High-frequency Keˆ internal friction peaks in polycrystalline aluminum and magnesium near the melting points at longitudinal and torsional resonance

By using a modified piezoelectric ultrasonic composite oscillator technique (M-PUCOT), the temperature dependence of Young’s modulus, shear modulus and related internal frictions of 4 N polycrystalline aluminum and magnesium were measured from room temperature to 900 K at longitudinal and torsional resonance under several tens of kilohertz. The grain boundary internal friction peaks ( K e ˆ peaks) accompanied by large moduli relaxations were observed near the melting point (~0.86 T _m and 0.78 T _m for Al and Mg respectively) in both vibration modes. The activation energy of grain boundary relaxation is almost the same for the two vibration modes, which is 31.2 kcal/mol for aluminum and 36 kcal/mol for magnesium. The peak values of grain boundary internal frictions are significantly dependent on the vibration mode, and for both materials, the torsional peak is about 1.5 times higher than that of longitudinal peak. Finally, the experimental results were evaluated by using existing grain boundary slipping models (including 3D and 2D Zener’s model, Budiansky and O’Connell’s model and Ghahremani’s model). Results show that the periodic hexagonal grains sliding model (Ghahremani’s model) can reproduce the experiments best.