关键词： Additive manufacturing   Selective laser melting   316L stainless steel   High-throughput experiments   Processing maps  

摘要： In the present study, a combination of high-throughput (HT) and low-throughput (LT) techniques was used to rapidly determine the processing window and generate processing maps for Selective Laser Melting (SLM) of 316L stainless steel. The HT method includes the fabrication of hundreds of hex nut-shaped specimens, each processed with a unique combination of laser power, scanning speed, and hatch spacing. An easily removable scaffolding permitted rapid sample extraction from the base plate, thus saving machining cost and time. Hardness and immersion density measurements were used for HT characterization to identify a processing window for maximum strength and density. Within the defined processing window, a low-throughput (LT) microstructural interrogation of specimens were performed. The microstructural analysis included quantification at various length scales (i.e., grains size and morphology, texture, primary dendrite arm spacing, and melt pool geometry analysis). Microstructure-based processing maps as a function of volumetric energy density were generated. The combination of HT and LT methods produced a predictive relationship between hardness and primary dendrite arm spacing using a Hall-Petch relationship. A model is proposed to explain the dependence of microstructure on the melt pool geometry. The HT method can be applied for the microstructural design of SLM-fabricated components in other alloys.

摘要： This dissertation demonstrates that substantial ductility improvement is possible for low-manganese transformation induced plasticity steel compositions through the quenching and partitioning heat treatment approach using a Gleeble thermo-mechanical simulator. Two investigated compositions had unique microstructures and mechanical behavior from an identical applied quenching and partitioning process. Electron backscattered diffraction analyses indicate that Comp-2 and Comp-5 both contained retained austenite which resulted in enhanced ductility. The face-centered cubic phase (austenite) more efficiently mitigates strain incompatibilities when located at martensitic grain boundaries known for hot spots and damage initiation. This location effect leads to enhanced ductility and improved toughness in a lean, transformation induced plasticity steel. However, the increase in ductility in Comp-2 and Comp-5 is limited; the partitioning of carbon cannot stabilize austenite to reach strength/ductility targets set by the Department of Energy. Comp-2 and Comp-5 lack sufficient manganese to stabilize austenite to a higher degree. Chem-2A will be explored to determine if the partitioning stage can stabilize austenite closer to the martensite finish temperature. Periodic intercritical annealing will be applied to Chem-1A to see if mechanical properties can be increased further than current research values. Ultimately, through literature, Manganese is proven to be a more effective austenite stabilizer than carbon, and with tailored heat-treatment, the DOE targets can be reached.

关键词： 扫描透射电子显微术   STEM-Z衬度像   二元合金颗粒   核壳结构  

摘要： 核壳结构二元合金纳米颗粒因其金属间的协同作用具有一些优异的性质,以Au@Pd为例,它是一种应用广泛的催化剂颗粒,但目前制备过程中过厚的Pd壳层会降低Pd前驱体的利用效率、催化性能,如何实现对包覆层厚度的调控是研究重点。本文利用STEM-Z衬度像技术,以Au@Pd为研究对象,分别制备了包覆层为1、4、6的Au@Pd纳米催化颗粒。根据STEM-Z衬度像原理,计算出了最优的探测器角度,θ1、θ2分别为41.34 mrad及266.03 mrad,在此条件下二者散射截面比为2.93。确定了不同反应条件下的包覆层厚度,并基于表面结构探究了其生长机理及模式:当Pd壳层<4时,Pd的生长符合F-M模式,在4~6层时,其生长模式符合S-K模式,当包覆层>6层时,已经过渡为V-W模式,表面会出现明显的岛状团簇。通过调控前驱体用量实现了对包覆层的精准调控,确定了当包覆层为1层时,纳米颗粒其比表面积可达100.9 m2/g,基于Pd计算的质量电流密度可达13.2 A/mg,电流密度可达13.1 mA/cm2,优于目前商用Pd/C催化剂。实现了对Au@Pd纳米合金颗粒的可控合成。

关键词： 长波红外   量子阱   选择定则   光吸收   耦合效率  

摘要： 在长波红外探测领域中,GaAs/AlGaAs量子阱材料由于其均匀性和低成本,极具应用前景,然而,子带间跃迁选择定则的的限制导致量子阱不能吸收垂直入射光,通过引入微结构是增强光吸收的主要方法。本文在简述了量子阱原理及在长波红外波段的应用前景后,综述了近年来提高光耦合效率的一些新颖微结构,指出它们各自的优势以及存在的不足,分析了今后量子阱材料微结构的发展方向。

关键词： 纳米银   纤毛虫   超微结构   毒性效应  

摘要： 本文利用透射电镜观察纳米银对冠突伪尾柱虫(Pseudourostylacristata)、浮萍棘尾虫(Stylonychialemnae)、小腔游仆虫(Euplotes aediculatus)三种纤毛虫超微结构的影响,结果表明:(1)浮萍棘尾虫的表膜结构受到破坏,另外两种细胞表膜未受到损伤。(2)冠突伪尾柱虫的线粒体结构破坏最严重,外膜裂解,嵴断裂瓦解;小腔游仆虫的线粒体破坏程度次之,外膜完整,嵴断裂;浮萍棘尾虫线粒体形态较完整。(3)冠突伪尾柱虫细胞核受损最严重,核膜、染色质均有瓦解现象。初步推测,纳米银对纤毛虫具有较大的生物毒性,且毒性作用与纤毛虫种类相关。

