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

Abstract

New insight into the temperature dependent evolution of internal strain in the austenite phase during the martensitic phase transformation in NiTi shape memory alloys is provided via classical molecular dynamics simulations that employ well-established interatomic potentials for NiTi. It is shown, for the first time, that the developed strain tensor in the austenite phase is tetragonal in nature, with exponential temperature-dependence. Equally importantly, it is found that the developed internal strain (parallel to the habit plane) in the austenite varies linearly with the evolving martensite phase fraction. Interestingly, the Richard's equation is found to describe the temperature dependence of the martensite phase fraction as well as the internal strain components parallel to the habit plane in the austenite phase. An analysis of the temperature dependent phonon dispersion of strained austenite revealed the competition between phonon softening of the TA 2 branch and internal strain that leads to stabilization of the austenite phase in the two phase regime.

Atomistic simulation of shape memory effect (SME) and superelasticity (SE) in nano-porous NiTi shape memory alloy (SMA)

“…B19 variants are only observed in porous NiTi. As reported in other MD simulation studies [7,9,10,34], metastable B19 can only be stabilized due to the presence of surfaces, and thus only observed in porous NiTi, not in non-porous NiTi. Further details on the bond length based order parameter and evolution and estimation of different phases can be found in [4,[7][8][9][10]34] and in the supplementary material.…”

“…This section provides the information about the interatomic potential, MD simulation process in LAMMPS [32,33] and on the characterization techniques used to separate the spatial domain of austenite phase from that of martensite. A brief description of the simulation methods is given since they are well-established and described in details in the literature [9,10,34].…”

“…Different parameters of this EMA potential are estimated by fitting the properties (obtained from first principles calculations) of the B2 phase at 0 K, with the potential cut off radius = 4.2 Å [6,36,37]. It has been shown in several studies [6,7,9,10,34,36,37] that this modified potential provides accurate values of the lattice constants and energies of different phases of NiTi, when compared to those obtained from ab-initio calculations [36,37]. Further, other studies confirm that this modified potential predicts appropriately (i) the phase transformation (PT) process due to the temperature i.e.…”

“…shape memory effect (SME) and super-elasticity (SE) [4,[7][8][9][10]36,37], (ii) transformation temperatures and stress values [4,6,8,9,34,36,37], and the phase fraction evolution kinetics and the stress-strain responses [4,6,8,9,34,37]. Since this potential is well documented in the literature [6,7,9,10,34,36,37], further details are not provided here; rather, a brief description is provided in the supplementary material.…”

“…The evolution of martensite phase fraction is found to follow the Richards equation [39], i.e., Eqns. (1), with respect to temperature [7,9,10,34,39], where the parameters entering the equation depend on the porosity. Eqn.…”