The Creep and Fracture Properties of Additively Manufactured Inconel 625

Abstract

Abstract High temperature creep tests of additively manufactured (AM) nickel-based superalloy 625 (IN625) and wrought IN625 were conducted at 650 ˚C and 800 ˚C over the stress range of 65 MPa to 658 MPa. Thermal treatments were conducted for both AM and wrought IN625 samples prior to creep testing: either solution heat-treated or hot isostatically pressed and, additionally, long-term cyclic heat-treatments (LHT) at 650 ˚C for 6 months and 1 year. AM IN625 showed equal or even higher creep strength than wrought IN625 for all heat treatments. However, AM IN625 exhibited poor ductility compared to wrought IN625 under all creep testing conditions, and the ductility decreased after the LHT. Both AM and wrought IN625 obtained some additional strength after the LHT. The amount of extra strength in the alloys was generally proportional to the matrix volume fraction of γ’’ phase (650 ˚C) and δ phase (800 ˚C). The creep analysis suggests that dislocation climb is the rate controlling mechanism for creep. Atomic probe tomography revealed that oxygen content at the grain boundaries of creep-deformed AM IN625 was too small to cause any embrittlement. Nano-secondary ion mass spectrometer analysis found strong sulfur segregation at Al2O3/matrix interfaces. Fracture is intergranular where Al2O3 (that forms as a result of oxygen absorption by the powder particles before additive manufacturing) is sometimes located. Cracking can occur from these interfaces and repeated sulfur diffusion to the crack tip was the foremost possibility to explain the poor ductility of AM IN625 within the temperature range tested.