Simulation Study Of Effects Of Thermal History Of Liquid Metal Nickel On The Solidification Microstructure Evolution

In this thesis, with the molecular dynamics method, the solidification processes ofliquid metal Ni under different initial metal temperatures and initial isothermal stepnumbers are simulated. By means of the different microstructural description methodsof the pair distribution function g（r） curve, the average atom energy of system,bond-type index method, cluster-type index method （CTIM） and the3-D visualizationmethod, the formation properties and evolution mechanisms of microstructures duringthe solidification processes of liquid metal Ni under different conditions are deeplystudied.The solidification processes of liquid metal Ni under different initial melttemperatures are simulated. The effects of different initial melt temperatures on themicrostructures during the solidification processes are investigated. The results showthat the initial melt temperature plays a critical role in the transitions ofmicrostructures. When the system consisting of10000Ni atoms with different initialmelt temperature （2073K,2023K,1973K,1923K,1873K and1823K） cooled down atcooling rate of1×10¹²K/s, the system are always crystalline structures with moreFCC and less HCP, but the number of FCC and HCP clusters are quite different. Whenthe initial melt temperature decreases, the average atomic energy of system increases,the percent of1421bond type and number of FCC cluster decrease, the percent of1422bond type and number of HCP cluster increase. This means that when the systemwith a higher initial melt temperature, the average atom energy will be lower, and thesystem tends to form a full FCC structure. Through the3-D visualization, it is foundthat the atoms of the same cluster are gathered in the same layer when the system withrelative higher initial melt temperature, and these layers would be scattered when theinitial melt temperature decreases. For the system consists of10000Ni atom, we usethe different number of isothermal step to indicate the different initial thermal history.We can obtain6groups of data （for50000，100000，150000，200000，500000and1000000steps） after the system cooled down at cooling rate of1×10¹²K/s. Byanalyzing the data, the majority of crystalline structures transformed from FCCâ†'HCPâ†'FCC, the structure of system with initial isothermal step of50000,100000,150000and200000are always crystalline with majority of FCC and minority of HCP; thestructure of system with initial isothermal step of500000is crystalline with majority of HCP and minority of FCC; when the isothermal step increases to1000000, thestructure of system turn to be majority of FCC and minority of HCP again. Thenumber and proportion of FCC and HCP cluster are quite different when the stepnumber varys. It is demonstated that the initial isothermal step has an importantinfluence to the microstructure evolution in the system.