Study On Influence Of Segregation Bands On Spallation Behavior Of 304 Stainless Steel

As a typical representative of 300 series stainless steel,304 stainless steel possesses excellent mechanical properties and corrosion resistance,and thus it has a wide range of applications in industry,civil field,etc.However,there are always banded structures(alloying element segregation bands)in the microstructure of 304 stainless steels.Up to now,there has been no consensus on the influences of banded structure on the mechanical properties of the material under quasi-static loading,and there exists no reports on the influences under the high strain rate loading.Therefore,it is worth studying in-depth.In the present study,X-ray diffractometer,scanning electron microscopy,electron backscatter diffraction and energy dispersive spectrometer are used to characterize and analyze the banded structures in the material.It verifies that the existence of the banded structures is due to the segregation of alloying elements such as Cr,Ni during the solution process.Single stage light gas gun is applied in the spallation experiments under high strain rate loading of the 304 stainless steel.During the experiment,the velocity history profile of the sample free surface particles is detected by the Doppler laser velocimeter,and the softly recovered spallation specimens are characterized by electron microscopy.Based on the above results,the influences of the banded structures on the spallation behavior of the material are analyzed and discussed.Via analyzing the particle velocity curve of the free surface,it can be concluded that the banded structures have no obvious influences on the elastic limit which is approximately 1.1 GPa,whereas they have evident influences on the spall strength.Segregation bands and the interfaces between segregation bands and the matrix is the weak zones in the materials,and thus damages are easy to form at such locations,resulting in no increase in the spall strength as peak stress increases,which stays approximately at 2.3 GPa.Through analyzing the results of electron microscopy characterization of softly recovered spallation specimens,it is found that the banded structures has significant effects on the spallation damage under high strain rate loading.Firstly,the majority of banded structures penetrate through grain boundaries.As the weak zones in the material,abundant crystallographic defects accumulate in the interior of the segregation bands,and the interfaces between the segregation bands and matrix act as the transition region,whose mechanical properties are weaker than those of matrix/grain boundary.Therefore,segregation bands and the interfaces between the segregation bands and matrix are the preferred nucleation spots.Secondly,the orientations of segregation bands impose restrictions on the nucleation,growth and mutual coalescence of micro-voids and micro-cracks;as a result,the distribution of micro-cracks is consistent with segregation band distribution,i.e.cracks propagate along banded structures.In addition,statistics of spall damages are performed.It can be inferred that when the impact direction is parallel to the banded structure,the material exhibits larger damage degree,but the propagation and coalescence of cracks along the transverse direction(which is perpendicular to the impact direction,i.e.along the spall plane)is few.Thus,under the impact speed of ~480 m/s,the material can withstand the tensile stress induced by the impact so that it cannot be split into two pieces.When the impact direction is perpendicular to the banded structure,even though the material exhibits a smaller damage degree,cracks propagate along the transverse direction.They are apt to coalesce contributing to the formation of larger cracks.Under the relatively high impact speed of ~460 m/s,the material is split into two pieces along the spall plane.In addition,based on the pre-deformation of 304 stainless steel,the effect of deformed martensite on the spallation behavior of the material was studied.After a series of experiments,it is found that the elastic limit of the material increases with the increase of the predeformation,and the spall strength decreases with the increase of the pre-deformation.At the same time,the damage morphology in material is also affected by the deformation martensite introduced by the pre-deformation.The nucleation and growth of micro-cracks are related to the distribution of martensite.