Investigation Of Microstructure And Property Of The Fe-Mn-Si-based Alloy Including Nb And C

Fe-Mn-Si-based alloy has good mechanical properties and low production cost,showing great potential for applications in fasteners,building seismic dampers,and other fields.However,the shape recovery rate of Fe-Mn-Si-based alloys prepared by conventional processes is low,which makes it difficult to meet the performance requirements for large-scale applications.To improve the performance,researchers have proposed thermomechanical training,alloying,heat treatment,and second-phase precipitation.Among them,the second phase precipitation is economical and effective,which has attracted a lot of attention.However,there are a lot of problems still need to be further clarified.This study focused on Fe-Mn-Si shape memory alloy containing NbC precipitated phase,investigated the precipitation and regulation mechanism of NbC and the influence mechanism of shape memory effect,and explored the way to optimize the performance of Fe-Mn-Si shape memory alloy.In this paper,Fe-21Mn-6Si-9Cr-5Ni-1NbC alloy was processed using different solid solution treatments,aging treatments,and deformation aging treatments.Optical microscopy(OM),transmission electron microscopy(TEM),scanning electron microscopy(SEM)and X-ray diffraction(XRD)were used to characterize the microstructure,analyze the phase composition,and study the microstructure evolution and the characteristics of the second phase precipitation under different treatment processes.The shape recovery effect was tested by bending method to investigate the influence mechanism.The following main conclusions were drawn:The original hot-rolled specimens retained the deformation characteristics after solution treatment at 1050 ℃ for 1 h.With increasing solution time and temperature,the austenite grains changed to equiaxial shape.In the solutionized alloy,ε-hot martensite,lamellar dislocation and twin crystal structures were formed.In alloys treated at higher temperatures for longer time,ε-hot martensite of different orientations cross-collide to form α′ martensite.With increasing the solution temperature and time,the recovery strain and shape recovery rate of the alloy decrease.For the alloy solutionized at 1050°C for 1 h,the shape recovery rate is as high as 60.35% when the pre-strain is 2%,and the recovery strain is up to 2.29% when the pre-strain is 8%.The aged alloy obtained grain morphology close to equiaxed,while uneven grain growth occurred.As the solution temperature increases from 1050 °C to 1200 °C,the grain size gradually increased after 800 °C aging treatment.Based on the classical nucleation and growth theory and considering the effect of alloying elements,the precipitation model of the second phase of NbC in Fe-Mn-Si-based shape memory alloy was established.The simulations show that the precipitated phase obtained by aging after solution at higher temperatures is larger in size and higher in volume fraction.For the alloy solutionized at 1050 °C for 1 h and then aged at different temperatures for 10 min,the size and volume fraction of the precipitated phase did not change significantly.The simulation results are in good agreement with the experiments.The recovery strain is close to or better than that of the original hot-rolled specimens,and the shape recovery rate is higher than that of the original hot-rolled specimens.The large number of crystal defects within the deformed alloy promotes the precipitation of the NbC second phase compared to the aged alloy,resulting in a higher NbC content in the deformation-aged alloy.At the same time,deformation aging inhibits the formation of α′ martensite,which can be attributed to the orientation ofε-hot martensite and the fewer cross-collisions between them.A large number of NbC precipitated particles coexist with a high density of stacking faults in the deformation-aged alloy.Transmission electron microscopy analysis revealed that the twins in the deformation-aged alloy consisted of fine martensite laths.The optimal process for deformation aging was 1050°C-1h solution + shot peening + 800°C-2h aging,and the highest shape recovery rate was 95.28%,which was significantly higher than that of the aged alloy.