Monotonic Tension And Tension-compression Cyclic Deformation Of High Manganese Austenitic Twip Steel

Chemical composition, grain size and loading rate are important factorsthat affect mechanical properties of materials. The major aim of this article isto explore in detail the effects of alloy element Al, grain size and strain rateon the monotonic tension and tension-compression cyclic deformationbehavior of high manganese austenitic TWIP steels under strain control modeat room temperature. The main results are as follows:1. Different types of serrations, depending on strain rate, occur in boththe coarse-and fine-grained Al-free steels during monotonic tensile loadingunder strain-controlled mode, whereas no serrated flow has been identified inthe Al-containing steel. Both the coarse-and fine-grained Al-free steels showa positive strain rate sensitivity of the critical strain in the high strain rateregion typified by type A serrations, yet a negative strain rate sensitivity inthe low strain rate region typified by type C serrations; as compared to thecoarse-grained Al-free steel, the fine-grained Al-free steel suppresses theserrated flow by increasing the critical strain for the onset of serrations,especially in the low strain rate region.2. The tensile strength and strain hardening rate of high manganese austenitic TWIP steels decrease with increasing tensile strain rate, showingnegative strain rate sensitivity. The variation of strain hardening rate iscontrolled by deformation mechanisms and microstructural evolution whichdepend greatly on the apparent stacking fault energy.3. Under cyclic loading, the cyclic stress response of high manganeseaustenitic TWIP steels is closely associated with the imposed strain amplitude,initial microstructural state, grain size and addition of aluminum. The Al-freeTWIP steel exhibits very strong cyclic hardening at all strain levels, whereasthe Al-containing TWIP steel showed cyclic hardening only when the strainamplitude is higher than a certain critical value, and exhibits cyclic softeningafter the critical value.4. Low-cycle fatigue lives of high manganese austenitic TWIP steelsdepend not only on the material tensile strength and ductility, but also on thecyclic plastic deformation mechanism. The relationship between elastic strainamplitude or plastic strain amplitude and the number of cyclic reversals up tofracture conforms to the Basquin or Coffin-Manson formula. Therefore, thefatigue performance parameters acquired in present article can be applied topredict the strain fatigue life of high manganese austenitic TWIP steels.5. A certain amount of slip bands are generated in the deformed grains of high manganese austenitic TWIP steels, and the number and width of slipbands depend on the applied strain amplitudes and number of cyclicdeformation. During cyclic plastic deformation, slip mode gradually transfersfrom one slip system to two or even three slip systems with increasing theimposed strain amplitude.