Papers by Keyword: TWIP

Abstract: At temperatures of 290 K and 77 K, the phase composition and mechanical properties ofnonequiatomic medium-entropy (MEA) alloys Fe40Mn40Co10Cr10 and Fe50Mn30Co10Cr10 werecompared in the coarse-grained (CG) and nanostructured (NS) states, in which additionaldeformation mechanisms are activated under load: phase transformations in the MEAFe50Mn30Co10Cr10 (MEA TRIP) and twinning in the MEA Fe40Mn40Co10Cr10 alloy (MEA TWIP). Itis shown that in the NS state in both alloys, in contrast to the CG state, a complete phase transitionfrom the fcc to the hcp phase is observed, the content of which weakly depends on the temperatureand the number of torsion revolutions during high-pressure torsion (HPT). The transition from theCG to the NS state leads to an increase in the microhardness (in the NS MEA TWIP by 3.7 and inthe NS MEA TRIP by 2.25). In the CG state, a thermally activated character of plastic deformationis observed for both alloys in the temperature range of 290 – 77 K. In the NS state, MEA TWIPremains plastic under active compression deformation at 290 K and 77 K, whereas in NS MEATRIP under similar conditions, macroscopic plasticity is absent. Tensile deformation up to 50 % at30 K in the CG state for both alloys leads to a significant decrease in the absolute values of Young'smodulus over the entire temperature range.

Abstract: To increase the safety of steels in high performance cases like crash energy absorption, even better properties of the materials are necessary. To advance this research, a TWIP and a TRIP steel were combined in a laminated composite via roll bonding at 450 °C with the goal of using accumulative roll bonding (ARB) in later research to further enhance the properties reaching an ultra-fine-grained material. Two different TWIP layer thicknesses (2 mm and 3 mm) were experimentally roll bonded with a 3 mm thick TRIP layer each using a 4-high rolling mill. A modular Python-based simulation incorporating coupled solving of ordinary differential equations of the temperatures and the horizontal stress changes of the layers were implemented to predict deformation and bonding behavior. Simulated results matched well with experimental data in terms of final geometry and temperature, while roll force deviations indicated the need for the refining of the used model. Furthermore, experimentally asymmetric layer relationships at the beginning and the addition of a thin (10 µm) Ni interlayer were found to enhance bond strength in high-strength steel laminates.

Abstract: Metastable β-type titanium alloys are highly suitable for use as structural biomaterials applied to hard tissue, i.e., as cortical bone (hereafter, bone) replacing implants. However, their mechanical biocompatibitities, such as the Young’s modulus, strength and ductility balance, fatigue strength, resistance against fatigue crack propagation and fracture toughness, require improvenent for increased compatibility with bone. Through deformation, the metastable β-phase in a metastable β-type titanium alloy is transformed into various phases, such as α’ martensite, α” martensite, and ω-phases with exact phase depending by metastable β-phase stability. In addition, twinning is also induced by deformation. Deformation twinning effectively enhances the work hardening in the metastable β-type titanium alloy, leading to increased strength and ductility. This improvement is accompanied by with other deformation-induced transformations including the appearance of deformation-induced martensite and ω-phase transformation. The enhancement of the mechanical biocompatibility of various materials using the abovementioned deformation-induced transformation is described in this paper, for both newly developed metastable β-type Ti-Mo and Ti-Cr alloys for biomedical applications.

Abstract: Al addition in TWIP steel not only reduces the specific weight but also increases the stacking fault energy which strongly affects the deformation mechanisms. Hot rolled air cooled TWIP steel with low Al content (1.61 wt. %) reveals duplex microstructure comprising austenite with ferrite, whereas steel with higher content of Al (3.56 wt. %) reveals fully austenite microstructure. It is evident that nano-twins are formed within austenite grain after 50% cold deformation. TWIP steel with the duplex microstructure exhibits an excellent combination of strength and ductility. Hardness and tensile strength values of air cooled steel specimens increase with a concomitant lowering of total elongation with the application of cold deformation. However, steel with low Al content shows higher hardness and tensile strength along with lower elongation as compared to the TWIP steel having higher Al content.

Abstract: Metal Matrix Composites (MMC) based on a TRIP (TRansformation Induced Plasticity)- or TWIP (TWinning Induced Plasticity)-steel matrix reinforced with MgO-partially stabilized zirconia (Mg-PSZ) are an interesting research field as both components exhibit a deformation-induced or stress-assisted martensitic phase transformation and twinning, respectively. The present work deals with the fatigue characteristics of a reinforced CrMnNi-steel as a function of the ceramic particle size. Therefore, the particles were classified into three grades (grade 1: <10 μm; grade 2: 10-30 μm; grade 3: 30-50 μm) whereas the volume fraction concerning the composite material was kept constant at 10 vol.%. The composites were produced using the hot pressing technique. The tests were performed under total strain control in a range of 0.2% ≤ Δε_t ≤ 1.2%. The microstructure of fatigued specimens was examined using scanning electron microscopy.

