Enhanced strength of Inconel 718 by high rate severe plastic deformation

Abstract

Inconel 718 is Ni-based superalloy commonly used for manufacturing engine components in aerospace industry. Nickel-based super-alloys are particularly well suited for service in high temperature environment where resistance to creep, corrosion, and thermal shock are of primary requirements. Alloy 718 forms various nano-precipitates which endow higher strength to the material and thermal stability to the microstructure up to 650 °C due to grain boundary pinning of these nano-precipitates. In order to further improve the strength of the alloy at elevated temperatures, we utilized high rate severe plastic deformation (HRSPD). Machining was used to impose HRSPD on the alloy, and the chip thus formed was used to study the response of thermomechanical process. Machining led to significant microstructural transformation and resulted in “bi-modal” grain size distribution. Grain refinement led to significant increase in strength. Further improvement in strength was attained after extended heat-treatment at 600 °C for 10 h. This increment in strength can be attributed to the formation of nano-precipitates which results in obstructing dislocations movement and pinning of grain boundaries. However, it is known that deformed microstructure with very fine grains saturated with dislocations, results in brittle behaviour of alloy rendering them unusable for most applications. In order to alleviate this problem, post-process heat-treatment was designed. Three different temperatures were selected for short heat-treatment at 700, 800, and 900 °C for 15 min. This short heat-treatment resulted in varying amount of recrystallization of the matrix and precipitation and ripening of γ″ and γ′ phase. Transformation in microstructure and ensuing properties after thermomechanical processing are discussed in this paper.