纳米3Y-TZP 陶瓷薄板超塑性成形研究

采用化学沉淀法制备了纳米级3Y－TZP粉体，制得了粒径尺寸为8－16nm，少团聚的粉体。热压烧结得到相对密度为97％，晶料尺寸为100nm－250nm的3Y－TZP陶瓷片。通过研究成形温度对成形性能的影响，最终采有超塑成形工艺制了半球形陶瓷零件。     

Effect of DC electric field on the tensile deformation of ultrafine-grained 3Y-TZP at 1450–1600 °C

Two test types were employed to determine the effect of DC electric field E = 5–8.3 V cm⁻¹ applied in the loading direction on the tensile flow stress of 3Y-TZP (d_o = 0.44 μm) at 1450–1600 °C. In type a, the field was applied continuously throughout the tensile test; in type b, the field was alternately applied and removed during the course of the tensile test. The field reduced the flow stress in both test types, but with an appreciably larger effect in the type a tests. The field, however, had no discernable effect on the stress exponent $n=\mathrm{\partial }\log \dot{\epsilon }/\mathrm{\partial }\log \sigma$ determined by strain rate change tests. The reduction in flow stress in the type b tests reversed upon removal of the field. It was interpreted to result from both Joule heating and a decrease in the energy to form the rate-controlling Zr⁴⁺ vacancies in the space charge zone adjacent to the grain boundaries. The calculated width of the space charge zone λ (=1.5–7.8 nm) and the ratio of the applied electric potential to the reduction in stress Δϕ_a/Δσ (=18–19 μV/MPa) were in accord with predictions. The larger reduction in the flow stress in the type a tests is attributed to a greater retardation of grain growth by the field.     

Superplastic deformation of duralumin LY12CZ under an electric field

The general superplastic deformation of many alloys had been studied exhaustively. Recently, the superplastic deformation of some alloys has been achieved by the application of an electric field to the process. Conrad and his colleagues have obtained the change of the flow stress and the strain-rate exponent during the superplastic deformation of 7475 Al alloy subjected to an external dc electric field. However, it is not clear how the electric field will affect the elongation and the growth of cavities quantitatively, and whether the type of cavities will change because of the existence of the external electric field.

In this paper, the optimum process parameters for the super plastic deformation of duralumin LY12CZ, without any pre-treatment, subjected to an external dc electric field have been obtained, LY12CZ (corresponding to ASTM 2024) being used widely in the aeronautic-astronautic industry. Furthermore, the effect of the electric field on the superplasticity and the superplastic-deformation conditions has been investigated. The experimental results show that the application of the electric field: (i) decreases the flow stress by 10–25% and the degree of cavitation at the onset of the fracture by 11% approximately; (ii) reduces the rate of strain-hardening; and (iii) increases the strain-rate sensitivity exponent by 26–38%. It is important to note that: (iv) the elongation of the superplastic deformation of LY12CZ under the electric field increases by 14–45% as compared with that without the electric field; (v) the equivalent strain at the onset of the rapid growth of the cavities increases by 25% with the application of the electric field; and (vi) spherical cavities exist in the specimens after the tensile test when applying the electric field, as opposed to the V-type cavities for the general superplastic deformation of LY12CZ. From these investigations, the optimum process parameters for the superplastic deformation of LY12CZ under an electric field are as follows: the deformation temperature is 471°C and the initial strain rate is 0.333 × 10⁻³ s⁻¹.     

Effect of electric current pulse on grain growth in superplastic deformation of 2091 Al-Li alloy

1　INTRODUCTIONThecharacteristicthatelectriccurrentpulsepro motesatomdiffusionanddislocationmotionwasgrad uallyrecognizedfromthefailureanalysisonintegrat edcircuitin 1970s[110 ] .Theelectriccurrentpulsewasusedtoincrease plastic property ,improvetheformabilityofless deformablemetals[11,12 ] ,andstudytheeffectsofcurrentpulseonrecrystallization[1318]respectivelyin 1980s .Inthisrespect ,ConradandLaihavedonealotofwork ,inwhichCu ,AlandTiweremainexperimentalmaterialsfordeformationemployedwitha…     

超塑变形对钛合金激光叠焊接头微观组织的影响

对超塑变形前后TC4钛合金激光叠焊接头的微观组织进行观察与分析。结果表明:在超塑变形过程中,焊缝的马氏体针状组织转变为层片状,变形过程中层片状组织被打断,超塑变形温度越高,层片状组织越碎化;超塑性变形应变速率对钛合金显微组织有重要影响,随着应变速率减小,母材α相的数量减小,而晶间β相数量逐渐增加,且两相都有等轴化趋势,但由于应变速率的减小,使得晶粒有一定程度长大;TC4钛合金激光焊焊缝组织由原始针状组织逐渐发展为层片状组织是超塑变形过程中激光焊焊缝剪切应力、相转变以及变形过程中动态再结晶等因素共同作用的结果。     

High-temperature deformation of Al2O3/Y-TZP particulate composites

Al₂O₃/Y-TZP particulate composites with compositions of between 20 and 80 vol.% Y-TZP were produced by tapecasting, lamination, and sintering. The processing methods employed resulted in fine grain sizes with only small variations between the composites produced. The resulting particulate composites were tested in compression at a temperature of 1350 °C over strain rates from 10⁻⁵ to 3.16×10⁻⁴ s⁻¹. Microstructural changes during testing were minor. Stress exponents were measured to the range from approximately two to three, which are consistent with published data on similar materials from tensile experiments. Models of composite creep behavior are compared to the experimental data over the full range of compositions studied. A constrained isostrain model is found to provide better predictive capabilities than either an unconstrained model, an isostress model, or a rheological model. Furthermore, the constrained isostrain model provides the most reasonable predictions for creep rates of 100% Al₂O₃ and 100% Y-TZP materials.     

