Significant improvement of yield strength by work hardening and back stress strengthening in duplex Fe-Cr-Ni-based alloy

Abstract

The microstructural characterization of duplex Fe-Cr-Ni-Al alloy processed through vacuum arc melting, solid solution treatment and subsequent cold rolling was investigated, and the mechanism underlying the microstructure evolution (grain shape, dislocation density and dislocation character), work hardening behavior and back stress generation was discussed. As compared with ferrite (δ), the grain aspect ratio (GAR) of austenite (γ) exhibits more significant change, and the dislocation density in γ phase presents faster growth after cold rolling. Besides, the dislocation character in γ phase undergoes more profound transformations from screw type to edge type simultaneously. The change of dislocation character could facilitate the dislocation accumulation behavior at grain boundary (GB) in terms of their tendentious interaction with crystal defects. The changes above indicate that the γ phase suffers from more severe plastic deformation during cold rolling compared with that of δ. More pronounced work hardening behavior and more intense interaction between dislocation and GB have occurred in γ phase. The application of cold rolling significantly improves the yield strength from 302.53 MPa to 1113.29 MPa, and the ultimate tensile strength from 679.41 MPa to 1145.85 MPa, corresponding to an increase of 267.99% and 68.65%, respectively. Quantitative analysis on work hardening in γ phase in terms of macroscopic scale was carried out. The results show that the strain-hardening exponent of the γ phase (0.51) is significantly higher than that of the δ phase (0.39) and the growth rate of dislocation strengthening contribution is 45.62% and 21.09% for γ and δ phase, respectively, which further indicates that more intense hardening behavior has occurred in γ phase. More complicated interaction on dislocation activity and slip transfer between individual grains are discussed in terms of the geometrical compatibility factor m′. The lower m′ at δ/γ GB results in back stress generation and suppressed dislocation activity at grain boundary, while the larger m′ indicates easily slip transfer and exerts less inhibition on the dislocation motion. The back stress generation at GB is responsible for work hardening during cold rolling and results in enhanced yield strength.

Graphical Abstract

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