Controlling Mechanical Behavior of TWIP Steels by Tuning Texture and Stacking Faults

Metallurgical and Materials Transactions A - We developed an Fe–22Mn–0.6C–1.5Al TWIP steel and investigated how thermo-mechanical processes affect the mechanical properties. In...     

Fe Mn C系TWIP钢的拉伸应变硬化行为研究

 通过试验研究了Fe Mn C系TWIP钢的拉伸应变硬化行为。结果表明：试验钢在拉伸过程中的应变硬化表现为阶段性多n值行为。其应变硬化机制有两种：在0.4%～3%的应变区间为位错强化阶段,10％～50%的应变区间为孪晶强化阶段。在这两个区间内n值皆为定值,而在4％～10％的应变区间n值呈不断上升趋势。     

Microstructures and Mechanical Properties of Fe-Mn-（Al, Si） TRIP/TWIP Steels

The mechanical properties and microstructure of two low carbon high manganese steels with 23.8% （No. 1） and 33% （No. 2） （mass percent） of manganese were investigated. The results showed that No. 1 steel possesses high strength and high plasticity, and No. 2 steel has a relatively high strength and extraordinary plasticity. The No. 1 steel exhibits both TRIP （transformation induced plasticity） and TWIP （twin induced plasticity） effects during the deformation; while only TWIP effect appeared under the same deformation condition for No. 2 steel. The comparison between the microstructures and mechanical properties of two steels was made, and the strengthening mechanisms were also analyzed.     

Effect of V on Hot Deformation Characteristics of TWIP Steels

Twinning induced plasticity (TWIP) steels, which rely on high Mn contents to promote twinning as the deformation mechanism, exhibit high ultimate strengths together with outstanding combinations of ultimate strength and ductility. In terms of mechanical properties, one of the most important microstructural features is grain size. The knowledge of the kinetics of recrystallization mechanisms, i.e., dynamic recrystallization (DRX) and static recrystallization (SRX), can be used in order to control the grain size of the final product by a proper rolling schedule design. The focus of this work is the characterization of the DRX kinetics of two TWIP steels. The basic composition of the steels is Fe–21Mn–0.4C–1.5Al–1.5Si, and one of them is further alloyed with 0.12% V. With this objective, compression tests were carried out at 900, 1000, and 1100°C and strain rates ranging from 1 × 10^?1 s^?1 to 1 × 10^?4 s^?1. Furthermore, metallographic observation by optical microscopy (OM) was done to assess the evolution of grain size for the different deformation conditions. According to the results, the existence of V in the composition does not affect the hot flow behavior of the steel, although recrystallization fraction and recrystallized grain size decrease for the V‐containing steel.     

Constitutive Modeling of Hot Deformation Behavior of the AA6063 Alloy with Different Precipitates

The current study proposes a simple constitutive model that integrates the kinetics of precipitation during static aging and the kinetics of precipitate dissolution during preheating to deformation temperature to predict the hot flow behavior of AA6063 alloy. The model relates the flow behavior of the age-hardenable alloy to the alloy chemistry, thermal history as well as deformation temperature, strain, and strain rate by means of a physically based model. Different aging conditions, including supersaturated solid solution and overaging conditions with different deformation parameters, were assessed. Each part of the model was in good agreement with those of experimental and other model results published in the literature.     

碳含量对Fe-Ni-Mn-Si-C系TWIP钢应变硬化行为的影响

采用单向拉伸、金相组织观察、TEM等方法,研究了3种不同碳含量Fe-Ni-Mn-Si-C系TWIP钢的拉伸应变硬化行为。研究结果表明,随着碳含量的增加,合金的强度和伸长率均提高,当碳质量分数为1.0%时,该TWIP钢具有较好的综合力学性能,强塑积达到83 160 MPa.%。3种TWIP钢的真应力-真应变曲线均不完全遵...     

