Superplastic Deformation Conditions Research Articles

Cavity of Al-Cu-Mg Alloy during Superplastic Deformation

According to the characteristic of appearing cavitation in the metals during superplastic deformation, the influence of strain rate on cavity evolvement, the influence of cavity on superplastic deformation capability, and the formation, development process of cavity were investigated for Al-Cu-Mg alloy (i.e. coarse–grained LY12). The results show that: ①The pore nucleation occurs not only at triangle grain boundaries, but also along nearby the second phase particles, and even within grains. The cavities at the triangle grain boundaries are present in V-shape, others near the second phase particles and within grains are present in O-shape. These cavities may result from disharmony slippage of grain boundaries. ②The tendency of cavity development decreases with increasing of strain-rate. In lower strain-rate condition, though Al-Cu-Mg alloy has better superplasticity, many big cavities in the specimen may reduce the room temperature properties of the alloy. In higher strain-rate condition, Al-Cu-Mg alloy has certain superplasticity and room temperature properties as well as few cavities forming. By analyzing, viscous layer on grain boundaries is very thin and grain sizes can be refined during their extruding and rotating each other in higher strain-rate superplastic deformation condition. ③Growth and coalescence of cavity are the main reason of the superplastic fracture of Al-Cu-Mg alloy. And small and a certain amount of cavities with dispersion and independence state are very useful to crystal boundary slippage.

An OIM Analysis on the Deformation Mechanism in Hot Compressed AZ31 Magnesium Alloy

Orientation mapping based on EBSD technique was applied to analyze the rules of orientation evolution of grains in AZ31 magnesium alloy. Results show that not only under deformation strain rate of 1×10-2s-1, but under 4×10-4s-1(the superplastic deformation condition), grains in all samples with initial textures rotate gradually to near basal orientation ({0002} || compression plane) at different ways, and basal texture becomes stronger with increasing strain, which indicates plastic slip plays an important role during hot deformation. Otherwise, no evident non-basal pyramidal slip of <a+c> as some studies mentioned was observed in the sample with the initial basal texture, and the basal orientation is kept unchanged during the deformation process, which suggests that basal slip is the uppermost plastic slip mechanism in this sample. In addition, the phenomenon of viscous laminar flow was observed in the sample with initial basal texture.

Microstructure and age hardening response of cast Al-Mg-Sc-Zr alloys

Al-Mg-Sc-Zr alloys have been investigated in recent years [1–3]. Sc and Zr additions to Al-5%Mg alloy were shown to refine the as-cast grain structure, increase strength of rolled sheets and reduce sub-grain size [4]. A series of alloys containing 0.30–0.35%Sc, 0.10– 0.16%Zr and 1.1–6.3%Mg were produced in rolled sheet form and shown to possess higher tensile properties compared to conventional Al-Mg based alloys [3]. In addition, the grain size of Zr containing alloy was found to be more stable at high temperatures than that of Zr-free Al-3%Mg-0.2%Sc alloy under superplastic deformation conditions [5]. The effect of Mg content in the Al-Mg-Sc-Zr alloy system on as-cast grain structure and hardening on direct aging has not been studied systematically. Such a study is considered important, since it can lead to the development of alloys with improved castability and weldability combined with high strength compared to conventional casting alloys. Also hardening on direct ageing of the as-manufactured components may be an added advantage in situations where a separate solution treatment may not be possible or may be undesirable due to problems associated with residual stresses and distortion. Chemical compositions of various alloys investigated are given in Table I. The alloys are designated by the nominal Mg content, and will be referred as such. The starting materials used in making the alloys were: 99.85% pure Al, Mg-30%Zr and Al-2%Sc ingots. The alloys were melted in an air induction melting furnace under flux cover, superheated to 760 ◦C, cooled to 700 ◦C and cast in dried M.S. molds as 25 mm diameter × 200 mm long ingots. Samples for metallography and age hardening studies were extracted leaving 25 mm height from the bottom of the ingots. Standard metallographic procedures were used. Samples were aged without prior homogenization or solution treatment in muffle type furnaces with job temperature variations within ±2 ◦C. Vickers hardness using 5 kg load was measured on the cross section of the ingots. Foils were

Modeling of microstructure and constitutive relation during superplastic deformation by fuzzy-neural network

In this paper, an adaptive fuzzy-neural network model has been established to model the microstructure evolution and constitutive relation of 15 vol.% SiCp/LY12 aluminum composite during superplastic deformation. This network integrates the learning power of neural networks with fuzzy inference systems. During the training process of the network, the back-propagation learning algorithm is applied to optimally adjust the weight coefficients of the neural network and the parameters of the fuzzy membership functions. Then, the trained network is used to predict the microstructure evolution and constitutive relation of 15 vol.% SiCp/LY12 aluminum composite during superplastic deformation. The predicted results agree very well with the experimental data of the test samples. On the basis of the good prediction ability of the proposed fuzzy-neural network, the constitutive relation and microstructure of 15 vol.% SiCp/LY12 aluminum composite under various superplastic deformation conditions have also been calculated and analyzed.

The effect of layer number on the superplasticity of laminate 7475/2091Al alloy

The layer number is one of the important structure parameters in laminate material. For laminate 7475/2091, the laminate with same thickness but different layer number from 2 to 20 were processed and studied. Experiment results show that the layer number of the laminate has a strong influence on the superplasticity of the laminate. Microscopy observations indicate that a metallurgical bonding at interface is obtained between two components in the laminates. There is a diffusion effect region on each side of a bonding interface. Cavities nucleate and grow there, especially at bonding interfaces in optimum superplastic deformation condition. The cavity evolution during superplastic deformation is an important factor that affects total elongation rate of the laminate.

