Determination of the magnetic signature due to the presence of a nonlinear material property gradient in a nickel-based superalloy by thermoelectric means

Abstract

It was showed that the thermoelectric coupling in a metallic medium can be applied as a viable means of inspecting inhomogeneities and defects in materials. This nondestructive sensing is carried out by using magnetometers to monitor the thermoelectric currents around the inspected area when a temperature gradient is induced inside the sample. In the NDE thermoelectric technique, the detection probability of defects in metals depends on the thermoelectric background signature produced by the intrinsic texture and inhomogeneity of the material. The measured magnetic signature is due to a combination of both effects. This paper investigate s the magnetic signature produced by a microscopic inhomogeneity such as precipitation and/or segregation when the material properties exhibit a non linear spatial variation in the cross section of a slender bar. An analytical method has been developed for calculating the magnetic field produced by the quadratic spatial dependence of the material properties. Experimental data from an inhomogeneous nickel-based superalloy 825 sample aged at 650 °C for 312 h were correlated with the theoretical predictions and fully verified the analytical model. Finally, the relationship between the strength of the magnetic signature and the complex microstructural and chemical features of this popular high-strength superalloy was studied.