Superplastic Alloy Research Articles

Сверхпластичность и твердофазное соединение наноструктурированных материалов. Часть I. Влияние размера зерна на твердофазную свариваемость сверхпластичных сплавов

Сверхпластичность и твердофазное соединение наноструктурированных материалов. Часть I. Влияние размера зерна на твердофазную свариваемость сверхпластичных сплавов

Energy Absorption Capability of Zn-22Al Superplastic Alloy Foam Manufactured through Melt Foaming Process

Energy Absorption Capability of Zn-22Al Superplastic Alloy Foam Manufactured through Melt Foaming Process

A theoretical approach to finite strain superplasticity and some of its applications

The plastic flow of a polycrystal is analyzed assuming grains so fine that the rate-limiting process is grain boundary sliding, and grains readily accommodate their shapes by slip to preserve spatial continuity. We derive a closed-form constitutive equation relating strain, strain rate, stress and temperature that fits the experiment within the experimental uncertainties. The results are applied to titanium and aluminium superplastic alloys to show how the theory can shed light on the phenomena taking place inside the material when being superplastically stretched and why it finally breaks.

Energy Absorption Capability of Zn-22Al Superplastic Alloy Foam Manufactured through Melt Foaming Process

Superplastic Zn-22Al alloy foams were manufactured through the melt foaming process using titanium hydride powder as a foaming agent. The compressive properties of the Zn-22Al foams were investigated under quasi-static and dynamic loading conditions. Experimental results show that the flow stress and the energy absorption of the Zn-22Al foam significantly increased with increasing the strain rate. At high strain rate region, the energy absorption of the Zn-22Al foams is also larger than that of conventional aluminum foams. These behaviors are due to superplastic deformation of cell walls.

Finite Element Modeling of Superplastic Forming of Tubular Shapes

Most of the work done on superplastic forming is related to sheet metal forming. Very limited studies have been directed toward investigating the superplastic tube forming process. In this work, Finite Element (FE) simulations are carried out in order to simulate the superplastic tube forming process. The analysis is conducted for the superplastic magnesium alloy AZ31 at 400°C. The results clearly demonstrate that there is a significant difference between tube forming and sheet metal forming in terms of forming pressure profiles. In addition, the effects of tube radius, free forming length, and contact on the tube forming process are investigated.

Superplasticity for Lightweight Metal Foams

Potential applications of superplasticity for lightweight metal foams are reviewed in this paper. Metal foams have been used for lightweight structures, biomedical implants, filters, heat exchangers, sound absorbers and mechanical damping devices. Superplasticity has advantages on metal foaming process and mechanical properties of metal foams. Four examples of metal foams combined with superplastic forming are presented; (i) Enhanced foaming under internal stress superplastic condition, (ii) Superplastic diffusion-bonding of metal foams, (iii) Superplastic forming and foaming and (iv) Superplastic Zn-22Al alloy foam for energy absorbing material.

Embedment of HA into Superplastic Ti-6Al-4V: Effects of Implantation Temperature

In this research, we present a method for the embedment of hydroxyapatite (HA) into superplastic Ti6Al4V where the elements of HA and Ti6Al4V diffuse into each other to form a high quality implanted layer. The implantation process into superplastic titanium alloy was divided to two steps; first, the superplastic forming of Ti6Al4V was carried out using high-temperature compression test machine. In the second step, HA was embedded into superplastic Ti6Al4V by using a special designed clamp with an initial compressive load at high temperatures. The EDX and Line Scanning analyses indicate that the resulted film is composed of elements of HA and titanium alloy and due to the diffusion of HA and Ti6Al4V elements into each other, a good embedment is achieved. The elemental diffusion of HA and Ti6Al4V increases by increasing the temperature of the implantation process therefore the thickness of the implanted layer and the hardness of the embedded surface increase with the temperature.

The Effects of Specimen Geometry on the Accuracy of Tensile Testing of Metallic Superplastic Materials

This work investigates the sensitivity of a superplastic material’s tensile test to the major geometrical parameters of the selected test specimen. This required generating a large number of specimens by systematically varying the gauge length, gauge width, grip length and width of a standard geometry. The specimens were prepared from a moderately superplastic AZ31B-H24 magnesium alloy sheet and were then stretched at a selected rate and temperature. Deformation in each specimen was tracked via an electrochemically-etched fine grid which was particularly used to quantify the amount of material flow from the grip into the gauge region. The consequences of the latter on the accuracy of measured stresses and strains were correlated back to the corresponding geometrical parameters. Ultimately, the results were utilized to set the guidelines for selecting the optimum parameters in a “proper” specimen, for testing the unique class of superplastic materials.

