Twinning-induced Plasticity Steel Research Articles

Introduction and mechanical evaluation of a novel 3rd-generation medium manganese AHSS with 86 GPa% of PSE

Reduction in weight and energy consumption is among the most concerning subjects in the automotive industry. To date, advanced high-strength steel (AHSS) has been one of the best solutions to reach this objective. However, this material involves high chemical composition and production processing costs. This paper introduces a new grade of third-generation medium Mn AHSS with extraordinary mechanical properties. Regarding its product of strength and elongation (PSE), i.e., 86 GPa%, this is the highest value of PSE of an AHSS reported until now to the best of the author's knowledge. Also, because of using 13 wt% of Mn, lower Mn has been used than conventional twinning-induced plasticity (TWIP) steels, and mechanical properties have been improved. The samples were fabricated using a vacuum induction furnace (VIM) and heat-treated at four different temperatures to perform austenite reversion transformation (ART). The best sample (i.e., the one with the highest PSE) was microstructurally evaluated and compared to the as-rolled one. In addition, the effect of different heat treatment temperatures on mechanical properties was observed. The difference in mechanical properties is mainly related to different austenite stabilities and fractions. Intercritical annealing formed lamellar austenite layers with varying levels of stability. These layers caused the TRIP to stay active during the whole deformation process. Since this structure can also delay necking, high elongation was achieved along with high strength.

The influence of temperature on the strain-hardening behavior of Fe-22/25/28Mn-3Al-3Si TRIP/TWIP steels

The influence of temperature and stacking fault energy (SFE) on the strain-hardening behavior and critical resolved shear stress for twinning was investigated for three Fe–22/25/28Mn–3Al–3Si wt.% transformation- and twinning-induced plasticity (TRIP/TWIP) steels. The SFEs were calculated by two different methods, density functional theory and statistical thermodynamic modeling. The dislocation structure, observed at low levels of plastic deformation, transitions from “planar” to “wavy” dislocation glide with an increase in temperature, Mn content, and/or SFE. The change in dislocation glide mechanisms from planar to wavy reduces the strain hardening rate, in part due to fewer planar obstacles and greater cross slip activity. In addition, the alloys exhibit a large decrease in strength and ductility with increasing temperature from 25 to 200 °C, attributed to a substantial reduction in the thermally activated component of the flow stress, predominate suppression of TRIP and TWIP, and a significant increase in the critical resolved shear stress for mechanical twinning. Interestingly, the increase in SFE with temperature had a rather minor influence on the critical resolved shear stress for mechanical twinning, and other temperature dependent factors which likely play a more dominant role are discussed.

Microstructural and mechanical analysis of double pass dissimilar welds of twinning induced plasticity steel to austenitic/duplex stainless steels

Double pass dissimilar welds between the twinning induced plasticity (TWIP) steel and austenitic stainless steel (ASS) AISI 304 L and duplex stainless steel (DSS) 2205 were studied theoretically and experimentally. A finite element (FE) thermal model linked to the liquid fraction calculation was used to estimate the optimal heat input level. The metallographic characterization of dissimilar weldments TWIP-SS 304 L and TWIP-DSS 2205 disclosed a priori the lack of hot cracking in the fusion zone (FZ) and liquation cracks in the heat-affected zone (HAZ). Later, the root bend test results corroborated a lack of fusion in the FZ-HAZ interface of the TWIP-DSS 2205 weld (TWIP steel side). The FE model predicted the heat accumulation in the SS 304 L and DSS 2205, which produced variation in the thermal gradient. The thermal gradient changes induced the ferritic-austenitic (FA) solidification mode in TWIP steel and SS 304 L and the massive austenite formation by epitaxial growth in the FZ-HAZ interface of the DSS 2205 side. Microhardness profiles corroborated the formation of homogenous FZs in both dissimilar welded joints. The microhardness increase/decrease was related to the solidification mode change in the FZ-HAZ interface. The mechanical strength and ductility of the TWIP-SS 304 L weld decreased 26% and 40%, respectively, compared to the reference autogenous similar weldment SS 304 L. The planar solidification growth detected in the FZ-HAZ interface was responsible for mechanical properties loss.

