Al Twinning-induced Plasticity Steel Research Articles

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.

Corrosion fatigue behavior of Fe-16Mn-0.6C-1.68Al twinning-induced plasticity steel in simulated seawater

The nature of corrosion fatigue of twinning-induced plasticity (TWIP) steel in a simulated seawater environment was investigated through various tests. Results indicated that TWIP steel has high corrosion fatigue susceptibility, which is mainly caused by anodic dissolution at grain boundaries and twin boundaries. However, uniform plastic deformation tends to inhibit corrosion fatigue. The influence of plastic deformation on corrosion fatigue of TWIP steel was clarified through the aspects of microstructure evolution, mechano-electrochemical effect, fatigue damage, etc. Findings reveal that plastic deformation or metallurgical design for high grain boundary strength will improve the resistance of corrosion fatigue for TWIP steel.

The mechanism of mechanical twinning near grain boundaries in twinning-induced plasticity steel

The mechanism of mechanical twinning in the vicinity of a grain boundary, which has been considered as a twin nucleation site, was investigated using Fe-17.5Mn-0.58C-1.5Al twinning-induced plasticity steel. When tensile strain was 0.02, multiple slips occurred near grain boundaries. The interaction between dislocations on multiple slip planes resulted in stacking faults composed of Shockley and Frank partial dislocations at the locations slightly away from grain boundaries. When the strain reached 0.05, mechanical twins formed near grain boundaries to grow toward the interior of grains. This observation result matched well with twinning mechanism proposed by Miura, Takamura, and Narita.

Revealing the mechanical properties and microstructure evolutions of Fe–22Mn–0.6C–(x)Al TWIP steels via Al alloying control

Three kinds of Fe–22Mn–0.6C–(x)Al (wt%) Twinning-Induced Plasticity (TWIP) steels were designed by changing the Al content. Uniaxial, unloading-reloading and stress-relaxation tensile tests were carried out to evaluate the mechanical properties. Meanwhile, electron channeling contrast imaging (ECCI), transmission electron microscopy (TEM) and X-ray tomography (XRT) techniques were used to investigate the microstructure evolutions. Firstly, it was found that the Al addition reduced the back stress level, suppressed twinning capability, and increased the stacking fault energy (SFE), leading to concession of work-hardening rates. Secondary, the Al addition improved the friction stress level and promoted the short-rang order (SRO) effect, which brought the transformation of dislocation slip from wavy to planar mode. Last but not least, the dynamic strain aging (DSA) effect brought the pinning of dislocations, forming high dislocation activation volume (> 100 b3). With increasing plastic strain, the deformation twins and SRO effect restricted dislocations gliding, resulting in low dislocation activation volume (< 50 b3). The present study could further illustrate the effects of SFE, SRO and DSA on the mechanical properties and microstructure evolutions of Fe–Mn–C–(Al) system TWIP steels via Al addition.

Determination of liquidus temperatures in high-alloyed Fe–C–Mn–Si–Al steels using differential thermal analysis

The liquidus temperature of the Fe–C–Mn–Si–Al alloy was investigated by using an improved differential thermal analysis method, which effectively tackles down the manganese evaporation in the course of differential thermal analysis experiments for high-manganese twinning-induced plasticity (TWIP) steels at high temperature. It was found that the liquidus temperature is more strongly dependent on the silicon content than expected. By considering the high manganese content in the Fe–C–Mn–Si–Al TWIP steels, the effect of carbon content on the depression coefficient of manganese should not be ignored, which has considerable impact on the liquidus temperature. An equation was summarized to effectively predict the liquidus temperature for a wide range of high-manganese steels. Meanwhile, the prediction results of the equation are consistent with the experimental results, as well as those results acquired from ThermoCalc.

