Shear and shuffling accomplishing polymorphic fcc gamma -> hcp epsilon -> bct alpha martensitic phase transformation

XS Yang and S Sun and HH Ruan and SQ Shi and TY Zhang, ACTA MATERIALIA, 136, 347-354 (2017).

DOI: 10.1016/j.actamat.2017.07.016

Martensitic transformation (MT) has extreme science merits and engineering significance. However, the underlying displacive atom collective movements for the transition from face-centered cubic structure (fcc-gamma) austenite to body-centered tetragonal structure (bct-alpha) martensite has not been uncovered due to the lack of directly experimental evidence. Here, we examined the Plastic Deformation-Induced Martensitic Transformation (PDIMT) from fcc-gamma to bct-alpha in AISI 304 stainless steel by High-resolution Transmission Electron Microscopy (HRTEM). The HRTEM observations exhibit a novel polymorphic fcc-gamma -> hcp-epsilon -> bct-alpha PDIMT mechanism, which is further confirmed by the Molecular Dynamics (MD) simulations. The transition from fcc-gamma to hcp-epsilon is accomplished by gliding Shockley partial dislocations on every second (111)gamma planes. The transition from hcp-epsilon to bct-alpha is executed by gliding half- Shockley partial dislocation dipoles on every second (0001)epsilon planes and the gliding is simultaneously accompanied by atom shuffling. The dipole shear is conducted in a sandwich manner, meaning that a half Shockley partial dislocation glides on one side of a (0001)epsilon plane and its partner of the dipole glides on the other side of the same (0001)epsilon plane. The novel findings will have great impact on the micro structural control in metals and alloys by PDIMT and stimulate innovative ideas to understand other solid phase transition mechanisms. (c) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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