Research Projects

Tailoring of grain size has been one of the most important strengthening mechanisms in metallic materials. But how does the variation of grain size influence diffusion and hence kinetically driven transformations in alloys? This question has been attempted sporadically and the studies have been limited to using radiotracer approach. A systematic impact of grain size during interdiffusion has not been detailed in the reported literature. Interdiffusion is frequently encountered in engineering components and devices, which is often an assembly of different types of materials, for e.g., in flip-chip technology, bond coatings, brazing, diffusion bonding and laminate structures. Phase transformations in the interdiffusion zone (IDZ) are often key to the mechanical integrity and performance of the assembly. The present proposal targets to perform detailed quantitative assessment of grain size and temperature effects during interdiffusion involving medium entropy alloys (MEAs). Diffusion couples of Al/CoCrNi and NiAl/CoCrNi will be prepared and annealed in a wide range of temperature for different times. Appropriate thermo-mechanical processing routes will be used to produce several grain sizes of CoCrNi alloy ranging from ultra-fine grained to very coarse regime. Phase identification and composition profiles in the IDZ will be obtained using EPMA. Correlative microscopy approach using scanning/transmission electron microscopy ((S)TEM) and atom probe tomography (APT) will be used to precisely capture phase formation even at nanoscale. Using the experimental composition profiles of diffusing elements, a CALPHAD-based optimization procedure will be developed to predict effective interdiffusion coefficients (combination of lattice and GB diffusivities) of diffusing species (Al, in particular) as a function of temperature, composition, and average grain size. Using a systematic variation of grain size of the CoCrNi, while keeping a fixed grain size of Al or NiAl, we will develop a numerical optimization procedure to deconvolute the effective diffusivities of species into lattice and GB contributions. We will use this procedure to obtain the effective GB diffusivity of Al as a function of temperature and grain size for a given composition. CoCrNi MEA shows exceptional mechanical properties at low and high temperatures, possesses a thermally stable single-phase FCC structure, and has high deformability, which makes it an ideal alloy for the present study. Al and NiAl are chosen as other end members to ensure (a) the studies can be performed in wide temperature interval (Al upto 600 ⁰C, NiAl up to 1000 ⁰C) (b) the outcomes of studies are relevant to important technological applications such as diffusion bonding and bond coats. The success of the proposed work will lead to the development of a robust framework that can be used to analyse and optimize the design of IDZ in superalloys as well as design of various welds and joints used in nuclear power plants.