Internal nitridation of Alloy 690 during creep deformation at 1100 °C

This study reports internal nitridation of alloy 690 during creep deformation at 1100 & DEG;C. A series of creep tests were conducted at stresses ranging from 7 to 18 MPa, which corresponded to 0.3-0.7 of the alloy's yield stress at 1100 & DEG;C. The alloy exhibited normal creep response at 15 and 18 MPa whereas at 7 and 12 MPa, oscillatory creep were observed. The alloy exhibited creep exponent of about 8 and failed by cavitation at all the stresses studied. A higher value of the creep exponent was attributed to the cavitation dominant failure mechanism. Needle shaped nitride precipitates were observed near cracks, which were identified to be of trigonal Cr2N phase. The Cr2N particles formed near cracks which extended to the free surface. These precipitates formed as a result of internal nitridation of the freshly exposed crack surface. The phase stability analysis of the nitride phases in Ni-Cr-10Fe alloys in equilibrium with nitrogen using the Thermo-Calc software supported the formation of the Cr2N phase at a N2 fugacity of 0.79 bar. Atmospheric nitrogen permeated through the creep cracks linked to the outer sample surface. However, the precipitation of nitride particles appeared to have little effect on creep life of the alloy as the precipitation occurred towards the end of tertiary creep regime. The creep fracture at the lowest stress was dominant by transgranular cleavage of stable crack propagation, but tended to be erratic crack propagation at higher stresses.