TWIP Steel Research Articles

Revealing extraordinary plasticity in single crystal Fe-15Mn-10Cr-8Ni-4Si alloy

An extraordinary elongation of over 156 % was achieved in single crystal Fe-15Mn-10Cr-8Ni-4Si alloy, which shows γ → ε → α’ transformation. Tensile tests were conducted along the 〈414〉, 〈111〉, and 〈100〉 directions, and the deformed microstructure was characterized with electron-backscattering diffraction (EBSD). Elongation in 〈414〉 orientation (156 %) was highest as ever recorded in metallic alloy—approximately twice that of conventional TWIP steels. Mechanical behavior in 〈414〉 is characterized with slow work hardening, with upward curvature in the stress–strain curve. EBSD analysis revealed 45 % of ε-martensite and significant γ-twin microstructure components in 〈414〉. In initial stage, deformation was marked by planar slip and a sluggish γ → ε transformation, activating a single ε-martensite variant with a Schmid factor of 0.5. Later stages witnessed crystal rotation towards 〈112〉, generating multiple shear bands and distinct ε variants. Fractures predominantly occurred along the 〈011〉 direction, with the fracture surface exhibiting a ductile nature.

Constitutive description of distortional hardening in a TWIP steel: Addressing differential hardening under nonlinear strain paths

The present study aims to describe the in-plane differential hardening behaviour of the twinning induced plasticity sheet metal TWIP980 under various stress states, including uniaxial tension, plane strain tension, and pure shear, particularly focusing on non-proportional loading conditions. The true stress–strain curves for each stress states were inversely obtained from their corresponding load–displacement curves and modeled using a differential hardening model that can accommodate all three stress states simultaneously on plastic work (density) contours. For non-proportional loading tests, oversize specimens were initially stretched under uniaxial tension up to a 10% pre-strain along the rolling, diagonal, and transverse directions, respectively. Subsequently, the three stress states were applied to subsize specimens cut from the deformed oversize specimens along the rolling direction. To describe the hardening behaviours during non-proportional loading, a homogeneous anisotropic hardening model was adopted and calibrated using two-step uniaxial tension tests. Subsequently, the differential hardening model was successfully incorporated into the homogeneous anisotropic hardening model to describe both the differential hardening and the strain path change-induced hardening behaviours under the two-step loadings, i.e., uniaxial tension to pure shear and uniaxial tension to plane strain tension. Both experimental and simulation results underscore the necessity to consider differential hardening under non-proportional loading conditions.

Microstructure evolution and work hardening behavior of medium/high carbon Fe–Mn–C TWIP steel during tensile deformation at 298 K and 223 K

In this paper, a comparative investigation was conducted on the tensile properties, microstructure evolution, and work hardening behavior of Fe18Mn0.6C and Fe18Mn1.0C TWIP steels at 298 K and 223 K by tensile tests and multi-scale microscopic characterization. With decreasing deformation temperature, two steels exhibit increased yield strength and ultimate tensile strength. However, the total elongation decreases for both, with Fe18Mn0.6C steel showing a more pronounced decrease. The work hardening behavior of two steels at 298 K and 223 K was analyzed. It is observed that the work hardening rate of both steels is higher at 223 K compared to 298 K, attributed to enhanced dislocation accumulation and the formation of deformation twins or martensite at lower temperatures. Particularly, Fe18Mn1.0C steel exhibits a continuous upward stage in work hardening at any temperature, driven by its higher dislocation density. On the other hand, Fe18Mn0.6C steel at 233 K shows a relatively stable trend in work hardening rate, attributed to the presence of martensite generated during deformation, which enhances its work hardening ability. Furthermore, the contribution of different strengthening mechanisms to the total flow stress was evaluated. The calculation results highlight dislocation strengthening as the primary source of flow stress in two steels at different temperatures. These findings provide theoretical support for understanding the mechanical properties of Fe–Mn–C TWIP steel at low temperature and optimizing the material design.

