Study On The Microstructures And The Mechanical Properties Of Mg-Gd-Y-Zr Alloys

Mg-Gd-Y-Zr alloys are widely used in national defence,spaceflight and aviation industry currently for its excellent properties. However, its casting character and formability are poor. In this paper a series of Mg-Gd-Y-Zr alloys were prepared by LFEC (Low Frequency Electromagnetic Casting) processing, and many research work were carried out to develop the alloys, including as-cast microstructures and mechanical properties, homogenization processing, hot compression behaviors, extrusion technology. In addition, the effects of heat treatment conditions on the microstructures and the mechanial properties of the extrusion rods were also investigated in detail.The main constituents in as-cast Mg-Gd-Y-Zr alloys by LFEC are a-Mg, Mg24(GdY)5, Mg3Gd, Mg2Gd and Mg5Gd. There is no new phase formed after adding 1% Ag, which is solid soluted in matrix. The second phases appear as block, granule and flake. Most of them are in the grain boundaries and small part is in intracrystillines. The suitable homogenization technological condition is 500℃×8h.The flow characteristics of Mg-Gd-Y-Zr alloys during hot compressing and and their microstructures showed that dynamic recrystallization (DRX) takes place during the compression.The true-stress/true-strain curves were obtained and the apparent activation energy was given out according to the constitutive equations, and the effects of deformation temperature, strain rate and strain on the microstructures were discussed. The results indicate that the apparent activation energy for Mg-10Gd-3Y-0.6Zr-1Ag alloy is 180kJ/mol, and its suggested hot deformation temperature range is 360℃-400℃.The extrusion experiment results showed that high aging mechanical properties of Mg-Gd-Y-Zr alloy would be achieved as extruded in lower temperature due to its dramatic refinement effect. The suitable second ectrusion temperature was determined as 360℃by a series of experiments. The mechanical properties of the alloy in T5 condition are higher than those in T4 and T6 conditions. After aging at 200℃for 68h, the tensile strength is 465MPa, the yield strength is 398MPa, and the elongation is about 8%.The results also indicate that strain ageing could improve mechanical properties of Mg-Gd-Y-Zr alloy as the pre-strain capacity ranges in 3-7.5%.The tensile strength is 480MPa, the yield strength is 431MPa, and the elongation is about 7% after aging at 200℃for 71h in 3% pre-strain capacity.