高温多向异步轧制对LZ91镁锂合金组织和力学性能的影响EI北大核心CSCD

采用异步轧制、多向异步轧制、高温异步轧制、高温多向异步轧制四种不同的方式轧制双相镁锂合金板材。通过光学显微镜、MTS E43拉伸试验机和X射线衍射仪观察不同工艺轧制后合金的显微组织、力学性能以及织构特征,综合分析温度和轧制方向条件耦合对镁锂合金组织和力学性能的影响。结果表明:四种轧制工艺可以使α-Mg相沿轧制方向伸长,同时沿着轧制方向法向细化。高温多向异步轧制后α相厚度最低为2.6μm。多向异步轧制后材料的屈服强度、抗拉强度、伸长率分别为149,167 MPa,14.5%,其综合力学性能最优。多向轧制使双峰织构沿ND方向45°偏转,高温轧制使双峰织构由基极向RD方向偏转的角度降低。轧制后样品R-cube织构组分最强,高温多向异步轧制使β-Li相轧制织构转变成为{001}〈100〉织构,有利于{011}〈111〉滑移系发生多滑移。     

盒维数的肠道压力数据分形分析

对肠道压力数据进行分形特性的分析,通过对结肠压力数据进行预处理,采用盒维数计算正常受试者和异常受试者结肠压力数据的分形维数.结果表明,该方法基本能够区分正常的和异常的结肠压力信号,可以作为判断肠道动力性能的辅助手段.     

Al-Mg合金的晶界内耗

研究了退火、淬火状态下 Al-Mg 合金的晶界内耗。实验表明,随 Mg 含量的增加,晶界内耗峰的峰宽和弛豫强度单调下降,峰位开始移向高温,后又移向低温;高温淬火处理能压低 Al-2.4wt-%Mg的晶界峰,但对其它低 Mg 含量试样不敏感,一定温度的保温能使压低的晶界峰回升。     

微孔聚丙烯的分形特征

泡孔结构具有分形特征,可以用分形维数来描述其形态复杂性。文中应用MATLAB编写的数盒子算法,计算了应用超临界二氧化碳作为发泡剂间歇发泡制备的微孔聚丙烯的盒维数,并讨论了二值化阈值对分形维数的影响。结果表明,聚丙烯微孔结构具有典型的分形特征,分形维数与微孔聚丙烯的泡孔密度有关,在一定阈值下,分形维数随着泡孔密度的增加而增加。分形维数是发泡聚丙烯微孔结构的几何特性之一,体现了聚丙烯在特定条件下的发泡能力。     

分形结构金属铜的制备与分析

从仿生的观点出发，选择具有分形结构的叉枝藻（海藻的一种）作为基底材料，经预处理后用化学沉积方法，先在海藻基底上沉积得到一层铜金属，之后再用电沉积方法，使基底表面的金属铜层加厚到一定程度（30～50μm）。最后通过烧蚀去除包裹在铜金属中的海藻，从而得到较为纯净的具有分形结构的金属材料，采用计盒维数法计算出该金属材料的分维数（在准二维上）为1.59。     

材料断口分形维数分析

本文分析了应用分形维数描述材料断口轮廓形貌几何特征的局限性。结果指出，分形维数对于材料断口粗糙度的变化是不敏感的，该参数与材料断口轮廓几何形态和断裂韧性之间不存在确定性的关系。     

水泥断口表面形貌的分形维数定量表征研究

利用分形几何学理论定量表征了几种水泥断口表面的微观形貌特征,并且考察了断口表面分形维数与其抗压强度的关系.结果表明,水泥断口的表面分形维数介于2～3之间,且线性回归分析的相关系数均大于0.98,强的相关性表明实验所选用的水泥断口具有明显的分形特征;水泥的抗压强度与其断口表面分形维数值呈正相关关系.     

分形公差理论及其分形维数的计算方法

 从产品性能公差设计的角度探讨分形公差理论的建模、分析、设计等有关方法, 着重研究分形维数的计算理论和工程方法。     

基于分形插值函数的分形插值曲面的变差与计盒维数

分形插值是拟合数据的一种新方法,它可以反映出曲线和曲面上的粗糙性质.本文介绍了基于仿射分形插值函数的分形插值曲面的构造方法,给出了连续函数中心变差的概念,讨论了中心变差与变差、中心变差与计盒维数之间的关系.研究了这类分形插值曲面所对应的二元连续函数中心变差的性质,并根据二元连续函数中心变差与函数图像计盒维数之间的关系,得到了这类分形插值曲面的计盒维数.     