210 FSPによるZn-22Al超塑性合金の組織制御(金属/金属基複合材料(1),ものづくりにおける基礎研究と先端技術の融合)

Friction stir welding (FSW) is a relatively new solid-state joining process primarily for aluminum alloys. FSW has been generating interest because of its association with friction stir processing (FSP), a new technique that essentially employs FSW tooling. FSP is being explored as a thermo-mechanical processing technique to transform a heterogeneous microstructure into a more homogenous microstructure. However, very little information is available regarding FSP characteristics of superplastic alloys. In this study we discuss the trials of Friction Stir Processing (FSP) on Zn-22Al superplastic alloy. Specially, the effects of FSP parameters on strength anisotropy and in-plain anisotropy have been researched. The results were discussed in terms of residual microstructure, mechanical properties and the superplastic properties.

Colossal anelasticity in polycrystals deforming under conditions of diffusional creep

Since diffusional creep viscosity depends strongly on the grain size, a variability in the grain size in a polycrystal can generate significant internal stresses. Upon unloading, the non-uniform internal stresses recover, leading to anelastic strains. The anelastic strains can be very large, approaching 100 times the elastic strains. We solve the case of bimodal viscosity in closed form to highlight the features of this type of anelasticity. The results are compared to the anelastic behavior of two-phase superplastic alloys. In such alloys the spatial variability in the diffusional viscosity can arise from the variability in the activation energy of grain boundary diffusion, which, because of its Arrhenius nature, can produce a much higher degree of variability than the grain size.

Relationship between loss-modulus and homologous temperature in superplastic alloys

A study of loss modulus values was conducted for three different metal alloys, in both superplastic and non-superplastic condition, using Dynamic Mechanical Analysis (DMA). Results showed a direct relationship between loss modulus values and the homologous superplastic temperature for each of the three different metal alloys that were studied.

Analysis of the texture of superplastic carburized duplex stainless alloy

We have investigated the effect of superplastic carburizing on the texture of duplex stainless (DS) alloy. The diffusion velocity of carbon in the superplastic DS alloy is much higher than that in non-superplastic DS alloy that has the same chemical composition. In carburized layer, the segregation of Cr can be observed and Cr-carbides precipitate at the grain boundary, in particularly, the triple-point intersection of grain boundaries. The precipitation of Cr-carbides in stainless alloys is disadvantageous because it causes a decrease in its corrosion resistance.

室温超塑性Zn-Al 合金の開発と制振ダンパーへの適用

室温超塑性Zn-Al 合金の開発と制振ダンパーへの適用

Study of the Li-insertion/extraction process in LiFePO 4/FePO 4

The structural properties of LiFePO 4 prepared by the hydrothermal route and chemically delithiated have been studied using analytical electron microscopy and Raman spectroscopy. High-resolution transmission electron microscopy and selected area electron diffraction measurements indicate that the partially delithiated particles include LiFePO 4 regions with cross-sections of finite size along the ac-plane, as a result of tilt grain boundary in the bc-plane, and dislocations in other directions. Only the boundary along the bc-plane is accompanied by a disorder over about 2 nm on each side of the boundary. The Raman spectrum shows the existence of both LiFePO 4 and FePO 4 phases in the shell of the particles at a delithiation degree of 50%, which invalidates the core–shell model. This result also invalidates the recent model according to which each particle would be single-domain, i.e. either a LiFePO 4 particle or a FePO 4 particle. On the other hand, our results, like prior ones, can be understood within the framework of a model similar to the spinodal decomposition of a two-phase system, which is discussed within the framework of morphogenesis of patterns in systems at equilibrium. Both end-members, however, are well crystallized, suggesting a recovery similar to that observed in superplastic alloys, with dynamics that are due to the motion of nucleation fronts and dislocations, and not due to a diffusion phenomenon associated with a concentration gradient.

