Twinning-induced Plasticity Steel Research Articles

Effects of cooling process on the microstructure and mechanical properties of a novel high-V high-Mn TWIP steel for cryogenic storage tank applications

In the present work, a novel high-V high-Mn twinning induced plasticity (TWIP) steel for cryogenic storage tank applications was developed. Furthermore, innovative two-stage cooling processes encompassing ultra-fast cooling and the subsequent furnace cooling were proposed. Effects of cooling process on the microstructure and mechanical properties of the experimental steel were investigated. It was observed that with the change in cooling process (the initial temperature of furnace cooling rised gradually), the dwell time of hot-rolled plates above 600 °C was extended. The extension in dwell time facilitated the precipitation of secondary phase particles, leading to the change in average diameter and volume fraction of secondary phase particles. The yield strength increased with the increasing initial temperature of furnace cooling due to the combined effect of small-sized secondary phase particles inside the grains and large-sized ones located at the grain boundaries. The occurrence of intergranular cracking, primarily caused by the presence of large-sized secondary phase particles at the grain boundaries, was the main factor responsible for the significant reduction in total elongation. The small-sized secondary phase particles inside the grains inhibited deformation twinning, while the large-sized ones at the grain boundaries weaken the binding force between grain boundaries, which ultimately resulted in a decrease in cryogenic impact toughness. The implementation of two-stage cooling processes effectively mitigated the trade-off between yield strength and cryogenic impact toughness, offering a novel approach and valuable insights for optimizing the strength and toughness characteristics of cryogenic high-Mn steels.

Discovery of microsegregation-aided transformation and twinning-induced plasticity in low Mn steel through directed energy deposition of functionally graded materials

Functionally graded materials (FGMs) combining two dissimilar steels, stainless steel 316 L and high-strength low-alloy steel, were additively manufactured using directed energy deposition. High-throughput characterization of the dissimilar steel FGM led to the discovery of a low Manganese (Mn<1 wt.%) TRIP (transformation-induced plasticity) and TWIP (twinning-induced plasticity) steel with a metastable microstructure enabled by additive manufacturing. Microsegregation from non-equilibrium solidification caused phase stability and stacking fault energy heterogeneity, leading to TRIP and TWIP effects in the as-built condition without heat treatment. Tensile testing of the new as-built TRIP and TWIP steel resulted in ultimate tensile strength of 960 MPa, yield strength of 415 MPa, total elongation of 26 %, and a unique strain hardening rate that increases after yielding. We compare experimental measurements of microsegregation with thermodynamic modeling to discuss the impact of microsegregation on phase stability, stacking fault energy, and solidification cracking susceptibility in additively manufactured FGMs. The highlights of this work include the discovery of a novel pathway for achieving TRIP and TWIP effects in additively manufactured steels without heat treatment. This work also shows that the TWIP effect can be introduced without high Manganese content playing a critical role in adjusting stacking fault energy.

Effect of Al on the low cycle fatigue deformation behavior and microstructure evolution of Fe–22Mn-0.6C TWIP steel

The dependence of low cycle fatigue (LCF) behavior and microstructure evolution of Fe–22Mn-0.6C twinning-induced plasticity (TWIP) steel on the alloying element Al has been investigated under fully reversed total strain amplitude control with a nominal strain rate of 0.006 s−1 and strain amplitudes ranging from 0.002 to 0.010 at room temperature. The results indicate a strong dependence of LCF response and evolved microstructure on the addition of Al, owing to its effect on the stacking fault energy (SFE). At all strain amplitudes, the 3Al steel with a higher SFE exhibits a visibly lower cyclic hardening and a higher cyclic softening, resulting in a lower cyclic yield strength than the 0Al steel. As compared with the 0Al steel, the relatively higher SFE of the 3Al steel causes higher accumulated plastic strain, decreased slip planarity and weaker reversibility of dislocations, thereby decreasing the fatigue life. The optical microstructure is featured by increase in slip band density with increasing strain amplitude for both steels. However, the slip band density of the 3Al steel is higher than that of the 0Al steel. Moreover, cyclic loading promotes the formation of dislocation cells, rough labyrinths as well as planar dislocation arrays. Deformation twins are observed only in the 0Al steel with a visible lower critical twinning stress due to its low SFE as compared to the 3Al steel.

