Intercrystalline Thermal Currents as a Source of Internal Friction

Abstract

An experiment has been designed to detect the contribution of intercrystalline thermal currents to the internal friction of polycrystalline metals. In accordance with a theory developed by one of the writers (C.Z.), the internal friction is a maximum when the vibration is partly isothermal and partly adiabatic with respect to adjacent grains. By passing in small steps from the nearly isothermal case of very small grain size through maximum internal friction to the nearly adiabatic case of large grain size, one can detect the relative importance of the intercrystalline thermal currents. Such an experiment has been performed on single phase 69—31 brass, with mean grain size ranging in small steps from 0.0006 cm to 0.4 cm, and with frequencies of 6000, 12,000 and 36,000 cycles per second. Not only was a maximum obtained with the anticipated grain size, but the maximum is of a larger order of magnitude than the background upon which it is superimposed. The internal friction in the extreme isothermal case ($Q>\mathrm{}300,000\mathrm{}$) was lower than has ever been observed for metals; in the extreme adiabatic case it approached the low values obtained for single crystals. This experiment indicates that in annealed nonferromagnetic metals at room temperature, intercrystalline thermal currents are the dominant cause of internal friction measured at small strains, aside from possible macroscopic thermal currents.