Yield Point Phenomenon Research Articles

Multiscale Finite Element Analysis of Yield-point Phenomenon in Ferrite–Pearlite Duplex Steels

Multiscale Finite Element Analysis of Yield-point Phenomenon in Ferrite–Pearlite Duplex Steels

Experimental and numerical investigation of the yield point phenomenon and strain partitioning behavior in a dual-phase steel with lamellar structure

Severe rolling techniques show potential for enhancing the strength and ductility of metallic materials, but the resulting microstructural anisotropy affects the mechanical behavior. The study investigated the anisotropic yield point phenomenon (YPP) and strain partitioning behavior in a dual-phase steel with ultrafine lamellar structure, fabricated through severe cold- and warm-rolling procedures. Experimental results show that tensile specimens aligned parallel to the rolling direction exhibit YPP and higher ductility, while those oriented normal to the rolling direction show continuous yielding behavior and decreased ductility. Using Digital Image Correlation techniques, we studied the evolution of the strain distribution on the surface of the gauge segment, revealing that the strain within the Lüders band was almost constant during the Lüders band propagation. Strain-induced martensitic transformation (SIMT) was observed within the Lüders band and the volume fraction of martensite keeps almost constant during the Lüders band propagation. Numerically, we integrated the mechanisms of SIMT and YPP into a dislocation density-based J2 plasticity constitutive framework. Simulated tensile tests indicate a smaller Lüders strain resulted from the initial higher strain hardening rate. Additionally, SIMT suppresses local instability and therefore promotes the stable propagation of Lüders bands. Simulations verify that the lamellar orientation and SIMT significantly affect the strain hardening rate and void growth. Our findings provided a new perspective on the intrinsic mechanism of the anisotropic YPP and ductility in lamellar dual-phase steels.

Cryogenic Tensile Behavior of Ferrous Medium-entropy Alloy Additively Manufactured by Laser Powder Bed Fusion

The emergence of ferrous-medium entropy alloys (FeMEAs) with excellent tensile properties represents a potential direction for designing alloys based on metastable engineering. In this study, an FeMEA is successfully fabricated using laser powder bed fusion (LPBF), a metal additive manufacturing technology. Tensile tests are conducted on the LPBF-processed FeMEA at room temperature and cryogenic temperatures (77 K). At 77 K, the LPBF-processed FeMEA exhibits high yield strength and excellent ultimate tensile strength through active deformation-induced martensitic transformation. Furthermore, due to the low stability of the face-centered cubic (FCC) phase of the LPBF-processed FeMEA based on nano-scale solute heterogeneity, stress-induced martensitic transformation occurs, accompanied by the appearance of a yield point phenomenon during cryogenic tensile deformation. This study elucidates the origin of the yield point phenomenon and deformation behavior of the FeMEA at 77 K.

Multiscale Finite Element Analysis of Yield-point Phenomenon in Ferrite–Pearlite Duplex Steels

Multiscale Finite Element Analysis of Yield-point Phenomenon in Ferrite–Pearlite Duplex Steels

In-situ SEM/EBSD investigation of tensile deformation-driven microstructural changes in low-carbon HSLA steels with ferrite-bainite microstructures

In this study, we aimed to investigate how microstructural constituents affect the plastic deformation behavior of low-carbon HSLA steels during a uniaxial tensile test using the in-situ SEM/EBSD technique. We fabricated three types of low-carbon HSLA steels, each with distinct ferrite-bainite microstructures, by altering the thermomechanical control processing conditions. All the steels were composed primarily of polygonal ferrite (PF) and bainitic microstructures. As the finish rolling temperature and the cooling rate decreased, the volume fraction of PF increased. The in-situ SEM and EBSD analysis during deformation revealed a greater concentration of plastic strain and an increased mobile dislocation density in the PF than in the bainitic microstructures. Most notably, as the volume fraction of the bainitic microstructure increased, so did the degree of strain gradient between the soft PF and the hard bainitic microstructure. This suggests that at the initial yield stage the soft PF plays a more significant role in mitigating the yield point phenomenon than the hard bainitic microstructure. Our findings offer a deeper understanding of the microstructural factors that influence plastic deformation, and may serve as a guide for designing optimal microstructures to ensure the mechanical properties of low-carbon HSLA steels for a range of applications.

