Deformation Behavior Study Of High Manganese TRIP/TWIP Steels

High manganese TRIP/TWIP steel is an important component of advanced high strength steel.Due to its excellent impact resistance and the absence of low temperature tough-brittle transition,it has gradually become an ideal material for service under extreme stress conditions such as high strain rate,low temperature high strain rate and low temperature cyclic load.However,the deformation behavior and the deformation twinning mechanism of TRIP/TWIP steel under high strain rate when collision happens are under-researched.Against this background,this paper investigated three typical FeMn-Al-Si high manganese steels to explore the deformation behavior of high manganese TRIP/TWIP steel under different deformation conditions by using X-ray diffraction(XRD),scanning electron microscopy(SEM),electron back scattering diffraction(EBSD)and transmission electron microscopy(TEM).Combined with quasi-static deformation,Hopkinson dynamic impact and in-situ TEM deformation experiments,the deformation behavior of high manganese TRIP/TWIP steel under different deformation conditions was studied.The positive strain rate sensitivity mechanism and austenitic twinning mechanism of high manganese TRIP/TWIP steel were revealed.A reversible phase transition was observed between austenite and ε-martensite.The phase transition model,invariant line and habit plane were determined.It is proved that ε-martensite twin is an indispensable prerequisite for reversible phase transition.The main results are as follow:(1)The strain rate sensitivity of Fe-20/25/29Mn-3Al-3Si TRIP/TWIP steel under different deformation conditions was systematically studied.The yield strength increases with the increase of strain rate and shows a positive strain rate sensitivity.The higher the strain rate,the stronger strain rate sensitivity.The higher the temperature,the stronger the strain rate sensitivity of the material.(2)TEM results show that multiple deformation twinning can be induced during dynamic impact deformation of Fe-29Mn-3Al-3Si TWIP steel.The critical twinning stress of Fe-29Mn-3Al-3Si TWIP steel and shear stress of different sliding systems in the same grain are calculated,respectively.The results show that different twinned systems can be activated simultaneously to form a large number of deformation twins at the initial stage of dynamic impact deformation,which become the main reason that TWIP steel has higher yield strength and positive strain rate sensitivity during dynamic impact.(3)The deformation products of Fe-20Mn-3Al-3Si TRIP steel under quasi-static compression deformation and dynamic impact deformation were characterized,respectively.The deformation mechanism of quasi-static and dynamic deformation was revealed,respectively.The main deformation mechanism of Fe-20Mn-3Al-3Si TRIP steel in quasi-static deformation is martensitic transformation.In addition to martensitic transformation,ε-martensitic twining is also the main deformation mechanism during dynamic impact deformation.(4)The deformation behavior of Fe-25Mn-3Al-3Si steel under different dynamic impact deformation was studied,and the dynamic impact deformation mechanism at different temperatures was summarized.Martensitic transformation and twinning are responsible for plasticity deformation at room temperature.Perfect dislocation glide andγ-austenite twinning become the main deformation mechanism of the samples during dynamic impact deformation at 300 °C.When the deformation temperature of dynamic impact is-80 °C,the main deformation mechanism is reversible transformation of ε-martensite(35)γ-austenite and twinning.The sequence of reversible transformation is γ-austenite → ε-martensite → ε-martensite twin → reversed austenite and reversed austenite twin.Increasing strain rate or decreasing temperature is beneficial to the reversible phase transition.Based on HRTEM results,a phase transition model for reversible phase transition was proposed and ε-martensitic twin was proved to be an indispensable prerequisite for reversible phase transition.Two new orientations relation between austenite and ε-martensite has been observed during reversible phase transition.The quasi-O-line model has been used to calculate transformation crystallographic parameters,such as invariant lines and habit planes.These new orientations relation belong to irrational orientation.Habit planes of austenitic and ε-martensite in irrational orientation Ⅰ are(1? 1.58????? 1.58)and(0,2.49 1),respectively.The calculated results agree well with the experimental results.However,the calculation results of irrational orientation relation Ⅱ have a large error with the experimental results.(5)The crack propagation behavior and dislocation slipping of Fe-29Mn-3Al-3Si TWIP steel was observed by in-situ TEM strain technique.For austenite TWIP steel,stacking faults or austenite twins formed in the local area at the front of the crack can strengthen the material,which lead to the "zigzag" morphology of the crack propagation.At the initial stage of deformation,the extended dislocation structure can not only planar slip,but also can be a lot of cross-slip.A large number of cross slips leads to the formation of secondary twins.The formation of primary twins is related to the dislocation emitted from grain boundary.The positive strain rate sensitivity mechanism and the reasons why there is no lowtemperature tough-brittle transition phenomenon of TRIP/TWIP steel were clarified by systematically studying the evolution law of microstructure under dynamic impact loads and austenite twinning mechanism of high manganese TRIP/TWIP steel,which provides theoretical guidance for the selection and design of materials for safety structural parts of car body or components under low-temperature dynamic loads.