Surface wrinkling of the twinning induced plasticity steel during the tensile and torsion tests

‘Heterogeneous twinning’ is defined as plastic deformation due to the formation and progress of twins resulting in surface wrinkles on the deforming part when the initial grain size is relatively large compared to the typical size of the part. In the case of a Twinning Induced Plasticity (TWIP) steel with an initial grain size of ∼160 m, the heterogeneous twinning generated visible wrinkles, an orange peel effect, under medium uni-axial strains. The heterogeneous twinning did not occur in the material subjected to high shear strains. The complications resulting from this phenomenon on strain hardening characterization of the TWIP steels using two commonly used mechanical tests, tensile and torsion are discussed along with some experimental aspects of heterogeneous twinning.     

Analysis of the tensile behavior of a TWIP steel based on the texture and microstructure evolutions

The texture and microstructure evolutions of a fine-grained TWIP steel subjected to tensile tests at room temperature were investigated in relation to the mechanical behavior. This steel combines both high ductility and strength owing to the TWIP effect. Also the steel exhibits a high strain hardening rate that evolves according to five stages, which are related to the microstructure and texture evolutions and characteristics. The formation of nano-twins in the initial stage of deformation leads to an increase in strain hardening rate. The development of the pronounced <1 1 1> fiber in the tensile direction sustains mechanical twinning and maintains the strain hardening rate on a high level. The resulting microstructure exhibits several types of twin configurations and sub-boundaries with high misorientations due to intense activities of dislocation glide. The twin volume fraction was estimated to be 9% at the final stage of tensile deformation. The new orientations generated by mechanical twinning do not change considerably the final texture.     

Strain hardening behavior of a Fe–18Mn–0.6C–1.5Al TWIP steel

The strain hardening behavior of a Fe–18Mn–0.6C–1.5Al TWIP steel was investigated through the modified Crussard–Jaoul (C–J) analysis and microstructural observations. The strain hardening rate obtained by modified C–J analysis was high up to the critical strain of 37% and then greatly decreased with further strain. The electron backscatter diffraction (EBSD) observation showed that the deformation twinning rate is greatly decreased beyond about 34% strain, indicating that the reduced strain hardening rate at the large strain region is attributed to the deceleration of deformation twinning rate. The volume fraction of twinned region was increased with tensile strain due to the increase in the number of deformation twins not to the lateral growth of each deformation twin.     

Dependency of deformation twinning on grain orientation in an FCC and a HCP metal

Twinning plays important roles in HCP metals and those FCC metals with low stacking fault energy. The structural difference of two types of metals makes quite different contributions of twinning to plasticity. The variety of grain orientation in polycrystalline metals causes the inhomogeneous occurrence of twinning and further distinct transformation kinetics of twinning as strain increases and texture develops. This changes finally the work hardening behavior and mechanical properties. This paper reveals the dependency of twinning on grain orientation in an FCC TWIP (twinning induced plasticity) steel with high Mn content and in a magnesium alloy using electron-backscatter-diffraction (EBSD) technique, and analyzes the characteristics of twinning in the two types of metals by Schmid factor calculation. In addition, the relation of twinning and shear banding, as well as their influence on properties are discussed.     

An investigation into the mechanical behavior of a new transformation-twinning induced plasticity steel

In recent years, the transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels have been the focus of great attention thanks to their excellent tensile strength-ductility combination. Accordingly the mechanical behavior of an advanced microalloyed TRIP–TWIP steel, the compression tests were conducted from 25 to 1000 °C. This experimental steel shows a high compressive strength of 1280 MPa with the yield strength of 385 MPa as well as an outstanding strain hardening rate of about 14,000 MPa at the 25 °C. In addition the results indicate that the plastic deformation in the range of 25–150 °C is controlled by both the strain-induced martensite formation and mechanical twinning. However the mechanical twinning has been speculated as the only deformation mechanism in the temperature range of 150–1000 °C. This as well has led to an outstanding grain refinement via grain partitioning. The occurrence of mechanical twinning at such high temperatures is a novel observation in this grade of TRIP–TWIP high manganese steels.     

