The grain size dependence of the tensile properties of a TWIP steel has been determined for a wide range of grain sizes obtained by grain growth after complete recrystallization of cold rolled material. The near-linear stress-strain behaviour typical of either TWIP steels or other materials that deform by twinning has been observed, the work hardening rate being larger for the smaller grain sizes. The Hall-Petch slope increases as a function of strain, from 350 MPa μm1/2 for the yield stress to 630 MPa μm1/2 for the maximum uniform strain in the tensile tests, ε  0.40. Profuse twinning is observed in deformed specimens by means of FIB-ISE.

Fatigue behavior of four high-Mn (18 to 22 wt pct Mn) steels, after cold rolling and soft annealing, were investigated in reversed plane bending in a high-cycle regime. The surfaces of fatigued specimens were examined using an optical microscope, a scanning electron microscope (SEM), and an atom force microscope (AFM). It was discovered that the chemical composition of the steel had only a minor influence on fatigue behavior, and the fatigue limit (FL), i.e., the stress amplitude leading to more than 2AE10 6 cycles to failure, of all steels is about 400 MPa. This is about 42 to 48 pct of their tensile strength whose value is comparable to that of austenitic stainless steel and 780 MPa transformation induced plasticity (TRIP) steel. Mechanical twinning does not occur in the course of cycling; however, intense slip bands are formed. Fatigue cracks tend to nucleate at quite an early stage of fatigue life and most commonly on grain and annealing twin boundaries due to intersections of slip bands and boundaries.Savings in fuel consumption as well as the safety of passengers demand that automotive manufacturers use new materials to enable weight reductions of vehicles and improve crash resistance. In this context, high-Mn steels showing twinning induced plasticity effect (TWIP) are a promising type of material providing high tensile strength in combination with extremely good ductility. The superductility (total elongations between 60 and 95 pct) with high tensile strength (600 to 1100 MPa) of TWIP steels are related to the tendency of mechanical twinning in straining due to a low stacking fault energy (SFE) on the order of 25 mJ/m 2 . [1,2] Several investigations [3,4] have been carried out to determine the mechanical properties of TWIP steels (Fe-Mn-Al and Fe-Mn-Al-Si) in tensile loading at low and ambient temperatures. Fabrication properties such as formability and weldability have also been studied and reported. [5]

Hot rolled, laboratory-cast, TWIP steel samples (5.4 mm thick) of 22% Mn - 0.6% C (in mass-%) were cold rolled to different reductions (from 40 % to 70 %) and subsequently isothermally annealed for various times at temperatures ranging from 450º C to 1100º C. The evolution of recrystallization and grain growth was followed by control of the softening kinetics complemented by metallographic, OIM and microtexture observations. A map of the recovery, recrystallization and grain growth in the temperature-time space was obtained. In all instances, the grain size at the end of recrystallization was very fine, D ≤ 2 µm and larger grain sizes were the result of grain growth. A range of grain sizes 2 µm ≤ D ≤ 50 µm was covered by the grain growth experiments. A phenomenological grain growth equation that is useful for the annealing control of this steel was derived from the measurements.

RESUMEN:Se ha estudiado la influencia de la velocidad de deformación y tamaño de grano en las propiedades mecánicas de un acero TWIP austenítico de composición 22% Mn, 0,6% C (% en masa). Para las velocidades de deformación de 9,4 s −1 y 265 s −1 a temperatura ambiente, se ha observado un comportamiento cuasi-lineal en las curvas tensión-deformación, típico en estos materiales que deforman por maclaje. Se ha mostrado que a altas velocidades de deformación la región con tasa de endurecimiento constante observada en aceros TWIP disminuye notablemente. Además, se ha obtenido la relación de Hall-Petch para cada velocidad de deformación. La pendiente de Hall-Petch K HP incrementa en función de la deformación en todos los casos. La dependencia de K HP con la velocidad de deformación podría ser causada por el calentamiento adiabático. ABSTRACT:Tensile properties and strengthening mechanisms of a TWIP steel at high strain rate: Hall-Petch relationship. The influence of strain rate and grain size on the mechanical properties of a 22% Mn, 0.6% C (mass %) austenitic TWIP steel has been studied. A typical quasi-linear stress-strain behaviour of TWIP steels that deform by twinning has been observed at strain rates of 9.4 s −1 and 265 s −1 and room temperature. At high strain rates, the constant work -hardening rate region typically observed in TWIP steel clearly shortens. In addition, the Hall-Petch relationship has been obtained for each strain rate. The Hall-Petch slope K HP increases as a function of strain in all cases. The dependence of the K HP on the strain rate could be adiabatic heating.

The optical Freedericksz transition for linearly polarized light at normal incidence is studied in mixtures of nematic E7 and cholesteric C15 in cells coated for homeotropic alignment. The reorientation process is found to be dramatically different from the case of pure nematic samples showing the phenomenon of optical phase locking and large hysteresis. These effects are ascribed to the occurrence of self-induced stimulated light scattering, which does not occur in pure nematics.