ALLOY DESIGN AND MICROSTRUCTURE ENGINEERING RESEARCH GROUP
Bonding to Achieve Strength
Grain Boundary and Interfaces
Grain boundary engineering (GBE) has become an important research stream since this can lead to enhancement of metallurgical properties related to grain boundaries without changing the alloy. GBE has been achieved through replacing high-energy boundaries (Random High Angle Grain Boundaries (RHAGBs)) by low energy (special CSL or twin) boundaries. Quantification of special boundaries has been possible through the development of Coincidence Site-Lattice (CSL) model. Development of geometrical and mathematical relations to CSL model has increased the possibility of identifying special boundaries and differentiating it from RHAGBs. Experimental study of structure of grain boundaries has become possible through the advent of SEM-EBSD (Electron Backscatter Diffraction) techniques. Orientation Imaging Microscopy (OIM) has been found to be very useful in this perspective.
Until now, GBE is realized through replacing RHAGBs by low Σ CSL where Σ is the reciprocal density of coincidence (Σ ≤29). Annealing twin boundaries that have low energy with Σ3 CSL has been considered as a special boundary because of its low energy. Through thermo-mechanical processing, these Σ3 CSL boundaries have been made to interact with-in themselves through strain induced boundary migration (SIBM). This interaction leads to the production of new twins such Σ9 and Σ27 because of geometrical restrictions. Newly produced twin boundaries interact with other twins to multiple twin and this process is called Multiple Twinning. This kind of thermos-mechanical treatment led to the enhancement of metallurgical properties such as corrosion, cracking, sliding and segregation which mostly affect or percolate through RHAGBs. More recently, twinning related domain (TRD) analysis has been incorporated to quantify multiple twinning and extent of GBE. In this, TRD is defined as an entity containing grains connected by twin boundaries. It has been clearly indicated in our recent works that larger TRD is a hallmark of a GBE microstructure and as a result remarkably enhances the metallurgical properties related to percolation. Also, fractal analysis has been included to quantify the RHAGBs connectivity in GBE microstructure. Here, lower fractal dimension signifies disrupted connectivity.
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