Deformation Lenses in a Bonding Zone of High-Alloyed Steel Laminates Manufactured by Cold Roll Bonding

The combination of strength of transformation-induced plasticity (TRIP) steel and ductility of twinning-induced plasticity (TWIP) steel can be achieved by manufacturing laminated composites via cold roll bonding (CRB). Work hardening of the surface before CRB produces deformation lenses (DLs), which play significant role in bonding, but are reported rarely in the literature. The present work aimed to study the DLs at the bonding interface of the laminated composite made of high-alloy TRIP and TWIP steels manufactured by CRB. The DLs and interfaces were investigated by means of scanning and transmission electron microscopy, roughness measurement, tensile and peel tests. Laminates showed ultimate tensile strength up to 900 MPa and elongation up to 45% maintaining the layer’s integrity up to failure. The TWIP–TWIP interface has shown higher maximum peel strength (up to 195 N/cm) than that of a TRIP–TWIP interface (up to 130 N/cm), which was found to be in direct proportion to the overall area of DLs. Bonding of the laminate layers was found to occur between DL fragments.

Research on Microstructures and Properties of High-Strength Anticorrosion Twinning-Induced Plasticity Steels

The microstructures, mechanical properties, and anticorrosion properties of Fe–25Mn–Cr(12, 18 wt.%)–CN TWIP steels compared with nominal Fe–22Mn–0.6C TWIP steel (TWIP-ref) in the solid-solution state were investigated in the present study by using X-ray diffraction (XRD), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), tensile tests, and potentiodynamic polarization measurement, etc. The results show that the Fe–Mn–Cr–CN TWIP steels have excellent mechanical properties and anticorrosion properties compared to the TWIP-ref steel. The strain-hardening rate of the 12CrN steel is lower than that of the TWIP-ref steel because of the relatively late generation of high-density deformation twins, which resulted in less tensile strength increments during the tensile test. Moreover, M23C6 precipitates enriched with Mn and Fe formed in the Fe–Mn–Cr–CN steel as the Cr content increased, which had an important influence on the mechanical properties and anticorrosion properties. In the Fe–Mn–Cr–CN TWIP steels, because of the influence of the M23C6 precipitation in the 18CrN sample, the yield strength (σYS) is increased slightly, while the ultimate tensile strength (σUTS) and elongation to failure are decreased. In addition, 18CrN has poor corrosion resistance because of the formation of M23C6 precipitation (i.e., it has relatively negative corrosion potential (Ecorr) and a smaller corrosion current density (Icorr)).

The Influence of Precipitation, High Levels of Al, Si, P and a Small B Addition on the Hot Ductility of TWIP and TRIP Assisted Steels: A Critical Review

The hot ductility of Transformation Induced Plasticity (TRIP) and Twinning Induced Plasticity (TWIP) steels is reviewed, concentrating on the likelihood of cracking occurring on continuous casting during the straightening operation. In this review, the influence of high levels of Al, Si, P, Mn and C on their hot ductility will be discussed as well as the important role B can play in improving their hot ductility. Of these elements, Al has the worst influence on ductility but a high Al addition is often needed in both TWIP and TRIP steels. AlN precipitates are formed often as thin coatings covering the austenite grain surfaces favouring intergranular failure and making them difficult to continuous cast without cracks forming. Furthermore, with TWIP steels the un-recrystallised austenite, which is the state the austenite is when straightening, suffers from excessive grain boundary sliding, so that the ductility often decreases with increasing temperature, resulting in the RA value being below that needed to avoid cracking on straightening. Fortunately, the addition of B can often be used to remedy the deleterious influence of AlN. The influence of precipitation hardeners (Nb, V and Ti based) in strengthening the room temperature yield strength of these TWIP steels and their influence on hot ductility is also discussed.

