FORMATING

Marc Bernacki, Nathalie Bozzolo and Roland Logé of the Centre for Material Forming (CEMEF, Mines-Paristech, Sophia Antipolis) propose to team with Profs. Anthony Rollett and Gregory Rohrer, of the Materials Science & Engineering Department at Carnegie Mellon University (CMU, Pittsburgh, USA). The focus of the investigation will be on 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 properties such as corrosion resistance and fatigue life through control of the grain boundary network. GBE typically involves repeated cycles of deformation and annealing but little research has been done on how these steps result in optimized microstructures. We will focus on detailed characterization of the early stages of the recrystallization process as a function of initial grain size, annealing temperature, particle content, prior strain (or stored energy) 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 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 distribution of the locations where it takes place are key to understanding the process. The Intellectual Merit of the proposal resides therefore in the hypothesis-based approach to determining the underlying mechanisms of twin formation that enable GBE to be accomplished. Essentially all previous work has focused on before-and-after characterization and which boundaries optimize properties. By contrast, our recent work clarifies the importance of considering grain boundary networks over and above fractions of “special boundaries”. Note that the hypothesis, if demonstrated, will also explain why large deformations do not promote GBE. Previous work has focused solely on fractions of special boundaries whereas the proposed work will also examine the grain boundary network, which appears to be essential for property improvement. 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 company to investigate nanocrystalline nickel, as well with Suter’s group that works with 3D synchrotron-based characterization. CEMEF has hot stage microscopy and advanced simulation tools for the entire deformation plus annealing process. The US and French teams have a strong track record of collaboration and are strongly synergistic with experience in measuring and simulating microstructural evolution in 3D, and characterizing grain boundary properties on the US side, and, on the French side, the development of in-situ high temperature heating stage for the SEM (up to 1200°C) that enables to observe the evolution a given area of the investigated microstructure. The experience with materials is similarly complementary with an emphasis on basic materials on the US side to set against knowledge of engineering alloys on the French side. Successful delineation of the key mechanisms for GBE has the potential to expand the range of application of the process. Several groups in the US make use of superalloys and will therefore be likely to be interested in the outcome of that fundamental work, such as Integran, Pratt & Whitney, General Electric, Bechtel Bettis, and Rolls Royce. CEMEF has many French industrial collaborators, especially those from the aeronautics field who make extensive use of Nickel-base superalloys, e.g. Aubert & Duval, Snecma and Manoir Industrie.