Strain-induced solid-state coating of TWIP steel sheets with zinc

We propose the strain-induced solid-state coating process of twinning-induced plasticity (TWIP) steel with zinc, achieved through concurrent rolling of TWIP steel and zinc sheets. Our study explores the effects of strain, strain rate, and rolling temperature on the morphology and intermixing at the Zn-TWIP steel interface. We found that two simultaneous mechanisms are responsible for the formation of a joint interface between the sheets in the rolling temperature range of 100–380 °C, namely mechanical intermixing at the interface and accelerated diffusion along moving dislocations and other defects introduced during rolling. Since it is impossible to deconvolute these mechanisms, we introduced the concept of an ‘apparent diffusion coefficient’ to represent the diffusion-like mass transfer resulting from mechanical intermixing and diffusion. These diffusion coefficients were determined by primary electron energy variation method in scanning electron microscope and by energy-dispersive X-ray spectroscopy in transmission electron microscope. Notably, these diffusion coefficients were found to be higher than the grain boundary diffusion coefficients of Zn in γ-Fe, primarily due to the dynamic nature of interface microstructure formation during rolling.

Atomic-scale understanding of twin intersection rotation and ε-martensite transformation in a high Mn twinning-induced plasticity steel

Twin intersections have been reported to significantly impact on macroscopic properties by relaxing localized stress concentration and accommodating strain. Here the atomic-scale structural characterization of the twin intersection region in a deformed high Mn twinning-induced plasticity (TWIP) steel was conducted using high-resolution transmission electron microscopy. Detailed microstructural features were revealed at the twin intersection region, including twin intersection rotation, low angle grain boundary and ε-martensite. The twin intersections were observed to preserve the face-centered cubic structure, which exhibits deviation angles of ∼0–15o with respect to the barrier twin depending on the localized stress concentration. Interestingly, the ε-martensite was observed in the vicinity of the twin intersection, which shows a wedge shape, distinct from the usual plate-like deformation-induced martensite. Therefore, we identified here a novel stress relaxation mechanism at the twin intersection region. Based on the detailed microstructural characterization of the twin interaction region at the atomic scale, the fundamental dislocation mechanisms of twin transmission and interaction were discussed. The research thus advances the understanding of twin intersection rotation and ε-martensite transformation by twin-twin interactions.

Cross-slip of extended dislocations and secondary deformation twinning in a high-Mn TWIP steel

Twinning-induced plasticity (TWIP) steels have become important materials in industry owing to the good combination of strength and ductility and high strain hardening rate. The excellent mechanical properties are highly related to the glide of dislocation and deformation twinning. However, the cross-slip behavior of the extended dislocation and the mechanism of deformation twinning are still controversial. Here, the partial dislocation motion and austenite twinning of a high-Mn steel at the early stage of deformation were investigated using in-situ tensile transmission electron microscope (TEM) technique. Results show that a large number of plane glide and cross-slip of extended dislocations can occur at the early stage of deformation. Extended dislocation nodes can be formed as a result of the reaction between adjacent extended dislocations on the same glide plane. In-situ tensile TEM experiments confirm two cross-slip models of partial dislocation: (1) the Friedel-Escaig model, cross-slip based on constriction of extend dislocation and re-dissociation; (2) the Fleischer model, cross-slip involving Lomer-Cottrell dislocation. Based on experimental results and energy calculations, it can be confirmed that the formation mechanism of austenite primary deformation twin induced by partial dislocations is different from that of secondary deformation twin. Grain boundary emits partial dislocation into the grain to form stable stacking fault which induces austenite primary deformation twin. The formation of secondary deformation twin is related to cross-slip of extended dislocation. Only the cross-slip of an extended dislocation containing a 90° partial dislocation can induce the formation of secondary deformation twin by introducing the Frank partial dislocation.

