EFFECTS OF THE ANNEALING CONDITIONS ON THE OXIDATION BEHAVIOR OF FE-36NI ALLOYS

Invar alloys with typical composition Fe-36%Ni were oxidized in air at temperatures of 823-1073 K with changing annealing conditions prior ot the oxidation test, i. e. in vacuum annealing (10** minus **2 Torr) and in hydrogen annealing (dew point equals 293 K). The oxidation behavior of these alloys is evaluated by observing impurity segregation, dislocation density and oxide film composition. The results obtained are as follows: (1) The growth rate of the oxide film ( alpha -Fe//2O//3/Ni//xFe//3// minus //xO//4, 0 less than x less than 3) was found to follow parabolic law. It was two to three times higher on the hydrogen annealed samples than on the vacuum annealed samples. (2) The hydrogen annealed samples were found to have a high density of dislocations on their surfaces and no segregation of silicon, chromium or other impurities was found thereon.