【作者】 王旭；

【导师】 张俊善； 雷明凯；

【作者基本信息】 大连理工大学 ， 材料表面工程， 2007， 博士

【摘要】 强流脉冲离子束（High-Intensity Pulsed Ion Beam—HIPIB）技术起源于20世纪70年代末期的惯性约束核聚变和高能密度物理研究。近年来，它作为一种新型的材料载能束表面改性技术，受到了广大材料科学工作者的广泛关注。鉴于HIPIB技术在材料表面工程领域的应用研究尚处于初级阶段，并且金属材料表面辐照改性是目前HIPIB技术在材料表面工程应用研究中的一个热点问题，本文系统研究了不同参数的HIPIB辐照对316L不锈钢表面结构、表面性能及基体力学性能的影响，旨在揭示其变化的本质原因，并探索合适的辐照参数，进而为其它材料的改性提供参考。利用TEMP-6型HIPIB装置，采用聚合物阳极的单极脉冲模式外磁绝缘离子二极管产生由70％的H离子和30％的C离子组成的加速电压为300 kV，束流密度为100、200和300 A／cm²（波动不超过20％），脉冲宽度为75 ns的强流脉冲离子束，分别辐照316L不锈钢1、5、10次。采用扫描电子显微镜（SEM）、X射线衍射仪（XRD）、透射电子显微镜（TEM）和电子探针（EPMA）观察表面形貌、鉴定表面层相组成和微观组织结构及分析表面元素分布变化。对辐照前、后样品，分别进行显微硬度测量，摩擦磨损、氧化、电化学腐蚀、抗疲劳及高温蠕变实验。结合表面结构的变化，研究不同辐照参数对上述各种性能的影响规律，探讨其改性机理，并确定了针对316L不锈钢各性能的最佳辐照改性条件。SEM观察结果表明，316L不锈钢表面经HIPIB辐照后随辐照强度的增大或辐照次数的增加均呈现出光滑化趋势。烧蚀坑面积对束流密度的变化敏感，其数量则受辐照次数变化的影响更加显著。采用一维非稳态热传导模型对试样最外表面温度进行了估算，发现HIPIB处理会将316L表面快速加热至其熔点甚至沸点温度，导致原始表面机械磨痕熔化、发生选择烧蚀和液滴喷射。XRD结果显示，辐照处理后316L表面无新相生成，但由于辐照引起的大温度梯度和高应力，导致试样表面层形成了择优取向，且这种趋势随束流密度的增大或辐照次数的增加越发显著。TEM观察发现，试样表面熔化层形成了非晶结构和纳米马氏体，在热激波、压缩应力波和C离子注入的共同作用下，熔化层以下的热影响区内形成了大量位错亚结构和孪晶。根据EPMA分析结果提出了一种新的烧蚀坑形成机制，316L表面层中包含强挥发性S元素的MnS夹杂在辐照瞬间发生了低熔点元素的选择性烧蚀，形成以原始MnS夹杂为中心的烧蚀坑。显微硬度测量结果显示，由于试样表面形成了非晶和纳米晶结构，近表层产生了大量位错胞和孪晶等亚结构，导致HIPIB辐照处理后的316L不锈钢表面层显微硬度有所提高，且沿截面都出现了微硬度双峰的现象。相同次数（10次）辐照后，表面硬度及截面硬度最大值均出现在束流密度为200A／cm²的样品上；相同辐照强度下（200A／cm²），试样表面显微硬度和截面显微硬度最大值均随辐照次数的增加而逐渐增大。由于样品表面的光滑化和显微硬度的提高，不同参数的HIPIB辐照处理均使316L不锈钢的表面摩擦系数降低，磨损量减少。试样表面耐磨性与硬度值保持了很好的对应关系，其中，200A／cm² 10次辐照后试样的表面耐磨性最好。HIPIB中30％的C离子注入后，在氧化温度下将占用样品表层中大量的Cr去生成碳化物，从而导致316L表面贫铬，优先发生了铁的氧化，抗高温氧化性能随辐照强度的增大或辐照次数的增加急剧下降。700℃氧化100h后，原始试样氧化程度轻微，表面生成了致密的Cr₂O₃保护膜，冷却过程中没有出现剥落迹象；低束流密度和辐照次数少的试样表面生成了瘤节状Fe₂O₃，剥落现象轻微；高束流密度和辐照次数多的样品表面生成了由Fe₂O₃和Cr₂O₃交替构成的氧化层，剥落现象严重。在表面光滑化、表面层非晶化和晶粒细化以及杂质元素选择性烧蚀的共同作用下，HIPIB辐照处理显著提高了316L不锈钢在0.5mol／L的H₂SO₄溶液中的电化学腐蚀性能，且辐照强度变化对自腐蚀电位的影响更加显著，而自腐蚀电流密度则对辐照次数变化更加敏感。疲劳和蠕变实验结果均说明，中、低强度的HIPIB辐照处理后，316L不锈钢试样表面的光滑化、表面层的非晶化和晶粒细化以及大量位错亚结构的形成，起到了阻碍位错运动、抑制表面裂纹产生和阻止裂纹扩展的作用，从而提高了其室温疲劳寿命，改善了其高温蠕变性能。与硬度测量结果一样，辐照相同次数（10次）后疲劳和蠕变断裂寿命的最大值也都没有出现在辐照强度最强的样品上，这不仅源于过高的辐照强度会引起试样表面剧烈的沸腾和蒸发，使改性层所剩无几，同时还在于高束流密度条件下样品表面大多数烧蚀坑中心会形成微裂纹，作为裂纹源将直接导致材料室温抗疲劳性能和高温蠕变性能的下降。在相同束流密度条件下（200A／cm²），辐照后试样的疲劳、蠕变断裂寿命和稳态蠕变速率均随辐照次数的增加呈现抛物线式的变化规律。更多还原

【Abstract】 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.更多还原