| Mg alloys have found important applications in the aerospace,defence and other industries due to their low density,high specific strength and high specific strength.As the lightest structural materials,the Mg-Li alloy has great potential used in aerospace compared with other magnesium alloys because of its higher specific strength.The structure of Mg-Li alloys will change with rising the content rising of Li element,resulting in better formability.However,its use is limited due to its poor mechanical properties at elevated temperature and unclear creep behavior.In this thesis,the microstructure of MA21 alloy was analyzed by means of OM,SEM,TEM,XRD,EDS,EBSD.The tensile and creep performance tests were carried out at RT~150 ? and the crept microstructures were examined.The creep mechanisms were determined by calculating stress exponent and activation energy at different temperature and stress state.The MA21 alloy has a certain grain orientation after rolling.The ?-Mg phase appears flat crystal grains on the TD direction.Compared with ? phase,?-Li phase was less,and wrapped around the ?-Mg phase.The ratio of ? phase to ?-Li phase was 65:35.In the ?-Mg phase,there are a number of Al Li compounds as long strips.The Al Li phase precipitated in ?-Li phase as particles with diameter less than 6 ?m.The precipitates is consisted of MgLi2 Cd phase with less than 200 nm,mixed MgLi2 Cd and AlLi with diameter in 200-700 nm as well as MgLi2 Cd ? AlLi and MgLi Zn with diameter ranging from 700 nm to 6?m.There are strong(0001)texture in TD direction.The tensile test results show that the strength decreases and the elongation increases with increasing the temperature.But there is an anomaly from 75 ? to100 ?.The yield strength at 100 ? is higher than 75 ? and the elongation is below75 ?.The reason is the microstructural change occurring at this temperature range.Fractured surfaces show that ?-Mg phase fracture mainly as cleavage platform and?-Li phase is dominated by dimple.And the fracture mechanism changes with increasing temperature.It is performed cleavage platform and a small amount of dimple at RT~100?.When temperature is higher than 125 ?,the cleavage fracture is replaced by dimples and tearing ridge.The fracture mode changes from brittleness to ductile.The results of creep test show that the temperature and stress have a significant influence on the steady-state creep rate.After creep at 30?,the dislocation density increases in the ? phase and the precipitation phase increases in the ? phase.When the temperature is less than 100 ?,the creep fracture is related to the strain rate.When the strain rate is faster,a small amount of cleavage platform will appear.The fracture mechanism is a ductile fracture mechanism with cleavage fracture.The fracture mechanism is ductile fracture mechanism when the strain rate is slower.When temperature is greater than 100 ?,the fracture mechanism gradually transforms into microporous aggregation type fracture mechanism.Crept at 125?,the precipitation zone appears at the grain boundaries.The creep mechanism of MA21 alloy is controlled by temperature and stress.When the temperature is 30?~100?,the creep stress exponents are 4~5,indicating that the creep is controlled mainly by dislocation creep mechanism with discontinuous precipitation.When the temperature is 125?and the creep stress exponent near 3,the creep mechanism is solute drag creep.Crept with low stress at150?,the creep stress exponent is close to 2,suggesting that the creep mechanism is mainly grain boundary slip accompanied by a small amount of diffusion creep mechanism.With the increase of stress,creep stress exponent is close to 5,creep mechanism changes into climb-controlled creep. |
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