Effect Of Grain Boundary Characters On The Precipitation Of Carbide In Inconel Alloy 600

Alloy 600（Ni-15Cr-9Fe,wt.%）is a face centered cubic structure nickel-based alloy,which is widely used as steam generator tubes in pressurized water reactor nuclear power plants due to its excellent corrosion resistance and mechanical properties.Corrosion damage of heat transfer tubes often occurs in PWR nuclear power plants in practical applications.Intergranular attack（IGA）and intergranular stress corrosion cracking（IGSCC）are the main failure mechanisms of steam generator tubes during service.It is known that the chromium depletion at grain boundary caused by carbide precipitation is the main reason of the IGA and IGSCC in nickel-based superalloys and austenitic stainless steels.Therefore,in order to prolong the service life of the steam generator tubes,it is necessary to further study the precipitation of the carbides at the grain boundaries in Alloy 600.In this work,the effect of aging temperature and grain boundary character on the precipitation of carbide in Inconel Alloy 600 after grain boundary engineering treatment was studied by electron backscatter diffraction technique（EBSD）,scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.The growth mechanism of carbide near the grain boundary was analyzed by high resolution transmission electron microscopy（HRTEM）.The following conclusions can be drawn:（1）The grain boundary character and aging time affect the precipitation morphology of carbide at grain boundaries in Alloy 600.During the aging time of 715°C,the carbides precipitated at theΣ3_c grain boundaries are the least;the plate like carbides precipitated near theΣ3_i grain boundaries andΣ9 grain boundaries;the coarse carbides precipitated at theΣ27 grain boundaries and the random grain boundaries.The ability to precipitate carbides on different types of grain boundaries is:Σ3_c<Σ3_i<Σ9<Σ27≤random.All carbides continue to grow as the aging time increases.（2）Aging temperature also has significant effect on the precipitation morphology of grain boundary carbide.During the aging time of 800°C,the carbides precipitated at each grain boundary grow significantly during the short period of time,and the growth rate is much higher than that during aging at715°C.There is no obvious plate like carbide near theΣ3_i grain boundaries andΣ9 grain boundaries,but only the coarse carbide particles precipitated on those grain boundaries.The ability of precipitation of carbides on different types of grain boundaries is also:Σ3_c<Σ3_i<Σ9<Σ27≤random.（3）The structures of carbide precipitated at different types of grain boundary are different.During the aging,two types of carbides,M₂₃C₆ and M₇C₃,are mainly precipitated at the grain boundaries.During the aging time of715°C,There are few and fine M₇C₃ carbides precipitated at theΣ3_c grain boundaries,which does not rule out the presence of a small amount of M₂₃C₆carbides;the M₂₃C₆ carbide precipitated near theΣ3_i grain boundaries;M₂₃C₆carbides precipitated near theΣ9 grain boundaries at the early stage of aging time,and M₇C₃ carbides precipitated near the other types of grain boundary fragments.The majority of M₇C₃ carbides precipitated at theΣ9 grain boundaries at the late stages of aging time.Coarse M₇C₃ carbide particles precipitated at theΣ27 grain boundaries and random grain boundaries.During the aging time of 800°C,the structures of carbide precipitated at different types of grain boundaries were almost unchanged.But due to the limited observable area of the TEM samples,the types of carbides at theΣ3_i grain boundaries need to be further studied,and the M₇C₃ carbides was found at theΣ9 grain boundaries.（4）All M₂₃C₆ carbides have the complex face centered cubic structure andcoherent orientation relationship with the matrix,the lattice constant is approximately three times as that of Alloy 600.The M₇C₃ carbide has the hexagonal structure,without coherent orientation relationship with the matrix.During the aging treatment,the growth mechanism of M₂₃C₆ and M₇C₃carbides is different.The M₂₃C₆ carbides grow irregularly into grains along grain boundaries,and there are fuzzy regions in the lattice fringes at the interface between carbide and the matrix.The M₇C₃ carbides do not have specific coherent orientation relationship with the matrix,which precipitates randomly at the grain boundaries.Carbide particles show the regular four-sided morphology,and there is no transition zone at the interface between carbide and the matrix.The M₂₃C₆ carbide may be converted into M₇C₃ carbides by in-situ conversion,and vice versa.In addition,stacking faults and twin structures are common in a single carbide particle.（5）The precipitation of carbides easily causes chromium depletion near the grain boundaries.The higherΣvalue of the grain boundaries,the coarser carbides precipitated at grain boundaries,which can cause more severe chromium depletion near the grain boundaries.During the aging time of 715°C,with the increasing aging time,the widths of the chromium-depleted zones near the grain boundaries increase,while the depths of chromium-depleted zones increase and then decrease.When the sample is aged for 50 hours,the depths of chromium-depleted zones near different types of grain boundaries are reduced to different extents.During the aging time of 800°C,the chromium depletion at the grain boundaries is not serious,and it is less than that at all types of grain boundaries at the aging of 715°C.With the increase of the aging time,the depths of the chromium-depleted zones near grain boundaries gradually decrease,and the difference between the chromium content near the grain boundaries and the chromium content of the matrix is smaller,and there is no obvious chromium depletion.