固态反应周期层片型结构分析

Osinski等人在1982年发现的固态反应周期层片型结构至今仍然缺乏令人满意的理论描述．由于层片间存在相互吻合的形貌特征，因此该结构的形成被认为与固态反应过程中产生的扩散应力有关．高倍扫描背反射电子像显示，周期出现的层片不是单相的，而是由双相构成，层片中簇状的、而非颗粒状的非扩散相连续分布在互扩散相内．实验证实了本文作者关于周期片层结构形成机理的界面力学失稳模型     

CoCrFeMnNi高熵合金作为中间层的Cu/304不锈钢扩散连接研究

将CoCrFeMnNi高熵合金作为中间层,采用真空固态扩散方法实现Cu/304不锈钢的连接,通过SEM、EDS以及显微硬度测试,研究温度对扩散反应机理及性能的影响,采用Fick第二定律计算Cu/Fe原子在高熵合金中的扩散系数,借助XRD以及高熵合金中固溶体相形成判据分析扩散界面的相组成。结果表明:在800～900℃下,高熵合金与Cu和304不锈钢分别实现了稳固连接,界面处发生了元素的互扩散,随着温度的升高,Cu/Fe在高熵合金中的平均扩散系数增加;Cu/CoCrFeMnNi高熵合金和CoCrFeMnNi高熵合金/304不锈钢扩散界面处均未形成脆性金属间化合物;扩散界面处硬度呈连续变化趋势。研究表明,CoCrFeMnNi高熵合金是一种可用于Cu/304不锈钢异种材料扩散连接的阻挡层材料。     

铝铜合金准固态力学行为和凝固过程应力应变及热裂纹数值模拟

作者研究了铝铜合金准固态力学行为和凝固过程应力应变及热裂纹形成规律,得到了以下主要结果:(1)阐述了准固态断裂应变和低强塑性区的物理本质和意义及准固态力学性能与热裂倾向性的关系;(2)确立了铝合金准固态建模方法,建立了Al-5．14%Cu合金准固态流变模型和力学本构方程;(3)模拟了有热节拘束端棒形件和应力框凝固过程应力、应变和热裂纹的形成规律。     

扩散连接接头行为数值模拟的发展现状

扩散连接技术是一门边缘科学,涉及材料、扩散、相变、界面反应、接头应力应变等各种行为,工艺参数多,虽然已经进行了大量的试验研究,但却对各种材料的连接机理尚未有明确的认识,为此人们试图借助于计算技术,对接头行为进行数值模拟,以便找到共同规律,对扩散连接过程及质量进行预测与实时控制。本文主要内容就是概括地介绍了国内外关于扩散连接接头行为数值模拟的发展现状;主要包括界面孔洞消失过程、接头元素扩散与反应层的形成、接头变形与应力行为的数值模拟,以使人们能够定性或半定量的分析扩散连接因素对接头性能的影响。     

40Cr/QCr0.5超塑性固态焊接过程中的原子扩散

40Cr钢经激光表面淬火预处理后，与QCr0．5在56．6～84．9MPa预压应力下，加热至750-800℃，保温7．5min，经120～180S短时压接即可实现高质量的固态焊接。对接头区组织、成分、显微硬度的观测表明，焊接过程中发生了C原子从40Cr侧向QCr0．5侧的扩散，Cu原子从QCr0．5侧向40Cr侧的扩散。激光淬火组织的超细化、淬火组织在焊接加热过程中的扩散型相变、以及预压力下的塑性变形均为原子的快速扩散提供了条件。     

扩散温度对TC4合金表面Cu/Ni复合镀层结构及腐蚀性能的影响

论文采用扩散热处理研究了Cu/Ni/Ti复合镀层不同温度下的扩散行为,分析了扩散层结构,并讨论了扩散过程对镀层结构及腐蚀性能的影响。结果表明：由于Cu/Ni/Ti原子之间的互扩散,形成稳定的扩散层,可以有效提高镀层表面耐蚀性能;随着热扩散温度上升到700℃,膜层结构致密,在扩散层中形成了NixTiy金属间化合物及少量的CuxTiy金属间化合物,镀层表面的耐蚀性最好;温度升高到800℃时,在膜层界面处引发了Kirkendall效应,所形成的Kirkendall空位相互聚集长大,形成裂纹或孔洞,使得镀层疏松多孔,从而低了耐蚀性。     

冷拔残余应力应变对钢丝氢扩散过程的影响

利用有限元法对高强度珠光体钢丝冷拔残余应力应变进行有限元分析,在此基础上利用有限差分法计算应力应变同时诱导下的氢扩散浓度分布规律,并比较其与只考虑残余应力诱导氢扩散模型所得氢浓度分布规律的不同.结果表明,由于冷拔加工过程中钢丝表面和内部变形速率的差异,冷拔后在钢丝表面产生较大的残余拉应力和大量的塑性应变.残余应力应变的...     

