Fatigue enhancement of metals operating at T>0.5Tmelting

Results and underpinning microstructure analysis of wrought IN718, AM IN718 and CMSX-4 materials show a new surface-treatment technology provides retention of residual stress and enhanced fatigue performance after hundreds of hours exposure to temperatures exceeding 0.5Tmelting. The process involves applying laser peening with multi-millimeter deep penetration interposed with thermal annealing. The moniker Laser Peening plus Microstructure Engineering (LP+TME) identifies the process. Microstructure analysis of AM IN718 shows the process generates high density dislocations and that the novel interposed thermal annealing creates precipitates that block the dislocations, enabling retention of residual stress and thus enhanced fatigue performance. Jet engines and gas turbine fuel efficiencies are limited by thermally initiated fatigue and creep of hot-section components. Surface treatments do not work in these hot-sections because residual stresses they impart relax quickly at elevated temperatures. Advanced power generation technologies such as supercritical CO2 turbines and new concept nuclear reactors rely on metals capable of higher operating temperatures. Additive manufactured metals have potential to impact all above systems but require advancing fatigue and creep performance at for long duration elevated temperatures, often with corrosion exposure. LP+TME variants in materials are showing from 50% up to 100% of peening residual stresses surviving after 600oC to 700oC thermal and corrosion exposure and for durations of up to 350 hours. Fatigue tests of materials show as much as 130% increase in fatigue strength improvement. Results directly contrast equivalently exposed samples conventionally shot or laser peened which severely degrade in residual stress and fatigue performance. The impact of LP+TME processing potentially enables higher operating temperatures and improved fuel efficiencies for jet engines and gas turbines. Internal US Department of Energy analysis suggests that a 40oC increased operating temperature enables 1% increase fuel efficiency resulting in $7B annual savings and CO2 reduction equivalent to removing 2 million automobiles. .