Revealing deformation mechanisms in additively manufactured Alloy 718: Cryogenic to elevated temperatures

Abstract

Tensile behavior of Alloy 718, fabricated through laser powder directed energy deposition, was examined over a wide temperature range (−195, 25, 200, 425, 650, 870, and 980 °C). The tensile properties, deformation mechanisms, and their temperature dependence were specifically investigated. The microstructure and mechanical properties were characterized in fully heat treated condition; i.e., first stress relieved, then homogenized, then solution annealed, and finally 2-step-aged. Tensile results showed thermally stable behavior from room temperature up to 650 °C, similar to other additively manufactured and wrought counterparts. Rapid reductions of strength with increasing test temperature were observed at −195 to 25 °C and 650–870 °C ranges, which were respectively ascribed to a significant reduction in deformation twinning density and the coarsening of γ″-precipitates as confirmed by electron microscopy. The fully heat treated microstructure also contained populous Mo/Nb-rich carbides within grain interior which were observed to be the dominant mechanism of fracture and responsible for the temperature insensitive ductility up to 650 °C. At 870 and 980 °C, sliding between dynamically recrystallized grains became significant resulting in an increased ductility.