Atomistic Study on Size Effects in Thermally Induced Martensitic Phase Transformation of NiTi

Abstract

The atomistic study shows strong size effects in thermally induced martensitic phase transformation evolution kinetics of equiatomic NiTi shape memory alloys (SMAs). It is shown that size effects are closely related to the presence of free surfaces; thus, NiTi thin films and nanopillars are studied. Quasi-static molecular dynamics simulations for several cell sizes at various (constant) temperatures are performed by employing well-established interatomic potentials for NiTi. The study shows that size plays a crucial role in the evolution of martensite phase fraction and, importantly, can significantly change the phase transformation temperatures, which can be used for the design of NiTi based sensors, actuators, or devices at nano- to microscales. Interestingly, it is found that, at the nanometer scale, Richard’s equation describes very well the martensite phase fraction evolution in NiTi thin films and nanopillars as a function of temperature.

Evolution of internal strain in austenite phase during thermally induced martensitic phase transformation in NiTi shape memory alloys

“…This section provides information on the MD simulation process in LAMMPS [32,33] and on the characterization techniques used to separate the spatial domain of austenite from that of martensite phase. A brief description of the simulation methods is given since they are wellestablished and described in details in the literature [5][6][7][8]13,14,[34][35][36][37][38].…”

“…MD simulations were performed within the LAMMPS [32,33] software framework, employing a previously proven well effective interatomic potential [5][6][7][8]13,14,38,37,[34][35][36] for capturing the phase transformation in the NiTi SMA. Here, simulations are performed in all three spatial dimensions (3D).…”

“…Thus, instead of showing distinct values (like delta function) at +1.0 (for B19 phase) or -1.0 (for B2 phase), this bond length based order parameter for B19and B2 phase exhibits a narrow Gaussian distribution around +1.0 and -1.0, respectively [5][6][7][8]35]. It has been observed that this order parameter shows Gaussian distribution around a mean of μ χ =± (1 ± αΔT ± ε ¿ ) , and with standard deviation of σ χ =√(αΔT ) 2 +(ε ¿ ) 2 .…”

“…The spread in the Gaussian curve of the martensite (B19) phase provides a measure of uncertainty in the martensite phase fraction evolution. A more detailed description of the martensite phase fraction calculation based on this order parameter is provided in Mutter and Nielaba [8,35] and in other relevant studies by the authors [5][6][7].…”

“…austenite (B2) to martensite (B19) and vice versa) phase transformation in NiTi SMA has been exploited widely [1,[4][5][6][7]. It is observed that, the evolution and growth rate kinetics of different phases in NiTi SMAs at the continuum level differs from that at the nanometer scale, mainly because of the vast amount of free surfaces at the nanometer scale [6][7][8]. The thermally induced martensite phase transformation process along length scales is studied in [6], i.e.…”