A team of investigators from the Materials Science & Engineering Department at Carnegie Mellon University (CMU) and the Centre for Material Forming (CEMEF) at the Paris School of Mines in Sophia Antipolis, France, examines the mechanisms of annealing twin formation that enable Grain Boundary Engineering (GBE) of materials to be accomplished. GBE is being applied increasingly as a process that seeks to improve a wide range of properties such as corrosion resistance and fatigue life through control of the grain boundary content of the material. GBE typically involves repeated cycles of deformation and annealing but little research has been done on how these steps result in optimized microstructures. This research focuses on detailed characterization of the early stages of the recrystallization process as a function of initial grain size, annealing temperature, particle content and prior strain level in order to determine the mechanisms that give rise to high fractions of desirable boundaries but most especially the break-up of the network of general high angle boundaries by annealing twins. There is also good evidence that the desired increases in the twin fraction are easiest to obtain at low to moderate strains, which is strongly associated with the Strain Induced Boundary Migration (SIBM) mechanism. This research offers the opportunity to test the hypothesis that the GBE process depends on a combination of SIBM with twinning, and that the spatial and textural distributions of the locations where it takes place are key to understanding the process. CMU has the capability to zoom in on particular regions of interest and characterize them with in-situ serial sectioning by using a dual-beam FIB-SEM equipment. CMU will also partner with Integran USA to investigate microstructural evolution in both nanocrystalline nickel and pure Ni. The CEMEF group, supported by the French National Research Agency (ANR), has hot stage microscopy and advanced simulation tools for the entire deformation plus annealing process. Key new information to be gained includes data on twin formation during annealing and the topology of grain boundary networks, as opposed to only measuring fractions of "special boundaries".
The hypothesis-based approach addresses the underlying mechanisms of twin formation that enable GBE to be accomplished. Most work so far has focused on before-and-after characterization, which has optimized properties but offers no understanding or control of the GBE process. The hypothesis, if verified, will also explain why large deformations do not promote GBE because other recrystallization mechanisms arise, such as abnormal coarsening of sub-grain networks. Successful delineation of the key mechanisms for GBE has the potential to expand the range of application of the process. Several parts of the industrial sector in the US make use of superalloys and will therefore be likely to be interested in the outcome of the work, such as Integran, Pratt & Whitney, and General Electric.