Conditions for superplastic deformation

Conditions for superplastic deformation

Conditions for superplastic deformation

The model of the superplastic deformation based on a concept of cooperative nature of grain-boundary sliding is proposed. According to the model, the superplastic deformation requires the formation of bands of the coherent shear. Analysis of the temperature dependence of the stress interval limits for superplastic flow leads to a notion about two types of threshold stress, providing a theoretical basis for a new interpretation of some ambiguous experimental results.

A New Theoretical Concept for Micrograin Superplasticity Providing the Prediction of the Optimum Conditions for Superplastic Deformation

The superplasticity phenomenon is considered as a particular example of the structure-mechanical behavior of polycrystals during plastic deformation. Based upon the common principles of the plastic deformation of polycrystal materials the fundamental principle of achieving high plasticity is suggested. This principle is assumed to reflect the basic nature of the superplasticity phenomenon. On the basis of the concept involved the experimental procedure to find out the full set of the optimum conditions for superplastic deformation is developed. The applicability of the method suggested is demonstrated on the example of the intermetallic compound TiAl.

Large-scale flow as an indicator of superplasticity

A model is proposed for superplastic deformation of materials, based on the concept of cooperative grain-boundary slip. The conditions for superplastic deformation are obtained as conditions for coherent shear bands. Analysis of the temperature dependence of the limits of the stress interval for superplastic flow is used as a basis for the introduction of two types of threshold stress that elucidate the cause of the ambiguity in the interpretation of exisiting experimental results.

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.

Predicting fatigue crack initiation life of an aluminium alloy at low temperatures: Lu, B. and Zheng, X. Fatigue Fract. Eng. Mater. Struct. (Dec. 1992) 15 (12) 1213–1221

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.

The influence of geometrical constraint on cyclic, elastoplastic, notch root behavior and fatigue microcrack initiation: Tregoning, R. L. Diss. Abstr. Int. (Nov. 1992) 53 (5) 412

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.

Control of Superplastic Cavitation by Hydrostatic Pressure

It has been shown that the application of hydrostatic gas pressures during superplastic deformation of fine grained 7475 Al can entirely prevent the intergranular cavitation normally encountered at atmospheric pressure. A critical ratio of hydrostatic pressure to flow stress may be defined for each superplastic forming condition above which virtually no cavitation occurs. In superplastic deformation conditions where intergranular cavitation plays a significant part in final tensile rupture, the superplastic ductility may be improved by the application of hydrostatic pressures. Similarly, detrimental effects of large superplastic strains on service properties may be reduced or eliminated by the application of suitable hydrostatic pressures during superplastic forming. In this case, superplastically formed material may have the same design allowables as conventional 7475 Al sheet.

Microstructural observations of superplastic cavitation in fine grained 7475-Ai

Intergranular cavitation is an important consideration in the successful development of a commercially viable superplastic forming process for the high strength aluminum alloy, 7475. This work examined the microstructural features involved in the initiation stages of cavity formation. The observations suggest that, with the optimum superplastic deformation conditions, cavity nucleation is generally the rate determining step in the overall development of cavitation with strain. Cavities do not generally form at even the largest of the common single phase inclusion particles unless forming conditions are such that the flow stress significantly increases. It appears that, as well as local stress concentrations, additional effects are required, such as temperature induced particle decohesion and internal gas evolution, in order that cavities may grow to stable sizes. Such conditions may exist at certain two phase inclusion particles in the 7475 Al alloy. Suitable modifications to the standard alloy processing may therefore be devised which result in even lower rates of cavitation at the optimum superplastic forming conditions.

Investigation of structural changes during superplastic deformation of Zn-22% Al alloy by replica locating technique

One and the same microregions of surface of Zn-22% Al alloy specimen before and after superplastic deformation at 150 and 250°C and strain rates from 7 × 10 −5 to 2.4 × 10 −1 s were photographed. Grain rearrangement in this alloy has the main topological features of Ashby-Verrall grain switching model and is always accompanied by the appearance of newly formed regions on a specimen surface which arise preferably on the transversal grain boundaries. Each of these regions is a continuation of an initial grain of zinc β-phase (outgrowth on it) along tension axis or a new β-grain on the transversal grain boundary. The contribution of grain elongation to the total specimen elongation ( γ G ) was evaluated by measuring the size variations of each grain (out of ~ 200 grains) along tension axis taking into account the sign of such variations. In optimum conditions of superplastic deformation of Zn-22% Al alloy with 1.25 μm grain size, γ G value and consequently the contribution of intragrain dislocation slip are very small ( γ G = 6% at 250°C and ge = 1.7 × 10 −3 s). The narrow folded areas along the grain boundaries of aluminium phase are the result of dislocation slip localized in the grain ‘mantle’.

Grain-boundary sliding and intergranular cavitation during superplastic deformation of α/β brass

Intergranular and interphase cavitation in binary alpha/beta brass has been investigated in tension at 600° C under conditions of superplastic deformation. The sites for nucleation of cavities has been studied by quantitative metallography and the cavities are observed to nucleate preferentially atα-β interfaces. The process of cavitation is associated with grain boundary sliding and cavity nucleation occurs at points of stress concentrations in the sliding interfaces. Measurements of grain and phase boundary sliding at various interfaces demonstrate that sliding occurred onα-β boundaries more readily than onα-α andβ-gb interfaces. The predominance ofα-β interface cavitation is believed to be as a result of greater sliding at theα-β boundary and of an unbalanced accommodation of sliding adjacent to this type of boundary.

Structural conditions for superplastic deformation of alloy ZhS6KP

The best conditions for deformation in the superplastic mode of Ni-base ZhS6KP were studied. It was found the mechanism of superplastic deformation in this alloy is characterized by intergranular slip under conditions of diffusional mass transfer. (JRD)