Size effects in NiTi from density functional theory calculations

Abstract

We use density functional theory to characterize how size affects the relative stability of thin NiTi slabs of different crystal structures and its implication on the martensitic phase transition that governs shape memory. We calculate the surface energies of B2' phase (austenite), B19 (orthorhombic), B19' (martensite) and a body centered orthorhombic phase (BCO), the theoretically-predicted ground state. We find that (110) B2 surfaces with in-plane atomic displacements stabilize the austenite phase with respect to B19' and BCO, thus slabs with such orientations are predicted to exhibit a decrease in martensite transition temperature with decreasing thickness. Our calculations predict a critical thickness of 2 nm, below which the transition would not occur. The opposite trend is observed in slabs with atomic displacements along the surface normal: the phase transformation temperature increases with decreasing size.

Compositional dependence of structures of NiTi martensite from first principles

“…In the BCO structure, the angle corresponding to the monoclinic angle in B19 0 is approximately 107°, which is larger than that experimentally observed value of $97° [2][3][4][5]. The stability of the BCO structure has been confirmed by several theoretical studies using first-principles calculations [6][7][8][9][10][11]. However, the BCO structure has not been experimentally observed.…”

Interatomic potential for the NiTi alloy and its application

“…The previous QE calculations [2,20] with ultrasoft pseudopotentials (USPP) plus generalized gradient approximation (GGA) have offered good descriptions of structural parameters and energy differences among B2, B19, B19 0 and BCO phases. We use the same parameter settings for QE as Vishnu et al [20] to calculate the energies of all structures for about 10 volumes around the equilibrium volume.…”

“…The previous QE calculations [2,20] with ultrasoft pseudopotentials (USPP) plus generalized gradient approximation (GGA) have offered good descriptions of structural parameters and energy differences among B2, B19, B19 0 and BCO phases. We use the same parameter settings for QE as Vishnu et al [20] to calculate the energies of all structures for about 10 volumes around the equilibrium volume. Since ab initio and empirical potential calculations have different reference states, ab initio energy e E for a given element or compound Ni 1Àn Ti n needs to be shifted to empirical potential energy E according to the following equation, E Ni 1Àn Tin ¼ e E Ni 1Àn Tin À ð1 À nÞðE Ni À e E Ni Þ À nðE Ti À e E Ti Þ; ð4Þ…”

Ab initio simulations of phase stability and martensitic transitions in NiTi

“…Considerable computational work has been performed to understand the phases of NiTi and related materials. In particular, density functional theory (DFT) studies [9][10][11][12][13][14][15][16][17][18][19][20][21][22] have provided many insights into the energetics and properties of NiTi; but they have also generated new unanswered questions. For example, DFT formation energies for B2 are in good agreement with experiments; [23][24][25][26] however, B2 is predicted to be dynamically unstable at T = 0, i.e.…”