关键词： 球磨   非均匀结构   镁基纳米复合材料   软相   硬相   碳化硅  

摘要： 研究了通过一步球磨的粉末冶金工艺制备Si Cp/Mg非均匀纳米复合材料的机理。采用扫描电子显微镜(SEM)、X射线衍射仪(XRD)、能谱仪(EDS)和电子背散射衍射(EBSD)对SiCp/Mg复合粉体的形貌演化过程、非均匀纳米复合材料的物相组成和微观结构特征进行了分析。结果表明:在球磨的过程中,附着在镁颗粒表面的纳米碳化硅粉体嵌入镁颗粒的表层形成核壳结构;复合粉体芯部不仅没有SiC颗粒,而且未受到研磨球的直接作用,使得复合粉体芯部仍然保持为尺寸较大的粗晶,将这种复合粉体进行烧结就能获得非均匀纳米复合材料;通过调节球磨时间,可以对非均匀纳米复合材料的微观结构进行调控。

关键词： 催化剂   碳纳米管   微反应器   流化床   纳米材料  

摘要： 采用共沉淀法制备碳纳米管催化剂,考察了制备方法对催化剂微结构以及碳纳米管形貌的影响。结合微反应器混合均匀、反应快速等优势,在实验中将其代替传统的滴定设备作为反应设备,将制备出的催化剂放置于流化床中进行碳纳米管的生长制备。借助于BET测试、扫描电镜、透射电镜和拉曼光谱等方法对催化剂和碳纳米管进行表征。结果表明:传统滴定反应制备的催化剂团聚现象明显、颗粒大小不一;生长的碳管有序度相对较低,管径跨度大,主要集中在10~18nm之间。微反应器制备的催化剂的比表面积大,表面形貌呈褶皱型且铁铝组分之间的相互作用力强;生长的碳管石墨化程度高,管径分布跨度小,主要集中在11~15nm之间。

关键词： 碳化钛涂层   裂解碳   功能梯度材料   有限元分析  

摘要： 为提高面向等离子体C基材料耐侵蚀性能,通过化学气相沉积工艺在碳/碳复合材料(C/C)表面制备了一系列TiC/C复合涂层。分别通过扫描电子显微镜(SEM)和X射线光电子能谱(XPS)表征了HT-7托卡马克中长脉冲等离子体放电后的不同涂层表面的微观结构和成分变化。结果表明,在富C的TiC/C涂层中观察到明显的侵蚀坑洞,但未发现裂纹。相反在纯TiC涂层中未观察到明显的侵蚀凹坑,但是观察到大的裂纹。基于这些结果,进一步设计制备了TiC/C功能梯度材料,并通过有限元计算了梯度材料内残余的热应力和变形,结果表明,引入TiC/C作为过渡层可显着降低材料内残余热应力,其最大值降低至474 MPa,同时变形程度大大降低,这有利于避免形成大的裂纹和杂质进入等离子体环境。

关键词： Intermetallic compound   Microstructural evolution   Dissimilar materials joint   Interface layer   Thermal and mechanical   Coupled loading  

摘要： The mechanism of the interface layer formation between steel and aluminum dissimilar materials is unclear by now. An interfacial bonding was investigated by joining steel sheets to 6061 aluminum sheets under a thermal and mechanical coupled loading. The effects of temperature and zinc coating on microstructural evolution and shear strength of the interface layer were both studied. The experimental results showed that the interface layer between the uncoated steel and aluminum was mainly composed of FeAl3 intermetallic compound (IMC). The shear strength of the joint increased with temperature. The existence of the zinc coating was able to lower the temperature to form the interface layer and improve its shear strength. The interface layer between the zinc coated steel and aluminum was mainly composed of Fe2Al5Znx and Al-Zn eutectic phases with a lower micro hardness. That suggested that the brittleness of the interface layer composed of the Fe-Al IMC could be avoided by modify the zinc coating and temperature. A zinc foil was added between steel and aluminum sheets to amplify the interface layer for microstructure observation. Based on the results an interface bonding model of steel and aluminum dissimilar materials considering the temperature and zinc coating was proposed.

关键词： Triply periodic minimal surfaces (TPMS)   Additive manufacturing   AlSi10Mg   Selective laser melting   Thermo-mechanical properties  

摘要： There has been an increasing interest in designing new types of architected metallic cellular structures (metallic meta-structures) for various engineering applications, such as thermal management devices, due to the advancements in metallic additive manufacturing technologies. In this work, the microstructure and mechanical properties of as-built and heat-treated additively manufactured AlSi10Mg triply periodic minimal surface (TPMS) sheet-based Schoen's I-graph - Wrapped Package (IWP) cellular structures are studied. Tensile coupons of the base material and IWP cellular structures are fabricated using laser powder bed fusion 3D printing technique, and thermomechanical tests are carried out. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), micro-computed tomography (micro-CT), and optical microscopy were utilized to visualize the surface morphology, fracture surfaces of the tensile coupons, grain orientation maps, and internal morphology of the samples. Micro-CT and experimentally measured relative densities of the fabricated cellular structures were found to be less than designed mainly due to the lack of proper fusion of metallic powder on complex surfaces leading to voids, especially at low relative densities. As-built samples undergo compression tests at 25 degrees C and exhibited brittle fracture while heat-treated samples undergo compression tests at 25 degrees C and 150 degrees C and exhibit more ductile behavior which is attributed to the grain growth during heat treatment, which was determined through the study of the EBSD maps. The tensile strength of the base material decreases with the increase of testing temperature, which is associated with a significant increase in elongation at fracture.