Influence of the Starting Microstructure on the Hot Deformation Behavior of a Low Stacking Fault Energy Ni-based Superalloy

Metallurgical and Materials Transactions A - The influence of varying fractions of primary gamma prime precipitates on the hot deformation and annealing behavior of an experimental Nickel-based...     

Low Cycle Fatigue Behavior and Deformation Mecha-nism of TWIP Steel

Low cycle fatigue behavior of TWIP （twinning induced plasticity） steel was investigated in axial symmetric tension-compression cyclic loading pattern. Fracture surfaces and microstructures were examined by optical, scanning electron and transmission electron microscopes. It was found that the fatigue life at the strain amplitude of 0.4 % is up to 15 000 cycles, which is much longer than TRIP780 and HSLAS00 steels. The strain hardening and softening features are significant until the strain amplitude comes to 1.25 ~. Persistent slip bands and tiny mechanical twinning layers were observed after fatigue deformation. Deformation mechanism of TWIP steel at low cycle fatigue process is not only twinning, but a complex of both twinning and persistent slip bands.     

Strain Hardening Behavior of High Performance FBDP,TRIP and TWIP Steels

Based on n‐value differential equation and microstructural observation, strain hardening behaviors of FBDP, TRIP, and TWIP steels during uniaxial tension were investigated. TRIP steel exhibits both superior strength and ductility than FBDP steel, and TWIP steel displays much higher total and uniform elongations in comparison to FBDP and TRIP steels. The instantaneous n values of FBDP and TRIP steels increase at small strains, reach a maximum value, smoothly decrease at higher strains, and then rapidly drop up to the specimen rupture. The strain hardening of TRIP steel persists at higher strains where that of FBDP steel begins to diminish. TWIP steel exhibits gradually increased instantaneous n values over the whole uniform plastic deformation, implying that TWIP steel shows a much larger strain hardening capability than FBDP and TRIP steels.     

Effects of Twin-Dislocation and Twin-Twin Interactions on the Strain Hardening Behavior of TWIP Steels

 In the present paper, tensile tests of Fe-30Mn-5Si -2Al steel were carried out for different strains of 0.05, 0.14, 0.26, and up to the strain-to-failure in order to observe the evolution of microstructure during deformation and investigate the strain hardening behavior. Three-stage strain hardening behavior was observed in this steel during tensile test. In stage I, planar dislocation structure was observed by TEM to be the main deformation mechanism, and low strain hardening rate exponent was exhibited. Primary deformation twinning occurred in stage II, and the strain hardening rate exponent increased due to the blockage of dislocations’ motion by twin boundaries. In stage III, the strain hardening rate exponent had increased to be higher than 0.5. The obstacle effect of twin boundaries and twin-twin interaction had been observed by TEM, and the interactions between primary and secondary twins were found to cause the additional hardening in addition to the obstacle effect on dislocations’ motion, which led to the twinning induced plasticity effect in the later stage of deformation.     

A Stacking Fault Energy Perspective into the Uniaxial Tensile Deformation Behavior and Microstructure of a Cr-Mn Austenitic Steel

A Cr-Mn austenitic steel was tensile strained in the temperature range 273 K (0 °C) ≤ T ≤ 473 K (200 °C), to improve the understanding on the role of stacking fault energy (SFE) on the deformation behavior, associated microstructure, and mechanical properties of low-SFE alloys. The failed specimens were studied using X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy. The SFE of the steel was estimated to vary between ~ 10 to 40 mJ/m² at the lowest and highest deformation temperatures, respectively. At the ambient temperatures, the deformation involved martensite transformation (i.e., the TRIP effect), moderate deformation-induced twinning, and extended dislocations with wide stacking faults (SFs). The corresponding SF probability of austenite was very high (~10^?2). Deformation twinning was most prevalent at 323 K (50 °C), also resulting in the highest uniform elongation at this temperature. Above 323 K (50 °C), the TRIP effect was suppressed and the incidence of twinning decreased due to increasing SFE. At elevated temperatures, fine nano-sized SF ribbons were observed and the SF probability decreased by an order (~10^?3). High dislocation densities (~10¹⁵ m^?2) in austenite were estimated in the entire deformation temperature range. Dislocations had an increasingly screw character up to 323 K (50 °C), thereafter becoming mainly edge. The estimated dislocation and twin densities were found to explain approximately the measured flow stress on the basis of the Taylor equation.     

