Mechanical Behavior Of Fe-20Mn-3Si-3Al Steels At Different Strain Rates And Temperatures

With the development of economy, the demand for automobile gradually increases. Meanwhile, in response to the recently increasing fuel cost and more strict regulations for safety and CO2 emission, new design concept is forthcoming for the construction of car with reduced specific weight, enhanced energy absorption and safety. Consequently, the simultaneous enhancement of strength and ductility of steels is still a important topic of automobile field. TWIP (Twinning induced plasticity) steels have received high interest in recent years due to their outstanding mechanical properties, such as good formability and super energy absorption capability. Hence, TWIP steels have the potation to improve the crash resistant and light-weight of transportation system.On the other hand, the mechanical behaviors of steels for the construction of car parts at high strain rates and various temperatures are important properties since there are different conditions at which materials deformed with impact load or high strain rate, such as metal forming and car accidents. For the late case, it concerns with the safety and property of person. Consequently, it is essential to research the mechanical behaviors of steels deformed under various temperatures and strain rates.In present study, mechanical properties of TWIP steel with a chemical composition of Fe-20Mn-3Si-3Al-0.045C(wt%) have been investigated. Tensile test were performed at extensive temperature range (25?,50?,100?,150?,200?,250? and 300?) and various strain rates (1×10-4s?1×10-3s-1?1×10-2s-1 and 1×10-1s-1). Based on the obtained results, the effect of temperature and strain rate on the mechanical properties of this material has been discussed. The microstructures of material after deformation have been observed using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The fundamental mechanism of fracture is also explored. The obtained results can shed a light on the working and the further improving mechanical properties of TWIP steels.Under low strain rate, the TWIP steel exhibits good ductility, in spite of the low strength. Both the yield stress and tensile strength increase with an increase in strain rate while the ductility follows a contrary way. The mechanical properties of this steel become poor with increasing temperature. For instances, the yield stress and tensile stress of the steel are 510MPa and 860MPa while the ductility reaches 56% under a strain rate of 1×10-s1 at room temperature. However, with increasing strain rate up to 1×10-1s-1, the yield stress goes up to 630 MPa and the tensile stress is as high as 970 MPa, in spite of the ductility of 42%. The tensile stress of the steel decreases to 764 MPa under 1×10-1s-1 at 300?. Obviously, the obtained results indicate the steel has presented remarkable strain rate sensitivity during deformation. Strain rate and temperature effect strain rate sensitivity factor and work hardening factor, both of them are increasing with the strain rate and temperature, and vice versa.At low temperature, the twinning process and the interaction between twin and dislocation play a key role during the deformation of the steel. However, the density of twin decreases and slip would dominate the deformation process with increasing temperature up to 300?. The fracture mechanism is investigated by conducting SEM observation on the necking part of deformed samples. It is noted that, with increasing strain rate, dimple gradually becomes small and shallow on the fracture surface of the sample. Meantime, the area of shear lip diminishes while fiber region increases. Due to a high strain rate, the speed of slip can't catch up with the tensile rate. Thus, the combination of small dimples is inhibited and brittle fracture is promoted, resulting in a poor ductility. On the contrary, the dimples become bigger and deeper and the area of shear lips increases with an increase in temperature. The size and distribution of dimples become more evenly. The measured poor ductility can be related with the density of dislocations decreases with the fact that increasing temperature, which results in an enhanced annihilation of dislocations. At the same time, the twinning process is inhibited with increasing temperature. Consequently, a lower strength and poor ductility are observed for the present steel.