Multi-Dimensional Site-Specific and Correlative Studies of High-Strength High-Toughness Naval Steels

For the project period April 1, 2015 through March 31, 2013, we are proposing to use a fundamental scientific physical metallurgy approach to elucidate the microstructure together with the underlying phase transformations during processing, to develop an understanding of the favorable combination of mechanical properties, for the base QLT 10 wt.% Ni steel as well as welded, weld simulated, and mechanically deformed samples, including low-cycle fatigue and high-speed deformation. Specifically, we are proposing to perform research in the following three areas: (1) The first task involves a detailed characterization of the microstructural elements, at all relevant length scales, and identifying all the phase transformations responsible for the microstructures providing the beneficial mechanical properties of QLT-treated 10 wt.% Ni steel, in collaboration with Dr. X. Jie Zhang (NSWCCD). For comparative purposes, we will also perform selected experiments characterizing HSLA-115 [ ]. (2) The second task is dedicated to an understanding of the effect of welding on the microstructure and mechanical properties of 10 wt.% Ni steel, collaborating with Prof. John DuPont (Lehigh University). This work builds on and continues our past research on welding of NUCu-140 [ - ]. (3) The third task, centering on fatigue and high-speed deformation, expands and continues our studies of adiabatic shear band (ASB) formation in high-strength high-Ni steels [ ], collaborating with Prof. K. Sharvan Kumar (Brown University) and Dr. X. Jie Zhang (NSWCCD). All of the proposed research will involve heavy usage of atom-probe tomography (APT) at Northwestern University, which provides microstructural information on a sub-nanoscale capturing local phase composition, precipitation, and segregation at grain and phase boundaries. Additionally, we will employ light optical microscopy (LOM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), to follow the kinetics of phase transformations and resulting microstructural features at different length scales. The microstructure will be compared and correlated with mechanical properties, primarily microhardness, tensile testing and Charpy V-notch toughness for selected samples. Computational thermodynamics (utilizing ThermoCalc and relevant data bases) and first-principles calculations (via density-funtional theory and general gradient approximation methods) will be used to model and understand the energetics behind the phase transformation and interfacial segregation processes. We will correlate the microstructural and atomistic results obtained utilizing these methodologies with the mechanical properties (yield strength, ultimate tensile strength, plasticity, toughness, etc.) of the steels being investigated.