Executive Summary : | The present project aims to generate multi-gradient hierarchical metallic microstructures with gradients in composition, stacking fault energy, grin size, twin thickness and frequency, dislocation density and crystallographic texture using a combination of diffusion bonding and surface severe plastic deformation techniques. A detailed microstructural characterization of the multi-gradient microstructure will be carried out followed with detailed mechanical testing including tensile testing and strain rate jump tests at different length scales to establish the process-microstructure-mechanical property paradigm in gradient metallic microstructures. To this end, plates of pure copper and Cu-30% Zn (alpha brass) will be diffusion bonded to get a compositional gradient sample with decreasing zinc content and increasing stacking fault energy from the surface of the sample to the centre (14-78 mJ/m2). These samples would be subjected to cold rolling and annealing to get samples with compositional gradient with almost equiaxed grain size. These coupons would be subjected to surface severe plastic deformation using surface mechanical attrition treatment (SMAT) and surface mechanical grinding treatment (SMGT) to obtain surface nanocrystalline layer along with a gradient in grain size and dislocation density from the surface to the centre. SMAT provides a nanocrystalline thin layer on the surface while SMGT provides a thicker sub-micron surface layer thus yielding two distinct microstructural gradients for similar compositional gradients. A detailed microstructural analysis of the processed sample would be carried out using scanning electron microscopy based energy dispersive spectroscopy for composition gradient, electron backscatter diffraction for microstructure and micro-texture gradient and nanoindentation for capturing the gradient in mechanical properties from periphery to the centre. Room temperature tensile test and strain rate jump test will be performed along with load reversal tests to determine backstress for different samples. This will be followed with constitutive modelling to explain the mechanical performance of the alloy and few in situ electron backscatter diffraction experiments will be performed to study the micro-mechanisms of deformation and damage and will be complimented by full field crystal plasticity simulations. Finite element method based simulations guided by full field simulations on different regions of the microstructure will be performed to capture the mechanical response of the multi gradient copper-brass gradient sample to firmly establish the hierarchical gradient microstructure-mechanical behaviour linkage in model FCC alloys. |