A Novel Surface Nanocrystallization and \(SNH)\ Process for Improved Fatigue and Wear Resistance

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The project is on a novel surface nanocrystallization and hardening (SNH) process wherein metallic parts are impacted with high-energy balls under a controlled atmosphere. It will be suitable for both large and small engineering components and offer superior fatigue and wear properties derived from the synergy of the tailored microstructure, the smooth gradients of both composition and grain size, and the desired residual stress distribution. The University of Connecticut (UConn) and the University of Tennessee (UT) have joined together to investigate the key issues related to the SNH process. UConn will bring in expertise in the high-energy ball milling and SNH process, while UT will provide expertise in the area of fatigue mechanisms and modeling. A major goal of the study are to develop and construct a special SNH machine with independent control of a variety of the SNH process variables such as the surface grain size, grain-size gradient, composition gradient, formation of new compounds, residual stresses, and surface finishing. The objective of the project is to perform systematic and comprehensive experiments to establish the processing maps for surface nanocrystallization of the Ti-6-4 alloy including detailed microstructure and dislocation density studies using high-resolution transmission electron microscopy (HREM). Based on the mechanisms established, analytical and numerical models will be developed to predict the general trend of surface nanocrystallization. Mechanical properties of the SNH-processed Ti-6-4 alloys will be quantified using nanoindentation, pin-on-disc wear, and high-cycle fatigue tests. The fatigue mechanisms established in these studies will be used as a basis to develop theoretic models for the prediction of fatigue crack initiation and propagation behavior in the material with surface nano-grains and a grain-size gradient.The work under this grant attempts to develop bulk nanocrystalline metallic components with superior mechanical properties through the development and testing of a new process known as surface nanocrystallization and hardening (SNH). The ultra fine grain size in the surface with appropriate grain size gradient into the bulk is expected to result in improved fatigue life of the structural components. The project plans include participation of undergraduate and high school students along with graduate researchers.