Crystallography, Morphology, and Martensite Transformation of Prior Austenite in Intercritically Annealed High-Aluminum Steel

The crystallography and morphology of the intercritical austenite phase in two high-aluminum steels annealed at 850 degrees C were examined on the basis of electron backscattered diffraction analysis, in concert with a novel orientation relationship determination and prior austenite reconstruction algorithm. The formed intercritical austenite predominantly shared a Kurdjumov-Sachs-type semicoherent boundary with at least one of the neighboring intercritical ferrite grains. If the austenite had nucleated at high-energy sites (such as a grain corner or edge), no orientation relationship was usually observed. The growth rate of the austenite grains was observed to be slow, causing phase inequilibrium even after extended annealing times. The small austenite grain size and phase fraction were consequently shown to affect martensite start temperature. Both steels had distinct variant pairing tendencies under the intercritically annealed condition.