Investigations On Deformation Mechanism And Microstructure-Mechanical Property Of Fe-Mn-Si-Al TWIP Steels Under Dynamic Loading Conditions

Fe-Mn-Al-Si-C twin-induced plasticity(TWIP)steel is kind of steels which would be produce large amounts of twins during plastic deformation,while twinning results in “pinning” effect for dislocations and the movement of dislocatons were suppressed.On the other hand the generation of twins will refine grains,reducing the free travel of dislocations and cracks.It is precisely because TWIP steel has the above deformation mechanisms so that it has a high strength and elongation.In addition,TWIP steel also has good low temperature toughness and low alloy density.These excellent performances of the TWIP steel materials has caused a wide range of interest in advanced transportation systems,particularly in the modern automotive and high-speed rail.TWIP steel has a high impact absorption energy which can keep the passengers far from harm at the greatest extent when an accident happened.The low alloy density of TWIP steel for reducing the own weight of cars and trains can achieve a lot of advantages: economic benefits,energy-saving,emission-reduction and the environmental protection.Compared to Conventional steel materials,TWIP steel materials can work in cold environment conditions and can be applied to refrigeration technology in the storage and transportation of liquid gases owning to the low temperature toughness of TWIP steel.The TWIP steel also can be applied to generator rotor and other areas owning to the non-magnetic of TWIP steels.The application prospect will be wider with production technology improvement and development.However,the study of Fe-Mn-Si-Al twin-induced plasticity steel is not systematic enough,and there are many technical problems to be solved.The influence of the composition on the microstructure,the mechanism of the plastic deformation,the change of the structure with the different deformation conditions and the anti-oxidation behavior should be thoroughly investigated.In the thesis,the twin-induced plasticity steel was systematically studied under dynamic loading and quasi-static loading by using XRD,SEM,EBSD and EPMA.The main original works of this thesis are presented here:(1)The research of the deformation behavior of TWIP steel under high-speed gas impact loading;(2)The microstructure,deformation behavior and properties of the TWIP steel were investigated under dynamic equal channel angular pressing loading;(3)The microstructure mechanism of the TWIP steel was studied by means of XRD in-situ tensile test;(4)The deformation behavior of TWIP steel at different strain rates under quasi-static tensile test.The systematic research of the TWIP steel provides theory and guidance for the further discussion and the industrial production and as a substitute of comprehensive mechanical property steel.This paper has achieved the following results:(1)The strain rate has a great influence on the deformation behavior of the TWIP steels.The elongation gradually decreases with the increase of strain rate ?.The low strain rate(<10-4/s)is not significant for the study of this experimental TWIP steel.(2)There is no phase transition in the process of gas-gun impacting and D-ECAP.It is also austenitic structure.Under the gas-gun impacting,the dislocations increase with the increase of velocity,but the increase speed of dislocations under low velocity impacting is greater than that of high speed impacting.The main reasons are that the temperature and twins of high speed impacting is higher.The slip of dislocations are easier to carry on.Lattice strain of(200)peak belongs to the contraction state.The(220)peak belongs to tensile state.It is consistent with the change of the peak intensity.A small amount of deformation twins and weak zeta fiber texture are formed during gas-gun impacting.During D-ecap,there are more dislocations and twins inside the material.The lattice strain is negative,indicating that the grains In the contraction state and the interplanar spacing decreases.In D-ecap,a strong zeta fiber texture and a weak gamma fiber texture are formed.(3)There is no phase transition in the process of in-situ XRD tensile test.With the increase of strain,a large number of twins and dislocations appeared inside the material.The lattice strain increases with the increase of strain.