Stress Relaxation across Grain Boundaries in Metals

In order to elucidate further the concept of relaxation of shear stress across grain boundaries in metals, the temperature dependence of internal friction and rigidity modulus of 99.991 percent aluminum have been measured as a function of frequency of torsional vibration and as a function of grain size of the specimen. It has been found that for the same specimen, an increase of frequency of vibration shifts the internal friction curve and the rigidity relaxation curve ($Q^{-1\mathrm{}}$ and $\frac{G}{G_U}$ versus temperature) to higher temperatures; and when the frequency of vibration is kept constant, a change in grain size of the specimen has the same effect as a change of the frequency of vibration The observed internal friction and rigidity relaxation can be expressed as functions of the parameter $(\mathrm{G}.\mathrm{S}.\mathrm{})\times f\times \exp \left(\frac{H}{\mathrm{RT}}\right)$, where (G.S.) is the grain size or average grain diameter of the specimen, $f$ is the frequency of vibration, and $H$ is the heat of activation. It is shown that all these observed phenomena are necessary manifestations of the stress relaxation across grain boundaries arising from the viscous behavior of the grain boundaries in metals, which behavior has been demonstrated by previous anelastic-effect measurements.