Microstructure and mechanical properties of ZK21 magnesium alloy fabricated by multiple forging at different strain rates

Multiple forging (MF) was carried out on as-cast ZK21 magnesium alloy to an accumulated strain of ∑Δε=2.64 at different strain rates. The microstructure and mechanical properties of the MFed samples were investigated. The initial grains should be significantly refined after MF due to dynamic recrystallization (DRX), including twin induced DRX (TDRX) at initial grain core and rotation DRX (RDRX) at initial grain boundaries. However, the microstructure and properties of the samples MFed at different strain rates were significantly different owing to the strain rate sensitivity of twinning and DRX. In low strain rate MFed (LSRMF) samples, it was difficult to obtain a complete DRX microstructure due to low twin density, which resulted in a limited improvement in mechanical properties. However, the high strain rate MFed (HSRMF) samples formed an island-like ultrafine DRX structure at grain cores with average grain size of ∼0.3 μm owing to higher twin density, while honeycomb-like coarse DRX grains also formed at initial grain boundaries due to an adiabatic temperature raise. This novel mixed structure of honeycomb-like coarse DRX grains and island-like ultrafine DRX grains in HSRMFed sample showed substantial improvements in mechanical properties. Mechanical testing gave an ultimate tensile strength (UTS) of 341 MPa and an elongation of 25% at ∑Δε=2.64, HSRMF has therefore identified as a potential technique for producing stronger and more ductile wrought ZK21 alloy.