Showing papers on "Strain hardening exponent published in 2020"

Real-time observations of TRIP-induced ultrahigh strain hardening in a dual-phase CrMnFeCoNi high-entropy alloy.

Abstract: Strategies involving metastable phases have been the basis of the design of numerous alloys, yet research on metastable high-entropy alloys is still in its infancy. In dual-phase high-entropy alloys, the combination of local chemical environments and loading-induced crystal structure changes suggests a relationship between deformation mechanisms and chemical atomic distribution, which we examine in here in a Cantor-like Cr20Mn6Fe34Co34Ni6 alloy, comprising both face-centered cubic (fcc) and hexagonal closed packed (hcp) phases. We observe that partial dislocation activities result in stable three-dimensional stacking-fault networks. Additionally, the fraction of the stronger hcp phase progressively increases during plastic deformation by forming at the stacking-fault network boundaries in the fcc phase, serving as the major source of strain hardening. In this context, variations in local chemical composition promote a high density of Lomer-Cottrell locks, which facilitate the construction of the stacking-fault networks to provide nucleation sites for the hcp phase transformation.

Strain-hardening and suppression of shear-banding in rejuvenated bulk metallic glass

Abstract: Strain-hardening (the increase of flow stress with plastic strain) is the most important phenomenon in the mechanical behaviour of engineering alloys because it ensures that flow is delocalized, enhances tensile ductility and inhibits catastrophic mechanical failure1,2. Metallic glasses (MGs) lack the crystallinity of conventional engineering alloys, and some of their properties-such as higher yield stress and elastic strain limit3-are greatly improved relative to their crystalline counterparts. MGs can have high fracture toughness and have the highest known 'damage tolerance' (defined as the product of yield stress and fracture toughness)4 among all structural materials. However, the use of MGs in structural applications is largely limited by the fact that they show strain-softening instead of strain-hardening; this leads to extreme localization of plastic flow in shear bands, and is associated with early catastrophic failure in tension. Although rejuvenation of an MG (raising its energy to values that are typical of glass formation at a higher cooling rate) lowers its yield stress, which might enable strain-hardening5, it is unclear whether sufficient rejuvenation can be achieved in bulk samples while retaining their glassy structure. Here we show that plastic deformation under triaxial compression at room temperature can rejuvenate bulk MG samples sufficiently to enable strain-hardening through a mechanism that has not been previously observed in the metallic state. This transformed behaviour suppresses shear-banding in bulk samples in normal uniaxial (tensile or compressive) tests, prevents catastrophic failure and leads to higher ultimate flow stress. The rejuvenated MGs are stable at room temperature and show exceptionally efficient strain-hardening, greatly increasing their potential use in structural applications.

Stress-strain curves of metallic materials and post-necking strain hardening characterization: A Review

Abstract: Fatigue Fract Eng Mater Struct. 2019;1–17. Abstract For metallic materials, standard uniaxial tensile tests with round bar specimens or flat specimens only provide accurate equivalent stress–strain curve before diffuse necking. However, for numerical modelling of problems where very large strains occur, such as plastic forming and ductile damage and fracture, understanding the post‐necking strain hardening behaviour is necessary. Also, welding is a highly complex metallurgical process, and therefore, weldments are susceptible to material discontinuities, flaws, and residual stresses. It becomes even more important to characterize the equivalent stress–strain curve in large strains of each material zone in weldments properly for structural integrity assessment. The aim of this paper is to provide a state‐of‐the‐art review on quasi‐static standard tensile test for stress–strain curves measurement of metallic materials. Meanwhile, methods available in literature for characterization of the equivalent stress–strain curve in the post‐necking regime are introduced. Novel methods with axisymmetric notched round bar specimens for accurately capturing the equivalent stress–strain curve of each material zone in weldment are presented as well. Advantages and limitations of these methods are briefly discussed.

