Insights into the role of Nb segregation on grain boundary structural transition and mechanical response in a Ni-Nb system

The role of Nb segregation on the grain boundary (GB) structural transformation and mechanical response has been studied in a binary Ni-Nb system through atomistic simulation. To investigate the effect of dopant con-centration on the GB structural transitions, a sigma 5 (310) GB has been created with the help of molecular statics simulation. Thereafter, hybrid Monte Carlo and Molecular dynamics (MD) simulations have been performed to segregate Nb atoms at the sigma 5 (310) GB. At lower dopant concentrations, the Nb atoms preferably segregated at the tip of the kite structure. With an increase in dopant concentration and temperature, the Nb atoms segregated at other sites in the GB as well as in the matrix and the GB structure becomes disordered. The effect of doping on the mechanical behavior has been investigated by applying constant strain rate along X-direction through MD. The stress-strain curve of the Ni-Nb system at 300 K revealed an increase in the strength and ductility with the dopant concentration up to a certain limit (0.4 at. % Nb) and decreased thereafter. The superior mechanical response of the Ni-0.4Nb system is attributed to the formation of stacking faults and sessile dislocations during deformation. Stacking faults delay the formation of cracks while sessile dislocations impede the dislocation motion, resulting in higher ductility and strength. Contrarily, nucleation of voids and cracks takes place at a lower extent of strain, resulting in an early fracture in the pure Ni. However, the strength of the doped specimen is found to be lower than the undoped one at 700 K due to the early initiation of cracks in the disordered GB region of the former specimen. The comprehensive understanding of the effect of interfacial segregation on the structural and mechanical behaviour would open up new avenues for alloy design and the development of advanced materials.