| Precipitation-strengthening has already been utilized in many alloy systems,becoming oneof the most powerful strengthening methods.High strength nanoprecipitate-strengthened steels developed by nanoprecipitates strengthening instead of carbon,can obtain excellent comprehensive mechanical properties of high strength and large ductility.The high strength and good ductility of nanoprecipitate-strengthened steels are mainly attributed to dispersed nanoscale precipitates and microstructure control.The crystal structure,volume fraction and size of nanoprecipitates have great impact on the mechanical properties.Moreover,if the matrix microstructure of the nanoprecipitate-strengthened steels cannot well match with the nanoscale precipitates,it is difficult to obtain excellent comprehensive properties.To solve the above issues,the Cu/Ni Al nanoprecipitate-strengthened steels are investigated in this thesis.Firstly,eatablishing the relationship between the microstructure and mechanical properties in the steel,thereby we can obtain the optimal microstructure.The three-dimensional atom probe technique(3D-APT)and small-angle neutron scattering(SANS)are used to characterize the evolution of nanoprecipitates during aging treatment.Then,the precipitation strengthening after different aging treatments,and the relationship between precipitation-strengthening and work hardening can be detailedly investigated.By studying the nucleation and precipitation mechansims of Cu/Ni Al nanoprecipitates,the mechanism of Mo additions on effect of the precipitation sequence and thermostability of nanoprecipitates is systematically illuminated.By changing the Mn content,a novel nanoprecipitate-strengthened dual-phase steel is developed with a tensile strength of?1.4 GPa and an elongation of?28%.Finally,the deformation mechanisms of the steels are studied by in-situ neutron diffraction,revealing the essential mechanism of the strength-ductility synertistic improvement induced by hardening martensite.The following conclusions are drawn:(1)The nanoprecipitate-strengthened steels can form the sorbate or martensite microstructureafter different solution treatments,and the steels with the fine lath martensite can obtain the best comprehensive mechanical properties.Aging treatment has no effect on the microstructure,and only affects the size and number density of uniformly distributed nanoprecipitates through changing the solid solubility and diffusion coefficient of solutes,thereby affecting the precipitation-strengthening.(2)During aging treatments,Cu/Ni Al nanoprecipitates show an evolution of the increase in size and decrease in number density.Until and at peak aging,the precipitation strengthening is mainly contributed by shearing mechanisms,among which order strengthening and modulus strengthening of the nanoscale precipitates play the most significant role in enhancing the yield strength.Beyond peak aging,shearing mechanisms is no longer valid,and the Orowan mechanism becomes the dominant one inducing the decrement of the precipitation-strengthening.The nanoprecipitates are independent on the dislocation density,and dislocation density has no effect on precipitation hardening.The work hardening and precipitation strengthening are independent,and the complex strengthening approach of work hardening and precipitation strengthening can be used to develop a nanoprecipitate-strengthened steel with a tensile strength of 2.02 GPa and an elongation of?7%.(3)The addition of Mo has no effect on the microstructure of the nanoprecipitate-strengthened steel,however,it changes the precipitation mechanism and thermostability of Cu/Ni Al co-precipitation.For Mo-free steel,a Ni Al pior-precipitation mechanism is formed which can be expressed as:supersaturated solid solution?incubation period?Ni Al?Ni Al+Cu,and nanoprecipitates with the incontinuous nucleation mechanism increase in size and decrease in number density with aging time increasing.However,doping Mo in the steel changes the Ni Al prior-precipitation to Cu prior-precipitation mechanism,which can be expressed as:“supersaturated solid solution?incubation period?Cu?Cu+Ni Al”,and the continuous nucleation in the Mo-doped steel makes a synchronous increment in both the mean radius and number density of nanoprecipitates with the increase in aging time.The research indicates that uniformly-distributed Mo in the Fe matrix,on the one hand,can retard the formation of Ni Al-enriched nanoprecipitates while promote the formation of Cu-enriched nanoprecipitates through the attractive effects between Mo and Ni/Al atoms and the repulsive effect between Mo and Cu atoms.On the other hand,the addition of Mo reduces the diffusion coefficient of Ni,Al and Cu atoms by almost two orders of magnitude.The decreased diffusion coefficients of Cu,Ni and Al in Mo-doped steel,delay the nucleation and growth of both Ni Al and Cu nanoprecipitates leading to the change of nucleation mechanism and the improvement of the nanoprecipitate's thermostability.(4)The change of Mn concentration from 1.5 wt.%to 6 wt.%has no effect on the microstructure in the steel,but doping 9 wt.%Mn in the steel induces the formation of austenite and martensite.During aging treatment,nanoprecipitates just form in the martensite of the dual-phase steel.Moreover,the increment of Mn content also accelerate the coarsening of nanoprecipitates,making the appearance of peak aging time in advance.In addition,precipitation-strengthening is enhanced after the increment of Mn content from 1.5 wt.%to 6wt.%,meanwhile resulting in a reduction in ductility.However,two-phase frame structure in the dual-phase steel with 9 wt.%Mn can make a strength-ductility synergy.(5)In the nanoprecipitate-strengthened dual-phase steel,hard martensite forms a strong skeleton frame with the soft austenite involved in the frame,forming a frame structure.During aging treatment,all the solute elements are distributed uniformly in the austenite,while high number density of Cu and Ni Al nanoprecipitates are formed in the martensite,and the average radius of the Cu and Ni Al nanoprecipitates after aging for 0.5 h are?1.7 nm,and their number densities are?1.0×1024 m-3,which effectively improve matensitic strength.The results of in situ neutron diffraction indicate that the hard martensite phase instead of soft austenite phase controls the yielding behavior of the steel with the frame-structure,thereby the precipitation-strengthening can doubly enhance the yield strength of the dual-phase steel.Meanwhile,the gap between the critical yielding stresses for various crystal planes in martensite is narrowed,thus the plasticity of the martensite phase is improved in the early uniform deformation stage through the synchronously deformation and rotation of martensite grains.In the late uniform deformation,the transfer of the dynamic stress and strain from the hard phase to soft one compels the cooperative deformation of the two phases,which form a strain hardening plateau together with the transformation induced plasticity(TRIP)effect of the soft austenite,contributing to the ductility.The cooperative deformation and the dynamic stress/strain partitions can also effectively suppress the strain localization at the phase interface,retarding the crack initiation. |
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