The processing, properties, and applications of high-strain-rate superplastic materials

Recent research and development on high-strain-rate superplasticity has led to the concept of an accommodation helper mechanism and a new opportunity for superplastic forging. In this article, works about processing for ultrafine-grained materials and deformation mechanisms and high-strain-rate superplasticity applications are reviewed. Mamoru Mabuchi earned his Dr.Eng. in materials science and engineering at Kyoto University in 1997. He is currently a senior researcher at the National Industrial Research Institute of Nagoya. Kenji Higashi earned his Dr.Eng. in materials science and engineering from Osaka Prefecture University in 1982. He is currently a professor at Osaka Prefecture University. Dr. Higashi is also a member of TMS.     

GRAIN SHAPE EFFECT IN SUPERPLASTIC DEFORMATION

The method controlling grain shape in TMT processing and the effect of grain shape on char-acteristic parameters in superplastic deformation were discussed.The accommodation velocityof grahl boundary sliding,which is the dominant mechanism in superplastic deformation,andthe contribution of each mechanism to the total strain,as influenced by grain shape,were ana-lyzed.Grain shape has been shown to be an essential structural factor for superplasticity.Thenan analysis was made about the effect of grain shape on the region transition strain rate sothat a new concept,critical aspect for superplasticity,was worked out.These predictions werecompared with the measured results in an Al-Zn-Mg alloy.     

Microstructural evolution and plastic flow behavior of As-equal channel angular pressed 5083 Al alloy in low temperature superplasticity regime

In this study, the plastic flow behavior of ultrafine grained 5083 Al alloy fabricated by severe plastic deformation was examined in conjunction with microstructural evolution during deformation in the low temperature superplasticity regime. The present investigation was aimed at providing a better understanding of the nature of the low temperature superplasticity of ultrafine grained metallic materials. For this purpose, an ultrafine grained structure was introduced into the commercial 5083 Al alloy by equal channel angular pressing. A series of tensile tests was performed on the as-equal channel angular pressed samples at the initial strain rates of 10^?5–10^?2 sec^?1 and temperatures of 498–548 K, belonging to the low temperature superplasticity regime. The relationship between the true stress and true strain rate showed a sigmoidal behavior in a double logarithmic plot. The superplastic elongation was obtained within the limited intermediate strain range of 10^?4–10^?3 sec^?1 at 523 and 548 K. The microstructural examination and analysis of plastic flow curves revealed that low temperature superplasticity of the present alloy was attributed to dynamic recrystallization. In addition, necking instability during low temperature superplastic deformation of the alloy was discussed by applying Harts necking instability criterion.     

Superplastic deformation behavior and hot-processing map of the TiNp/2014 Al composite

The superplastic deformation behavior and hot-processing map of the TiN_p/2014 Al composite were investigated based on tensile tests conducted at various temperatures (773 K, 798 K, and 818 K) with various strain rates (0.033, 0.167, 0.33, and 0.67 s^-1). The results revealed that the influence of strain on the energy dissipation map is negligible. The optimal superplastic deformation parameters corresponding to the peak power dissipation efficiency of 0.65 differ from those corresponding to the maximum elongation of 351%. For the superplastic deformation of TiN_p/2014 composite, the deformation activation energy is much higher than that for the lattice self-diffusion in pure aluminum, which can be explained by the combination of mechanisms including grain (subgrain) boundary sliding accommodation, interface sliding accommodation, liquid-phase helper accommodation and load transfer. To avoid voids and wedge cracks, two obvious instability domains in the hot-processing maps should be avoided. The hot-processing maps obtained can approximately, but not accurately enough, optimize superplastic deformation parameters of the TiN_p/2014 Al composite.     

