Research On High Strength Mg-Y-Gd-Zn-Mn Alloys With Long-period Stacking Ordered Structures

As the lightest metallic structural material, magnesium alloys have perfect properties of low density, high specific strength, specific stiffness and excellent processing performance, but the low strength and plasticity and poor high-temperature mechanical properties also limit their wide-scale applications in engineering filed. Due to the formation of LPSO in rare earth magnesium alloys recently, the ambient and elevated temperature properties have been improved significantly, which have attracted a wide variety of interests.In this wok, Mg-Y-Gd-Zn-Mn alloys containing LPSO phase were fabricated by conventional casting technology. OM, XRD, SEM, TEM were utilized to analyze the microstructural evolution and phase constituent. The formation mechanism and evolution law of LPSO phase, corresponding mechanical performances with increasing Y content were studied. The microstructural evolution and changes of mechanical properties through heat treatment and direct extrusion were also discussed. Especially, how the morphology, distribution and quantity of LPSO phase affecting the mechanical properties were investigated in detail. The results are summarized as follow:1, With the increasing of Y content, the microstructures was refined and the quantity of LPSO phase increased. However, the excessive Y addition coarsened the 18R-LPSO phase. The as-cast 1.0%Y and 1.5% alloys contained ?-Mg matrix and Mg3Zn3RE2 phase, a few Mg12 REZn phase was also observed in 1.5%Y alloy. When Y content was 2.0%~2.5%, the microstructure was comprised of ?-Mg matrix, Mg24(RE,Zn)5 phase and Mg12 REZn phase.2, The mechanical properties of as-cast 2.0%Y alloy were the highest, which valued about 221 MPa, 8.2% and 77.9HB respectively. However, the mechanical properties of 2.5%Y alloy degraded, it may result from the coarse 18R-LPSO phase.3, After heat treatment at 500? for 50 h, the eutectic phase dissolved and broken to spherical particles, For the 1.5%~2.5%Y alloys, a few 18R-LPSO compound decomposed, fine lamellar 14H-LPSO precipitated in ?-Mg matrix.4, The mechanical performances of four solution treated alloys shows a trend that increased firstly and decreased later. After aging treatment at 200? for 55 h, the 2.0%Y alloy obtained the aging peak, the tensile strength increased to 261 MPa.5, After direct extrusion, the grain size of 2.0%Y alloy decreased remarkably. The 18R-LPSO and 14H-LPSO phase were bent with large kinking angle. The fiber-like 18R-LPSO and 14H-LPSO phase were formed along the extrusion directiong. Dynamic recrystallization(DRX) occured and DRXed grain size was inhomogeneous. The finer grains were in favor of distributing in the vicinities of LPSO phase. The tensile strength and elongationg to fracture of extrusion processed 2.0%Y alloy were valued about 400 MPa and 17.8% respectively, the high strength can be attributed to the finer fibe-like LPSO phase, tiny cracked particle phase and refined DRXed grains.After aging treatment at 200? for 20 h, a higher strength were obtained with tensile strength of 415 MPa and elongation of 17.5%.