This research ultimately aims at developing a more complete understanding of triple junction character and how the geometric character of the triple junction relates to the observed properties and local character of the microstructure. A triple-junction distribution function (3DF) will be developed that characterizes the complete space of triple junctions without assuming a priori that a certain type of boundary or junction is special. 3DF measurements will be obtained using grain-boundary engineered Cu and the Inconel alloys 600 and 617. The 3DF will include the functional dependence on crystallite lattice misorientations, triple line orientations with respect to the lattice and grain-boundary plane orientations. The complete function lies in an extremely large space making statistically reliable measurements difficult to obtain. This problem will be overcome by focusing on triple junctions associated with twin boundaries, thereby reducing the dimensionality of the function. The proposed analysis applies specifically to grain-boundary engineered alloys that have a preponderance of coherent twins, but the technique can be easily extended to include all triple junctions. Microstructural characterization to this extent has yet to be employed in investigations of grain-boundary engineering and offers a real opportunity to gain a more complete understanding of the mechanisms involved in formation of triple junctions. Finally, carbide and void distributions in alloy 617 will be characterized using the 3DF.
NON-TECHNICAL SUMMARY: Polycrystalline materials subjected to stress at high temperatures for long periods of time often develop brittle phases or even voids and cracks in certain regions that are most susceptible to this type of microstructural damage. Such materials are used in conventional fossil-fuel and nuclear power plants, for example, and these alloys are the primary focus of this project. To be able to predict the lifetime of such materials, the distribution of grain boundaries and their intersections (triple junctions), where damage is most likely to occur, should be quantified. To this end, the difficulty in measuring triple-junction distributions has prevented reliable characterization of these features in real materials. The present work focuses on the development of a triple junction distribution function that will enable researchers to properly characterize regions that are most susceptible to damage and distinguish them from those that are least susceptible. With this information, more realistic predictions of the useful life of engineering materials can be made, and process engineers will be able to design fabrication procedures that result in the most damage resistant microstructures. Undergraduate and graduate students will be involved through experience in the laboratory and development of curriculum.
Polycrystalline materials that are subjected to stress at high temperatures for long periods of time often develop brittle phases or even voids and cracks in certain regions that are most susceptible to this type of microstructural damage. Such materials are used in conventional fossil and nuclear power plants, for example, and these alloys are the primary focus of this project. To be able to predict the lifetime of such materials, the distribution of grain boundaries and their intersections (triple junctions) where damage is most likely to occur should be measured. Figure 1 shows a schematic of the parameters involved in the function. To this point, the difficulty in measuring triple junction distributions has prevented reliable characterization of these features in real materials. The present work has developed a technique to measure and represent a triple junction distribution function that will enable researchers to properly characterize regions that are most susceptible to damage and those that are least susceptible. Figure 2 shows an image containing the actual distributions of triple junctions as well as those obtained from the technique developed in this study. The correspondence of the functions is very close, both in position of the triple junction parameters as well as intensity of the values. Both graduate and undergraduate students have been involved in developing the algorithms necessary to perform this work and have made measurements showing how certain special boundaries avoid damage. Some of the measurements were made inside an electron microscope on an in-situ heating stage, so the structure could be observed as it evolved. This led to a greater understanding of the effect of heating rate on the development of special types of boundaries and triple junctions. The special boundaries were the focus of the present study with the algorithms being developed for these specific types of structures. The work performed, however, can be generalized to a complete distribution of triple junctions regardless of the character of individual boundaries.
