SiCw/LD2Al复合材料超塑变形协调机制的研究

为了研究金属基复合材料超塑变形时液相的作用,分析了金属基复合材料界面处的局部应力.当由滑移引起的界面处局部应力值高于外界施加于界面处应力值时,复合材料在含有液相的状态下超塑变形能力最大;反之,变形能力在不含液相的纯固相状态最大.因此,可以认为塑性变形过程中当有应力集中产生时,为了缓解应力集中,液相必需存在才能取得超塑性.当无应力集中时,超塑变形不需要液相的存在.上述理论与实验结果是一致的.     

Cu-Be/Cu-Zn层状金属基复合材料的冷轧变形行为及界面过渡层演变

高铍含量的铍铜(Cu-Be)合金时效后抗拉强度可达1400 MPa以上,伸长率却不到5％,呈现显著的强度-塑性倒置关系,严重影响了合金服役的安全可靠性.高强Cu-Be合金塑性变形时产生的局部应变集中现象是导致其低塑性的根本原因,将层状非均质构型设计的思想运用于Cu-Be合金,构建Cu-Be/Cu-Zn层状金属基复合材料,可以有效减少该现象的产生,有望获得高强塑性的层状金属基复合材料.运用塑性变形法制备层状金属基复合材料简单易行,受到广泛关注.前人对层状金属基复合材料轧制变形规律的研究主要集中在复合材料金属组元方面,对界面过渡层变形规律研究较少.本工作利用真空热压复合及后续冷轧变形的方式制备了Cu-Be/Cu-Zn层状金属基复合材料,利用光学显微镜(OM)、场发射扫描电镜(FE-SEM)结合能谱仪(EDS)、显微维氏硬度计对Cu-Be/Cu-Zn层状金属基复合材料冷轧变形行为及界面过渡层的演变进行了研究.研究结果表明,Cu-Be/Cu-Zn层状金属基复合材料冷轧前金属层间界面基本呈平直状,界面结合良好且无裂纹、孔洞等缺陷.当冷轧压下率不超过50％时,Cu-Be/Cu-Zn层状金属基复合材料发生不均匀的宏观变形,Cu-Zn层在板材厚度方向的变形量明显大于Cu-Be层和界面过渡层,当冷轧压下率为35％时,界面过渡层的厚度仅减小8.3％,不均匀的塑性变形导致Cu-Be/Cu-Zn界面由平直状态变为波浪状态;当冷轧压下率超过65％时,层状金属基复合材料内部发生均匀、协调的变形,各层厚度基本按照总冷轧压下率变化.不同冷轧压下率下,显微硬度最高的均为过渡层,其次是Cu-Be层,而Cu-Zn层的显微硬度最低.这是因为在层状金属基复合材料冷轧变形过程中,界面过渡层主要起到协调变形的作用,处于显著剪切应力状态,会产生额外的背应力强化.本工作探讨了界面过渡层在Cu-Be/Cu-Zn层状金属基复合材料冷轧过程中的宏观变形以及强化机理,有助于进一步阐明层状金属基复合材料塑性加工变形规律并合理制定其塑性加工工艺.     

金属基复合材料的高应变速率超塑性

综述并评论了金属基复合材料的高应变速率超塑变形机制，描述了金属基复合材料在高应变速率超塑变形中的一些理化现象，说明了变形过程中的各种影响因素，总结了具有高应变速率超塑性能的金属基复合材料及其性能，并指出了在金属基复合材料的高应变速率超塑性研究方面的不足。     

7B04铝合金超塑性变形行为

针对7B04铝合金开展了变形温度为470~530℃,应变速率为0.0003~0.01s~(-1)的高温超塑性拉伸实验,研究了材料的超塑性变形行为和变形机制。结果表明,7B04铝合金的流动应力随着变形温度的升高和应变速率的降低而逐渐减小,伸长率随之增加;在变形温度为530℃,应变速率为0.0003s~(-1)时,7B04铝合金的伸长率达到最大1105%,超塑性能最佳;应变速率敏感性指数m值均大于0.3,且随变形温度的升高而增加;在500~530℃的变形温度范围内,m值大于0.5,表明7B04铝合金超塑性变形以晶界滑动为主要变形机制;变形激活能Q为190kJ/mol,表明7B04铝合金的超塑性变形主要受晶内扩散控制;7B04铝合金超塑性变形中在晶界附近有液相产生,且适量的液相有利于提高材料的超塑性能。     

