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.