It has been well established that in superalloys grain boundary serration has been occurred by the formation of the second phase and also it has been reported that in 20-11P austenitic stainless steel, the grain boundary serration is occurred due to the formation of the precipitation on the grain boundary.
Therefore, many researchers have investigated to verify the precipitation mechanism including the driving force for the grain boundary serration. Recently, it has been found that grain boundary serration occurs at the early stage of the aging treatment prior to the formation of $M_{23}C_6$ precipitates in an AISI austenitic stainless steel. Moreover, after the serration, it is observed that the morphology of carbides formed along the serrated grain boundary is changed from triangular to planar. In spite of the fact that the grain boundary serration is one of the most important factors to determine the mechanical properties in many alloys, most of the investigators have merely stduied the grain boundary serration based on the phenomenological observations without considering the mechanism of the grain boundary serration.
In this study, to verify the mechanism for the grain boundary serration, the factors affecting the occurrence of the grain boundary serration has been investigated and the mechansim for the grain boundary serration is to investigate on the basis of the formation activation energy of the grain boundary serration in the view point of the kinetics, using AISI 316 stainless steel. Furthermore, on the basis of the relationship between the grain boundary serration and low index planes and experimental results, the mechanism for the grain boundary serration has been propoesed.
In 316 stainless steel, some grain boundaries are observed to be considrably serrated while some grain boundaries are observed to be maintained straight shape even though heat treatment condition is same. To verify the reason why grain boundary serration is partilly occurred under same heat treatment condition, the grain boundary serratin behavior of the CSL and random grain boundaries have been investigated. It is found that the grain boundary serration is strongly related with the grain boundary characteristics, i.e. in the case of the CSL grain boundaries, the initial straight grain boundary is remained to be straight while the random grain boundary is considerably serrated after aging treatment at 760℃ for 1hr. To investigate the reason why the grain boundary serration is observed only in random grain boundary, the misorientation angle between the neighboring grains is measured. The misorientation angle is decreased after occurrence of the grain boundary serration. It is expected that since the random grain boundaries witha high misorientation angle may have higher grain boundary energy, they may be serrated to decrease the boundary energy. On the other hands, because the CSL boundaries of lower misorientation angle have lower grain boundary energy, they may not have enough driving force for the boundary to be serrated.
Among the factors affecting the occurrence of the grain boundary serration, it has been invesigated the effect of the carbon on the occurrence of the grain boundary serration. To verify the effect of the carbon, three kinds of 316 stainless steels with different carbon content are used. It is observed that the higher the carbon contents, the more the fraction of the grain boundary serration and also get the direct evidence that the grain boundary serration is strongly affected by the carbon atom, the sample with the lowest carbon contents is carburized to increase carbon content. It is observed that the grain boundary is considerably serrated after carburizing. These results idicate that carbon atoms are strong alloying element to accelerate the occurrence of the grain boundary serration in AISI 316 stainlee steel.
It has been reported that the grain boundary serration is strongly related to test temparature. Therefore, the relashionship between the grain boundary serration and the aging temperature has been investigated in 316 stainless steel. It is observed that the grain boundary serration depends on the certain aging temperature range. In the range from 650 to 870℃, grain size is slightly increased and most of the grain boundaries are observed to be serrated while grain size is drastically increased and straight grain boundaries are observed more than 880℃. The relationship between the grain growth and grain boundary serrationis investigated using the samples with different grain size. In the case of the largest grain size(about 200m), straight grain boundaries are observed while in the case of normal grain size(about 55m), most of grain boundaries are observed to be serrated. And also, distribution of the misorientation angle in each sample is measured by EBSD. It is observed that the number of the low angle boundaries is drastically increased in the case of the former as compared with that of the latter. It is suggested that the driving force for the reducing of the total energy on the given alloy system is to be grain growth more than 880℃ while that is to be grain boundary serration on the temperature range from 650 to 870℃.
In general, most of investigations for the grain boundary serration has not been focused on the view point of kinetics but thermodynamics. Therefore, grain boundary serration has been investigated on the view point of the kinetics. Using the Arrhenius equation, the formation activation energy for the grain boundary serration is calculated. The activation energy of the grain boundary serration is found to be 148±20kJ/mole in AISI 316 stainless steel. It is interesting to note that the formation activation energy of grain boundary serration (Q = 148±20kJ/mole) is closely similar to the value of the activation energy for carbon lattice diffusion (Q = 142kJ/mole) in γ-iron. Therefore, comparing the activation energy for grain bounry serration with several kinds of activation energies in 316 stainless steel and γ-iron, the mechanism for the formation of the grain boundary serration in AISI 316 stainless steel is somehow suggested to be related with the carbon lattice diffusion.
Finally, relashionship between the grain boundary serration and low index planes of neighboring grains has been investigated. It is found that the grain boundary serration occurs along the low index planes which have small angle between the plane normal vector of the grain boundary and neighboring grains. This reult indicates that the grain boundary serration lies along the low index plane which has small angle between the plane normal vector of the grain boundary and neighboring grains to reduce the grain boundary energy and to minimize the moving energy of the grain boundary so from these all obsevations, it is expected that these results provide the basis of developing new alloys with good mechanical properties.
