Fundamental Mechanisms in Stress-Aided Variant Selection of Nanoscale Precipitation

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NON-TECHNICAL SUMMARYNickel-based superalloys possess combined high strength and corrosion resistance during service at elevated temperatures. Due to these important characteristics, they have been widely used in high-performance combustion engines, such as gas turbines, thermal and nuclear power plants. In order to achieve superior mechanical properties at high temperatures, precipitation hardening has been widely employed to strengthen superalloys. This program will focus on understanding and controlling the orientation of precipitates with respect to the alloy matrix, i.e., variant selection, in a nickel-based superalloy. The modulated mechanical properties by selected variant during the stress-assisted aging process will be also investigated. Insights gained from the program will advance the understanding of strengthening mechanisms as well as provide related industries with new guidance to improve the mechanical properties of alloys. The program will not only promote the progress of science but will also advance the national prosperity, and welfare due to the contribution to the aerospace, thermal and nuclear industry, where the enhanced performance of high temperature gas turbine components can significantly increase the efficiency of aircraft engines and power plants. In addition, this award will encourage women and underrepresented groups in engineering and enhance community and outreach activities through creative learning modules and workshops among other university programs. TECHNICAL SUMMARYThe mechanical properties of metallic materials are closely related to their internal microstructure. In particular, the morphology of precipitate particles including their shape, orientation and distribution can critically determine the properties of phase-separated alloys. Variant selection refers to the formation of a particular precipitate orientation with respect to the alloy matrix. Using Ni-based alloys as an example, the formation mechanisms of variant selection of coherent nanometer-sized precipitates at the nucleation and early growth stage, and the corresponding modulated mechanical properties will be investigated. We aim to answer the following fundamental questions: -How can one control the initiation of variant selection through varying important materials and processing parameters; -How to achieve anisotropic strengthening by selecting variants of which the desirable slip systems are in the preferential direction? The technical effort is featured by a closely integrated computational and experimental approach to (1) clarify the effect of materials parameters in variant selection at the nucleation and early growth stage during thermomechanical treatment, (2) fabricate single-crystalline samples, and (3) investigate the anisotropic strengthening during tensile and creep deformation. This program will be focused on nickel-based alloys due to its wide-spread applications. However, the fundamental mechanism elucidated is applicable to most alloys that contain coherent precipitates, e.g., novel refractory alloys.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.