Twinning-induced Plasticity Steel Research Articles

Fatigue resistance evaluation of high Mn-TWIP steel through damage mechanics: A new method based on stiffness evolution

The work presented here deals with the implementation of a new methodology that allows fast and reliable determination of the fatigue strength. It is based on monitoring the specimen stiffness changes at different stress levels, as an indicator of the evolution of fatigue damage. This new rapid fatigue test uses techniques available in many laboratories, as the DIC (Digital Image Correlation) technique and common extensometers. Moreover, the obtained data are easier to handle than infrared cameras or acoustic emission systems data, and the experimental procedure to determine the fatigue limit is more evident than in the self-heating method.Experiments have been conducted in TWIP (Twinning Induced Plasticity) steel, a material used for lightweighting the structural parts of vehicles. With their excellent energy absorption capacity, TWIP steels can satisfy the part requirements in terms of crash performance, while their high tensile strength can deal with the cyclic loads acting on chassis parts. Therefore, many efforts focus on improving the fatigue strength of TWIP steels through pre-straining and/or surface treatments. However, finding the best way to improve the fatigue resistance requires time and resources that often hinder the development of the material. For this reason, a TWIP steel has been selected to check the new rapid fatigue test. The prediction made using the proposed approach is validated by comparison with conventional staircase results and fatigue crack growth standardised tests. The good agreement allows proposing the new method as a fast and efficient way to determine the fatigue resistance in metals.

An Investigation on Microstructures and Mechanical Properties of Twinning-Induced Plasticity Steels Prepared by Directional Solidification

An Investigation on Microstructures and Mechanical Properties of Twinning-Induced Plasticity Steels Prepared by Directional Solidification

Influence of carbon addition on mechanical properties of Fe–Mn–C twinning-induced plasticity steels

Influence of carbon addition on mechanical properties of Fe–Mn–C twinning-induced plasticity steels

Ultrafine-grained twinning-induced plasticity steel prepared by mechanical alloying and spark plasma sintering

A twinning-induced plasticity (TWIP) steel was prepared by mechanical alloying and spark plasma sintering (SPS), and its microstructure and mechanical behavior were examined. The ultrahigh microhardness and considerable tensile strength of the sintered bulk sample were mainly attributed to grain-size strengthening and particle strengthening by numerous Mn-rich particles dispersed in the matrix. Micropores coupled with large internal inclusions easily produced cracks in the bulk sample under uniaxial tensile stress, resulting in poor ductility with rapid intergranular brittle fracture. The ductility of the material showed significant improvement with the elongation increased from 4% to 13% after annealing the bulk sample at 850 °C for 1 h. The size of the inclusions in the annealed sample dramatically reduced and several small dimples with high densities and depths were formed, presenting a mixed fracture mode in which intergranular fracture was the dominant mechanism, whereas transgranular fracture was the supplementary mechanism.

Consequences of Deep Rolling at Elevated Temperature on Near-Surface and Fatigue Properties of High-Manganese TWIP Steel X40MnCrAl19-2

Due to pronounced work-hardening induced by the complex interplay of deformation mechanisms such as dislocation slip, twinning and/or martensitic phase transformation, high-manganese steels represent a class of materials well-suited for mechanical surface treatment. In the present study, the fatigue behavior of a high-mangsanese steel showing twinning-induced plasticity (TWIP) effect at room temperature (RT) was investigated after deep rolling at 550 °C. Results are compared to a former study discussing the behavior after RT deep rolling. Evolution of the near-surface microstructure was analyzed by scanning electron microscopy (SEM), microhardness measurements and residual stress depth profiles obtained by X-ray diffraction (XRD). Both uniaxial tensile tests and uniaxial tension-compression fatigue tests have been conducted in order to rationalize the macroscopic material behavior. Following deep rolling at 550 °C, SEM measurements employing electron backscatter diffraction (EBSD) revealed a heavily deformed surface layer as well as localized deformation twinning. Specimens showed inferior hardness and residual stress depth profiles when compared to RT deep rolled counterparts. Tensile tests indicated no difference between the conditions considered. Fatigue properties however were improved. Such behavior is rationalized by a more stable residual stress state induced by dynamic strain aging.

Influences of carbon concentration on microstructure and tensile properties of Fe–18Mn–9Cr–2Al–xC steels

As an effort to develop corrosion resistant and highly formable austenitic steels, Fe–18Mn–9Cr–2Al–(0–0.5)C steels in wt. % were designed and their microstructures and tensile properties were investigated with particular attention being paid to the effects of carbon concentration. The carbon-free steel consisted of the austenite, δ-ferrite and ε-martensite phases in as heat-treated condition while those of carbon-added steels had an austenite single phase with the (Fe,Cr)23C6 carbides at grain boundaries. When tensile strained at room temperature, all the steels exhibited extensive deformation twinning, resulting in large work-hardening and excellent ductility. Deformation twinning was suppressed at 300 °C, and dynamic recrystallization took place partially at grain boundaries when strained at 600 °C. The addition of Cr and hence formation of carbides in carbon-added steels have little influence on plasticity of the conventional twinning-induced plasticity steels, suggesting the possibility of developing Cr-bearing TWIP steels with both corrosion resistance and excellent formability.

