A Study On The Low ? Coincidence Site Lattice Boundaries Associated With Phase Transformation In Pure Iron

Iron products are used in all aspects of daily life,with high commercial value.However,intergranular corrosion will lead to the performance degradation of this kind of material and cause huge losses to the national economy.From the point of view of grain boundary engineering,this work aims to find a method that can significantly improve the intergranular corrosion resistance of body-centred cubic iron-based alloys(such as ferritic stainless steel).In this work,99.999%purity high purity iron was used as experimental material,and the micro structure before and after phase transformation was tested and analyzed by electron backscatter diffraction(EBSD).The microstructure of low ? 3,? 5,? 7 and ? 9 grain boundaries was obtained the change law of proportion.Furthermore,the five-parameter analysis method based on EBSD and stereology is used to characterize the grain boundary plane distribution(GBPD)and the grain boundary inter-connection(GBIC)of grain boundary obtained by combining with crystallographic analysis.The main results of these researches(i)Effects of ?-y-? phase transformation on the low ?coincidence site lattice boundaries in a high purity iron;(ii)Strain-induced phase transformation and its effects on the low ? coincidence site lattice boundaries in pure iron;(iii)The low ? coincidence site lattice boundaries in pure iron after rolled at upper ? region followed by ?-y-? PTP,has discussed.The results show that the ?-y-? transformation path significantly affects the proportion of ?3 grain boundary,grain boundary plane distribution(GBPD),and the grain boundary interconnection(GBIC).The results show that the relative frequency of ?3 grain boundary only increases 2.5%of the total grain boundary in high purity iron before phase transformation,3.2%after one-time ?-y-? phase transformation,4.6%after second-time ?-y-? phase transformation,and 6%after third-time ?-y-? phase transformation.Where ?3 boundaries were randomly distributed before ?-y-? PTP and highest concentration has been found on {011} planes with a maximum MRD value of 2.07 after the ?-y-? PTP.?5 boundaries were found concentrated on {013} planes before and after the first phase transformation ?-y-? PTP with a maximum MRD value of 1.61.Other ?7 and ?9 boundaries were mainly oriented on {111}and {1 2 2} planes after phase transformation.And frequency increase with the increase of phase transformation times.However,it is not found that the proportion of ? 5,?7,and ?9 grain boundaries vary greatly in the distribution of misorientation.The results show that ?-y-? PTP has an impact on both the frequency and the GBIC of ?3 boundaries,but it doesn't affect the formation of coherence ?3 boundaries,because no {112}/{112} GBIC has observed.The strain-induced transformation has a certain influence on the proportion of ?3 grain boundary,grain boundary plane distribution(GBPD),and grain boundary inter-connection(GBIC).The results show that the frequency of ?3 grain boundary accounts for 6%of the total grain boundary after hot rolling at 920?,increases to 7%after hot rolling at 935?,and still accounts for 7%after hot rolling at 940?.After hot rolling at different temperatures,the grain boundary inter-connection GBIC {011}/{011} match type is formed.Hot rolling at the same ?-temperature region has little effect on the proportion of ?5,? 7,and ?9 grain boundaries.The results show that the hot rolling in ? upper-temperature region has a significant effect on the proportion of ?3 grain boundaries,grain boundary plane distribution(GBPD),and the grain boundary inter-connection(GBIC).The results show that the frequency of ?3-grain boundary increases significantly after holding at 930? for 5 min,accounting for 11.5%of the total grain boundary;when the holding time is extended to 15 min,the frequency of ?3-grain boundary reaches a maximum of 12%;when the holding time is extended to 30 min,the frequency of ?3grain boundary decreases to 11.5%.In the ? upper-temperature region,the ratio of ? 5,? 7,and ?9-grain boundaries have little effect on the grain boundary,so it is not considered.The calculation based on the density of coincident site lattice shows that the {0 11}/{011} GBIC of ?3 grain boundary has higher coincidence density,lower grain boundary energy,and more stable grain boundary structure.The intergranular corrosion's resistance of the material can be significantly increased by changing the processing method,especially by using proper ?-y-? phase transformation cycle treatment to improve the {011}/{011}and{12}/{112} GBICs of ?3 grain boundary.The research results have important theoretical significance and practical engineering application value for significantly improving the intergranular corrosion resistance of BCC Fe-based alloys(such as ferritic stainless steel).It is also found that the density of {013}/{013} GBIC of ? 5 grain boundary and {114}/{114} GBIC of ? 9 grain boundary are relatively high,but the three different processes in this present work have little effect on the development and formation of?5,?7 and ?9 grain boundaries...