Low temperature superplasticity in a friction-stir-processed ultrafine grained Al–Zn–Mg–Sc alloy

Friction stir processing (FSP) was used to create a microstructure with ultrafine grains (0.68 μm grain size) in an as-cast Al–8.9Zn–2.6Mg–0.09Sc (wt.%) alloy. The ultrafine grained alloy exhibited superplasticity at relatively low temperatures and higher strain rates. Optimum ductility of 1165% at a strain rate of 3 × 10⁻² s⁻¹ and 310 °C was obtained. Enhanced superplasticity was also achieved at a temperature as low as 220 °C. Experimentally observed parametric dependencies and microstructural examinations indicated that the operating deformation mechanism might be the Rachinger grain boundary sliding accommodated by intragranular slip. The FSP microstructure became highly unstable at 390 °C onwards, thus, affecting ductility adversely. In situ transmission electron microscopy heating was used to understand the instability phenomenon, which has been attributed to the drop in particle pinning forces due to the dissolution of metastable precipitates and microstructural heterogeneity.     

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%...     

Superplasticity in fine-grained AZ61 magnesium alloy

The effect of grain refinement on the superplasticity of ingot-processed magnesium alloy was investigated. From the AZ61 material with a linear intercept grain size of 5 μrn, which was obtained by the multi-rolling process at an elevated temperature, tensile elongation over 400% could be achieved at 10^-3s^-1 at 400°C with a maximum value of 560% at 2x10^-4s^-1 at the same temperature. It was found that grain boundary diffusion controlled grain boundary sliding and pipe diffusion controlled slip creep govern the plastic flow at low and high strain rate ranges, respectively. A deformation map for pure magnesium was constructed to examine the effect of grain size and flow stress on deformation behavior at elevated temperature. The superplastic formability of Mg alloys was demonstrated by forming an AZ61 sheet into a hemi-sphere.     

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)     

Advances and applications in high strain rate superplasticity

High strain rate superplasticity (i.e., superplastic behavior at strain rates over 10^-2 s^-l) has been observed in many metallic materials such as aluminum alloys and their matrix composites and it is associated with an ultra-fine grained structure of less than about 3 pm. Its deformation mechanism appears to be different from that in conventional superplastic materials. A new model was considered from the viewpoint of the accommodation mechanism by an accommodation helper such as a liquid or glassy phase. The new mechanism was proposed in which superplasticity was critically controlled by the accommodation helper both to relax the stress concentration resulting from the sliding at grain boundaries and/or interfaces and to limit the build up of internal cavitation and subsequent failure. The possibility of the industrial applications was demonstrated for high-strain-rate superplasticity.     

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.     

The Effects of Stress State and Cavitation on Deformation Stability During Superplastic Forming

The current available models describing superplastic deformation do not account for a number of important characteristics, leading to the current limited predictive capabilities of deformation and failure. In this work, the effects of cavitation and stress state on deformation stability during superplastic forming are investigated using Finite Element simulations. The simulations are performed using constant strain rate forming and using a proposed optimization approach based on a multiscale failure criterion that accounts for stress state, geometrical necking, and microstructural evolution including grain growth and cavitation. The simulations are conducted for the superplastic copper-based alloy Coronze-638 and the superplastic aluminum alloy Al-5083 which are known to develop significant cavitation during deformation. The results clearly show the importance of accounting for microstructural evolution during superplastic forming, especially when the state of stress is biaxial. Furthermore, the results highlight the effectiveness of the proposed optimization technique in reducing the forming time and maintaining the integrity of the formed parts. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Seattle, WA, June 6-9, 2005.     

Analysis of Plastic Flow Instability During Superplastic Deformation of the Zn-Al Eutectoid Alloy Modified with 2 wt.% Cu

The aim of this work is to analyze the plastic flow instability in Zn-21Al-2Cu alloy deformed under 10^?3 s^?1 and 513 K, which are optimum conditions for inducing superplastic behavior in this alloy. An evaluation using the Hart and Wilkinson–Caceres criteria showed that the limited stability of plastic flow observed in this alloy is related to low values of the strain-rate sensitivity index (m) and the strain-hardening coefficient (γ), combined with the tendency of these parameters to decrease depending on true strain (ε). The reduction in m and γ values could be associated with the early onset of plastic instability and with microstructural changes observed as function of the strain. Grain growth induced by deformation seems to be important during the first stage of deformation of this alloy. However, when ε > 0.4 this growth is accompanied by other microstructural rearrangements. These results suggest that in this alloy, a grain boundary sliding mechanism acts to allow a steady superplastic flow only for ε < 0.4. For ε values between 0.4 and 0.7, observed occurrences of microstructural changes and severe neck formation lead to the supposition that there is a transition in the deformation mechanism. These changes are more evident when ε > 0.7 as another mechanism is thought to take over.     