Effects of constrained groove pressing on mechanical properties of a TWIP steel

The evolution of mechanical properties and microstructure of a twinning induced plasticity (TWIP) steel sheet during constrained groove pressing (CGP) was investigated. It was found that by imposing CGP, the volume fraction of twins, hardness and yield strength increase significantly, while ductility decreases. The results of the pin-on-disk wear test showed that the CGP process improves the wear resistance of the TWIP steel, where oxidation is the main wear mechanism. The effect of annealing treatment at various temperatures on the hardness evolution of the CGP-treated samples was also investigated. Moreover, a modified work-hardening model was applied to predict the flow behaviour of TWIP steel during the severe plastic deformation. The model successfully predicts the flow curve and measured dislocation density and twin evolution.

Ultrasonic Assessment of the Influence of Cold Rolling and Recrystallization Annealing on the Elastic Constants in a TWIP Steel.

The evolution of the elastic constants, , and Poisson’s ratio and acoustic birefringence of a Fe-0.5 wt% C-21.5 wt% Mn twinning-induced plasticity (TWIP) steel with reduction by cold rolling and recrystallization annealing was assessed from measurements of the times of flight of ultrasonic waves propagating along the thickness of the rolled plates. As the reduction increased, changes in the elastic constants resulted in a steadily increasing orthotropy, which was clearly shown by Poisson’s ratio and acoustic birefringence. Although optical metallography and hardness measurements showed that partial or full recrystallization is attained after annealing at 600 °C and 700 °C, the ultrasonic measurements revealed that a high level of orthotropy remains.

Effect of grain size on dynamic strain aging behavior of C-bearing high Mn twinning-induced plasticity steel

In this work, the effects of grain size on the tensile properties, serrated flow, Portevin-Le Châtelier (PLC) band evolution, dislocation density, and microstructure evolution of Fe–22Mn-0.6C steel were investigated using digital image correlation (DIC), X-ray diffraction (XRD), transmission electron microscope (TEM) and monotonic tensile tests. Results show that the yield strength, tensile strength, and work hardening value of Fe–22Mn-0.6C steel increase whereas its elongation decreases with decreasing grain size. The critical strain of serration decreases whereas the amplitude of serration increases with decreasing grain size. Moreover, the plateau of serrations is larger and that the strain concentration within PLC bands is more severe in fine-grained steel than in coarse-grained steel. The dynamic strain aging (DSA) behavior of the steel is enhanced by grain refinement. The speed of PLC band movement decreases faster in fine-grained steel than in coarse-grained steel, which results in a decrease in plasticity. XRD and TEM analyses show that the high dislocation density and unique twin substructure may be the main reasons for the strong DSA phenomenon observed in fine-grained steel.

Design and Test Verification of Energy Absorption Material in the Soft Landing Gear for “Tianwen 1” Mars Probe

The landing gear is a key component for soft-landing on Mars, which should use high energy-absorption and excellent thermal stability material to absorb the impact energy of Mars probe. The energy absorption pull rods made by directionally solidified twinning-induced plasticity (TWIP) were utilized for the soft landing gears of Mars lander. The mechanical behavior and microstructure evolution of TWIP steel were investigated. The results show that TWIP steel can sustain the high plasticity of 600 MPa ultimate true stress and 72% true strain. The landing dynamic simulation and experiment results of soft-landing gear drop testes demonstrate that that Mars landing system is able to adapt to the complex topography conditions because of the high energy absorption ability of pull rods.

Effect of silver in thermal treatments of Fe-Mn-C degradable metals: Implications for stent processing

Twinning-induced plasticity (TWIP) steels are considered excellent materials for manufacturing products requiring extremely high mechanical properties for various applications including thin medical devices, such as biodegradable intravascular stents. It is also proven that the addition of Ag can guarantee an appropriate degradation while implanted in human body without affecting its bioactive properties. In order to develop an optimized manufacturing process for thin stents, the effect of Ag on the recrystallization behavior of TWIP steels needs to be elucidated. This is of major importance since manufacturing stents involves several intermediate recrystallization annealing treatments. In this work, the recrystallization mechanism of two Fe-Mn-C steels with and without Ag was thoroughly investigated by microstructural and mechanical analyses. It was observed that Ag promoted a finer microstructure with a different texture evolution, while the recrystallization kinetics resulted unaffected. The presence of Ag also reduced the effectiveness of the recrystallization treatment. This behavior was attributed to the presence of Ag-rich second phase particles, precipitation of carbides and to the preferential development of grains possessing a {111} orientation upon thermal treatment. The prominence of {111} grains can also give rise to premature twinning, explaining the role of Ag in reducing the ductility of TWIP steels already observed in other works. Furthermore, in vitro biological performances were unaffected by Ag. These findings could allow the design of efficient treatments for supporting the transformation of Fe-Mn-C steels alloyed with Ag into commercial products.

