Shear and shuffling accomplishing polymorphic fcc γ → hcp ε → bct α martensitic phase transformation

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-γ) austenite to body-centered tetragonal structure (bct-α) 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-γ to bct-α in AISI 304 stainless steel by High-resolution Transmission Electron Microscopy (HRTEM). The HRTEM observations exhibit a novel polymorphic fcc-γ → hcp-ε → bct-α PDIMT mechanism, which is further confirmed by the Molecular Dynamics (MD) simulations. The transition from fcc-γ to hcp-ε is accomplished by gliding Shockley partial dislocations on every second (111)γ planes. The transition from hcp-ε to bct-α is executed by gliding half-Shockley partial dislocation dipoles on every second (0001)ε 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)ε plane and its partner of the dipole glides on the other side of the same (0001)ε plane. The novel findings will have great impact on the microstructural control in metals and alloys by PDIMT and stimulate innovative ideas to understand other solid phase transition mechanisms.

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