Deformation mechanisms of Inconel-718 at the nanoscale by molecular dynamics

Abstract

Ni-based super alloy Inconel-718 is ubiquitous in metal 3D printing where a high cooling rate and thermal gradient are present. These manufacturing conditions are conducive to high initial dislocation density and porosity or voids in the material. This work proposes a molecular dynamics (MD) analysis method that can examine the role of dislocations, cooling rates, voids, and their interactions governing the material properties and failure mechanisms in Inconel-718 using the Embedded Atom Method (EAM) potential. Throughout this work, three different structures – nanowires (NWs), nanopillars (NPs), and thin-plates – are used. The strain rate is varied from 10⁸ s⁻¹ to 10¹⁰ s⁻¹ and the temperature is varied from 100 K to 800 K. Different cooling rates ranging from 0.5 × 10¹⁰ K s⁻¹ to 1 × 10¹⁴ K s⁻¹ are applied. Our results suggest that the high cooling rates create regular crystalline structures which result in high strength and ductility. In contrast, the lower cooling rates form a non-crystalline structure that exhibits low strength and a brittle nature. This brittle to ductile transition is observed solely due to the cooling rate at the nanoscale. Elimination of voids as a result of heat treatment is reported as well. Shockley dislocation is observed as the key factor during tensile plastic deformation. Increasing strain rates result in strain hardening and a higher dislocation density in tension. Our computational method is successful in capturing extensive sliding on the {111} shear plane due to dislocation, which leads to necking before fracture. Furthermore, notable mechanical properties are revealed by varying the temperature, size and strain rate. Our results detail a pathway to design machine parts with Inconel-718 alloy efficiently in a bottom-up approach.