颗粒增强金属基复合材料中界面附近位移场测量的探索

本文采用数字散斑相关法对高能超声法制备的颗粒增强金属基复合材料在拉伸变形过程中颗粒与基体的界面处的位移进行了测量与分析，。实验结果表明，界面处所发生的位移变化要大于颗粒和基体内部所发生的位移，说明在颗粒与基体合金的界面处存在着导致应变集中的缺陷，从而导致了颗粒增强金属基复合材料失效过程中常常出现颗粒与基体在界面处脱粘的损伤形式。     

短纤维增强金属基复合材料基体中的应力分布及其变形特征

基体行为是影响随机分布短纤维增强金属基复合材料力学性能的一个重要因素。本文作者将在短纤维复合材料单纤维三维模型的基础上, 借助于弹塑性有限元分析方法, 研究在加载过程中, 不同纤维位向和界面结合状态下基体中的应力分布情况及基体的变形特征。研究表明, 该类复合材料中基体的应力分布情况和变形特征将受到纤维位向改变和界面结合强弱的显著影响。      

微机模拟研究金属基复合材料拉伸断裂行为与纤维强度分布的关系

纤维强度分布的变动系数CV与金属基复合材料制造工艺、纤维及界面特性等直接相关。本文用微计算机模拟法研究了CV对长纤维增强金属基复合材料拉伸断裂行为的影响,针对某—B/A1复合材料所作模拟结果指出,当其他条件相同,CV≤5％时,材料断裂形式为非累积型;当CV≤15％时,断裂形式为累积型;而当CV值处于5％与15％之间时,表现为两种断裂的混合形式。纤维所能承受的平均应力值随着CV增大而单调减小。     

TC6钛合金的超塑变形机制研究

提出了一种全新的超塑性研究方法：基于m值的高效超塑变形机制。采用该方法对TC6钛合金进行高温拉伸实验,研究其超塑性能,并与最大m值法超塑性进行比较分析。实验结果表明,细晶组织的该合金具有优良的超塑性,最佳变形温度在900℃,最大m值超塑变形可以获得20倍的最大延伸率;基于m值的高效超塑变形可以显著提高超塑成形效率,在获...     

金属基复合材料拉伸断裂过程及其分析模型

本文对金属基复合材料拉伸断裂过程及其分析模型进行了述评。内容包括纤维增强复合材料中直力分布、复合材料中裂缝萌生与扩展的微观机制、关于断裂纤维附近应力集中的分析方法、界面结合状态及界面反应对复合材料断裂特性的影响等。并提出了分析模型的发展趋势。     

金属和陶瓷界面对复合材料特性的影响

对粉末冶金，挤压铸造和液相铸造工艺制备的各种陶瓷增强铝基，钛基复合材料中金属和陶瓷的界面特性，显微结构，界面对复合材料力性能的影响进行了初步的探讨。研究结果指出，金属和陶瓷界面存在机械结合和反应结合的不同特性，并在受载破坏时表现出不同的断裂形貌，复合材料界面上存在析出相割裂了陶瓷增强相与基体界面的联系，降低了界面结合强度，陶瓷增强相表面的物理、化学状态及基体的化学成分对金属基复合材料的特性具有重要     

Deformability of a SiCw/6061Al composite during high strain rate compression at elevated temperatures

The deformability of SiCw/6061Al composite during high strain rate compression has been investigated at elevated temperatures around the solidus of the matrix alloy. The results show that the maximum deformability was obtained at 580°C which is near the solidus of the matrix. Analysis of the results indicates that the composites deformed at 580°C have the largest strain rate sensitivity (m value) and the lowest threshold stress, both of which lead to the maximum deformability. Microstructure observation shows that microcracks were formed at the interfaces in the composites deformed at 540°C and 620°C, whereas, in the composite deformed at 580°C, microcracks were rarely found because of the low stress concentration at the interfaces due to the presence of a small amount of liquid. It is suggested that the presence of an adequate amount of liquid phase gives rise to the effective accommodation required for grain boundary sliding for the composite, and thus directly affects the deformability of SiCw/6061Al composite.     

