Development of the high-strength ductile ferritic alloys via regulating the intragranular and grain boundary precipitation of G-phase

A typical G-phase strengthened ferritic model alloy (1Ti:Fe-20Cr-3Ni-1Ti-3Si, wt.%) has been carefully studied using both advanced experimental (EBSD, TEM and APT) and theoretical (DFT) techniques. During the classic “solid solution and aging” process, the superfine (Fe, Ni)₂TiSi-L2₁ particles densely precipitate within the ferritic grain and subsequently transform into the (Ni, Fe)₁₆Ti₆Si₇-G phase. In the meanwhile, the elemental segregation at grain boundaries and the resulting precipitation of a large amount of the (Ni, Fe)₁₆Ti₆Si₇-G phase are also observed. These nanoscale microstructural evolutions result in a remarkable increase in hardness (100–300 HV) and severe embrittlement. When the “cold rolling and aging” process is used, the brittle fracture is effectively suppressed without loss of nano-precipitation strengthening effect. Superhigh yield strength of 1700 MPa with 4% elongation at break is achieved. This key improvement in mechanical properties is mainly attributed to the pre-cold rolling process which effectively avoids the dense precipitation of the G-phase at the grain boundary. These findings could shed light on the further exploration of the precipitation site via optimal processing strategies.