In the present paper, the microstructural changes associated with phase transformation and the straining behavior in polycrystalline structures dueing transformation superplastic deformation are investigated. In-situ observations of microstructural changes during Ac₃transformation on rapid heating in pure iron have been carried out with a high temperature optical microscope and a dark field reflection high temperature microscope. Both of hot-stage microscopes were specially designed for this study. The distribution of superplastic strain has been examined by microscopic strain analyses by using a micro-grid pattern with 12.7μm intervals. The main results are as follows:
(1) Ac₃ transformation process at a heating rate under 50K/s is predominantly nucleation of austenite grains at the prior ferrite grain boundaries and triple points. A subsequent growth of grains into the prior ferrite matrices is observed. The growth of austenite grains is not always isotropic under a tensile stress.
(2) In the initial stage of transformation, superplastic strain is induced by the sliding at γ/α transformation interface along the prior ferrite grain boundaries. The observed superplastic strain is also associated with the grain rotation, corresponding to the growth of austenite grains which surround ferrite grains.
(3) In the intermediate stage of transformation, the sliding deformation is generated at the migrating transformation interface associated with the growth of austenite grains. Accumulated strain by the sliding is left within the previously transformed region.
(4) These observations suggest that the sliding mechanism at the migrating transformation interface is a principal mechanism of transformation superplasticity.

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Superplasticity of δ/γ duplex stainless steels has been studied in relation to the microstructural change during deformation in order to elucidate the role of the second phase particles. In the case of deformation in the δ/γ duplex phase region, local strain concentration within the δ-ferrite matrix due to dispersion of relatively hard γ particles and the subsequent recrystallization occur repeatedly. When the alloys containing high Cr and Mo are deformed at temperatures around 900°C, σ phase particles precipitate via eutectoid decomposition of δ-ferrite into new γ and σ phases, and the γ/σ duplex phase structure forms. The dynamic recrystallization of γ phase matrix occurs locally and intermittently, and extremely large superplasticity is also obtained. Prior cold work largely accelerates these processes, and leads to equiaxed δ/γ or γ/σ duplex structure in the early stage of deformation. The superplasticity in both cases can be obtained by subtle balance between the local strain hardening and the subsequent recrystallization, and thus the flow stress exhibits large strain rate dependence. This model can also explain granular appearance of the fracture surface as a coalescence of microvoids induced by the individual hard phase particles. In order to obtain large superplasticity, the microstructure should consist of hard particles embedded within a soft matrix and the amount of hard particles should be at least greater than 10% in the case of γ/δ duplex structure. The structure with hard phase matrix within which the soft second phase particles disperse do not exhibits superplasticity.

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Superplasticity of a microduplex stainless steel with a relatively small amount of elongated austenite phases is studied. Specimens used for this study were prepared by different manufacturing histories: one was an as hot-rolled strip worked by a planetary mill which can provide a very high rolling speed (up to 170s^-1) followed by water cooling, the other was an as cold-rolled strip. High temperature tensile tests and microstructural observations were conducted.
In spite of the characteristic microstructure that was not considered as suitable for superplasticity, under a certain condition both the as hot-rolled and as cold-rolled plates showed superplastic behavior. Observed total elongation was over 500% in the strain rate range between 1.67×10^-3 and 8.33×10^-3s^-1. Elongation above 3000% was obtained under a specific condition. The m-values in the superplastic state were larger than 0.3. In the relation between total elongation and initial strain rate, at least two ductility peaks were shown. This seemed to be related to the superplastic deformation mechanism of the steel. From the microstructural observation and the estimation of activation energy for superplastic flow, it was considered that sigma phase transformation contributed to the grain refinement which was accelerated by accumulated strain induced by water cooling or cold working.

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A study has been conducted to develop the composite sheets produced by combined superplastic rolling and joining technique of steel and superplastic alloys which can be used for the damping material at high temperatures. The superplastic (s.p.) alloys used are Zn-22%Al alloy, Al-6%Cu-0.5%Zr alloy and Al-12.9%Mg₂Si alloy and the composite sheets consist of the three layers of steel and superplastic alloy (e.g., steel-s.p. alloy- steel, s.p. alloy-steel-s.p. alloy). The results obtained are as follows. The damping capacity of the steel-Zn-22%Al alloy composite sheet can be raised several times higher than the steel at room temperature. The damping capacity increases with elevating temperature. Above 200°C, the damping capacity exceeds 0.1.
On the basis of these results, the damping mechanism of the composite sheet and the design of the composite damping sheet of steel and superplastic alloy are investigated. The effect of the joining interface of the composite sheet on damping capacity can not be found. The damping capacity of the composite sheet can be calculated from a simple model and is generally in agreement with the measured one. From the calculated results, when the layer number of the composite sheets is five, the best damping capacity is obtained.

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The strain while undergoing allotropic transformation has been considered to be an important component for the transformation superplasticity. The fundamental characteristics of the strain due to a “both ways” passage through the (α+β) field under low stress, ε_tr, . has been investigated for a commercially-pure titanium.
The remarkable strain was found to occur in certain temperature range during heating and also cooling, in which the transformation takes place. The amount of ε_tr was changed by the rate of heating and cooling, T, showing a maximum at a rate of 0.1K/s. The strain generating during heating was always larger than that during cooling in the present study. This has been considered to be attributed to the strain of the transformation itself. The values of ε_tr increased with an increase in the applied stress. It was also shown that the orientation between the stress axis and the rolling direction had an influence on ε_tr. This has been explainable qualitatively in terms of both the orientation dependence of ε_tr under stress free condition and also the texture of the specimen.
A possible mechanism for the transformation superplasticity will be discussed on the basis of the stress dependence of ε_tr.

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A new superplastic Al-Mg-Cu alloy is described. The effect of copper addition on superplastic properties of Al-Mg base alloy (AA5083) has been studied. By addition of 0.6%Cu, a high elongation of about 700% is obtained in deformation at a strain rate of 2.8×10^-3s^-1 and 550°C. Typical mechanical properties of formed parts made from the new alloy are 340MPa in tensile strength and 25% in elongation. The present alloy sheet has a potential for general applications in building and construction because of its properties very similar to the conventional 5000 system alloys and high productivity in the conventional casting and rolling process.

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Quantitative analysis of the emulsification phenomena, in top and combined blowing processes, was carried out using a two dimensional cold model. The effect of lance height and/or position of bubblers, located at the bottom of the vessel, on the quantity of metal droplets entrapped in the emulsion was studied. It was found that, in the absence of bottom bubbling the lance height had a considerable effect on the degree of emulsification, while under conditions of combined blowing maximum metal emulsion formation can be achieved when the bubblers are located underneath the impingement zone. The increase in entrapment can be as much as an order of magnitude greater than that produced by top blowing only.

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Effects of the volume fraction of γ phase on {111} texture development have been investigated by making use of several extra low carbon sheet steels which were intercritically annealed in alpha plus gamma two phase region. The results obtained are summarized in the following.
(1) {111} texture developed with increase in volume fraction of γ phase. {111} component had two maxima; first at the γ fraction below 50% and second at the fraction above 60%.
(2) The similar relation to (1) was obtained between the Lankford value and γ fraction.
(3) The development of {111} annealing texture was stronger in a thinner specimen than in a thicker one, and on the surface than on the midsection.
(4) During slow cooling after intercritical annealing, {111} component increased and {100} component decreased.
(5) In the residual α phase, {111} texture evolved with increase in γ fraction, and had a maximum between 60 and 90% of transformed γ phase.
(6) These results were reasonably understood on the basis of a new theory, proposed by the present authors, on the formation mechanism of the transformation texture.

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