A Novel Approach to Structure Modification of Brasses by Combination of Non-equilibrium Heat Treatment and Friction Stir Processing

A non-equilibrium heat treatment was used to produce secondary phase (β) in the single phase brass (α), which caused the modification of the grain structure and mechanical properties after friction stir processing. For this aim, the single phase brass plate containing 37 wt pct Zn was heated at 810 °C for 1 hour, and then quenched in water, which caused the formation of non-equilibrium secondary β phase. After the non-equilibrium heat treatment, bead on plate friction stir processing was employed. The origin of the modification by β phase was studied using high resolution electron backscattered diffraction and transmission electron microscopy. The results showed that the average grain size, yield strength, and strain-hardening exponent were changed, respectively, from 4.8 µm, 166 MPa and 0.28 to 2.1 µm, 213 MPa and 0.25 in the presence of β phase. The β phase promoted the discontinuous dynamic recrystallization by particle-stimulated nucleation mechanism, and then transformed to α resulting in a finer microstructure with more random texture. In addition, the nanometer-sized β particles were retained at the grain boundaries, which reduced their mobility and hence the grain growth was inhibited.