Deformation of single crystal Hadfield steel by twinning and slip

The stress–strain behavior of Hadfield steel (Fe, 12.34 Mn, 1.03 C, in wt%) single crystals was studied for selected crystallographic orientations ([1̄11], [001] and [1̄23]) under tension and compression. The overall stress–strain response was strongly dependent on the crystallographic orientation and applied stress direction. Transmission electron microscopy and in situ optical microscopy demonstrated that twinning is the dominant deformation mechanism in [1̄11] crystals subjected to tension, and [001] crystals subjected to compression at the onset of inelastic deformation. In the orientations that experience twinning, the activation of multiple twinning systems produces a higher strain-hardening coefficient than observed in typical f.c.c. alloys. Based on these experimental observations, a model is presented that predicts the orientation and stress direction effects on the critical stress for initiating twinning. The model incorporates the role of local pile-up stresses, stacking fault energy, the influence of the applied stress on the separation of partial dislocations, and the increase in the friction stress due to a high solute concentration. On the other hand, multiple slip was determined to be the dominant deformation mechanism in [1̄11] crystals subjected to compression, and [001] crystals deformed under tension. Furthermore, the [1̄23] crystals experience single slip in both tension and compression with planar type dislocations. Using electron back-scattered diffraction patterns, macroscopic shear bands (MSBs) were identified with a misorientation of 9° in the compressed [1̄11] single crystals at strains as low as 1%.