Effect of Carbon Fraction on Stacking Fault Energy of Austenitic Stainless Steels

The effect of C fraction (C/N) on stacking fault energy (SFE) of austenitic Fe-18Cr-10Mn steels with a fixed amount of C?+?N (0.6?wt pct) was investigated by means of neutron diffraction and transmission electron microscopy (TEM). The SFE were evaluated by the Rietveld whole-profile fitting combined with the double-Voigt size-strain analysis for neutron diffraction profiles using neutron diffraction. The measured SFE showed distinguishable difference and were well correlated with the change in deformation microstructure. Three-dimensional linear regression analyses yielded the relation reflecting the contribution of both C?+?N and C/N: SFE (mJ/m²)?=??C5.97?+?39.94(wt pct C?+?N)?+?3.81(C/N). As C fraction increased, the strain-induced ?????? martensitic transformation was suppressed, and deformation twinning became the primary mode of plastic deformation.     

Weak Beam TEM Study on Stacking Fault Energy of High Nitrogen Steels

The nature of the high work‐hardening rate of nitrogen bearing steels was examined focusing on the stacking fault energy (SFE). The dislocation configuration and the width of dissociated dislocations were evaluated in various kinds of austenitic stainless steels with and without nitrogen, using the weak beam method. Nitrogen addition resulted in changing the dislocation configuration from tangled to planar. Nitrogen was, however, found to increase the SFE rather than decrease as reported previously and the SFE can be formulated as a function of chemical composition, SFE(mJ/m²) = 5.53 ‐ 0.16 (wt%Cr) + 1.40 (wt%Ni) + 17.10 (wt%%N). These results indicate that dislocation planarization by nitrogen addition is inadequately explained in terms of SFE.     

Tensile Deformation Behavior of Fe-Mn-Al-C Low Density Steels

Room temperature tensile tests of Fe-Mn-Al-C low density steels with four different chemical compositions were conducted to clarify the dominant deformation mechanisms.Parameters like product of strength and elongation,as well as specific strength and curves of stress-strain relations were calculated.The microstructures and tensile fracture morphologies were observed by optical microscope,scanning electron microscope and transmission electron microscope.The tensile behavior of low density steel was correlated to the microstructural evolution during plastic deformation,and the effects of elements,cooling process and heat treatment temperature on the mechanical properties of the steels were analyzed.The results show that the tensile strength of steels with different cooling modes is more than 1 000 MPa.The highest tensile strength of 28Mn-12Al alloy reached 1 230 MPa,with corresponding specific strength of 189.16 MPa·cm~3·g~(-1),while the specific strength of 28Mn-10 Al alloy was 178.98 MPa·cm~3·g~(-1),and the excellent product of strength and elongation of 28Mn-8Al alloy was over 69.2 GPa·%.A large number of ferrite reduced the ductility and strain hardening rate of the alloy,while the existence of κ carbides may improve the strength but weaken the plasticity.Some fine κ carbides appeared in the water-quenched specimen,while coarse κ carbides were observed in the air-cooled specimen.High temperature heat treatment improved the decomposition kinetics of γ phase and the diffusion rate of carbon,thus speeded up the precipitation of fine κ carbides.The dominant deformation mechanism of low density steel was planar glide,including shear-band-induced plasticity and microbandinduced plasticity.     