Crystal Orientation Dependence of the Portevin–Le Chatelier Effect in Instrumented Indentation: A Case Study in Twinning-Induced Plasticity Steels

Instrumented indentation can be effectively used to investigate the Portevin–Le Chatelier (PLC) effect at small scales. It has been shown that the PLC effect in single crystals may depend on the crystal orientation, yet the underlying mechanism is unclear. Here, the orientation dependence of the PLC effect was systematically studied by conducting instrumented indentation tests in the [001]-, [101]- and [111]-oriented grains of a polycrystalline twinning-induced plasticity steel. It is found that the crystal orientation does not affect the PLC effect at relatively high indentation strain rates. In contrast, there is a strong orientation dependence at lower rates, with enhanced difficulty in the formation of serrations in the order of the [001], [111] and [101] orientations. This finding contradicts the previous proposals of the orientation effects, which are associated with the dislocation waiting time. On the basis of both the orientation and rate effects observed here, we proposed that the crystal orientation influences the occurrence of serrations in instrumented indentation by affecting the number of activated slip systems and, therefore, the probability of finding sufficient dislocation sources to accommodate the plastic avalanche.

Homogeneously introducing more and thinner nanotwins by engineering annealing twin boundaries: A TWIP steel as an example

It is a challenge to homogeneously introduce more and thinner nanotwins into bulk materials. Our results showed that more and thinner deformation nanotwins could be more homogeneously introduced in a bulk solution-treated FeMnSiC twinning-induced plasticity (TWIP) steel after significantly reducing boundaries of annealing twins by thermomechanical processing that consisted of three cycles of deformation at room temperature (RT) and subsequent annealing. After introducing the recovery-annealed deformation nanotwins by 30% deformation at RT and annealing at 673 K, the three-time-trained steel showed a similar ductility comparable to the solution-treated steel but obtained 125 MPa higher in yield strength. The engineering annealing twins will open a promising pathway for strengthening bulk TWIP steels by homogeneously introducing thinner recovery-annealed deformation nanotwins.

Hydrogen-induced ductilization in a novel austenitic lightweight TWIP steel

This study reported a novel hydrogen-induced ductilization in an austenitic lightweight twinning-induced plasticity (TWIP) steel (Fe-1C-25Mn-5.5Al-3Si, in wt. %). This phenomenon was irrelevant to strain rate or global work hardening rate. In fact, the stacking fault energy (SFE), 49.5 mJm−2, of the steel is an appropriate value for hydrogen-induced ductilization. This SFE was reduced by hydrogen and deformation twin was enhanced in the hydrogen-rich surface layer during deformation. Such twinning-strengthened surface limits formation of strain localization and allows extra deformation. Besides, this SFE is high enough so that martensitic transformation and hydrogen-enhanced intergranular cracking were absent, making the steel free from hydrogen embrittlement. This work provides a new concept that SFE/twinability engineering not only prevents hydrogen embrittlement but enables hydrogen-induced ductilization.

The tensile properties and microstructure evolution of cold-rolled Fe–Mn–C TWIP steels with different carbon contents

Fe–18Mn-1.0C and Fe–18Mn-0.6C twinning-induced plasticity steels are cold rolled at various rolling reductions, and their tensile properties, evolution of dislocation density, and twinning behavior during tensile deformation were systemically investigated. With increasing the cold rolling reduction, the yield strength and tensile strength of both steels increase obviously, especially for Fe–18Mn-1.0C steel; concurrently, the elongation to fracture decreases largely while this decrease is not as significant for Fe–18Mn-1.0C steel as for Fe–18Mn-0.6C steel. Furthermore, dislocation density and the amount of deformation twins increase with increasing rolling reduction and tensile strain. Such increase is more obvious for Fe–18Mn-1.0C steel than for Fe–18Mn-0.6C steel. A maximum (or saturation) dislocation density and a maximum (or saturation) amount of deformation twins are observed in specimens when they are deformed to fracture. Compared with Fe–18Mn-0.6C steel, the saturation values of dislocation density and amount of deformation twins are obviously larger in Fe–18Mn-1.0C steel, regardless of cold rolling reductions. The contributions of lattice friction stress, dislocations, twins and dynamic strain ageing (DSA) to flow stress were quantitatively analyzed using a parametric model. Dislocation strengthening and twin boundaries strengthening are found to play a dominant role in increasing the flow stress during tensile deformation, and the contribution from DSA is negligibly small.

