Investigating annealing twin formation mechanisms in face-centered cubic nickel

The origins and formation mechanism(s) of annealing twin boundaries in face centered cubic (FCC) materials have remained unclear for over a century in the material science community. Although these interfaces are readily observable through simple means such as optical microscopy, the appearance of these highly faceted, straight boundaries remains an open scientific question. Annealing twins play important roles in material properties, where the low energy structure of the unique boundary provides improved integranular properties in materials achieved through grain boundary engineering (GBE). This enables metals exhibiting higher corrosion resistance, higher strength, more fatigue resistance, crack propagation resistance, and other superior properties for use in pressure vessels, turbine blades, and so forth.;Annealing twin boundaries arise with the heat treatment of low-to-medium stacking fault energy FCC metals in both the recrystallization and grain growth regimes. While a number of formation theories have been proposed in the past, such as the growth accident model, grain encounter model, grain boundary dissociation model, and stacking fault packets, none have fully satisfied the scientific community due to a lack of direct observation of a twin boundary formation event. However, by performing ex-situ annealing experiments over a single recrystallization cycle as well as grain growth, new data has been generated to directly identify the formation of a twin boundary and to better understand annealing twin formation mechanisms.;Two techniques were utilized to obtain the ex-situ information on annealing twin formation in high purity (99.999 %) FCC nickel. The first was scanning electron microscopy coupled with an electron backscatter detector to obtain individual crystal orientations necessary for misorientation studies. Ex-situ heat treatments were performed to monitor the growth of grains in the same area during the recrystallization of nickel specimen cold-rolled to 25 % and annealed at 490 °C. The second set of data analyzed came from high-energy x-ray diffraction microscopy performed at a synchrotron x-ray source on a high purity nickel microstructure undergoing grain growth at 800 °C. This non-destructive technique provided the three-dimensional crystallographic characterization of a microstructural volume and enabled the ex-situ monitoring of the same volume with applied thermal treatments. The microstructure evolution during both recrystallization and grain growth was observed and the formation of new annealing twins was detected in both regimes.;The role of the crystallographic interface(s) in the microstructure that resulted in annealing twin formation was studied. Previous work did not have the extensive crystallographic orientation data available nor were the stereological 2D and 3D techniques available to obtain the full description of a grain boundary character of a material. The orientation of the grain boundary plane that resulted in the formation of an annealing twin event was investigated, as each proposed annealing twin formation mechanism would promote annealing twin formation on different planes, namely {111} and non-{111}.;It was found that the processes of twin formation during recrystallization and grain growth were different from one another. During recrystallization, annealing twins are readily formed at the recrystallization interface and are left behind within recrystallizing grains, where the driving force is presumably the stored energy of the deformed matrix. However, during grain growth, annealing twins formed exclusively at triple junctions. Furthermore, the driving force for annealing twin was the replacement of higher energy grain boundaries with lower energy grain boundaries.;During recrystallization, annealing twin boundaries formed from migrating planes that are close to {111}. In contrast, during grain growth, there were no unique relationships found for the grain boundaries at the triple junction. However, the triple line was found to lie close to the {111} plane of the parent grain that exhibits the twin. Furthermore, the effect of temperature on annealing twin formation was studied for recrystallization at temperatures from 350 to 550 °C and for grain growth at temperatures from 550 to 950 °C. Here it was found that temperature had little effect on the twin content in either regime, provided the same change in grain size was achieved. Although the crystallographic findings support the growth accident mechanism of annealing twin formation, the absence of a temperature dependence runs counter to this idea.;The results in this dissertation are the first to show the role of crystallography in annealing twin formation in a bulk, annealed microstructure. It is proposed that annealing twin formation is primarily dependent on the number of appropriate grain boundaries and triple lines for nucleation in the microstructure. More specifically, to maximize the population of annealing boundaries in the microstructure, the focus should be on maximizing the number of {111} planes on the recrystallization front during recrystallization and triple lines that lie in the {111} plane of a grain during grain growth.