火花等离子烧结技术制备的WC/Co纳米硬质合金

研究了火花等离子烧结工艺与YG10、YG12两种纳米硬质合金性能的关系.然后采用火花等离子烧结技术制备了硬质合金功能梯度材料,该材料由纳米WC/10%Co、纳米WC/12%Co、微米WC/15%Co混合粉以及不锈钢圆片烧结而成.显微硬度压痕显示该材料各层间的应力较小.     

叠层加压SPS烧结制备梯度硬质合金

采用叠层加压SPS烧结法制备致密的YG10/YG20梯度结构硬质合金。通过调整WC粒度和添加微量元素B来调整烧结温度,使含钴量低的粉末与含钴量高的粉末的烧结温度相近。研究了烧结温度对致密度、组织形貌、显微硬度和断裂韧性的影响,分析了沿梯度截面上C,Co,W等成分、显微硬度的变化及YG10/YG20界面的结合情况。结果表明:原始WC粒度为1μm的YG10+0.05%B混合粉末和9μm的YG20混合粉末都能在1100℃~1160℃烧结致密,相对密度达到99%以上,晶粒尺寸均匀,梯度界面结合良好,没有开裂现象。低钴端的硬度达到了15500MPa~16000MPa,高钴端的硬度为11100MPa;在294N载荷的作用下低钴端的断裂韧性为12.62MPa·m1/2,而高钴端在这一载荷的作用下没有出现裂纹,断裂韧性较高,从而实现了硬质合金一端具有高硬度,另一端具有良好的韧性的有机结合。     

Graded plasma spraying of premixed metalceramic powders on metallic substrates

The mismatch between the thermal expansion coefficients of ceramics and metals and the differential stresses it causes at the interface create problems in metal to ceramic joining. Research has been con-ducted to solve this problem in thermal barrier coating technology. Previous studies have considered met-al-ceramic multilayers or graded-coatings, which include a metallic bond coat. In this study, a graded plasma-sprayed metal-ceramic coating is developed using the deposition of premixed metal and ceramic powders without the conventional metallic bond coat. Influences of thickness variations, number, and composition of the layers are investigated. Coatings are prepared by atmospheric plasma-spraying on In-conel 718 superalloy substrates. Ni-Cr-Al and ZrO₂ -8 % Y₂O₃ powders are used for plasma spraying. Ad-hesive and cohesive strength of the coatings are determined. The concentration profile of the elements is determined by x-ray energy-dispersive analysis. The microstructure and morphology of the coatings are investigated by optical and scanning electron microscopy (SEM). Results show that the mixed metal-ce-ramic coating obtained with the deposition of premixed powders is homogeneous. The morphology and microstructure of the coatings are considered satisfactory.     

离心力作用下Al/SiCp系统二维凝固过程的数值模拟

采用数值计算方法对离心浇铸下Al/SiCp系统的二维凝固过程进行了研究，给出了各种因素对最终颗粒分布的影响，结果表明，从外侧面冷却时凝固件中颗粒的分布由内侧向外侧可依次分为以下区域：颗粒体积分数为零区、颗粒梯度区、颗粒堆积区以及最外侧的梯度薄层。外部介质对流换热系数减小时，颗粒堆积区和颗粒体积分数近似为零区范围将增大，而颗粒梯度区范围则会减小；颗粒尺寸增大或铸模转数增加时，也会产生同样情况。     

PSZ／Mo功能梯度材料的有限元设计

利用有限元分析的方法，依据拉应力和比应力最小原则对PSZ／Mo功能梯度材料进行了优化设计，确定了FGM的最佳形状分布因子、层数和每一层的厚度。并对最优成分分布FGM在稳态隔热状态下所产生的温度场和热应力进行了计算，得出进行PSZ／Mo功能梯度材料优化设计所需考虑的主要因素。     

Crashworthiness design for functionally graded foam-filled bumper beam

Automotive bumper beam is an important component to protect passenger and vehicle from injury and damage induced by severe collapse. Recent studies showed that foam-filled structures have significant advantages in light weight and high energy absorption. In this paper, a novel bumper beam filled with functionally graded foam (FGF) is considered here to explore its crashworthiness. To validate the FGF bumper beam model, the experiments at both component and full vehicle levels are conducted. Parametric study shows that gradient exponential parameter m that controls the variation of foam density has significant effect on bumper beam’s crashworthiness; and the crashworthiness of FGF-filled bumper beam is found much better than that of uniform foam (UF) filled and hollow bumper beam. The multiobjective optimization of FGF-filled bumper beam is also performed by considering specific energy absorption (SEA) and peak impact force as the design objectives, and the wall thickness t, foam densities ρ_f1 and ρ_f2 (foam densities at the end and at mid cross section, respectively) and gradient exponential parameter m as design variables. The Kriging surrogate modeling technique and multiobjective particle swarm optimization (MOPSO) algorithm were implemented to optimize the FGF-filled bumper beam. The optimized FGF-filled bumper beam is of great advantages and it can avoid the harmful local bending behavior and absorb more energy than UF filled and hollow bumper beam. Finally, the optimized FGF-filled bumper beam is installed to a passenger car model, and the results demonstrate that the FGF-filled bumper beam ensures the crashworthiness performance of the passenger car while reduces weight about 14.4% compared with baseline bumper beam.     

基于自然元法和精细积分法的功能梯度材料瞬态热传导问题求解

为促进无网格法分析技术在热传导分析中的应用,提出空间离散采用自然单元法、时间离散采用精细积分法求解功能梯度材料瞬态热传导问题的数值计算方法.在计算过程中,取高斯点的材料参数模拟功能梯度材料特性的变化.温度场采用自然邻接点插值形函数进行离散插值.数值算例验证该数值算法的正确性和有效性.     

C语言源程序的自动评判系统

自动评判系统为通过编译C语言源程序生成的可执行程序创建进程,输入适当的测试数据,对输出结果与预设答案进行比较和评判。提出了通过匿名管道进行数据输入输出的实现方法,分析了自动评判机制中的主要问题及解决方法,重点对分级评判进行了探讨,进而提出了四级评判机制。     

基于FDTD法的电磁仿真的优化算法

在对研究对象所在空间进行电磁仿真的过程中,如果采用经典的均匀网格划分法处理具有复杂结构的物体(例如在同一物体上,某些部位之间的尺寸差别很大),势必造成网格数量巨大,计算成本过大的问题.为解决这个问题,提出了一种基于FDTD算法的渐变非均匀网格划分技术,在保证计算精度的基础上比均匀网格划分法占用更少内存和计算时间.仿真结果表明,该改进方法明显提高了计算效率.     

一种支持分级用户访问的文件分层CP-ABE方案

文件分层的密文策略基于属性的加密（FH-CP-ABE）方案实现了同一访问策略的多层次文件加密,节省了加解密的计算开销和密文的存储开销.然而,目前的文件分层CP-ABE方案不支持分级用户访问,且存在越权访问的问题.为此,提出一种支持分级用户访问的文件分层CP-ABE方案.在所提方案中,通过构造分级用户访问树,并重新构造密文子项以支持分级用户的访问需求,同时消除用户进行越权访问的可能性.安全性分析表明,所提方案能够抵御选择明文攻击.理论分析和实验分析均表明,与相关方案相比,所提方案在计算和存储方面具有更高的效率.