Anomalous strengthening by supersaturated solid solutions of selectively laser melted Al–Si-based alloys

Abstract: To identify the dominant contributing factor in the anomalously high strength of Al–Si-based alloys fabricated by selective laser melting (SLM), microstructural characteristics of a SLM-built Al–10Si–0.3 Mg alloy (AlSi10Mg) and their changes upon annealing at elevated temperatures were investigated. The as-built AlSi10Mg alloy exhibits a peculiar microstructure comprising of a number of columnar α-Al (fcc) phase with concentrated Si in solution. Numerous nano-sized particles were observed within the α-Al matrix. At elevated temperatures, a number of Si phase (diamond structure) precipitates consumed the solute Si in the columnar α-Al phase, but the microstructure of the α-Al matrix changed slightly. After annealing at elevated temperatures, the tensile strength of the as-built AlSi10Mg alloy substantially decreased accompanied by a reduction in the strain hardening rate. The supersaturated solid solution of the α-Al phase containing numerous nano-sized particles enhanced the strain hardening, resulting in the anomalous strengthening of the SLM-built AlSi10Mg alloy. The microstructural features were formed due to rapid solidification at an extremely high cooling rate in the SLM process, which provides important insights into controlling the strength of Al–Si-based alloys fabricated by SLM.

Strength-ductility synergy of selective laser melted Al-Mg-Sc-Zr alloy with a heterogeneous grain structure

Abstract: In this work, a Sc/Zr modified Al-Mg alloy was processed by both selective laser melting (SLM) and directed energy deposition (DED). Due to different precipitation behavior of primary Al3(Sc,Zr)-L12 nucleation sites, a heterogeneous grain structure was formed in SLMed sample, which consisted of ultrafine equiaxed grains bands and columnar grains domains, while a fully equiaxed grain structure was obtained in DEDed sample. Tensile results showed that the as built SLMed sample had a good combination of strength and ductility. The yield strength of SLMed sample (335 ± 4 MPa) was about 2.8 times that of DEDed sample (118 ± 3 MPa), however, the ductility in uniform elongation (23.6 ± 1.9%) was still comparable to that of DEDed sample (23.8 ± 2.6%). Based on the relationship between the heterogeneous grain structure and strain hardening behavior, the strength-ductility synergy mechanism of the SLMed Al-Mg-Sc-Zr alloy was discussed. Stress partitioning tests showed that the contribution of back stress hardening to flow stress was higher in SLMed sample than DEDed sample, while effective stress hardening showed an opposite trend. Despite the overall strain hardening ability of SLMed sample was limited by the high dynamic recovery rate of ultrafine equiaxed grains, additional back stress hardening, which was caused by strain partitioning between equiaxed grains bands and columnar grains domains, improved its strain hardening ability and resulted in the good combination of strength and ductility.

PET FRP-concrete-high strength steel hybrid solid columns with strain-hardening and ductile performance: Cyclic axial compressive behavior

Abstract: This paper presents the results of an experimental program on the behavior of fiber-reinforced polymer (FRP)-concrete-high strength steel solid columns (FCSSCs), with an outer polyethylene terephthalate (PET) FRP tube and an inner circular high-strength steel (HSS) tube, under cyclic axial compression. A PET FRP tube has a much larger rupture strain and a larger FRP hoop strain efficiency, leading to an excellent ductility of PET FRP-confined concrete. The HSS tube, which has a good deformation compatibility with the PET FRP-confined concrete in FCSSCs, provides a much larger longitudinal load carrying capacity and a larger confinement to the core concrete compared with a normal strength steel tube. The experimental results demonstrated that the axial load carrying capacity of an FCSSC is much larger than the summation of the axial load resistance of the hollow steel tube and that of the concrete-filled FRP tube; the buckling of the HSS tube is also prevented so that its post-yield material strength is effectively utilized. It is found that cyclic load-strain envelope curves lie closely to the corresponding monotonic load-strain curves, and repeated loading cycles increase the plastic strain while decrease the reloading new stress. The existing stress-strain model fails to provide an accurate prediction on the cyclic axial behavior of concrete under combined PET FRP-steel confinement.

Effect of solution temperature on static recrystallization and ductility of Inconel 625 superalloy fabricated by directed energy deposition

Abstract: Solution treatment is a very important method to modify the microstructure and mechanical properties of additive-manufactured nickel-based superalloy. This study mainly investigated the effect of solution temperature on static recrystallization and ductility of Inconel 625 superalloy fabricated by directed energy deposition (DED). The microstructural results showed that static recrystallization occurred after solution treatment, and as the solution temperature increased, the recrystallization volume fraction in the alloy gradually increased. When the solution temperature reached 1200 °C, static recrystallization occurred more completely. The tensile results indicated that the elongation and strain hardening exponent of Inconel 625 improved with the solution temperature increased, while the yield strength decreased. The deformation behaviors during the tensile process of different samples were obtained by digital image correlation (DIC), and the results showed that the uniform plastic deformation ability was strengthened with the solution temperature increased. In addition, due to the decrease of dislocation density, the yield strength decreased, too. Besides, the increase of strain hardening exponent delayed the appearance of the necking and the dissolution of the Laves phase caused the ductility increasing.

