Study On The Structure And Properties Of 316L Stainless Steel Irradiated By High-intensity Pulsed Ion Beam

Recently, High-intensity pulsed ion beam (HIPIB) treatment has received extensive attention as a new technology for surface modification of materials. It has roots in inertial confinement nuclear fusion and high-energy density physics research from seventies of last century. The application of HIPIB technique in material surface engineering is inchoate and the surface irradiation of metallic materials is a hotspot issue now. In this dissertation, the investigations about the influences of HIPIB irradiation on the surface structure, surface properties and matrix mechanical performances of 316L stainless steel have been studied systematically, in order to explore the essential reasons of modification, and search for the appropriate parameters so as to provide references for other materials.The HIPIB irradiation was carried out in the TEMP-6 type HIPIB apparatus operating in unipolar mode. In the TEMP type ion source by using an ion diode magnetically insulated by an external-magnetic field (MID) with perforated polyethylene anode, the main ion species of ion beam were about 70%H⁺ and 30%C⁺. Irradiation of the targets was performed under the conditions: ion energy E = 300 keV, current density J_i = 100, 200 and 300 A/cm² (the fluctuation of current density was limited to no more than 20% from shot to shot), pulse duration r= 75 ns and shot number N = 1, 5 and 10 shots. The surface morphology and the phase structure in the near surface region of original and treated samples were analyzed with scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). Electron probe microanalysis (EPMA) was used to study the distribution of elements on the irradiated surfaces. The microhardness, wear resistance, oxidation and corrosion resistance, fatigue and creep tests of all the samples have been examined. In combination with the structure changes in the surface layer of the irradiated sample, we investigated the effects of different parameters on above properties, discussed the mechanism of the modification, and finded the best irradiation conditions for diversified performances of 316L stainless steel.As can be seen in the SEM images, HIPIB irradiation smoothed the surface of 316L stainless steel. The polishing marks disappeared gradually due to more intense or repeated melting and severe ablation with increasing the ion current density or the shot number. The area of the craters was more impressible to the energy density per shot and the number of it was more dependent on the shot number. The one-dimensional heat-flow modeling was used to estimate the temperature of the irradiated surface during HIPIB interaction with the target. It is found that HIPIB irradiation heated the surface soon at least exceeded the melting point of 316L and induced the melt of the scratches, the selective ablation and the droplets ejection. The XRD patterns showed that there were no new phase appeared on the treated surfaces after the irradiation by HIPIB. A preferred orientation formed in treated samples is caused by extremely high temperature gradient and stresses, which arise from the interaction between HIPIB and the targets. And this tendency became more and more remarkable with the increase of the power density or the shot number. The results of TEM observation indicated that amorphous and nano-martensite formed in the thin molten surface layer, and there were a lot of cellular dislocation substructures and twins under the irradiated surface due to the cooperation of the thermomechanical stress, the recoil impulsed compression wave and the injected carbon ions induced by the HIPIB irradiation. The EPMA analysis suggested a new mechanism of the 316L cratering process. The origin of the cratering was the MnS. During the interaction between HIPIB and the target, selective ablation of MnS occurred on the treated surfaces and formed the crater. This is because sulphur is a volatile alloying element of relatively low vaporization temperature.It can be clearly seen that the microhardness in the near surface region of irradiated samples was increased compared to the control one, and the profiles of the cross-section along the line perpendicular to the surface of the treated samples all have a typical two-peak distribution. This result should be own to the amorphous and nano-structure on the treated surface, and the dislocation substructure and twins in the heat-affected region rooted from the irradiation by HIPIB. After irradiation by HIPIB with 10 shots, the maximum microhardness appeared in the sample irradiated at 200 A/cm². At a fixed ion current density of 200 A/cm², the maximum of microhardness increased gradually with increasing the shot number. The irradiated samples have a lower friction coefficients and more durable surfaces than that of the original sample because wear resistance usually increases with smoothing and hardness, and the best tribological property presented to the sample treated at 200 A/cm² with 10 shots.After HIPIB irradiation, the 30% injected C ions segregated below the treated surfaces, which reacted with abundant Cr in the near surface layer to form carbide during the oxidation process. Due to the oxidation of Fe occurred precedently, the oxidation resistance of the irradiated samples decreased drastically. After exposure at 700℃for 100h, the original 316L stainless steel was oxidated slightly and formed a compact Cr²O₃ protective film, the oxidation product of the sample treated at lower ion current density or lesser shot number was nodular Fe₂O3, and the sample irradiated at higher intensity or more shot numer formed a oxidated layer alternately by Fe₂O₃ and Cr₂O₃. During the experiments, nearly no spallation of the oxidation scale was observed on the original sample. However, spallation was severe gradually on the surfaces of the irradiated specimens with increasing the irradiation intensity or the shot number. Under the cooperation of the smooth surface and the grain refinement with the selective ablation of impurities, the electrochemical corrosion resistance of 316L stainless steel in 0.5 mol/L H₂SO₄ solutions was improved significantly by the HIPIB irradiation. It can be concluded that the dependence of the potential was greater on the irradiation intensity and the current density was more sensitive to the shot number.The fatigue and creep experimental results illuminated that the fatigue life and creep rupture life of 316L stainless steel can be proiongated by the irradiation of HIPIB at lower and moderate intensity. At the same time, the steady creep rate of the treated specimens was reduced. This can be attributed to the smoothing of the surface and the existence of the preferred orientation in the surface layer of irradiated specimens. The smoothed surface has less initiators of crack, which can restrain the production of the surface cracks, so as to prolongate the fatigue and creep rupture life of the treated specimens. The presence of a preferred orientation implied the treatment creates an intense compression wave and high dislocation density in the surface layer of the irradiated specimens, which hinders the dislocations movement. According to the microhardness, the best fatigue and creep properties were not appeared on the sample irradiated at the maximum current density after 10 shots. This is because of micro-crack formed in the center of craters owed to the excessive disturbance of molten surface layer from local liquid evaporation and droplet ejection. It is observed that the influence of HIPIB irradiation on creep property was similar to that on the fatigue life of the irradiated specimens, which changed parabolically with increasing HIPIB shot number at 200 A/cm².