The isochronal y—m transformation of Fe-ISi alloy was measured by high-resolution dilatometry. According to the variation of the ferrite formation rate, an abnormal y—>a phase transformation was recognized, while normal reaction, i.e. one peak continuous reaction, was also detected. The occurrence the one or the other type of y—?a transformation strongly depends on the grain size: the transformation type changes from abnormal to normal with decreasing grain size. In the abnormal transformation process the first stage of the transformation corresponds to the first peaks in the transformation rate, which are not thermally activated.
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