Superplastic behaviour and microstructure evolution of a fine-grained TA15 titanium alloy

The fine-grained microstructure of TA15 titanium alloy was prepared through two-step forging technology combined with high and low temperatures, and a transnormal superplastic elongation of more than 2000% was obtained. The superplastic behaviour and microstructure evolution were systematically researched at different temperatures and strain rates through superplastic tensile test. The results indicate that the fine-grained TA15 alloy exhibits superplasticity at temperatures of 760–980°C and initial strain rates from 1.1 × 10⁻² to 5.5 × 10⁻⁵ s⁻¹. The optimal superplastic conditions are 940°C and 3.3 × 10⁻⁴ s⁻¹, in which the average elongation is 2526% and the maximum elongation is 2743%. During superplastic deformation, dynamic recovery and recrystallization occur obviously, and the corporate effect of strain hardening and recrystallization softening decides the superplastic ability directly.     

Study on high strain rate superplasticity of A 6061Al alloy composite reinforced with 30 Vol.% AlN particulate

An investigation on the superplastic behavior of 30 vol.% AlNp/6061Al composite prepared by powder metallurgy (PM) techniques was carried out. Superplastic tensile tests of the composite were performed at strain rates ranging from 10° to 10⁻³ s⁻¹ and at temperatures from 823 to 893 K. A fine-grained structure prior to superplastic testing was obtained by hot rolling after extrusion. The highest total elongation to failure of 438% was achieved at a temperature of 863 K and at an initial strain rate of 1.67×10⁻¹ s⁻¹ and the highest value of the strain rate sensitivity index (m) was 0.42 for the composite. Differential thermal analysis (DTA) was used to investigate the possibility of any partial melting in the vicinity of optimum superplastic temperatures. The formation of a liquid phase is attributed to the melting of the Al-Si eutectic phase at the surface of the AlN particulates at elevated temperatures, as determined by electron probe microanalysis (EPMA). The influence of the microstructure and the fracture behavior on the superplastic behavior of the composite was studied by transmission electronic microscopy (TEM) and scanning electron microscopy (SEM). A large number of matrix filaments were present at the fracture surfaces of the specimens when superplastic deformation of the composite was performed under the optimum superplastic test conditions. The presence of dislocations and fine recrystallized grains in the test specimens suggested that they play an important role in the high-strain-rate superplasticity for this composite.     

材料超塑性研究的现状与发展

对20世纪90年代以来国内外超塑性的研究工作进行了介绍和评述.主要介绍了新的超塑材料的开发及超塑性的应用,概述了国内外超塑性研究的最新进展,并对超塑性研究的热点问题进行了评述,重点评述了用超塑成型方法制作大型铝合金汽车零件、用分子动力学模拟超塑变形中的晶界滑动、新材料和纳米材料的超塑性开发及超塑微成形的研究等国内外超塑性研究的新进展.展望了超塑性的发展趋势,指出应开发材料的低温或高速超塑性,重视超塑性流动过程的理论研究,进一步拓展超塑性的应用领域.     

High strain rate superplasticity of rolled AZ91 magnesium alloy

The high strain rate superplastic deformation properties and characteristics of a rolled AZ91 magnesium alloy at temperatures ranging from 623 to 698 K（0.67Tm-0.76Tm） and high strain rates ranging from 10^-3 to 1 s^-1 were investigated.The rolled AZ91 magnesium alloy possesses excellent superplasticity with the maximum elongation of 455% at 623 K and a strain rate of 10-3 s-1,and its strain rate sensitivity m is high up to 0.64.The dominant deformation mechanism responsible for the high strain rate superplasticity is still grain boundary sliding（GBS）,and the dislocation creep mechanism is considered as the main accommodation mechanism.     

Superplasticity in electrodeposited nanocrystalline nickel

Electrodeposited nanocrystalline Ni films were processed with different levels of S, to evaluate the role of S on superplasticity. All the materials exhibited high strain rate superplasticity at a relatively low temperature of 777 K. Microstructural characterization revealed that the S was converted to a Ni₃S₂ phase which melts at 908 K; no S could be detected at grain boundaries. There was no consistent variation in ductility with S content. Superplasticity was associated with a strain rate sensitivity of ～0.8 and an inverse grain size exponent of ～1, both of which are unusual observations in superplastic flow of metals. Based on the detailed experiments and analysis, it is concluded that superplasticity in nano-Ni is related to an interface controlled diffusion creep process, and it is not related to the presence of S at grain boundaries or a liquid phase at grain boundaries.     

