Research On Co-precipitation,Elemental Segregation And Strength-ductility Of Nano-precipitation Strengthened Steels

Nano-precipitation strengthened steel is a new type of ultra-high-strength steels,which has ultra-high strength,excellent corrosion resistance,low cost,energy-saving and eco-friendly metris.The steels are strengthened by the precipitation strengthening of nanoparticles besides traditional strengthening,where co-precipitation behaviors have paid more attentions.However,it generally sacrifice ductility while precipitation-strength enhances the strength at room temperature.Controlling the aging behavior to balance the ductility and strength by nanoscale co-precipitates is a critical scientific question.In this thesis,base on Cu-rich precipitates(CRPs),NiAl-type precipitates and carbide precipitates,three kinds of co-precipitation strengthened steels(Cu-steel,Cu/Ni-steel and Mo-steel)were designed and the aging behaviors were studied by tailoring intercritical tempering,different Mo content and external high magnetic field.Microstructure of matrix,morphology and compositional evolution of co-precipitates were observed by using optical microscopy,transmission electron microscopy and atom probe tomography(APT).The relationship between co-precipitation,elemental segregation and strength-ductility was discussed,which provides fundermental supports for the R&D of nanoprecipitation strengthened steels.The main conclusions are as follows:(1)The effects of tempering and aging treatments on the strength and ductility of the Cu steel,Cu/Ni steel and Mo steel were studied.Tempering and aging in Cu/Ni steels remarkably increased the strength with seriously sacrificing of ductility because larger-sized blocky CRPs segregated along the grain boundaries.Cu partitioning along grain boundaries was partially was do minated,resulting in brittle fracture in almost all of aged Cu/Ni steels.Mo-steel,however,showed superior strength and ductility than Cu/Ni steel because Mo addition reduced the possibility of blocky CRPs by decreasing Cu partitioning along grain boundaries,and promoted the formation of hierarchical co-precipitates where large-sized coarsening co-precipitates contributed to the ductility and small-sized secondary co-precipitates had significant strengthening,thus resulting in the improvement of strengthening-ductility.The optimal conditions of multistage heat treatment are 650? or 675? tempering 10 min followed by 550? aging for 2 h.(2)The influence of Mo addition on the co-precipitation behaviors of co-precipitates including Cu-rich precipitates and NiAl particles in three Mo-bearing multicomponent nanostrengthened steels was investigated.The results indicated that the increase of Mo content suppressed the nucleation of Cu-rich and NiAl precipitates,resulting in lower nucleation densities of Cu-rich and NiAl precipitates in 0.8Mo steel at the initial stage of aging.And the increase of Mo content supressed the growth of Cu-rich precipitates during the growth of precipitation,resulting in significantly lower growth rate Cu-rich precipitates in 0.8Mo steel than that in 0.1 Mo steel,but the increasing Mo content had negligible effect on the growth process of NiAl particles.In addition,Mo addition delayed the structural transformation rate of the Cu/NiAl co-precipitation from the adjacent structure to the core-shell structure during the aging process.(3)The effects of high magnetic field on the strength and ductility of nano-precipitated steel and the precipitation behaviors of Cu-rich and NiAl particle were studied.It was found that high magnetic field of 12T promoted the nucleation of Cu-rich and NiAl particles in the Cu/Ni steel,resulting the reduced radius and the increased number density of the co-precipitates.The application of 12T high magnetic field promoted the enrichment of Ni in the large-sized co-precipitation,resulting in a larger radius of the NiAl particles.In summary,high magnetic field increased the formation of hierarchical co-precipitates.