| The precipitation strengthened steels are one kind of key structural materials,which are widely used in construction bridges,precision instruments,large vessels,aerospace and other important fields.The properties of the steels are mainly dependence on the strengthening of high density precipitates with different scales.Thus the regulation of precipitation strengthening is the key to improve the properties of precipitation strengthened steels.In this thesis,a new strengthening ferrite steel by NiAl precipitates and a nano-precipitated strengthening steel with co-precipitates of NiAl and Cu-rich particles were choosen to investigate the influencing mechanism of precipitation behaviors modified by Mo alloying and high magnetic field annealing using Thermo-Calc thermodynamic modelling and various characterization methods including SEM,TEM,XRD,DSC and APT to analyze microstructure of alloy matrix,morphology,composition and structural evolution of precipitates.The main conclusions are as follows:(1)In the process of Mo alloying,a portion of Mo was solid soluted in the matrix,inducing the lattice distortion and strengthening matrix,but the precipitation strengthening of Mo was different in different precipitation-strengthened steels.In the new NiAl strengthening ferrite steel,Mo precipitated Laves phase by co-precipitating Fe and Ni at grain boundaries and pinned dislocations.In the nano-precipitated strengthening steel,Mo precipitated carbides by coprecipitating C and strengthened the matrix.(2)An appropriate amount of Mo element alloying promoted the coherent strengthening of NiAl particles and α-Fe matrix in the novel ferrite steel.The small-sized Laves phases formed in the novel ferrite steel with low Mo contents pinned dislocations,but excessive Mo element generated a large number of Laves phases,resulting in intergranular fracture.The lowest misfit was estimated by XRD tests when the Mo content was ranged between 2wt.%and 3.5wt.%.(3)The alloying of Mo element promoted the nucleation of the co-precipitates in the nanoprecipitated strengthening steel,reduced the diffusion coefficient of Cu and Ni,effectively prolonged the nucleation period of the precipitates,delayed the coarsening process of the precipitates,and slowed down the structural evolution of precipitates.(4)The interaction of Mo alloying and high magnetic field promoted the high density and homogeneous precipitates in nano-precipitated strengthening steel.However,the effects of Mo alloying and high magnetic field on NiAl particles and Cu-rich precipitates were different.In the nucleation stage,the addition of Mo prolonged nucleation periods and effectively controlled the size of precipitated phases.In the growth coarsening stage,however,high magnetic field increased the diffusion coefficient of Ni element and accelerated the coarsening of NiAl particles,but had no obvious influence on the Cu-rich precipitates. |