High-strain-rate superplasticity of the Al–Zn–Mg–Cu alloys with Fe and Ni additions

During high-strain-rate superplastic deformation, superplasticity indices, and the microstructure of two Al–Zn–Mg–Cu–Zr alloys with additions of nickel and iron, which contain equal volume fractions of eutectic particles of Al₃Ni or Al₉FeNi, have been compared. It has been shown that the alloys exhibit superplasticity with 300–800% elongations at the strain rates of 1 × 10^–2–1 × 10^–1 s^–1. The differences in the kinetics of alloy recrystallization in the course of heating and deformation at different temperatures and rates of the superplastic deformation, which are related to the various parameters of the particles of the eutectic phases, have been found. At strain rates higher than 4 × 10^–2, in the alloy with Fe and Ni, a partially nonrecrystallized structure is retained up to material failure and, in the alloy with Ni, a completely recrystallized structure is formed at rates of up to 1 × 10^–1 s^–1.     

Superplastic behavior of a TiAl-based alloy

Superplastic behavior under the conditions of a temperature range from 850 to 1075°C and strain rates varying from 8×10⁻⁵ to 1×10⁻³ s⁻¹ was investigated for Ti–33Al–3Cr–0.5Mo (wt%) alloy with a very fine grain size obtained by the multi-step thermal mechanical treatment. The results show that the TiAl-based alloy with a hot-deformed fine grain size possesses good superplasticity. It exhibits a strain rate sensitivity coefficient of 0.9 at a strain rate of 3×10⁻⁵ s⁻¹ and temperature from 1000 to 1075°C. Moreover, the strain rate sensitivity coefficient is stable during the hot deformation, and a tensile elongation of 517% was obtained at 1075°C and a strain rate of 8×10⁻⁵ s⁻¹. The superplastic behavior of the present fine-grained TiAl-based alloy can be explained by the local strain hardening and high m value during the tensile deformation. Microstructure evolution in the superplastic deformation was also discussed.     

Superplasticity in intermetallic alloys and ceramics

Surprisingly, certain materials which are commonly seen as brittle may exhibit superplastic behavior under the proper conditions, and the ability to use superplastic forming methods on intermetallics and ceramics opens a realm of possibilities. By using unique processing techniques, superplasticity has been observed in such advanced materials as nickel silicides (Ni₃Si), yttria-stabilized tetragonal zirconia (YTZ) and an Al₂O₃-reinforced YTZ composite (Al₂O₃/YTZ).     

Superplastic deformation behaviour of friction stir processed 7075Al alloy

Commercial 7075Al rolled plates were subjected to friction stir processing (FSP) with different processing parameters, resulting in two fine-grained 7075Al alloys with a grain size of 3.8 and 7.5 μm. Heat treatment at 490 °C for 1 h showed that the fine grain microstructures were stable at high temperatures. Superplastic investigations in the temperature range of 420–530 °C and strain rate range of 1×10⁻³–1×10⁻¹ s⁻¹ demonstrated that a decrease in grain size resulted in significantly enhanced superplasticity and a shift to higher optimum strain rate and lower optimum deformation temperature. For the 3.8 μm 7075Al alloy, superplastic elongations of >1250% were obtained at 480 °C in the strain rate range of 3×10⁻³–3×10⁻² s⁻¹, whereas the 7.5 μm 7075Al alloy exhibited a maximum ductility of 1042% at 500 °C and 3×10⁻³ s⁻¹. The analyses of the superplastic data for the two alloys revealed a stress exponent of 2, an inverse grain size dependence of 2, and an activation energy close to that for grain boundary self-diffusion. This indicates that grain boundary sliding is the main deformation mechanism for the FSP 7075Al. This was verified by SEM examinations on the surfaces of deformed specimens.     