An investigation to the microstructural evolution of Fe–29Mn–5Al dual-phase twinning-induced plasticity steel through annealing

The potential effects of twinning induced plasticity have been taken into consideration to further improve the mechanical properties of advanced high-strength steels. Accordingly the high-Mn twinning-induced plasticity (TWIP) steels with austenite–ferrite dual-phase microstructure have been developed. In the present study, the influence of cold rolling and post-annealing treatments on the microstructural evolution and mechanical behavior of a group of dual-phase TWIP steel as a function of annealing time have been investigated. The mechanical behavior of processed materials has been examined through applying a set of low strain rate (0.001 s⁻¹) compression tests at room temperature. The austenite recrystallization characteristics during various annealing conditions are explained through proper microstructural examinations. The results indicate that as the recrystallization proceeds with annealing time the related yield stress deceases. The rapid drop of yield stress in short annealing periods is related to the onset of recrystallization. The yield stress variation diminishes as the annealing duration increases. This is attributed to the formation of austenite side-plates which may balance the softening effects of restoration processes.     

Stacking fault energy and tensile deformation behavior of high-carbon twinning-induced plasticity steels: Effect of Cu addition

Three experimental fully austenitic high-carbon twinning-induced plasticity (TWIP) steel grades were produced and the stacking fault energy (SFE) was investigated based on the thermodynamic modeling approach. The SFE of Fe–20Mn–xCu–1.3C (x = 0, 1.5 and 3.0) steels varied from 24.36 to 28.74 mJ m⁻² at room temperature. In order to study the correlation between the SFE and the mechanical behavior of TWIP steels, tensile tests were performed at room temperature and the deformed microstructures were examined at different strain levels by transmission electron microscopy. The Cu additions resulted in a remarkable increase in total elongation without a slight loss of tensile strength. In addition, the critical strain for serration start on the tensile stress–strain curves (i.e. required strain to generate mechanical twinning) was found to increase with increasing Cu content. Transmission electron microscope (TEM) observations also indicated that the occurrence of mechanical twinning was suppressed by increasing the Cu addition. The strain hardening mechanism and the superior ductility in deformation are dominated by the interaction of twins and dislocations. The mechanical behavior of TWIP steels is related to the Cu addition, the SFE, the interaction of twins and dislocations.     

State-of-the-knowledge on TWIP steel

Abstract

High Mn twinning induced plasticity (TWIP) steel is a new type of structural steel, characterised by both high strength and superior formability. TWIP steel offers an extraordinary opportunity to adjust the mechanical properties of steel by modifying the strain hardening. The use of TWIP steel may therefore lead to a considerable lightweighting of steel components, a reduction of material use and an improved press forming behaviour. These key advantages will help implement current automotive vehicle design trends which emphasise a reduction of greenhouse gas emissions and lowering of fuel consumption. In addition, high strength TWIP steel will effectively contribute to weight containment in vehicles equipped with hybrid and electric motors, as these are considerably heavier than conventional motors. The present review addresses all aspects of the physical metallurgy of the high strength TWIP steel with a special emphasis on the properties and key advantages of TWIP sheet steel products relevant to automotive applications.     

Crystallographic texture evolution and martensite transformation in the strain hardening process of a ferrite-based low density steel

The strain-induced martensite transformation is of great importance in the strain hardening process of ferrite based low-density steel.Based on the microstructure analysis,the texture evolution and martensite transformation behavior in the strain hardening process were studied.The results show that martensite transformation accompanied by TWIP effect and high density dislocations maintains the con-tinuous hardening stage.As the strain increases,the texture of retained austenite evolves towards the F orientation{111}〈112〉,which is not conducive to martensite transformation.After the strain of 5％,the number of austenite grains with high Schmid factor orientations is gradually increased,and then signif-icantly reduced when the strain is over 10％due to the occurrence of martensitic transformation,which results in a high martensitic transformation rate.However,the unfavorable orientation and the reduced grain size of austenite slow down the martensite transformation at the final hardening stage.Moreover,because of the coordination deformation of austenite grains,strain preferentially spreads between adja-cent austenite grains.Consequently,the martensite transformation rate in strain hardening process is dependent on the orientation and grain size evolution of austenite,leading to a differential contribution to each strain hardening stage.     

Equal channel angular pressing of a TWIP steel: microstructure and mechanical response

A Fe–20.1Mn–1.23Si–1.72Al–0.5C TWIP steel with ultrafine grain structure was successfully processed through equal channel angular pressing (ECAP) at warm temperature up to four passes following the B _C route. The microstructure evolution was characterized by electron backscattered diffraction to obtain the grain maps, which revealed an obvious reduction in grain size, as well as a decrease in the twin fraction, with increasing number of ECAP passes. The texture evolution during ECAP was analyzed by orientation distribution function. The results show that the annealed material presents brass (B) as dominant component. After ECAP, the one pass sample presents A ₁* and A ₂* as the strongest components, while the two passes and four passes samples change gradually toward \( B/\bar{B} \) components. TEM analysis shows that all samples present twins. The twin thickness is reduced with increasing the number of ECAP passes. Nano-twins, as a result of secondary twinning, are also observed in the one and two passes samples. In the four passes sample, the microstructure is extensively refined by the joint action of ultrafine subgrains, grains and twins. The mechanical behavior was studied by tensile samples, and it was found that the yield strength and the ultimate tensile strength are significantly enhanced at increasing number of ECAP passes. Although the ductility and strain hardening capability are reduced with ECAP process, the present TWIP steel shows significant uniform deformation periods with positive work hardening rates.     

