Synchrotron Radiation Study On Microstructure Evolution Of High Performance Mg-Gd Alloys

With the decrease of global resources and energy,and the increasing attention of the international community to lightweight,energy saving and emission reduction,cast magnesium rare earth alloys,as the lightest metal structure material,has been widely concerned on the fields of aerospace,navigation,automobile and other industries for their excellent high temperature strength,good plasticity,strong creep resistance and damping seismic resistance.The mechanical properties of alloy castings depend largely on the formation and evolution of solidification microstructure of the characteristic structure.Therefore,it is especially important to understand the morphology and growth behavior of the solidification structure in different conditions for the implementation of control measures,the optimization of the production process,the assurance of the internal quality of the alloy and the improvement of the casting performance.In this paper,the Mg-Gd alloys with the hexagonal close-packed?H.C.P?structure are chosen as the research object,and the microstructure characteristics,such as grain size,dendrite arm spacing,solute distribution,dendrite growth,fracture,drift and so on are analyzed under different characteristic structures,cooling rates and compositions by using synchrotron X-ray tomography technology and constant temperature gradient.The experiment found that under the same temperature gradient and different characteristic structures,there is a huge gap in the evolution process of microstructures.The characteristic structure has a great influence on the growth rate of dendrites,in which the T-type structure has an inhibitory effect on the growth of dendrites,while the variable cross-section promotes the growth of dendrites;the solute in the T-type structure tends to concentrate towards the bottom of the boss,but there is no excessive lack of solute in the middle of the T-type structure,and the solute in the variable cross-section structure is concentrated at the shoulder.Besides the rectangular structure,dendrite arm breaks and drifts in the T-type structure and the variable cross-section structure.The microstructure evolution of Mg-15Gd-1Zn?wt.%?alloys during solidification was observed in real time and dynamically by synchrotron X-ray tomography technology at different cooling rates.When the temperature gradient is checked,the dendrite growth rate of Mg-15Gd-1Zn?wt.%?alloys increases with the the increase of cooling rate;the relationship between the secondary dendrite arm spacing and the cooling rate index,the secondary dendrite arm spacing decreases rapidly with the increase of cooling rate;At different cooling rates,the relationship between the tip radius is R_0.025K/s>R_0.05K/s>R_0.1K/s>R_0.25K/s>R_0.5K/s,Mg-Gd-Zn alloys dendrite tip radius decreases with increasing cooling rate.Under the constant temperature gradient and cooling rate,it is noted that Zn can promote the growth rate,arm spacing and tip radius of dendrites in Mg-15Gd?wt.%?alloy.The addition of 1%of Zn can greatly improve the arm spacing and tip radius of the secondary dendrite,but the effect on the growth rate of dendrite is not obvious in Mg-15Gd?wt.%?alloys.The above experiment results of Mg-Gd alloys perfect and verify the solidification theory of metal alloy,especially provide the first-hand experimental data for the solidification process of metal under situ observation.As a new idea and experimental method to study the evolution process of solidification microstructure of alloys,synchrotron radiation imaging technology makes up for the problems that traditional experimental and characterization technologies cannot describe the solidification microstructure of alloys in real time,dynamically,quantitatively and accurately due to its limitations.