Superplastic testing of the aluminium-lithium alloy 8090 has been performed in the temperature range 485°C to 550°C. Up to 530°C, raising the test temperature has the effect of increasing the m-value and decreasing the flow stress. Changes in superplastic forming characteristics and microstructure have been followed during straining using a variety of techniques. Jump-strain rate testing reveals that in the optimum temperature range (515°C to 525°C).there is a steady decrease in m-value. Cavitation also occurs, but can be suppressed effectively by applying a hydrostatic pressure during testing. Raising the test temperature above this optimum range to 550°C produces a significant decrease in the m-value and a more rapid fall-off with strain. These observations are explained in terms of recrystallisation and grain growth, and the effectiveness of precipitates in pinning grain boundaries at the various temperatures.

Superplastic aluminium alloys may be divided into two categories: those which recrystallise dynamically during superplastic deformation, such as the Supral alloys (2004 SPF) and 8090 SPF and, secondly those in which the fine grain necessary for superplasticity is produced by static recrystallisation during thermo-mechanical processing.

Both 7475 SPF and 5083 SPF belong to the second category.

This paper describes the superplastic forming characteristics of 5083 SPF and its post-formed mechanical properties.

Some typical applications will be shown to demonstrate the degree of complexity which is possible with this non-heat treatable alloy.

8090SPF alloy sheet has been in commercial production for about three years. This paper describes some of the property statistics of the commercially produced material, including tensile and superplastic results.

Properties have been determined for test components formed with back pressure with different levels of superplastic strain. Results from tensile and corrosion tests are presented.

This paper deals with the new development of both SPF of an Al alloy and SPF/DB of an Ti alloy.

Based on a theoretical model, a pressure versus time curve was determined for optimizing the SPF parameters. The effects of applied back pressure and reverse billowing on the performance of 7475 and Al-Cu-Li-Mg-Zr alloys are discussed.

Some essential technological factors which affect the quality of the SPF/DB of the Ti-6A1–4V alloy have been studied, such as the effect of an o layer on the strength of the base metal, the gas filling method and the pre-forming of parts with curved surfaces etc. Examples of applications to the manufacturing of airplane spars and undercarriage doors are also shown in this paper.

Based on current theories of diffusion and creep cavity growth (or closure) at high temperatures, a physical analysis of diffusion bonding for the Ti–6Al–4V titanium alloy is given. The diffusion bonding is mainly pictured as a two—stage process:

1. The shrinkage process of cavities with shape change from elliptical to cylindrical;

2. In the second stage, the radii of the cylindrical cavities arc reduced and eliminated.

In addition, according to experimental work and previous theoretical models of diffusion bonding, a new model has been established in this paper, which can be used to predict the effect of process variables, such as temperature, pressure, surface roughness and bonding time on the bonding degree for Ti–6Al–4V by using mechanisms of creep and diffusion.

Recent advances in superplasticity of L12 intermetallic alloys are reviewed. Results show that superplastic elongations of over 500% can be obtained in this group of materials. These intermetallic alloys include nickel silicide, Ni3 Si, and nickel aluminide, Ni3Al. In this review, the superplastic properties of these alloys are presented. The microstructural characteristics, e.g., grain size, phases, and texture, are described and correlated to the superplastic properties. The effects of alloying additions, temperature, and strain rate on the superplastic properties of these materials are discussed. In addition, the superplastic deformation mechanisms are proposed. Comparison of the superplastic characteristics of the intermetallic alloys are made with metal alloys and ceramic materials.

Two representative titanium aluminides containing 48 and 52 at. pct Al were produced by a mechanical alloying (MA) process for the purpose of attaining fine microstructures. For the MA apparatus, an attritor mill and a conventional ball mill were both used and the results compared. Mechanically alloyed powders were consolidated by hot pressing. In addition to morphological changes in the powders during MA, microstructures and mechanical properties of the consolidated products were studied. In particular, the MA alloy with 48 at. pct Al comprised of both TiAl and Ti 33Al phases showed a high strain rate sensitivity exponent, above 0.3. This suggests a possibility of developing superplastic deformation.