Modeling of distortional hardening including plane strain tension and pure shear for a TWIP steel

This paper focuses on calibrating and modeling of distortional hardening behaviours in twinning induced plasticity steels. True stress-strain curves for uniaxial tension, plane strain tension, and pure shear specimens are inversely identified from corresponding load-displacement curves. The study reveals that accurately predicting the hardening behaviours of TWIP980 steel under plane strain tension and pure shear stress states is challenging with an isotropic hardening model, and a negative hydrostatic effect for TWIP980 is observed through shear testing. A novel distortional hardening model is proposed to simultaneously accommodate the three stress states on the contours of plastic work. Coefficients of the distortional hardening model are calibrated at discrete levels of plastic work and then interpolated to describe the distortion of the initial yield surface. The model is then expanded to consider the true stress-strain curves under uniaxial tension along 0, 45 and 90-degree directions, as well as under the plane strain tension along the 0-degree direction simultaneously. This expansion explicitly incorporates the three true stress-strain curves under uniaxial tension, with the curve of plane strain tension captured by an evolutionary exponent related to plastic work. The developed distortional hardening models demonstrate reasonable reproduction of load-displacement curves for TWIP980 steel under uniaxial tension, plane strain tension, and pure shear stress states.

Notched Tensile Fracture of a Fe-15Mn-0.6C-2Al Twinning Induced Plasticity Steel at Room Temperature

The tensile fracture behavior of an Al-bearing TWIP steel was investigated by conducting a series of tensile tests on smooth and notched specimens with different notch geometries, focusing on the effects of evolution of the stress triaxiality and the effective strain during deformation. The flow curve and digital image correlation (DIC) analysis evidenced suppression of dynamic strain aging due to Al addition, and therefore, the effects of local inhomogeneous deformation associated with Portevin-Le Chatelier (PLC) band on fracture could be excluded. The smooth specimen fractured with negligible necking despite the absence of PLC bands. As a result, the effective strain was uniform through the gage section and the stress triaxiality (<i>η</i>) of ~0.33 was nearly unchanged over the entire cross-section up to the maximum load. This led to the fracture surface of the smooth specimen being entirely covered with fine equiaxed dimples. For notched specimens, the fracture strain was drastically reduced with decreasing notch radius, indicating the high notch susceptibility of the steel. The effective strain of the notched specimens was the highest at the edge of the notch root, regardless of the notch radius, so cracks first developed at the surface of the notch root. Although the <i>η</i> at the center of the notched specimens (0.40~0.48 depending on the notch radius) was higher than that of the smooth one, the center of the fracture surface of all notched specimens exhibited dimple features that were very similar to the smooth one, even in size. In contrast, in spite of the same <i>η</i> of ~0.33, fractography at the edge of the notched specimens revealed a fracture mode transition from dimple fracture to void sheet fracture to quasi-cleavage fracture as the notch radius decreased. The present results were rationalized in terms of the local evolution of stress triaxiality and effective strain during deformation, which were analyzed using the finite elemental method and DIC technique. It can be said that the fracture mode of TWIP steel, showing limited necking, was more influenced by the distribution and/or gradient of stress traiaxiality and effective strain rather than their local absolute values - that is, the severer their gradient is, the easier the quasi-cleavage fracture occurs.

Characterization of microstructures and mechanical properties of laser welded TWIP steel plate

In the present study, laser welding of medium-thick TWIP steel plates was studied and the microstructures and mechanical properties of the welded joints obtained at varied welding speeds were characterized. It is shown that a full penetration was achieved in the welded joint, which exhibits a keyhole-shaped configuration. The welded joint displays a single austenitic phase and typical solidification structure composed of cellular, columnar and dendrite grains. With increasing the welding speed, the grains in both the central fusion zone and adjacent heat-affected zone were refined. The latter was attributed to inadequate recrystallization because of decreased heating time at elevated welding speed. The mechanical properties of all welded joints were lower than those of base metal, in which the decline of elongation was more serious than that of strength. This is attributed to the formation of thick columnar crystals and the Al2O3 core-shell structured nanoscale precipitates in the grains of the fusion zone. Moreover, there existed a certain degree of non-uniformity in the microstructures and mechanical properties along the thickness direction of welding seam, and the samples in the upper area of welding seam showed better comprehensive properties than the lower ones. When welded at a moderate speed as 1.0 m/min, the welding seams exhibited the best mechanical properties and the least difference across the thickness direction of the welding seam. This is because the moderate welding speed resulted in the most beneficial macro- and micro-structures as well as grain boundary characteristics.

