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Ni-free and Ni-bearing steels were designed to elucidate the effect of Ni on the free energy of bainitic transformation. Steels with two distinct nano-bainite structures were obtained through conventional one-stage and two-stage isothermal treatments. The effect of work hardening and accompanying strain on martensitic transformation of the two nano-bainite structures was also studied. Microstructure of both the steels consisted of bainitic ferrite and film-like and block-like retained austenite (RA). The total carbon concentration, degree of Fe–C cluster formation, and stability of RA were significantly affected by the free energy of bainitic transformation. The presence of Ni leads to reduction in the free energy of bainitic transformation, refinement of bainite structure, and extension of bainite nucleation regime. The carbon-content in block-like and film-like RA microstructure of Ni-bearing steel was 5.5% and 19.1%, respectively, which was greater than Ni-free steel. The load-displacement behavior indicated superior strain hardenability and higher strain hardening rate (SHR) in Ni-bearing steel. The uniform nanostructure of Ni-bearing steel promoted the formation of stable film-like RA between bainitic ferrite, leading to higher strain hardening rate and stronger transformation-induced plasticity (TRIP) effect at large strain. The Ni-bearing steel was characterized by superior plasticity (13.7%–10.3%) and product of tensile strength and % elongation (25.9 GPa·% to 20.9 GPa·%) in comparision to the Ni-free steel. [ABSTRACT FROM AUTHOR]

A commercial scale Mg-7Gd-2Y–1Zn-0.5Zr alloy extrusion rod with synergetic high yield strength of 350 MPa and fracture toughness K IC of 22.4 MPa m1/2 was fabricated by direct-chill casting, hot extrusion and subsequent aging treatment at 200 °C for 108 h. The microstructures and mechanical properties of the as-extruded and peak-aged alloys were investigated, and the fracture toughness and fracture mechanisms before and after aging treatment were also analyzed. The results showed that the as-extruded alloy consisted of long period stacking ordered (LPSO) phase and bimodal matrix microstructure composed of fine dynamic recrystallized (DRXed) grains and coarse deformed grains. Microstructural analysis near fracture surfaces indicted that twinning was activated in the coarse deformed grains during fracture toughness test, which may be conducive to the improvement of the fracture toughness of the alloys. The lamellar-shaped LPSO phases enhanced the fracture toughness since the formation of secondary cracks parallel to the lamellar LPSO can cause significant energy dissipation and deflection of main crack. The micro-cracks were initiated and propagated in the bulk LPSO phases, which may deteriorate the fracture toughness. Large amount of nano β' phase precipitated in the T5 peak-aged alloy significantly improved the strength of the alloy. Fractography analysis indicated that fracture mechanism changed from ductile fracture of the as-extruded alloy to quasi-cleavage fracture of the peak-aged alloy. The precipitation of β' would suppress twin growth and induce the microcracks initiated at the DRXed grain boundaries and in the deformed grains, which led to slight decrease of K IC. [ABSTRACT FROM AUTHOR]