离子辐照的316L不锈钢中空洞密度的定量化电子显微镜测试

A quantified TEM analysis of number density of cavities formed in 316L stainless steels irradiated with energetic ions

316L奥氏体不锈钢经过高剂量氦注入及高能铁离子辐照后,产生高密度空洞.本文采用聚焦离子束(FIB)技术从辐照后的316L不锈钢试样上制备出透射电子显微镜(TEM)样品,通过TEM定量分析样品薄区的空洞密度及分布状态,进而计算出样品的辐照肿胀率.该定量表征结果的准确性依赖于样品薄区厚度的精确测量.然而,Kelly和Allen早期提出的基于会聚束电子衍射(CBED)的膜厚测量技术无法直接应用于TEM样品较薄微区的厚度测量(厚度<0.6ζ_g,ζ_g为衍射束g对应的消光距离).本文提出对同一样品较厚区域CBED花样进行测量拟合出特定衍射束的消光距离,再将该参数应用于待测薄区同一取向CBED花样的测量结果,从而估算样品薄区的厚度.讨论了CBED方法相对于电子能量损失谱(EELS)方法在透射样品厚度测量上的优势.以离子辐照后的316L不锈钢样品薄区厚度测量及空洞密度定量分析为实例,探讨了本文所提出的基于CBED技术的薄区厚度测量方案在实验过程中的主要影响因素及优化途径.

The number density and the statistic analysis of defects, cavities and bubbles are important structural parameters in the characterization of the radiation resistant properties of nuclear materials. Transmission electron microscope （TEM） is a powerful tool for the quantitative characterization of irradiation defects. The accuracy of the quantitative characterization is based on the precise measurement of the thickness for the micro-region. The traditional thickness measurement method based on convergent beam electron diffraction （CBED） with tow-beam approximation pro- posed by Kelly and Allen is a classic technique. But, this technique can not directly measure the thickness of thinner sample with thickness less than 0.6~g （~g is the extinction distance of the diffraction beam ~, ）. It is limited in the experiment and post-data process for the thickness measurement of thinner samples. However, for the irradiated materials with high density of cavities or defects, the thinner sample preparation is necessary for the clear observa- tion and precise statistic analysis of the high-density cavities or defects. Therefore, it is urgently needed to develop the TEM-based technology for the thickness measurement of thinner samples. In this work, we use irradiated 316L austenitic stainless steels as research sample. High-density cavities are formed in 316L austenitic stainless steels implanted by high-dose He and then irradiated with high-energy Fe13＋ ions. Using focused ion beam （FIB） technique, TEM sample is prepared from the irradiated 316L austenitic stain- less steels. To measure the thickness for the thinner micro-region of the 316L stainless steel TEM sample, we pro- pose a new approach based on the CBED technology proposed by Kelly and Alien. Firstly, the extinction distance of specific diffraction beam is evaluated in the thicker micro-region by using Kelly and Allen methodology, and then this parameter is used to calculate the thickness of thinner micro-region of the same sample in which the CBED is recorded under the orientation as that of the thicker region. Using this approach, the density of the cavities and the irradiation swelling ratio in a thinner micro-region of the sample of 316L stainless steels obtained through FIB are quantitatively characterized by TEM. The experimental results confirm that our proposed CBED-based approach is suitable for the thickness measurement of the thinner 316L stainless steel samples with thickness about from 20 to 50 nm. Furthermore, to present the advantage of CBED-based method for thinner-region thickness measurement, we compare it with the thickness measurement method based on electron energy loss spectroscopy （EELS）. The thick- ness measurement results by using EELS are the total thickness of the TEM samples, including the thickness of the surface amorphous layer introduced through the sample preparation. In contrast, the CBED-based method can pro- vide the intrinsic thickness of the crystalline samples without the influence of surface amorphous layer. Finally, we discuss the experimental factors which influence the thickness measurement results of thinner samples by using our proposed experimental approach and the related optimization methods. It is envisaged that our proposed approach can be extended to the micro-region quantitative statistic analysis of different type of defects, such as dislocation loops, cavities, bubbles and stacking fault tetrahedrons in ion irradiated crystalline materials.