A series of tensile tests, Charpy impact tests, optical microscopy observations, and field emission-scanning electron microscopy examinations, were carded out to investigate the mechanical properties and microstructur...A series of tensile tests, Charpy impact tests, optical microscopy observations, and field emission-scanning electron microscopy examinations, were carded out to investigate the mechanical properties and microstructural evolution of 20Cr32NilNb steel. Experimental results indicate that the as-cast microstructure of the steel typically consists of a supersaturated solid solution of austenite matrix with a network of interdendritic primary carbides （NbC and M23C6）. In the ex-service samples, large amounts of secondary carbides precipitate within austenite matrix. Besides the growth and coarsening of NbC and M23C6 carbides during service condition, the Ni-Nb silicides known as G-phase （Nil6Nb6Si7） are formed at the interdendritic boundaries. The microstructural evolution results in the degradation of the mechanical properties of the ex-service steel. In addition, the precipitate rate of G-phase, depending in part on Si content, varies greatly for the 20Cr32NilNb steel, which plays a key role in the long-term microstructural stability of the steel. Based on the X-ray diffraction data, time-temperature-transformation curve for the steel is obtained from the aged specimens.展开更多
A typical G-phase strengthened ferritic model alloy(1Ti:Fe-20Cr-3Ni-1Ti-3Si,wt.%)has been carefully studied using both advanced experimental(EBSD,TEM and APT)and theoretical(DFT)techniques.During the classic“solid so...A typical G-phase strengthened ferritic model alloy(1Ti:Fe-20Cr-3Ni-1Ti-3Si,wt.%)has been carefully studied using both advanced experimental(EBSD,TEM and APT)and theoretical(DFT)techniques.During the classic“solid solution and aging”process,the superfine(Fe,Ni)_(2)TiSi-L2_(1)particles densely precipitate within the ferritic grain and subsequently transform into the(Ni,Fe)_(16)Ti_(6)Si_(7)-G phase.In the meanwhile,the elemental segregation at grain boundaries and the resulting precipitation of a large amount of the(Ni,Fe)_(16)Ti_(6)Si_(7)-G phase are also observed.These nanoscale microstructural evolutions result in a remarkable increase in hardness(100-300 HV)and severe embrittlement.When the“cold rolling and aging”process is used,the brittle fracture is effectively suppressed without loss of nano-precipitation strengthening ef-fect.Superhigh yield strength of 1700 MPa with 4%elongation at break is achieved.This key improvement in mechanical properties is mainly attributed to the pre-cold rolling process which effectively avoids the dense precipitation of the G-phase at the grain boundary.These findings could shed light on the further exploration of the precipitation site via optimal processing strategies.展开更多
The duplex stainless steels(DSSs)are susceptible to thermal ageing embrittlement due to the spinodal decomposition and Gphase precipitation in the ferritic phase.This study presents a ternary(Fe-Cr-Ni)phase-field mode...The duplex stainless steels(DSSs)are susceptible to thermal ageing embrittlement due to the spinodal decomposition and Gphase precipitation in the ferritic phase.This study presents a ternary(Fe-Cr-Ni)phase-field model for the simulation of spinodal decomposition with concurrent G-phase precipitation.Two Cahn-Hilliard equations and one Ginzburg-Landau equation are used in the model to describe the diffusion of Cr,Ni,and the growth of G-phase,respectively.The model is able to generate a spinodally-interconnected structure with G-phase particles near theα-α′interfaces,similar to experimental observations.The kinetic synergy between spinodal decomposition and G-phase precipitation is discussed.The simulation results indicate that Gphase can enhance the evolution of spinodal decomposition by occupying the volume where the decomposition could otherwise occur,and that the system’s elastic strain energy is largely contributed by G-phase rather than spinodal decomposition.These results would help in better understanding the states of the materials for plant structural integrity assessment and life management.展开更多
基金financially supported by the National Natural Science Foundation of China(No.50775107)the China Petrochemical Corporation(No.315007)the Innovation Program for Graduate Students in Jiang Su Province of China(No.KYLX15_-0800)
文摘A series of tensile tests, Charpy impact tests, optical microscopy observations, and field emission-scanning electron microscopy examinations, were carded out to investigate the mechanical properties and microstructural evolution of 20Cr32NilNb steel. Experimental results indicate that the as-cast microstructure of the steel typically consists of a supersaturated solid solution of austenite matrix with a network of interdendritic primary carbides （NbC and M23C6）. In the ex-service samples, large amounts of secondary carbides precipitate within austenite matrix. Besides the growth and coarsening of NbC and M23C6 carbides during service condition, the Ni-Nb silicides known as G-phase （Nil6Nb6Si7） are formed at the interdendritic boundaries. The microstructural evolution results in the degradation of the mechanical properties of the ex-service steel. In addition, the precipitate rate of G-phase, depending in part on Si content, varies greatly for the 20Cr32NilNb steel, which plays a key role in the long-term microstructural stability of the steel. Based on the X-ray diffraction data, time-temperature-transformation curve for the steel is obtained from the aged specimens.
基金This work was financially funded by the National Natural Science Foundation of China(Nos.51971082 and 52001098)the National Post-doctoral Program for Innovative Talents(No.BX20200103)the China Post-doctoral Science Foundation(No.2020M681092).The authors would like to thank Dr.Ivan Povstugar at ZEA-。
文摘A typical G-phase strengthened ferritic model alloy(1Ti:Fe-20Cr-3Ni-1Ti-3Si,wt.%)has been carefully studied using both advanced experimental(EBSD,TEM and APT)and theoretical(DFT)techniques.During the classic“solid solution and aging”process,the superfine(Fe,Ni)_(2)TiSi-L2_(1)particles densely precipitate within the ferritic grain and subsequently transform into the(Ni,Fe)_(16)Ti_(6)Si_(7)-G phase.In the meanwhile,the elemental segregation at grain boundaries and the resulting precipitation of a large amount of the(Ni,Fe)_(16)Ti_(6)Si_(7)-G phase are also observed.These nanoscale microstructural evolutions result in a remarkable increase in hardness(100-300 HV)and severe embrittlement.When the“cold rolling and aging”process is used,the brittle fracture is effectively suppressed without loss of nano-precipitation strengthening ef-fect.Superhigh yield strength of 1700 MPa with 4%elongation at break is achieved.This key improvement in mechanical properties is mainly attributed to the pre-cold rolling process which effectively avoids the dense precipitation of the G-phase at the grain boundary.These findings could shed light on the further exploration of the precipitation site via optimal processing strategies.
基金the National Key Research and Development Program of China(Grant No.2017YFB0702201)。
文摘The duplex stainless steels(DSSs)are susceptible to thermal ageing embrittlement due to the spinodal decomposition and Gphase precipitation in the ferritic phase.This study presents a ternary(Fe-Cr-Ni)phase-field model for the simulation of spinodal decomposition with concurrent G-phase precipitation.Two Cahn-Hilliard equations and one Ginzburg-Landau equation are used in the model to describe the diffusion of Cr,Ni,and the growth of G-phase,respectively.The model is able to generate a spinodally-interconnected structure with G-phase particles near theα-α′interfaces,similar to experimental observations.The kinetic synergy between spinodal decomposition and G-phase precipitation is discussed.The simulation results indicate that Gphase can enhance the evolution of spinodal decomposition by occupying the volume where the decomposition could otherwise occur,and that the system’s elastic strain energy is largely contributed by G-phase rather than spinodal decomposition.These results would help in better understanding the states of the materials for plant structural integrity assessment and life management.