Investigating the dislocation reactions on \u03a33{111} twin boundary during deformation twin nucleation process in an ultrafine-grained high-manganese steel

Some of ultrafine-grained (UFG) metals including UFG twinning induced plasticity (TWIP) steels have been found to overcome the paradox of strength and ductility in metals benefiting from their unique deformation modes. Here, this study provides insights into the atomistic process of deformation twin nucleation at Σ3{111} twin boundaries, the dominant type of grain boundary in this UFG high manganese TWIP steel. In response to the applied tensile stresses, grain boundary sliding takes place which changes the structure of coherent Σ3{111} twin boundary from atomistically smooth to partly defective. High resolution transmission electron microscopy demonstrates that the formation of disconnection on Σ3{111} twin boundaries is associated with the motion of Shockley partial dislocations on the boundaries. The twin boundary disconnections act as preferential nucleation sites for deformation twin that is a characteristic difference from the coarse-grained counterpart, and is likely correlated with the lethargy of grain interior dislocation activities, frequently seen in UFG metals. The deformation twin nucleation behavior will be discussed based on in-situ TEM deformation experiments and nanoscale strain distribution analyses results.

Effects of Crystal Orientation on Deformation Twinning and Dislocation Slip in Single Crystal Micro-pillars of a Twinning-Induced Plasticity Steel

Effects of Crystal Orientation on Deformation Twinning and Dislocation Slip in Single Crystal Micro-pillars of a Twinning-Induced Plasticity Steel

Statistical study on the effects of heterogeneous deformation and grain boundary character on hydrogen-induced crack initiation and propagation in twining-induced plasticity steels

The initiation and propagation of hydrogen-induced cracks (HICs) in twining-induced plasticity (TWIP) steel were quantificationally explored based on the HIC statistics at a macro level and a micro level by slow strain rate tensile (SSRT) with in-situ hydrogen-charging and electron microanalysis. The heterogeneous deformation and grain boundary character determines a HIC initiation. The slip transfer parameter m′ < 0.93 and the minimum residual Burgers vector of residual dislocations on a grain boundary br > 0.19 were the critical condition for HIC initiation. Moreover, HICs prone to initiate and propagate along random grain boundaries (RGBs), while the Σ3 grain boundaries and vice.

Direct Formed High-Strength Bolt with Hot-Rolled Twinning-Induced Plasticity Steel Using Its High Strain Hardening Rate

Direct Formed High-Strength Bolt with Hot-Rolled Twinning-Induced Plasticity Steel Using Its High Strain Hardening Rate

Low-cycle fatigue life and plasticity mechanisms of a Fe−15Mn−10Cr−8Ni−4Si seismic damping alloy under cyclic loading at various temperatures

A recently developed Fe−15Mn−10Cr−8Ni−4Si (FMS) seismic damping alloy has a superior fatigue life at room temperature due to reversible dislocation motion associated with the γ → ε martensitic transformation. The effect of temperature on the low-cycle fatigue life (Nf) and the associated plasticity mechanisms of the FMS alloy were evaluated between 253 and 393 K. The longest Nf of 18,029 cycles was obtained at 333 K (at the total strain amplitude of 1%). The Nf was dependent on the plasticity mechanisms, which were subdivided into three temperature regions with respect to the upper temperature limits for stress-assisted and strain-induced martensitic transformation (Msσ and Md, respectively). At temperatures below Msσ, the Nf exceeded 5,000 cycles, where ε-martensite was dominant and a long period stacking ordered structure was formed. The longest Nfvalues of over 10,000 cycles were obtained in the dual γ/ε-phase formed between Msσ and Md, while Nf decreased rapidly as the deformation temperature increased beyond Md. The effect of temperature on the Nf of the FMS alloy was comparable to that of Fe–28Mn–6Si–5Cr shape memory alloy, and to the chemical composition dependence of Nf of Fe–30Mn–(6 – x)Si–xAl (x = 0-6) transformation- and twinning-induced plasticity (TRIP/TWIP) steels. A new set of thermodynamic parameters was established to calculate the Gibbs free energy difference between the phases (ΔGγ→ε) and the stacking fault energy of austenite (ΓSFE). The superior Nf was associated with the cyclic strain-induced martensitic transformation when ΔGγ→ε was between –50 and 100 Jmol−1, while reversible martensitic transformation relied on a ΔGγ→ε of ∽0 Jmol−1.

