A non-linear model for internal stress superplasticity

Most models of internal stress superplasticity predict a linear relationship between the applied stress and the plastic strain per cycle, and are only valid at low applied stresses. In the present paper, we extend the original linear theory of phase transformation superplasticity by Greenwood and Johnson [1] and derive a non-linear closed-form solution valid over the whole range of stresses, from the low-stress regime (where a linear relationship between strain and stress is predicted in agreement with the model by Greenwood and Johnson (Proc. R. Soc. Lond., 1965,283, 403), to the high-stress regime (where the strain increases non-linearly as the applied stress approaches the yield stress of the weaker phase). The model is found to be in agreement with literature data on transformation superplasticity of iron spanning both stress regimes. Furthermore, the model is adapted to the case where internal stresses are produced by thermal expansion mismatch: it is compared to experimental literature data for metals with anisotropic thermal expansion (Zn and U) and for metal matrix composites with inhomogeneous thermal expansion (AlSiC).     

纳米多晶体Ashby-Verrall超塑性模型的分子动力学模拟

应用分子动力学方法以纳米晶铝为例模拟了纳米多晶体的Ashby-Verrall超塑性模型的变形过程。在OK～400K的温度下,大倾角的晶界较容易开动,符合Ashby-Verrall模型;但小倾角的晶界难于开动。温度升高至600K,纳米晶粒界面熔化,产生微液相,较小倾角的晶界也可随液相流动而开动;晶粒随着液相的流动旋转到有利于滑移的方向,完成Ashby-Verrall模型的变形。此时单个晶粒的变形机制符合Gifkins的“心部和表层”模犁。     

一种耦合晶粒长大的超塑性本构及数值模拟

本文基于弹粘塑性有限元技术 ,将经过修正的晶粒长大模型耦合到材料粘塑模型中 ,得到一个考虑晶粒行为的超塑材料本构模型。利用所建立的考虑晶粒效应的本构模型和经典的Backofen本构模型 ,分别对Ti 6Al 4V圆杯进行超塑性成形模拟 ,其厚度分布结果表明 ,新本构能很好地描述晶粒敏感型材料的超塑性能。     

快速超塑性的研究与应用

综述了高应变速率超塑材料种类、变形机理和应用技术的最新进展。高应变速率超塑材料主要是铝基复合材料及铝合金,最近,对镁合金、纳米材料、钛合金高应变速率超塑性能的研究也已开始。高应变速率超塑性在工业中的应用已经起步,例如快速超塑成形技术、一模多件技术等,可以实现中等批量、甚至大批量生产,但是主要集中在铝合金上。未来激光辅助超塑成形技术、电塑性辅助超塑成形技术值得期待。     

Hydrodynamic flow of the eutectic under the conditions of superplasticity

In this paper we present original experimental data indicating the manifestation of a hydrodynamic mode of deformation under the conditions of superplasticity. The investigations have been performed on a cast alloy Sn-38 wt % Pb at room temperature. The manifestation of the hydrodynamic mode is related to the high level of internal stresses arising in the process of tension in connection with the occurrence of structural and phase transformations stimulated by plastic deformation in the initially metastable alloy and caused by a preliminary swaging of the samples.     

High-strain-rate superplasticity in aluminum-matrix composites

Recent progress in high-strain-rate superplastic forming of aluminum metal-matrix composites has highlighted the potential of these materials for the mass production of complex shapes. This article reviews the current scientific understanding of this subject and identifies the areas where future work is needed to develop the technology to the manufacturing stage.     

Transformation superplasticity in Zircadyne 705

The zirconium alloy Zircadyne 705 (main alloying addition: 2.5 wt.% Nb) was thermally cycled from 900 °C (100% β-phase) to 710 °C (80% α-phase and 20% β-phase), resulting in strain increments after each cycle that are linearly proportional to stress up to 2 MPa. Tensile elongations in excess of 240% were achieved without fracture. The Newtonian flow behavior and high ductility indicate that transformation superplasticity is the dominant deformation mechanism. The superplastic strain increment decreases as the cycling amplitude and period decrease, in general agreement with existing transformation superplasticity models. This paper was presented at the International Symposium on Superplasticity and Superplastic Forming, sponsored by the Manufacturing Critical Sector at the ASM International AeroMat 2004 Conference and Exposition, June 8–9, 2004, in Seattle, WA. The symposium was organized by Daniel G. Sanders, The Boeing Company.     

Fabrication of metal-matrix composites by application of the superplasticity effect

The production of continuous-fiber metal-matrix composites (MMCs) with improved mechanical properties by solid-state consolidation under superplastic conditions was investigated. The finite-element method was used to model MMC consolidation, which was experimentally studied on a sample of boronaluminum composite. It was shown that the deformation properties of a matrix exert considerable influence on metal flow geometry and its strain-stress state. The superplasticity effect provides the straightening of a matrix flow front and the localization of an intense deformation zone. It also facilitates the removal of pores in the final stage of hardening and considerably decreases local stresses on the fiber surface. The features of matrix material flow during consolidation of composites were considered. It was shown that the matrix deformation during composite consolidation takes place in the form of cooperative grain-boundary sliding and intragranular sliding. The mechanism of matrix deformation determines a type of fiber-matrix reaction at the interface, or, alternatively, the type of fiber-matrix interface interaction depends on the intensity of localized deformation in the given area. Identification of the interface structure was performed by acoustic emission.     

