Variant selection in grain boundary nucleation of bainite in Fe-2Mn-C alloys

The effects of transformation temperature and carbon content on variant selection of bainite structure in Fe-2mass%Mn-C alloys with carbon content ranging from 0.2 to 0.75 mass% were investigated at transformation temperatures between 673 and 773 K. Single variant of bainitic ferrite (BF) was nucleated at the austenite grain boundaries in all the alloys transformed at 773 K and the 0.2 mass% C alloy transformed at 673 K. Multiple variants tended to be selected in nucleation at γ grain boundaries with increasing carbon content at 673 K. Variant selection rules in the nucleation of BF at γ grain boundaries were analyzed with respect to (1) a near Kurdjumov-Sachs (K-S) orientation relationship with both sides of austenite grains, parallel relationships of (2) growth direction and (3) habit plane to austenite grain boundary plane, and (4) plastic accommodation of transformation strain. The analyses revealed that rule (1) is the strongest among the rules investigated, and the fraction of BF that satisfies rule (1) is higher at lower carbon content and higher transformation temperature. The fractions of BF that satisfy rule (2) are low under all the conditions investigated, while the effects of rule (3) and (4) increase at higher carbon content. The variant selection rules observed indicate that higher austenite strength with higher carbon alloys results in different variant selection from those that appear in low carbon steel and also result in an enhancement of self-accommodation.

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