Dynamic strain aging behavior of Inconel 625 alloy processed via directed energy deposition

Abstract

Additive manufacturing of the Inconel 625 alloy has become popular for the maintenance, repair, and overhaul of gas turbine systems. However, deformation instabilities are observed in the alloy at elevated temperatures, which degrade the mechanical properties of metallic components. Several studies have reported various possible mechanisms that induce deformation instabilities in Inconel 625 alloy, including segregation and the presence of secondary phases. However, it is still difficult to explain the origin of the deformation instability by combining all the microstructural changes during plastic deformation. Therefore, we investigated the deformation instabilities of an Inconel 625 alloy specimen manufactured by directed energy deposition, by combining direct observation methods and first-principle density functional theory (DFT) calculations. The DFT calculation results revealed that the stacking fault energy (SFE) of the specimen reduced at elevated temperatures. In addition, solute elements (Cr, Co, and Fe) cause chemical short-range ordering (CSRO) in the matrix, and the CSRO extended from 1 nm (for 10 % deformation) to 2–3 nm (for 30 % deformation) at 923 K. Consequently, both SFE reduction and CSRO evolution enhanced the slip planarity, making it easier to dissociate partial dislocations at elevated temperatures. Because Lomer-Cottrell locks are induced by {111} partial dislocations in Inconel 625 alloys, frequent dislocation locking and unlocking behaviors occur at elevated temperatures compared to room temperature. Therefore, frequent Lomer-Cottrell locking resulting from enhanced slip planarity induces deformation instabilities in the Inconel 625 alloy specimens at elevated temperatures.