To study the effect of Al on superplasticity of ultra-high carbon steel (UHCS), 3–5%Al was added to the UHCS, and superplastic behavior was measured at 700–800°C. To produce the fine and superplastic microstructure, an isothermal rolling at 700°C was used, and the grain size of ferrite was less than about 2μm in diameter after the process.

Significant improvement in superplasticity was observed; the strain-rate sensitivity was high upto a strain-rate of 10−2 s−1, and 402% elongation was measured at 750°C and at an engineering strain-rate of 5000%/min(i.e., 8.3×10−2s−1). It has been determined that the Al-addition leads to a finer grain size, which, in turn, improved the superplastic properties of the ultra-high carbon steels.

Experiments were conducted on a superplastic aluminum-based alloy containing 6% Cu and 0.5% Zr. Specimens were pulled in tension under selected experimental conditions and then polished and examined with an optical microscope connected through a video camera to an image monitor and a personal computer. A software package was used to provide quantitative information on the cavity morphologies including the number of cavities in a selected area, their size, their deviation from a circle, and the orientation of their longest length with respect to the tensile axis. Results are presented showing the change in cavity morphology with elongation and the effect of testing at different strain rates.

The superplasticity of an Al-Li-Cu-Zr alloy has been studied under uniaxial tension and biaxial bulging conditions using hydrostatic pressure. An optimum tensile elongation of 850% at a strain rate of 3×10−4 /sec was obtained at 758 K. The maximum true thickness strain was 1.22 for a biaxially deformed specimen under the same conditions. It was shown that strain rate dependent flow hardening was related to necking profile. It was observed that cavity growth during superplastic deformation depended on the type of loading. Under biaxial bulging conditions, the cavities grew by forming cavity fissure networks along the grain boundaries. It was found that, by the superimposition of hydrostatic pressure, cavitation was retarded and the superplastic ductility was increased in both tensile and biaxial deformation.

At 515° C with a constant strain-rate of 6.7× 10−3 min−1,tensile specimen of LC4 aluminum alloy with an initial average grain size of 8.7μm exhibited good superplasticity. A fracture elongation of 4649% was obtained. Under such deformation conditions, the m-δrelationship belongs to the mL=mmax type , with a maximum m value of 0.78 at 94% strain. The m-δ relationship can be expressed as δI(%)= (CI ×0.0067(mI−0.5) −1) × 100. During superplastic deformation, three kinds of structural evolution take place, namely the formation of precipitate-free zones adjacent to grain boundaries due to diffusional creep, grain coarsening and cavity generation. The m-δ relationship of superplasticity of the alloy mentioned above can be explained on the basis of the structural evolution during superplastic deformation.

The influence of the strain hardening coefficient,γ,and the strain rate sensitivity exponent, m, on the onset and growth of necking has been pointed out in many models about plastic instability controlled by the mechanical properties of materials. In this paper, the instability parameter I (=(l-γ-m)/m) proposed by Hart is used to relate the values of m (=∂lnσ/∂lnέ) and γ (=∂σ/(σ·∂ε)) for some superplastic aluminium alloys. These values of m and γ were calculated from the load-elongation curve at various strain-rates in constant strain-rate tests with periodic m determinations by strain-rate departures of small magnitude. It will be shown that the strain hardening due to grain growth contributes to an increase in the stability of plastic deformation during superplastic flow. However the decrease of m due to dynamic grain growth with strain causes superplastic deformation to be unstable near the onset of fracture. The transition of I values from negative to positive coincides with a decrease in m value and flow stress.

It is concluded that the development of necks can be characterized by an instability parameter I which include the effect of strain hardening caused by strain induced grain growth.