Effect of the thermal field on the microstructure of dissimilar welded joints between TWIP steel and 2205 duplex stainless steel

This study reported a theoretical-experimental analysis of dissimilar metals duplex stainless steel (DSS) grade 2205 and twinning induced plasticity (TWIP) steel joined. Welded joints were performed in autogenous mode and using filler metal (ER2209) by the pulsed gas tungsten arc welding (GTAW-P) process. Critical weld regions were characterized metallographically using LOM and SEM. Microstructural changes such as the morphology and contents of ferritic and austenitic phases were correlated to the temperature ranges estimated numerically by a finite element (FE) model, which considered the variation of the percentage dilution in the fusion zone (FZ). The average variation between experimental and numerical results was up to 6.75%. SEM chemical analysis and line scanning analysis corroborated the diffusion of Al, Mn, Cr and Ni in the FZ of the autogenous joint. Small concentration gradients promoted a homogenous chemical composition of the ferritic and austenitic phases in the filler metal joint. The FZ exhibited an austenitic phase coarsening due to the elevated temperatures (1506°C–1803 °C, DSS 2205 side). Wetting problems were observed in both weldments producing prominent interfaces between the welded metal (WM) and the heat-affected zone (HAZ). Other weld defects such as underfilling, undercuts and lack of fusion were also detected. When a high TWIP steel volume was melted (filler metal joint), WM hardness increased compared to the autogenous joint. The hardness increment (2–5% in the DSS 2205) combined with the lack of fusion brought about the failure of the filler metal joint during the bend test.

Influence of laser power on microstructure and mechanical properties of laser welded TWIP steel butted joint

The twinning-induced plasticity (TWIP) steel is a newly developed which possesses fantastic mechanical properties and high potential for anti-crash weight-optimized structural applications. In this study, the weldability of 2 mm thick TWIP steel plate by laser welding method was investigated. The welded joint exhibited fully austenitic structure, and the weld seam consisted three types of solidification structures: columnar dendrite, cellular crystal and equiaxed dendrite. The microsegregation of C and Mn elements occurred in the interdendritic regions during the solidification of the weld seam. The toughness of the heat-affected zone (HAZ) was higher than that of the weld seam. The microhardness diagram of cross-section of welded joints presented V-shaped under different laser power. The microhardness of equiaxed dendrite zone was higher than that of the weld seam and coarse grain heat affected zone (CGHAZ). The average strain and tensile strength increased first and then decreased with the rising of laser power. Further study demonstrated that the fracture of the laser welding TWIP steel joints during tensile test presented ductile mode characteristics. The results show that the appearance and performance of the welded joint are optimal with laser power of 1800 W in present study, and the tensile strength and total elongation are 915 MPa and 48%, respectively.

Reveal Hydrogen Behavior at Grain Boundaries in Fe–22Mn–0.6C TWIP Steel via In Situ Micropillar Compression Test

In this study, the effect of hydrogen on dislocation and twinning behavior along various grain boundaries in a high-manganese twinning-induced plasticity steel was investigated using an in situ micropillar compression test. The compressive stress in both elastic and plastic regimes was increased with the presence of hydrogen. Further investigation by transmission electron backscatter diffraction and scanning transmission electron microscope demonstrated that hydrogen promoted both dislocation multiplication and twin formation, which resulted in higher stress concentration at twin–twin and twin–grain boundary intersections.

Effect of pre-strain on hydrogen embrittlement of high manganese steel

The effect of deformation twins on the hydrogen embrittlement (HE) of a twinning-induced plasticity (TWIP) steel with various pre-strain levels, coupled with subsequent annealing treatment was systematically investigated by a slow strain rate tension test after cathodic hydrogen charging. The results revealed that specimens with high twin volume fraction had relatively better HE resistance. The high-density twin boundary not only plays an important role in hindering hydrogen diffusion, but also can refine the grains and inhibit the accumulation of hydrogen, so as to relieve stress concentration and avoid premature fracture of the specimen.

