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

The microstructure evolution of Inconel 625 additively manufactured by laser powder bed fusion during creep tests at the temperature range 600-800 degrees 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 degrees 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 degrees C. The negative creep strain phenomenon was revealed at 600 degrees C, which was associated with the intense precipitation of the gamma '' 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 degrees C, numerous plate-like particles of the 8 phase precipitated inside of grains as well as coarse delta, 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 degrees C and 750 degrees 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 delta 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 degrees 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.

Automated summary

The microstructure evolution of Inconel 625 alloy during high-temperature creep tests was studied. It was found that the cellular microstructure of Inconel 625 is stable at 600 degrees C and exhibits negative creep strain. Plate-like particles and coarse precipitates were present inside the grains and at the grain boundaries in the samples tested at temperatures ranging from 700 to 800 degrees C.