Effects of volume fraction of fine grains on the tensile creep properties of a hot-deformed Mg-Gd-Y-Zr alloy

© 2020 Elsevier B.V. The creep properties of a hot-compressed Mg-9Gd-4Y-0.5Zr alloy with different volume fractions (VFs) of fine grains were investigated at 523 K, 548 K and 573 K. Based on the initial VFs of fine grains, the samples were divided into four groups including VF0 (VF = 0), VF0.2 (VF = 20%), VF0.5 (VF = 50%) and VF0.8 (VF = 80%) samples. The VF0.8 sample had the highest yield stress at room temperature, followed by the VF0.5 and VF0.2 samples, and the VF0 sample had the lowest. But after creep tests, the VF0.2 and VF0.5 samples had better creep resistance compared to the VF0 and VF0.8 samples at all the three temperatures. The creep life was shortest for the VF0 sample due to its lowest yield stress. The VF0.2 and VF0.5 samples were dominated by dislocation climb and pyramidal slip during creep testing. Deformation bands formed by pyramidal dislocations and obvious dynamic precipitation strengthened the creep resistance. Besides, the serrated grain boundaries were formed by the combination of coarse and fine grains, which absorbed dislocations and further improved the creep resistance. The VF0.8 sample was weakened by the precipitate-free zones (PFZs) formed along the high densities of grain boundaries. Although fine grains could improve yield stress, the excessive fine grains deteriorated the creep resistance owing to the heavy appearance of PFZs. Fortunately, the creep properties were successfully enhanced in this work through introducing mixed microstructures with both coarse and fine grains.