Microstructure and Mechanical Properties of a New TWIP Steel under Different Heat Treatments.

The effects of solution treatment and annealing temperature on the microstructure and mechanical properties of a new TWIP steel that was alloyed from aluminum (Al), silicon (Si), vanadium (V), and molybdenum (Mo) elements were investigated by a variety of techniques such as microstructural characterization and room tensile testing. The austenite grain size grew slowly with the increase in annealing temperature. The relatively weak effect of the solution treatment and annealing temperature on the austenite grain size was attributed to the precipitation of MC and M2C, which hindered the growth of the austenite grain. The plasticity of the TWIP steel in cold rolling and annealing after solution treatment was obviously higher than that in cold rolling and annealing without solution treatment. This was because the large-size precipitates redissolved in the matrix after solution treatment, which were not retained in the subsequently annealed structure. Through cold rolling and annealing at 800 °C after solution treatment, the prepared steel exhibited excellent strength and plasticity simultaneously, with a yield strength of 877 MPa, a tensile strength of 1457 MPa, and an elongation of 46.1%. The strength improvement of the designed TWIP steel was mainly attributed to the grain refinement and precipitation strengthening.

Study on the Low Plastic Behavior of Expansion Deformation of Austenitic Stainless Steel.

The plastic deformation of TWIP steel is greatly inhibited during the expansion process. The stress-strain curves obtained through expansion experiments and observations of fracture morphology confirmed the low plastic behavior of TWIP steel during expansion deformation. Through an analysis of the mechanical expansion model, it was found that the expansion process has a lower stress coefficient and a faster strain rate than stretching, which inhibits the plasticity of TWIP steel during expansion deformation. Using metallographic microscopy, transmission electron microscopy, and EBSD to observe the twin morphology during expansion deformation and tensile deformation, it was found that expansion deformation has a higher twin density, which is manifested in a denser twin arrangement and a large number of twin deliveries in the microscopic morphology. During the expansion deformation process, dislocation slips are hindered by twins, the free path of the slips is reduced, and dislocations accumulate significantly. The accumulation area is the initial point of crack expansion. The results show that the significant dislocation accumulation caused by the delivery of a large number of twins under expansion deformation is the main reason for the decrease in the plasticity of TWIP steel.

Effect of annealing temperature on carbide precipitation, microstructure and mechanical properties of Fe-Mn-C- (Cr, Mo, W, Ni, Nb, Ti, V) multi-alloying TWIP steel

The effect of annealing temperature on the microstructure, carbide precipitation and mechanical properties of a cold-rolled multi-alloying TWIP steel were investigated by means of Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) analysis, tensile and impact tests. As the temperature increases from 400 °C to 800 °C, the amount of carbides climbed to the peak at 700 °C and then decreased, having a strong opposite relationship with the impact energy, which was minimum 26 J at 700 °C. The main distribution of carbides underwent a transition from intragranular, intragranular + intergranular to intergranular, and the composition of carbides changed from (Nb, Ti and W) to (Cr, Mo and W). Compared with the cold-rolled sample, Fe21W2C6 precipitated out at low annealing temperature of 400 °C, when the tensile and yield strengths and elongation were all simultaneously improved due to the precipitation recovery annealing. And the 600 °C annealing sample exhibited identical yield and tensile strengths. Subsequently, the yield strengths decreased but the tensile strengths and elongation increased at 650 °C and 700 °C attributed to the interaction of carbide precipitation and recrystallization. The recrystallization started at 650 °C and finished at 750 °C. And Fe3Mo3C was identified at 750 °C.

