Research On Precipitation Behaviors Of Nucleation And Growth Of Co-precipitates In Copper-bearing Strengthened Steels

High-strength Cu-bearing steels are widely used in aerospace,vessel,automobile and other important fields.Co-precipitation with copper-rich precipitates(CRPs)and NiAl-type particles can strengthen steels more effectively without sacrificing of ductility than that of single precipitation,which provides a new strategy for the strengthening and toughening of highstrength steels.However,the precipitation order and structural evolution of co-precipitation,and the effect of universal carbides in steels on co-precipitation are still an open scientific issue.This thesis investigated the precipitation behaviors and properties of both Fe-2wt%Cu model alloy and two containing-co-precipitates steels(Fe-2wt.%Cu-5wt.%Ni-3wt.%Mn-1.5wt.%Al and Fe-2wt.%Cu-5wt.%Ni-3wt.%Mn-1.5wt.%Al-1wt.%Mo).Microstructure of matrix,morphology and compositional evolution of precipitates were observed by using optical microscopy,transmission electron microscopy and atom probe tomography(APT).The effect of Mo addition on co-precipitation is studied,which provides fundamentals for R&D of highstrength Cu-bearing steels.The main conclusions are as follows.(1)The activation energy values for the nucleation and growth of CRPs in Fe-Cu steel are 97.3KJ/mol and 182.6KJ/mol,respectively.The two activation energy values for the nucleation and growth of co-precipitates in the Fe-Cu-Ni-Mn-Al steel are 97.3KJ/mol and 182.6KJ/mol,respectively.(2)The matrix of Fe-Cu steel treated after solution and aging are all ferrite with the grain size of about 100?m,however,the matrix of Fe-Cu-Ni-Mn-Al and Fe-Cu-Ni-Mn-Al-Mo solution-treated steels are all martensites,but after aging,martensites transformed to polygonal ferrites with grain sizes of 30?m and 15?m,respectively.Aging time is almost incapable of coarsening the grain sizes of the three steels.(3)Both CRPs and NiAl particles can nucleate independently,but each of them is more favorable to nucleate at the heterogeneous interface.Each homogeneous nucleation rejects the heterogeneous elements,resulting in keeping away from each nucleation sites.The adjacency structure of co-precipitates is gradually formed.However,core-shell structures with Cu cores enclosed with NiAl shells are formed as the co-precipitates are coarsening.(4)Mo strongly impedes the diffusion of Cu and Mn,resulting in the delay of aging process and compositional evolution of co-precipitation in Fe-Cu-Ni-Mn-Al-Mo steel.Because the diffusion of Cu is inhibited,there is fewer heterogeneous nucleation sites for NiAl particles at the beginning,but homogeneous NiAl particle nucleate sites are much more than those in the Fe-Cu-Ni-Mn-Al steel.The sequence of co-precipitates during aging is as follows:CRPs+NiAl particles?Cu/NiAl adjacency structure+NiAl particles?Cu-core/NiAl-shell structure.(5)Mo₂C precipitates nucleated heterogeneously nucleate behand co-precipitates.At the beginning of aging,the size of Mo₂C is too small to offset the decline of solution strengthening;In the middle stage,Mo₂C grows up together with NiAl particles which form later,and partly offsets the strengthening decline owing to the coarsening of co-precipitation;At last,the coarsening of Mo₂C and NiAl particles accelerate the decline of hardness.