Polycrystallinity and grain size effects on tensile and creep behaviour of tungsten nanomaterial

Abstract

Though use of Tungsten (W) as nanomaterial has been increasing there is lack of knowledge on its failure behaviour and other mechanical properties at nanoscale. In order to investigate plasticity and creep mechanism at the nanoscale, single and polycrystalline nanowires and nanocubes of W are studied in this work. In case of single and polycrystalline nanowire, diameter, temperature, loading orientation, and grain size are varied to see the resulting impact on the deformation mechanism using EAM potential and a constant strain rate of 109 s−1. Results of the analyses indicate domination of twinning deformation over dislocation in single crystal nanowires. However, in polycrystalline nanowires, the failure mechanism is governed by the twin-grain boundary interaction. Additionally, strength of polycrystalline nanowires follow inverse Hall-Petch rule. In the latter part of the thesis, nature of creep in nanocrystalline tungsten and factors that govern creep mechanism such as grain size, temperature, and applied stress are studied through atomistic simulations. From the simulations, it has been observed that the creep mechanism in nanocrystalline tungsten is contingent on the applied stress as the creep mechanism varies from lattice diffusion to grain boundary diffusion creep eventually to dislocation-creep with higher stress. Moreover, temperature and grain refinement seem to aid the creep phenomenon at the nanoscale. It is also identified that for very large values of stress and temperature, the power-law fails to define the creep in nc-tungsten. To quantify the propensity and mechanism of creep in nanocrystalline tungsten, stress and grain size exponents are determined in addition to the time evolution of strain and mean square displacement. Finally, atomistic features of deformation are analyzed which evince the simulation results.