Delving into the intrinsic co-relation between microstructure and mechanical behaviour of fine-/ultrafine-grained TWIP steels via TEM and in-situ EBSD observation

To illustrate the microstructural factors of grain refinement for enhancing mechanical properties, the fine-/ultrafine-grained TWIP steels with a product of strength and elongation of ∼71 GPa·% were first prepared by combining rolling and stress relief annealing. Subsequently, the evolution of dislocations, stacking faults, and associated substructures of the fine-/ultrafine-grained TWIP steels was analysed by using in-situ EBSD tensile tests and TEM characterisation of the interrupted strain experiments. The results reveal that the excellent mechanical properties of the TWIP steels are attributed to dislocations and associated dislocation cells, dislocation walls, dislocation tangles, stacking faults and associated Lomer-Cottrell locks (LCs), nano-twins, primary and secondary twins and their interactions during plastic deformation. The density of geometrically necessary dislocations (GNDs) was evaluated based on the modified Ashby's model and compared with experimental results, indicating that grain size heterogeneity can promote the accumulation of GNDs, which facilitates the generation of subgrains and new boundaries to reduce the mean free path (MFP) of dislocations, thus enhancing strain hardening. Meanwhile, the interaction of lamellar primary and secondary twins in fine grains and the generation of stacking faults and nano-twins in ultrafine grains at higher strains can further promote strain hardening to elevate strength. Furthermore, the effects of grain orientation and grain size on the activation and evolution of dislocations and twins were elucidated. In ultrafine grains, twinning is strongly inhibited due to the elevated critical shear stress for twinning, resulting in more stacking faults and nano-twins, but fewer dislocation cells. The present work contributes to an in-depth understanding of the mechanical properties of fine-/ultrafine-grained materials to exploit their potential for industrial applications.

Microstructural Characterization and Corrosion Behavior of Similar and Dissimilar Welded Advanced High-Strength Steels (AHSS) by Rotary Friction Welding.

Advanced high-strength steels (AHSSs) are designed for meeting strict requirements, especially in the automotive industry, as a means to directly influence the reduction in the carbon footprint. As rotary friction welding (RFW) has many important advantages over other welding technologies, it plays an important role in the automotive sector. On the above basis, in this work, combinations of the first (complex phase (CP)), second (TWIP (TW)), and third (quenched and partitioned (Q&P)) generations of similar and dissimilar high-alloyed advanced steels have been joined by the RFW process. Having a specific microstructure, rods of CP/CP, Q&P/Q&P, CP/TW, and Q&P/TW steels were welded by employing a homemade adaptation machine under fixed parameters. Microstructural characterization has allowed us to corroborate the metallic bonding of all the tested advanced steels and to identify the different zones formed after welding. Results indicate that the welding zone widens in the center of the workpiece, and under the current friction action, the intermixing region shows the redistribution of solute elements, mostly in the dissimilarly welded steels. Furthermore, because of their complex chemistry and the different mechanical properties of the used steels, dissimilarly welded steels present the most noticeable differences in hardness. The TWIP steel has the lower hardness values, whilst the CP and Q&P steels have the higher ones. As a direct effect of the viscoplastic behavior of the steels established by the thermomechanical processing, interlayers and oxidation products were identified, as well as some typical RFW defects. The electrochemical response of the welded steels has shown that the compositional and microstructural condition mostly affect the corrosion trend. This means that the dissimilarly welded steels are more susceptible to corrosion, especially at the TWIP-steel interface, which is attributed to the energy that is stored in the distorted microstructure of each steel plate as a consequence of the thermomechanical processing during RFW.

Структурно-фазові характеристики та механічні властивості TWIP-сталі Fe-24Mn-11Al-1,4C