氮化硅陶瓷超塑性研究

以非晶氮化硅纳米陶瓷粉体为起始材料,以纳米氧化钇和氧化铝为添加剂液相烧结获得超塑性陶瓷块体材料,实现氮化硅陶瓷的超塑性拉伸和超塑性成形。氮化硅陶瓷的平均晶粒直径为280nm,在1550℃的较低温度,4.7×10-4/s的相对较高应变速率下,延伸率可达到110%,在成形速率0.2mm/min的条件下,可拉深成形出完好的球形和锥形零件。在室温断口上存在大量的细小的白色氮化硅晶粒;而高温断口上却几乎不存在这样的晶粒,这种不同可以用氮化硅陶瓷材料的超塑性变形机理来解释,也可以证明超塑性变形过程中晶界玻璃相的存在。     

Using intense plastic straining for high-strain-rate superplasticity

Ultrafine grain sizes may be introduced into bulk samples by using the intense plastic straining technique equal-channel angular pressing. This article describes the principles of equal-channel angular pressing and demonstrates the application of this procedure to attain ultrafine grain sizes in an Al-3Mg solid-solution alloy and a commercial cast Al-Mg-Li-Zr alloy. Provided there is stability of these ultrafine grains at elevated temperatures, as in the Al-Mg-Li-Zr alloy, equal-channel angular pressing may be used as a processing tool to achieve high-strain-rate superplasticity in materials that are not potentially superplastic. These results have important implications for reducing the long production times that are associated with the fabrication of complex parts using superplastic forming. Terence G. Langdon earned his Ph.D. in physical metallurgy at Imperial College, University of London, in 1965. He is currently professor of materials science and mechanical engineering at the University of Southern California, Los Angeles. Dr. Langdon is a member of TMS. Minoru Furukawa earned his D.Eng. in metallurgy at Kyushu University in 1988. He is currently an associate professor at Fukuoka University of Education, Munakata, Japan. Dr. Furukawa is also a member of TMS. Zenji Horita earned his Ph.D. in materials science at the University of Southern California in 1983. He is currently an associate professor of materials science and engineering at Kyushu University, Fukuoka, Japan. Dr. Horita is also a member of TMS. Minoru Nemoto earned his D.Eng. in materials science and engineering at Tohoku University in 1966. He is currently a professor of materials science and engineering at Kyushu University, Fukuoka, Japan. Dr. Nemoto is also a member of TMS.     

大块非晶合金的超塑性研究进展

对大块非晶合金在过冷液相区内的超塑性变形特点进行了评述.介绍了在过冷液相区内的流变行为和变形后的组织结构变化,并解释了超塑性变形的机理.同时,对大块非晶合金的超塑性成形技术发展现状况进行了概述.     

Effect of scandium on superplasticity of Al-Mg alloys

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Effect of alloying on superplasticity of two-phase brasses

Superplasticity characteristics of two-component and multicomponent brasses in the temperature range 525–600°C have been investigated at tension tests under the conditions of stepwise enhancement in the strain rate and when maintaining a constant strain rate of 1 × 10^–3 s^–1. The effective energy for activating superplastic deformation has been determined. It has been shown that brass alloyed with aluminum, tin, and iron exhibits large elongations and less porosity due to superplastic deformation. Changes in the granular structure and sample surfaces have been analyzed after deformation, and signs of grain-boundary sliding and intragrain deformation have been revealed in the alloys studied.     

Effect of the solid-solution composition on the superplasticity characteristics of Al-Zn-Mg-Cu-Ni-Zr Alloys

The structure and superplasticity characteristics of the Al-Zn-Mg-Cu-Ni-Zr alloys with different amounts of Zn and Mg have been analyzed. It has been demonstrated that for the formation of a micrograined structure and superplasticity effects, a bimodal size distribution of particles is required, i.e., the presence of coarse eutectic particles of Al₃Ni and dispersed Al₃Zr particles, and also the presence of highly alloyed solid solution is obligatory. In the case of a low-alloy solid solution in the alloys of this system, the recrystallization is hampered, both at heating and at the initial stage of the alloy deformation. An increase in the amount of Zn and Mg in the aluminum solid solution stimulates the formation of finer grains, a decrease in the yield stress, an increase in the strain-rate sensitivity m, and a growth in the values of relative elongation.     

退火工艺制度对5083铝合金板材超塑性的影响

针对普通(非细晶)5083铝合金轧制板材在超塑性成形生产过程中出现的平行于轧制方向的裂纹,通过实验研究发现这是由于轧制板材的各向异性导致t方向的超塑性较差引起的。为了减小各向异性对板材超塑性的影响,采用退火工艺对板材进行处理,并通过在不同温度进行退火实验寻找提高板料t方向超塑性的最佳退火热处理工艺。通过对普通5083铝合金板材在270~600℃进行退火热处理,发现500℃保温2 h后空冷退火工艺可明显减小各向异性对板材超塑性的影响,大大提高板材t方向的超塑性能,从而避免5083板材在超塑成形过程中因各向异性导致的裂纹缺陷。