Role of Grain Boundary Character on Dynamic Recrystallization Megan E. Frary, Boise State University, DMR OBJECTIVE AND APPROACH: One of the objectives of this project is to determine the role of special boundaries on DRX behavior, including whether nucleation happens preferentially at a particular type of boundary or triple junction, and then whether changing the population of special boundaries will change the macroscopic behavior. To this end, stainless steel 316L samples were processed to have similar grain sizes, but different initial special boundary fractions (32%, 37%, and 45%). We analyzed the microstructures of the samples after DRX testing over a range of temperatures and strain rates to determine where the nuclei are forming. Grain boundary character (i.e., general high angle boundary, or special boundary using the CSL classification for  < 29) was determined at each nucleated grain. The orientation of each nucleus, and the parent grains surrounding it, were determined using TSL OIM Analysis software; a custom routine in Mathematica used the orientations to determine grain boundary misorientation and grain boundary character. RESULTS OF NUCLEATION ANALYSIS: For the material with 37% special boundaries, almost 300 nucleation sites were observed and analyzed over the range of total strains. Here, 52% of the total nuclei were found at triple junctions (with the balance found along grain boundaries) and 56% of the nuclei at triple junctions were found at J0 triple junctions (i.e., a junction between three general boundaries). At the low strain values (5%), where nucleation is only just beginning, 80% of the nuclei were found at triple junctions. This indicates that triple junctions are the preferential site for the onset of dynamic recrystallization. Furthermore, despite the fact that the J0 junctions should statistically be about 25% of the population, these sites represent a much higher fraction of the sites for nucleation. Analysis of samples with different special boundary fractions shows that the number of nuclei observed decreases with increasing special boundary fraction, which is expected as these boundaries resist nucleation, effectively increasing the grain size. These experimental results can be compared to the results from computer simulations which also showed that triple junctions were the preferred nucleation location. As was seen experimentally, J0 triple junctions are preferred sites for nucleation, even when their population in the microstructure is taken into account. Results for nucleation location for samples with varying initial special boundary fraction (SBF). (left) Overall, the preferred site for a nucleation to occur is at triple junctions (blue vs. red). This is especially true for the low SBF. (right) More nuclei were found on J0 (all three boundaries are general) triple junctions than other junction types in all samples. The difference is smaller though in the high SBF sample where J0 are less populous. Samples tested to 15% strain with different initial special boundary fractions: (left) 32% and (right ) 45%. The red circles indicate where the nuclei are. Detailed analysis of many such micrographs reveals that the number of nuclei decreases with increasing SBF.

Koyuki Fritchman Megan Beck Chris Stifter Callum Poole Role of Grain Boundary Character on Dynamic Recrystallization Megan E. Frary, Boise State University, DMR In the past year, three undergraduate and one graduate student contributed to this project. Two of these students, Koyuki Fritchman and Megan Beck, are rising seniors majoring in Materials Science and Engineering and who have been working on the project for the past three years since they were freshmen. Largely because of the skills and experience they have gained through this project, each had a summer research experience this year: Megan participated in MIT’s Center for Materials Science and Engineering REU, while Koyuki was part of NIST’s SURF program in Gaithersburg, MD. Chris Stifter graduated in December 2012 with his B.S. in Materials Science and Engineering. Chris joined the team for this project in June 2009 and stayed on as a researcher contributing to this project until May He is now working as a Materials Process Engineer at Zodiac Aerospace in Santa Maria, CA. The final student to contribute to the project was Callum Poole, who is working toward a Masters of Engineering degree, which he should complete in fall At the TMS Annual Meeting in March 2013, Megan presented a poster on this work. Her poster received the Best Undergraduate Poster Award from the Structural Materials Division.