Investigation of mechanical properties of NiAl nanowires by means of molecular dynamics simulations

In this thesis, it has been modeled shape memory Ni-30Al alloy in a form of nanowires in order to investigate their stress-strain behavior by means of molecular dynamics simulations. Embedded atom method has been used to define the physical interactions between atoms. The simulations have been realized by using the LAMMPS simulator package having open source code. Molecular dynamics simulations have been done for the models with 11 different thicknesses, the same length and the same crystal orientation [001], but at different temperatures 0, 300K, 500K, 700K and 900K, and 10 different strain rates from 1m/s to 10m/s used to obtain stress-strain relations. By using the stress-strain curves the values of Young's modulus, yield strength, ultimate strength and fracture points of nanowires were obtained. Temperature, thickness and strain rate dependencies of these mechanical parameters were determined. The results were also discussed comparatively using the results found in the literature. It has been determined that the Young's modulus of shape memory Ni-30Al nanowires change nonlinearly with temperature, i.e. they increase first up to 500K temperature at which the phase transition starts, and then decrease with temperature. The Young's module of the fine nanowire with 1.44nm thickness was determined as 72GPa for a strain rate of 1m/s and at 300K. At the same conditions and for the same nanowire the yield strength and the ultimate stress were 3.15Gpa and 5Gpa, respectively. The Young's modulus at 300K decrease as increasing thickness of nanowires. The Young's modulus for the thicker nanowire with 4.32nm thickness has a value of about 40Gpa. Similarly, the values of yield strength also decrease as increasing thickness of nanowires. Yield strength is 1.75Gpa for the thicker nanowire at 300K. The effect on the Young's modulus of the strain rate is very complex and exhibits a non-linear behavior depending on the nanowire thickness. This result reveals that the thickness of nanowire is the most important factor affecting the formation of the dislocations in the wire, and so the mechanical properties.