Dendrite Coarsening In Mg-gd Alloys By Synchrotron Radiation Imaging

Magnesium alloys are widely used in engineering because of their low density,high specific strength and good casting properties.The mechanical properties of magnesium alloys largely depend on the microstructure formed during solidification.Dendrite is the most common microstructure during solidification of metal materials,and dendrite coarsening will affect dendrite morphology,dendrite arm spacing and solute segregation,which has an important impact on the final solidification structure.Synchrotron radiation imaging technology has the advantages of strong penetration,high resolution and non-destructive.It is a powerful tool to characterize the microstructure changes during alloy solidification.Therefore,the study of dendrite coarsening behavior by synchrotron radiation imaging technology during solidification of magnesium alloys is of far-reaching significance for understanding the formation of microstructure of magnesium alloys.Taking Mg-Gd alloy as the research object,the solidification process of the alloy was dynamically characterized by synchrotron radiation imaging technology.The dendrite coarsening law under different cooling rate,different solute content and different casting structure was studied.The effects of different conditions on the dendrite coarsening mode,the change of secondary dendrite arm spacing(SDAS),solute change were analyzed.The main works are as follows:(1)Taking Mg-15Gd-1Zn alloy as the research object,the effect of cooling rate on dendrite coarsening was studied.It is found that the dendrite coarsening behavior is different under different cooling rates.The migration of dendrite arms at low cooling rate R=0.025 K/s is beneficial to the coarsening mode of dendrite arm coalescence.When R=0.05 K/s,there are anaxial dendrites without obvious primary arm,and the secondary arm of this dendrite can melt from the root.At higher cooling rate R=0.1 K/s,the dendrite arm fragments combine with other dendrite arms to coarsen the dendrite arms.The higher the cooling rate,the less likely the small arm melting will occur,while the root fragment and dendrite arm coalescence will occur more easily.The higher the cooling rate,the smaller the secondary dendrite arm spacing,and the smaller the increasing rate of the secondary dendrite arm spacing.The higher the cooling rate,the greater the segregation ratio in the process of dendrite coarsening.(2)Taking Mg-15 Gd,Mg-15Gd-1Zn and Mg-14Gd-2Zn alloys as research objects,the effect of alloy composition on dendrite coarsening was studied.It is found that the alloy composition affects the coarsening of dendrites by affecting the morphology and size of dendrites.It is found that after adding Zn,the proportion of small arm melting increases,while the proportion of root fragment decreases,and the proportion of dendrite arm coalescence changes little;The more Zn is added,the faster the secondary dendrite arm spacing increases,the larger the final secondary dendrite arm spacing,and the greater the segregation ratio of Gd in the process of dendrite coarsening.(3)Taking Mg-15 Gd alloy as the research object,the effect of structural shape of alloy casting on dendrite coarsening was studied.The change of casting structure will cause convection,which will lead to the redistribution of solute and temperature,and then affect the dendrite growth morphology and coarsening.In the samples with variable cross-section structure and T-shaped structure,the solute is easy to deposit in the small cross-section,which hinders the dendrite growth and coarsening,resulting in the small size and change rate of dendrite arm spacing,while the size and change rate of dendrite arm spacing in the large crosssection are relatively large.