Properties Of Twinning-induced Plasticity Steel Research Articles

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

Temperature Effect on Twin Initiation during Equal-Channel Angular Pressing and Mechanical Properties of Twinning-Induced Plasticity Steel

To examine the effect of temperature on twinning initiation and strain accumulation, a twinning-induced plasticity (TWIP) steel with chemical composition of Fe-25Mn-1.66Si-1.23Al was successfully prepared and subjected to severe plastic deformation (SPD) with equal-channel angular pressing (ECAP) at room temperature and warm temperature (300 °C), respectively. The microstructure was analyzed using different techniques, while its mechanical properties were measured by performing micro-tensile tests before and after the ECAP procedure. With such analyzed microstructure after ECAP procedure, the grains of the investigated TWIP steel are found to be elongated and the grain size decreases sharply especially at room temperature, compared to that before the ECAP procedure. The transmission electronic microscope (TEM) micrographs confirmed the existence of twins in all ECAP extruded conditions; however, at room temperature and after multiple passes of ECAP process at a higher temperature, it is easier to initiate secondary twins with a complex morphology. One pass of ECAP at room temperature can cause a strain effect of two passes at 300 °C. Strain hardening behavior was analyzed through ln(dσ/de) − lnσ plot; the results show that the annealing samples present a relatively high strain hardening capability while the strain hardening capability of ECAP-ed samples reduces gradually with the amount of total strain applied by the ECAP.

Drawing Direction Effect on Microstructure and Mechanical Properties of Twinning-Induced Plasticity Steel During Wire Drawing

The effect of drawing direction on microstructure and mechanical properties in twinning-induced plasticity (TWIP) steel during wire drawing has been investigated to improve drawability for wire rod applications. The steel wires subjected to unidirectional (UD) and reverse-directional (RD) wire drawing were analyzed. The yield and tensile strength processed by the RD were slightly higher than those by the UD, whereas the ductility and drawability have inverse features, which is totally different from the results of plain carbon steel and Al wire. Although final shear strain of wires deformed by the RD is lower than that by the UD, the RD wires had a more complex stress history and a higher texture component at the surface area, which encouraged twinning rate, resulting in fast exhaustion of ductility, finally earlier fracture of wires. In contrast to the materials deformed by mainly slip mechanism, the RD process cannot improve the ductility and drawability in TWIP steel due to the stress state dependency of twinning mechanism. Also, based on the different strengthening behaviors between pearlitic and TWIP steels during UD and RD, it is induced that microstructure and applied stress state simultaneously control the mechanical properties.

Improving the high-cycle fatigue properties of twinning-induced plasticity steel by a novel surface treatment process

A novel method named as surface spinning strengthening (3S) is proposed to achieve gradient microstructures in the surface layer of a high-Mn twinning induced plasticity (TWIP) steel. A hardened layer with depth larger than 1 mm is found in the 3Sed sample, accompanied by a gradient grain-size transition from ~300 nm to ~50 µm. In comparison with the as-received sample, the high-cycle fatigue property of the graded sample is enhanced mainly by suppressing the initiation of fatigue cracks, which has its origin from the structure and strength gradient.

Recovery of tensile properties of twinning-induced plasticity steel via electropulsing induced void healing

The effects of electropulsing treatment (EPT) and annealing on the tensile behaviors of twinning-induced plasticity (TWIP) steel after interrupted tension are compared. It is found that EPT effectively restores the tensile property of specimens with macro voids completely, while annealing could not. The selective heating and thermal compressive stress induced by EPT around voids make them readily to be healed while keeping the matrix unchanged. The healing effect of voids improves the strain-hardening capability of TWIP steels and leads to the recovery of strength and elongation of the EPT specimen, relative to those annealed counterparts.

Properties of Resistance Spot-Welded TWIP Steels

High manganese TWIP (twinning-induced plasticity) steels are particularly attractive for automotive applications because of their exceptional properties of strength combined with an excellent ductility. However, the microstructure and properties of TWIP steels are affected by excessive thermal cycles, such as welding and heat treatment. This paper deals with characterization and understanding the effect of welding current and time on the mechanical properties and microstructure of the resistance spot welded TWIP steel. For this purpose, weld nugget diameter was evaluated and the hardness, tensile shear strength of the weldment, and failure mode of samples were also determined. It has been found that the tensile shear strength of the samples increased with increasing welding current and welding time without expulsion, which reduces the strength of the weldment. Tensile shear samples failed by a partial interfacial fracture mode for low-heat input welds. The pullout fractures were observed with a sufficient heat input without expulsion.

