Magnesium metal and its alloys are widely used in industry,especially,as biodegradable materials are highly suitable for biomedical applications.Since macroscopic properties and service behaviors of materials are mainly determined by their microstructures,it is very important to in depth understand the melting structure of pure magnesium and its evolution process in solidification process.In this work,a molecular dynamic simulation studyis performed with embedded atom method potential at different cooling rates to investigate the rapid solidification process of liquid magnesium,and the microstructure evolution and phase transition mechanisms are systematically analyzed by using E-T curves,pair distribution function g (r),Honeycutt-Anderson (HA) bond-type index method,cluster-type index method (CTIM-3) and three-dimentional (3D) visualization method,respectively.It is found that the cooling rate plays an important role in the evolution of microstructures,especially;from HA bond index method,CTIM-3 and 3D visualization method,the microstructure details of crystalline or amorphous structures in the system are displayed quite clearly with temperature decreasing.Meanwhile,it can be easily found how some basic clusters interconnect to form a larger one in the system. For short,some local configurations under different conditions at four typical temperatures are also given to show the difference in microstructure on a relatively large scale.At a lower cooling rate of 11011 K/s,the evolution of metastable bcc structure is obviously consistent with the Ostwald's step rule in the system,meaning that the bcc structure is first formed preferentially and then dissociated largely,and eventually the stable crystalline structures are formed mainly with the predominant hcp structure and fcc structure,and coexisting along with remaining partial bcc structure.At a middle cooling rate of 11012 K/s,the crystallization process is slower,the bcc initially is formed at lower temperature, suggesting that the crystalline process is postponed,and the coexisting structures is still formed with the predominant hcp structure and fcc,bcc structures,but lacking in the larger grains,due to the competitions among the hcp,fcc and bcc structures.Finally,for a higher cooling rate of 11013 K/s,amorphous magnesium is formed with basic amorphous clusters characterized by 1551,1441 and 1431 bond types and there is not a predominant structure,although a small number of medium or long range orders come out.In addition,there surely exists a critical cooling rate for forming amorphous structures in a range of 11012-11013 K/s.From the evolution of bcc,it is also suggested that short range orders in super-cooling liquid give birth to bcc structure and the process can be avoided by simply speeding up the cooling rate to a critical one.