Extraordinary strength–ductility–toughness in Fe–0.08C plain low-carbon steel via introducing weblike martensite: Towards the third generation

The effect of large-strain cold rolling followed by intercritical annealing on the microstructure and mechanical properties of plain low-carbon steel was investigated. The grain size of ferrite reduced from 38 μm (for the as-received steel) to about 4 μm (for the dual-phase steel). The martensite phase was connected in 3D space in the dual-phase steel and a weblike structure of martensite was created. The stress-strain curve of dual-phase steel represents the continuous yield behavior without any distinct evidence of the yield point phenomenon. By introducing weblike martensite into the microstructure and refining α grains, the yield strength, ultimate tensile strength, and tensile toughness significantly increased to 740 MPa, 1062 MPa, and 193 J cm−3, respectively. The fracture surface of the DP sample had a dimpled appearance, indicating a ductile fracture mode. The results of the present work indicated that a low-cost plain low-carbon DP steel composed of weblike martensite and fine-grained ferrite exhibits extraordinary strength–ductility–toughness balance. Therefore, the dual-phase steel produced in the present work, by moving towards the third generation, can find wide applications in the automotive industry.

Effect of Intercritical Annealing Time on the Microstructure and Mechanical Properties of Dual‐Phase Steel Processed by Large‐Strain Asymmetric Cold‐Rolling

Herein, the effect of intercritical annealing time on the microstructure and mechanical properties of dual‐phase steel processed by large‐strain asymmetric cold‐rolling is studied. It is observed that the martensite islands are uniformly distributed in the ferrite phase in the microstructures of dual‐phase steels due to performing the asymmetric cold‐rolling before intercritical annealing treatment. As the intercritical annealing time increases up to 10 min, the fraction of martensite increases. By increasing the holding time and fraction of martensite, the carbon content of the martensite phase is decreased. The short‐term intercritical annealing eliminates the yield point phenomenon. However, intercritical annealing at 860 °C for 20 min leads to the reoccurrence of a yield point phenomenon. Increasing the intercritical annealing time to 10 min improves the yield strength to 505 MPa and ultimate tensile strength to 834 MPa. However, the strength decreases sharply after the holding time of 20 min. There is a perfect linear relationship between the mechanical properties and the fraction of martensite. Ductile failure is observed at the center of the fracture surfaces of dual‐phase steels while shear failure occurs at the edges of the fracture surfaces.

Effects of Temperatures of Rolling and Annealing on Microstructures and Tensile Properties of Low Carbon Ferritic Low Density Steels

The microstructural modification and the effect of cold/warm rolling and annealing on tensile properties of ferritic low density steels containing different amounts of carbon (0.0035 mass%, Steel 1 and 0.04 mass%, Steel 2) and aluminium (6.8 mass%, Steel 1 and 9.7 mass%, Steel 2) were investigated with respect to amount of ferrite, grain size and formation of Fe3Al intermetallic. The Steel 1 was deformed by cold rolling at room temperature and Steel 2 was deformed through warm rolling at 250°C to avoid prior cracks formation. After rolling, samples of Steels 1 and 2 were annealed at 900°C for 5 minutes. The yield strength, ultimate tensile strength and ductility increase while yield ratio decreases with annealing. This is due to formation of Fe3Al intermetallic and reduction of ferrite grain size. Due to high interstitial Carbon in Steel 2, yield point phenomenon was observed which may reduce the formability of the steel. The cracks started during tensile test at the ferrite grains and ferrite grain boundaries and the fracture took place in the quasi cleavage mode in Steels 1 and 2. The grain refinement and the microstructural features due to rolling and annealing are accountable for the good combination of strength and ductility of the both steels.

