Mechanism Research Of Nanaoscale Composite Precipitates In Fe-Cu-Ni-Al-Mn Steel

With the development of modern industry and scientific technology, the strength improvement of the steel materials becomes a subject of concern. Precipitation strengthening is an important way to improve the strength of steel materials. Fe-Cu steels are commercial importance due to their high strength, which is derived from Cu precipitates in these steels after solutionizing and thermal aging. When the composite addition of Ni, Al and Mn elements are introduced into Fe-Cu steels, multi-strengthening phases are formed in these Fe-Cu-Ni-Al-Mn steels, and precipitation strengthening is more obvious. Research on the composite precipitation strengthening in Fe-Cu-Ni-Al-Mn steels, has an important significance to the development of high strength steels.The evolution mechanisms of composite precipitates are studied by Vickers hardness(VHN), optical microscopy(OM), high resolution transmission electron microscopy(HRTEM), atom probe tomography(APT) and precipitation kinetics. When the Cu-rich phase and Ni(Al, Mn) phases coexisting, the effects of precipitation on the strength are also studied, The main results can be summarized as follows:(1) The effect of Mn on the precipitation evolution and mechanism of Cu-rich phase. The microhardness of Fe-Cu-Mn steel reaching the maximum value is faster than that of Fe-Cu steel during isothermal aging process, the micro hardness of the Fe-Cu-Mn steel decreases quickly than that of Fe-Cu steel during the over-aged state, which indicated that the process of precipitation strengthening increases with the addition of Mn. The APT results indicates that the number density of Cu-rich phase in Fe-Cu-Mn steel is higher than that in Fe-Cu steel at early aging. At late aging, the number density of Cu-rich phase in Fe-Cu-Mn steel is lower than in Fe-Cu steel, while the size of Cu-rich phase in Fe-Cu-Mn steel is bigger than in Fe-Cu steel. These results show the growth and coarsening of Cu-rich phase are promoted due to the addition of Mn. The Mn element can increase chemical driving force, decrease interface energy, and increase nucleation rate in the Fe-Cu-Mn steel. At the same time, Mn with faster diffusion rate will accelerate the diffusion of Cu, so that the growth and coarsening of Cu-rich phase are accelerated. With aging time increase, the crystal structure of Cu-rich phase will transform from bcc to fcc, which results to lots of defects and Mn atom segregation around defects. At last, the Cu Mn-rich phase occurs spinodal decomposition, and layered structures are formed due to the alternate arrangement of flaky Mn-riched phase and Cu-riched phase.(2) The effect of Ni Al on the precipitation evolution and mechanism of Cu-rich phase. The peak hardness of Fe-Cu-Ni-Al steel is much higher than that of Fe-Cu steel, and the range of peak hardness duration of Fe-Cu-Ni-Al steel is also wider than that of Fe-Cu steel. which indicates that the effect of precipitation strengthening increased with addition of Ni and Al. At the same aging time, the size of Cu-rich phase in Fe-Cu-Ni-Al steel is smaller than that in the Fe-Cu steel, and the density of Cu-rich phase in Fe-Cu-Ni-Al steel is higher than that in Fe-Cu steel. At early aging, the nucleation rate of Cu-rich phase is increased, the Cu-rich/α-Fe interface offers particle and energy for the nucleation of Ni Al phase, and the core-shell structure forms with the core of Cu-rich phase and the shell of Ni Al phase. Which stable precipitation phases, retardes its coarsening, and maintain good precipitation strengthening effect. With aging time increase, the separation of Ni Al and Cu-rich phase occurs, and the micro hardness decreases.(3) Spherical Ni Al Mn Cu-rich phase in Fe-Cu-Ni-Al-Mn steel is first precipitated during ageing. With ageing time increase, Ni Al Mn Cu-rich phase decomposed into Ni(Al,Mn) phaes and Cu-rich phase, and these two precipitation phases are mutually dependent. The process of precipitation evolution during the aging of Fe-Cu-Ni-Al-Mn steel can be summarized as: Ni Al Mn Cu-rich solute cluster? Ni(Al,Mn) phase + Cu-rich phase. Whatever the addition of Mn or Ni and Al will accelerate the nucleation of Cu-rich phase. While Mn stimulate the growth and coarsening of Cu-rich phase, but Ni Al inhibit the growth and coarsening of Cu-rich phase. So, the nucleation rate of four different steel is: Fe-Cu-Ni-Al-Mn > Fe-Cu-Ni-Al > Fe-Cu-Mn > Fe-Cu. The growth rate is: Fe-Cu-Mn > Fe-Cu >Fe-Cu-Ni-Al > Fe-Cu-Ni-Al-Mn. And the coarsening rate is: Fe-Cu-Mn > Fe-Cu > Fe-Cu-Ni-Al-Mn > Fe-Cu-Ni-Al.(4) The precipitation characterization of Cu-rich phase and Ni(Al,Mn) in two-phase region of steel. The residual austenite is Unavoidable presenting after Fe-Cu-Ni-Al-Mn steel quenching. When aged 1 hour at 500 ℃, no precipitate phase is discovered in the residual austenite investigated by APT, while precipitate phases can be discovered at martensite and martensite/residual austenite interface, and a precipitation depleted zone occurs in martensite near phase boundary. The equivalent radius, spacing, and concentration of the strengthening phases at phase boundary are larger than that inside martensite phase. In addition, the trend of Ni Al phase separateing from Cu-rich phase at the boundary is larger than that inside martensite phase. The above results are attributed to lots of defects at phase boundary, which promote the growth of precipitate phases at phase boundary, and result in precipitation phase along with different growth stages at the boundary and inside martensite phase, respectively.