Evolution of microstructures, dislocation density and arrangement during deformation of low carbon lath martensitic steels

Abstract: In this paper, the role of solute carbon on the strengthening and work hardening behavior of lath martensite was studied by analyzing the microstructures and dislocation density in the undeformed and deformed conditions. An increase in carbon content from 0.18% to 0.30% decreases the martensite start (Ms) temperature, leading to refinement of both the block and lath widths. Although reduction of the “effective grain size” is observed via Electron Backscatter Diffraction (EBSD) and Electron Channeling Contrast Imaging (ECCI) techniques, this effect is considered secondary in increasing the strength of lath martensite with increased carbon content. The higher strength is attributed mainly to the phase transformation-induced dislocation density in the high-carbon martensite. Comparing this total dislocation density calculated using a Convolutional Multiple Whole Profile (CMWP) fitting procedure with the estimated geometrically necessary dislocations (GND) from the misorientation distribution of EBSD analysis, it appears that a high fraction of the dislocations in lath martensitic steel is GND. Furthermore, the analysis of the samples strained to a different level suggests that the dislocation density shows minimal change during deformation, whereas the dislocation arrangement rapidly decreases at the beginning of the plastic deformation. Finally, the strain hardening behavior of the lath martensitic steel is quantitatively described by considering lath width, dislocation density, and dislocation arrangement parameters through the α coefficient in Taylor's equation.

Mechanism of low temperature deformation in aluminium alloys

Abstract: This study investigates differences in the deformation mechanisms between room temperature (296 K) and cryogenic temperatures (77 K) and their advantages for low temperature formability in alloys EN AW 1085, EN AW 5182 and EN AW 6016. Compared to room temperature behaviour, tensile tests showed an overall increase in yield strength, ultimate tensile strength and uniform elongation with differences among the principal alloy types. In general, the improved mechanical properties result from higher strain hardening rates at lower temperatures. The application of an extended Kocks-Mecking approach showed a significant reduction of the dynamic recovery and suggested higher dislocation densities upon cryogenic deformation. This was confirmed via in-situ synchrotron experiments, which also reveal a higher proportion of screw dislocations. Moreover, kernel average misorientation maps from electron backscattered diffraction and in-situ cryogenic deformation in a transmission electron microscope displayed a more uniform dislocation arrangement with a reduction of slip lines and less highly misaligned areas after deformation at lower temperatures. Supported by a detailed characterization of the microstructure and its dislocation structure, the associated fundamental mechanisms we reveal, which are at the origin of the exceptional improvement in mechanical properties, are extensively discussed.

Enhancing the mechanical performance of PA6 based composites by altering their crystallization and rheological behavior via in-situ generated PPS nanofibrils

Abstract: Polyamide 6 (PA6) is a popular engineering thermoplastic due to its impressive mechanical and wear resistance properties. However, its poor notched impact performance at room temperature limits its application. Although melt blending with elastomeric materials can improve PA6's toughness, this comes at the expense of its tensile strength and stiffness. In this work, a unique in-situ fibrillation technique was demonstrated to improve the impact performance of PA6 without sacrificing its stiffness. PA6-based in-situ nanofibrillar composites, containing polyphenylene sulfide (PPS) nanofibrillar domains with an average diameter around 60 nm, were produced combining melt compounding and hot stretching. Then, the effect of this fibrillar network on the mechanical properties of PA6 composites was investigated under uniaxial tensile deformation and notched impact. Results indicated no decrease in the tensile modulus in the presence of PPS nanofibrils; however, the nanofibrillar network did induce a significant difference in the stress-strain curves with the evolution of multiple necking and strain hardening. This behavior was explained by the formation of transcrystalline structures and a small crystal size in the presence of the fibril network. The evolution of multiple necking and strain hardening was correlated with the improved elasticity of the nanofibrillar composite through rheological analysis as well. Notched Izod impact tests on the composites demonstrated that 3 wt% PPS nanofibrils improved the impact strength by ~85% compared to neat PA6. Overall, this study gives insight into the design of PA6 composites with tuned impact strength and stiffness through a simple and scalable production method.