Improvement of tensile properties of low-carbon steels via short-time intercritical annealing

The effect of low volume fraction formation of martensite on the tensile properties of low carbon steel was evaluated. First, steel samples with ferrite-cementite microstructure were produced. The thermomechanical treatment used included austenitizing at 1000 °C and then quenching in ice brine solution, tempering the obtained martensitic structure for 1 h at 650 °C, 80% cold rolling, and re-tempering for 2 h at 650 °C. In order to form a low volume fraction of martensite, steel samples with ferrite-cementite microstructure were intercritically annealed for 30 seconds at 740 °C. As a result of intercritical annealing treatment, 6.2% martensite was formed. The results of tensile test showed that the formation of 6.2% martensite led to the elimination of yield point phenomenon and Lüders banding, decrease of yield stress and increase of true stress at maximum load, while true uniform strain did not change significantly. The work hardening rate also increased significantly. Based on the results of modeling of the flow behavior with the Holloman equation, the work hardening capability of the steel sample including ferrite-cementite decreased after a certain plastic strain, while the work hardening capacity remained constant with the formation of a low volume fraction of martensite in the microstructure.

The anomalous annealing hardening behaviors in commercial pure Tantalum foil

Annealing softening phenomenon is usually observed in pure metals. However, in this study, the anomalous annealing hardening behavior is found in commercial pure Tantalum (Ta) foil. When the cold-rolled Ta foils are annealed at a temperature lower than 700 °C, both the yield strength and ductility gradually increase with the rise in annealing temperature, resulting in an anomalous "low-temperature annealing hardening" phenomenon. The yield strength of Ta foils annealed at 700 °C reaches the maximum value of about 750 MPa. This phenomenon in Ta foil is closely related with the shear bands and the pinning behavior between interstitial atoms and dislocations. When annealing temperature is higher than 700 °C, the yield strength decreases. When annealing temperature exceeds 900 °C, recrystallization occurs in Ta foil. Anomalous yield point phenomenon appears in the recrystallization foils during stretching, which is also related with the pinning of dislocations by interstitial atoms. As annealing temperature further increases, the volume fraction of surface grains gets larger. And {112}<111> slip systems are preferred over {110}<111> slip systems in the surface grains.

Improvement of strength and ductility synergy and the elimination of the yield point phenomenon of ultrafine-grained Mg–Al–Zn–Ag alloy sheet

Strength and ductility synergy in ultrafine-grained AZQ310 alloy sheet was successfully improved at the expense of grain boundary segregation via multi-pass rolling. Before rolling, the as-extruded alloy has high yield strength (TYS, 382 MPa) but poor elongation (4.1%) mainly due to the ultra-fine grains (414 nm) and the co-segregation of Al/Zn atoms at grain boundaries. More importantly, this non-equilibrium segregation dies away during the rolling process, and the sheet exhibits a tensile yield strength of 370 MPa, an ultimate strength of 422 MPa, and an elongation of 9.8% along the rolling direction after 5th pass rolling. The results show that the high strength-ductility synergy is mainly attributed to the ultra-fine grains and residual dislocations. Furthermore, it is surprising that the yield point phenomenon (YPP) prominently in as-extruded sample is weakened and finally eliminated in as-rolled sheets. Microstructure evolution in the room temperature tensile test manifests that the underlying reason for the YPP is the stress-induced grain growth and grain boundary segregation, which in turn expounds the difficulty in the refining of ultrafine-grained Mg alloys by traditional processing methods.

Bimodal grain structure formation and strengthening mechanisms in Mg-Mn-Al-Ca extrusion alloys

The effects of small additions of calcium (0.1% and 0.5%11All compositions in weight percentage.) on the dynamic recrystallization behavior and mechanical properties of as-extruded Mg-1Mn-0.5Al alloys were investigated. Calcium microalloying led to the formation of Al2Ca in as-cast Mg-1Mn-0.5Al-0.1Ca alloy and both Mg2Ca and Al2Ca phases in Mg-1Mn-0.5Al-0.5Ca alloy. The formed Al2Ca particles were fractured during extrusion process and distributed at grain boundary along extrusion direction (ED). The Mg2Ca phase was dynamically precipitated during extrusion process, hindering dislocation movement and reducing dislocation accumulation in low angle grain boundaries (LAGBs) and hindering the transformation of high density of LAGBs into high angle grain boundaries (HAGBs). Therefore, a bimodal structure composed of fine dynamically recrystallized (DRXed) grains and coarse unDRXed regions was formed in Ca-microalloyed Mg-1Mn-0.5Al alloys. The bimodal structure resulted in effective hetero-deformation-induced (HDI) strengthening. Additionally, the fine grains in DRXed regions and the coarse grains in unDRXed regions and the dynamically precipitated Mg2Ca phase significantly enhanced the tensile yield strength from 224 MPa in Mg-1Mn-0.5Al to 335 MPa and 352 MPa in Mg-1Mn-0.5Al-0.1Ca and Mg-1Mn-0.5Al-0.5Ca, respectively. Finally, a yield point phenomenon was observed in as-extruded Mg-1Mn-0.5Al-xCa alloys, more profound with 0.5% Ca addition, which was due to the formation of (101¯2)extension twins in unDRXed regions.

