采用2D-XRD研究TFA-MOD法制备的Nb5+掺杂YBCO薄膜的形核

通过在YBCO前驱液中掺入Nb5+,在制备的YBCO中发现生成了BYNO纳米颗粒,且BYNO存在着外延和随机2种取向。通过2D-XRD对YBCO/BYNO复合薄膜的形核进行了研究,发现复合薄膜在低温分解时其成分和纯YBCO一致,而在750℃到775℃之间时BYNO开始形核,并且它以外延和随机取向共存。而YBCO的形核则发生在820℃及其保温过程中,且YBCO呈现完全的(00l)取向,BYNO的存在并不会对YBCO的取向造成影响。此外还对形核过程的方程式进行了分析。     

超声振动对近共晶Al-Si活塞合金凝固组织的影响

研究了超声振动对Al-11.5Si-4Cu-2Ni-1Mg-0.45Fe合金凝固组织的影响。结果表明,经超声振动的合金中,初生硅形貌由长条状转变为多边形状,并得到了显著细化。其中空化效应产生的大过冷度引起的异质形核和生长时间缩短是初生硅细化的主要原因。随着超声功率的增加,初生硅由长条状转变为短棒状,α-Al₅FeSi相由块状转变为半圆形态。共晶硅和金属间化合物形态的演变和细化主要是由空化效应引起的固体破碎、均匀的溶质分布、均匀的温度场及共晶生长时间的缩短造成的。     

MgO-B_2O_3-SiO_2渣晶化动力学

应用差热分析（ＤＴＡ）技术研究了Ｃｒ２Ｏ３、Ｌａ２Ｏ３、ＣｅＯ２、Ｖ２Ｏ５、Ｓｉ３Ｎ４、ＳｉＣ、ＣａＦ２、ＭＯｘ对ＭｇＯＢ２Ｏ３ＳｉＯ２渣中含硼组分析晶活化能的影响。结果表明,Ｃｒ２Ｏ３、ＣｅＯ２、ＣａＦ２、ＳｉＣ、ＭＯｘ作为晶核剂加入后,可降低析晶活化能,有利于含硼组分以结晶相析出,是有效的晶核剂;Ｌａ２Ｏ３和Ｖ２Ｏ５对析晶活化能没有影响;Ｓｉ３Ｎ４则增强玻璃网络稳定性,提高析晶活化能。所以Ｌａ２Ｏ３、Ｖ２Ｏ５和Ｓｉ３Ｎ４作为晶核剂是无效的。     

钢中珠光体相变机制的研究进展

简要总结了钢中珠光体相变的机制，特别是珠光体形核的晶体学和长大的动力学。基于形核时的最小形核能和渗碳体与铁素体“合作”生长的要求，珠光体通常在奥氏体晶界形核并只向某一晶粒内生长，因而晶界珠光体与相邻晶粒存在特定取向关系而与生长进入的晶粒无特定取向关系。晶体学研究表明，珠光体形核的领先相与碳浓度及渗碳体与铁素体间的取向关系(分别为Pitsch-Petch、Bagaryatsky或lsaichev)无关。最近的研究发现，具Bl结构的非共格杂质粒子界面是过共析钢中晶内珠光体形核的有效位置，初步的结果表明，晶内珠光体形核的原因在于杂质粒子生长造成的局部区域碳贫化及低能的珠光体，杂质界面取代高能的奥氏体，杂质界面。特定过冷度下的层片间距与正向界面推移速率的关系是珠光体生长动力学的主要问题。基于珠光体与奥氏体间的无序界面假设和稳态扩散方程分别发展了以体扩散控制和以界面扩散控制为主的动力学理论。两种理论均与实验结果符合较好，但后者似更合理。最近的实验在珠光体中发现了生长台阶与结构台阶，表明生长动力学方程需要修正。     