电解加工中磁场对电场分布的影响

探讨如何将磁场叠加到电解加工装置上，使间隙电场按有利于提高加工精度的方式分布，分别进行了静态和动态磁场的试验。测量、分析后发现，只有在叠加磁场方向垂直于电场方向且N极指向电场叠加磁场时，对电场均布有较明显的作用。     

Influence of the radii of quartz capillaries on the rising height of water in an electric field

In quartz macrocapillaries of various radii, discharge fields of identical intensity result in an additional rise of water in different ways. This is caused by different changes in the properties of the liquid and solid phases, as well as the strong influence of the fields.     

5083铝合金最佳超塑性变形行为

采用最大m值法研究5083铝合金不同轧制方向的超塑性。在500℃～535℃温度范围内,得到其试样延伸率以及最佳变形温度为525℃,同时对比恒速度法、恒应变速率法在525℃的超塑性能。实验结果表明,5083铝合金轧制后,晶粒产生各向异性,导致不同轧制方向的延伸率有显著差异。在温度500℃、525℃和535℃下采用最大m值法拉伸,其纵向试样延伸率分别为264%、331%、317%,而横向试样延伸率则分别为98%、129%、119%,纵向试样延伸率显著大于横向试样延伸率。在温度525℃下,5083铝合金基于最大m值法的拉伸效果最好,其纵向试样最大延伸率为331%,拉伸时间为3846s;在相同温度下,用恒速度法、恒应变速率法拉伸,其纵向试样最大延伸率分别为316%、302%,而拉伸时间分别为9141s、12602s,最大m值法的延伸率略大于恒速度法、恒应变速率法的延伸率,但最大m值法的拉伸时间较恒速度法和恒应变速率法有大幅缩减。     

Superplastic deformation of 15 vol% SiCp/LY12 aluminum composite under an electric field

In this paper, influence of an external electric field on mechanical performance of deformation, cavity formation, and fracture mechanisms has been investigated during superplastic deformation of 15 vol% SiC_p/LY12 (LY12 matrix corresponds approximately to ASTM 2024) aluminum composite. The experimental results show that the appropriate electric field makes the strain-rate sensitivity index increase and the superplasticity improve. The results obtained by scanning electron microscopy (SEM) show that the nucleation growth and linkage or coalescence of cavities is restrained by applying an external electric field; meanwhile, fracture transforms from intergranular tear to typical superplastic failure, that is, fracture by formation and coalescence of cavities at particles and boundaries.     

The finite-element deformation modeling of superplastic Al-8090

The deformation behaviors of an Al-Li-Cu alloy (Aluminum 8090) during uniaxial and biaxial stress states were modeled using the nonlinear finite element analysis package ABAQUS 5.8. Two different material models were used for the purpose of comparison and to propose a valid and accurate superplastic material deformation behavior. Using a power law strain hardening model, the material was modeled with either constant or variable properties of strain-rate sensitivity and strain-hardening exponent. Results from the numerical models were compared to experimental results by Chen and Huang, who investigated the uniaxial and biaxial states of stress. Experimental data were primarily used to acquire the needed uniaxial material parameters for the model. The strain-rate sensitivity and strain-hardening exponent were dependent on the strain and strain rates. Comparing the two material models for both stress states indicated that the material model with variable properties better represented the true behavior of the material when compared to the experimental results. For more information, contact Noha M. Hassan, the American University in Cairo, 113 Kasr El Aini St., P.O. Box 2511, Cairo 11511, Egypt; +202-797-5336; fax +202-797-7565; e-mail nhassan@vt.edu.     

陶瓷超塑性变形机理研究进展

由于微纳米陶瓷材料在超塑性成形方面的潜在应用,近年来,关于陶瓷材料超塑性的研究已成为陶瓷领域研究的热点之一。关于陶瓷超塑性变形机理,目前普遍认为微观结构对陶瓷超塑性变形产生重要影响,细小晶粒之间晶界相互滑移在陶瓷超塑性变形过程中发挥重要作用,并且在改善和发展纳米陶瓷的超塑性方面已经取得了明显的进步。大多数学者对于陶瓷超塑性的变形机理的研究,目前还主要集中于晶粒局部变形的一些基本原则。文章在总结陶瓷超塑性影响因素和晶界滑移模型的基础上,以研究的氮化物陶瓷为例,对陶瓷超塑性变形机理进行了分析与探讨。