Experimental study on low cycle fatigue behavior of TWIP steel under cyclic shear loading condition

This study investigated the low-cycle fatigue (LCF) behavior of Twinning induced plasticity (TWIP) steel based on the twin bridge shear specimen and cyclic shear loading condition. The cyclic shear experiments were conducted with different strain amplitudes, and we compared their softening and hardening behaviors, cyclic stress-strain curves, and fatigue life. Through studying the crack morphology and the damage mechanism of shear specimens the role of cyclic shear stress on the damage evolution of TWIP under cyclic shear conditions was shown. The results indicate that the cyclic hardening and softening capacities of TWIP steel are highly affected by the strain amplitude. With increasing the load amplitude, the hardening rate increases, and its fatigue life is negatively correlated with the strain amplitude. Meanwhile, the hardening rate is positively correlated with the strain amplitude, and the softening rate is negatively correlated. The magnitude of strain amplitude affects the ratio of cyclic softening and cyclic hardening periods. Through analyzing the obtained microstructure characteristics, it is found that there is a high density of dislocations around the crack zone which causes the final failure of TWIP.

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.

Superior energy absorption characteristics of additively-manufactured hollow-walled lattices

Designing innovative lightweight architected materials with enhanced mechanical properties and superior energy absorption is a long-term pursuit to protect against impact-induced damage. Compared with dense-walled lattices, innovative engineering designs indicate that hollow-walled lattices exhibit unprecedented combinations of mechanical properties and functionalities with minimal weight. This paper proposed a novel hollow-walled lattice structure with variable cross-sections that can improve energy absorption significantly. A series of hollow-walled lattices were additively manufactured by the selective laser melting technique with the twinning-induced plasticity steel. The quasi-static uniaxial compressive tests were conducted to study the mechanical property and energy absorption capacity. Experimental results indicated that the hollow-walled lattices with a constant cross-section and those with a variable cross-section can improve the specific energy absorption by 29.2 % and 135.6 % compared with the conventional dense-walled lattices. Besides, finite element models of hollow-walled lattices were established by using the FE package LS-DYNA to investigate the effects of various structural parameters on mechanical properties and energy absorption capacity. A comprehensive parametric analysis based on the validated finite element models shows that the maximal ratio of the diameter to thickness in hollow-walled lattices leads to the optimal energy absorption capacity for the lattices with the same relative density. In addition, three typical tailored parameters, including the necking ratio, the necking local coefficient and the ratio of the thickness to diameter of variable struts, were tailored to achieve a better energy absorption capacity of hollow-walled lattice structures. Finally, the novel proposed hollow-walled lattice with a variable cross-section was also applicable in other lattices to enhance energy absorption capacity.

Simulation and Microstructural Analysis of Twin-Induced Plasticity Steel Cylinder Deep Drawing.

This study investigated the stress-strain behavior and microstructural changes of Fe-Mn-Si-C twin-induced plasticity (TWIP) steel cylindrical components at different depths of deep drawing and after deep drawing deformation at various positions. The finite element simulation yielded a limiting drawing coefficient of 0.451. Microstructure and texture were observed using a scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The research revealed that the extent of grain deformation and structural defects gradually increased with increasing drawing depth. According to the orientation distribution function (ODF) plot, at the flange fillet, the predominant texture was Copper (Cu){112}<111> orientation; at the cylinder wall, the main textures were Copper Twin (CuT) and Goss (G) orientations; at the rounded bottom corner of the cylinder, the primary texture was τ-fiber (<110>//TD), with its strength increasing with deeper drawing.

