| Magnesium alloys have high specific strength and specific stiffness,but the mechanical properties at elevated temperature,especially the poor creep properties,affect the development of magnesium alloys in the aerospace field.The development of high strength and heat-resistant magnesium alloys has been a hit of research.In this paper,the X-ray diffractometer,optical microscope,scanning electron microscope and transmission electron microscope were used to analyze the microstructure of the Mg-6Gd-3Y-0.5Zr alloy after the cast,aging and creep.The high temperature tensile and high temperature are carried out by the Instron5500 R electronic universal material testing machine and the SWT-2504 high temperature creep enduring machine respectively.The tests were carried out to analyze the mechanical properties and creep properties at elevated temperature of Mg-6Gd-3Y-0.5Zr alloy.The mechanical properties,creep behavior,and the microstructure,fracture,stress coefficient and activation energy after the cree p were analyzed,and the creep mechanism of Mg-6Gd-3Y-0.5Zr alloy under different temperatures and stresses was determined.At the same time,the creep behavior of ZM6 alloy is compared,and the difference of creep mechanism is analyzed.The gain size of as-cast Mg-6Gd-3Y-0.5Zr alloy is about 78.7μm,which is equiaxed with different sizes.The precipitates are mainly composed of two morphologies of skeletal eutectic phase and tiny lumps.The eutectic phase is Mg24Y5 +Mg5Gd phase,which is mainly located in the triangular grain boundaries.The eutectic phase is the face centered cubic structure.The tiny lump is a particle that enriching the Zr element.The average grain size after solution and aging treatment is reduced to 53.75μm.There are still tiny lumps in the interior of the grains and grain boundaries.The aging treatment can precipitate the elliptic spherical phases in the interior of the gains.When the temperature is higher than 250 oC,the tensil strength and yield strength decreases wth the rise of temperature.And the elongation incre ase with the increase of pemperature.At the room temperature,Tensile fracture mechanism belongs to the mixed quasi cleavage fracture;along with the temperature increase,fracture mechanism turns into microporous gathered ductile fracture.After the creep test,a thick long strip second phase is precipitated at the grain boundary,and the size and morphology of the precipitated phase have changed compared with the aging state.The stress provides conditions for precipitated phase to nucleate and grow up by preciptitaing dislocation lines and original direction.After creep,there are cavities and wedge-shaped cracks in the grain boundaries,they expands and grows into cracks,which lead to fracture of the alloy.When the temperature is 200 oC,250oC and 275 oC,the creep stress exponent of Mg-6Gd-3Y-0.5Zr alloy is 5.3,5.16 and 4.69,respectively.Dislocation climb ing mechanism is dominant.The activation energy under different stresses is within the range of 229-325 k J/mol,and the Mg-6Gd-3Y-0.5Zr alloy belongs to the precipitation strengthening alloy,which corresponds creep mechanism for cross slip mechanism.When the temperature is 200 oC,the creep stress expontent of ZM6 alloy is 3.39,and dislocation slip mechanism is dominant at this time,while the stress expontent at 225 oC and 250 oC is 2.27 and 2.68 respectively,which corresponds creep mechanism for grain boundary sliding mechanism at this time.The activation energy of ZM6 is higher than 140 k J/mol,indicating that the creep mechanism of ZM6 alloy is also related to the mechanism of cross slip. |
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