Thermoelastic Relaxation Due to Chemical Gradient in γ-γ' Ni₃Al Materials

Abstract
Internal friction measurements performed in two types of γ-γ' Ni₃Al structures (an AM₁ superalloy with periodic distribution of 1 µm and a non-periodic biphased material with grain size in the order of 100 µm) bring out a high amplitude relaxation peak located at about 10 Hz, independently of the measurernent temperature. The characteristics of the peak (amplitude, frequency position and temperature range observation) and experimental conditions (torsion tests) do not allow to interpretate this effect in terms of classic thermoelastic relaxation due to transverse or intercrystalline thermal currents. We propose a model of thermal relaxation under confined chemical gradient related to the elaboration of this kind of biphased materials ; the chemical potential appears as a new internal variable for thermomechanical equilibrium conditions and implies local temperature gradients under stress annihilated by delayed thermal currents. The expression of the relaxation time is a function of the chemical gradient and of the thermal diffusivity.