative Design of a Load-Path Adapted Fibre Reinforced Composite (FRP) Transporter Rear Door

The demand for weight reduction in the automotive, aerospace, and railway industries is at an all-time high. Fibre-reinforced composites (FRC) have proved their suitability with their excellent combination of strength and weight. The only limiting factor in adapting FRC for mass-production in industries is the high costs of composite materials like carbon fibre reinforced plastics. A design workflow, suitable for FRC’s and their available manufacturing technologies has been proposed by adopting a progressive approach. A multi-material component design has been optimized using Altair Hyperworks and OptiStruct to reduce weight and maintain stiffness and strength as compared to its conventional metallic version. The layup of the substrate laminate has been optimized using special optimization tools in OptiStruct in three stages. The determination of load-paths is followed by the optimization of the base laminate under given load cases. Subsequently, the geometry of injection-moulded stiffeners was determined by employing topology optimization considering the composite layup, obtained in previous steps.

Optimizing transformation-induced plasticity in CoCrNi alloys by combined grain refinement and chemical tuning

Grain refinement increases yield strength, but usually at the cost of ductility reduction. In the present work, we explored the strategy of grain refinement and composition tuning in CoCrNi alloys to optimize mechanical properties. Grain refinement was found to strongly suppress deformation-induced martensitic transformation in metastable CoCrNi alloys, which was utilized to modulate the transformation-induced plasticity (TRIP) effect in combination with composition tuning guided by theoretical calculations. We demonstrated in a non-equiatomic CoCrNi TRIP medium-entropy alloy (MEA) that our approach resulted in an excellent combination of strength and ductility. The proposed strategy is expected to be useful in exploring superior mechanical properties of MEAs and high-entropy alloys in varying systems.

Influence of cellular subgrain feature on mechanical deformation and properties of directed energy deposited stainless steel 316 L

The fundamental deformation mechanism responsible for the exceptional combination of strength and ductility exhibited by SS 316 L material manufactured using directed energy deposition (DED) based additive manufacturing (AM) technique was studied. First, the stress-strain behaviors under uniaxial tensile testing of specimens containing different intensities of cellular subgrain feature were obtained. The stress-strain behaviors showed large differences with different intensities. Then, the microscopic deformation under in-situ tensile testing was characterized. At intragranular length scale, results revealed an extensive formation of a heterogeneous surface texture with a characteristic length scale and morphology consistent with the geometry of the cellular subgrain feature. The results also revealed that the high strain hardening rate regime of the macroscopic stress-strain behavior is primarily driven by deformation slipping, not deformation twinning. Finally, the evolution of the slip bands under uniaxial tensile testing was characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Results revealed a barrier effect of cellular subgrain feature on slip band evolution. For a given macroscopic strain the width, depth, and spacing of slip band formation varied with the intensity of the cellular subgrain feature. The heterogeneous texture formation and barrier effect on slip band evolution observed in this study were clear evidence of the extensive influence of cellular subgrain feature on microscopic deformation and mechanical properties. Additionally, these remarkable observations revealed that dynamic slip band refinement induced by cellular subgrain feature is the fundamental mechanism that facilitates the exceptional combination of strength and ductility of DED manufactured SS 316 L • Cellular subgrain feature of DED 316 L produces a textured deformation and disrupts slip band evolution substantially. • Causes dynamic slip band refinement resulting in high strain hardening rate. • High strain hardening rate results in excellent combination of strength and ductility. • Early-stage plastic deformation is dominated by deformation slipping, not deformation twinning.

Strengthen and Toughen the Low‐Alloyed Steel by Direct Tempforming

Improving strength without sacrificing toughness is a permanent request for the development of advanced engineering materials. Herein, a novel energy‐saving strategy is developed to obtain strong but tough low‐alloyed steels. A medium‐carbon steel (42CrMo) is used for demonstration. Warm deformation (tempforming) is applied to the hot‐rolled and air‐cooled 42CrMo steel. After tempforming, the microstructure consists of nano‐sized carbides and highly textured ultrafine grains. The tensile strength of the tempformed steels can reach 1.2 GPa, while the impact toughness is higher than 100 J even at temperatures as low as −80 °C, which are one order of magnitude higher than the counterpart steel after tempering. The present work sheds light on the design and fabrication of high‐performance engineering steels with an excellent combination of strength and toughness.