Temperature and grain size dependence of superplasticity in a Zn−0.3wt.%Al alloy

The superplastic deformation behavior of quasi-single phase Zn-0.3 wt. %Al was investigated. A series of load relaxation and tensile tests was conducted at various temperatures ranging from RT (20 °C) to 200 °C. The recently proposed internal variable theory of structural superplasticity was applied. The flow curves obtained from load relaxation tests were shown to consist of contributions from interface sliding (IS) and accommodating plastic deformation. In the case of quasi-single phase Zn-0.3 wt.% Al alloy with an average agrain size of 1 μm, the IS behavior could be described as a viscous flow process characterized by a power index of M_g=0.5. A large elongation of about 1400% was obtained at room temperature and the strain rate sensitivity parameter was about 0.4. Although relatively large-grained (10 μm) single phase alloy showed a high value of strain rate sensitivity comparable to that of fine-grained alloy at very low strain rate range, IS was not expected from the analysis based on the internal variable theory of structural superplasticity at room temperature. As the temperature increased above 100 °C, however, the contribution from IS was observed at a very low strain rate range. A high elongation of ∼400% was obtained in a specimen of 10-μm-grain-size at 200 °C under a strain rate of 2×10⁻⁴/sec. Jointly appointed at Center for Advanced Aerospace Materials (CAAM)     

Superplastic deformation of a heat-resistant Al–Cu–Mg–Ag–Mn alloy

The superplastic behavior and deformation mechanism of a heat-resistant Al–Cu–Mg–Ag–Mn alloy prepared by ingot metallurgy was investigated by using optical microscopy, scanning electron microscopy and transmission electron microscopy. It is shown that the Al–Cu–Mg–Ag–Mn alloy shows good superplastic properties at temperatures higher than 450 °C and strain rates lower than 10^?2 s^?1. A maximum elongation-to-failure of 320% was observed at 500 °C and 5 × 10^?4 s^?1, where the corresponding strain rate sensitivity index m is 0.58 and the flow stress σ is 5.7 MPa. Microstructure studies revealed that the observed superplastic behavior resulted from severe grain elongation rather than grain boundary sliding. It is suggested that this phenomenon may provide a new concept for developing superplastic materials.     

Superplasticity in a fine-grained beta-silicon nitride ceramic containing a transient liquid

Compressive deformation in a dense fine-grained β-Si₃N₄ sintered with 7 wt% cordierite was investigated over a wide range of temperatures (1450–1650°C) and strain rates (1×10⁻⁵–1×10⁻³/s). The superplastic characteristics and the results of microstructural examination are presented. These indicate that: (a) fine-grained β-Si₃N₄ ceramics are capable of high rates of deformation (about 10⁻⁵–10⁻⁴/s) at 1550°C without strain hardening occurring; (b) remarkable phase and microstructural evolutions, such as formation of elongated and strongly textured Si₂N₂O grains occurred during deformation; (c) a transient liquid phase present in this material system which facilitated the formation of Si₂N₂O grains enhanced the superplastic flow, and well-aligned Si₂N₂O grains formed in situ did not have a detrimental impact on the superplasticity; and (d) the mechanism of deformation is grain boundary sliding accommodated by diffusion-controlled solution–precipitation creep.     