TWIP steels belong to a number of modern innovative structural materials that combine a high level of mechanical characteristics and low density, which brings their specific strength closer to the level of titanium alloys. Such steels usually do not contain deficient components and they are suitable for wide commercial industrial production. The most high-strength alloys include around 20-30 % Mn (wt.), around 10 % (wt.) Al and 1% (wt.) C. Such chemical composition brings them closer to the three-phase region, in which austenite, ferrite and k-carbides can simultaneously exist. According to this, it is possible to change mechanical characteristics of such alloys in a wide range only by heat treatment. Although in practice the best results are shown by a combination of deformation and heat treatment. The Manuscript presents a study of changes in the structural and phase characteristics and mechanical properties of the Fe-24Mn-11Al-1,4C alloy, prepared by electroslag melting, in its initial state, after deformation and annealing, and after deformation with subsequent hardening and aging. A fractographic analysis of the studied samples destruction places was also carried out. It is shown that the original sample obtained by electroslag remelting has the highest plasticity and the lowest strength, which is explained by combination of largest alloying of austenite and as-cast microstructure. The deformed and annealed sample is characterized by a pronounced fine-grained structure in which carbides are separated along the grain boundaries. Such a sample exhibits a strength greater than 1.1 GPa and the lowest ductility of 4.26 %. The deformed sample after quenching and aging has a similar structure, but the carbide inclusions are much finer, and the alloying rate of austenite is higher than the previous one. Such a sample shows the highest strength of more than 1.2 GPa and plasticity of 8.6 %. Keywords: TWIP-steels, casting, deformation and heat treatment, structural and phase characteristics, fractography, mechanical properties.

Experimental and numerical determination of the temperature of TWIP steel during dynamic tensile testing

One of the most important factor affecting properties of TWIP steels is the heat generated from plastic work during deformation. As the significant temperature rise can lead to a reduction or cessation of the main deformation mechanism – twinning. Nevertheless, the determination of the exact temperature value appears to be especially difficult – mainly under dynamic conditions. The state of knowledge in this subject is not extensive, while in the case of deformation under dynamic conditions it is completely limited. That is why this paper presents the attempt to determine the amount of heat generated during deformation of the austenitic high-Mn TWIP steel – Fe–21Mn-2.55Al (wt.%) at the strain rate of 170 s−1. Firstly, the complexity of the issue was discussed. Then, two methods such as IR thermography and FEM were applied to determine this temperature. Moreover, the DIC technique was used to calculate the elongation of the sample and the strain rate obtained with the use of flywheel machine. The investigated steel exhibited a maximum true stress of ca. 1000 MPa, the deformation limit reached over 0.4, while the temperature in the necked region exceeded 200 °C. Similar temperature values in the necked area were obtained by means of FEM. Unfortunately, the IR measurements at relatively high frame rates can only be achieved for sub-window mode (reduced area) and a limited temperature range, while in the FEM necking starts when the sample is artificially narrowed. Both mentioned methods can be applied for determination of the temperature, but awareness of their limitations is essential.

Strain rate-dependent plastic behavior of TWIP steel investigated by crystal plasticity model

A strain rate-dependent crystal plasticity finite element (CPFE) model was developed to describe the plastic behavior of TWIP steel over a wide range of strain rates, encompassing quasi-static to dynamic loadings. The developed model captures the effect of strain rate on the mechanisms governing dislocation motion and temperature rise. The mechanical behavior and underlying micro-mechanisms of TWIP steel during compression at different strain rates were then systematically investigated by combining the CPFE model with experimental tests. The results demonstrate that the developed model can accurately predict the mechanical response of TWIP steel over a wide range of strain rates from 10−3 s−1 to 104 s−1. It was observed that the yield stress presents a gradual increase with increasing strain rates below a threshold value of applied strain rate, beyond which the yield stress increases sharply. The stress drop after yielding under high strain rates is found to be attributed to the decreased viscous drag stress by increased dislocation density. Further quantitative analysis results show that the contribution of deformation twinning to the macroscopic flow stress is negligible for both high and low strain rates, and the strain hardening behavior of TWIP steel is mainly dominated by the forest dislocation stress.

Effect of Heat Treatment on Microstructure and Mechanical Properties of Cold-Rolled 17Mn-1.58Al TWIP Steel

Effect of Heat Treatment on Microstructure and Mechanical Properties of Cold-Rolled 17Mn-1.58Al TWIP Steel

Effect of Cr content in temperature-dependent mechanical properties and strain hardening of a twinning-induced plasticity steel