Effect of crystalline grain structures on the mechanical properties of twinning-induced plasticity steel

In order to improve the mechanical properties of twinning-induced plasticity steel, the grain morphology was tailored by different solidification technologies combined with deformation and heat treatment processing routes. Three typical grain morphologies, i.e., equiaxed, columnar as well as equiaxed/columnar grains were formed, and their mechanical behaviors were comparatively studied. Among the three materials, the equiaxed grain material exhibited the highest strength but the lowest plasticity. Depending on the grain size, the smaller the grain size, the higher the strength, but the lower the elongation. The columnar grain material possessed the most excellent plasticity but the weakest strength. These properties presented a non-monotonic dependence on the dendrite spacing, and the moderate spacing resulted in the optimum combination of strength and plasticity. The equiaxed/columnar grain coexisted material showed interesting properties, i.e., the strength and plasticity were just between those of single grain-shaped materials. The three materials also presented different strain hardening behaviors particularly in the uniform deformation stage. The equiaxed grain material showed a constant strain hardening rate, while the columnar grain and equiaxed/columnar grain materials showed a progressively increasing rate with increasing the true strain. The introduction of equiaxed grains into the columnar grain material obviously enhances the strength but weakens the plasticity of the material. However, it seems that an appropriate amount of equiaxed grains will provide the material an optimum combination of strength and plasticity.

Effect of reduction of area on microstructure and mechanical properties of twinning-induced plasticity steel during wire drawing

The effect of reduction of area (RA), 10%, 20%, and 30%, during wire drawing on the inhomogeneities in microstructure and mechanical properties along the radial direction of Fe-Mn-Al-C twinning-induced plasticity steel has been investigated. After wire drawing, the deformation texture developed into the major and minor duplex fiber texture. However, the texture became more pronounced in both center and surface areas as the RA per pass increased. It also shows that a larger RA per pass resulted in a higher yield strength and smaller elongation than a smaller RA per pass at all strain levels. Although inhomogeneities in microstructure and mechanical properties along the radial direction decreased with increasing RA per pass, there existed an optimum RA per pass for maximum drawing limit. Insufficient penetration of strain from surface to center at small RA per pass (e.g., 10%) and high friction and unsound metal flow at large RA per pass (e.g., 30%) all resulted in heterogeneous microstructure and mechanical properties along the radial direction of drawn wire. On the other hand, 20% RA per pass improved the drawing limit by about 30% as compared to the 10% and 30% RAs per pass.

Grain refinement effect on cryogenic tensile ductility in a Fe–Mn–C twinning-induced plasticity steel

Cryogenic mechanical properties of twinning-induced plasticity steels of various grain sizes were investigated by the tensile tests. Specimens with coarse grain sizes, e.g. 10, 23 and 37μm, showed a brittle fracture at 123K. The embrittlement was suppressed by a grain refinement to 3.5μm by cold rolling and recrystallization. Furthermore, a grain refinement to 460nm by warm caliber rolling suppressed the embrittlement more effectively compared with the grain refinement via cold rolling and recrystallization treatment. The grain refinements improved the tensile elongation, yield strength, and ultimate tensile strength significantly at 123K. It was found that the suppression of the embrittlement was caused by the reductions in the amount of ε-martensite.

Strain rate sensitivity of C-alloyed, high-Mn, twinning-induced plasticity steel

The mechanical properties of twinning-induced plasticity (TWIP) steels are often assumed to be solely due to the reduction of the mean free path of glide dislocations resulting from deformation twinning. Other mechanisms may also play an essential role: Mn-C cluster formation, planar glide, pseudo-twinning, short range ordering, and dynamic strain ageing. The present contribution offers a critical analysis of the mechanical properties of high-Mn TWIP steels, especially in terms of Dynamic Strain Aging (DSA) and Static Strain Aging (SSA). The presentation offers new insights into the properties of TWIP steels which were obtained by using new experimental techniques such as in-situ strain analysis and high sensitivity infrared thermo-graphic imaging.

Effect of Nitrogen on Stacking Fault Formation Probability and Mechanical Properties of Twinning-Induced Plasticity Steels

Twinning induced plasticity (TWIP) steels containing nitrogen as the interstitial alloying element were designed, and the effects of N and Al on stacking fault energy (SFE), microstructure, and mechanical properties were investigated by using X-ray diffraction and transmission electron microscopy to determine the stacking fault formation probability. In addition, the microstructure and mechanical properties have been characterized. The results indicate that both N and Al decrease the stacking fault formation probability due to the increase of the SFE. The probability of localized stacking fault determined by a new method using electron diffraction spot shift reveals that stress- or strain-induced martensitic transformations (γfcc → ehcp → α′bcc) during deformation may consume a large number of stacking faults and give rise to a more marked decrease in the stacking fault formation probability of TWIP steels with lower SFE than of higher one(s). Based on their magnitude of SFEs, the temperature-dependent mechanical properties of four TWIP steels were analyzed.