往复挤压ZK60合金的低温准超塑性(英文)

通过2道次往复挤压制备细晶ZK60合金,在443~523K和初始应变速率为3.310-4~3.310-2s-1的范围内测试合金的低温超塑性。结果表明:往复挤压ZK60合金的平均晶粒尺寸约为5.0m,分布于基体内的破碎二次相颗粒和沉淀颗粒尺寸分别为不大于175nm和50nm。该合金具有低温准超塑性,在523K和3.310-4s-1应变速率下伸长率最大,为270%;在443和473K时,应变速率敏感系数m小于0.2;在523K时m为0.42。当温度不高于473K和523K时,超塑性变形激活能分别不高于63.2kJ/mol和110.6kJ/mol。当低于473K时,主要的超塑性流变机制为晶内滑移;在523K时,主要的超塑性变形机制为晶界滑移,由晶界扩散控制的位错蠕变为主要的兼容机制。     

Superplastic behavior of a powder metallurgy TiAl alloy with a metastable microstructure

Superplasticity in a powder metallurgy (P/M) TiAl alloy (Ti–47Al–2Cr–1Nb–1Ta) with a metastable B2 phase coexisted with a fine-grained γ+α₂ duplex structure has been studied. Alloy samples were tested at temperatures ranging from 650 to 1100°C, and at strain rates ranging from 10⁻⁶ to 10⁻⁴ s⁻¹. An elongation of over 300% was obtained at a strain rate of 2×10⁻⁵ s⁻¹ and at a temperature of 800°C, which is close to the ductile-to-brittle transition temperature of the alloy. This is in contrast to the prior observations of superplastic behavior of TiAl alloys in which a typical temperature of 1000°C is usually required for achieving superplasticity. It is suggested that the occurrence of low-temperature (800°C) superplasticity in the present alloy is primarily due to the presence of a metastable B2 phase in addition to a fine-grained (α₂+γ) duplex microstructure. The metastable B2 phase continues to decompose into fine-grained α₂ and γ phases, which promotes grain boundary sliding during superplastic deformation. The retained fine B2 grains accommodate the sliding strains to reduce the propensity of cavitation at grain triple junctions and thus delay the cavitation and fracture process.     

Effects of strain accumulation on the superplastic deformation behavior of 7075 Al alloy

The superplastic deformation behavior of a fine-grained 7075 Al alloy has been investigated within the framework of an internal variable theory for inelastic deformation. The theory takes the dislocation glide process within and across the grain boundaries (grain matrix deformation (GMD)) as the major accommodation mechanism for the grain boundary sliding (GBS). The flow curves were obtained by performing a series of load relaxation tests at the various prestrain values to examine the effects of accumulated strain on the superplastic deformation behavior. The most significant result obtained in this study is that the grain boundary characteristics change gradually with the strain accumulation from an initially Newtonian viscous flow signified with the power index value of M _g =1 to a non-Newtonian flow with the value of M _g =0.5 commonly observed in the various microduplex alloys such as Ti-6Al-4V. The variation of GBS characteristics with the prestrain is then examined by observing the microstructural evolution with the strain through the use of transmission electron microscopy (TEM).     

Er、Zr微合金化5083铝合金的超塑性

针对5E83合金(Er、Zr微合金化5083合金),采用超塑性拉伸试验、扫描电镜(SEM)、电子背散射衍射(EBSD)和透射电镜(TEM),探究了Er、Zr微合金元素、晶粒尺寸、变形温度、应变速率对合金超塑性的影响。通过再结晶退火、空冷和水冷的搅拌摩擦加工(FSP),分别获得了晶粒尺寸为7.4、5.2、3.4μm的完全再结晶组织,作为初始状态进行超塑性拉伸。结果表明,初始晶粒尺寸越细小,超塑性伸长率越高。当晶粒尺寸>5μm时,超塑性变形过程晶粒粗化缓慢,细化初始晶粒可显著提高超塑性;而当晶粒尺寸<5μm时,超塑性变形过程晶粒粗化严重,进一步细化初始晶粒对超塑性的提高有限。不同变形温度、应变速率的超塑性拉伸结果显示在变形温度为450～540℃、应变速率为1.67×10^-4～1.67×10^-1 s^-1,超塑性伸长率随变形温度和应变速率的提高呈现先上升后下降再上升的趋势;变形温度为520℃、应变速率为1.67×10^-3 s^-1条件下,水冷FSP态合金获得最大伸长率330%...