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.     

Electron backscattered diffraction study on superplastic properties of coarse-grained Fe-27 at% Al

An electron backscattered diffraction technique has been used to investigate crystallographic features of a superplastic coarse-grained Fe-27 at% Al alloy. Alloy samples studied have been tensile tested in a temperature range between 600 and 800°C in air under an initial strain rate of 1×10⁻⁴ s⁻¹. As a result of dynamic recovery and recrystallization, the grain structure undergoes four major transitions: subgrain-boundary formation, grain-boundary migration, formation and growth of recrystallized grains. A model based on the microstructural evolution is described. Subgrains form during an initial stage of high-temperature deformation when deformation is conducted at low temperature (600°C). Upon further deformation at 700°C, grain boundaries migrate, resulting in the formation of new grains. When deformation is made further to a larger elongation or at even higher temperature (800°C), dynamic recovery and recrystallization occur significantly, resulting in grain refinement and hence superplasticity. Refined grains thus formed maintain crystallographic relationships with parent grains.     

Phase Composition and Superplastic Behavior of a Wrought AlCoCrCuFeNi High-Entropy Alloy

A cast AlCoCrCuFeNi high-entropy alloy was multiaxially forged at 950°C to produce a fine homogeneous mixture of grains/particles of four different phases with the average size of ~2.1 μm. The forged alloy exhibited unusual superplastic behavior accompanied by a pronounced softening stage, followed by a steady-state flow stage, during tensile deformation at temperatures of 800°C–1000°C and at strain rates of 10^?4–10^?1 s^?1. Despite the softening stage, no noticeable strain localization was observed and a total elongation of up to 1240% was obtained. A detailed analysis of the phase composition and microstructure of the alloy before and after superplastic deformation was conducted, the strain rate and temperature dependences of the flow stress were determined at different stages of the superplastic deformation, and the relationships between the microstructure and properties were identified and discussed.     

Constitutive modeling and processing map for elevated temperature flow behaviors of a powder metallurgy titanium aluminide alloy

The flow behaviors of PM titanium aluminide alloy were studied by isothermal compression simulation test. The apparent activation energy of deformation was calculated to be 313.53 kJ mol^?1 and a constitutive equation had been established to describe the flow behavior. Processing map was developed at a strain of 0.7. With an increase of strain, two domains can be found: dynamic recrystallization and superplastic deformation, which are further confirmed by microstructural observations. The dynamic recrystallization occurs extensively at 1000 °C and 10^?3 s^?1, with a peak efficiency of 50%, and the superplastic deformation occurs at 1100 °C and 10^?3 s^?1, with a peak efficiency of 60%. At a strain rate higher than 10^?1 s^?1, the alloy exhibits flow instability.     

Superplasticity of alloy V96Ts

Conclusions Alloy V96Ts with a fine-grained structure (grain diameter 5 ) exhibits superplasticity in tension at strain rates of 2.8·10^–4 to 5.5·10^–3 sec^–1 at 460–470°. In the superplastic condition the samples elongate evenly without necking.Ordzhonikidze UfaAviation Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metalov, No. 3, pp. 55–56, March, 1978.     

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.     

Microforming of superplastic 5083 aluminum alloy

The mechanical behavior of superplastic 5083 aluminum alloy during microforming process was investigated by finite element analysis.A micro V-groove die was modeled to analyze the effects of forming time,load and temperature on the microformability of the 5083 aluminum alloy.First,the microformability of the 5083 aluminum alloy was estimated using a microformability index.The simulation results show that the microformability increases with the forming load,time and temperature increasing.Superplasticity of the 5083 aluminum alloy during microforming using the V-groove die was also investigated in terms of the effective strain rate.The results show that the superplasticity of the 5083 aluminum alloy occurs in a specific part of the material for a specific period during the microforming process depending on the forming conditions and the microformability index.     

High strain rate superplasticity in a commercial Al–Mg–Sc alloy

It was shown that an Al–5.7%Mg–0.32%Sc–0.3%Mn alloy subjected to severe plastic deformation through equal-channel angular extrusion exhibits superior superplastic properties in the temperature range of 250–500 °C at strain rates ranging from 1.4 × 10⁻⁵ to 1.4 s⁻¹ with a maximum elongation-to-failure of 2000% recorded at 450 °C and an initial strain rate of 5.6 × 10⁻² s⁻¹.     

