Effect of Friction Stir Processing on the Kinetics of Superplastic Deformation in an Al-Mg-Zr Alloy

The effect of friction stir processing on the superplastic behavior of extruded Al-4Mg-1Zr was examined at 350 °C to 600 °C and at initial strain rates of 1×10−3 to 1 s−1. A combination of a fine grain size of 1.5 μm and high-angle grain boundaries in the friction stir-processed (FSP) alloy led to considerably enhanced superplastic ductility, much-reduced flow stress, and a shift to a higher optimum strain rate and lower optimum temperature. The as-extruded alloy exhibited the highest superplastic ductility of 1015 pct at 580 °C and an initial strain rate of 1×10−2s−1, whereas a maximum elongation of 1280 pct was obtained at 525 °C and an initial strain rate of 1×10−1s−1 for the FSP alloy. The FSP alloy exhibited enhanced superplastic deformation kinetics compared to that predicted by the constitutive relationship for superplasticity in fine-grained aluminum alloys. A possible origin for enhanced superplastic deformation kinetics in the FSP condition is proposed

Analysis of damping in particle-reinforced superplastic zinc composites

The damping behavior of superplastic zinc (SPZ) participate composites with up to 42.5 vol pet spherical TiC particles (3 /im in diameter) was studied in the 25 °C to 330 °C temperature range using a low frequency torsion pendulum. The observed damping at room temperature was modeled as a combination of a diffusion-controlled dislocation relaxation and a grain boundary relaxation. Addition of TiC produced a lower dislocation damping contribution at room temperature, but this loss was offset by an increased contribution from the grain boundary relaxation. An increase in the elastic modulus was also observed for the composite. The validity of a theoretical model for predicting changes in the grain boundary relaxation peak temperature resulting from the introduction of large nondeforming particles was tested. This study demonstrates that grain sliding in SPZ alloys occurs by cooperative sliding of grain clusters containing three to five grains. The activation energy for this process was found to be 111 kJ/mole (1.15 eV), which is in agreement with previously published values for grain sliding in SPZ. A second internal friction peak at a temperature just below the eutectoid transformation temperature was also observed and this peak was associated with recrystallization