Effect of creep deformation on the microstructure evolution of Inconel 625 nickel-based superalloy additively manufactured by laser powder bed fusion

Abstract

The microstructure evolution of Inconel 625 additively manufactured by laser powder bed fusion during creep tests at the temperature range 600–800 °C and under a constant stress of 100 MPa was studied using scanning and transmission electron microscopy, X-ray diffraction and dilatometry. Before creep tests, the samples were stress relieved at 980 °C for 1 h and the initial as-built microstructure was preserved. The results show that the cellular microstructure of the Inconel 625 LPBF is stable even after 2000 h of creep at 600 °C. The negative creep strain phenomenon was revealed at 600 °C, which was associated with the intense precipitation of the γʺ phase along the cell boundaries and in their interior. XRD, dilatometry and TEM studies revealed that the negative strain is caused by a precipitation-assisted stress-relaxation process enhanced by rearrangement of a dislocation substructure. In the samples creep tested at the temperature range 700–800 °C, numerous plate-like particles of the δ phase precipitated inside of grains as well as coarse δ, Laves phase, M6C and M23C6 particles at the grain boundaries were present. Based on the microstructural analysis of the samples from the interrupted creep test at 700 °C and 750 °C it was concluded that the creep deformation in the secondary stage is controlled by the thermally activated dislocation movement and its effective hindering, mainly by the plate-like δ phase precipitates densely distributed inside the grains. In addition, as a consequence of the diffusion of vacancies, the formation of cavities along grain boundaries occurs. The analysis of the sample creep ruptured at 800 °C revealed that the formation of microcracks at the grain boundaries and the intergranular fracture mode in the tertiary creep of Inconel 625 LPBF is controlled mainly by the diffusion mechanism.