Internal Friction and Elastic Constants of Sintered Titanium

Abstract
Using a Marx-oscillator standing-wave resonance method, we measured the internal friction Q^-1 of sintered titanium containing up to 26 volume-percent voids (c). The surprising Q^-1-versus-c curve shape, an exponential increase, lead us to remeasure Q^-1-versus-c by acoustic-resonance spectroscopy, which gave similar results. We hypothesized that both void shape and void size change with c. For the void shape, we confirmed this hypothesis by measuring longitundinal and transverse sound velocities v_l and v_t and comparing them with Mori-Tanaka model predictions. As c increases from 0 to 0.26, effective void shape changes from near spherical to oblate spheroidal with an aspect ratio near 0.05. Optical microscopy confirmed the void-size change. We outline a model based on wave-scattering theory that explains the observed Q^-1-c behavior. Increased particle size with increased c provides the dominant factor for the exponential Q^-1 increase.