| Fe-Mn-Al-Si TRIP/TWIP steels with high Mn content not only havehigh strength and excellentductility but also have good fatigue properties and high impact resistanceability. The high-cycle fatigue strength of Fe-29Mn-3Al-3Si TWIP steel was examinedby means of high-cycle fatigue experiment with stress control. The effects of grain size and pre-strained condition on high-cycle fatigue properties of Fe-29Mn-3Al-3Si TWIP steel wereinvestigated by means of scanning electron microscopy (SEM), electron back-scatterdiffraction technology (EBSD) and transmission electron microscopy (TEM). On the other hand, the impact toughness of Fe-25Mn-3Al-3Si TRIP/TWIP steel was studied at various temperature sbythe Charpy V-notch impact test, and different characterization methods including opticalmicroscopy (OM), SEM, EBSD and TEM were used to characterize themicrostructures of impact specimens.The main results are showed as follows:(1) Fe-29Mn-3Al-3Si TWIP steel has a good fatigue performance. The fatigue strength of coarse grained (CG) samples is 376 MPa, which is higher than the yield strength. However, the fatigue limit strength of fine grained (FG) samples is 344 MPa, which is a little lower than the yield strength. Within a range of grain size, the largergrain size is, the betterfatigue propertiesare.(2) The fatigue performance of Fe-29Mn-3Al-3Si TWIP steel is improved greatly by cold rolling with 15% reduction. The fatigue strength of the pre-strained fine-grained samples (15%FG) is 586MPa, which is higher than that of the FG samples, and the increment is242MPa.While the fatigue strength of the pre-strained coarse-grained samples (15% CG) is 477MPa,which is also higher than that of CG samples, and the increment is 101 MPa. By pre-strained treatment, the fatigue strength of 15%FG samples is much higher than that of 15%CG samples.(3) The micro structures of the fatigue fracture are examined by EBSD and TEM. It shows that a large number of lamellar structures were observed in the region near the fracture, and the lammellar structures are deformation twins and slip bands. There aretwinning induced plasticity (TWIP) effect and dislocation glide in fatigue process. For CG samples, the interaction between twin and dislocations hinderscrack propagationand improves the fatigue properties of the TWIP steel.(4) The deformation mechanism of CG samples during pre-strained treatment is TWIP effect and dislocation glide, producing profuse deformation twins and dislocation structures. While the deformation mechanism of FG samples during pre-strained treatment is dislocation glide and no deformation twins are observed. For 15%FG samples, the dislocation structuresare retainedafter fatigue test, which means the dislocation structures still play a key role in hindering the movement of dislocationsand increasingthe critical stress value of crack initiationduring fatigue process. Meanwhile, dislocation cellsand dislocations aggregate near the grain boundarycan also hinder the crack growth.(5) The impact absorbing energy of Fe-25Mn-3Al-3Si TRIP/TWIP steel at various temperatures from -190? to 200? maintain 40 J.This steelexhibits ductile fracture over a temperature range from cryogenic to elevated temperatures, andthe ductile-brittle transition does not happen.(6) The test temperature affects the deformation mechanism in impact deformation.The dislocation glide is the main deformation mechanism at high temperature, and the TWIP effect gradually preferswith temperature decreasing to room temperature. When temperaturecontinues to decrease, the transformation induced plasticity (TRIP) effect gradually prefers at low temperatures. Moreover, Fe-25Mn-3Al-3SiTRIP/TWIP steel exhibits excellent impact toughness due to the TWIP/TRIP effect at low temperatures. The combined effects of TRIP and TWIP enhance the impact toughness at low temperatures and the impact toughness does not decrease even at liquid nitrogen temperature. |
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