Rapid drop in ductility of the bimodal-structured Mg–15Gd binary alloy during early aging

A bimodal-structured Mg–15Gd binary alloy with 45% volume fraction of elongated grains and 55% of dynamically recrystallized (DRXed) grains is fabricated by the extrusion process. The precipitating behavior correlating with the evolution of mechanical properties is systematically characterized during the subsequent aging treatment at 200 ℃. The extruded alloy presents an outstanding strength with tensile yield strength of 466 MPa and ultimate tensile strength of 500 MPa at peak aging condition, while the elongation drops from 9.2% in extrusion state to 3.1%. It is found there obviously exist a rapidly decreasing range of ductility at the early stage of aging. Just during this time, the nano precipitates form preferentially at lamellar dislocation boundaries (LDBs) within the elongated grains, but there is no dense and uniform precipitation in the matrix. The results suggest that the low elongation in the aged Mg–15Gd alloy is mainly attributed to the nano precipitates prior formed at the LDBs with a high density in the elongated grains. The related mechanism has been clarified.