Phase Transformation Behavior Of Aluminum Under High Pressure: A Molecular Dynamics Study

As the most abundant metal element in the earth’s crust,aluminum(Al)has wide application in aerospace,nuclear energy,electronic and construction due to its excellent electrical and thermal conductivity,ductility and low density.Al has a facecentered cubic(FCC)structure at normal temperature and pressure,and its cell structure exhibits complex deformation behavior as the pressure increases.However,due to the limitation of experimental conditions,the mechanism of phase transformation of Al under high pressure and the evolution of microstructure during phase transformation are not clear.Molecular dynamics(MD)simulation methods can reveal the physical nature of experimental phenomena from microstructure,which the results can clearly guide material design.In this thesis,the phase transition behavior of Al under different hydrostatic pressures(P)(0 ～ 100 GPa)is systematically investigated using the MD method.Based on the variety of thermodynamic quantities,characterization of phase structure,and microstructural evolution characteristics,the phase transition behavior of Al under high pressure can be divided into four parts according to pressure 0-14 GPa,15-21 GPa,22-25 GPa,and 26-100 GPa,respectively.This thesis will discuss the phase transition behavior in different pressure ranges from three aspects:thermodynamic changes,phase structure characterization,and microstructure evolution characteristics.The results obtained can provide a reliable theoretical basis for optimizing the performance design of Al.In this thesis,the variation of potential energy and phase transition temperature(T)of Al under different P were first investigated.During the heating and cooling process from 50 K to 3500 K,the potential energy increases(decreases)with the increasing(decreasing)temperature,and a sudden variety of potential energy represents a structural phase transition.When 0 ≤ P ≤ 14 GPa and 15 ≤ P ≤ 21 GPa,the Al undergoes one phase transition during the heating and cooling process(melting and solidification).And the T increases with the increasing pressure.When 22 ≤ P ≤25 GPa,two phase transitions occur during the heating process.The T of the first phase transition decreases with increasing pressure,while the T of the second phase transition increases with increasing pressure.The cooling process undergoes a phase transition,and the T increases with increasing pressure.When 26 ≤ P ≤ 100 GPa,one phase transition occurs during the heating and cooling processes respectively,and the T increases with increasing pressure.In addition,this thesis explores the effects of model size and heating/cooling rate on potential energy changes and phase T.The results indicate that the potential energy changes of Al with different model sizes are the same as the T;Under different heating/cooling rates,the T of Al will exhibit a deviation of 0.44%.The above results also indicate that the phase transition results generated by the model size and heating/cooling rates adopted in this thesis are reliable.The phase structure was characterized by X-ray diffraction(XRD)and radial distribution function(RDF).The results show that the phase transition pathways of Al during the heating/cooling processes under different pressure ranges.The phase transition of FCC → liquid → FCC,FCC → liquid → body-centered cubic phase(BCC),FCC → BCC → liquid → BCC and BCC → liquid → BCC occurs under 0 ≤P ≤ 14 GPa,15 ≤ P ≤ 21 GPa,22 ≤ P ≤ 25 GPa and 26 ≤ P ≤ 100 GPa,respectively.Finally,the microstructure evolution of Al during phase transformation was studied within different pressure ranges.When 0 ≤ P ≤ 14 GPa,the thermal vibration of Al atoms causes transition from FCC to liquid during the heating process.During the cooling process,there are two types of defects in the FCC structure formed by liquid phase crystallization: {1 1 1}<1 1 2?> twin and {1 1 1} stacking faults.Two adjacent {1 1 1}<1 1 2?> twin crystals exhibit an angle of 70.5°,which accompanied by the generation of edge dislocations.Due to the variation of temperature,the relative motion of edge dislocations can cause defects to disappear;When 15 ≤ P ≤ 21 GPa,the FCC structure undergoes the transition to liquid phase due to the thermal vibration of Al atoms during the heating process.During the cooling process,the BCC phase gradually stabilizes within the system with the increase of pressure,and the increase of pressure will lead to the increasing defects within the FCC structure.HCP phases formed by fivefold twins and multi-layer {111} stacking faults can be observed under 15 GPa.Lattice distortion can increase the angle between adjacent twins within the fivefold twin;When22 ≤ P ≤ 25 GPa,FCC first forms BCC nuclei(Bain path)through continuous lattice changes,and then grows BCC nuclei through atomic diffusion during the heating process.The {1 1 2}<1 1 0>twins will form in the BCC phase.When BCC undergoes the transition to liquid phase,it preferentially become disordered at the {1 1 2}<1 1 0> twins.