Deformation behaviour of a cast Fe–19Al–5Cr-0.04Zr alloy at room and intermediate temperatures

The mechanical properties of a Fe–19Al (at.%) alloy containing 5 at.% Cr as the main alloying element are presented and discussed. While the alloy behaves brittle at room temperature and 100 °C, tensile stress-strain curves at temperatures between 200 °C and 500 °C show strain hardening recovery effects similarly to those of fcc twinning induced plasticity (TWIP) steels. It is supposed that a combination of dislocation glide and formation of internal boundaries in the submicron range leads to similar strain hardening stages at intermediate temperatures (200 °C–500 °C) in the FeAl alloy as usually observed in TWIP steels at room temperature. The maximum strain hardening rate after recovery reaches very high values of about G/19 at 400 °C. Low-cycle fatigue (LCF) tests at 400 °C and 500 °C up to strain amplitude values of 0.6% and stress-relaxation tests do not reveal high back stresses or pseudoelasticity in the FeAl alloy. This indicates that the deformation structures responsible for the strain hardening rate recovery remain stable upon stress relief.

Strong resistance to Zn-assisted liquid metal embrittlement of austenitic-TWIP/martensitic-HSLA multi-layered steel sheets additively manufactured by laser cladding

As a novel design idea to overcome a shortcoming of liquid metal embrittlement (LME) occurred frequently in twinning-induced plasticity (TWIP) steel, multi-layered steel (MLS) sheets consisting of austenitic TWIP and high-strength low-alloy (HSLA) steel grades have been suggested. Two- and four-time clad MLS sheets (C2 and C4) were fabricated by a powder-fed laser additive manufacturing (AM) process using HSLA powders on the TWIP surface, and the LME was investigated in various resistance-spot-welding-time stages. The C2 and C4 sheets consisted of the austenitic TWIP substrate and tempered-martensitic HSLA-clad layer, and the HSLA layer had two or four sublayers having different C and Mn contents reduced step-wise from the TWIP/HSLA interface. LME cracks formed in the C2 spot-welded for 240 or 380 ms, whereas they did not form at all in the C4 spot-welded for 380 ms. This different LME susceptibility depended mainly on the HSLA-sublayer microstructures beneath the Zn coating after their high-temperature exposure during the spot welding. Since LME cracks propagated mainly along prior austenite grain boundaries, the whole austenite existed in the austenite range played a key role in determining whether the LME cracking occurred or not. The total volume fraction of reverted austenite (appeared as fresh martensite at room temperature) and retained austenite was very high at about 68% in the C2 welded for 240 ms, thereby leading to the LME cracking. It was lower than 20% in the C4 even under the severe welding condition of 380 ms, which provided the reason for no LME cracking.

Investigation of dynamic tensile mechanical responses and deformation mechanism at high strain rates in a TWIP steel

In view of the potential demand for the materials with excellent dynamic mechanical behavior in protective construction, the dynamic tensile mechanical properities and deformation mechanism of twinning-induced plasticity (TWIP) steel at various high strain rates were investigated in this paper by combining test research and numerical simulation. In order to explore the impact of strain rates on mechanical behaviors within the tested range, a split Hopkinson tensile bar (SHTB) was utilized to measure the mechanical responses of TWIP steel ranging from 1000 s−1 to 6500 s−1. The Johnson–Cook (J-C) dynamic constitutive model was established based on the mechanical test results, and the mechanical properties were studied by using ANSYS/LS-DYNA. EBSD and TEM were employed to investigate the microstructure evolution of TWIP steel under dynamic loading. The results demonstrate that both strength and plasticity increase with increasing strain rate, demonstrating positive strain rate sensitivity (PSRS) which is primarily due to the activation of multiple twin systems at higher strain rates. As the strain rate increases, the mean free path of dislocation slip reduces, causing a higher strain hardening rate. The critical shear stress required for twinning is almost unchanged with an increase in strain rate, while the shear stress required for dislocation increases consistently. As the strain rate exceeds 3000 s−1, the critical shear stress required for dislocation becomes greater than that for twinning. With increasing strain rate, the proportion of secondary twins in TWIP steel rises and the deformation twins are bent when the strain rate is 6500 s−1 owing to the effect of dislocations and stacking faults. The production of nanoscale twins at higher strain rates is a beneficial phenomenon, as it enhances both strength and plasticity. The deformation twins become thinner with an increase in strain rate, leading to a rise in the stress needed for the motion of dislocations to pass the twins, which contributes to the improvement of strength in TWIP steel. The slip systems that deforms at a disadvange have new orientations because of defromation twins. The established J-C model is appropriate well for analyzing the mechanical properties of TWIP steel in the tested range of strain rates accoring to the fingdings of test and numerical simulation. Establishing the connection of microstructure and macroscopic behavior of TWIP steel could provide a guidance for the its further study and the application in the fields of impact resistance and anti-explosion.

