Influence of Processing Method on the Grain Boundary Character Distribution and Network Connectivity
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Watanabe first discussed "grain boundary design and control" in reference to the manipulation of the relative fractions of "special" and "random" boundaries in order to improve certain bulk materials properties [1 ]. Since that time, such manipulations have become known as grain boundary engineering. Grain boundary engineering has been demonstrated to be a viable means of improving certain properties of low to medium stacking fault energy FCC materials such as austenitic stainless and microalloyed steels [2,3], nickel and nickel-based alloys [2, 415], and lead alloys [16-18]. The susceptible properties are typically grain boundary controlled, such as corrosion and stress corrosion cracking [2, 4-18], creep and cavitation [19-22], and weldability [23]. Recent advances in grain boundary engineering have resulted from a number of factors. One of the most significant is the commercialization of a scanning electron microscopy (SEM) based technique to accurately and rapidly characterize crystal misorientations between grains in order to determine the GBCD [24-26]. Less than a decade ago, these misorientations were determined through time consuming transmission electron microscopy (TEM) or electron channeling within the SEM [27,28]. With automated EBSD hardware and software, it is now possible to acquire approximately 10,000 data points per hour, thus allowing characterization of a statistically significant number of grain boundaries in a reasonable time frame. Additional factors contributing to the recent advances in grain boundary engineering are reports by Palumbo et al. [2-13, 16-18] of the optimization of the GBCD through practical thermomechanical processing schedules and the recognition that improvements in the special fraction can play a crucial role in Mat. Res. Soc. Symp. Proc. Vol. 586 © 2000 Materials Research Society
controlling the properties. A demonstration that boundary properties depend on misorientation has been presented in these proceedings by Bedrossian et al. [29] who have studied the susceptibility of individual grain boundaries to corrosion by coupling automated EBSD with atomic force microscopy (AFM). The AFM observations have identified the sites of localized attack observed in the EBSD as random grain boundaries. The deepest attack occurred at certain triple junctions composed of three random grain boundaries and no attack was observed at the .3 boundaries. A number of observations of this type have revealed a correlation between misorientation and localized corrosion processes [29]. These findings, coupled with our recent experimental results, suggest that increasing the special fraction is a necessary, but insufficient condition to assure property improvements that depend on intergranular processes. This is because the GBCD is a scalar quantity and does not contain details of the connectivity of the random grain boundary network. In order to improve properties, it appears imperative that the random grain boundary network be disrupted, which is accompanied by an increase in the GBCD. Researc
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