  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.