陶瓷材料超塑性研究进展

超塑性是细晶陶瓷在高温下的固有属性。本文综述了陶瓷材料超塑性的一般特征和氧化钇稳定四方相氧化锆多晶陶瓷(Y-TZP)的形变机理及最新研究进展。解释了不同纯度Y-TZP陶瓷在Ⅰ区存在巨大差异的原因以及杂质特征对应力指数的影响。从能量的观点进一步分析了陶瓷材料超塑变形过程中的控速机制。对共价键陶瓷Si₃N₄、SiC的超塑性特征以及晶间玻璃相在超塑变形中的作用进行了概括。此外,还总结了其它陶瓷材料,包括Al₂O₃及其复合陶瓷、纳米陶瓷的研究进展及发展方向。     

Critical shear stress produced by interaction of edge dislocation with nanoscale inhomogeneity

According to the Mott and Nabarro’s model, the contribution to the critical shear stress of the material caused by the interaction between edge dislocations and nanoscale cylindrical inhomogeneities with interface stresses is obtained. The influence of the radius and the volume fraction of the inhomogeneity as well as the interface stresses on the critical shear stress is investigated. The important result is that, if the interface stress is considered, a maximum of the contribution to the critical shear stress produced by this interaction may be obtained when the radius of the inhomogeneity reaches a critical value.     

Studies of effect of phase volume ratio on transfer of ionizable species across the water/ 1,2-dichloroethane interface by a three-electrode setup

The electrochemical behavior of pyridine distribution at the water/1,2-dichloroethane interface with variable phase volume ratios (r = Vo/Vw) was investigated by cyclic voltammetry. The system was composed of an aqueous droplet supported on a Ag/AgCl disk electrode covered with an organic solution or an organic droplet supported on a Ag/AgTPBCl disk electrode covered with an aqueous solution. In this way, a conventional three-electrode potentiostat can be used to study an ionizable compound transfer process at a liquid/liquid interface with a wide range of phase volume ratios (from 0.0004 to 1 and from 1 to 2500). Using this special cell we designed, only very small volumes of both phase were needed for r equal to unity, which is very useful for the investigation of the distribution of ionizable species at a biphasic system when the available amount of species is limited. The ionic partition diagrams were obtained for different phase volume ratios.     

Mesoscopic grain boundary sliding as the rate controlling process for high strain rate superplastic deformation

Important features observed during high strain rate superplastic deformation are enumerated. Starting from the premise that the phenomenon of structural superplasticity in different classes of materials results when grain boundary sliding that develops to a mesoscopic scale (defined to be of the order of a grain diameter or more) controls the rate of flow, the particular case of high strain rate superplasticity is explained. The rate equation developed is validated using experimental results concerning 5 alloy systems in which an ultra-fine grain size is developed by thermomechanical processing and retained in a similar condition during superplastic deformation by fine, grain boundary pinning particles and 3 alloy composites in which the volume fraction of the reinforcing constituent is significant (15–25%). It is demonstrated that the analysis results in estimates for the externally measured strain rates that are within a factor of two, in addition to providing a physically meaningful free energy of activation for the rate controlling process. This approach explains superplastic flow in different classes of materials in terms of a single rate controlling mechanism of deformation, viz., mesoscopic grain boundary sliding, with the help of a few constants that have the same values for all systems. The system-dependent variables of threshold stress needed for the onset of mesoscopic boundary sliding and free energy of activation are obtained directly from superplasticity stress–strain rate data, without external inputs.     

