Deformation mechanisms of Inconel-718 at nanoscale by molecular dynamics

TL;DR

This study uses molecular dynamics simulations to analyze how dislocations, cooling rates, voids, and temperature affect the deformation and failure mechanisms of Inconel-718 at the nanoscale, revealing a brittle-ductile transition driven by cooling rate.

Contribution

It introduces a novel MD analysis method to examine nanoscale deformation mechanisms of Inconel-718 considering dislocations, cooling rates, and voids, providing insights into failure processes.

Findings

Higher strain rates increase dislocation density and strain hardening.

High cooling rates lead to high strength and ductility.

Cooling rate influences brittleness and void healing during solidification.

Abstract

Ni-based superalloy Inconel-718 is ubiquitous in metal 3D printing where high cooling rate and thermal gradient are present. These manufacturing conditions are conducive to high initial dislocation density and porosity or void in the material. This work proposes a molecular dynamics (MD) analysis method that can examine the role of dislocations, cooling rates, void, and their interactions governing the material properties and failure mechanism in Inconel-718 using Embedded Atom Method (EAM) potential. Three different structures: nanowire (NW), nanopillar, and nanoplate are used throughout this work. Initially, strain rates are varied from 10^8s^-1 to 10^10s^-1 keeping the NW diameter and temperature constant at 3.17 nm and 300K respectively. Compressive loading is applied to a 7.04 nm nanopillar by applying a constant strain rate of 109 s^-1 while temperature is varied from 100K to 700K. Different cooling rates ranging from 0.5x10^10 K/s to 1 x 10^14 K/s are applied to nanoplates (with and without a central void). The size of the central void is kept fixed at 2.12 nm. Increasing strain rates in tension not only results in strain hardening but also increase in dislocation density. Our computational method is successful to capture extensive sliding on {111} shear plane, which leads to significant necking of the alloy before fracture due to dislocation. The high cooling rates creating non-equilibrium structure leads to high strength and ductile behavior. On the other hand, the low cooling rate forming well defined crystalline structure causes low strength and brittle behavior. This brittle to ductile transition is observed solely due to cooling rate. Cooling rate may diminish the void by healing the structure during solidifications process. Subsequent mechanical properties by varying temperature and size are also presented in detail.