Conglin Zhang

Microstructures of pure nickel induced by high-current pulsed electron beam irradiation

The characteristics of surface feature and microstructure of the near-surface of nickel were investigated after high current pulsed electron beam (HCPEB) irradiation. Based on a physical model, the temperature profile was simulated. The depth of heat-affected zone, the initial melting position, heating rate and quenching rate were computed. The initial coarse-grained structure on the surface was refined, of which the size is about 70 nm. The TEM observations indicate that dislocation wall, sub-dislocation wall, twin and twin boundary steps were achieved on the surface of material after HCPEB irradiation. Furthermore, huge numbers of vacancy defect clusters including dislocation loops, stacking fault tetrahedras (SFT) and voids were also formed within the sublayer of the irradiated surface. The fact that supersaturation vacancies migrated towards the surface along grain boundaries and/or various dislocation configurations led to the formation of microporous feature on the irradiated surface.

Crater Formation on the Surface of Pure Metal and Alloy Irradiated by High Current Pulsed Electron Beam

Abstract A detailed investigation concerning the formation mechanism of craters of pure metal and alloy surface after high-current pulsed electron beam (HCPEB) processing has been carried out. After HCPEB irradiation, typical craters were homogeneously distributed on the entire surface of 3Cr13 stainless steel, resulting from local sublayer melting and eruption through the solid outer surface. Comparatively, almost no craters were created of pure zirconium, which was associated with the shallow melting site and incomplete treated surface. Furthermore, a large number of ultrafine grains were formed on the irradiated surface, indicating that surface melting was the dominant interaction mechanism between HCPEB irradiation and pure zirconium. It is apparent that “crater free” phenomenon plays a dominant role in surface modification of zirconium and zirconium alloys.

Effect of thermocycling on the microstructure of Ti-6Al-4V alloy in simulated low Earth orbit space environment

The study of material damage and its physical mechanisms, the deep understanding of material performance degradation processes, and the improvement of key materials and devices used in a space environment are undoubtedly of critical scientific importance. This work investigated the mechanical properties and microstructures of the Ti-6Al-4V alloy subjected to thermocycling in a simulated low Earth orbit (LEO) space environment by using microhardness tests, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results revealed an initial increase in hardness with the number of cycles, followed by a subsequent decrease. After 300 thermal cycles, many cavities were formed at the two-phase interface of the Ti-6Al-4V alloy. The dominating structures were transformed into dislocation cell substructures of larger size. After 500 cycles, the central structures in the sample were subgrains evolving by dislocation cells, as well as intragranular twins and jogs structures. The relationship between microstructural evolution and thermal fatigue in LEO space environment was discussed, which may help predict the fatigue damage of materials.摘要本文通过模拟空间环境中冷热循环实验条件, 利用光学显微镜、 X射线衍射仪、 透射电子显微镜及显微硬度仪分析了低地球轨道冷热循环作用下TC4钛合金的微观结构状态和性能的演化行为. 实验结果表明, 随着循环次数的增加, 样品的硬度迅速增加到最大值, 在随后的冷热循环过程中样品出现了循环软化现象; 冷热循环300次后, α和β相的界面出现大量孔洞, 样品中的典型结构转变为尺寸较大的位错胞状亚结构, 晶界容易成为应力集中的区域并萌生微裂纹; 500次循环样品中的典型结构为位错胞演化而成的亚晶以及晶内的孪晶和割阶结构. 本文详细讨论了低地球轨道冷热环境下TC4钛合金微观结构的演变规律和冷热疲劳之间的关系.