Hot working processing and microstructure characterisation of as-cast high manganese TWIP steel

ABSTRACT The hot compression of the as-cast Fe-21Mn-0.7C–0.1Si twinning induced plasticity (TWIP) steel was investigated at the deformation temperature range of 950–1100 °C and strain rates of 0.01–5 s−1, using a GleebleTM thermo-mechanical simulator, to determine the hot deformation behaviour and analyse the recrystallisation mechanism. The results showed that the flow curves were characterised by yield-point-elongation (YPE) and dynamic recrystallisation (DRX) as the principal restoration mechanisms. The activation energy (Q) was calculated to be 394.975 KJ mol−1, which implies that the recrystallisation is sluggish in the whole range of the deformation conditions. Furthermore, the processing maps were generated according to the dynamic material model (DMM). The processing map was subdivided into different domains for the microstructural observations. The ideal hot deformation parameters of as-cast high Mn TWIP steel were obtained in the deformation condition range of 1087–1098 °C/10−1.2-10−0.17 s−1. Moreover, the microstructure analysis revealed that the DRX grains were twinned and nucleated through the bulging mechanism at the serrated grain boundaries, accompanied by twin boundaries migration created in the DRX grains by growth accidents, which contributed significantly to the growth of the DRX.

Production of Ultrahigh‐Strength and High‐Ductility TWIP Steel by Precipitation of β‐Mn during Large Deformation during Warm‐Rolling

Herein, twinning‐induced plasticity (TWIP) steel having large deformation is rolled at different rolling temperatures to improve the tensile strength and retain a certain plastic deformation capacity. Based on X‐ray diffraction and transmission electron microscope analysis, β‐Mn is found as the precipitate at the grain boundary during the warm‐rolling process (500–650 °C). To investigate the impact of β‐Mn on the tensile properties, the microstructure of the TWIP steel rolled at the temperature value of 600 °C is observed by carrying out electron backscatter diffraction and scanning electron microscope measurements. The intergranular β‐Mn phase can help the material to accumulate geometric necessary dislocation (GND) density, inhibit crack propagation, as well as improve the strength and plasticity of the material. Once TWIP steel is warm‐rolled above the temperature value of 600 °C, and serrated flow appears in the tensile process, which is also conducive to improving the material properties.

Microstructural evolution and deformation behavior of friction stir welded twin-induced plasticity steel

The weldability of twin-induced plasticity (TWIP) steel with ultra-high strength via friction stir welding (FSW) technique was investigated, and microstructural evolution and deformation behavior of whole and micro-zones of FSW TWIP joint were studied for the first time. The results showed that the content of recrystallized grains in the stir zone (SZ) increased from 10.5% of basal material (BM) to 14.2%, and that of heat affected zone (HAZ) increased to 78.6%. The percentage of annealing twins decreased from 26.8% in BM to 11% in SZ, while increased to 35% in HAZ. Compared with the BM, the ultimate tensile strength and yield strength of the FSW joint increased to 1036 and 550 MPa, respectively, reaching 106.7% and 110.9% of BM, respectively. The elongation of the entire joint was 50.5%, which was lower than that of BM due to the nonuniform deformation during the tensile test. The engineering strain was mainly concentrated in BM and SZ and transferred to each other during the tensile test, while the engineering strain in HAZ was always the lowest. Finally, the tensile fracture occurred in the SZ. The order of ultimate tensile strength of micro-zones in the FSW joint was as follows: HAZ > BM ≈ SZ. The order of yield strength was as follows: HAZ > BM > SZ.

