Co-Precipitation Mechanisms Research Of Cu-rich And NiAl Phases In Steel

With the unprecedented environmental challenges,it is critical to develop advanced ultra-high strength steels to dramatically reduce the amount of common low-strength steels in use at the present time.Among the various strengthening mechanisms,precipitation hardening has been proven to be a most effective method to improve the strength of steels and Fe-based alloys.In particular,the co-precipitation of multiple types of nanoparticles can lead to a superior combination of different good properties.Therefore,the research of co-precipitation strengthening is of great significance for the development of high strength steels.Recently,there has been an increasing interest in the co-precipitation of Cu-rich and NiAl nanoparticles of high strength steels,which offers a promising way to achieve a good combination of high strength,good ductility,good weldability and relatively low cost.The precipitation evolution and strengthening effects of precipitates in the Fe-based steels were studied by Vickers hardness,tensile test,scanning electron microscope(SEM),transmission electron microscopy(TEM)and atom probe tomography(APT).The relationship among the synergistic effects of the elements,the co-precipitation of precipitates and strengthening effects were disscussed.The main results can be summarized as follows:(1)The Fe-CuNiAl steel reaches the hardness peak earlier than the Fe-NiAl steel.Moreover,it shows a higher hardness increment in the ageing peak state,combined with a wider hardness plateau than the Fe-NiAl steel.These indicate that Cu accelerates the process of precipitation strengthening of NiAl phase and the co-precipitation of Cu-rich phase and NiAl phase enhances the age-hardening ability.APT results reveal that the addition of Cu plays an important role in promoting the nucleation of NiAl nanoparticles in the intial period,thus accelerating the nucleation,growth and coarsening process of NiAl phase.The growth rate of NiAl phase in the Fe-CuNiAl steel is lower than that of the Fe-NiAl steel in the ageing process,which is caused by the slower diffusion rate of Cu than Ni and Al in ?-Fe.After the short ageing time(0.1 h),the Cu-rich phase firstly precipitates in the Fe-CuNiAl steel.With increasing ageing time,the NiAl phase begins to precipitate.When the ageing time increases to 4 h,the Cu-rich phase and NiAl phase form a core-shell structure,with the Cu-rich phase in the core and the NiAl phase in the shell.After ageing for 128 h,Cu-rich phase and NiAl phase separete,because the structure of Cu-rich phase changes from bcc to fcc,while the NiAl phase always maintains the B2 structure.(2)The time to reaching and maintaining the peak hardness of the Fe-CuNiAlMn steel are both less than that of the Fe-CuNiAl steel during 500 ? isothermal ageing process,indicating that the addition of Mn accelerates the process of precipitation strengthening.APT results reveal that Mn increases the nucleation rate of the precipitates in the ageing time(0.5 h),which improves the effect of precipitation strengthening in the early ageing.However,the addition of Mn element has little influence on the peak hardness increment,it appears that Mn addition does not significantly improve the effect of precipitation strengthening.The diffusion coefficient of Mn in the matrix is far higher than that of Cu and Ni.Moreover,Mn can displace A1 in the NiAl phase and produce lattice defects,resulting in accelerating the diffusion of elements.Therefore,Mn promotes the growth and coarsening of precipitated phase,accelerates the growth and coarsening process of precipitates and further accelerates the separation of the Cu-rich phase and NiAl phase.(3)The Fe-CuNiAlMn steel is faster to reach peak hardness than the Fe-CulowNiAlMn steel,and compared with the as-quenched state,the peak hardness increment of the Fe-CuNiAlMn steel is also higher than that of the latter,indicating that the increase of Cu content accelerates the process of precipitation strengthening and enhances the effect of precipitation strengthening.APT results reveal that the increase of Cu content can promote the segregation of Cu,Ni,A1 and Mn elements in the matrix at the early period of ageing(0.1 h).Both steels form co-precipitates of Cu-rich phase and NiAl phase during isothermal ageing.However,there is independent NiAl phase in the Fe-CulowNiAlMn steel,while individual Cu-rich phase in the Fe-CuNiAlMn steel.With increasing ageing time,only co-precipitates are found in both steels.The precipitation sequences in the two steels are different.The precipitation sequence in the Fe-CulowNiAIMn steel can be expressed as "supersaturated solid solution ? NiAl ?NiAl + Cu".But the Fe-CuNiAlMn steel has a different precipitation sequence of"supersaturated solid solution ? Cu? Cu + NiAl".This is related to the ratios of Ni/Cu and AI/Cu in steel.(4)The Fe-CuNiAlMn steel was aged at different temperatures for 4 h.When the ageing temperature is 350 ?,there is no significant change in hardness compared with the as-quenched state.With the increase of temperature,the hardness increases gradually and reaches the peak value at 500 ?,and then decreases.This is owing to no segregation of Cu,Ni,A1 and Mn at the ageing temperature of 350 ?;as the temperature increases,Cu,Ni,Al and Mn elements begin to segregate and form Cu-rich phase and NiAl phase.Their sizes increase gradually,and number densities decrease,and the position of Cu-rich phase and NiAl phase transforms from core-shell to adjacent distribution.When the ageing temperature increases to 650 ?,there is only Cu-rich phase in the steel.