Stacking-fault energy, mechanical twinning and strain hardening of Fe-18Mn-0.6C-(0, 1.5)Al twinning-induced plasticity steels during friction stir welding

The effect of friction stir welding (FSW) on the microstructure, stacking-fault energy (SFE) and strain hardening rate (SHR) of Fe-18Mn-0.6C-(0 and 1.5)Al (wt.%) twinning-induced plasticity steels using three welding speeds (50, 100 and 200 mm min−1) was investigated. The yield strength of the FSWed 0Al and 1.5Al steels improved due to both grain boundary strengthening by grain refinement and dislocation hardening by the introduction of dislocations with an increase in the welding speed. Their SHR with three stages and without the yield drop increased due to the active mechanical twinning and the introduction of dislocations during the FSW when the welding speed was increased. Among the 0Al steels, 0Al-200 steel with a fine grain exhibited more active twinning than the coarse-grained specimen (0Al-50), which is contrast to the 1.5Al steel. Regardless of the specimens, the slight increase in the SFE, which was attributed to both the shear strain energy caused by the introduction of dislocations and the excess free energy by the grain refinement during the FSW, leads to an increase in the critical twinning stress (σtw). Despite the fine grain of the 0Al steel, the origin of its active twinning was the highly increased yield strength relative to σtw, and the promoted dislocation interactions, giving rise to an increase in the number of sites at which twin nucleation occurred.

Negative to positive transition of strain rate sensitivity in Fe-22Mn-0.6C-x(Al) twinning-induced plasticity steels

In this study, it is found that the high-manganese austenitic FeMnC twinning-induced plasticity (TWIP) steel exhibits negative strain rate sensitivity (SRS), which is contrary to other face-centered cubic (fcc) metals or alloys. The negative SRS phenomenon of Fe-22Mn-0.6C (wt%) TWIP steel is considered to be suppressed or even convert to be positive through adding Al element. Meanwhile, the relation between tensile strength and uniform elongation of the FeMnC(Al) TWIP steels displays a transition from trade-off to synchronously improvement with decreasing the strain rate and Al content. It is suggested that the negative SRS phenomenon of FeMnC TWIP steel can be mainly attributed to the twinning mechanism assisted by dynamic strain aging (DSA). This mechanism can also explain well the negative to positive transition of SRS in FeMnC(Al) TWIP steel with the addition of Al element.

The effects of grain size on yielding, strain hardening, and mechanical twinning in Fe–18Mn–0.6C–1.5Al twinning-induced plasticity steel

The objective of the present study was to investigate the influences of grain refinement on yielding, strain hardening, and mechanical twinning during tensile deformation in Fe-high Mn twinning-induced plasticity (TWIP) steel. For this purpose, Fe–18Mn–0.6C–1.5Al TWIP steels with average grain sizes of 2, 10, and 50μm were tensile tested at room temperature, and their stress–strain and strain hardening rate curves, dislocation densities, and microstructures were measured and analyzed by means of transmission electron microscopy and neutron diffractometry. The stress–strain curves showed a transition from continuous to discontinuous yielding with grain refinement, which was due to a lack of mobile dislocations, not due to mechanical twinning or martensitic transformation. The grain refinement increased the dislocation density, caused the planar to non-planar slip, and retarded primary and secondary mechanical twinning. The strain hardening rate–strain curves of TWIP steels used were able to be divided into five stages by the slope change. Until the stage III, dislocation hardening was predominant; at the stages IV and V mechanical twinning became more contributive to strain hardening. The suppression of both planar dislocation slip and mechanical twinning by grain refinement is most likely due to the increase in the back stress of dislocations on a slip plane, which was caused by the rapid accumulation of dislocations by plastic deformation in the fine-grained TWIP steel. A high level of back stress narrows the width of stacking faults, facilitates the cross slip of dislocations, and reduces the interactions between partial dislocations required for mechanical twinning.

An effective stacking fault energy viewpoint on the formation of extended defects and their contribution to strain hardening in a Fe–Mn–Si–Al twinning-induced plasticity steel

A deformation-dependent stacking fault energy (SFE) viewpoint is invoked to interpret the low strain rate tensile deformation of a Fe–27Mn–2.5Si–3.5Al austenitic steel at room temperature by using X-ray diffraction analyses and transmission electron microscopy (TEM) observations. The effective SFE of austenite increased with strain from ∼18 to 40mJm−2, and it was reasoned that this was the result of the increase in strain energy of the stacking faults (SFs) per unit area due to its dependence on the dislocation character and density. Twinning was observed at 2% strain and confirmed to occur by a glide mechanism of a6〈121〉 Shockley partial dislocations, leading to the formation of overlapping intrinsic–extrinsic SF pairs representing a three-layer twin embryo, revealing periodic dislocation contrast in TEM. The early onset of twinning is attributed to the unusually low critical twinning stress of the steel, ∼200MPa. In spite of twinning, the stacking fault probability (Psf) of twinned austenite was remarkably low (∼10−4) at low strains, but increased moderately (to ∼10−3) up to failure strain. At the emergence of twinning, the corresponding perfect dislocation density was low (∼1014m−2) but was well above the critical dislocation density required for twinning occurrence. Dislocation character analysis indicated that increasing deformation gradually changed the dislocation character from edge to screw type. The microstructural parameters of the steel estimated in direct or indirect relation to its SFE could explain its flow stress and strain hardening behavior.

