The role of bimodal-grained structure in strengthening tensile strength and decreasing yield asymmetry of Mg-Gd-Zn-Zr alloys

This paper studied the role of bimodal-grained structure in strengths and asymmetry behaviors of Mg-15Gd-1Zn-0.4Zr (wt%) alloys under the as-extruded, compressive-yielded and fractured conditions. The bimodal-grained structure consisting of fine recrystallized grains and coarse un-recrystallized grains formed when the alloys were extruded with relatively low temperature. Compared with the equiaxed structure, the bimodal-grained structure possessed higher tensile strength attributed to the fine recrystallized grains plus the strong and hard un-recrystallized texture with its basal planes nearly parallel to the loading direction. However, the bimodal-grained structure weakened the ultimate compressive strength, because the un-recrystallized texture is favorable for tensile twining in the compressive test. Eventually, the aged sample with the bimodal-grained structure exhibited ultra-high tensile yield strength and ultimate tensile strength of 465 MPa and 524 MPa, respectively. Especially, the inhomogeneous bimodal-grained structure with stronger texture resulted in extraordinary low compressive-tensile yield asymmetries of 0.99 and 0.98 compared with those (from 1.09 to 1.48) of the homogeneous equiaxed structures with relatively random textures, because the fine recrystallized grains in the bimodal-grained structure undertook major deformation by slips and suppressed twining before the yield points in the mechanical tests. The relationship between the microstructures and properties allows us to produce high tensile strength Mg components by applying low extrusion temperature and obtaining bimodal-grained structure.