Study of internal stresses in a TWIP steel analyzing transient and permanent softening during reverse shear tests

Recent Bauschinger-type tests conducted on a twinning-induced plasticity (TWIP) steel highlights the important contribution of internal stresses to work hardening [1]. Along this line we present Bauschinger experiments in a Fe–22Mn wt%–0.6C wt% TWIP steel. The mechanical behavior upon load reversal shows transient and permanent softening effects. Determination of the internal stress from the magnitude of the permanent softening yields a contribution to work hardening of the order of 20%. Analysis of the transient softening, during strain reversal, indicates that internal stress is consistent with reported data on high carbon spheroidized steels.     

Characterization of different work hardening behavior in AISI 321 stainless steel and Hadfield steel

In order to distinguish the difference between AISI 321 stainless steel and Hadfield steel in work hardening behavior, both the Hollomon analysis and the differential Crussard–Jaoul analysis were used to determine the strain hardening exponent as a function of the strain. The results showed that the differential Crussard–Jaoul analysis characterized the discrepancy between AISI 321 steel and Hadfield steel in work hardening behavior more accurately than the Hollomon analysis. The work hardening of AISI 321 stainless steel resulted mainly from interactions of dislocations. When the true strain was rather low, the work hardening of Hadfield steel also resulted mainly from interactions of dislocations. At high strains, twinning would occur in Hadfield steel. It was the occurrence of twins that led to unusual work hardening at larger strains in Hadfield steel.     

Work hardening associated with ?-martensitic transformation, deformation twinning and dynamic strain aging in Fe-17Mn-0.6C and Fe-17Mn-0.8C TWIP steels

The tensile properties of carbon-containing twinning induced plasticity (TWIP) steels and their temperature dependence were investigated. Two steels with carbon concentrations of 0.6% and 0.8% (w/w) were tensile-tested at 173, 223, 273, 294, and 373 K. Three deformation modes were observed during tensile testing: ?-martensitic transformation, deformation twinning, and dynamic strain aging. The characteristic deformation mode that contributed to the work hardening rates changed with the deformation temperature and chemical compositions. The work hardening rate in the carbon-containing TWIP steels increased according to the deformation modes in the following order: ?-martensitic transformation > deformation twinning > dynamic strain aging.     

The effect of low temperature annealing on the mechanical behavior of cold rolled dual-phase Twinning-Induced Plasticity steel

In recent years, Twinning-Induced Plasticity (TWIP) steels with high specific strength have been developed to mainly address the unsaturated demands of transportation industries for weight reduction. To achieve the exclusive mechanical properties of TWIP steels, the understanding of their thermomechanical processing (TMP) behavior is highly necessitated. In the present work, the influence of cold rolling and post-annealing treatments on the mechanical behavior of a new dual phase (γ + α) TWIP steel have been studied. The microstructural studies indicated the presence of deformation twins in the deformed state of material. Annealing the as-rolled experimental alloy could result in the formation of Widmanstätten austenite within the ferrite grains at 500 °C. The nearly constant yield stress at high annealing durations was attributed to the opposite effects of recovery and Widmanstätten austenite formation.     

Hot deformation behavior of a Nb-containing 316LN stainless steel

A Nb-containing 316LN stainless steel was compressed in the temperature range 900–1200 °C and strain rate range 0.01–10 s^?1. The mechanical behavior has been characterized using stress–strain curve analysis, kinetic analysis, processing maps, etc. The microstructural evolution was observed and the mechanism of flow instability was discussed. It was found that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate. On the contrary, the efficiency of power dissipation increased with them; Flow instability was manifested as cracking and flow localization; The hot deformation equation and the relationships between deformation condition and dynamic recrystallization grain size and fraction were obtained; For Nb-containing 316LN stainless steel, the favorite nucleation sites for dynamic recrystallization are in sequence of triple point, grain boundary, twin boundary and intragranular deformation band; The suggested processing window is given.     

