Committee Chair

Mahtabi Oghani, Mohammad Javad, 1982-

Committee Member

Ibrahim, Hamdy; Yadollahi, Aref, 1984-; Bhosale, Rahul

Department

Dept. of Computational Science

College

College of Engineering and Computer Science

Publisher

University of Tennessee at Chattanooga

Place of Publication

Chattanooga (Tenn.)

Abstract

With the growing demand for high-temperature shape memory alloys (HTSMAs), NiTi-based HTSMAs have gained more attention for implementation in applications at elevated temperatures. Among NiTi-based alloys, NiTiHf has shown to have great potential, by having high transformation temperatures, lower preparation cost and good thermal stability. However, until today, most studies conducted on NiTiHf have focused on a limited range of compositions due to the difficulties and high cost of the experimental studies. Therefore, there exists a lack of comprehensive research on the thermo-mechanical behavior of these HTSMAs. Computational simulations are a very cost-effective and feasible approach for addressing this shortcoming. Among computational methods, molecular dynamics (MD) simulation as an atomistic method offers comprehensive means to explore microstructural phenomena that govern the behavior of material. An essential requirement for performing MD simulations is interatomic potential which serves as the constitutive equations of MD and determine the forces and interactions between atoms. Since no applicable interatomic potential has been developed for NiTiHf, there has not been any progress in MD studies on NiTiHf alloys. Therefore, in this study, a Second Nearest-Neighbor Modified Embedded Atom Method (2NN MEAM) interatomic potential has been developed to accurately represent the NiTiHf ternary system. Initially, the parameters of constituent unary and binary potentials were calibrated by fitting their reproduced results of physical properties to DFT results. Then, the final ternary MEAM potential was checked for reliability and transferability by performing MD simulations. The results showed that the developed potential can accurately capture temperature-induced and stress-induced martensitic phase transformation in NiTiHf. In addition, the lattice parameters and formation energy of different compositions of NiTiHf were obtained and compared with experimental and DFT (Density Functional Theory) results showing a good agreement. Furthermore, using the developed MEAM potential, MD simulations were conducted to analyze the influence of precipitates on the superelasticity and shape memory effect of NiTiHf alloy. The results showed that in the presence of H-phase precipitates, the transformation temperatures increase. In addition, the thermal cycling of NiTiHf under constant stress was simulated and it was found that reducing the temperature rate results in a narrower thermal hysteresis.

Degree

Ph. D.; A dissertation submitted to the faculty of the University of Tennessee at Chattanooga in partial fulfillment of the requirements of the degree of Doctor of Philosophy.

Date

12-2023

Subject

Alloys--Effect of high temperatures on; Alloys--Thermomechanical properties

Keyword

High-temperature shape memory alloy; NiTiHf; MEAM potential; Molecular dynamics; H-phase precipitate; Thermal actuation

Document Type

Doctoral dissertations

DCMI Type

Text

Extent

xvi, 99 leaves

Language

English

Rights

http://rightsstatements.org/vocab/InC/1.0/

License

http://creativecommons.org/licenses/by/3.0/

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