Twinning behavior and hydrogen embrittlement of a pre-strained twinning-induced plasticity (TWIP) steel

The twinning behavior and hydrogen embrittlement (HE) of a twinning-induced plasticity (TWIP) steel were investigated for different pre-strains at different strain rates. The lower pre-strain rate led to thinner twins and higher twin volume fraction. The influence of the twin characteristics on HE sensitivity was analyzed from statistical data on the evolution of surface crack characteristics during secondary straining period after cathodic hydrogen charging. Based on the ratio of crack initiation sites caused by the intersection of twins and grain boundaries, it was found that thinner twins in twin bundles of similar thickness promoted the generation of hydrogen-induced cracks (HICs).

Precipitation behavior of carbides and cryogenic toughness in heat-affected zone of high-Mn twinning-induced plasticity steel welded joint

The newly designed cryogenic high-Mn steel plates (Fe-23Mn-1.5Al-0.45C) were joined by metal inert gas (MIG) multi-pass welding process. To evaluate the inhomogeneity of microstructure and cryogenic toughness in heat-affected zone (HAZ) of welded joint, the impact samples were prepared by zonal sampling method. The microstructure features and embrittlement mechanism in HAZ of welded joint were systematically investigated. The results reveal that grains obviously grew in the coarse-grain HAZ (CGHAZ). C element segregated and nano-sized lamellar and granular (Cr, Mn) 7 C 3 carbides precipitated at the grain boundary in HAZ, and the size of precipitate carbides in CGHAZ and fine-grain HAZ (FGHAZ) was about 100 nm and 40 nm, respectively. The interface between (Cr, Mn) 7 C 3 and γ-austenite matrix in CGHAZ was incoherent with the lattice misfit of 45.13%. Higher fraction of low angle grain boundaries (LAGBs) appeared in CGHAZ (25.89%) and FGHAZ (11.99%), compared with the base metal (4.33%). Such microstructural differences can degrade the impact toughness in CGHAZ. From cryogenic Charpy impact tests, the impact toughness in different zones of the joint at each test temperature follows this sequence: base metal > fine-grain HAZ > coarse-grain HAZ > fusion line. These results can provide a fundamental understanding and guidance on the brittle failure of cryogenic high-Mn steel welded joint during service.

Thermodynamic activity of FeO in CaO–SiO2–MnO–FeO–MgO molten slags

ABSTRACT To understand the thermodynamic characteristics of the melting process for high manganese twinning-induced plasticity steel, the activity coefficient of FeO in molten slag of MnO–CaO–SiO2–FeO–MgO slag system with up to 52 mass% MnO was measured at 1450°C by the experiments of the iron equilibrium between liquid silver and molten slag under the mixed gas atmosphere of CO, CO2 and Ar. The effects of MnO, FeO and the basicity on the activity coefficient of FeO in molten slag were discussed. The relationships between the activity coefficients of FeO, the concentrations of FeO and MnO and the basicity ((mass% CaO + mass% MgO)/mass% SiO2) were investigated by the regression analysis method. The results show that the activity coefficient of FeO was proportional to the reciprocal of FeO concentration, and increased slightly with the increasing MnO, but slowly decreased by increasing the basicity under the research conditions.

Microstructure Evolution and Recrystallization Temperature Change of Cold-Rolled Fe–19Mn–0.6C Twinning-Induced Plasticity Steel during Annealing

Ultra-high twinning-induced plasticity (TWIP) steel is receiving increasing attention in the automobile industry. Self-designed Fe–19Mn–0.6C TWIP steel was subjected to reveal the relationship between microstructures, which were related to recrystallization starting/ending temperature and cold rolling. The results indicated that initial deformation twins, secondary deformation twins, and nano-twins were successively generated in rolled TWIP steel with the increase of cold rolling, deformation twins, and dislocations, as well as with the elongation of grains. The elements remained uniformly dispersed rather than agglomerated in the twin crystals and grain boundaries. The recrystallization starting temperature changes of TWIP steel were 500–525, 400–425, 400–415, and 400–410 °C at cold rolling deformations of 25%, 50%, 75%, and 88%, respectively. Furthermore, the obtained corresponding recrystallization ending temperature changes were 580–600, 530–550, 520–540, and 500–520 °C, respectively. The linear relationship between cold deformation and hardness suggests that cold rolling can increase dislocation density and thus facilitate improving the hardness of TWIP steel.