Effects Of Ni And Al On Precipitation Of Nano-size Cu-rich Phase In Fe-Cu Alloy

Given the unique precipitation mechanism of copper that does not need to form intermetallic compounds like NbC,VC etc,Cu has received extensive attention in the development of high-strength and low-alloy(HSLA)steels.Cu-rich nano-precipitation strengthened HSLA steels possess high strength,high toughness,good weldability and corrosion resistance,and are widely used in shipbuilding industry,marine engineering equipment,automotive industry,construction steel,heavy engineering structure,high-pressure transportation pipeline,as well as the bridge,high pressure container,and petroleum industries.The precipitation of nano Cu-rich phase in Cu-containing steel during the aging process is the main factor causing the strengthening effect,so it is of great significance to investigate the corresponding mechanism of Cu precipitation during the aging process.To understand the fundamental the aging precipitation strengthening behavior of copper-containing steel,the entire aging precipitation process and the related mechanism need to be investigated systematically,which would also provide theoretical and practical foundation support for the development and application of a new generation of Cu alloyed steel.In this paper,two Cu-containing steel was prepared using a vacuum induction furnace and a solution and aging process was carried out using a laboratory vacuum tube furnace.The mechanical properties were tested,and the microstructure and precipitate morphology of the solid solution and aging process were observed via metallographic microscope,field emission scanning electron microscope,and high-resolution transmission electron microscopy.Consequently,The evolution process of the microhardness,structure and precipitate during the aging process were elucidated through the combination of the above experimental results.Firstprinciples were used to calculate the binding energy between Cu,Ni and Al,and the statistical data of the precipitates were used to explore the effect of Ni and Al co-addition on the precipitation reaction.At the same time,the strengthening mechanism in the aging process was discussed.The lattice parameter changes of the matrix during aging were explored using X-ray diffraction Rietveld structural refinement and first-principles calculations,and also the evolutions of dislocation density were investigated using structural refinement combined with WH method,then the precipitation mechanism of Cu-rich precipitates were elucidated.The results show that the room temperature organization of 1# experimental steel after solid solution water cooling at 900℃ is polygonal ferrite,and the room temperature organization of 2# experimental steel is polygonal ferrite and lath martensite,with the extension of aging time at 500℃,The matrix structure has continuously recrystallized.Simultaneously,the hardness values of the experimental steels showed an overall trend of first increasing and then decreasing,and all reached the peak hardness at 60 minutes of aging.Due to the co-addition of Ni and Al elements in 2 # experimental steel,the microhardness is the same as 1 # Fe-Cu alloy increases significantly,and the peak aging hardness can be maintained for a longer time.Moreover,the co-addition of Ni and Al elements significantly increases the number density of nano-Cu-rich precipitates and delays the growth rate of the precipitates during aging,and improves the thermal stability of the nano-Cu-rich phase.Through theoretical calculation of the shear strengthening mechanism between dislocations and nano-precipitated phases,it is believed that the difference in shear modulus between the matrix and the precipitates is the main reason for matrix strengthening.XRD Rietveld structure refinement and first-principles lattice constant and mixed energy calculation results show that with the continuous precipitation of nano-Cu-rich phases,the lattice parameters of the matrix continue to decrease,which is consistent with the structure of the hardness change curve before the aging peak.The microstrain and dislocation density of the substrate also showed a downward trend,and due to the influence of Ni and Al co-addition on the precipitation reaction of the aging process,the time when the dislocation density became stable was delayed.