Microstructural Evolution During Aging Treatment Of Ni-based Alloys And Characteristics Of Oxide Films Formed In High Temperature Water

Ni-based alloys have been widely used in the primary circuit of the pressurized water reactor nuclear power plants,such as the control rod drive mechanism tubes and steam generator heat-exchanger tubes are made of Ni-based alloys.The Ni-based welding metals have been used for the welding of dissimilar metals/alloys for using in the primary circuit of the pressurized water reactor.These Ni-based alloys are subjected to high temperature and high pressure primary water during long-term operation,exhibiting potential stress corrosion cracking issues.Both the microstructure and the oxidation behavior can affect the stress corrosion cracking sensitivity in high temperature water environment.The alloy compositions play important roles in the microstructure evolution and surface oxidation behavior during the process of material service.In this work,the microstructural evolution and its influence on the properties of Ni-based alloys during the long time thermal aging process were investigated.The characteristics of the oxide film on the Ni-based alloys in high temperature water environment were also investigated.This work is aiming at providing the references for understanding performance and evaluating the reliability of Ni-based alloys after long term service in high temperature water environments.The work in this paper consists of two main parts.In the first part,the Ni-Cr-xFe alloys with different Fe contents were treated by long time aging at constant temperatures,aiming at achieving long-range ordering.The microhardness,transmission electron microscope and three dimensional atom probe were adopted to characterize the long range ordering phase Ni2Cr.Slow strain rate tests were conducted in high temperature water environment.Based on the experiments,the influencing factors on the ordering process were investigated.The effects of ordering on the mechanical properties and stress corrosion cracking behavior in simulated pressurized water reactor primary water were studied.In the second part,the scanning electron microscope and transmission electron microscope were adopted to characterize the oxide film properties of Ni-Cr-xFe(x from 0 to 10 at%)alloys in simulated pressurized water reactor primary water environments.The effects of Fe content in the alloy on the morphologies and compositions of the oxide film were analyzed.The main results are shown as follows:(1)The model alloys(Ni33Cr,Ni33Cr1Fe,Ni33Cr3Fe,Ni33Cr5Fe and Ni25Cr)were solution annealed at 1100 ℃ to get the disordered samples.The numerical number in the alloys represents approximately the atomic percentage.The microstructure and microhardness of solution annealed samples were similar.The solution annealed samples were aged at 360 ℃,400 ℃ and 500 ℃ to get the ordered samples.No long range ordering phase Ni2Cr could be detected in all samples aged at 360 ℃ until the aging time reached 17500 h.While the aging time reached 9000 h at 400 ℃,the microhardness of Ni33Cr and Ni33Cr1Fe samples increased remarkably,showing that the ordering transformation had happened in the samples.When the aging tests were conducted at 500 ℃,the microhardness of samples Ni33Cr and Ni33Cr1Fe increased remarkably after the aging time reached 20 h.When the aging time reached 120 h at 500 ℃,the increasing rate of the microhardness decreased.The microhardness reached the highest level after 1200 h aging time,and the increasing rate of the microhardness became very low.The formation process of the long range ordering phase Ni2Cr consisted of ordering incubation period,ordering process and saturation value of ordering.During the ordering incubation period,the microhardness increased slowly.The incubation time decreased and the ordering transformation rate increased with the aging temperature increased from 360 ℃ to 400 ℃ and 500 ℃.(2)For the solution annealed samples aged at 400 ℃,the long range ordering phase Ni2Cr precipitated in samples Ni33Cr and Ni33Cr1Fe after 9000 h of aging.The increasing rate of the microhardness of Ni33Cr was higher than that of Ni33Cr1Fe.The increasing rates of the microhardness of Ni33Cr3Fe and Ni33Cr5Fe were low and similar to the rate before 9000 h of aging.For the solution annealed samples aged at 500 ℃,the increasing rate of microhardness of Ni33Cr was higher than that of Ni33Cr1Fe,and the microhardness value of Ni33Cr was higher than that of Ni33Cr1Fe during the ordering process.After the ordering transformation reached a saturation value,the microhardness values of Ni33Cr and Ni33Cr1Fe became similar.These results indicated that the Fe content in Ni-Cr-xFe alloys significantly affected the incubation period and the transformation rate of the ordering.About 1 at.%of Fe content significantly delayed the transformation rate of ordering,but did not have significant effect on the incubation period of ordering.About 3 at.%and 5 at.%of Fe content prolonged the incubation period of ordering remarkably.(3)For the Ni-Cr binary alloys aged at 400 ℃,the increasing rate of microhardness of Ni25Cr was low and almost constant during the whole aging period,which was similar to that during the incubation period of ordering for Ni33Cr.The ordering transformation occurred easier in the Ni33Cr with Ni/Cr atomic ratio about 2:1 than in Ni25Cr,with the incubation period of ordering remarkably longer in Ni25Cr.(4)The values of microhardness of Ni33Cr and Ni33Cr1Fe alloys could reach the saturation values after ageing at 500 ℃ for a long enough time,indicating the saturation effect of the ordering transformation.After the tensile tests at strain rate of 10-4/s in air at room temperature,the yield strength and the ultimate tensile strength of the ordered sample were obviously higher than those of the solution annealed sample.Brittle fracture areas could be found on the fractures of the ordered samples.After the tensile tests at strain rate of 10-4/s in high temperature water environment,the similar effects of ordering on mechanical properties could be observed.These results indicated that ordering transformation increased the strength and sensitivity to brittle fracture.(5)The fracture surface of the ordered Ni33Cr sample after slow strain rate testing at strain rate of 10-4/s in simulated pressurized water reactor primary water with 18 cc(STP)H2/kg dissolved hydrogen at 310 ℃ showed stress corrosion cracking characteristics.The intergranular cracking area distributed along one side of the fracture edge,and the transgranular cleavage cracking area distributed between the intergranular cracking area and the ductile cracking area.The stress corrosion cracking resulted in the brittle intergranular and transgranular cracking.The results indicated that the ordering transformation increased the stress corrosion cracking sensitivity of Ni33Cr alloy.(6).The iron content in Ni-Cr-xFe(x=0-10 at.%)alloys strongly affected the characteristics of oxide films after 978 h of immersion in the simulated pressurized water reactor primary water environment at 310 ℃.Increasing the iron content in the alloys increased the amount of iron-bearing polyhedral spinel oxide particles in the outer oxide layer and increased the local oxidation penetrations into the alloy matrix from the chromium-rich inner oxide layer.The effects of iron content in the alloys on the oxide film characteristics after 500 h of immersion were less significant than those after 978 h.Iron content increased,and chromium content decreased,in the outer oxide layer with increasing iron content in the alloys.Increasing the immersion time facilitated the formation of the local oxidation penetrations along the matrix/film interface and the nickel-bearing spinel oxides in the outer oxide layer.(7)The oxide films that formed on Ni-Cr-xFe alloys(x=0-9 at.%)in an oxygenated simulated pressurized water reactor primary water environment were characterized.Iron in Ni-Cr-xFe alloys had a strong effect on the oxide film characteristics.Increasing the iron content in the alloy increased the porosity and thickness of the outer layer significantly.Increasing the iron content in the alloy increased the amount of spinel needle-like oxides and decreased the amount of atactic NiO particles in the outer layer.Increasing the iron content in the alloy slightly increased the thickness of the inner layer and the porous area in the inner layer.Cellular oxide in the inner layer was Ni-rich in the center and Cr-rich at the edge.