On The Grain Boundary Character Distribution And Its Evolution Mechanism In Alloy 690

The nickel-based Alloy 690,with main composition Ni-30Cr-10Fe(wt.%),is currently replacing Alloy 600(Ni-16Cr-9Fe) as a steam generator tube material in pressurized water reactors,because of its superior resistance to intergranular stress corrosion cracking(IGSCC).But with prolonged service lifetime and improved performance being demanded by the nuclear energy industry,the need to improve the resistance to IGSCC and intergranular stress corrosion(IGA) in Alloy 690 should also be considered.Grain boundary structure has long been known to play a critical role in many material properties,such as precipitation,corrosion,and cracking.Grain boundary engineering(GBE),which was under the light of "grain boundaries design and control" proposed by Watanabe,developed a lot and was fruitful during the last two decades.The grain boundaries related properties of materials can be enhanced by exercising control over the population of low∑CSL(∑≤29) grain boundaries,as defined by the coincident site lattice(CSL) model.A large body of works proved that the grain boundary character distribution(GBCD) can be altered via proper thermomechanical treatments.Some face-centered cubic(fcc) metal materials with low to medium stacking fault energy,such as Pb-base alloy,Ni-base alloy,OFE copper,and austenitic stainless steel,were successfully applied this concept to promote∑3~n(n=1,2,3....) boundaries formation,and their properties were greatly enhanced.This may be an approach to further improve the resistance to intergranular corrosion of Alloy 690 which is also a f.c.c material with low stacking fault energy.The effects of thermomechanical treatments on GBCD in Alloy 690 were studied by optical microscope(OM),scanning electron microscope(SEM),electron backscatter diffraction(EBSD) and orientation imaging microscopy(OIM).The microstructure evolution of Alloy 690 during annealing at 1100℃after various cold rolling were observed and the microstructures in the partially recrystallized state were analyzed in detail.Based on these observations and analyzing the mechanism of the GBCD manipulation was elucidated.Finally,the intergraunlur corrosion testing was carried out on the specimens with different GBCD to reveal their intergraunlur corrosion resistance,and the mechanism involved.It is possible to reach the following general conclusions that are given below:(1)In Alloy 690,small strain(5%) and subsequent high temperature(1100℃) annealing for short time(5 min) can produce high proportion of low∑CSL grain boundaries(more than 70%),which mainly are∑3~n.In this case,the large size twin-induced grains-clusters(several hundred microns in dimension) constitute the microstructure.The proportion of Low∑CSL grain boundaries and the size of the grains-cluster decreased with the increase of strain.The starting state,i.e.solution annealed or aged after solution annealed,,didn't affect the GBCD obviously. Multi-cycle treatment didn't show different effects on the GBCD apart from single cycle treatment.(2) Based on the observation of partial and full recrystaUization state,it is clear that the nucleation density and multiple twinning are the key factors affecting the GBCD.The grains-cluster is formed by continuous chain of twinning events starting from a single nucleus of recrystallization,and with the associated presence of multiple∑3~n boundaries.As a consequence,all the grains inside the same cluster have close∑3~n misorientations even at a long distance.That the mean size of the grains-clusters and proportion of low∑CSL boundaries decrease with the increasing strain,is caused by the increasing nucleation density of recrystallization with the increase of strain.(3)In the case of high proportion of low∑CSL boundaries,connectivity of random boundaries network is very sensitive to the applied criterion of CSL.The connectivity of random boundary network is not fragmented obviously when the Palumbo-Aust criterion is used,whereas substantially interrupted when the Brandon criterion is applied.Almost all those low∑CSL boundaries(Brandon criterion) appear on the network of the random boundaries have very large deviations.Hence, the marked discontinuity of random boundaries according to Brandon criterion is not reliable.(4) When the proportion of low∑CSL boundaries is enhanced,the high resistance to mass loss during intergranular corrosion testing is attributed to the large size of the grains-cluster and its zigzag shape,rather than the unreliable discontinuity of the random boundary network.