Atomistic simulation of mechanical behaviours of nanocrystalline AI alloys with grain boundary segregation
Date of Issue2017-11-23
School of Mechanical and Aerospace Engineering
Bulk nanocrystalline (NC) metals and alloys demonstrating very high ultimate strength compared to conventional coarse-grained materials have a great potential for structural applications. However, their usage is limited due to low ductility and premature intergranular fracture at ambient conditions. An increase in resistance of NC metallic materials against cracking along grain boundaries (GBs) is one of the important tasks of materials science. It is believed that the possible method for their mechanical property improvement is to utilize GB segregation hardening which can be expected only for materials having very dense GB networks. Therefore, the PhD research aims to understand the mechanical behaviours of NC aluminium (Al) with GB segregation of various alloying elements through numerical simulation. The effects of different additional elements in GBs of NC Al on the elastic-plastic, fracture and cyclic behaviours are systematically studied with the consideration of the embrittlement potency, stress-strain relation, structure evolution and deformation mechanisms of the materials via molecular statics (MS) and molecular dynamics (MD) simulations. In order to estimate the ability of impurity elements such as Fe, Co, Cu, Ti, Mg and Pb to resist brittle GB separation in NC Al, the GB segregation energy and the embrittlement potency for the mentioned elements located at different GBs of Al bi-crystals are calculated using the MS simulation. It is shown that the Co atom in GBs of the Al bi-crystals has a noticeable positive effect on the GB strength, while Pb has a negative effect. These results are compared with those obtained by conducting the MD simulation of the stress-strain test for NC binary Al alloys with GB segregation. It is found that GB can be strengthened (weakened) by GB segregation of Fe or Co (Cu or Pb), but is nearly not affected by those of Ti or Mg. The results of the MD simulation are in a good agreement with those obtained from the MS simulation. Furthermore, the effect of GB segregation of Co, Fe, Ti, Mg or Pb on the elastic behaviour of NC binary Al alloys is investigated via the MD simulation. The alloying element distribution and temperature effects are considered. It is shown that alloying can significantly affect the elastic moduli of NC Al. The decrease in the atomic radii of additional elements increases rigidity of the alloys with GB segregation. It is revealed that the shear modulus of the Al-Co alloy with Co atoms in GBs does not undergo significant changes with the increase in temperature in comparison with the NC Al and the other considered alloys. The simulation results show that GB segregation can strongly affect not only the elastic behaviour but also plastic and fracture behaviours of the NC binary Al alloys. For example, it is found that within the considered alloying elements, the GB segregation of Co, Ti or Pb atoms improve the ductility of NC Al, while its strength can be increased only by the addition of Co or Fe. The failure of the NC Al alloys with GB segregation of Fe or Mg occurs via intergranular fracture, while the other additional elements in GBs do not lead to cracking until 40% shear and tensile deformation. The difference in deformation behaviours of the alloys is further studied via the analysis of deformation mechanisms. Two main deformation mechanisms are identified to be responsible for the plastic deformation of the NC Al having GB segregation during its shear loading along the GB plane: GB sliding (GBS) and GB motion (GBM) which normally work simultaneously during a deformation process. In comparison with these two deformation mechanisms, a contribution from dislocation sliding is not so pronounced. A uniaxial tensile loading perpendicular to GB planes of Al promotes dislocation activity and twinning rather than GBS and GBM. It is revealed that in this case, an increased strength of the NC Al alloys with GB segregation can be explained by suppression of the perfect dislocation sliding and twinning and activation of the partial dislocation slip in a structure. It is observed that such deformation behaviour can reduce the crack propagation rate and residual strain during material cyclic loading. This study contributes to the understanding of the deformation and damage behaviours of NC metallic materials with GB segregation, which is essential for their potential applications.