Superplastic Mn-Si-Cr-C duplex and triplex steels: Interaction of microstructure and void formation

Duplex and triplex microstructures consisting initially of ferrite plus carbide or of martensite, ferrite plus carbide, respectively, can undergo strain induced austenite formation during superplastic deformation at 30 K below Ae1 (Ae1: equilibrium pearlite-austenite transformation temperature) and low strain rate (e.g. 2×10−3 s−1). The effect leads to excellent superplasticity of the materials (elongation ~500%, flow stress <50 MPa) through fine austenite grains (~10 µm). Using a deformation temperature just below Ae1 leads to a weak driving force for both, carbide dissolution and austenite formation. Thereby a sufficient volume fraction of carbides (1-2 µm, 15 vol%) is located at austenite grain boundaries suppressing austenite grain growth during superplastic deformation. Also, void nucleation and growth in the superplastic regime are slowed down within the newly transformed austenite plus carbide microstructure. In contrast, austenite grains and voids grow fast at a high deformation temperature (120 K above Ae1). At a low deformation temperature (130 K below Ae1), strain induced austenite formation does not occur and the nucleation of multiple voids at the ferrite-carbide interfaces becomes relevant. The fast growth of grains and voids as well as the formation of multiple voids can trigger premature failure during tensile testing in the superplastic regime. EBSD is used to analyze the microstructure evolution and void formation during superplastic deformation, revealing optimum microstructural and forming conditions for superplasticity of Mn-Si-Cr-C steels. The study reveals that excellent superplasticity can be maintained even at 120 K above Ae1 by designing an appropriate initial duplex ferrite plus carbide microstructure.