Effect of High Tensile Strain Rate on the Evolution of Microstructure in Fe-Mn-C-Al Twinning-Induced Plasticity (TWIP) Steel

Fe-17.8Mn-0.52C-0.5Al TWIP steel has been investigated under high-strain rate conditions. Twinning along with stacking faults and high dislocation densities in the austenite matrix has been evaluated by X-ray diffraction line profile analysis and transmission electron microscopy. The samples strained at 100 s−1 show a gradient in the evolution of the dislocation density along the gage length except the fracture end where the density shows a decrease. In case of the samples strained at 1 s−1, the evolution of density shows attainment of a near-saturation stage. Electron backscatter diffraction analysis shows that the decrease in the dislocation density as well as near-saturation stage is due to dynamic recovery as well as dynamic recrystallization at region near the fracture end. The dynamically recrystallized grains are related to the deformed matrix through twin relationship.

Pre‐Blast Strengthening of Fe–18Mn–0.6C–1.5Al TWIP Steel

Armored vehicles are primarily designed to provide protection against blast and ballistic events and secondly to improve the manoevrability in battlefield. The manoevrability can be materialized by weight reduction of the vehicles, which also leads to the decrease in operating cost. To obtain improved performance without compromising blast and ballistic properties, explosion bulge testing was adpoted in this work to strengthening a austenitic twinning‐induced plasticity (TWIP) steel. The pre‐blast technique significanly reduced the grain size of the material which increased the hardness.The electron backscattered diffraction (EBSD) was conducted to study the microstructure and the dominant deformation mechanism was found to be dislocation slips. As a result, it was found that soft materials like TWIP steel lost a minimal toughness but showed strongly work‐hardened after blasts. Based on this result, it is possible that the lifecycle of armor vehicles could be significantly expanded and the repair cost could be greatly reduced.

Microstructure evolution and strengthening mechanisms of Fe–23Mn–0.3C–1.5Al TWIP steel during cold rolling

Abstract The effect of cold rolling on the microstructure evolution and mechanical properties of Fe–23Mn–0.3C–1.5Al twinning-induced plasticity (TWIP) steel was studied. The extensive mechanical twinning subdivides the initial grains into nanoscale twin lamellas. In addition, the formation of deformation micro bands at e >40% induces the formation of nanostructured bands of localized shear. It is demonstrated that the mechanical twinning is notably important for dislocation storage within the matrix, as the twin boundaries act as equally effective obstacles to dislocation glide as conventional high-angle grain boundaries. However, the contribution of the grain size strengthening to the overall yield stress (YS) is much smaller than that of the deformation strengthening, which plays a major role in the superior work-hardening behavior of TWIP steels. A very high dislocation density of ~2×10 15 m −2 is achieved after plastic deformation with moderate strains. The superposition of deformation strengthening and grain boundary strengthening leads to an increase in the YS from 235 MPa in the initial state to 1400 MPa after 80% rolling.

TEM investigation of the formation mechanism of deformation twins in Fe–Mn–Si–Al TWIP steels

The microstructure of a Fe–Mn–Si–Al twinning-induced plasticity (TWIP) steel exhibiting remarkable work hardening rate under uniaxial tensile deformation was investigated using transmission electron microscopy to uncover the mechanism(s) controlling the nucleation and growth of the mechanically induced twins. The results show that the stair-rod cross-slip deviation mechanism is necessary for the formation of the twins, while large extrinsic stacking faults homogenously distributed within the grains could act as preferential sources for the activation of the deviation process. The influence of such features on the thickness and strength of the twins and the resulting mechanical behaviour is discussed and compared to similar works recently performed on Fe–Mn–C TWIP steels.

&lt;i&gt;In Situ&lt;/i&gt; Neutron Diffraction Measurements of the Deformation Behavior in High Manganese Steels

Deformation behavior of high Mn TWIP (twinning induced plasticity) steels was observed using neutron diffraction. Two kinds of specimens were prepared; 0 and 2 wt% of Al TWIP steels. The lattice strains and peak widths of hkl grains were measured under tensile loading. The results provide an insight into the influence of the Al contents on the deformation behavior associated with the microstructure changes in TWIP steels.