Effects of cerium addition on the microstructure, mechanical properties and strain hardening behavior of TWIP steel Fe-18Mn-0.6C

The effects of cerium (Ce) on the microstructure, mechanical properties and strain hardening behavior of Fe-18Mn-0.6C twin-induced plasticity (TWIP) steel were investigated by electron backscatter diffraction, X-ray diffraction and transmission electron microscopy. Compared with the addition of Al, the addition of Ce significantly reduced the stacking fault energy of TWIP steel, which promotes the formation of deformation and annealing twins. The dynamic strain aging behavior of TWIP steel was inhibited by Al and Ce atoms during plastic deformation. The optimal mechanical properties were obtained when 0.015 wt% of Ce was added, resulting in a tensile strength of 1023 MPa; and an elongation was 92%. The interaction of twin variants in the Ce-containing alloys exhibited an X shape, which was significantly different from the T shape in the non-Ce containing alloy. This is mainly attributed to the fact that Ce promotes dislocations near grain boundaries, where for the nucleation of deformation twins at grain boundaries is favored. Compared with the T-shaped twin variants, the dynamic Hall-Petch effect caused by the X-shaped staggered twin variants was stronger, which improved the strain hardening ability of TWIP steel. • The addition of Ce can improve the stacking fault probability and increase the fraction of twin in TWIP steel. • The initial strain can be reduced during the strain hardening equilibrium stage after the addition of Ce. • Compared with T-shape, the interaction of twin variants in TWIP steel modified by Ce tends to X-shape. • The dislocation entanglement near grain boundaries is easily induced by the addition of Ce in TWIP steel.

Effect of twin thickness on the resistance of dislocations to pass through the twin interfaces:

The presence of coherent twin boundary (TB) in polycrystalline materials results simultaneous ultrahigh strength and ductility by controlling the deformation behavior with grown-in nano-twins and mechanical twinning. In twinning-induced plasticity (TWIP) steels, deformation twins are reported to nucleate at grain boundaries and grow in the thickness direction during tensile deformation, resulting in high work hardening due to the dynamic microstructure changes. The mean free path of dislocations in the matrix reduces by increasing the number of newly formed deformation twins as these are considered to be strong obstacles to dislocation glide. The interaction of dislocation with TBs has great importance in achieving optimum materials prosperities; however, the influence of the deformation twin thickness on deformation mechanism in TWIP steels has not yet been understood. In this study, in order to understand the impact of deformation twin thickness on the material strength, we investigate the interaction of 60 degrees lattice dislocations with deformation twins with different thicknesses in the matrix of Cu with lower stacking fault energy through molecular dynamics (MD) simulations and find an optimum twin thickness of strength in the systems. Finally, we propose a new type of size-dependent strengthening mechanism in TWIP steels.

Influence of hydrogen on deformation and fracture mechanisms in austenitic steel types

In the present work, the hydrogen/material interaction with two austenitic steel types, created via a completely different alloying strategy, is investigated. The two steels include 304L austenitic stainless steel (ASS) and 18Mn-0.6C twinning-induced plasticity (TWIP) steel. Constant extension rate tensile tests are performed to evaluate the influence of hydrogen on the deformation and fracture mechanisms. A reference condition without hydrogen is compared to a seven days electrochemically hydrogen precharged condition. 304L ASS shows transformation to α’-martensite without hydrogen, while hydrogen increases the α’-martensite fraction and additionally enables the transformation to ε-martensite. The fracture surface has a transgranular, quasi-cleavage appearance. The TWIP steel shows deformation twinning without hydrogen, while abundant ε-martensite transformation is observed in the presence of hydrogen. The fracture type is intergranular due to the high cracking sensitivity of the grain boundaries.

