Study On The Nanoprecipitation Behavior And The Resultant Strengthening And Ductilizing Mechanism In Fe-Ni-Al Based Ultrastrong Steels

Next-generation high-performance structural materials are required for lightweight design strategies and advanced energy applications.Maraging steels,combining a martensite matrix with nanoprecipitates,are a class of high-strength materials with the potential for matching these demands.Their outstanding strength originates from semi-coherent precipitates,which unavoidably exhibit a heterogeneous distribution that creates large coherency strains,which in turn may promote crack initiation under load.In this dissertation,a counterintuitive strategy was introduced for the design of ultrastrong steel alloys by high-density nanoprecipitation with minimal lattice misfit and high cutting stress.The novel grade of maraging steels was designed via replacing the essential but high-cost alloying elements cobalt and titanium with inexpensive and lightweight aluminum,which is combined by monitoring the Mo and Al concentration to minimize the lattice misfit.The steels with a composition of Fe-18Ni3A14.5Mo0.8Nb0.08C are heat-treated by a simplified procedure which leads to formation of nanoprecipitates with an extremely high number density(more than 1024 per cubic metre)and small size(about 2.7 ± 0.2 nanometres).It was found that these highly dispersed,fully coherent precipitates,showing very low lattice misfit with the matrix and high anti-phase energy,strengthen alloys without sacrificing ductility.The novel class of steels thus showed a strength of up to 2.2 GPa and good ductility(about 8.2%).The nanoprecipitation behavior in Fe-Ni-Al based alloys was optimized by experimental investigations.It was found that the high driving force for the precipitation and the extremely low diffusivity of Mo,which was ejected from the Ni(Al,Fe)phases,enforced the nuclei to adjust its composition to an meta-stable state,i.e.,a lowered concentration of Al in precipitates.This modification further decreases the nucleation barrier and thus promotes formation of nuclei with an extreme number density.The low diffusivity and the minimized interfacial energy of the fully coherent nanoprecipitates significantly suppress the local capillary-driven coarsening in the grown process,resulting in the high thermal stability of the high-density nanoprecipitates.The extreme homogeneity of the precipitate microstructure largely contributes to the superior plasticity of the novel steel class,the deformation behavior was then investigated.It was found that nanoprecipitates with high ordering effect profoundly enhance the yield strength of the steels,and simultaneously enable the massive moving dislocations to glide through precipitates and to further penetrate dislocation networks and lath boundaries at the high applied stress.This behavior indeed induces dramatic dislocations multiplication which produces the strong strain-hardening effect and further promotes the macroscopic deformation.In summary,this dissertation not only reported a counterintuitive strategy for the design of ultrastrong steel alloys,but also proposed an alternative approach to ductilize the ultrahigh-strength alloys strengthened by coherent nanoprecipitates,which always encounter two critical problems,i.e.the susceptibly to cleavage fracture at the ambient temperature and the rapid losing of toughness and ductility with strength.