Hot deformation and processing maps of an Fe3Al intermetallic alloy

The deformation behavior of an Fe–28Al–5Cr–0.08Zr–0.04B (at.%) intermetallic alloy under hot compression conditions was characterized in the temperature range of 600–1100 °C and strain rate range of 0.001–100 s⁻¹. Processing maps were calculated to evaluate the efficiency of the hot working and to recognize the instability regions of the flow behavior. The investigated alloy possesses the optimum hot-working conditions at 1100 °C and 0.001 s⁻¹, since the material undergoes dynamic recrystallization to produce a fine-grained structure with a high fraction of high-angle boundaries (∼70%). At lower temperature the material exhibited “large grained superplasticity” with a peak efficiency of ∼60% at 1000 °C and 0.001 s⁻¹. These parameters are the optimum ones for superplastic working of that alloy. The occurrence of large grained superplasticity is attributed to the formation of a subgrain structure within the large original grains and higher strain-rate sensitivity. The material also exhibits flow instabilities due to flow localization at lower temperatures (<700 °C) and higher strain rates (>0.1 s⁻¹).     

Low temperature superplasticity of a fine-grained ZK60 magnesium alloy processed by equal-channel-angular extrusion

Fine-grained ZK60 magnesium alloy with the grain size of 1.4 μm was processed by equal-channel-angular extrusion. The material exhibited low temperature superplasticity. The normalized plot suggested that the present material had equilibrium grain boundaries at the superplastic temperature in contrast to typical materials processed by severe plastic deformation.     

Effect of Various SPD Techniques on Structure and Superplastic Deformation of Two Phase MgLiAl Alloy

MgLiAl alloy containing 9 wt% Li and 1.5% Al composed of hexagonal α and bcc β phases was cast under protecting atmosphere and hot extruded. Various methods of severe plastic deformation were applied to study their effect on structure and grain refinement. Rods were subjected to 1–3 passes of Twist Channel Angular Pressing TCAP (with helical component), cyclic compression to total strain ε?=?5 using MAXStrain Gleeble equipment, both performed at temperature interval 160–200 °C and, as third SPD method, KOBO type extrusion at RT. The TCAP pass resulted in grain refinement of α phase from 30 μm down to about 2 μm and that of β phase from 12 to 5 μm. Maxstrain cycling 10?× up to ε?=?5 led to much finer grain size of 300 nm. KOBO method performed at RT caused average grain size refinement of α and β phases down to about 1 μm. Hardness of alloy decreased slightly with increasing number of TCAP passes due to increase of small void density. It was higher after MAXStrain cycling and after KOBO extrusion. TEM studies after TCAP passes showed higher dislocation density in the β region than in the α phase. Crystallographic relationship (001) α|| (110) β indicated parallel positioning of slip planes of both phases. Electron diffraction technique confirmed increase of grain misorientation with number of TCAP passes. Stress/strain curves recorded at temperature 200 °C showed superplastic forming after 1st and 3rd TCAP passes with better superplastic properties due to higher elongation with increasing number of passes. Values of strain rate sensitivity coefficient m were calculated at 0.29 after 3rd TCAP pass for strain rate range 10^?5 to 5?×?10^?3 s^?1. Deformation by MAXStrain cycling caused much more effective grain refinement with fine microtwins in α phase. Superplastic deformation was also observed in alloy deformed by KOBO method, however the value of m?=?0.21 was obtained at lower temperature of deformation equal to 160 °C and deformation rate in the range 10^?5 to 5?×?10^?3. Tensile samples deformed superplastically showed grain growth and void formation caused by grain boundary slip. Summarizing, all methods applied resulted in sufficient grain refinement to obtain the effect of superplastic deformation for alloys of two phase α?+?β structure.     

Plastic flow characteristics of ultrafine grained low carbon steel during tensile deformation

In order to explain steady-state plastic deformation, i.e. the absence of strain hardening in ultrafine grained low carbon steel during tensile deformation, steel of different ferrite grain sizes was prepared by intense plastic straining followed by static annealing and then tensile-tested at room temperature. A comparison between the ferrite grain size of ultrafine grained steel and the dislocation cell size of coarse grained steel formed during tensile deformation revealed that uniform dislocation distribution with high density and cell formation were unlikely to occur in this ultrafine grained steel. This is ascribed to the fact that the ultrafine grain size is comparable to or smaller than the cell size at the corresponding stress level. In addition, from a consideration of dynamic recovery, it was found that the characteristic time for trapped lattice dislocations to spread into the grain boundaries was so fast that the accumulation of lattice dislocation causing strain hardening could not occur under this ultrafine grain size condition. Therefore, the extremely low strain hardening rate of ultrafine grained low carbon steel during tensile deformation is attributed to the combined effects of the two main factors described above.