Effect of Yb addition on the microstructure and mechanical properties of ZK60 alloy during extrusion

The effect of Yb addition on the microstructure and mechanical properties of ZK60 alloy during extrusion was investigated. With the addition of Yb, bulk MgZnYb phases formed at grain boundaries during solidification and were partially broken into large particles (>1 μm diameter) during extrusion which promoted dynamic recrystallization (DRX) by particle-stimulated nucleation (PSN). However, the Yb atoms dissolved in grains played an important role in the morphology evolution of nanosized MgZn₂ precipitates from rod-shaped to spherical particles by suppressing the preferential growth of MgZn₂ precipitates along the [0001] direction inside grains not subjected to DRX during extrusion. The presence of numerous spherical MgZn₂ precipitates suppressed DRX and impeded the movement of dislocations inside elongated grains not subjected to DRX, causing deformation to be concentrated inside the grains subjected to DRX. The extruded ZK60-2Yb alloy exhibited superior mechanical properties (393 MPa ultimate tensile strength, 367 MPa yield strength, and 17.3% elongation) to the ZK60 alloy extruded under identical conditions. The excellent mechanical properties were mainly attributed to a bimodal grain microstructure, which consisted of coarse elongated non-DRX grains and fine equiaxed DRX grains, and precipitation strengthening.