Research On The Special Grain Boundaries And Their Effects On Hydrogen Resistance Of Fe-Ni Based Alloys

Due to high strength,high ductility,good fracture toughness and good hydrogen resistance performance,the precipitation strengthened Fe-Ni based austenitic alloys(Fe-Ni based alloy)are widely used as structural materials in turbine disk,high pressure hydrogen storage vessel,pipeline and so on,in the fields of petrochemical,nuclear energy and hydrogen energy.However,with the strain localization caused by γ’ phase and interaction of hydrogen with dislocation,these alloys exhibit higher hydrogen embrittlement(HE)susceptibility than single phase austenitic alloys,and the fracture mode will change from ductile dimple fracture to brittle-appearing intergranular fracture.In present work,without changing chemical compositions of Fe-Ni based alloys,grain boundary engineering(GBE)is used to restrain the initiation and propagation of hydrogen-induced intergranular cracking to reduce the sensitivity of hydrogen embrittlement and to improve the hydrogen resistance performance.Effects of thermomechanical treatment and slow cooling heat treatment on the grain boundary character distribution and the formation and evolution of low ∑CSL boundary and serrated grain boundary(SGB)have also been investigated.The main research works and conclusions are as follows:1.The effects of thermomechanical process parameters(including strain,annealing temperature and time)on the fraction of low ∑CSL boundaries and grain boundary character distribution(GBCD)were invested.After 5%deformation+980℃/60min annealing thermomechanical treatment,the proportion of low ∑CSL boundaries can be increased to over 70%,and the connectivity of random grain boundaries(RGBs)network would be broken,which is attributed to the formation of annealing twins and boundary migration.The process of grain boundary(GB)migration during annealing is analyzed by EBSD.During the annealing process,∑3ic boundary has high migration ability,and when it encounters other ∑3 boundary,∑9 boundary will be formed.In addition,∑3 meeting with ∑9 would more likely lead to ∑9+∑3→∑3 reaction,which will cause ∑3 regeneration.The unconnectivity of RGBs network is attributed to high fraction of special triple junctions(∑3-∑3-∑9 and ∑3-∑9-∑27,etc.)and the formation of RGB/∑boundary.When RGB meets with low ∑CSL grain boundary,the new RGB/∑boundary will be formed.If the ratio(RGB/∑)≤ 29,the connectivity of RGBs network can be interrupted.2.The effect of GBCD and strain rate on the Fe-Ni based hydrogen resistance performance were invested.After GBE treatment,the improvement of hydrogen resistance can be attributed to not only the high fraction of SBs but also the disruption of RGBs network.As the former has high resistance to hydrogen-induced cracking initiation and the later can suppress the propagation of cracks.At low strain rate,hydrogen can keep the pace of the movement of dislocations and adequate hydrogen atoms can be transported to the boundaries.As a result,hydrogen concentration at SBs will reach its threshold value which will induce SBs to crack,but the propagation of cracks can be restrained.3.The effect of GBE on the formation of η phase at GB was invested.The reason for GBE inhibiting precipitation of η phase can be attributed to not only introducing high fraction of SBs but also breaking the connectivity of RGB networks.As nucleation and growth of η phases on SBs are difficult due to the lower Ti concentration and slow diffusion rate on SBs,and the disruption of RGB networks will further reduce supply of Ti atoms to the η phase significantly.4.The formation mechanism of SGB and its effect on hydrogen-induced intergranular cracking in Fe-Ni based alloy were studied.SGB can be introduced into Fe-Ni based alloy after slow cooling treatment(980℃/1.5h+1℃/min→920℃,air cooling).During slow cooling at high temperature,quite amount of Ni,Ti and Al atoms can diffuse to GBs and segregate on them,in order to decrease the total free energy.While,γ’phase is easy to precipitate and grow at RGBs during air cooling from 920℃,due to higher Ni,Ti and Al atom concentration on them.RGBs exhibit higher mobility,so the growth of γ’ phase will easy to promote the RGB migration,as a result,the straight RGB will change into a convoluted one.Compared with straight GBs,SGBs show higher resistance to hydrogen-induced cracking initiation and propagation.5.Fe-Ni based alloy with high fraction of ∑3n boundaries and SGBs has been obtained with the treatment:5%deformation+980℃/1h+1℃/min→920℃+air cooling.The fraction of ∑3n boundaries can reach to 74.2%and parts of straight GBs have been changed into wavy morphology,which will introduce more special triple junctions and further hinder the crack propagation.