Grain size altering yielding mechanisms in ultrafine grained high-Mn austenitic steel:Advanced TEM investigations

The underlying mechanism of discontinuous yielding behavior in an ultrafine-grained(UFG)Fe-31Mn-3Al-3Si(wt.％)austenitic TWIP steel was investigated by the use of advanced TEM technique with taking the plastic deformation mechanisms and their correlation with grains size near the macroscopic yield point into account.Typical yield drop mechanisms such as the dislocation locking by the Cottrell atmo-sphere due to the presence of interstitial impurities cannot explain the origin of this phenomenon in the UFG high-Mn austenitic TWIP steel.Here,we experimentally revealed that the plastic deformation mechanisms in the early stage of deformation,around the macroscopic yield point,show an obvious association with grain size.More specifically,the main mechanism shifts from the conventional slip in grain interior to twinning nucleated from grain boundaries with decreasing the grain size down to less than 1 μm.Our observation indicates that the grain size dependent deformation mechanisms transition is also deeply associated with the discontinuous yielding behavior as it could govern the changes in the grain interior dislocation density of mobile dislocations around the macroscopic yield point.     

On deformation twinning in a 17.5% Mn-TWIP steel: A physically based phenomenological model

TWinning Induced Plasticity (TWIP) steel is a typical representative of the 2nd generation advanced high strength steels (AHSS) which exhibits a combination of high strength and excellent ductility due to the deformation twinning mechanisms. This paper discusses the principal features of deformation twinning in faced-centered cubic austenitic steels and shows how a physically based macroscopic model can be derived from microscopic-level considerations. In fact, a dislocation-based phenomenological model, with internal state variables including dislocation density and micro-twins volume fraction describing the microstructure evolution during deformation process, is proposed to model the deformation behavior of TWIP steels. The originality of this work lies in the incorporation of a physically based model on twin nucleation and volume fraction evolution in a conventional dislocation-based approach. Microstructural level experimental observations with scanning electron microscope (SEM) and transmission electron microscope (TEM) techniques together with the macroscopic quasi-static tensile test, for the TWIP steel Fe-17.5 wt.% Mn-1.4 wt.% Al-0.56 wt.% C, are used to validate and verify the modeling assumptions. The model could be regarded as a semi-phenomenological approach with sufficient links between microstructure and the overall mechanical properties, and therefore offers good predictive capabilities. Its simplicity also allows a modular implementation in finite element-based metal forming simulations.     

Effects of grain size on compressive behaviour in ultrafine grained pure Mg processed by equal channel angular pressing at room temperature

Ultrafine-grained pure magnesium with an average grain size of 0.8 μm was produced by refining coarse-grained (980 μm) ingot by multi-pass equal channel angular pressing (ECAP) at room temperature with the application of a back pressure. The compressive deformation behaviour at room temperature depended on grain size, with deformation twinning and associated work hardening observed in coarse-grained Mg, but absent in the ultrafine grained material as decreasing grain size raised the stress for twinning above that for dislocation slip. The ultrafine grained Mg showed good plasticity with prolonged constant stress after some initial strain hardening.     

Effect of chemical composition on work hardening of Fe—Mn—C TWIP steels

The work hardening behaviour of Fe-Mn-C twinning induced plasticity (TWIP) steels with a wide compositional range has been investigated. Based on the consideration that twinning provides a dynamic composite effect resulting in high work hardening rate in TWIP steels, the present work proposes a model to describe such behaviour as a function of chemical composition. The model predictions are in good agreement with experimental observations.     

Influences of Nb-microalloying on microstructure and mechanical properties of Fe–25Mn–3Si–3Al TWIP steel

The Fe–25Mn–3Si–3Al TWIP steel was microalloyed by niobium in this paper, and the appropriate heat treatment and cold rolling processes were drafted in order to improve the poor yield strength of the steel. The results show that the yield strength of the steel increases from 320 MPa to 445 MPa, and the tensile strength increases from 680 MPa to 795 MPa, but the uniform elongation decreases from 65% to 55%. Nb addition can strongly hinder the growth of recrystallized grains, moreover Nb atoms react with C atoms to form nanoscale NbC precipitations, and these precipitations can block the dislocation motion, and then the yield strength and initial work hardening ability of Fe–25Mn–3Si–3Al steel is clearly improved. Furthermore, the strain-induced twinning is still a major deformation mechanism for the Nb-microalloying TWIP steel, and the twinning induced plasticity (TWIP) effect ensures a satisfactory ductility for the steel. Finally, the modified TWIP steel obtains a better match between the strength and plasticity by the joint action of precipitation strengthening and TWIP effect.