This study aims at assessing and analyzing the effect on the mechanical behavior at temperatures between 25 °C and 350 °C of the Cr additions (5 % and 10 %) to a TWIP steel (Fe–22Mn-0.6C). We conducted tensile tests and characterized the microstructure after deformation by light microscopy, X-ray diffraction (XRD) and FEG-SEM-EBSD. Different temperature-dependent strengthening mechanisms were determined. Mechanical twinning, Dynamic Strain Aging (DSA), and slip bands were the predominant mechanisms, with a combination of them depending on the stacking fault energy (SFE). The SFE decreases with the chromium content and increases with temperature, influencing the percentage of mechanical twinning generated during plastic deformation and consequently, affecting the strain hardening rate (SHR). There exists a direct relationship between strain hardening rate with temperature of deformation and mechanical twinning. DSA was observed and its effect on strain hardening was analyzed in detail.

Integrity evolution during dissimilar welding of TWIP steel

The integrity evolution induced by manufacturing involving dissimilar TWIP and mild steel weld spots was investigated. Focus was given to the effect of manufacturing parametrization on controlling the dilution of the alloying elements and the resulting weld integrity. Samples manufactured with different conditions were characterized by chemical and phase composition, morphology, and mechanical properties. The findings showed that the distribution of chemical composition and metallurgical features are sensitive to the welding parameters. The influence of manufacturing on weld morphology was noticed. Manganese distribution is affected by the welding cycle, thus leading to austenite destabilization and brittle behavior upon a quasi-static tensile shear strength test. A processing set was proposed to control manganese dilution and martensite transformation.

Achieving superplastic TWIP steel welded joint via vacuum electron beam welding

A crack-free high-Mn TWIP steel welding joint with comparable strength and superplastic was obtained via vacuum electron beam welding (VEBW) where the microstructural evolution, mechanical behaviors and strengthening mechanism of welding joint were investigated. The results indicated that the welding joint with a typical nail-shaped welding cross-section was divided into three zones including the base metal (BM), heat-affected zone (HAZ) and fusion zone (FZ). Massive fine AlN-type precipitates with the hexagonal crystal structure were observed in welding join. The orientation relationship between the γ-austenite matrix and AlN precipitates was determined as [001]γ//[2110]AlN. In FZ, small-sized cellular grains or columnar grains with consistent growth directions were combined into large-sized columnar grains with obvious <001> preferential orientation. Compared with the base mantal, the yield strength, ultimate strength and elongation of the VEBWed specimen were slightly reduced by 6.03%, 4.7% and 13.23%, respectively. However, the impact toughness was obviously decreased from the 87.90 J–68.97 J after the VEBW process, which was mainly attributed to the brittle AlN precipitates, inhomogeneous microstructure and back stress induced by the plastic mismatch between gradient grains. The strength contributions of different regions to overall strength including of BM, HAZ and FZ in welding joint were calculated. Here, the dislocation strengthening and precipitation strengthening were the dominant strengthening mechanisms of FZ, which was attributed to the high level of density and massive fine AlN precipitates induced by micro-elements segregations during the VEBW process.

Effect of Heat Treatment on the Mechanical Properties of Cold-worked TWIP Steel

Effect of Heat Treatment on the Mechanical Properties of Cold-worked TWIP Steel

Microstructure, strain hardening behavior, segregation and corrosion resistance of an electron beam welded thick high-Mn TWIP steel plate

In the present work, the microstructure, strain hardening behavior, segregation and corrosion resistance of a high-Mn TWIP steel joint fabricated by electron beam welding (EBW) method were investigated. It was observed that in the fusion zone (FZ), columnar dendritic structure and cellular structure were formed and the texture component of ⟨100⟩∥X-axis was developed. Annealing twinning events hardly occurred during the solidification of weld metal since few Ʃ3 and Ʃ9 grain boundaries existed in the FZ. The 0.2% yield strength (YS) of the joint is comparable to that of the base metal (BM). Deformation twinning played an important part in increasing the strain hardening rate. C, Mn and Si elements were found to segregate in the interdendritic regions, nevertheless, Al element was discovered to concentrate in the centres of the dendrites. The microsegregation of alloying elements and the formation of (Fe,Mn)3C cementites increased the corrosion sensitivity of interdendritic regions, which led to the occurrence of preferential corrosion in these regions and then reduced the corrosion resistance of the joint.