Molecular Dynamics Simulation For Characteristic Of Nanostructured Mo And FeAl Under Uniaxial Tensile Strain

In the present thesis, with the analytic embedded atom method (EAM), the mechanical properties and behavior, such as Young’s modulus, deformation mechanism, phase transformation and fracture manner of nanocrystallines and nanowires are studied systematically. The results are expected to have significant guide functions on their applicability in areas such as chemical and biological sensors, electronic and/or optical devices and micromachines.The structure of Au nanocrystalline has been studied by "quasi-static straining" It is found that the structure of grain boundaries in nanocrystalline is almost independent on average grain size. With reducing grain size, the fraction of grain boundaries increases. The grain boundaries possess not only higher short-range order but also definite long-range order. The plastic deformation of Au nanocrystalline mainly carries out by the grain boundaries sliding and grain rotation.We have studied the mechanical properties, deformation mechanism, phase transformation and fracture behavior of Mo nanocrystalline by "continuous straining" The results show that the Young’s modulus increases linearly with increased grain size or decreased temperature. The flow stress and the tensile strength decreases with decreased grain size, but increases dramatically with decreased temperature. Grain rotation, grain boundary sliding or migration, dislocation motion and intergranular activities are observed in the deformation process. Twinning is regarded to be a secondary mechanism. Stress-induced phase transitions from bodycentered cubic to face-centered cubic (fcc) and hexagonal close-packed (hcp) structures take place locally. Moreover, an increased strain rate delays the appearance of fcc atoms, but increases the peak fraction of fcc atoms. At0K intergranular brittle fracture occurs through formation and convergence of nanocracks at grain boundaries and the fracture strain increase with decreased grain size. At300K ductile fracture occurs through formation of nanocracks, their transformation into pores, growth of pores and formation of local necks between large pores.The mechanical property and phase transformation of polycrystalline (PC) Mo nanowires have been studied at room temperature by "continuous straining". The results show that the nanowires display the ductile characteristic through the formation of the neck and atom-thick contact before failure. The deformation behavior of the polycrystalline nanowires is similar to single-crystalline fee metallic nanowire, while rather different from three-dimensional nanocrystalline Mo. We find the deformation behaviors and fractures of the nanowires depend heavily on the grain size and length-to-diameter ratio (LDR). When fixing the number of the grains, the nanowires with smaller grain size or LDR may exhibit superplasticity behaviors due to an amorphous rearrangement of atomic positions. However, the nanowires with larger grain size and LDR rupture at the strain of39%～76%and keep most crystalline structures. For the moderate grain size (3.90and4.92nm) nanowires, the stress-strain relation displays a distinct zigzag curve. The new phase nucleation and facture occur at the grain boundaries, which indicates the main deformation mechanism of PC nanowire is controlled by grain boundary effect. The tensile strength and fracture strain of nanowire increase correspondingly with increasing strain rate. At strain rate above2.0%ps-1, the superplasticity behaviors take place. During the deformation process, two types of structure transformation are observed in PC nanowires:(1) the body-centered cubic (bcc) configurations transform into others configurations, subsequently the others configurations transform into fcc or hcp configurations;(2) the fcc configurations convert into others configurations, then into bcc or hcp configurations. The failure heavier and phase transformations alike to PC nanowires occur also in SC nanowires. But the bcc configurations in the SC nanowire can transform into fcc configurations completely. Our simulation results show that the Young’s modulus of the SC Mo nanowire is about33%higher than that of PC Mo nanowire. Moreover, the main deformation mechanism of the SC nanowire is controlled by surface effect. Based radial distribution function and average energy curves, stress-induced and energy-controlled phase transformation mechanism have been clarified clearlyo in SC nanowires.The effects of strain rate, temperature and size on mechanical properties and behaviors of FeAl alloy nanowires and deformation processes have been studied by "quasi-static straining". The uniaxial tensileness simulation of FeAl alloy nanowire indicates the tensile deformation process of nanowire can be described as four characteristic regimes:elastic-plastic deformation of B2FeAl, the B2to body-centered tetragonal (BCT) transition, linear deformation and fracture of BCT FeAl. The nanowires exhibit brittle properties at low stain rate, whereas at high strain rate, they behave as ductile materials. The fracture strain increases with increased strain rate. The brittle to ductile transition (BDT) is observed in the nanowires with increasing temperture and the BDT temperatures lie in about1150K. Less than the temperature, the fracture strain decreases with increasing temperature, whereas more than the temperature, the fracture strain increases. The Young’s modulus, critical stress and strain decrease with increasing temperature.