Oxidation Characteristics and Thermal Expansion of Inconel Alloy 718 from 300 K to 1273 K

Abstract

The measurements of the coefficient of thermal expansion (CTE) of Inconel alloy 718 are reported for the temperature range of 300 K to 1273 K and compared with the CTE of Ni. For Ni, CTE (in units of 10m6 K-l) varies from 12 at room temperature to 19 at 1273 K with a sharp peak at the magnetic transition Tc = 630 K. For the 718 alloy, CTE increases monotonically with temperature from 11 at 300 K to a value of 16 at 925 K. Above 925 K, CTE increases sharply reaching a peak value of 28 at 1150 K above which temperature, CTE again falls, matching the value of Ni at the higher temperatures. X-ray diffraction studies of alloy 718 at various stages of annealing, aging and CTE measurements show that the above heat treatments oxidize lhe sample, producing FeNbOq and (Cr,Fe)203 as surface oxides, contracting the lattice of the austinitic fee phase and depleting the F-Ni3Nb phase. Thermogravimetric measurements show that weight gain due to oxidation begins near 700 K, with the maximum rate of weight gain occurring between 925 K and 1075 K and somewhat lower rate at the higher temperatures. The sharp increase in the CTE of alloy 718 above 925 K is therefore most likely associated with its oxidation and the resulting degradation of its austinite and &NigNb phases.