Effect of the Deformation Twin Density on the Corrosion Behavior in TWIP Steel

Based on its excellent tensile strength-ductility property combination,twinning-induced plasticity (TWIP) steel shows great potential in applications for structural components in automobile industry.The aim of this research is to investigate the corrosion resistance properties and corrosion mechanism under room temperature in TWIP steel.The influence of the deformation twin density on corrosion property was primarily considered by salt spray test.The specimens used in the investigation are as-annealed and as-deformed respectively.The microstructure and corrosion resistance property were characterized by scanning electron microscope (SEM),optical microscope (OM) and so on.There are some annealing twins distributed randomly in austenitic grains in the as-annealed specimen.After the specimen was subjected to tensile experiment,the density of the deformation twins increased sharply,which are different from the annealing twins in size and morphology.It was found that the corrosion potential of the as-annealed is lower than that of the as-deformed and the corrosion current density behaves contrarily.After immersed in 5% NaCl solution salt spray for 48h,the as-deformed showed a bit better than the as-annealed in corrosion resistance.With the time prolonged,the gap between the two specimens in corrosion resistance increased rapidly.The corrosion morphologies varied in color and shape.Further investigation,carried out by SEM and EDS,indicated that as-annealed and the as-deformed followed pitting corrosion and uniform corrosion mechanism respectively.The reason for the difference in corrosion mechanism is possibly the presence of the deformation twins.The deformation twins formed during the tensile test refine grains by way of segmentation.The twin boundaries largely belong to the coincidence site lattice (CSL),which is on lower energy state.It suggests that the twins not only play a role in strengthening,but also improve effectively the corrosion resistance in TWIP steel.     

Constitutive Modeling of Inherently Anisotropic Sand Behavior

A plasticity constitutive framework for modeling inherently anisotropic sand behavior is presented within a modified form of critical state soil mechanics. A second-order symmetric fabric tensor, Fij, describes the material inherent anisotropy, and a scalar-valued anisotropic state variable A is properly defined in terms of a joint invariant of Fij and the stress tensor. The location of the critical state line in the plane of void ratio and effective mean normal stress is not fixed but depends on A, rendering the soil dilatancy also a function of A. In addition, the plastic modulus is made a function of A. The incorporation of these two modifications in terms of A in an existing stress-ratio bounding surface model, achieves the successful simulation of both the contractive and dilative responses of sand over a wide range of variations in stress and density as shown by experimental data. Of particular significance are the results which exhibit the drastic effect of different principal stress orientations in reference to the material axes of anisotropy.     

Dependence of Deformation Twinning on Grain Orientation and Texture Evolution of High Manganese TWIP Steels at Different Deformation Temperatures

 Mechanical properties, microstructure and texture evolution were studied in two tensile-deformed high manganese TWIP steels at different temperatures. Special attention was paid to the effects of deformation temperature and grain orientation on twinning behavior. The results showed that, at -70 ℃ and at room temperature, both twins and hexagonal martensite were found in a lower manganese steel of 26Mn. With deformation temperature rising, twins became less and they disappeared at 500 ℃. Strong <111> texture appeared at 300 ℃, while it weakened at 500 ℃ due to the low strain rate and higher stacking fault energy. EBSD measurement revealed the dependence of deformation twinning on grain orientation at all test temperatures.     

钒微合金化对高锰TWIP钢热加工行为的影响

重点研究钒微合金化高锰(20%~30%)TWIP钢的静态软化和应变诱导析出行为,考察了不同钒含量(0.1%、0.2%)和碳含量(0.2%、0.6%、1%)的影响,目标是确定钒在热轧后还能有进一步析出强化作用的条件(如:成分、热加工参数)。在700~1100℃温度范围内进行了双道次扭转试验,还对卷取进行了模拟(从700℃到550℃)。采用先进的分析方法,如通过EBSD来确定晶粒尺寸和再结晶分数,采用TEM来分析析出物。结果表明,在热加工温度范围内,钒的析出相对迟缓,而且只在20%Mn-0.6%C-0.2%V或30%Mn-1%C-0.1%V的成分条件下发生。将碳含量降低到0.2%时,只在650℃和700℃模拟卷取后才能观察到析出物。另外,在有应变诱导析出发生的情况下,回复、再结晶和应变诱导析出之间有着复杂的相互影响。静态再结晶被明显推迟,导致回复对软化动力学具有重要影响。