Abstract
To study the effect of tensile stress on the high temperature oxidation of alloys, wire specimens of Ni-20Cr-4Al alloy were oxidized in a creep testing machine under constant loads of zero to 14.7 MPa at 1237 K.
The results of the study indicated that under the tensile stresses of about 7.4 MPa protective thin oxide film of Al2O3 formed firmly and the oxidation rate showed a minimum. However, increasing the tensile stress beyond this stress range caused a severe oxidation penetrating the alloy matrix. From the present study, two mechanisms for the stable formation of a protective thin film of Al2O3 under these tensile stresses are proposed as follows.
(1) Tensile stress induced in a specimen by the constant load relieves the compressive stress being induced in the glowing oxide film, preventing the spalling of a thin oxide film of Al2O3 on the specimen surface.
(2) The tensile stress causes a significantly increased diffusion rate of Al in the alloy matrix, and the protective oxide film of Al2O3 easily forms on the surface of alloy.