高固相率MB15镁合金半固态压缩力学行为研究

通过MB15镁合金半固态等温压缩试验,研究了高固相率半固态材料的力学行为,提出了触变强度是高固相率半固态金属在稳态变形过程中的触变点的应力,即半固态金属固体骨架所能承受的最大正应力,并分析了加热温度、应变速率、保温时间、固相晶粒大小、晶粒圆整度及材料本身的强度等因素对半固态触变强度的影响,提出了触变强度的存在条件.结果表明,半固态触变强度随着加热温度的升高、应变速率的降低及保温时间的延长而降低,随着固相晶粒的减小、晶粒圆整度的增加及材料本身强度的增加而增加,当半固态材料内部的固相颗粒相互连结形成固体骨架时,存在触变强度.     

焊接残余应力对氢扩散影响的有限元模拟

利用ABAQUS有限元软件开发的氢扩散耦合计算程序,对焊态和焊后热处理状态下残余应力对氢扩散的影响进行了数值模拟,并与无应力状态下的氢扩散进行了比较．结果表明,存在焊接残余应力梯度时,氢向高应力区富集,在热影响区附近,有一个氢浓度低谷,这是氢向高应力区长程扩散所致．经过焊后热处理,应力松弛效果明显,残余应力大幅下降,对氢扩散的影响也大大降低,氢的最高浓度降低了近40％．因此,焊后热处理可以有效降低材料在氢环境下开裂的敏感性．     

TC4／QAl10-3-1-5扩散接头残余应力的有限元数值模拟

采用有限元分析软件ANSYS对TC4与QAl10-3-1．5直接扩散连接和填加Ni中间层扩散连接接头残余应力分布进行了数值模拟。结果表明，对接头强度有害的轴向拉应力巩的最大值出现在靠近接头边缘的界面脆性相及焊缝TC4侧的狭小区域内，且拉应力σy随着向中心轴的靠近而迅速减小并逐渐转为压应力，因此该区域是接头的薄弱区；采用Ni中间层时，应力分布梯度相对缓和一些，最大残余应力出现在中间层Ni与TC4之间的金属间化合物NiTi、Ni3Ti上；而随着中间层厚度的增加，接头残余应力也相应的增大。中间层厚仅存在一个最佳值。     

Programmed strengthening of crystalline materials using copper and aluminum as an example

Fundamental propositions of the well-known method of programmed strengthening of crystalline solids are described. It is shown that when mechanical (deformation) and thermal (tempering, annealing, and aging) actions are combined, the loading rate is determined by the rate of diffusion and microshear processes of relaxation of internal stresses in regions of defect accumulation and by the formation of stable complexes as a result of directional diffusion of point defects.     

Sn-Zn-Nd钎料焊点高温时效界面组织演变

研究了150 ℃时效条件下,经过360,720以及3 000 h以上时效处理后Sn-Zn-Nd钎料焊点界面组织的变化. 结果表明,焊点在长期时效过程中界面层成分的变化主要由原子的扩散机制主导,随着界面附近各原子浓度的变化,界面层的成分从焊后的单一Cu₅Zn₈层逐渐转变为成分复杂的Cu-Zn-Sn多元结构. 在时效过程中,由于界面层体积的增加会受到钎料基体的阻碍,因此在界面两侧的晶界处产生了压应力. 这些应力在时效过程中不断累积,当累积到一定程度后,使得金属间化合物层表面出现了孔洞和裂纹,从而导致了焊点性能的恶化.     

On the mechanism of diffusion-induced recrystallization: Comparison between experiment and molecular dynamics simulations

Recent experimental results on diffusion-induced recrystallization (DIR) in size-mismatched thin film metallic diffusion couples are summarized in this paper. As the most striking feature, newly formed grains reveal a preferred concentration which is characteristic for a given couple. Based on a suggested thermoelastic interpretation, stress in front of a migrating grain boundary is calculated from observed characteristic compositions. A remarkable relation between derived stresses and shear strength of respective parent matrix is discovered: plane stress in the diffusion zone amounts consistently to about 80% of the ideal shear strength. To elucidate this relation, molecular dynamics simulations are performed in the copper–gold system applying the embedded-atom method. Simulations indicate a break of coherency within the nanometric diffusion zone when a critical diffusor concentration is reached. Both experiment and simulation show in close agreement that maximum stress in the diffusion zone is of the order of the ideal shear strength and hence far above the yield strength of the material. It is deduced that the limit of coherency controls the observed characteristic concentration levels of DIR.     

Fundamental role of nanoscale structural level of plastic strain in solids

A conceptual substantiation of the necessity to consider the nanoscale structural level of plastic strain in the physics of plasticity and strength of solids is presented. It is inferred that the fundamental mechanism of plastic strain is represented by local structural transformations (of the type of rearrangement of atomic clusters of various configurations) that occur in a loaded solid in local zones of tensile normal stresses. This mechanism determines generality of the nature of all possible kinds of plastic deformation of solids.     