Effect of gauge length in superplastic tensile tests

There is an incentive to obtain data on the mechanical behaviour of superplastic materials to model superplastic forming processes, in particular to help reduce forming times. The usual method involves tensile tests at elevated temperatures without extensometry. However, inhomogeneous deformation has a large effect on the apparent stress–strain behaviour and elongation to failure. Strain distributions have been measured in tensile specimens of superplastic Al–4·5Mg alloy with different gauge lengths, and on a non-superplastic Al–1Mn alloy, all tested at 530°C. There was considerable drawing-in of material from the specimen ends to the gauge in the superplastic alloy, but not in the Al–Mn material. The draw-in meant that the apparent elongation of the Al–4·5Mg alloy almost doubled when the gauge length to width ratio was reduced from 4 to 1 and the apparent stress–strain behaviour was also significantly affected by the gauge geometry.

Texture and High-Angle Boundary Formation during Deformation Processing

Among the phenomena leading to formation of high-angle boundaries during deformation processing at low homologous temperatures is the subdivision of prior grains and formation of deformation bands. Evidence for this phenomenon during processing of AA2004, a superplastic aluminum alloy, is reviewed; fragmentation of deformation bands leads to equiaxed grains and high-angle boundaries that support superplasticity. In addition to subdivision, groups of grains undergo lattice rotation toward one or the other variant of the β orientation fibers during plane-strain deformation of pure aluminum by ambient temperature rolling. The formation of equiaxed grains from banded structures during simple shear by equal channel angular pressing (ECAP) is also considered.

Effect of additions of zirconium, chromium, and nickel on the structure and characteristics of superplasticity of alloys of the Al-Zn-Mg-Cu system

By optical and transmission microscopy as well as by X-ray structural analysis, the structure and properties of alloys of the Al-Zn-Mg-Cu system with Ni, Zr, and Cr additions during superplastic deformation are investigated. It is shown that introduction of 0.26% Zr into the alloys leads to the formation of a nonrecrystallized or partially recrystallized structure in the billets. Such materials manifest the effect of superplasticity at t = 515°C in a range of deformation rates of (0.15–1.0) × 10−2 s−1, and the relative elongation reaches 450–680%.

Superplasticity of an AZ91 Magnesium Alloy

The superplastic deformation characteristics of the AZ91, the mostly used magnesium alloy, were investigated at various strain rates in the interval from 3x10-5 to 1x10-2 s-1 and temperature of 420 °C. To prepare superplastic alloys thermo-mechanical treatment was used. Cast materials were heat-treated in two stages, after homogenization at 415 °C for 10 h were submitted to the precipitation annealing at temperature in the range of 200-380 °C for 10 h, and deformed by hot extrusion. Microstructure of samples was observed by the light microscope Olympus. Strain rate sensitivity parameter m has been estimated by the abrupt strain rate changes method. The strong strain rate dependence of the m-parameter was found. The highest elongation to failure, 584%, was found for the samples aged at 380 °C. Possible physical mechanisms of the superplastic flow are discussed.

Constitutive equation for superplastic Ti–6Al–4V alloy

Superplasticity is the capability of some materials to exhibit large plastic deformations prior to failure. Structural superplasticity is observed in fine-grained alloys (the average grain size does not exceed 10 μm) under proper conditions of: • high temperature (greater than about one-half the absolute melting point); • a controlled strain-rate (within the strain-rate range 10 −4 to 10 −2 s −1). Commercial applications of superplastic forming are restricted to aluminium and titanium alloys. In the aircraft and automotive industries, superplastic forming shows promise as a main approach for producing light, complex-shaped parts. This paper describes a method to determine the material constants of superplastic alloys from a free forming test at constant pressure. In the finite element simulation the constitutive equation based on power law with hardening variables containing significant physical elements is chosen to fit the true stress, true strain and true strain-rate obtained from experimental data for Ti–6Al–4V at 1200 K. The finite element simulation of the free forming process is used to examine the validity of the suggested method.

Rheological Analogies of Superplastic and Viscous-Plastic Materials Flow

There are presented original experimental results of studying rheological behavior of Vitralloy 1 (Zr41Ti14Cu12.5Ni10Be22.5), which shows in the certain strain rate range stable linear viscous (Newtonian) flow. Comparative analysis of these results and the results related to other superplastic alloys as well as to crystallizing metallic melts has been carried out.