Abstract: The automotive TWIP steels are high-Mn austenitic steels, with a relevant C content, which exhibit a promising combination of strength and toughness, arising from the ductile austenitic structure, which is strengthened by C, and from the TWIP (TWinning Induced Plasticity) effect. The microstructure of the low-alloy Q&P steels consists of martensite and austenite and is obtained by the Quenching and Partitioning (Q&P) heat treatment, which consists of: austenitizing; quenching to the T_q temperature, comprised between M_s and M_f; soaking at the T_p partitioning temperature (T_p being equal to or slightly higher than T_q) to allow carbon to diffuse from martensite to austenite; and quenching to room temperature. The fatigue behavior of these steels is examined both in the as-fabricated condition and after pre-straining and welding operations, which are representative of the cold forming and assembling operations performed to fabricate the car-bodies. Moreover, the microscopic fracture mechanisms are assessed by means of fractographic examinations.

Abstract: A TWinning Induced Plasticity (TWIP) steel was cold rolled to 42% thickness reduction followed by isochronal annealing for 300 s between 600-850 °C. Bulk texture evolution during recrystallisation was investigated by X-Ray Diffraction. While the development of the α-fibre after cold rolling is typical of low stacking fault energy materials, anomalously higher intensities were noted for the Goss ({110}) compared to Brass ({110}) orientations. Upon recrystallisation, the main rolling texture components were retained and ascribed to nucleation at orientations close to those of the deformed matrix followed by annealing twinning which leads to crystallographically identical variants. Unlike previous texture investigations on austenitic steels, the relatively homogeneous deformation microstructure and uniform distribution of subsequent nucleation sites led to the retention of the F ({111}) orientation. Moreover, the firsthand observation of the Rotated Copper ({112}) orientation in TWIP steel is attributed to the second order twinning of the A ({110}) orientation.

Abstract: TWinning Induced Plasticity (TWIP) steels have been recently developed as a promising material for automotive applications. In the present work the recrystallisation behaviour of 42% cold-rolled Fe-24Mn-3Al-2Si-1Ni-0.06C TWIP steel was investigated during isochronal annealing for 300s via microhardness testing, Electron Back-Scattering Diffraction (EBSD) and uniaxial tensile testing. EBSD internal misorientation data corroborates recrystallised fraction estimates from microhardness measurements. Annealing twins play an important role during recrystallisation by bulging at the deformed grain boundaries during nucleation and generating twin related orientations. During uniaxial tension, the recovered condition recorded three work hardening regions while all partially recrystallised samples exhibited four regions. A modified Hollomon scheme is suggested to account for the effect of strain on microstructure refinement.

Abstract: By analysis of TWIP Steels with different manganese content, the results showed that the microstructures and properties had been changed with different Mn content. The elongation of the tested steel with 22.5% Mn was high for 55.5 % and n value of that reached to 0.360. When Mn content of the tested steel was 17.9%, the yield and tensile strength were higher and its elongation was lower for the tested steel than that of the tested steel with 22.5% Mn. The microstructures of the tested steel with high Mn content were austenite before and after being stretched at room temperature. Mn content was decreased and the microstructure of the tested steel after being stretched had a small amount of martensite transformation at room temperature. That is to say, double effect with TWIP and TRIP had occurred, but TWIP effect was dominant. TWIP effect increased plasticity and strain hardening capacity to improve formability. TRIP effect was mainly to improve strength so as to further attain the strength of the tested steel.

Abstract: At different temperatures ranging from ‑60°C to 200°C a cast CrMnNi-TRIP steel was deformed by uniaxial tension. The resulting microstructure was investigated using XRD, EBSD and LOM. The correlation of the phase transformation with the deformation temperature was examined. Depending on temperature, a transition in the deformation mechanisms was observed. Starting with the generation of deformation bands, accompanied by martensitic phase transformation, followed by twinning, the deformation mechanism turned to conventional dislocation glide with raising temperature. Between -60°C and 20°C the TRIP (TRansformation Induced Plasticity)-effect is the dominating deformation mechanism, whereas between 20°C and 200°C the TWIP (Twinning induced plasticity) effect is observed. The geometrical arrangement of martensite within the microstructure is considered within this study. The amount of α'-martensite is mainly responsible for the hardening rate and the resulting mechanical properties.