Porous nickel–titanium alloy prepared by gel-casting

To explore the preparation of porous nickel–titanium alloy with excellent properties, larger size and complex shape, the premixed powder of Ni and Ti with atomic ratio of 1:1 was shaped by gel-casting. The effects of solids loading and the content of dispersant on flow ability of nickel–titanium slurry and the mechanical properties of nickel–titanium sintered body were studied. The drying models under different solids loading were also discussed.The results show that the viscosity of slurries significantly increases with an increase in solids loading. After a proper process of drying, the green body with complex shape is obtained. The sintered body with porosity rate reaching up to49.5 % and compression strength reaching to 364.74 MPa could meet the basic demands of implant materials.     

Study on cracking of welding overlay based on the theory of diffusion-induced stress

Hydrogen diffusion in the wall of hydrogenation reactor for three situations,i.e.operating,normal shutdown and abnormal shutdown,was numerically simulated based on the finite element program-ABAQUS.The formula of diffusion-induced stress was deduced for model of the thin walled cylinder.Distribution of diffusion-induced stress in the wall of hydrogenation reactor was studied.The results showed that the maximum stress was at the fusion surface between welding overlay and base metal,and which would increase w...     

The mechanism of periodic layer formation during solid-state reaction between Mg and SiO2

The as yet unresolved microstructure of the periodic layers formed in the reactive diffusion system Mg/SiO₂ was clarified by using high-resolution field-emission SEM. The periodic layered structure actually consists of the single-phase layer of Mg₂Si and the two-phase layer of (Mg₂Si + MgO) alternated within the reaction zone. According to the experimental observations and in line with the diffusion-induced stresses model, the mechanism controlling this phenomenon could be attributed to the stresses induced by the difference in interface growth rates of Mg₂Si and MgO phases within the layer. When the elastic deformation of the slow-growing aggregated-MgO phase reaches its elastic maximum, it will be split off from the reaction front by the neighboring Mg₂Si phase and a new periodic layer forms. The computer simulation results are coinciding well with the experimental data.     

Microstructology of solid-state reactions

The use of a combined thermodynamic and diffusion kinetic approach in predicting the product morphology developed during solid-state displacement reactions is exemplified by the interactions in the GaSb/Co and SiC/Me (where Me=Cr,Pt,Co) systems. The influence of mechanical stresses induced during the interaction on the formation of microstructures is demonstrated. It was shown that the manifestation of the effects accompanying reactive-phase formation in inorganic solids (like the Kirkendall effect and the net volume change during internal precipitation) causes the generation and relaxation of the stresses. This significantly contributes to the morphological evolution of the reaction zone. Different micro- and macrodefects (vacancies, dislocations, pores, cracks, etc.) that can be generated inside the diffusion zone can also affect (or even control) the course of the reaction and determine the topological arrangement of the product phases within the resulting microstructure.     

Measurement of self-diffusion coefficients in Li ionic conductors by using the short-lived radiotracer of 8Li

For the effective use of short-lived radioactive beams, soon to be available at the Tokai Radioactive Ion Accelerator Complex, the authors have developed a radiotracer method for diffusion studies in solids. The experimental test was performed by the measurement of the diffusion coefficients of Li in a sample of the compound βLiAl using an α-emitting radiotracer of ⁸Li (T_1/2=0.84 s). It was found that the time-dependent yields of the α particles from the diffusing ⁸Li that was initially implanted in the sample could be used as a measure of the diffusivity of the tracer in a nondestructive way. The method was applied to measure the self-diffusion coefficients of Li in βLiGa, and for investigating how the Li diffusion in the Li ionic conductors is affected by the concentration of atomic defects (i.e., the existence of the atomic vacancies of Li and the defects in Ga sites that are replaced by Li).     

Experimental evidence for diffusion creep in the superplastic 3 mol% yttria-stabilized tetragonal zirconia

Although there have been numerous studies on the high temperature deformation characteristics of the superplastic 3 mol% yttria stabilized tetragonal zirconia (3YTZ), the rate controlling deformation mechanism has not been identified unambiguously. In the present study, experiments were conducted on 3YTZ at high stresses and at coarser grain sizes than used conventionally for superplasticity. The experimental results reveal, for the first time, an intragranular dislocation motion controlled high stress regime that is independent of the grain size. With a decrease in stress, there is a transition to a Newtonian viscous deformation regime consistent with Coble grain boundary diffusion creep. At sufficiently low stresses, or in materials with finer grain sizes, there is a further transition to a grain size dependent interface controlled deformation regime. Analysis of the experimental data suggests strongly that superplastic flow in 3YTZ occurs by an interface controlled deformation mechanism.