| Magnesium alloy is one kind of light structural metal material,and its application fields extend wider and wider.Some parts of the magnesium alloy are used in elevated temperature.But the service temperature of most magnesium products are not higher than 120℃,because the mechanical properties fall down rapidly when the temperature is increased.It is a key point to develop novel heat resistant magnesium alloy system to expand the application of the alloys.In this thesis,Mg-9Gd(5Gd)-3Y and Mg-5Gd-0.5Sc are investigated.ICP,DSC, OM,SEM,TEM and EDS etc are used to study the manufacture technology of the alloys.The properties of the alloys at room temperature and higher temperature and the structure evolution at the high temperature are studied.The heat treatment technology of cast Mg-9Gd(5Gd)-3Y-0.3Zr alloy is decided. It is found that the solid solution and aging(T6) technology is suitable for both alloys. After the aging and at the peak aging condition,half coherentβ' phase precipitated in the two alloys.In high Gd content Mg-9Gd-3Y-0.3Zr alloy,the density and the volume fraction ofβ' phase are higher than that in Mg-5Gd-3Y-0.3Zr alloy.At 25℃the tensile strength and yield strength for Mg-9Gd-3Y-0.3Zr alloy are 314MPa and 256MPa,respectively and higher than that 230MPa and 167MPa for Mg-5Gd-3Y-0.3Zr alloy.Mg-5Gd-3Y-0.3Zr alloy has better plasticity and the percentage elongation of Mg-5Gd-3Y-0.3Zr alloy at room temperature is 9.9%,100% higher than that of Mg-9Gd-3Y-0.3Zr alloy.While the temperature is increased,the strength decreased for both alloys,but the elongations are improved.From the curves true stress to true strain at elevated temperature,the Mg-9Gd-3Y-0.3Zr alloy shows a typical dynamic recrystallization type.The alloy experienced different scale dynamic recrystallizations when the temperature increased. The mechanisms of the dynamic recrystallization of the alloy are nucleation of grain boundary migration under the strain,nucleation of twins and mergence and growth of subgrains.Extruded Mg-9Gd-3Y-0.3Zr and Mg-5Gd-3Y-0.3Zr are all suitable for direct aging(T5) technology.After the aging,there are plate like metastableβ' phase precipitates in both alloys and the precipitates have different directions forming a kind of net.The orientation relationship between the Mg matrix and theβ' precipitate is as[001]β',//[0001]α,(100)β'//(2(?)0)α,But the density and volume fraction of theβ' precipitates in Mg-9Gd-3Y-0.3Zr alloy are higher than that in Mg-5Gd-3Y-0.3Zr alloy.At 25℃the tensile strength and yield strength for the extruded Mg-9Gd-3Y-0.3Zr alloy are 375MPa and 335MPa,respectively and the elongation is 5.1%.At 300℃the tensile strength and yield strength for the extruded Mg-9Gd-3Y-0.3Zr alloy are 314MPa and 284MPa,respectively and the elongation reaches 18%.At the same temperature,the strength of Mg-5Gd-3Y-0.3Zr alloy is lower than that of Mg-9Gd-3Y-0.3Zr alloy,but the plasticity is better.The fracture mode of extruded Mg-9Gd-3Y-0.3Zr alloy with T5 aging condition is a kind of mixture fracture.At 200℃the plastic fracture is the main mode,but with ductile fracture together as mixture fracture behavior.Between 250 and 300℃the ductile fracture with micropore aggregation occurs.Between 200 and 300℃the fracture mode for Mg-5Gd-3Y-0.3Zr alloy(T5) is also the ductile fracture with micropore aggregation.The creep deformation of Mg-9Gd-3Y-0.3Zr alloy at a temperature range between 200 and 300℃was investigated.It showed that the creep rate of the alloy increased with the increasing of temperature and stress,and the average stress index is 3.62.The creep active energy increases when the temperature is raised.The creep active energy is 85.6 KJ/mol when the temperature between 200 and 250℃.The creep mechanism is dislocation creep associated with tube diffusion.The creep active energy is 245.5 KJ/mol when the temperature between 250 and 300℃.The creep mechanism is dislocation climb and grain boundary slip.In the creep deformation process,the temperature has significant influence on the precipitates transformation.Under the stress,the metastableβ' becomes coarse or neck shrank.β1 nucleates in coarsed or at the neck shrankedβ' precipitates.When the temperature reached 300℃,β' andβ1 transformed in situ to the stableβphase.Theβphase becomes coarsen with the creep time becoming longer.The influence of creep stress and time on phase transformation is less than that of creep temperature during creep process.β' becomes coarse and partially transforms toβ1 phase.In the creep process,the dynamic precipitation occurred under the conditions of stress and temperature. |