Atomic scale understanding of the role of hydrogen and oxygen segregation in the embrittlement of grain boundaries in a twinning induced plasticity steel

High strength twinning induced plasticity (TWIP) steels have potential for applications in the automotive industry. However, they are prone to hydrogen embrittlement (HE) and galvanic corrosion. Here, we report on the susceptibility towards HE and oxidation of a model Fe 27Mn 0.3C (wt%) TWIP steel by atom probe tomography. We measured the segregation of hydrogen and oxygen at grain boundaries, which appears correlated with manganese depletion. Our study suggests a correlation between HE and oxidation mechanisms in TWIP steels, which can combine to favor the previously reported hydrogen enhanced decohesion of grain boundaries.

The effect of loading direction and pre-deformation on the low-cycle fatigue behavior of high-manganese twinning-induced plasticity steels

The low-cycle fatigue performance of high-manganese twinning-induced plasticity (TWIP) steels with the specimen axial direction of 0°, 22.5°, 45°, 67.5°, and 90° to the rolling direction is investigated in the range of the total strain amplitudes (Δε/2) of 0.6 % ∼ 4.4 % and two types of pre-deformed conditions (i.e., tension and tension-unloading). Experimental results show that the dependence of the fatigue life on the specimen orientation is obvious for lower strain amplitudes, and it gradually reduces with the increase of the applied strain level. The 0° orientations show the highest sensitivity to the pre-deformation process, and the tension-unloading pre-deformation exhibits the most severe change of the texture at fracture.

Microstructure, properties and crystallographic orientation of novel austenitic Fe–26Mn-3.4Cr-0.4C steel under hot torsion process

An experimental austenitic twin-induced plasticity (TWIP) steel was deformed under hot torsion tests. Two different total equivalent strains (2.2 and 2.6) were employed in order to raise its yield strength while keeping reasonable toughness. The material was isothermally strained at 900 °C with a low strain rate of 0.05 s−1. The effects of torsional deformation on the microstructure, texture and mechanical properties of the TWIP steel were investigated and compared with the as-received samples. X-Ray Diffraction (XRD) measurements were performed to identify the phases present in the metal before and after the thermomechanical process. Scanning electron microscopy (SEM), Electron backscatter diffraction (EBSD), Orientation distribution function (ODF) analysis and tensile experiments complemented the investigation of this research. The main deformation mechanism changed from conventional sliding inside the grain to nucleated twinning from the grain boundaries, which led to the decrease of the grain size to less than 1 μm (UFG - Ultrafine Grains). The results indicate that the transition in the deformation mechanisms is dependent on grain size associated with the flow profile of the material. This was due to changes in the dislocation density within the grain and the movement of dislocations around the yield point. No martensitic transformation was detected during the strain and neither from the twinning mechanism in the microstructure after the torsion deformation. The best balance between high strength and good ductility (920 MPa and 52%) was obtained after the total equivalent strain of 2.6. It was verified that the employed thermomechanical schedule affects the displacement of the substructure with the formation of sub-grains influencing the hardening behavior, texture and yield strength of the metal.

Nonmetallic inclusion characterizations and effects on impact toughness anisotropy of TWIP steel with different Ce additions

The effects of cerium (Ce) addition (0, 0.015% and 0.045%) on the microstructure, inclusions and impact toughness of Fe-18%Mn-0.6%C twinning-induced plasticity (TWIP) steel were investigated. The results show that the addition of Ce has no significant effect on the texture of TWIP steel. The grain size of TWIP steel decreases, and the volume fraction of twins increases slightly with Ce addition. Typical MnO-MgO-MnS complex inclusions and MnS were observed in TWIP steel without Ce. The inclusions gradually transform from the long strip MnS-Ce oxysulfide complex inclusions to ellipsoidal Ce oxysulfides with the increase in Ce addition from 0.015% to 0.045%. Notably, the transverse impact toughness of TWIP steel with 0.015% Ce addition is much lower than that of the other samples and exhibits serious anisotropy, and these differences are attributed to the stronger deformation ability of the complex inclusions with MnS and Ce oxysulfides compared with Ce oxysulfides. Fine grain size and ellipsoidal inclusions lead to excellent isotropic properties in TWIP steel with 0.045% Ce addition.