Segregation enabled outstanding combination of mechanical and corrosion properties in a FeCrNi medium entropy alloy manufactured by selective laser melting

• A face-centered cubic single-phase FeCrNi medium entropy alloy with a high Cr (~35 at.%) content was successfully manufactured by selective laser melting (SLM). • This as-SLMed MEA presents a hierarchical microstructure of coarse columnar grains and submicron cell structures. Cr, and also interstitial C , are found segregated to the cell boundaries. • The as-SLMed FeCrNi MEA shows a good combination of strength (σ0.2 = 745 MPa, σUTS=1007 MPa) and ductility (εf=31%), as well as a superb corrosion resistance ( i corr =0.06 μA/cm 2 ). Selective laser melting (SLM) has the advantage in preparing supersaturated solid solutions due to its unique thermal field and high solidification rate. In this study, a face-centered cubic single-phase FeCrNi medium entropy alloy (MEA) with an ultrahigh Cr content (~35 at.%) was additively manufactured by SLM. The as-built MEA shows a hierarchical microstructure of coarse columnar grains and submicron dislocation cell structures, where the cell boundaries are probed segregated with Cr and C and decorated with nano carbides. The appearance of these dislocation barriers results in an excellent combination of strength ( σ 0.2 =745 MPa, σ UTS =1007 MPa) and ductility ( ε f =31%). The current MEA also shows a superb corrosion resistance with a corrosion current density of 0.06 μA cm −2 in 3.5 wt.% NaCl solution, which is far lower than that of 316 L. The high content of solutioned Cr in the MEA ensures sufficient Cr supply to form an integrated Cr 2 O 3 passive film, and the large number of cell boundaries acting as the diffusion channels lead to the fast formation of a stable passive film over the alloy surface.

Experimental and numerical evaluation of squeeze cast Al-Si-Cu-Ni-Mg alloy for piston applications

ABSTRACT Al-12% Si is the commercial alloy commonly used to make pistons by using gravity casting method. When this alloy was modified with Ni, Cu, and Mg (modified alloy), it showed an adequate fluidity during casting and achieved higher strength. However, the addition of alloying elements reduces its thermal characteristics. Therefore, an attempt has been made to improve the thermal conductivity through modifying the microstructure. It was done by changing the cast method to squeeze cast, adding 0.04%Sr (Sr-modified alloy) and post-heat treatment (T6 and T7). Results highlighted that the sample of gravity cast-Sr-modified alloy showed enhancement in thermal conductivity. Even though squeeze cast-Sr-modified alloy showed reduction in thermal conductivity of few percentage, it possessed better strength, however the heat treatment (T7 condition) showed a noticeable increment in thermal conductivity. Therefore, it is concluded that T7-heat-treated squeeze cast-Sr-modified alloy showed an excellent combination of strength and thermal conductivity making it an ideal material for piston applications. The piston component was studied by numerical steady state thermal analysis using ANSYS Workbench, obtained a maximum heat flux of 1.80 W/mm2 for Al-Si piston alloy aged for 11 h (T7), which is in agreement with the experimental results.

