Creep Performance and Microstructure Evolution of IN617 at 950 °C for VHTR Applications

IN617 was considered the primary material candidate for the IHX in the VHTR. Researching microstructure evolution during high-temperature creep of IN617 helped understand its fracture laws and guide IHX operation under creep loading. Creep tests of IN617 were conducted under 19 MPa, 24 MPa, 27 MPa, and 38 MPa at 950 °C. Creep rupture mechanisms of IN617 were discussed by correlating creep performance, microstructure characteristics and fracture-surface morphology. The results indicated that DRX, creep voids and brittle-phase precipitation were found under different stresses during microstructure observation, which would cause the specimen ductile rupture, intergranular rupture and brittle rupture, respectively. Specifically, under the highest stress 38 MPa, DRX occurred and grain size was decreased greatly to 11.1 μm from 99.7 μm. Fine grains were easy to migrate, causing significant plastic deformation and ductile rupture of specimens. With stress decreased to 27 MPa, grain boundaries became vulnerable and intergranular rupture occurred because intergranular carbides dissolved and their pinning effect was weakened. As stresses were lowered to 24 MPa and 19 MPa, nitrogen was diffused into specimens and brittle nitrides precipitated into continuous networks along GBs. The internal cracking of nitride networks caused brittle rupture. Meanwhile, steady creep rates were increased, and creep rupture lives were shortened greatly, especially under 19 MPa.