贝氏体相变机制的研究进展

简要地总结了清华大学贝氏体相变研究组在贝氏体相变领域的主要研究成果。包括以下方面 :①利用TEM、AFM、STM发现了钢及有色合金中的贝氏体超精细结构 ;②利用STM发现贝氏体表面浮突大多是由与超亚单元相对应的“帐篷型”小浮突群组成的 ;③利用TEM和STM证实了三维巨型台阶存在的普遍性和可动性 ;④发现钢中贝氏体碳化物在α/γ界面的γ侧形核并向奥氏体内生长 ;⑤概述了贝氏体的激发形核 台阶长大机制及其理论计算 ;⑥用前述相变机制解释了贝氏体的多层次复杂结构 ,以及上贝氏体、下贝氏体等不同形态的形成原因。     

二元包晶合金研究进展

综合评述了二元包晶合金凝固领域的有关理论和实验研究的进展情况.指出,目前研究的内容主要集中在固态方面,而对与凝固过程密切相关的液态二元包晶合金的结构和性质则涉及很少.因而从液态二元包晶合金的结构和性质入手,寻求液态和固态之间的相互关系,是一个值得研究的新课题.     

低碳微量铌钢形变过程中动态相变的特点

用热模拟变形实验研究了低碳微量铌钢形变中的动态相变以及形变后冷却中的相变行为,透射电镜观察了Nb（CN）的析出及对铁素体晶粒截径和体积转变量的影响。结果表明：含Nb钢动态相变中铁素体形核位置依次为原奥氏体晶界、铁素体,奥氏体的相界前沿直至奥氏体晶内,随着细小的应变诱导Nb（CN）析出在基体上弥散分布,铁素体的转变量大幅增加并且其相变长大趋势得到有效抑制,使得铁素体的长大在时间和空间上均受到限制,是一个以形核为主的过程,相变完成后铁素体晶粒截径约为2舯;而形变后冷却相变工艺中铁素体的形核位置主要为奥氏体晶界以及形变带,而大量弥散分布的Nb（CN）析出对细化铁素体晶粒的作用并不明显,是一个形核长大的过程,最终得到的铁素体晶粒截径约为7μm。     

An investigation of fatigue failure prediction of adhesively bonded metal/metal joints

A fracture mechanics-based model for fatigue failure prediction of adhesive joints has been applied in this work. The model is based on the integration of the kinetic law of evolution of defects originated at stress concentrations within the joint. Final failure can be either brittle (fracture toughness-driven) or ductile (tensile/shear strength-driven) depending on the adhesive. The model has been validated against experiments conducted on single-lap shear joints bonded with a structural adhesive. Three different kinds of adhesives, namely a modified methacrylate, a one-part epoxy and a two-part epoxy supplied by Henkel, have been considered and three different overlap lengths have been tested. Fracture toughness and fatigue crack growth properties of the adhesives have been determined with mode I tests. The number of cycles to failure has been successfully predicted in several cases. It is interesting to notice that in the case of joints loaded at the same average shear stress, the shorter the joint, the longer the duration. This fact is also captured by the model.     

Micromechanical modeling of the behavior of duplex stainless steels

The aim of the present paper is the micromechanical modeling of an aged duplex (austenite/ferrite) stainless steel. While the elastic properties of both phases are almost the same, the plastic mismatch between ferrite and austenite is high due to ferrite aging. Accounting for the complex micro and macrostructure of this material, three scales are investigated. The first one corresponds to each single phase. The second scale consists of a percolated (interwoven) network of both phases. The third scale is an aggregate of percolated bicrystals. For the first scale, each phase is modeled as a single crystal. In the second scale, F.E. simulations are performed on a unit cubic cell representing the percolated networks. A mean field model is then fitted in order to represent that bicrystal. This mean field model is used at the third scale to model the behavior of the aggregate of two-phase grains, using a model for multi-phase materials. At this scale, it becomes possible to represent the distribution of average ferrite stresses in each ferrite-austenite bicrystal. These variations are thought to be responsible for the heterogeneous damage nucleation observed in this material.     

考虑空穴多级形核的损伤材料内时本构关系

本文提出了多级形核的损伤模型,建立了无形核损伤变量与无形核损伤能释放率、形核损伤变量与形核损伤能释放率之间的损伤发展的内蕴时间演变关系。