Influence of Strength Mismatch on Microstructural Evolution and Mechanical Behaviors for Q390/Q690 Ferritic Steel Dissimilar Welded Joints

For the structural integrity of weldments, it is necessary to ascertain acceptable strength mismatch mode when dissimilar parent materials are welded together. Herein, multilayer and multi‐pass CO2 gas shielded welding is carried out to manufacture two dissimilar welded joints of Q390 and Q690 steels using two filler materials with different strength levels. A comparative assessment of both strength mismatch modes is done based on microstructural characterization and mechanical testing. The results show that the weld metal (WM) with predominantly fine acicular ferrite (about 80 pct) and granular bainite has a more excellent combination of strength and toughness compared to the WM mainly composed of polygonal ferrite and degenerated pearlite, and the weakest link of the welded joint has transferred from the WM to the Q390‐side heat‐affected zone. The acceptable strength mismatch relationship of the dissimilar welded structures is revealed, e.g., the hardness in the welded joint should present a uniform gradient distribution. Herein, the gradient difference in the equivalent yield strength (calculated from their hardness) between adjacent areas is ≈170 MPa.

High strength aluminum alloys design via explainable artificial intelligence

Here, we have approached to discover new aluminum (Al) alloys with the assistance of artificial intelligence (A.I.) for the enhanced mechanical property. A high prediction rate of 7xxx series Al alloy was achieved via the Bayesian hyperparameter optimization algorithm. With the guide of A.I.-based recommendation algorithm, new Al alloys were designed that had an excellent combination of strength and ductility with a yield strength (YS) of 712 MPa and elongation (EL) of 19%, exhibiting a homogeneous distribution of nanoscale precipitates hindering dislocation movement during deformation. Adding Mg and Cu was found to be the critical factor that decides the relative ratio of strength and EL. We also demonstrate an explainable A.I. (XAI) system that reveals the relationship between input and output parameters. Our A.I. assistant system can accelerate the search for high-strength Al alloys for both experts and non-experts in the field of Al alloy design.

Effects of deformation-induced martensitic transformation on cryogenic fracture toughness for metastable Si8V2Fe45Cr10Mn5Co30 high-entropy alloy

In this study, fracture toughness was evaluated at room and cryogenic temperatures for a metastable Si8V2Fe45Cr10Mn5Co30 (at.%) alloy, and roles of deformation-induced martensitic transformation (DIMT) on fracture behavior were investigated with respect to types of transformation mechanism and martensite morphology. The as-annealed alloy consisted mainly of face-centered-cubic (FCC) phase with athermal hexagonal-close-packed (HCP) martensite. The DIMT from FCC to body-centered-cubic (BCC) via intermediate HCP resulted in an excellent combination of strength and ductility by high strain-hardening effect. Considerably high fracture toughness and ductile-dimpled fracture mode appeared at room temperature, but the fracture toughness significantly deteriorated at cryogenic temperature. The BCC martensite exhibited a thick-plate morphology as a resultant product of stress-induced transformation from low-stability FCC, and the plates had an alternating variant characterized by a ladder martensite, based on a 60°-BCC-twin relationship with {112} twin planes. This plate-type BCC martensite had the fewer phase boundaries and geometrically necessary dislocations than the lath-type BCC martensite. In addition, cleavage {001}BCC-planes of two martensite blocks having the alternating variants were slightly deviated, thereby leading to the much lower toughness along with the brittle fracture mode of lamellar cleavage facets at cryogenic temperature. Thus, the present work suggests that the proper control of FCC stability is required to enhance the resistance to low-temperature embrittlement for securing cryogenic applications.

Exploring New Strategies for Ultrahigh Strength Steel via Tailoring the Precipitates

Ultrahigh-strength (UHS) steels have shown great potential in the field of high-end equipment manufacturing in demand of lightweight engineering and performance upgrade. A significant research effort has been directed toward the development of advanced UHS steels with excellent combination of strength and toughness. In the course of development, tailoring precipitates by means of composition design and process optimization is absolutely a critical module. In this mini review, typical UHS steels strengthened by carbides and intermetallics are systematically summarized and discussed. With the increase of strength, the toughness losses of UHS steels strengthened by carbides and intermetallics have been compared in detail. In particular, the in-depth mechanisms leading to various strength/toughness variation trends have been discussed, extracting the bottleneck in developing new-generation UHS steels containing merely one type of precipitate. Meanwhile, prospects on designing advanced UHS steels strengthened by coexisting dispersive precipitates have been proposed to achieve better performance.