Strengthening contributions of dislocations and twins in warm-rolled TWIP steels

A twinning-induced plasticity (TWIP) steel was processed by warm rolling with different reduction ratios to elevate yield strength while maintaining considerable ductility. To investigate the effect of prior warm rolling on the deformation mechanisms during subsequent tensile straining, the contributions of individual strengthening mechanisms to the tensile flow stress of the as-received (AR) and warm-rolled (WR) steels were decoupled, based on the measured dislocation density evolutions determined by synchrotron X-ray diffraction (XRD) experiments. Whereas dislocation multiplication invariably governs the maximum flow stress of the AR and WR steels, deformation twinning becomes increasingly important for the strain hardening of steels with larger warm rolling reductions. In addition, twinning kinetics is enhanced by the pre-existing high dislocation densities caused by warm rolling, leading to an enhancement of twinning-induced hardening. The present work provides further insight into the influence of warm rolling on the deformation mechanisms of TWIP steels.

Study on manganese volatilization behavior of Fe–Mn–C–Al twinning-induced plasticity steel

Abstract In the smelting process of high manganese steel, the volatilization of manganese will be accompanied. In this article, the volatilization of manganese in high manganese steel was studied by simultaneous thermal analyzer. The results show that the volatilization rate of manganese in high manganese steel increases with increasing temperature and holding time. It is proved by experimental study and data analysis that manganese volatilization follows the first-order kinetics model, and the empirical formula of manganese evaporation is derived. The volatile products of manganese were analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy. It was found that the volatile components of manganese mainly consisted of MnO, Mn3O4, Mn2O3, and MnO2. Combined with thermodynamics, the mechanism of manganese volatilization is further analyzed, and two forms of manganese volatilization in high manganese steel are revealed. One is that manganese atoms on the surface of high manganese steel and oxygen atoms in the gas form different types of manganese oxides and then volatilize at high temperature. The other way is that Mn atoms vaporize into Mn vapor and evaporate in high temperature environment, and then are oxidized into different types of manganese oxides. The results of theoretical calculation and experiment show that manganese volatilization is mainly in the first form.

Transformation-induced plasticity via γ → ε → α’ and γ → ε → γ martensitic transformations in Fe–15Mn–10Cr–8Ni–4Si alloy

Transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) are key mechanisms for achieving excellent strength–ductility in advanced high-strength steels (AHSSs). The Fe–15Mn–10Cr–8Ni–4Si alloy is an austenitic steel that exhibits high plastic fatigue durability due to reversible bidirectional γ→ε→γ martensitic transformation under push–pull cyclic loadings. In addition to the fatigue property, the Fe–15Mn–10Cr–8Ni–4Si alloy exhibits good strength–ductility balance (where the product of strength and ductility is 51.4 GPa%). In particular, its total elongation of 77% is comparable to those of TWIP steels. To elucidate the underlying mechanism for the TRIP-enhanced ductility, this study reports on the tensile deformation microstructure. The post-fracture microstructure is a γ-ε-α′ triple-phase structure that includes a significant amount of deformation-induced ε-martensite, whereas the frequency of γ-twinning was very limited. In addition to sluggish strain-induced ε-martensitic transformation (ε-TRIP), two-stage γ→ε→α′ martensitic transformation (two-stage TRIP) and bidirectional γ→ε→γ transformation (B-TRIP) under monotonic tensile loading were uniquely observed at intersections of the ε-martensite variants. The tensile property and microstructure of the Fe–15Mn–10Cr–8Ni–4Si alloy (where the Gibbs free energy difference ΔGγ→ε between γ-austenite and ε-martensite is −65.0 J/mol) are compared with those of the Fe–30Mn–(6−x)Si–xAl (x = 0, 1, 2, 3, 4, 5, 6 wt%) alloy system, with different ΔGγ→ε (−276.5–417.5 J/mol) resulting in different plastic deformation mechanisms (martensitic transformations or twinning). The three TRIP mechanisms with different transformation paths, i.e., γ→ε, ε→α’, and the reverse ε→γ, in the Fe–15Mn–10Cr–8Ni–4Si alloy are then discussed in terms of their contributions to its excellent ductility, which is comparable to that produced by TWIP.