Bainite nucleation mechanism and mechanical properties in low carbon bainite deposited metals with different nickel additions

Exploring the transformation mechanism of bainite is necessary to achieve the matching of high-strength and accepted damage tolerance for low carbon bainite deposited metals. In this work, the effect of Ni content on the bainite transformation mechanism and mechanical properties has been investigated systematically with Ni contents change from 4 wt. % to 9 wt. %. Scanning electron microscope, transmission electron microscope, and electron backscatter diffraction were used for the characterization of the microstructure. Results indicated that the nucleation method of bainite ferrite (BF) laths changed from autocatalytic nucleation to classical nucleation when the Ni content exceeded 5.5%. The deposited metals with BF laths nucleated by autocatalysis had excellent combinations of strength and toughness while the toughness severely deteriorated as the classical nucleation replaced the autocatalytic nucleation. The presence of autocatalytic nucleation resulted in the refinement of microstructure, and the density of high-angle grain boundaries had been improved effectively. In particular, apparent variant selection tendency in microstructure occurred accompanied by the disappearance of autocatalytic nucleation. Moreover, the strong variant selection tendency resulted in numerous identical variants growing next to each other and coarsening the microstructure. In particular, the key factor for the generating of autocatalytic nucleation was that the lower interfacial energy for the new BF laths nucleated through autocatalytic nucleation, and resulted in a lower nucleation activation energy. Therefore, autocatalytic nucleation tends to occur in the deposited metals with low bainite transformation speed due to enough time and space for new BF laths nucleates at existing BF lath.