A Study On Grain Boundary Plane Distributions And Grain Boundary Inter-connections In High-purity Iron

Iron and its alloys with BCC structure,as key engineering materials,are widely used in the field of industrial production.However,the fairly low resistance to intergranular corrosion largely limited its serving life.In this work,the structure and performance of grain boundary itself is the point-cut.High-purity iron(99.999%,mass fraction)is studied by stereology-based five parameter analysis(FPA),and the grain boundary inter-connection(GBIC)with crystallographic analysis to confirm the type of near singular boundaries(NSB).Then,the effect of annealing temperature,pre-strain,and deformation mode on the recrystallization GBPD,GBIC and the percentage NSB is investigated.Finally,the relationship between grain boundary structure and intergranular corrosion resistance is confirmed through grain boundary corrosion experiments.The major contents and results are as follows.The GBPD of high-purity iron with uniform microstructure was significantly distributed on {0 1 1}.The frequency of grain boundary with the GBIC as {0 1 1}/{01 1} reached a value of 2.6%,which was a kind of easily formed NSB in high-purity iron.The annealing temperature had a great effect on GBPD and GBIC of high-purity iron.When the annealing temperature was 730?,the GBPD was mainly distributed on {0 1 1},and the proportion of {0 1 1} / {0 1 1}GBIC,which was 3.45%,was the highest among the temperature range from 680? to 880?.At this temperature,the grain grown slightly compared with condition of the uniform microstructure,which indicated that the change of GBPD and GBIC was realized by a mechanism of local adjustment and resulting in the frequency of {0 1 1} GBPD and {0 1 1} / {0 1 1}GBIC was the highest.The amount of pre-strain(cold rolling)had a most significant effect on the GBPD and GBIC of high purity iron.When the pre-strain was 10%,the frequency of {0 11}/{0 1 1}-NSB after recrystallization 730 ? for 30 min reached a peak value of4.21%.The results showed that the grain boundary migration was mainly completedby the mechanism of strain induced boundary migration in the procession of recrystallization annealing.This is because the small deformation energy storage after10% pre-strain.The GBPD followed the thermodynamic law,which was the movement of grain boundary plane with high Miller index focusing on the {0 1 1}.Hence,The proportion of {0 1 1}/{0 1 1}-NSB obviously increased.The 80% thickness reduction by cross rolling and direct rolling had an adverse effect on the GBPD and GBID of the high-purity iron.The results indicated that the frequency of {0 1 1} GBPD and the proportion of {0 1 1}/{0 1 1}-NSB declined with the increment of recrystallization annealing time,which is lower than that of near singular grain boundary with the condition of uniform microstructure.Discussion pointed that recrystallization behavior was mainly nucleation and its growth during the procession of recrystallization annealing,which had a negative influence on grain boundary migration and reconstruction,and the formation of {0 1 1}/{0 1 1}NSB.The relationship between intercrystalline corrosion resistance and structure of grain boundary was confirmed by the grain boundary crrosion test and the method of overlapped pole figures.It was observed from the grain boundary corrosion experimental results that the planar coincidence site density(PCSD)of {0 1 1}/{0 11}-NSB was between the coherent ? 3 grain boundary and the high angle grain boundary,and the degree of corrosion of {0 1 1}/{0 1 1}-NSB was also between them.