Abstract
To study the effect of tensile stress on high temperature oxidation of alloys, wire specimens of a Ni–20Cr–4Al alloy were oxidized by using a creep testing machine under constant loads of zero to 14.7 MPa at 1273 K.
The results indicated that under the tensile stress of about 7.4 MPa a protective thin oxide film of Al2O3 formed firmly and the oxidation rate showed a minimum. However, an increase in the tensile stress beyond this stress range caused a severe oxidation penetrating into 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 induced in the growing oxide film, preventing the spalling of a thin oxide film of Al2O3 on the specimen surface.
(2) The tensile stress causes a significantly enhanced diffusion rate of Al in the alloy matrix, and the protective oxide film of Al2O3 easily forms on the surface of the alloy.
