Shape-Memory Transformations of NiTi: Minimum-Energy Pathways between Austenite, Martensites, and Kinetically Limited Intermediate States

Abstract

NiTi is the most used shape-memory alloy, nonetheless, a lack of understanding remains regarding the associated structures and transitions, including their barriers. Using a generalized solid-state nudge elastic band (GSSNEB) method implemented via density-functional theory, we detail the structural transformations in NiTi relevant to shape memory: those between body-centered orthorhombic (BCO) groundstate and a newly identified stable austenite ("glassy" B2-like) structure, including energy barriers (hysteresis) and intermediate structures (observed as a kinetically limited R-phase), and between martensite variants (BCO orientations). All results are in good agreement with available experiment. We contrast the austenite results to those from the often-assumed, but unstable B2. These high-and low-temperature structures and structural transformations provide much needed atomic-scale detail for transitions responsible for NiTi shape-memory effects.

Reliable thermodynamic estimators for screening caloric materials

“…In materials with structural and magnetic instabilities (or, more generally, "dimpled" potential energy surfaces), this assumption is invalid, at least near temperatures, where the crossover between states occur and key associated properties manifest. With the premartensitic instability 80 in AFM(111) B2 FeRh, similar (but smaller) to that in NiTi austenite, 16,17 care must be taken to calculate accurately the lattice entropy.…”

“…[19][20][21][22][23][24] Both B2 austenites (FeRh below 353 K and NiTi above 313 K) have a premartensitic instability with known unstable phonon modes. 16,17 While they both show elasto-and baro-caloric responses, FeRh also exhibits a giant magnetocaloric effect at its metamagnetic transition from an antiferromagnetic (AFM) to a ferromagnetic (FM) state at the critical temperature T c of 353 K, with a 1% decrease in density. 14 Properties of FeRh were extensively studied experimentally [12][13][14][19][20][21] and theoretically.…”

Nudged-elastic band method with two climbing images: Finding transition states in complex energy landscapes

“…We apply C2-NEB to study (A) a deformation of the martensitic structure in the NiTi shape-memory alloy; [7][8][9][10][11] (B) its more complicated transformation to the austenite; and FIG. 4.…”

Titaniumα−ωphase transformation pathway and a predicted metastable structure

“…3). For accuracy, we use the C2NEB method, 23,24 as tested on shape-memory transforms, 25,26 to verify each TS. First, we turn off climbing and then sample the path by equidistant images.…”