The effects of Si on the mechanical twinning and strain hardening of Fe–18Mn–0.6C twinning-induced plasticity steel

The stacking-fault energy (SFE), dislocation slip, mechanical twinning, strain hardening, and yield and tensile strengths were systemically investigated in Fe–18Mn–0.6C–1.5Si twinning-induced plasticity (TWIP) steel. The results were also compared with those for Fe–18Mn–0.6C and Fe–18Mn–0.6C–1.5Al TWIP steels. The SFE decreased by 4mJm−2 per 1wt.% Si. The addition of Si increased both the yield strength, due mainly to solid solution hardening, and the tensile strength, owing to the high strain hardening that occurred while maintaining a large elongation of over 60%. To examine this high strain hardening, especially at low strains, the volume fractions of the primary and secondary mechanical twins were quantitatively evaluated by combining the merits of electron backscattered diffractometry and transmission electron microscopy. The volume fractions of both the primary and secondary twins were the highest in the Fe–18Mn–0.6C–1.5Si TWIP steel, which had the lowest SFE of the three TWIP steels. In particular, the volume fraction of the secondary mechanical twins increased rapidly with the addition of Si. The contributions of dislocation storage, mechanical twinning and dynamic strain aging (DSA) to the strain hardening were also quantitatively evaluated in the three TWIP steels. The Si-added TWIP steel had the highest strain hardening, due mainly to the active primary and secondary twinning, and experienced negligible DSA. In contrast, the Al-added TWIP steel exhibited the lowest strain hardening due to the reductions in both the mechanical twinning and DSA.

Effect of Cu addition on the mechanical behavior of austenitic twinning-induced plasticity steel

The effect of the addition of copper on the mechanical behavior of a Fe–12%Mn–0.7%C–1.0%Al twinning-induced plasticity steel was investigated by analysis of the mechanical properties obtained in uniaxial tensile tests by means of a physically based constitutive model. The addition of copper was found to retard the kinetics of twin formation and influenced the type of serrations on the stress–strain curve. The copper additions also resulted in a remarkable increase in total elongation without a loss of strength.

On the Stacking Fault Energy of Fe-18 Pct Mn-0.6 Pct C-1.5 Pct Al Twinning-Induced Plasticity Steel

The stacking fault energy (SFE) of Fe-18 pct Mn-0.6 pct C-1.5 pct Al twinning-induced plasticity (TWIP) steel was measured using weak-beam dark-field imaging of dissociated dislocations observed in transmission electron microscopy. The SFE was found to be 30 mJ/m2. A relatively wide scatter was observed in the experimentally measured partial dislocation separation of screw dislocations. It is argued that the anomalously wide partial dislocation separation is due to the interaction of point-defect pairs involving interstitial C atoms with the strain field of the partial dislocations. Internal friction (IF) experiments were carried out to detect the presence of point-defect pairs that might affect the dislocation separation, and a clear Finkelshtein–Rosin (FR) peak related to point-defect pairs involving interstitial C atoms was observed in the IF spectrum.

Effect of Annealing Temperature and Alloying Elements on the Mechanical Properties of Fe-Mn-C TWIP Steels

Twinning-induced plasticity (TWIP) steels have attracted great attention due to their excellent mechanical properties of high tensile strength (over 800MPa) and high ductility (over 50%), which result from the high strain hardening due to the mechanical twin formation during plastic deformation. The purpose of this study is to investigate the effect of annealing temperature and alloying elements on the mechanical properties of Fe-18Mn-0.6C TWIP steel. In 1.5%Al TWIP steel with 0.123%Ti content, the average recrystallized grain size was reduced to 2.5 μm by cold rolling and annealing at 800˚C for 5 min, because of the pinning effect of the fine TiC carbides on grain coarsening. The tensile strength was decreased and the ductility was improved with the increase of the annealing temperature. However, a reversion of hardness and yield strength happened between 750˚C and 800˚C due to TiC and M3C type precipitation. 0.56% Ni added TWIP steel exhibited relatively lower yield strength, because Ni precipitates were not formed during the annealing process. When this specimen was annealed at 800˚C for 5min, the tensile strength and elongation were revealed at 1096MPa and 61.8%, respectively.