Ag-TCNQ分形生长现象研究

利用溶液化学反应法，在Ag-TCNQ过饱和度较大的条件下，发现了Ag-TCNQ的分形生长现象。研究表明，Ag膜厚度和与溶液反应的时间对样品的形貌有影响。Ag膜薄生成的样品稀疏，分形维数小；反之样品致密，分形维数大。反应完全时样品最致密，分形维数最大。SEM研究表明，这些分形生长主要是堆垛分形，分支堆积有序，表现出材料良好的自组织性。     

压下率对轧制单层晶极薄带晶界滑移特性的影响

采用退火态单层晶轧制铜箔为原料,进行不同压下率的箔轧.以多晶体位错滑移及塑性流动机制为基础,建立了考虑潜在硬化和晶格转动效应的率相关晶体滑移本构模型,分析了压下率对轧制单层晶极薄带晶界附近区域变形分布特性、取向演化和滑移系激活规律的影响,并探讨其机理.确定了合理的材料本构参数,铜箔拉伸实验与晶体塑性有限元模拟得到的应力-应变曲线一致.所建立的晶体塑性有限元模型,可很好的模拟最大压下率达到80%时轧制单层晶铜箔的变形过程.结果表明:1)由于晶粒形状、晶界及晶粒取向的作用导致晶内-晶间变形分布非均匀性;2)由于晶粒间复杂的相互作用导致晶粒取向主要绕横向(TD)进行旋转,且旋转角度和取向分散度随压下率的增加而增大;3)在晶内-晶间不同区域的滑移系启动存在显著差异,启动滑移系随压下率的增加而增多,当压下率小于等于60%时,在晶粒表层和晶界处,滑移系成对发生启动,当压下率达到80%时,表层和晶界处为多滑移系启动情形;4)滑移最先从晶粒表层和晶界处开始,然后向晶粒内部延伸.     

Solute segregation effect on grain boundary migration and Hall–Petch relationship in CrMnFeCoNi high-entropy alloy

Homogenisation has been used to eliminate solute segregation of CrMnFeCoNi high-entropy alloys (HEAs) before thermomechanical processing. To overcome the deficiencies caused by high-temperature homogenisation, we cancelled the homogenisation and studied the solute segregation feature and its effect on the grain growth and the mechanical property. The HEA forms equiaxed grains with the solute segregation in the range of tens of micrometres after cold rolling and recrystallisation. The grain growth in recrystallisation still abides by the classical grain growth kinetics, but with a higher power index of 3.33 and activation energy of 392.4?kJ?mol^?1 than those of the homogenised HEA, indicating a solute-drag effect. The relationship between the yield stress and grain size follows the Hall–Petch dependence with a higher intrinsic strength.     

一种新的铝合金晶界检测算法及应用

晶粒度检测是金相检验中的一个重要问题，在应用图像处理方法时体现为边缘检测，原有的边缘检测算法的各向同性的特点导致边缘提取的效果不理想，本文通过对图像边缘的分析，提出一种新的边缘检测算法，设计出一种各向异性，权重合理的边缘检测算子。与传统的边缘检测算法实验结果的比较显示了其优良的性能，在铝合金的晶界检测中取得了较好的效果。     

Solving oxygen embrittlement of refractory high-entropy alloy via grain boundary engineering

Refractory high-entropy alloys (RHEAs), particularly NbMoTaW RHEAs, exhibit outstanding softening resistance and thermal stability at ultra-high temperatures, but suffer from room-temperature brittleness, which severely limits their processability and thus practical application. In this study, we successfully achieved large plasticity of >10%, along with high strength of >1750 MPa in the NbMoTaW RHEAs via grain boundary engineering with the addition of either metalloid B or C. It was revealed that the room-temperature brittleness of the as-cast NbMoTaW RHEA originates from the grain-boundary segregation of the oxygen contaminant which weakens grain-boundary cohesion. The doped small-sized metalloids preferentially replace oxygen at grain boundaries and promote stronger electronic interaction with the host metals, which effectively alleviates the grain boundary brittleness and changes the fracture morphology from intergranular fracture to transgranular fracture. Our findings not only shed light on the understanding of the embrittlement mechanism of RHEAs in general, but also offer a useful route for ductilization of brittle HEAs.     

