Molecular Dynamics Simulations Of Nanocrystalline Ta & Fe Under Uniaxial Tension

Using molecular dynamics simulation, we have investigated the mechanical properties and the microstructural evolution of nanocrystalline Ta&Fe (grain size varies from 3.25 nm to 12.99 nm for Ta and varies from 2.82nm to 8.47nm) under uniaxial tensile loading. The results show that the elastic modulus of nanocrystalline Fe&Ta, with nearly zero porosity, has a linear relationship with the relative density of the samples. The tensile strength of both Fe and Ta increases as the grain size is increased for the grain size region studied, revealing a reverse Hall-Petch effect. The simulation results of nanocrystalline Ta indicate that the yield stress of Ta increases with increasing strain rate (2×10⁷-2×10⁹_s^-1) or decreasing temperature (10^-6-1500K). Grain rotation, grain boundary sliding or movement and dislocation migration are observed during the deformation process of Fe&Ta samples. The results of simulations performed at nearly zero temperature demonstrate that stress-induced phase transitions from body-centered cubic (BCC) to face-centered cubic (FCC) and hexagonal close-packed (HCP) structures take place during the deformation process of both Fe and Ta, and the maximum fraction of FCC atoms varies linearly with the tensile strength, from which we can conclude that a critical stress exists for the phase transition to take place. Strain rate may not influence the maximum fraction of FCC&HCP atom, however, it will influence the strain at which phase transition begins to occur. Inter-granular fracture was clearly observed in the deformation process of nanocrystalline Ta, whereas similar phenomenon did not occur remarkably during the deformation of Fe. In the range of simulations performed here, the strain at which the crack formation is initiated is seen to be not affected by the average grain size, but influenced by the strain rate and temperature. Twining is also clearly observed in the simulations of Ta.Similar to nanocrystalline FCC metals, the deformation mechanism of nanocrystalline BCC metals can be explained as being dominated by grain rotation, grain boundary sliding or movement and dislocation migration. Nonetheless, when the flow stress is great enough, phase transition will occur to accommodate the deformation of both Fe and Ta;in addition, during the deformation process of Ta, inter-granular fracture is also an effected mechanism through the formation of which a large component of the strain is accommodated. The two later mechanisms are quite different from what have been observed in the simulations of nanocrystalline FCC metals.