At 700°C, with strain rates (έ) of 0.02, 0.05 and 0.08 min−1, the superplastic m-δ curves of T10 steel determined were all of the type (the general type). Their Values of rr (0.241 → 0.252 → 0.267), mL (0288 → 0.241 → 0.252 ),mF (0236 → 0.251 → 0.268), δL(38→. 535 →.58%), and δF (84.7 → 92 → 99%) all increased with increases in value. At the same temperature with έ =0.1 min−1, an ascending type of curve, mL=mwn = mo(=0.245)<mx (=.0.249), δo (=0)= δL > δx (=94%) was obtained from the transformation of δL and mL into the δF and mL of the general type of curve. Similar results have been obtained at 730°C.

At 600°,650°, 700° and 730°C, with έ =0.05 min−1 the same general type of curves were obtained. Their values of mo. (0.213 → 0.230 → 0.252 → 0296), mL (0.19 5 → 0.208 → 0.241 → 0.256 ).mF (0.204 → 0.234 → 0.251 → 0.280). δL (44 → 49 → 53.5 → 63.5%)and δF (77 → 81 → 117 percnt) increased with increases in temperature. For the same strain rate, while at 780°C, an ascending type of curve, mL.= mmis = mo (= 0.198) < mF (0.236).δo (=o) = δL < δx (=73%)was obtained from the transformation of the general type of curve in the same way as mentioned above.

The shape constants, C values, which vary with strain have been calculated.

Symbols: Refer to the List of Symbols.

It has been expected that “High rate superplastic materials” will be developed for industrial applications. The Dorntype equation for high temperature deformation suggests that strain rate can be increased if the grain size is decreased. This means that grain refinement can effectively establish high strain rate superplastic materials.

It is well known that a high degree of grain size refinement will result from the addition of zirconium to Al-base alloys. Powder-metallurgical processing with rapidly solidified powders is also available for the improvement of superplasticity by both the refinement of the solidified structure and the maintenance of the stable fine structure of a 7475 Al alloy during recrystallization and deformation. Therefore. P/M 7475 Al alloys containing Zr up to 0.9 wt% were selected as candidate specimens. The objective of the present paper includes the clarification of the role and the effective amount of Zr to obtain high strain rate superplastic materials. As a result, the addition of 0.3%Zr or more is effective in grain refinement of the P/M 7475 Al alloy. However, alloys containing 0.7 and 0.9 wt%Zr only show superplasticity at 793K. The optimum strain rate is shifted to a higher range with increasing Zr. The alloy of 7475 Al-0.9%Zr shows the maximum elongation of 900% at the remarkably high strain rate of 3.3×10−1 s−1.

The deformation mechanism of such high stain rate superplasticity will be discussed briefly, by considering the effect of the fine particles of Zr on superplastic behavior.

IN90211 has exhibited superplastic elongations above 500% at high homologous temperatures (0.76–0.82 Tm). A high strain rate and flow stress for optimum elongation was measured (1–5/sec, 20–60 MPa, 425–485 °C). The apparent strain rate sensitivity of m≈0.25 differs from the usual m≈0.5 observations of superplastic deformation. An analysis of the data at several strains indicates a highly temperature dependent threshold stress is present, with either a n=2 or n=3 assumption for the stress exponent. The magnitude of the threshold stresses in IN90211 are smaller than usually observed in a dispersion strengthened matrix (1–20% instead of ≈50% of the Orowan stress). Experimental evidence from creep experiments supports the n=3 deformation mechanism as the rate limiting step of deformation.

Improvements of superplasticity in the high strain rate range have been achieved in a 7475 Al alloy with 0. 7 wt% Zr produced from rapidly solidified powder (PM7475-0.7Zr alloy), and the superplastic properties and microstructural changes with strain have been investigated.

