Molecular Dynamics Study On Phase Transition And Damage Fracture Of Metallic Materials

The phase transition and damage fracture rules of metal materials under dynamic loading and unloading processes are the hot and difficult subjects in the research fields of material physics, shock wave physics and weapons physics, and it involves the evolution and interaction of multi-scales microstructures. The dynamic physical modeling needs systemic knowledge on the interaction and evolution rules of these microstructures. With the development of simulation methods and calculation speed of computers, molecular dynamics simulation becomes an effective method for studying the evolution mechanisms of microstructures. In this thesis, we systematically investigate the non-equilibrium phase transition process under compression and damage fracture process under stretch in metals. Detailed results are as follows:The nucleation and growth mechanisms of phase domains under compression:The microscopic mechanisms of nucleation and growth under uniaxial uniform and impact compression are similar. The flatted-octahedral-structure (FOS) is the primary structural unit of the embryo nucleus and interface of the phase domain. The phase domains nucleate and grow up with the aid of the aggregation and slip of multiple FOSs. The phase transition process can be described by the following four stages:(ⅰ) Some atoms deviate from their equilibrium positions with the aid of thermal fluctuations to form FOSs with two different deformation directions in the local region of stress exceeding the phase threshold.(ⅱ) FOSs with different deformation directions aggregate to form a thin stratified structure like twin crystal configuration.(ⅲ) The thin stratified structure undergoes a relative slip to form the new hcp phase.(ⅳ)The phase domain grows up through the formation of new FOSs along the phase boundary. In addition, via comparing the evolution curves of initial single phase domain, we find that the growth rate of single phase domain depends on the loading way and occurrence time.The morphology and growth speed of phase domains under compression:In the impacted iron, the initial morphology of a single phase domain is ellipsoid-like; the three principal-axes directions are along[100],[011], and[011]; the three linear growth speeds along principal axes directions are all supersonic within a range of4×104～5×103m/s; the time evolutions of the length, surface area, and volume are approximately scaled as L～t0.465, A～t0.930, V～t1.395. Based on the order parameter theory of Ginzburg-Landau and the atomic evolution image which demonstrates the potential barrier is reduced with the help of interfacial atoms forming FOSs, we propose a phase transition model to clarify our derived growth law of the phase domains. In addition, via fitting the phase transition fraction curves, we find that the time evolution of phase transition fraction can be approximately scaled as:(ⅰ) Phase domains form and grow up independently f～t1.89(ii) Phase domains stop formation and interact with each other f～t1.23(iii) Phase domains collide with each other and the growth reaches saturation f～t0.80.The origin of dislocation creation and void nucleation under stretch:The dislocations originate from the double-layer defect clusters consisting of multiple FOSs. The process of dislocation creation can be described by three stages:(i) FOSs are randomly activated by thermal fluctuations;(ii) The double-layer defect clusters are formed by self-organized stacking of FOSs on the close-packed plane;(iii) The stacking faults surrounded by the Shockley partial dislocations are created from the double-layer defect cluster due to the relative slip of internal atoms. The voids originate from the vacancy strings created by the stacking faults intersection. The process of void nucleation can be described by two stages:(i) The vacancy strings are first created by the intersection of different stacking faults;(ii) Some appropriate vacancy strings transform into the voids by emitting dislocations. In addition, we demonstrate that our findings on the origin of dislocation creation and void nucleation are universal for a variety of FCC ductile metals.The evolution of damage structures in single crystal metal under stretch:Both the increase of temperature and decrease of strain rate reduce the yield strength, but the stress-strain curves separate prior to yield point under different temperatures; both increase of temperature and strain rate shorten the duration of the dislocation nucleation and slip stage; void nucleation needs not only lower yield strength but also lower fault energy. Contrasting the stress-strain curves and atomic evolution snapshots, the evolution processes of damage structures in single crystal metal under stretch can be divided into five stages:(i) The system elasticity deforms and the von Mises stress value linearly increases;(ii) Some defect clusters form along the loading direction and the von Mises stress value reaches peak value;(iii) Small dislocation loops nucleate from some larger defect clusters, then quickly multiply and move on slip plane, and the von Mises stress value relaxedly drops;(iv) Some voids nucleate in dislocation aggregation regions, then gradually grow up via emitting dislocations, and the von Mises stress value sharply drops;(vi) Different voids interact and coalesce to form larger voids, which result in the fracture of material, and the von Mises stressvalue reaches a minimum value.These results well explain some experimental phenomena, and establish a theoretical foundation for the mesoscopic description and macroscopic physical modeling of the relevant mechanical process.