LOW-TEMPERATURE MAGNETIC TRANSITION AND HIGH-TEMPERATURE OXIDATION ININCONEL ALLOY-718

X-ray diffraction and temperature dependent (5 K-380 K) magnetic measu rements have been carried out in INCONEL 718 superalloy before and aft er high temperature aging treatments. (INCONEL is a trademark of the I NCO family of companies.) The nominal composition of this alloy is Ni (52.5%), Cr (19.0%), Fe (18.5%), Nb (5.1%), Mo (3.0%), Ti (0.9%), Al ( 0.5%), Cu (0.15%), and C (0.08%), and it yields an x-ray diffraction p attern consisting of a fee phase with a = 3.5987 (3) Angstrom and an o rthorhombic phase associated with delta-Ni3Nb. It is concluded that th e fee pattern is due to both the gamma austenitic phase and gamma' Ni- 3(Al, Ti) phase of alloy 718. The standard annealing and aging treatme nt carried out in air at temperatures between 621 and 982 degrees C pr oduces surface oxides (Cr,Fe)(2)O-3 and FeNbO4 (which are easily remov ed by etching and polishing) and contracts the lattice. Magnetic measu rements show a distinct phase transition at T-c = 14 K, which has been attributed to the gamma'-Ni-3(Al, Ti) phase by the process of elimina tion and by observing that it has most of the characteristics of the w eak itinerant ferromagnet Ni74.5Al25.5. This transition may have some effects on the cryogenic applications of this alloy.