Heterostructure effect at the interface of maraging steel deposited upon carbon steel via directed energy deposition

To study the mechanical behavior of the interface region that appears when dissimilar metals are deposited by additive manufacturing process, maraging steel was deposited on S45C carbon steel via directed energy deposition. A 3.3 mm-thick plate obtained from the deposition containing four different regions in the vicinity of the interface (substrate/HAZ/dilution layers/feed material) was subjected to uniaxial tensile deformation parallel to the interface. The strength of the layered structure exceeded the theoretical strength calculated through the rule of mixture, while elongation to failure was almost the same as the carbon steel and yield point phenomena disappeared. The synergetic heterostructure effect of the layered structure was attributed to the formation of strain gradient and back stress strengthening effect due to the mechanical incompatibility between each region in the layered structure. Applying 220 °C preheating reduced the layered structure's elongation by assigning different strengths and residual stresses to each layer. The current result suggested that the mechanical incompatibility between domains in the interface region, including the residual stress field distribution, should be carefully controlled to improve the mechanical behavior of the interface region.

Yield-Point Phenomenon and Plastic Bands in Ferrite–Pearlite Steels

Lüders deformation is one type of discontinuous yielding in ferrite-pearlite steel. The yield-point phenomenon and localized plastic bands are two features of the Lüders phenomenon. It is believed that the yield-point phenomenon is related to the formation of plastic bands, but the correlation between them is unclear. In this study, this correlation was investigated by examining the global and local deformation behaviors in the tension processes of four ferrite-pearlite steels (carbon content, 0.05-0.3%; pearlite fraction, 1.2-32%) via an extensometer and digital image correlation (DIC) technique. The main obtained results are as follows: (1) the degree of yield drop decreased with an increase in the pearlite fraction (the magnitude of the yield stress drop was 8.6-0 MPa), and (2) a plastic band was formed at a certain stress level smaller than the upper yield stress; when the stress level was larger than 92% of the upper yield stress, the upper yield point disappeared.

The yield point phenomenon in ultrafine-grained dilute Mg-Zn-Y alloys

The present work is focused on the yield behaviour in dilute Mg-Zn-Y alloy prepared by the rapidly solidified ribbon consolidation technique. This processing method results in ultrafine-grained microstructure (average grain size below 1 µm) with Zn- and Y-rich stacking faults segregated in the basal planes. Despite the high yield strength and moderate elongation, a pronounced yield point phenomenon is observed during tensile loading at room temperature. The global and local microstructural changes, leading to the appearance of the upper and lower yield points, are highlighted by advanced image techniques and post-processing data analysis.

Yield Point Phenomenon Induced by Twinning Accompanied by Abnormal Grain Growth in Fine-Grained Copper

In this study, an equiaxed fine-grained copper specimen was fabricated by powder injection molding, followed by hot isostatic pressing. The specimen showed a yield point phenomenon with a Lüders-type deformation during a uniaxial tensile test. This study aimed to identify the cause of the yield point phenomenon in the fine-grained copper specimen. First, the Lüders band was designed to partially propagate within the gauge section through an interrupted uniaxial tensile test equipped with a digital image correlation system. The microstructures of the Lüders and non-Lüders band regions were compared by electron backscatter diffraction. It was observed that self-annealing was accelerated by the applied stress in the Lüders band region. Consequently, the microstructure of the Lüders band region transformed into a bimodal structure composed of fine grains and abnormally coarse grains containing numerous large twins. Identification of active twin variants in the abnormally coarse grains suggested that a significant tensile strain can be accommodated by this twinning. Based on these results, it was confirmed that the strain burst generated by the sudden occurrence of numerous large twins induces the yield point phenomenon in the fine-grained copper.

