Effect of substitutional and antisite defects on the mechanical and thermal properties of shape-memory alloy NiTi

Substitutional and antisite defects can play an important role in the thermodynamics and mechanical properties of intermetallic alloys. We compute the thermodynamic properties of shape-memory alloy nickel-titaniun in the B19' martensitic and B2 austenitic phases from molecular dynamics (MD) using a classical potential in the temperature range from 200K to 600K and composition range from 45 atomic percent to 55 atomic percent titanium. We developed and used a python framework to generate input files, run the simulations, and analyze the data, which we call Utilities To Execute Pipelines (UTEP). Thirty-nine lattice parameters and five temperatures were simulated for eleven distinct compositions around equiatomicity, each with four distinct defect concentrations on 16,000-atom supercells, for a total of 25,740 MD simulations of 2000 time steps each. We extracted temperature-dependent materials parameters from fits to the internal energies by fitting the Birch-Murnaghan equation of state. Thermodynamic properties such as the vibrational entropy were obtained from the materials parameters using the Moruzzi-Janak-Schwarz approximation. Finally, the configurational entropy due to the defects was obtained from the Warren-Cowley parameters up to fourth nearest neighbors. Defects are energetically unfavorable, but can become thermodynamically stabilized at high temperature, particularly on the nickel-rich side. The bulk modulus decreases with temperature in the nickel-rich side, while the opposite is observed on the titanium-rich side.