Mechanisms Governing the Thermal Stability of Microstructure and Residual Stress in Surface Treated Aero Engine Alloys

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TECHNICAL: The research will combine process parameter variations with novel and detailed atomic-scale characterization, quantitative analysis, modeling and simulation in a study of the thermal stability and relaxation behavior of microstructure and residual stress in surface-treated aero engine alloys. New insight will be gained into how changes in atomic configurations during advanced surface treatments like laser shock peening (LSP) and post-process annealing take place and the associated thermodynamic, kinetic, structural, mechanical, mechanistic and modeling features. The program has been developed in collaboration with GE Infrastructure Aviation (GEIA) and LSP Technologies (LSPT). The primary goal is to secure the required fundamental knowledge of the impact of high temperatures and thermal cycling on materials behavior and properties generated by processes like LSP and thereby advance the science and application base of advanced surface treatment processes to high temperature materials. The key elements of the research program are: 1) LSP processing of typical Ni-base aero engine alloys; (2) characterization of surface and sub-surface macro and micro residual strains/stresses and microstructural changes as a function of LSP process parameters using novel methods; (3) determination of thermal stability and relaxation of residual stresses (macro and micro) and microstructure evolution with time at high temperatures and modeling of the kinetics of relaxation; 4) developing robust modeling and simulation approach for establishing the effect of LSP on the residual stress distributions; and 5) integrating the research program with undergraduate/graduate education, and with outreach programs. An arsenal of powerful characterization tools will be used to disclose materials behavior. Apart from conventional XRD, depth-resolved SXRD measurements will be performed using the facilities at the APS/ANL, NSLS/BNL and SSRL to characterize the near- and sub-surface residual strains/stresses and degree of cold work. Thermal relaxation of both macro and micro residual strains/stresses and microstructure stability at selected temperatures as a function of time will be studied and the kinetics of relaxation modeled from the data obtained. Finite element modeling will be conducted to simulate the LSP effects and predict residual stresses and their evolution. Correlation of the results from the various studies will provide the required fundamental new insight into the structural, mechanical, mechanistic and property features generated by processes like LSP and thermal effects to extend their use to high temperature materials applications. NON-TECHNICAL: LSP is an emerging surface treatment technology like low plasticity burnishing that has shown the potential to provide engineering materials with the extraordinary set of ambient temperature properties required for advanced structural applications in transportation, propulsion and energy-intensive systems. While the potential for enhancement of properties at high temperatures with the treatments like LSP also clearly exists, much more fundamental research is needed to establish the science-based, mechanistic understanding of the thermal stability of microstructure and stress to extend the use of these types of processes to the high temperature regime. The broader impacts of the program also include aiding scientists in industry and government laboratories in the development and implementation of advanced surface treatment processes like LSP for high temperature structural applications, stimulating our youth to pursue doctoral studies and cultivating a new breed of scientists/engineers much better trained for advanced careers in the materials field.