Joining molybdenum to aluminium by diffusion bonding

The joining of molybdenum to aluminium and aluminium-copper alloy using diffusion bonding has been investigated. Bond strengths have been measured by means of a simple shear jig and the joint microstructures characterized by electron microscopy and electron-probe microanalysis. Successful joints were produced by using a copper foil interlayer to form a eutectic liquid during the bonding process which helped disrupt the oxide film on aluminium and promote metal diffusion across the joint interface. When bonding commercial-purity aluminium to molybdenum, the iron present as an impurity caused a ternary eutectic liquid to form and, after solidification of the liquid phase, a thin film of Al₇Cu₂Fe was left behind on the aluminium. Failure of this joint occurred at a shear stress of 75 MPa, with the fracture path contained within the aluminium. With super-purity aluminium, a binary eutectic liquid was produced and the ensuing interface reaction resulted in a multi-layered structure of molybdenum-containing phases. The bond failed at the molybdenum interface at a stress of 40 MPa. When bonding aluminium-copper alloy to molybdenum without a copper interlayer, general melting at the interface via eutectic phase formation did not occur and the interface showed only localized reaction. The joint failed by separation from the molybdenum, at a stress of 25 MPa. When, however, a copper interlayer was used, fairly thick regions of multi-layered molybdenum intermetallics formed and the remaining surface was covered by a layer of Al₇Cu₂Mo phase. Failure of this joint occurred at a stress of 70 MPa, mainly by separation at the molybdenum interface.     

Effect of Solid/Liquid Interface Morphology on Microstructure and Segregation of DSX40M Superalloy

The effect of solid/liquid intedece morphologies on the microstructure and segregation of a new type superalloy, DSX40M, was studied. It has been found that the primary arm spacing presents maximum value as the solid/liquid interface shape transforms from cellular to cellular-dendritic.As the alloy solidifies with a coarse dendritic interface, the solute segregation degree and the average size of the carbide reach the maximum values because of the widest mushy zone. A Zr-rich phase forms at this range. Within the solidificatin rate range of dendritic interface. the primary dendritic arm spacing and solute segregation decrease with the increasing of solidification rate and the Zr-rich phase disappears. It should be indicated that the change of the solid/liquid interface does nt vary the carbide type, but greatly affects the average size of the carbides.The quantitative results of the carbide size change in this alloy system with different solid/liquid interfaces is presented     

Compression Properties of Ti-Al-X Alloys in Relation to Site Occupancies of Alloying Elements in TiAl and Ti3Al

1-IntroductionGammaTiAla1loyswithpotelltialforengineer-ingapplicationsinvariablycofltainasmallamountofa2-Ti3Alphase.Severalrepresentativetypesofmi-crostructurecanbeobtainedthroughbothalloyingandprocessing,resultinginsignificantvariationinmechanicalproperties[1].Extensiveresearchinthelastdecadehasbeendirectedatfindingthecorre-lationbetweenthemicrostructuralparametersandproperties;theunderstandingofthefundamentalmechanismofalloyinghasreceivedlessattention.Theimportanceofthislatterissueisbestil…     

An experimental investigation of phase transformation superplastic diffusion bonding of titanium alloy to stainless steel

The solid-state direct diffusion bonding of a near α-phase titanium alloy to an austenitic stainless steel by means of the phase transformation superplasticity (PTSP) caused by the cycles of heating and cooling has been carried out. The test results showed that, under the conditions of T_max = 890°C, T_min = 800°C, cyclic number of heating and cooling N = 10 cycles, specific pressure P = 5 MPa, heating rate V_h = 30°C/s and cooling rate V_c = 10°C/s, the ultimate tensile strength of the joint reached its maximum value (307 MPa), and the bonding time was only 120 s. In the phase transformation superplastic state, the deformation of titanium alloy has a character of ratcheting effect and it accumulates with the cycles of heating and cooling. The observations of tensile fracture interface showed that both the brittle intermetallic compound (FeTi) and the solid solution based on β-Ti were formed on the interface, and the more in quantity and the smaller in size the solid solutions are, the higher the ultimate tensile strength is.