In this project, 10-20 µm thick magnesium alloy foils with varying Mg: Nb ratios will be used. These foils will be cut into rectangular samples (500 mm x 250 mm) and subjected to an Accumulative Fold-Forging (AFF) process, which involves multiple folding and forging steps to produce a layered ultra-fine-grained (UFG) magnesium alloy. The AFF process initially folds a 500×250×0.016 mm3 foil seven times to create 128 layers, each with a cross-sectional size of 31.25×31.25 mm2. Reinforcing nanoparticles or alloying agents can be introduced between the layers during folding. This process has the potential to produce materials with excellent hydrogen storage capabilities by leveraging the high density of interfaces and crystalline defects within the material to facilitate hydrogen diffusion and storage. Magnesium is particularly appealing for this purpose due to its lightweight nature and a unique hydrogen storage capacity of up to ~7.6 wt% in the form of MgH2. While previous studies mainly focused on HPT and ECAP for grain refinement and microstructural improvements, AFF offers the advantages of further grain refinement and the creation of a layered structure. Electron backscatter diffraction (EBSD) analysis will be used to understand crystallographic texture development during AFF. Hydrogen charging of samples will be carried out in a controlled reactor at 623 K and 50 bar pressure, followed by H-content measurements using a Bruker Galileo 8 analyzer. The phase composition will be analyzed using X-ray diffraction (XRD) with a Co anode and compared with the PDF-2 database.
This proposal aims to create, test, and model a nanocrystalline Mg/Nb alloy with millions of layers using an innovative SPD method. We will gain insights into grain-refinement mechanisms through advanced microstructural analysis using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and scanning transmission electron microscopy (STEM). Instrumented indentation offers a unique tool for measuring material properties, including dislocation activity, activation energy, indentation hardness, reduced modulus, creep, strain-rate sensitivity, and indentation fatigue. We shall use dislocation-based micro-mechanical models to understand flow stress and how various factors, such as precipitates, nano-layers, ultra-fine grains, and nano-sized particles, strengthen the material. This research bridges various scientific and technological disciplines, shedding light on material responses and mechanisms on both the nano and bulk scales. Instrumented indentation and micropillar compression provide valuable insights into interface-driven plasticity, dislocation interactions, and material failure under different conditions. Understanding dislocation/layer interactions and temperature-dependent indentation mechanisms can lead to accurate life prediction criteria and smart structural design. Instrumented indentation techniques offer a path to comprehending deformation mechanisms and designing structural materials more effectively. This innovative approach combines nanoscale structures with ambient-temperature deformation kinetics, benefiting nanocomposite manufacturing of strategic defense components. We will also assess the hydrogen storage performance of MgNb alloy counterparts, exploring the mechanisms behind hydrogenation and dehydrogenation stages. The layered composites formed by the AFF process increase the surface area available for hydrogen dissociation, and Nb nanoparticles act as catalysts. The ultrafine-grained structure facilitates hydrogen diffusion, thereby improving hydrogen storage kinetics. As hydrogen concentration in the Mg base increases, interactions between hydrogen atoms become crucial. Hydride phase nucleation and growth occur, impacting hydrogen storage capacity. The equilibrium pressure affects the coexistence of α and β phases, with reduced capacity above this pressure.