The PM 7475-0.7 Zr alloy has a cold rolling structure, but on heating to the superplastic temperature of 520°C, the alloy developed a very fine subgrain structure of average subgrain size of less than 1 μm. The fine structure was stabilized with metastable cubic Al3Zr precipitate dispersions, and there exist a large number of less than 5° misorientation subboundaries together with a small number of highly misoriented ones.

After 117% strain at the high strain rate of 2. 5×10−1 S−1 the subboundaries with the misorientation of 5° – 15° Were observed to increase. During the initial stage of the deformation, m value and the contribution of boundary sliding to the deformation also increased with strain, and thus the alloy behaved superplastically during the following deformation.

The improvements of superplasticity with strain in the early stage of the high strain rate deformation of the alloy were interpreted to be closely related with the increase in misorientation of subboundaries stabilized with finely dispersed metastable Al3Zr precipitates.

With AM/Ao = 1, the superplastic m- δ curves of Zn-Ni-Mg alloy have been studied. When T=220°C and έ (initial)=1.33×10−2 min−1, the curve was of the type. It was transformed into the mL =mmin =mo<mF,δL=δL<εF type of curve, when T=200°–230°C and έ (initial)=3.33×10−2 min−1 and further transformed the type at 240°–250°C with the same strain rate.

With AN/Ao<1 and >0.595, T=230°C and έ (initial)=3.33×10−2 min−1 the curves were all of the mL=mmax =mo <mF, δ.(=O)= δL < δF type. The δF values increased with increase in AN/Ao value. The effect mainly reflected on the CF value and the CL m-δ equation can be expressed to account for the notch effect as follows:

where and AN/Ao < 1 and > 0.595.

Symbols: Refer to the List of Symbols.

The influence of superplastic extrusion on the microstructures and the shape memory effect of the Cu-Zn-Al shape memory alloys has been investigated. The shape memory alloy Cu-26%wt%Zn-3.5wt%Al is superplastio with an index of strain rate sensitivity n = 0.48 at 600°C, at a strain of . After extrusion under the superplastio condition the miorostruotures are improved and no cavities are observed. The superplastically extruded specimens of this alloy were heat-treated to obtain the shape memory effect. These specimens indicate that no deterioration of shape memory effect of the alloy is induced by the superplastio extrusion and the shape memory effect of the alloy is somewhat improved.

The effect of Cerium addition on the superplasticity of Al-6Cu-0.35Mg-O.2Zr alloy is investigated. The superplastic elongation of the alloy is increased from 450% to 500–700% by the addition of 0.5wt% Cerium, with some decrease in superplastic tensile resistance, when the alloy is deformed at 789K(516°C) with a strain rate of 8.33×10−4S−1. Microstructure observation shows that the addition of Cerium decreases the grain growth rate both in static annealing and during superplastic deformation. This is considered to be the result of the enhanced pinning of fine Zirconium-rich precipitates and the additional pinning of Cerium-rich phase particles. TEM and X-ray diffraction analysis show that Cerium in the alloy forms Cerium-rich phases (AlgCu4Ce and Al4CuCe), and these phase particles join in the pinning of second phase particles on grain boundary, stablizing the microstructure. The addition of Cerium also affects the distribution of Zirconium and Zirconium Aluminide in the alloy.

The superplastic behaviour of Ti-6Al-4V alloy containing various amounts of hydrogen (0.07 ∼ 0.33wt%) has been investigated by tensile tests at temperatures of 800 ∼ 950°C. Results show that the solution of hydrogen in the alloy helps to reduce the flow stress with strain rate in the range of 1.67 ×10−4 ∼ 1.67×10−2 s−1 and makes the temperature of superplastlc deformation become Lower (810∼870°C). Nevertheless, it also causes a decrease of the strain rate sensitivity exponent and tensileelongation, However, by a suitable alloying with hydrogen (∼ 0.1wt%), the alloy can be made to undergo superplastlc deformation at a rather tow temperature (840°C) with an acceptable loss of superptastic ductility.