Superplasticity and grain boundary sliding in rolled AZ91 magnesium alloy at high strain rates

The superplastic deformation characteristics and microstructure evolution of the rolled AZ91 magnesium alloys at temperatures ranging from 623 to 698 K (0.67–0.76 T_m) and at the high strain rates ranging from 10⁻³ to 1 s⁻¹ were investigated with the methods of OM, SEM and TEM. An excellent superplasticity with the maximum elongation to failure of 455% was obtained at 623 K and the strain rate of 10⁻³ s⁻¹ in the rolled AZ91 magnesium alloys and its strain rate sensitivity m is high, up to 0.64. The dominant deformation mechanism in high strain rate superplasticity is still grain boundary sliding (GBS), which was studied systematically in this study. The dislocation creep controlled by grain boundary diffusion was considered the main accommodation mechanism, which was observed in this study.     

The roles of grain orientation and grain boundary characteristics in the enhanced superelasticity of Cu71.8Al17.8Mn10.4 shape memory alloys

Through texture and grain boundary control by continuous unidirectional solidification, the continuous columnar-grained polycrystalline Cu_71.8Al_17.8Mn_10.4 shape memory alloys were prepared and possess a strong 〈0 0 1〉 texture along the solidification direction and straight low-energy grain boundary. The alloys show excellent superelasticity of 10.1% improved from 3% for ordinary polycrystalline counterpart and with a tiny residual strain of less than 0.3% after unloading. There are some reasons for the enhanced superelasticity: (1) The martensitic transformation of all grains with strong 〈0 0 1〉-oriented texture occur at the same time under the tensile loading, which can avoid the significant stress concentration problem and transformation strain incompatibility at the grain boundaries due to the high elastic anisotropy in ordinary polycrystalline alloy. (2) High phase transformation strain can be obtained along 〈0 0 1〉 grain orientation. (3) Straight low-energy grain boundary and the absence of grain boundary triple junctions of continuous columnar-grained polycrystals can significantly reduce the blockage of martensitic transformation at the grain boundaries. These results provide a reference to structure design of high-performance polycrystalline Cu-based shape memory alloys.     

引线框架铜合金精轧薄板表面起皮剥落的数值分析

运用弹塑性有限元方法的平面应变分析模型，依据引线框架Cu-Fe-P合金精轧后表面起皮剥落处的显微组织特征，研究了Cu基体和Fe相颗粒界面附近的应力应变场分布。研究表明：铜基体与铁相的塑性行为及应力应变分布具有明显的不均匀性．界面区强烈的应力集中易造成界面开裂；精轧前Fe相周围的微裂纹在精轧变形时将进一步发展，直至发生引线框架扳材表面起皮剥落破坏。应避免热轧板坯中出现较大Fe颗粒。     

Production of very fine grained AISI 304 steel with high special grain boundary density by grain boundary engineering

Abstract

AISI 304 stainless steel was subjected to grain boundary engineering by applying cycles of calibre rolling and subsequent heat treatment. After three cycles the grain size started to decrease, and after the fourth cycle a very fine grained material having high fraction of special grain boundaries was produced. Due to the short heat treatment at 850°C, only partial recrystallization occurred after the first three cycles, which was proven by the large amount of low angle boundaries. The stored elastic strain energy helped the grain boundary movement and the formation of annealing twins in the fourth cycle, which caused the formation of very fine grained structure with a large amount of special grain boundaries.     

A novel etchant for revealing the prior austenite grain boundaries and matrix information in high alloy steels

To observe the prior austenite grain (PAG) boundaries in high-alloy steels, a novel etchant composed of oxalic acid, hydrogen peroxide, sodium bisulfite, and water has been developed. Etching with this system successfully revealed both the grain boundaries and the deformation bands within grains in high Co–Ni martensitic alloy steels. The carbon content of the alloys used in the test program varied from 0.23% to 0.35%, the cobalt content was in the range 9–13%, and the nickel content was in the range 8–11%. The specimens were austenitized at temperatures in the range 900–1200 °C. The rolling temperatures ranged from 700 °C to 1000 °C. The grain sizes in these alloys and those containing titanium were clearly revealed by optical microscopy following application of the new etchant.     

The role of grain boundary energy in grain boundary complexion transitions

Recent findings about the role of the grain boundary energy in complexion transitions are reviewed. Grain boundary energy distributions are most commonly evaluated using measurements of grain boundary thermal grooves. The measurements demonstrate that when a stable high temperature complexion co-exists with a metastable low temperature complexion, the stable complexion has a lower energy. It has also been found that the changes in the grain boundary energy lead to changes in the grain boundary character distribution. Finally, recent experimental observations are consistent with the theoretical prediction that higher energy grain boundaries transform at lower temperatures than relatively lower energy grain boundaries. To better control microstructures developed through grain growth, it is necessary to learn more about the mechanism and kinetics of complexion transitions.