Cavitation in Superplastic 7075Al Alloys Prepared via Friction Stir Processing

The effect of strain rate, temperature, and grain size on the cavitation of superplastically deformed 7075Al alloys prepared by friction stir processing (FSP) was systematically examined. The nucleation of cavities was generally observed to be associated with the grain triple junctions and coarse particles. While new cavities were continuously nucleated during deformation, the cavity density did not increase above a certain strain due to significant cavity coalescence and linkage. The density, size, and volume fraction of cavities increased with increasing initial strain rates from 1×10−2 to 1×10−1 s−1. Increasing the temperature from 450 to 510 °C resulted in a decrease in the cavity density. However, the specimen deformed at the optimum superplasticity temperature of 480 °C exhibited the lowest cavity volume fraction and ratio of large-size cavities. The decrease in the grain size from 7.5 to 3.8 μm resulted in a significant decrease in the density, size, and volume fraction of cavities. The growth of cavities was controlled by plasticity, and the cavity growth rate parameter, η, decreased with decreasing grain size and strain rate. FSP aluminum alloy exhibited lower cavity level and higher critical strain compared to a thermo-mechanically processed one