Effect of nitrogen content on the mechanical properties and deformation behaviors of ferritic-pearlitic steels

The effect of nitrogen content (0.001 wt%, 0.009 wt%, 0.017 wt%) on the microstructure, mechanical properties and deformation behaviors of a medium-carbon ferritic-pearlitic steel is investigated in this study. The addition of nitrogen is shown to have little effect on the ferrite-pearlite microstructure and only marginally coarsens it because austenite grain growth is promoted. Although nitrogen is a strong element for solid solution strengthening, nitrogen addition only marginally improves the strength of the steel and has little effect on the ductility because fine Fe3N phases are precipitated during the tensile process. The toughness of the steel is markedly decreased by the addition of nitrogen because the dislocation locking effect caused by interstitial nitrogen atoms is enhanced. When the nitrogen content increases from 0.001 wt% to 0.017 wt%, the yield-point phenomenon in the tensile process is first strengthened and then weakened. The yield-point phenomenon is closely related to the dislocation behaviors in proeutectoid ferrite, and the different yield-point phenomena under different nitrogen contents are related to the dislocation locking effect caused by interstitial nitrogen atoms and to the dislocation pinning effect caused by Fe3N phases.

Investigation of the effect of initial states of medium-Mn steel on deformation behaviour under hot stamping conditions

Medium-Mn (MMn) steels have received much research attention recently because their low austenitisation temperature enables low-temperature hot stamping (LTHS). However, the effect of the initial state of the material on the hot stamping performance is still unknown. In this study, the effect of different initial states on the deformation behaviour of a typical MMn steel during uniaxial tensile testing under LTHS conditions (deformation at 500–600 °C under strain rates of 0.01–1 s−1) are investigated using a Gleeble 3800 materials simulator; the final mechanical properties after austenitising and quenching are also examined. The microstructure of each material state before and after the LTHS heating cycle is characterised using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Of the three states investigated the hot rolled and annealed (HRA) state shows the best hot deformation performance represented by larger strain hardening exponent and higher total elongation, followed by the cold-rolled (CR) state, with the cold-rolled and annealed (CRA) state exhibiting the worst performance. The final mechanical properties, however, are very similar among the three states. In addition, the yield point phenomenon is found during hot deformation in both the CR and CRA states, and absent in the HRA state. The hot deformation behaviour has been discussed in terms of differences in microstructural properties, namely the grain size and its degree of heterogeneity.

Investigation of deformation behaviour with yield point phenomenon in cold-rolled medium-Mn steel under hot stamping conditions

Medium manganese (MMn) steels are a promising class of advanced high-strength steels (AHSS) with potential for application as vehicle panels in the automotive industry. In this study, the deformation behaviour of a representative cold-rolled MMn steel and its dependence on processing parameters are studied using uniaxial tensile testing under low-temperature hot stamping (LTHS) conditions, covering austenitisation soaking times of 60–600 s, deformation temperatures of 500–700 °C and strain rates of 0.01–5 s −1 . A yield point phenomenon is observed for the first time in the early stage of deformation at such temperatures in this MMn steel, which has a submicron grain size. The extent of the yield point phenomenon is reduced, and strain hardening capability and total elongation are enhanced, with longer austenitisation soaking times, which give coarser-grained microstructures. The yield point phenomenon also tends to be weaker at higher deformation temperatures and lower strain rates; under these conditions, both the flow stress and the degree of strain hardening decrease, while the total elongation is insensitive to differences in deformation conditions. The mechanisms for the deformation behaviour and its dependence on test conditions are discussed. In addition, a set of unified constitutive equations is established and calibrated using the experimental data to predict the deformation behaviour of the MMn steel under uniaxial LTHS conditions, and close agreement with experiment, including the yield point phenomenon, is obtained.

Yield point phenomenon in low-alloyed TRIP-aided steel: role of retained austenite

Multiphase TRIP-aided steels are generally characterised by a continuous yielding behaviour. The processing route involving a deformation step in the intercritical (ferrite + austenite) region showed an undesirable discontinuous yielding. This is attributed to the formation of fresh dislocations in the soft ferrite matrix that trap the diffusing carbon and form Cottrell atmospheres. The discontinuous yielding disappears when fresh mobile dislocations are introduced through transformation of the retained austenite to martensite.