The Evolution And Optimization Mechanism Of Grain Boundary Character Distribution Of Hastelloy C-276

Hastelloy C-276 alloy was the main manufacture material of nuclear power plant key components,which has been running in high temperature,high pressure and strong irradiation environments for a long time.Meanwhile,it was suffered from the corrosion of O-,Cl-and F-in coolant.This was required to have a good resistance to high temperature corrosion for the materials.However,the shorter service lifetime was still a serious problem for homemade Hastelloy C-276 materials in working conditions.In order to ensure the safety of nuclear power plant and prolong the material service lifetime,the need to improve the resistance to high temperature corrosion for Hastelloy C-276 alloy.Therefore,in the early 1990s,the idea of "grain boundaries design and control" was proposed by Watanabe to improve the materials performance,such as resistance to corrosion,creep and fatigue.This approach was employed to enhance the fraction of special boundaries via proper thermomechanical treatments,and the grain boundary character distribution(GBCD)can be optimized.Generally,the grain boundaries related properties of materials can be further improved.The effects of cold-rolled reduction and annealing time on GBCD in Hastelloy C-276 were studied by optical microscope(OM),scanning electron microscope(SEM),electron backscatter diffraction(EBSD)and orientation imaging microscopy(OIM).The microstructure evolution of GBCD during strain-annealing treatment was observed.With the microstructure evolutions were understood,the mechanism of GBCD optimization was clarified by in-situ observation.Based on those observations and results,a new cold-roll and two-step annealing treatment was proposed and the processing parameters were optimized.Finally,the GBCD optimized sample was examined by intergranular corrosion testing.The main conclusions are given below;(1)Cold-rolling and annealing treatment can be as an important processing to improve the GBCD of material.In Hastelloy C-276 alloy,the cold-rolling reductions were carried out from 2.5%to 90%and subsequent 1100 annealing for 15min.The result showed that the low strains(2.5%-15%)have an obvious effect on the fraction of special boundary.And the annealing time was optimized in this low strain,the total proportion of special boundary reaches 69.8%under optimal processing conditions:7.5%strain followed annealing at 1100? for 30min.Meanwhile,the grain boundary plane distributions of 3 and ?9 were best.However,the large cluster was not formed according to the connectivity of high angle boundary network was not break-up.This can be certified by ?(9+27)/?3 value.Therefore,the processing was needed to be optimized further.On the contrary,the larger strain(>15%)did not effect on the frequency of special boundaries.(2)Through in-situ microstructure observations with 7.5%strain followed annealing at 1100?,the movement of grain boundary migration and the evolution of incoherent twin boundaries were revealed during the annealing process.There were a lot of incoherent twin boundaries in grains during the low-strain annealing original 5min.As the high angle grain boundaries migrated into deformed matrix and removed annealing twins,it generated incoherent twins in the corresponding areas.And greater grains-cluster was formed during this process.Continue to annealing for 15min,many incoherent twin boundaries were disappeared and new coherent twin boundaries were formed.As well as the formed clusters became larger,the connectivity of high angle boundary network was recovery.This may be related to disappeared incoherent twin boundaries.At last annealing process,there was no change in GBCD,which means the stability of GBCD microstructure.(3)The effects of initial grain sizes and two-step annealing treatment on the GBCD were investigated by EBSD technology.The results showed that larger grain size was better for improving the fraction of special boundaries.And it contributes to form the larger clusters.With the same grain size,two-step annealing treatment can generate higher proportion special boundaries than one-step annealing treatment.Following more ?3n relative clusters were formed and the connectivity of high angle boundary network was disrupted effectively.Hence,the optimization process parameters were that 5%cold rolling reduction and annealed at 1000? for 5min,subsequent annealed at 1100? for 15min.The fraction of special boundary was up to 78.8%.While the connectivity of high angle boundary network was fragmented obviously,the size of grain-cluster was 200?m.Accordingly,the key factor of optimization was abundant incoherent 3 boundaries produced.Only incoherent ?3 boundaries were formed,it can be achieved the GBCD optimization.(4)The distribution of grain boundary carbides on different grain boundaries after aging treatment was investigated by SEM.Following that,the corrosion behaviors of grain boundaries were employed by intergranular corrosion test.The reason for special boundaries resisting to intergranular corrosion was revealed.It was observed that there was little carbide on coherent E3 boundaries and a few of carbides existed on incoherent ?3 boundaries.However,a large number of carbides were precipitated on high angle boundaries.Through the intergranular corrosion test,special boundaries exhibited an ability of intergranular corrosion resistance:the coherent ?3 boundary was better,the incoherent ?3 boundary was good,and ?9 boundary and ?27 boundary were worse.This is due to that those special boundaries could prevent carbides precipitation and reduce Mo depletion phenomenon,further to be resistant to intergranular corrosion.(5)The high temperature annealing treatment sample and GBCD optimization sample were conducted by intergranular corrosion testing.It was found that the GBCD optimization sample exhibits better intergranular corrosion resistance.This is mainly due to that there was little carbide precipitation at special boundaries and drastically reducing the proportion of high angle boundary.Instead,there are a few carbide precipitations on high angle boundaries in the high temperature annealing treatment sample.So the corrosion still happened.